Switchable capsule-luminescent pigments

Abstract

The present invention relates to a capsule-luminescent pigment comprising a luminescent substance and a photochemically switchable absorber as a molecular switch, wherein the luminescent substance and the photochemically switchable absorber are dissolved and / or dispersed as separate molecules in a polymer particle, the luminescent substance having an excitation spectrum, and the photochemically switchable absorber having a first absorption spectrum in a first switching state and a second absorption spectrum in a second switching state, wherein the first absorption spectrum of the photochemically switchable absorber does not significantly overlap with the excitation spectrum of the luminescent substance and the second absorption spectrum of the photochemically switchable absorber does significantly overlap with the excitation spectrum of the luminescent substance, so that the luminescence of the luminescent substance is considerably more intense in the first switching state of the photochemically switchable absorber than in the second switching state of the photochemically switchable absorber. The present invention also relates to a switchable luminescent printing ink, comprising a capsule-luminescent pigment as described herein, and to a value document, preferably a banknote, comprising a print having a first switchable luminescent printing ink as described herein.

Description

[0001] SWITCHABLE CAPSULE LUMINOUS PIGMENTS

[0002] Technical field

[0003] The present invention relates to a capsule luminescence pigment comprising a luminescent substance and a photochemically switchable absorber as a molecular switch, wherein the luminescent substance and the photochemically switchable absorber are dissolved and / or dispersed as separate molecules in a polymer particle, the luminescent substance has an excitation spectrum, the photochemically switchable absorber has a first absorption spectrum in a first switching state and a second absorption spectrum in a second switching state, the first absorption spectrum of the photochemically switchable absorber does not significantly overlap with the excitation spectrum of the luminescent substance, and the second absorption spectrum of the photochemically switchable absorber significantly overlaps with the excitation spectrum of the luminescent substance, so that in the first switching state significantly more intense luminescence of the luminescent substance occurs than in the second switching state of the photochemically switchable absorber.The present invention further relates to a switchable luminescent printing ink comprising a capsule luminescent pigment as described herein and to a security document, preferably a banknote, comprising an imprint with a first switchable luminescent printing ink 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 ("switch").

[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 and back again under UV or visible light. The two switching states have different chemical structures and thus differ primarily in their absorption spectra in the visible and UV spectral ranges (UV / 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 / 080653 Al.

[0011] CN 1 13 174004 A describes a switchable luminescent material based on

[0012] Tetraphenylporphyrin (TPP) and a diarylethene, where the TPP and the diarylethene are encapsulated in nanoparticles. The diarylethene emits green light in the switched-on state and dark light in the switched-off state; that is, 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.

[0013] CN 1 14 907 312 A describes a switchable luminescent material based on a diarylethene that is excited in the UV range and switches at 405 nm. Here too, the diarylethene itself emits. No other luminescent materials are present.

[0014] WO 2021 / 074 275 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 two. However, the synthesis of such specialized complexes is very complex and expensive.

[0015] 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 generally insufficient.

[0016] Furthermore, in Adv. Mater. 2017, 29, a printing ink with switchable luminescence based on a diarylethene and a ruthenium complex is described. In this case, the luminescent substance and the switch form a supramolecular unit; that is, the spatial proximity between the photochromic substance and the luminescent substance is established by chemical bonding, which, however, involves a complex chemical synthesis.

[0017] The Eur. J. Org. Chem., 2007, 2064 describes a security data storage device with switchable luminescence based on a diarylethene, in which the luminescence and switch groups are molecularly linked. Here, too, the necessary complex chemical synthesis of the substance is a disadvantage.

[0018] Consequently, previously described security printing inks with switchable luminescence using diarylethenes fall into two groups.

[0019] The first group comprises systems with a close molecular connection between the luminescence-

[0020] center and the photochromic center, such as diarylethenes with luminescent

[0021] Substituents, systems in which a luminescent substance and diarylethene are linked via a molecular spacer, a polymer strand with attached luminescent substance and diarylethene, luminescent metal-organic complexes with a diarylethene as a ligand, metal-organic frameworks with a luminescent substance and a diarylethene as a loading, and nanoparticles with a luminescent substance and a diarylethene as a loading. However, these systems all require very complex manufacturing and purification processes due to the formation of numerous byproducts and do not yet exhibit optimized printing and aging properties. Furthermore, nanoparticles are not well-suited for use in banknote printing ink.

[0022] The second group comprises printing inks with a mixture of a luminescent substance and a diarylethene. However, with these inks, it is not guaranteed that the luminescent substance and the diarylethene will not separate during the printing process, or be attacked to varying degrees by environmental influences or removed from the finished printed layer. Therefore, the aging resistance of the switchable luminescence effect is not ensured. Furthermore, the insufficient spatial proximity of the switch and the photochromic substance in such mixtures means that a sufficiently pronounced FRET energy transfer is not guaranteed (i.e., the switching depth is shallow), which renders the fading of the luminescence after the switch is activated unsuitable for use as a simple, quick, and unambiguously verifiable security feature.Furthermore, many known diarylethenes can only be switched at short UV wavelengths below 300 nm. Since such light sources are not widespread on the market, this also makes them unattractive for use as a security feature.

