System consisting of switchable encapsulated luminescent pigments

Abstract

The invention relates to a system consisting of at least two types of switchable encapsulated luminescent pigments, wherein the encapsulated luminescent pigments are polymer particles with a diameter of 0.1-1000 µm, each particle containing at least one luminescent substance and a photochemically switchable absorber; a first type of encapsulated luminescent pigment comprises a first luminescent substance which emits a first color in the VIS spectral range; a second type of encapsulated luminescent pigment comprises a second luminescent substance which emits a second color in the VIS spectral range, said first and second colors being different; and the two encapsulated luminescent pigments each comprise the same photochemically switchable absorber, said photochemically switchable absorber having a first and a second switched state, and the absorption spectrum of the photochemically switchable absorber greatly overlapping with the excitation spectra of the first and the second luminescent substance in the second switched state. The invention further relates to a switchable luminescent printing ink, comprising a system consisting of at least two types of switchable encapsulated luminescent pigments as described herein, and to a value document, preferably a banknote, having a switchable luminescent imprint, comprising a system consisting of at least two types of switchable encapsulated luminescent pigments as described herein.

Description

[0001] SYSTEM OF SWITCHABLE CAPSULE LUMINOUS PIGMENTS

[0002] Technical field

[0003] The present invention relates to a system of at least two types of switchable capsule luminescent pigments, wherein the capsule luminescent pigments are polymer particles with a diameter of 0.1-1000 pm, each containing at least one luminescent substance and a photochemically switchable absorber, wherein a first type of capsule luminescent pigment comprises a first luminescent substance emitting in a first color in the VIS spectral range; a second type of capsule luminescent pigment comprises a second luminescent substance emitting in a second color in the VIS spectral range, wherein the first and the second color are different;and the two capsule luminescent pigments each comprise the same photochemically switchable absorber, wherein the photochemically switchable absorber has a first and a second switching state, and the absorption spectrum of the photochemically switchable absorber in the second switching state significantly overlaps with the excitation spectrum of the first and the second luminescent material. The present invention further relates to a switchable luminescent printing ink comprising a system of at least two types of switchable capsule luminescent pigments as described herein, and to a security document, preferably a banknote, with a switchable luminescent print comprising a system of at least two types of switchable capsule luminescent pigments as described herein.

[0004] Technical background

[0005] Valuable documents and other valuables have long been protected against counterfeiting by being printed with inks that luminescent under UV light. Multicolored images created using several different luminescent inks, especially in the primary colors red, green, and blue, have a particularly high aesthetic and recognition value.

[0006] To further increase brand recognition, it is known to use luminescent printing inks with dynamically changing luminescence effects, particularly by exploiting the following effects:

[0007] - Combination of a rapidly decaying luminescent material with a phosphor, especially one with a different emission color. This, however, requires switching the excitation light on and off. Combination of several separately excitable luminescent materials. This, however, requires the use of multiple excitation wavelengths.

[0008] Combination of a luminescent substance with a photochemically switchable absorber.

[0009] Photochemically switchable absorbers (hereinafter also referred to as "switches") are particularly well-known examples of diarylethenes, which can be repeatedly and reproducibly switched from a first to a second switching state under UV or visible light. The two switching states have different chemical structures and thus differ primarily in their absorption spectrum in the visible spectral range (VIS range). Such switchable absorbers are also called photochromic substances. Compared to other photochromic substances, diarylethenes are distinguished by their resistance to aging and their non-toxicity, and they also achieve a particularly high number of switching cycles.The switching effect is based on the fact that one of the two different absorption spectra of the two switching states does not overlap with the excitation spectrum of an additionally present luminescent substance, while the other of the two different absorption spectra of the two switching states does overlap with the excitation spectrum of an additionally present luminescent substance. In the case of overlap, a non-radiative energy transfer, such as a Förster resonant energy transfer (FRET), can occur between the luminescent substance and the switch, which causes the luminescence of the luminescent substance to be "switched off." Since a certain amount of radiative energy is required for the switch molecules to change from the first to the second switching state under appropriate illumination (e.g., by UV light), the luminescence of the additionally present luminescent substance only gradually fades for the human eye.For example, if a substrate printed with a printing ink containing the switch and the luminescent substance, such as a valuable document, was held under a UV lamp for a certain period of time.

[0010] To optimize printing properties and aging resistance, it is also known to incorporate luminescent substances into polymer capsules, as described, for example, in WO 2017 / 080 653 Al.

[0011] CN 1 13 174 004 A describes a switchable luminescent material based on tetraphenylporphyrin (TPP) and a diarylethene, where the TPP and the diarylethene are encapsulated in nanoparticles. In the switched-on state, the diarylethene emits green light; in the switched-off state, it is dark, i.e., a color change in the luminescence can be observed. In this case, the diarylethene itself emits differently in the two switching states but has no influence on the luminescence of the TPP. The TPP itself always emits a static red light. Thus, only emission colors are mixed.

[0012] WO 2017 / 080654 A1 describes a system of differently colored encapsulated luminescent pigments with identical properties regarding excitation wavelength, aging and environmental resistance, lightfastness (also referred to here as "fatigue resistance"), and printing properties. In particular, several luminescent substances can be encapsulated together using energy transfer. The term "luminescent substances using energy transfer" means that a first luminescent substance is excited by radiation, for example, UV radiation, and the radiation emitted by the first luminescent substance excites a second luminescent substance. The second luminescent substance then emits radiation that determines the overall color impression of the encapsulated luminescent substances, since the luminescence of the first substance is essentially completely absorbed by the second luminescent substance and thus does not contribute to the overall color impression.However, WO 2017 / 080654 Al does not describe switchable luminescence of the encapsulated luminescent substances.

[0013] WO 2021 / 074275 Al describes lanthanide complexes with photochromic ligands, in particular diarylethenes. Here, a chemical bond exists between the luminescent substance and the photochromic ligand to achieve and ensure the spatial proximity required for FRET energy transfer between the luminescent substance and the photochromic ligand. Emission colors are only mixed in this process.

[0014] CN 1 06 978 164 A describes a mixture of an (organic or inorganic) perovskite and a diarylethene for switchable luminescence. However, the switch and the luminescent material are not encapsulated together, and the extinguishing of the luminescent material after switching the switch is insufficient. Furthermore, no luminescent materials are combined in this formulation.

[0015] A number of specific individual examples of switchable luminescent materials are known. However, to provide security features that are both easily verifiable in everyday use and aesthetically pleasing, a system of luminescent pigments would be desirable. In such a system, not only the excitation wavelength (for simple testing using only one wavelength, particularly 365 nm, as is common practice for security checks of banknotes at cash registers), aging and environmental resistance, lightfastness, and printing properties would be identical for all luminescent pigments, but also the wavelength and timescale for switching off, the conditions (temperature, illumination) and timescale for switching back on, as well as the aging properties of the switch. Ideally, the switching depth of the different pigments should also be comparable.

[0016] These requirements cannot be met with currently known switchable luminescent pigments (whether encapsulated or unencapsulated), because, firstly, different switchable absorbers must be used for luminescent substances with different emission wavelengths in order to achieve the necessary good overlap between the emission spectrum of the luminescent substance and the absorption spectrum of the (activated) switch. However, different switchable absorbers generally differ with regard to the aforementioned properties. Known systems with multiple luminescent pigments for producing luminescence-based security features with different color impressions are not switchable.

[0017] The present invention is therefore based on the objective of providing improved systems with multiple luminescent pigments compared to the prior art, which exhibit at least some, ideally all, of these advantageous properties. Furthermore, the present invention aims to provide improved luminescent printing inks that can be used to produce security features. Finally, the present invention aims to provide securities, preferably banknotes, that exhibit improved security features with the advantageous properties described above.

[0018] Summary

[0019] According to a first aspect, the present invention relates to a system of at least two types of switchable capsule luminescent pigments, wherein the capsule luminescent pigments are polymer particles with a diameter of 0.1-1000 pm, each containing at least one luminescent substance and a photochemically switchable absorber, wherein a first type of capsule luminescent pigment comprises a first luminescent substance emitting in a first color in the VIS spectral range; a second type of capsule luminescent pigment comprises a second luminescent substance emitting in a second color in the VIS spectral range, wherein the first and the second color are different;and the two capsule luminescent pigments each comprise the same photochemically switchable absorber, wherein the photochemically switchable absorber has a first and a second switching state, and the absorption spectrum of the photochemically switchable absorber in the second switching state significantly overlaps with the excitation spectra of the first and the second luminescent substance.

[0020] The present invention further relates, according to a second aspect, to a switchable luminescent printing ink comprising a system of at least two types of switchable capsule luminescent pigments, according to the first aspect of the present invention.

[0021] According to a third aspect, the present invention further relates to a security document, preferably a banknote, with a switchable luminescence print comprising a system of at least two types of switchable capsule luminescence pigments, according to the second aspect of the present invention.

[0022] Figures

[0023] Figure 1 shows the absorption spectra of a diarylethene switch (DAE 2) before and after switching (SZ1 and SZ2) as well as the excitation spectrum of a luminescent substance (LS2) in the range of 230 to 730 nm.

[0024] Figure 2 shows the absorption spectra of a diarylethene switch (DAE 1) according to circuit (SZ2) and the excitation spectrum of a luminescent substance (LS2) in the range of 300 to 450 nm.

[0025] Figure 3 schematically shows the switching on and off of luminescence in a type of capsule luminescence pigments from the system according to the invention when irradiated with UV light by switching the photochemically switchable absorber from the state “ON” to “OFF”.

[0026] Figure 4A shows various monochrome printing patterns, comprising a switchable luminescent printing ink according to the invention and a non-switchable luminescent printing ink, before the switching of a photochemically switchable absorber.

[0027] Figure 4B shows various monochrome print patterns, comprising a switchable luminescent printing ink according to the invention and a non-switchable luminescent printing ink, after the switching of a photochemically switchable absorber. Figure 5A shows various print patterns, comprising a switchable luminescent printing ink according to the invention and a non-switchable luminescent printing ink, before the switching of a photochemically switchable absorber.

[0028] Figure 5B shows various printing patterns, comprising a switchable luminescent printing ink according to the invention and a non-switchable luminescent printing ink, after switching a photochemically switchable absorber.

[0029] Figure 6 shows a sample of a monochrome, switchable melange fiber comprising a print with a switchable luminescent printing ink according to the invention and a non-switchable luminescent printing ink.

[0030] Figure 7A shows a visually recognizable pattern for a switchable color-changing fiber prior to the switching of a photochemically switchable absorber.

[0031] Figure 7B shows a visually recognizable pattern for a switchable melier fiber with color change after switching a photochemically switchable absorber.

[0032] Detailed description of the invention

[0033] The present invention is based, firstly, on the finding that the luminescence of a luminescent substance can be switched off even if an absorption band of a switch sufficiently overlaps with the excitation spectrum of the luminescent substance. This is surprising, since it has always been assumed that for a luminescent substance to be successfully switched off, there must be sufficient overlap between the absorption spectrum of the diarylethene switch and the emission spectrum of the luminescent substance so that a Förster resonant energy transfer (FRET) between the luminescent substance and the switch can occur, which then switches off the luminescence of the luminescent substance.Without being bound to any specific theory, it is assumed that when the excitation spectrum of the luminescent material overlaps with the absorption spectrum of the switch, excitation from the ground state to the excited state of the luminescent material with the excitation wavelength already results in an energy transfer to the corresponding absorption band of the switch, leading to the switching off of the luminescence of the luminescent material. The present invention is further based on the discovery of how one and the same switchable absorber can be used to switch different colored luminescence emissions. This allows for the provision of a system of switchable luminescent pigments emitting different colors, in particular red and green and blue, such as red and green, or red and blue, or green and blue, which behave identically with respect to their switching properties.The switching characteristics include, in particular, the wavelength (e.g., 365 nm) and intensity of the light required to switch off the luminescence (i.e., the switching wavelength from the first to the second switching state), the switching speed during the switch-off process, the wavelength for switching back from the second to the first switching state, especially in ambient light, the switching speed during the return process, and the aging and environmental resistance of the switch. Further optimization of the quantities of material used can also be used to equalize the switching depth of the various luminescent pigments.

[0034] The present invention is further based on the finding that a high switching depth, combined with advantageous stability and pressure properties, can be achieved by encapsulating the luminescent substance and a switch together in a capsule-luminescent pigment. This is surprising, since it has always been assumed that the switching effect is based on Förster resonant energy transfer (FRET) between the luminescent substance and the switch, as described above. Since it is known that any energy transfer between two molecules, and especially FRET, depends to a high degree on the distance between the molecules involved, it was not expected that simply encapsulating the mixed molecules in micrometer-sized polymer spheres would reproducibly lead to a sufficiently small distance and thus to a sufficiently high switching depth.

[0035] According to a first aspect, the present invention relates to a system of at least two types of switchable capsule luminescent pigments, wherein the capsule luminescent pigments are polymer particles with a diameter of 0.1-1000 pm, each containing at least one luminescent substance and a photochemically switchable absorber, wherein a first type of capsule luminescent pigment comprises a first luminescent substance emitting in a first color in the VIS spectral range; a second type of capsule luminescent pigment comprises a second luminescent substance emitting in a second color in the VIS spectral range, wherein the first and the second color are different;and the two capsule luminescent pigments each comprise the same photochemically switchable absorber, wherein the photochemically switchable absorber has a first and a second switching state, and the absorption spectrum of the photochemically switchable absorber in the second switching state significantly overlaps with the excitation spectra of the first and the second luminescent substance.

[0036] The system according to the invention comprises at least two types of switchable capsule luminescent pigments, wherein a first type of capsule luminescent pigment comprises a first luminescent substance emitting in a first color in the visible spectral range; a second type of capsule luminescent pigment comprises a second luminescent substance emitting in a second color in the visible spectral range, wherein the first and second colors are different. The system can comprise any number of further types of capsule luminescent pigments, for example, a third, fourth, and / or fifth type of capsule luminescent pigment, each of which comprises a further, for example, third, fourth, or fifth luminescent substance emitting in a third, fourth, or fifth color in the visible spectral range, wherein the colors of the first, second, and further, for example, third, fourth, and / or fifth luminescent substances are different.Each of the different types of capsule luminescent pigments thus has a different emission color, resulting from the luminescence of the first and second luminescent substances, and subsequently, for example, the third, fourth, or fifth. It is also possible for one or more types of capsule luminescent pigments to contain more than one, for example, two or three, different types of luminescent substances, each luminescent in a different color. This is then referred to as a mixture of luminescent substances. In this case, the emission color of the capsule luminescent pigment results from the additive color mixing of the contained luminescent substances, i.e., the mixture of luminescent substances. In this way, many types of switchable capsule luminescent pigments can be produced that emit in various colors, such as green, yellow, red, orange, etc.

[0037] For example, at least the second type of capsule luminescent pigment can comprise a further, third luminescent substance emitting in a third color in the visible spectral range, wherein the third color is preferably different from the first and second colors. In one embodiment, the first type of capsule luminescent pigment can also additionally comprise a further, fourth luminescent substance emitting in a fourth color in the visible spectral range, wherein the fourth color is preferably different from the first, second, and third colors.

[0038] It is also possible and preferred that, after stimulation, an energy transfer occurs, for example through

[0039] FRET can occur from the first or third to the second (or also from the second, third, or fourth to the first) luminescent substance, or to each of the further, for example, third, fourth, and / or fifth, luminescent substances. This influences the overall color impression of a type of switchable capsule luminescent pigment, which in these cases then depends significantly on the luminescence of the second or the further, for example, third, fourth, or fifth, luminescent substance. Systems consisting of two or three types of switchable capsule luminescent pigments are preferred, and systems consisting of three types of switchable capsule luminescent pigments are particularly preferred. However, the system according to the invention can also consist of four, five, or more types of switchable capsule luminescent pigments.

[0040] It was surprisingly discovered that the luminescences of all types of switchable capsule luminescent pigments can be switched simultaneously, i.e., at the same wavelength and radiation intensity, by using the same switchable absorber in all types of switchable capsule luminescent pigments, whose absorption spectrum in the second switching state significantly overlaps with the excitation spectrum of the first luminescent substance emitting in a first color, as well as with the excitation spectrum of the second luminescent substance emitting in a second color, and, if present, optionally also with the excitation spectra of any further luminescent substances, for example, emitting in a third, fourth, and / or fifth color.

[0041] For example, the system of capsule luminescent pigments according to the invention can comprise two types of capsule luminescent pigments, for example, a first type of UV-excitable, blue-emitting capsule luminescent pigment with a first luminescent substance, and a second type of green-emitting capsule luminescent pigment comprising a second luminescent substance that is also UV-excitable but, for example, emits green light. Due to the presence of the same switchable absorber in both the first and second types of capsule luminescent pigments, switching the absorber from the first to the second switching state in the first, blue capsule luminescent pigment can suppress or at least attenuate the luminescence of the first luminescent substance, so that the luminescence of this type of capsule luminescent pigment is (almost) completely extinguished.In the second, green capsule luminescent pigment, the luminescence of the second luminescent substance is suppressed or at least attenuated, so that the luminescence of this type of capsule luminescent pigment is (almost) completely extinguished. In the case of a possible energy transfer between two different luminescent substances within a type of capsule luminescent pigment, for example, from a first to a further (e.g., a third, fourth, and / or fifth) luminescent substance, switching the absorber also suppresses or at least attenuates the luminescence of the further luminescent substance, since the energy transfer cascade is interrupted as soon as the first luminescent substance absorbs energy. This only occurs if the energy transfer from the first luminescent substance to the absorber is more efficient than the energy transfer from the first luminescent substance to the further luminescent substance.In one embodiment, such energy transfer from the first to a further luminescent substance can occur within a type of capsule luminescent pigment. In another embodiment, however, the further luminescent substance cannot be excited by energy transfer from the first luminescent substance, so that no energy transfer from the first to the further luminescent substance takes place. In this case, the luminescence of the further luminescent substance cannot be switched off by switching the absorber, but remains. This naturally presupposes that the further luminescent substance can then be excited by a suitable excitation wavelength.

[0042] In another preferred embodiment, the system of capsule luminescent pigments according to the invention comprises three types of capsule luminescent pigments, for example, a first type of capsule luminescent pigment comprising a first luminescent substance emitting in a first color in the VIS spectral range, and a second type of capsule luminescent pigment comprising a second luminescent substance emitting in a second color in the VIS spectral range, wherein the first and second colors are different, as well as a third type of capsule luminescent pigment comprising a third luminescent substance emitting in a third color in the VIS spectral range, wherein the first, second, and third colors are different, and the three different capsule luminescent pigments each comprise the same photochemically switchable absorber.wherein the photochemically switchable absorber has a first and a second switching state and the absorption spectrum of the photochemically switchable absorber in the second switching state significantly overlaps with the excitation spectrum of the first luminescent substance and the excitation spectra of the second and third luminescent substances.

[0043] To ensure switching of the luminescence, the excitation spectra of the first and second luminescent substances, and optionally those of any further luminescent substances (e.g., the third, fourth, and / or fifth), must overlap with the absorption spectrum of the switchable absorber, but only in its second switching state, not in the first. In the first switching state, the luminescence is unaffected by the switchable absorber; in the second switching state, however, the luminescence is reabsorbed by the switchable absorber, or the energy is transferred non-radiatively, thus significantly reducing the luminescence intensity.To ensure significantly more intense luminescence of the first and second, and optionally any further (e.g., third, fourth, and / or fifth) luminescent materials in the first switching state of the switchable absorber than in the second switching state, the absorption spectrum of the switchable absorber in the first switching state should not significantly overlap with the excitation spectra of the first and second, and optionally any further (e.g., third, fourth, and / or fifth) luminescent materials. Conversely, the absorption spectrum of the switchable absorber in the second switching state should strongly overlap with that of the first and second, and optionally any further (e.g., third, fourth, and / or fifth) luminescent materials. This ensures the most efficient energy transfer possible.

[0044] A non-significant overlap of the first absorption spectrum of the switchable absorber (i.e., the switchable absorber before switching) with the excitation spectra of the first and second, and optionally, if present, the further, for example, the third, fourth, and / or fifth luminescent material, means that the first absorption spectrum of the switchable absorber, normalized to 1 at the wavelength of highest absorption in the range of 235 to 400 nm, exhibits at most one absorption in the wavelength range of an excitation band in the excitation spectra of the first and second, and optionally, if present, the further, for example, the third, fourth, and / or fifth luminescent material, which (i.e.,the absorption) at its highest point less than 40%, preferably less than 35%, more preferably less than 30%, even more preferably less than 25%, and particularly preferably less than 20%, of the maximum excitation of the first and second and, if present, optionally of the further, for example, the third, fourth and / or fifth luminescent material in its respective excitation spectrum normalized to the value 1 at the wavelength of the highest excitation in the range of 245 to 700 nm.

[0045] In this process, an excitation band of the first and second, and optionally, if present, of the further, for example, the third, fourth and / or fifth luminescent material, is defined as a wavelength range in which the excitation corresponds to at least 50%, for example, at least 60% of the maximum excitation of the first and second, and optionally, if present, of the further, for example, the third, fourth and / or fifth luminescent material.In this relevant excitation band of the first and second, and optionally, if present, the further, for example, the third, fourth and / or fifth luminescent material, lies the (local) absorption maximum of the switchable absorber after switching in the second switching state, wherein the difference between the absorption curves of the first and second switching states of the switch at the wavelength of the excitation maximum of the first and second, and optionally, if present, the further, for example, the third, fourth and / or fifth luminescent material, can be absolutely more than 40%, such as more than 50%; preferably more than 70%, more preferably more than 80%, and particularly preferably more than 90%, relative to the absorption maximum in the second switching state, wherein the first and the second absorption spectra are each normalized to the value 1 at the wavelength of the highest absorption in the range of 235 to 400 nm.In one embodiment, the second absorption spectrum of the switch in the second switching state can exhibit at least 120%, preferably at least 150%, more preferably at least 200%, and particularly preferably at least 250% of the absorption of the first absorption spectrum of the switch in the first switching state at the same wavelength of the highest excitation of the respective luminescent material, wherein the first and the second absorption spectrum are each normalized to the value 1 at the wavelength of the highest absorption in the range of 235 to 400 nm.In a further embodiment, the integral of the second absorption spectrum in the region of an excitation band of the respective luminescent material, for example from 310 to 400 nm, preferably 350-380 nm, particularly preferably at 365 nm, can be at least 120%, preferably at least 150%, more preferably at least 200%, and particularly preferably at least 250% of the integral of the first absorption spectrum in the respective same region of the respective excitation band of the respective luminescent material, wherein the first and the second absorption spectrum are each normalized to the value 1 at the wavelength of highest absorption in the range from 235 to 400 nm.

[0046] A significant overlap of the second absorption spectrum of the switchable absorber (i.e., the switchable absorber after switching) with the excitation spectrum of the first and second, and optionally, if present, the further, for example, the third, fourth, and / or fifth luminescent material, means that the second absorption spectrum of the switchable absorber, normalized to 1 at the wavelength of highest absorption in the range of 235 to 400 nm, exhibits absorption in the same wavelength range as an excitation band in the excitation spectrum of the first and second, and optionally, if present, the further, for example, the third, fourth, and / or fifth luminescent material, which (i.e.,The absorption at its highest point is at least 40%, preferably at least 45%, more preferably at least 50%, even more preferably at least 55%, and particularly preferably at least 60% of the maximum excitation of the first and second, and, if present, optionally of the further, for example, the third, fourth and / or fifth luminescent substances, in its excitation spectrum normalized to the value 1 at the wavelength of highest excitation in the range of 245 to 700 nm. The excitation band of the luminescent substances is defined as above.

[0047] It goes without saying that in the procedure described here for determining a non-significant or significant overlap of the first or second absorption spectrum of the switchable absorber with the excitation spectrum of the first and second, and optionally, if present, the further, for example, the third, fourth and / or fifth luminescent substance, the first and second absorption spectra are measured under the same conditions, for example, with the same settings on the measuring instrument, at the same concentrations, etc.

[0048] Figure 1 illustrates this. The graph labeled "DAE2 SZ1" (SZ = switching state) shows the first absorption spectrum of a diarylethene (DAE2) as described in the examples below, acting as a switch, i.e., a photochemically switchable absorber, before the switching. The absorption spectrum is normalized to 1 at the wavelength of highest absorption in the range of 235 to 400 nm. From approximately 430 nm and above, the absorption spectrum shows no absorption by the unswitched diarylethene. The graph labeled "DAE2 SZ2" is the second absorption spectrum of the diarylethene after the switching. Here, too, the absorption spectrum is normalized to 1 at the wavelength of highest absorption in the range of 235 to 400 nm. In relation to the absorption spectrum of the diarylethene before the circuit, the formation of an absorption band in the range of approximately 330 to 420 nm and another absorption band in the range of approximately 500 to 650 nm can be seen.The graph labeled "An LS2" (= excitation of luminescent substance 2) is the excitation spectrum of the luminescent substance. The excitation band is located in the range of approximately 330 to 430 nm, with the excitation band at which the excitation is at least 50% of the maximum excitation extending from approximately 350 to 410 nm. Figure 1 shows that the absorption spectrum of the unswitched diarylethene switch does not significantly overlap with the excitation spectrum of the luminescent substance, since only a non-significant absorption of the unswitched diarylethene switch is observed in the relevant range of 370 to 410 nm for the excitation of the luminescent substance. The absorption spectrum of the switched diarylethene switch overlaps significantly with the excitation spectrum of the luminescent substance due to the formation of an absorption band at approximately 330 to 420 nm, i.e., also in the range of 370 to 410 nm relevant for the excitation of the luminescent substance.It should be noted that Figure 1 shows an exemplary type of switchable capsule luminescent pigment. The second type of capsule luminescent pigment from the system according to the invention is structured accordingly, except that it comprises a different luminescent substance that differs from the luminescent substance in the first type of capsule luminescent pigment. The same applies optionally to any further capsule luminescent pigments that may be present, for example, a third, fourth, and / or fifth.

[0049] Figure 2 is a section of the wavelength range from 300 to 450 nm from Figure 1, but without the absorption spectrum of the switch in the first switching state, in which the respective maxima have been normalized to 1 (100%). It illustrates, firstly, that the maximum of the absorption band formed after switching the diarylethene switch lies in the region of the maximum excitation of the luminescent material. This is desirable for efficient energy transfer.

[0050] In particular, switching the switchable absorber results in a reduction of the luminescence intensity of the first luminescent substance by at least 10%, such as at least 20%, at least 30%, or at least 40%. Preferably, the reduction in luminescence intensity is at least 45%, such as at least 50%, at least 60%, or at least 70%. A reduction in luminescence intensity of at least 20% can be perceived, especially visually. The switching depth can be quantitatively determined by measurement using commercially available fluorescence spectrometers.

[0051] The wavelength at which the first luminescent substance can be excited is not inherently limited. However, for the use of the system according to the invention, comprising at least two types of switchable capsule luminescent pigments, in security applications such as for valuable documents, it is advantageous to select the excitation wavelength such that the luminescence of the luminescent substance is perceptible to the human eye. This will generally be the case with shorter excitation wavelengths. Excitation wavelengths in the UV range are preferred, for example, in the range of 310–400 nm, more preferably 350–380 nm. An excitation wavelength of 365 nm is particularly preferred, i.e., a wavelength commonly used in lamps for verifying the authenticity of banknotes at cash registers.

[0052] The wavelength at which the second luminescent substance can be excited is also not inherently limited. However, for the use of the system according to the invention in security applications, such as valuable documents, it is advantageous to select the excitation wavelength such that the luminescence of the second luminescent substance is perceptible to the human eye. To facilitate simple verification of a security application comprising the system according to the invention, the excitation wavelength for the second luminescent substance should preferably correspond to the excitation wavelength for the first luminescent substance, preferably in the range of 310–400 nm, such as 350–380 nm, or particularly preferably at 365 nm.

[0053] Regarding the excitation wavelength for the optional additional, for example third, fourth and / or fifth, luminescent substances, the same considerations apply as for the second luminescent substance. In a preferred embodiment, the optional additional, for example third, fourth and / or fifth, luminescent substance can be excited by the excitation wavelength of the first and / or second, preferably first and second, luminescent substance, for example in the range of 310-400 nm, preferably 350-380 nm, or particularly preferably at 365 nm.

[0054] The wavelength range in which the switching of the switchable absorber from the first to the second switching state can occur is not inherently limited, and in principle any suitable wavelength can be selected. However, as already explained above, it is advantageous and therefore preferred if the switching of the switchable absorber from the first to the second switching state can occur in the same wavelength range, or preferably even at the exact same wavelength, as the excitation of the first and / or second luminescent material. For example, the switching of the switchable absorber from the first to the second switching state and the excitation of the first and / or second, preferably both, luminescent materials can occur in the range of 310–400 nm, preferably in the range of 350–380 nm, and particularly preferably at an excitation wavelength of 365 nm.Simultaneous excitation of the first and / or second, preferably the first and second, luminescent material and switching of the switch is advantageous because then only one light / radiation source is required, which significantly simplifies methods for verifying the authenticity of valuable documents based on the system according to the invention. The same considerations apply accordingly to the luminescent materials in optional further, for example third, fourth and / or fifth, luminescent pigments.

[0055] According to a further advantageous embodiment, it is therefore preferred that, preferably in the wavelength range of 310-400 nm, the excitation spectra of the first and / or second, preferably first or third, luminescent material and the absorption spectrum of the switchable absorber overlap in the first switching state. In a further embodiment, the excitation spectrum of the first and / or second, and / or further, for example third, fourth and / or fifth, luminescent material, preferably the first, second and third, and the absorption spectrum of the switchable absorber overlap in the first switching state, preferably in the wavelength range of 310-400 nm.The light intensities required for switching are preferably comparable in the UV range to the excitation intensity of the first luminescent substance, so that both effects are triggered simultaneously with the UV lamps commonly used at checkout counters, and in the visible range at the level of typical room lighting. The intensity of the light source primarily influences the switching duration, but not the switching depth. The switching duration is defined as the time after which, with constant illumination from the excitation or switching light, the luminescence intensity no longer changes by more than 2%. The switching depth is the percentage difference between the initial luminescence intensity and the intensity reached after the switching duration. Another measure with which the luminescence properties of a luminescent substance can be described and quantified is the residual intensity (l. RThe residual intensity is defined as the percentage of the intensity (or switching depth) of the luminescence measured in the unswitched state after irradiation according to BWS3, compared to the intensity (or switching depth) measured in the unswitched state before irradiation. The residual intensity can be measured, for example, by quantitatively measuring a sample containing the respective luminescent substance(s) before irradiation using a commercially available fluorescence spectrometer. The obtained value is normalized to 100%, and the remaining residual intensity of the luminescence after reaching the Woll scale points is considered. For good visual perception of a change in luminescence with the human eye, a residual intensity of at least 30% is desirable. Accordingly, the capsule luminescent pigments used in the system according to the invention preferably have a residual intensity of at least 30%.Furthermore, for good optical perception of a luminescence change of different capsule luminescence pigments relative to each other by the human eye, it is desirable if the difference in residual intensities and residual switching depths of the different capsule luminescence pigments is at most 45%. Accordingly, the difference in residual intensities and residual switching depths of the at least first and second capsule luminescence pigments used in the system according to the invention is preferably at most 45%, for example at most 35%, more preferably at most 20%, and particularly preferably at most 10%.

[0056] For example, a VL-6.LC UV lamp equipped with a 6W UV tube from Vilber can be used to switch from the first to the second state and / or to excite the luminescent material. Preferably, under UV light of the specified intensity and at a distance of approximately 20 cm, a visually detectable switching depth of at least 10%, such as at least 20%, at least 30%, or at least 40%, preferably at least 10%, is achieved within a period of less than 30 s, preferably less than 20 s, and particularly preferably less than 10 s.

[0057] 45%, such as at least 50%, at least 60%, or at least 70%, has been achieved.

[0058] Figure 3 schematically illustrates the switching on and off of luminescence in a type of capsule luminescent pigment from the system according to the invention when irradiated with UV light by switching the photochemically switchable absorber (represented by a switch symbol) from the state “ON” to “OFF”.

[0059] The photochemically switchable absorber used as a molecular switch, i.e., the switch or switchable absorber as described herein, has a first absorption spectrum in a first switching state and a second absorption spectrum in a second switching state. Thus, the switchable absorber is limited in principle only in that it has two switching states, i.e., it is a photochromic substance, with the absorption spectra of the two switching states differing.

[0060] In principle, any suitable switchable absorber can be used that can be reversibly switched from a first switching state to a second switching state by radiation. A preferred class of suitable switchable absorbers are diarylethenes. In principle, any suitable diarylethene can be used in the capsule luminescent pigments of the system according to the invention.The selection of a suitable switchable absorber, in particular a diarylethene, for a specific application can be made by a person skilled in the art by taking into account the desired switching wavelength of the switchable absorber, in particular a diarylethene switch, and / or the excitation maximum of the first and / or second and, if present, optionally the third, fourth and / or fifth luminescent material (which, as described above, should overlap with the absorption spectrum of the second switching state of the switchable absorber) and / or the desired wavelength for switching (back) from the second to the first state. For example, the diarylethene can be a diheteroarylethene, preferably a dithienylethene, and particularly preferably a fluorinated dithienylethene. Suitable diarylethenes and their syntheses are described, for example, in JP 3 384 087 B2, JP 3 479 992 B2, JP 3 491 704 B2, JP Hll- 256 146 A, JP 2001- 254 074 A, JP 3 225 581 B2, and JP H09- 241 254 A, as well as in the articles by J.Mamiya, A. Kuriyama, N. Yokota, M. Yamada, T. Ikeda, Chem. Eur. J. 2015, 21, 3174, and by S. Kobatake, T. Yamada, K. Uchida, N. Kato, M. Irie, J. Am. Chem. Soc. 1999, 121, 2380. The following compounds have proven to be particularly suitable diarylethene switches:

[0061] The switchable absorber, in particular a diarylethene switch, preferably switches back from the second to the first state when illuminated with visible light, for example in the wavelength range of 400–700 nm. Furthermore, it is preferred that the switchable absorber, in particular a diarylethene switch, remains in its state in the dark at room temperature, i.e., neither switching from the first to the second state nor vice versa occurs. Preferably, the switchable absorber, in particular a diarylethene switch, can be switched from the second to the first state by the prescribed average illuminance of 500–750 lux at office workstations (DIN EN 12464-1).The switchable absorber, in particular a diarylethene switch, preferably switches automatically back from its second switching state to its initial state under daylight intensity of 500 lux within a period of less than 10 minutes, preferably less than 5 minutes, and most preferably less than 1 minute. This means that at least 95%, preferably at least 98%, of the initial intensity is achieved.

[0062] The first and second, as well as the optional third, fourth, and / or fifth luminescent substances, are not limited in principle, and any suitable luminescent substance can be used in the capsule luminescent pigments of the system according to the invention. For optimal applicability as a safety feature, the first luminescent substance is preferably selected such that it can be excited between 310 and 400 nm, preferably between 350 and 380 nm, and particularly preferably at 365 nm. The selection of the second and the optional third, fourth, and / or fifth luminescent substances is preferably based on the excitation wavelength of the first luminescent substance, for example, between 300 and 400 nm, between 350 and 380 nm, or at 365 nm. Furthermore, it is preferred that the luminescent substances emit visibly.The luminescent substance need not be a pure chemical compound, but can also be a mixture of several luminescent substances, for example, two, three, or four. In other words, the term "luminescent substance" as used herein also includes mixtures of luminescent substances, i.e., mixtures of two or more, for example, two, three, four, or five different luminescent substances. Suitable luminescent substances can be selected independently from the group consisting of diarylpolyenes, arylacetylenes, oxazoles, pyrazoles, benzazoles, anthrones, quinones, cyanines, rhodamines, oxazines, phenoxazines, thiazines, phenothiazines, perylenes, terylenes, coumarins, benzoxazinones or benzothiazinones, oxinates, aldazines, anthranilic acid derivatives, salicyclic acid derivatives, and mixtures thereof.Preferably, the luminescent substances are selected independently from the group consisting of perylenes, benzoxazinones, oxinates, benzthiazine, aldazines, anthranilic acid derivatives, salicyclic acid derivatives, oxazines, oxazoles, anthrones, and mixtures thereof. Suitable luminescent substances are described, for example, in WO 2006 / 014658 A2, US 2015 / 0132575 Al, and EP 2195395 Bl.

[0063] The luminescent substance and the switchable absorber, in particular the diarylethene switch, are preferably dissolved and / or dispersed as separate molecules in a polymer particle within the capsule luminescent pigments of the system according to the invention. Thus, there is no direct chemical bond between the luminescent substance and the switchable absorber, in particular the diarylethene, but rather they exist as separate molecules. The luminescent substance and the switchable absorber, in particular the diarylethene, can either be dissolved, for example, in a solvent enclosed within the capsule luminescent pigment, or dispersed, for example, in a polymer / polymeric matrix enclosed within the capsule luminescent pigment. It is understood that the term "separate molecules" in the context of the present invention also means that the diarylethene and the luminescent pigment are different from each other, i.e., they cannot be the same molecule.

[0064] Encapsulation means that the luminescent material and the switchable absorber, in particular the diarylethene, are together surrounded by a polymeric shell. The type of encapsulation is not restricted, as long as it is a polymer encapsulation. The polymer particle can be a core-shell particle, i.e., a polymer particle comprising a core of a first polymer and a shell of a second polymer, which may differ from the first polymer. In a preferred embodiment, the core consists of a first polymer and the shell of a second polymer that differs from the core polymer in at least one monomer. This core / shell structure additionally guarantees the chemical stability of the encapsulated components against external influences.External influences include, but are not limited to: humidity, aqueous environments, perspiration, grease, detergents, solvents, and chemical compounds such as alkalis, acids, alcohols, and acetone. For example, the capsule luminescent pigments can be obtained by encapsulation as described in WO 2017 / 080 653 Al (polymethyl methacrylate (PMMA) / melamine-formaldehyde resin (MF) coating) or WO 2017 / 080 656 Al (polyurethane (PUR) / MF coating). The processes described therein for the production of core-shell particles containing a luminescent substance in the core can be modified for the production of the capsule luminescent pigments by adding the switchable absorber, in particular the diarylethene switch, in addition to the luminescent substance in the core material production step.

[0065] For example, the capsule luminescent pigments can contain as a core a polymer selected from polystyrene (PS), polyacrylates, polyethylene (PE), polypropylene (PP), polycarbonates (PC), polyamides (PA), polyurethanes (PU), polyureas (PH), polyethylene terephthalate (PET), other polyesters, or mixtures thereof, and / or as a shell a condensation polymer selected from aminoplasts, phenolplasts, melamine-formaldehyde resins (MF), melamine-phenol-formaldehyde resins (MPF), phenol-formaldehyde resins (PF), urea-formaldehyde resins (UF), melamine-guanidine-formaldehyde resins, phenol-resorcinol-formaldehyde resins, or mixtures thereof. Preferably, the capsule luminescent pigments contain as a core a thermoplastic polymer, in particular a thermoplastic polymer selected from polymethyl methacrylate or polystyrene, and / or as a shell a melamine-formaldehyde resin.Particularly preferably, the capsule luminescence pigments according to the invention contain as a core a thermoplastic polymer selected from polymethyl methacrylate or polystyrene and as a shell a melamine-formaldehyde resin.

[0066] The proportion of the luminescent material in the core of the capsule luminescent pigments can be independently 0.1 to 10 wt%, preferably 0.2 to 8 wt%, more preferably 0.3 to 7 wt%, such as 0.5 to 5 wt%, 0.7 to 4 wt%, or 0.8 to 3 wt%, and particularly preferably 1 to 2.5 wt%, in each case based on the total weight of the core material. If the luminescent material consists of several different luminescent materials, the proportions of the respective individual luminescent materials can be correspondingly lower, for example, 0.01 to 10 wt%, 0.02 to 8 wt%, 0.05 to 5 wt%, or 0.7 to 4 wt%, in each case based on the total weight of the core material.

[0067] The proportion of the photochemically switchable absorber in the core of the capsule luminescence pigments can be independently 1 to 20 wt%, preferably 1.5 to 18 wt%, more preferably 2 to 17 wt%, such as 2.5 to 15 wt%, 3 to 13 wt%, or 3.5 to 10 wt%, and particularly preferably 4 to 8 wt%, in each case based on the total weight of the core material.

[0068] The capsule luminescent pigments of the system according to the invention have a mean diameter of 0.1 to 1000 pm, for example, from 0.05 to 500 pm, from 1 to 200 pm, or from 1 to 100 pm, preferably from 1 to 50 pm, for example, from 1 to 30 pm or 1 to 20 pm or 1 to 10 pm, more preferably from 0.2 to 10 pm or from 1 to 5 pm, and particularly preferably from 1 to 3 pm. The diameter is generally specified as the D99 value, i.e., 99% of all capsule luminescent pigments have the specified diameter or are smaller. The determination of the diameter and the D99 value is well known to those skilled in the art. For example, the diameter and the D99 value can be determined using laser scattering, for example, using the Cilas 1090 Particle Size Analyzer from 3P Instruments.

[0069] According to a second aspect, the present invention relates to a switchable luminescent printing ink comprising a system of at least two types of switchable encapsulated luminescent pigments according to the first aspect of the present invention. The system of at least two types of switchable encapsulated luminescent pigments contained in the switchable luminescent printing ink according to the invention can thus be any system of at least two types of switchable encapsulated luminescent pigments described above.

[0070] In principle, any printing ink can be used as the basis for the switchable luminescent printing ink according to the invention, in particular any printing ink suitable for printing security documents, for example an offset ink or an intaglio ink, and a capsule luminescent pigment as described herein can be incorporated to produce the switchable luminescent printing ink according to the invention. Suitable printing inks are known to those skilled in the art and are described, for example, in WO 2013 / 178 325 A2, EP 2 888 112 Bl, DE 10 2012 010 534 Al, WO 2018 / 197 039 Al and EP 3 660 110 Bl. The printing ink can preferably be oxidatively drying or curing, or UV-curing, and particularly preferably UV-curing.For example, the ink may preferably be an oxidatively drying or UV-curing offset ink, an oxidatively drying or UV-curing intaglio ink, an oxidatively drying or UV-curing screen printing ink, or an oxidatively drying or UV-curing flexographic printing ink. Offset inks, especially UV-curing offset inks, are particularly preferred.

[0071] According to a third aspect, the present invention relates to a security document with a switchable luminescence print, comprising a system of at least two types of switchable capsule luminescence pigments according to the first aspect of the present invention or a switchable luminescence printing ink according to the second aspect of the present invention.

[0072] The valuable document can in principle be any type of valuable document, for example a security document, a banknote or an identity document, and is preferably a banknote or an identity document, and especially preferably a banknote.

[0073] A security document typically comprises a substrate with optionally at least two opposing layers applied to it. Any substrate suitable for security documents can be used. These substrates are known to those skilled in the art and include, for example, paper substrates, substrates based on cotton fibers or mixed fibers, where mixed fibers may include, in particular, fibers from cotton, flax, linen, cellulose, and plastics, substrates based on plastic films, and impregnated and pre-coated substrates. Hybrid substrates can also be used, such as paper-plastic substrates or multilayer substrates, for example, film / paper / film substrates, as known from WO 2004 / 028825 A2. The substrate can be impregnated with polymers of any kind and is preferably impregnated with polyvinyl alcohol.

[0074] The layers optionally applied opposite each other on the substrate are generally intended to protect the substrate, for example, by repelling dirt and / or moisture. However, they can also alternatively or additionally provide a substrate with improved adhesion for further layers, such as a printing layer. Accordingly, the layers applied opposite each other on the substrate can be, for example, sizing layers and / or primer layers. Potential sizing layers and primer layers, as well as methods for applying them, are known to those skilled in the art. A sizing layer and / or primer layer can, for example, be formed by a physically drying lacquer layer. "Physically drying" means that drying occurs through evaporation and / or dispersion of the solvents or dispersion agents into the substrate.The production of suitable coatings is described, for example, in EP 2 634309 Al and WO 2004 / 072378 Al. Water-based dispersion coatings are particularly preferred. For example, a sizing layer may contain polyvinyl alcohol and / or polyurethane. Examples of suitable primer compositions are those based on acrylates, polyester acrylates, urethane acrylates, polyester polyurethanes, and acrylonitrile styrene polyurethanes. Water-based dispersions, especially water-based dispersions of aliphatic components, are particularly preferred.

[0075] The security document according to the invention further comprises a print with a switchable luminescent imprint, comprising a system of at least two types of switchable encapsulated luminescent pigments according to the second aspect of the invention. The print can be a full-surface or at least partially surface-surface printed layer applied to the security document, or a printed pattern, consisting of a first switchable luminescent printing ink according to the second aspect of the present invention, or a printing ink or printing ink mixture containing a system of at least two types of switchable encapsulated luminescent pigments according to the first aspect of the present invention. Parts or sections of a security document, such as a melange fiber, can also be printed with the switchable luminescent imprint.A printed design can, of course, also contain several printed patterns made from different printing inks, applied side by side or (partially) on top of each other, wherein at least one of the printing inks is a switchable luminescent printing ink according to the second aspect of the present invention, or a printing ink or printing ink mixture containing a system of at least two types of switchable encapsulated luminescent pigments according to the first aspect of the present invention. Likewise, in addition to at least one printed pattern made from a switchable luminescent printing ink, a further printed layer may also be applied to the entire surface or at least partially across the surface of the document. A printed design using more than one printing ink is preferred, wherein at least one of the printing inks is a switchable luminescent printing ink.The printing can be, for example, an intaglio print or a gravure print and can be applied directly to the substrate or to at least one, preferably both, of the two layers optionally applied opposite each other on the substrate or to any further layers of the security document.

[0076] Printing a valuable document with a switchable luminescent ink according to the second aspect of the present invention enables simple security verification of the document by visual inspection when the document is irradiated with a suitable wavelength, for example, 310 to 400 nm, preferably 350 to 380 nm, and particularly preferably 365 nm. The photochemically switchable absorber contained in the at least two types of switchable luminescent pigments from the system according to the invention switches from the first to the second switching state, as described above, when irradiated with a suitable wavelength. In the second switching state, the luminescence of the first and second luminescent substances, which are excited simultaneously, preferably at the same wavelength as the switch, is reduced by energy transfer in the first and second types of switchable capsule luminescent pigments.For visual inspection, the luminescence of the luminescent dyes gradually diminishes when the radiation source is switched on. This is noticeable as a darkening or change in the printed ink during visual inspection. After the radiation source is switched off, the switch resets under daylight, allowing further corresponding safety checks to be carried out.

[0077] In a preferred embodiment of the security document according to the invention with a luminescent imprint, the luminescence of the at least two types of capsule luminescent pigments can thus be switched off using light of the same wavelength, preferably between 310 and 400 nm, more preferably between 350 and 380 nm, and particularly preferably at 365 nm.

[0078] Preferably, the luminescent print of the security document according to the invention further comprises at least one additional luminescent color whose luminescence in the same wavelength range, preferably between 310 and 400 nm, more preferably between 350 and 380 nm, and particularly preferably at 365 nm, cannot be switched off and whose emission color in the VIS spectral range differs from that of the first and second switchable luminescent capsule pigments. In an alternative preferred embodiment, the luminescent print of the security document according to the invention further comprises at least one additional luminescent color whose luminescence in the same wavelength range, preferably between 310 and 400 nm, more preferably between 350 and 380 nm, and particularly preferably at 365 nm, cannot be switched off and whose emission color in the VIS spectral range corresponds to that of at least one of the first and second switchable luminescent capsule pigments.

[0079] The additional luminescent printing ink can contain the suitable luminescent substance or mixture of suitable luminescent substances as a simple component, i.e., as molecules freely incorporated into the additional luminescent printing ink, or alternatively and preferably encapsulated in the form of capsule luminescent pigments as described above, which, however, do not contain a switchable absorber. In this way, security features can be created in which some areas of the print darken upon irradiation with a suitable wavelength (i.e., the areas printed with the switchable luminescent printing ink according to the second aspect of the present invention) and some areas retain their luminescence unchanged (i.e., the areas printed with the additional, non-switchable luminescent printing ink). The security document according to the invention can, of course, also include prints with further printing inks.For example, printing with several, say, two, three, four, or five, switchable luminescent printing inks, as described in the second aspect of the present invention, which differ from one another, particularly with regard to their color appearance, is conceivable. Such printing with several, say, two, three, four, or five different switchable luminescent printing inks allows for the creation of complex patterns that only become visible under irradiation, for example, UV irradiation. Likewise, in complex color mixtures that produce luminescence in a specific color, the visually perceptible luminescence under irradiation, for example, UV irradiation, can be altered by selectively switching off individual components of the color mixture.The colors of the switchable luminescent printing inks according to the second aspect of the present invention, as well as the further non-switchable luminescent printing inks, can be adjusted by a person skilled in the art by mixing suitable luminescent colors by additive color mixing as required.

[0080] Part of the invention is thus a multi-colored, switchable luminescent print made from one or more luminescent printing inks with the inventive system of at least two switchable capsule luminescent pigments, as well as a security document with such a print. A system consisting of a switchable and a non-switchable luminescent printing ink is particularly advantageous, especially one with the same luminescent color tone with or without the photochemically switchable absorber. The luminescent color tone can be adjusted by combining the switchable luminescent pigments in the printing ink. This allows for the production of prints that initially exhibit a homogeneous color impression and a homogeneous luminescence intensity under UV illumination.With prolonged exposure to light, such as UV radiation, the luminescence of the switchable luminescent ink fades after a certain period, for example, after a few seconds (e.g., 5, 10, or 20 seconds), revealing a dark image against a light, luminescent background. Color-changing effects can also be achieved by combining switchable and non-switchable luminescent inks of different emission colors. In particular, combining one or more non-switchable (static) luminescent printing inks with one or more switchable luminescent printing inks of the same emission color can generate an image that changes shape or color under UV radiation.

[0081] Figures 4-7 show exemplary applications of the switchable according to the invention.

[0082] Luminescent printing ink in printed patterns and melange fibers. Figures 4A and 4B show monochrome printed patterns with shape changes using a switchable luminescent printing ink according to the second aspect of the present invention. For this purpose, three pairs, each consisting of a conventional UV-luminescent ink and a switchable UV-luminescent ink, are applied to a substrate, for example, a security document, in separate areas. The colors within a pair have the same color impression, while different pairs have different color impressions. Figure 4A shows the first state immediately after the UV light is switched on, i.e., before the photochemically switchable absorber has switched to any significant extent. The vertically hatched, white, and horizontally hatched areas describe three different color impressions.After a certain switching time (after the photochemically switchable absorber has switched), the second state shown in Figure 4B is reached, in which the hatched areas retain their respective colors from the first state (i.e., the simple, non-switchable luminescent printing ink). The black-marked areas lose their luminescence and become dark (i.e., the switchable luminescent printing ink). The luminescent color effect is created, in particular, by a mixture of different colored switchable luminescent substances or luminescent pigments through additive color mixing.

[0083] For example, a print in its initial state shows a blue, a green, and a turquoise luminescent square under UV light with a wavelength of 365 nm. After irradiation with UV light, the print shows rotated rhombuses in the corresponding luminescent colors blue, green, and turquoise. The black areas no longer luminesce.

[0084] Figures 5A and 5B show color-changing printed patterns using a switchable luminescent ink according to the second aspect of the present invention. Three pairs of inks, each consisting of a conventional UV-luminescent ink and a switchable UV-luminescent ink, are applied to separate areas of a substrate, for example, a security document. In the initial state shown in Figure 5A, immediately after switching on the UV light, i.e., before the photochemically switchable absorber has switched to any significant extent, all colors exhibit the same color impression (in Figure 5A, the white areas describe a uniform color impression). After a certain switching time, the state shown in Figure 5B is reached, in which the white areas retain their respective colors from the initial state (i.e., the simple, non-switchable luminescent ink).The hatched areas change their color appearance (the different directions of the hatching represent different luminescent color impressions) because one or more of the luminescent pigments used are extinguished (i.e., the switchable luminescent printing ink). The luminescent color impression arises primarily from a mixture of different colored switchable and non-switchable luminescent substances or luminescent pigments through additive color mixing.

[0085] For example, three white luminescent squares in the hatched area change color to red, blue, and orange respectively when exposed to prolonged UV light. The white areas remain white.

[0086] Figure 6 shows a possible pattern of a monochrome, switchable luminescent fiber, comprising a print with a switchable luminescent ink according to the invention and a non-switchable luminescent ink. A first, switchable luminescent ink (black areas) and a second, non-switchable luminescent ink (white areas) are incorporated into a luminescent fiber. The first and second luminescent inks have the same luminescent color. The second, non-switchable luminescent ink does not change its intensity upon irradiation, for example, UV irradiation, while the first luminescent ink darkens. The two luminescent inks consist, for example, of any mixture of blue, green, and red luminescent substances. The corresponding luminescent color is obtained by additive color mixing.

[0087] Figures 7A and 7B show a possible visually recognizable pattern for a switchable luminescent fiber with color change before (Figure 7A) or after (Figure 7B) the activation of a photochemically switchable absorber. A first, non-switchable luminescent printing ink (white areas), a second switchable luminescent printing ink according to the present invention (hatched areas, inclined to the right), and a third switchable luminescent printing ink (hatched area, inclined to the left) are incorporated into a luminescent fiber. At the beginning of irradiation, the first, second, and third luminescent printing inks have the same luminescent color, which is produced by the additive mixing of different luminescent pigments (Figure 7A). For example, the first luminescent printing ink does not change its intensity or color under UV irradiation, while the second and third luminescent printing inks do change their color (Figure 7B).In particular, a color change occurs in the second and third luminescent printing inks, as one or more of the pigments in the respective luminescent dye mixtures darken, thus changing the color composition. This can result in a color change to different colors in the final state.

[0088] For example, a white luminescent melange fiber turns pink and red in the hatched areas after irradiation with UV light. The white areas, however, continue to luminescent white.

[0089] Examples:

[0090] methods

[0091] To produce the printing inks, the pigments were incorporated into an offset printing ink (Sicpa Holding SA) using an Engelsmann JEL 25 / 53 ink rubbing machine (manufactured in 2013). The pigmentation level was 15% by weight. The printing inks were applied at a printing thickness of 1 g / m². 2Printed on security paper, the proofs were dried at 60 °C for 2 hours. They were then visually assessed for their switching time and depth. The switching depth was quantitatively determined using a commercially available fluorescence spectrometer. The proofs were then subjected to the lightfastness test described above and again assessed for switching time, switching depth, and residual intensity of the luminescent substance in its initial state.

[0092] Lightfastness and fatigue resistance were determined using the European Blue Wool Scale (BWS), commonly used for determining the lightfastness of absorption inks, e.g., analogous to standard EN ISO 105-B01:1999. However, instead of the (absorptive) color impression, the intensity of luminescence emission at the various points of the wool scale was determined. For this purpose, the respective luminescence intensity of the prints was quantitatively measured before irradiation using a commercially available fluorescence spectrometer and normalized to 100%. The remaining residual intensity of the luminescence after reaching the wool scale points was considered. To quantitatively assess lightfastness, fatigue resistance, and chemical resistance, a test procedure analogous to WO 2017 / 080654 Al was used. Both the luminescent material and the photochemically switchable absorber were required to pass the BWS test.The luminescent material is measured, particularly in the first switching state of the photochemically switchable absorber, and exhibits a residual luminescence intensity of at least 30% of the original luminescence intensity at "BWS3". The photochemically switchable absorber should be stable with respect to its switching depth, i.e., the switching depth at "BWS3" is at least 35% of the original switching depth.

[0093] Diarylethene syntheses 3,3'-(Perfluorocyclopent-l-ene-l,2-diyl)bis(2-methyl-6-nitrobenzo[b]thiophene) (DAE 2)

[0094] DAE 1 DAE 2

[0095] 69%

[0096] AC₂O (0.50 mL, 0.54 g, 5.29 mmol, 8.2 eq.) was dissolved in 6 mL AcOH (99.8%), cooled to 10°C, and 1,2-bis[2-methylbenzo[b]thiophen-3-yl]-3,3,4,4,5,5-hexafluoro-l-cyclopentene (DAE 1, TCI Chemicals) (300 mg, 0.64 mmol, 1.0 eq.) was added. Subsequently, 0.3 mL of conc. HNO₃ was added dropwise, the reaction mixture was heated to room temperature, and stirred overnight at room temperature. The reaction was stopped by the addition of 10 mL of NaHCO₃ and taken up in 20 mL of EtOAc. The phases were separated, and the aqueous phase was extracted with EtOAc (3 x 10 mL). The combined organic phases were washed with NaCl(aq) (1 x 20 mL), dried over Na₂SO₄, filtered, and the solvent was removed under vacuum. The product was purified by column chromatography (SiO₂, H / EtOAc = 5 / 1 -> 4 / 1 -> 1 / 1, UV) and isolated as a colorless solid (247 mg, 0.44 mmol, 69%).

[0097] Example 1:

[0098] Example a) Pigment system consisting of blue and green luminescent pigments switchable at 365 nm with a polymethyl methacrylate core and a melamine-formaldehyde shell

[0099] The blue switchable capsule luminescent pigment was prepared according to Example 2 of WO 2017 / 080 653 Al and contains 2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole) (LSI, C26H26N2O2S) as the luminescent substance dissolved in the core. 3,3'-(perfluorocyclopent-1-ene-1,2-diyl)bis(2-methyl-6-nitrobenzo[b]thiophene) (DAE 2, C23Hi2F6N2O4S2) was used as the diarylethene switch. The luminescent substance is present at 0.5 wt% and the diarylethene switch at 7.5 wt%.

[0100] The green switchable capsule luminescent pigment was prepared according to Example 2 of WO 2017 / 080 653 Al and contains / V-(2-(4-oxo-4H-benzo[d][l,3]oxazin-2-yl)phenyl)naphthalene-2-sulfonamide (LS2, C24H16N2O4S) as the luminescent substance dissolved in the core. 3,3'-(perfluorocyclopent-l-ene-l,2-diyl)bis(2-methyl-6-nitrobenzo[fa]thiophene) (DAE 2, C22H27NO4) was used as the diarylethene switch. The luminescent substance is present at 0.5 wt% and the diarylethene switch at 5 wt%. The resulting pigments luminesce blue or green under illumination at 365 nm, depending on their composition, and switch on when irradiated with light of a wavelength of 365 nm. LSI corresponds to a first luminescent substance and LS2 to a second luminescent substance.

[0101] The excitation band of the first luminescent substance, as defined above, lies in the range of 350–410 nm, while that of the second luminescent substance lies in the range of 247–411 nm. The absorption maximum of the diarylethene switch in its second state, within the spectral range relevant for excitation of the luminescent substance, is located at 372 nm and thus lies within the defined excitation bands of both luminescent substances. Furthermore, the absorption of the second switching state of the diarylethene at the excitation wavelength is more than 150% of the absorption of the first switching state of the diarylethene for both luminescent substances.

[0102] Due to the significant overlap of the absorption and excitation spectra of the diarylethene switch and the first and second luminescent substances, a good switching depth of 53% and 31%, respectively, is achieved for the blue and green switchable luminescent pigments (BWSO) and 19% and 22%, respectively, for BWS3. This switching depth is thus significantly above the desired threshold of 10%, and the difference in luminescence intensity is clearly visible. Furthermore, the difference between the respective switching depths is less than 45 percentage points. No difference in switching behavior or color shift is discernible when switching mixed colors from the two pigments. In the first switching state, the residual intensity of the luminescent substance (BWS3) is significantly higher than the desired residual intensity of 30% for a pigment according to the invention, at 44% for blue and 73% for green, and the deviation between the two values ​​is less than 45 percentage points.Thus, after exposure to the same light intensity, the printed colors lose approximately the same amount of luminescence intensity, and the relative ratio of the emission intensities of the two colors does not change for the eye. In particular, the pigments with 36% and 71% residual switching depth at BWS3 showed good fatigue resistance. The entire security feature can be uniformly recognized, and the system of the aforementioned switchable luminescent pigments is therefore ideally suited for use as a security feature.

[0103] Table 1 summarizes the experimentally obtained data for example aa.

[0104] Table 1. [1] Residual intensity of the luminescent substance at BWS3,

[0105] [2] in percentage points

[0106] A printing ink, Fl, was produced from blue and green luminescent pigments to create a turquoise (blue-green) color impression. For this purpose, the pigments were incorporated into an offset printing ink (Sicpa Holding SA) using an Engelsmann JEL 25 / 53 ink leveling machine (manufactured in 2013). The pigmentation level was 7.5% by weight for each pigment. The printing ink Fl was applied at a printing weight of 2 g / m². 2Printed on security paper, the proofs were dried at 60 °C for 2 hours. Subsequently, the proofs were visually assessed for their switching time and depth. In particular, the proofs exhibited exceptional fatigue resistance, switching depth, and uniform extinction of the luminescence for BWSO and BWS3 without any color shift during the switching process. The system of the aforementioned pigments is therefore ideally suited for use as a security feature, enabling a wide range of switchable luminescent colors that can be achieved through additive color mixing.

[0107] Example lb) Pigment system consisting of blue and green luminescent pigments switchable at 365 nm with a polymethyl methacrylate core and a melamine-formaldehyde shell

[0108] The blue switchable capsule luminescent pigment was prepared according to Example 2 of WO 2017 / 080 653 Al and contains CD311 (Honeywell) (LS3) as the luminescent agent dissolved in the core. 3,3'-(Perfluorocyclopent-l-ene-l,2-diyl)bis(2-methyl-6-nitrobenzo[fa]thiophene) (DAE 2, C23H12F6N2O4S2) was used as the diarylethene switch. The luminescent agent is present at 0.5 wt% and the diarylethene switch at 7.5 wt%.

[0109] The green switchable capsule luminescent pigment was prepared according to Example 2 of WO 2017 / 080 653 Al and contains / V-(2-(4-oxo-4H-benzo[d][l,3]oxazin-2-yl)phenyl)naphthalene-2-sulfonamide (LS2, C24H16N2O4S) as the luminescent substance dissolved in the core. 3,3'-(perfluorocyclopent-l-ene-l,2-diyl)bis(2-methyl-6-nitrobenzo[fa]thiophene) (DAE 2, C22H27NO4) was used as the diarylethene switch. The luminescent substance is present at 0.5 wt% and the diarylethene switch at 5 wt%.

[0110] The resulting pigments luminesce blue or green under illumination at 365 nm, depending on their composition, and switch under irradiation with light of the same wavelength. LS3 corresponds to a first luminescent substance, and LS2 to a second. The excitation band of the first luminescent substance, as defined above, lies in the range of 280–415 nm, while that of the second luminescent substance lies in the range of 247–411 nm. The absorption maximum of the diarylethene switch in the second state, within the spectral range relevant for excitation of the luminescent substance, is located at 372 nm and thus lies within the defined excitation bands of both luminescent substances. Furthermore, the absorption of the second switching state of the diarylethene at the excitation wavelength is more than 150% of the absorption of the first switching state of the diarylethene for both luminescent substances.

[0111] Due to the significant overlap of the absorption and excitation spectra of the diarylethene switch and the first and second luminescent substances, a good switching depth of 47% and 31%, respectively, is achieved for the blue and green switchable luminescent pigments (BWSO) and 18% and 22%, respectively, for BWS3. This switching depth is thus well above the desired threshold of 10%, and the difference in luminescence intensity is clearly visible. Furthermore, the difference between the respective switching depths is less than 45 percentage points. No difference in switching behavior or color shift is discernible when switching mixed colors from the two pigments. In the first switching state, the residual intensity of the luminescent substance (BWS3) is 31% for blue and 73% for green, exceeding the desired residual intensity of 30%, and the deviation between the two values ​​is less than 45 percentage points.Thus, after exposure to the same light intensity, the printed colors lose approximately the same amount of luminescence intensity, and the relative ratio of the emission intensities of the two colors does not change for the eye. In particular, the pigments with 38% and 71% residual switching depth at BWS3 showed good fatigue resistance. The entire security feature can be recognized uniformly, and the system of the aforementioned switchable luminescent pigments is therefore ideally suited for use as a security feature.

[0112] Table 2 summarizes the experimentally obtained data for example lb).

[0113] Table 2.

[0114] [1] Residual intensity of the luminescent substance at BWS3,

[0115] [2]In percentage points, a printing ink F2 was produced from blue and green luminescent pigments to create a turquoise (blue-green) color impression. For this purpose, the pigments were incorporated into an offset printing ink (Sicpa Holding SA) using an Engelsmann JEL 25 / 53 ink rubbing machine (manufactured in 2013). The pigmentation level was 7.5% by weight for each pigment. The F2 printing ink was applied at a printing weight of 2 g / m². 2Printed on security paper, the proofs were dried at 60 °C for 2 hours. Subsequently, the proofs were visually assessed for their switching time and depth. In particular, the proofs exhibited exceptional fatigue resistance, switching depth, and uniform extinction of the luminescence for BWSO and BWS3 without any color shift during the switching process. The system of the aforementioned pigments is therefore ideally suited for use as a security feature, enabling a wide range of switchable luminescent colors that can be achieved through additive color mixing.

Claims

REQUIREMENTS 1. A system of at least two types of switchable capsule luminescent pigments, wherein the capsule luminescent pigments are polymer particles with a diameter of 0.1–1000 pm, each containing at least one luminescent substance and one photochemically switchable absorber, wherein a first type of capsule luminescent pigment comprises a first luminescent substance emitting in a first color in the visible spectral range; a second type of capsule luminescent pigment comprises a second luminescent substance emitting in a second color in the visible spectral range, the first and second colors being different; and the two capsule luminescent pigments each comprise the same switchable absorber, wherein the absorber has a first and a second switching state, and the absorption spectrum of the absorber in the second switching state significantly overlaps with the excitation spectra of the first and second luminescent substances.

2. System according to claim 1, wherein excitation of the first and second luminescent material occurs at the same wavelength as switching the photochemically switchable absorber from the first to the second switching state.

3. System according to claim 1 or 2, wherein the system comprises a third type of capsule luminescent pigments, wherein the third type of capsule luminescent pigments comprises a third luminescent substance emitting in a third color in the VIS spectral range, wherein the third color is different from the first and second colors, and wherein the third type of capsule luminescent pigments comprises the same switchable absorber as the first and second types of capsule luminescent pigments.

4. System according to one of claims 1-3, wherein at least the first, the second and, if present, the third luminescent substance are excitable in the UV spectral range, preferably in the range of 300-380nm, more preferably in the range of 365-380nm, and particularly preferably at 365nm.

5. System according to one of claims 1-4, wherein the photochemically switchable absorber is a diarylethene.

6. System according to one of claims 1-5, wherein one of claims 1-6, wherein the diarylethene is a diheteroarylethene, preferably a dithienylethene, and particularly preferably a fluorinated dithienylethene.

7. System according to any one of claims 1-6, wherein the second absorption spectrum of the diarylethene in the second switching state at an excitation wavelength of the first, the second and, if present, the third luminescent substance exhibits at least 150% of the absorption of the first absorption spectrum of the diarylethene in the first switching state at the same excitation wavelength of the first, the second and, if present, the third luminescent substance.

8. System according to any one of claims 1-7, wherein the first, second and, if present, third luminescent agent is independently selected from the group consisting of diarylpolyenes, arylacetylenes, oxazoles, pyrazoles, benzazoles, anthrones, quinones, cyanines, rhodamines, oxazines, phenoxazines, thiazines, phenothiazines, perylenes, terylenes, coumarins, benzoxazinones or benzothiazinones, rare earth metal complexes, oxinates, aldazines, anthranilic acid derivatives, and salicyclic acid derivatives, and preferably is selected from the group consisting of perylenes, benzoxazinones, oxinates, benzthiazones, aldazines, anthranilic acid derivatives, salicyclic acid derivatives, oxazines, oxazoles, and anthrones.

9. System according to any one of claims 1-8, wherein the polymer particle is a core-shell polymer particle, preferably comprising as a core a polymer selected from polystyrene (PS), polyacrylates, polyethylene (PE), polypropylene (PP), polycarbonates (PC), polyamides (PA), polyurethanes (PU), polyureas (PH), polyethylene terephthalate (PET), other polyesters, or mixtures thereof, and / or comprising as a shell a condensation polymer selected from aminoplasts, phenolplasts, melamine-formaldehyde resins (MF), melamine-phenol-formaldehyde resins (MPF), phenol-formaldehyde resins (PF), urea-formaldehyde resins (UF), melamine-guanidine-formaldehyde resins, phenol-resorcinol-formaldehyde resins, or mixtures thereof. wherein the polymer particle is particularly preferably a core-shell polymer particle comprising as its core a thermoplastic polymer selected from polymethylmetharylate or polystyrene and / or as its shell a melamine-formaldehyde resin.

10. System according to one of claims 1-9, wherein the proportion of the luminescent material in the core is 0.1 to 10 wt.% and / or the proportion of the photochemically switchable absorber is 1 to 20 wt.%.

11. Switchable luminescent printing ink comprising a system of at least two types of switchable capsule luminescent pigments according to any one of claims 1-10.

12. A security document, preferably a banknote, with a switchable luminescent print, comprising a system of at least two types of switchable capsule luminescent pigments according to any one of claims 1-10 or the switchable luminescent printing ink according to claim 11.

13. A security document, preferably a banknote, with a luminescent imprint according to claim 12, wherein the luminescence of the at least two types of capsule luminescent pigments can be switched off using light of the same wavelength, preferably between 300 and 380 nm, more preferably between 350 and 380 nm, and particularly preferably at 365 nm.

14. A security document, preferably a banknote, with a luminescent print according to claim 12 or 13, wherein the luminescent print comprises at least one further luminescent printing ink, the luminescence of which is not switchable off in the same wavelength range, preferably between 300 and 380 nm, more preferably between 350 and 380 nm, and particularly preferably at 365 nm, and which differs in emission color in the VIS spectral range from the first and second switchable luminescent capsule pigments.

15. A security document, preferably a banknote, with a luminescent print according to claim 12 or 13, wherein the luminescent print comprises at least one further luminescent printing ink, the luminescence of which is not switchable in the same wavelength range, preferably between 300 and 380 nm, more preferably between 350 and 380 nm, and particularly preferably at 365 nm, and the emission color in the VIS spectral range corresponds to at least one of the first and second switchable luminescent capsule pigments.

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