[0023] However, switchable luminescent printing inks would be desirable, which would have at least some, ideally all, of the following properties:

[0024] Luminescence excitation and switching at 365 nm, i.e. a wavelength commonly used in UV lamps at checkout counters for security checks;

[0025] Automatic switching of the switch from the second to the first state by daylight; large change in luminescence intensity between the switching states (switching depth); aging resistance (environmental stability, lightfastness (also referred to here as "fatigue resistance")), i.e. that many switching cycles can be performed;

[0026] Compatibility with various printing inks; and ease of manufacture.

[0027] The present invention is therefore based on the objective of providing improved luminescent printing inks compared to the prior art, which exhibit at least some, ideally all, of these advantageous properties. Furthermore, the present invention aims to provide capsule luminescent pigments that can be used to produce such improved luminescent printing inks. Finally, the present invention aims to provide security documents, preferably banknotes, that have enhanced security features with the advantageous properties described above.

[0028] Summary

[0029] According to a first aspect, the present invention relates to a capsule luminescence pigment comprising a luminescent substance and a photochemically switchable absorber as a molecular switch, wherein the luminescent substance and the photochemically switchable absorber are dissolved and / or dispersed as separate molecules in a polymer particle, the luminescent substance has an excitation spectrum, the photochemically switchable absorber has a first absorption spectrum in a first switching state and a second absorption spectrum in a second switching state, the first absorption spectrum of the photochemically switchable absorber does not significantly overlap with the excitation spectrum of the luminescent substance, and the second absorption spectrum of the photochemically switchable absorber significantly overlaps with the excitation spectrum of the luminescent substance, so that significantly more intense luminescence of the luminescent substance occurs in the first switching state than in the second switching state.

[0030] The present invention further relates, according to a second aspect, to a switchable luminescent printing ink comprising a capsule luminescent pigment according to the first aspect of the present invention.

[0031] The present invention further relates, according to a third aspect, to a security document, preferably a banknote, comprising an imprint with a first switchable luminescent printing ink according to the second aspect of the present invention.

[0032] Figures

[0033] 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 from 230 to

[0034] 730 nm. Figure 2 shows the absorption spectra of a diarylethene switch (DAE 2) according to circuit (SZ2) as well as the excitation spectrum of a luminescent substance (LS2) in the range of 300 to 450 nm.

[0035] Figure 3a shows a monochrome print pattern comprising a switchable luminescent printing ink according to the invention and a non-switchable luminescent printing ink.

[0036] Figure 3b shows a monochrome print pattern comprising a switchable luminescent printing ink according to the invention and a non-switchable luminescent printing ink.

[0037] Figure 4A shows a print pattern with color change, comprising a switchable luminescent printing ink according to the invention and a non-switchable luminescent printing ink.

[0038] Figure 4B shows a print pattern with color change, comprising a switchable luminescent printing ink according to the invention and a non-switchable luminescent printing ink.

[0039] Figure 5 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.

[0040] Figure 6A shows a visually recognizable pattern for a switchable mélier fiber with color change before a switch is activated.

[0041] Figure 6B shows a visually recognizable pattern for a switchable variegated fiber with color change after the activation of a switch.

[0042] Figure 7 shows the results of a fatigue resistance test over 50 switching cycles for the diarylethene switch DA2 (European Blue Wool Scale 0).

[0043] Figure 8 shows the results of a fatigue resistance test over 50 switching cycles for the diarylethene switch DA2 (European Blue Wool Scale 3).

[0044] Figure 9 shows the absorption spectra of a diarylethene switch (DAE) before and after switching (SZ1 and SZ2) and the excitation spectrum of a luminescent material (LSI) in the range of 230 to 680 nm. Figure 10 shows the absorption spectra of a diarylethene switch (DAE) after switching (SZ2) and the excitation spectrum of a luminescent material (LSI) in the range of 300 to 460 nm.

[0045] Detailed description of the invention

[0046] The present invention is based, firstly, on the finding that the luminescence of a luminescent substance is 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 wanting to be bound to a specific theory, it is assumed that when the excitation spectrum of the luminescent substance overlaps with the absorption spectrum of the switch, excitation with the excitation wavelength from the excited state of the luminescent substance already leads to an energy transfer to the corresponding absorption band of the switch, which results in the luminescence of the luminescent substance being switched off.

[0047] 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.

[0048] According to a first aspect, the present invention relates to a capsule luminescent pigment comprising a luminescent substance and a photochemically switchable absorber as a molecular switch, wherein the luminescent substance and the photochemically switchable absorber are dissolved and / or dispersed as separate molecules in a polymer particle, the luminescent substance has an excitation spectrum, the photochemically switchable absorber has a first absorption spectrum in a first switching state and a second absorption spectrum in a second switching state, the first absorption spectrum of the photochemically switchable absorber does not significantly overlap with the excitation spectrum of the luminescent substance, and the second absorption spectrum of the photochemically switchable absorber significantly overlaps with the excitation spectrum of the luminescent substance, so that significantly more intense luminescence of the luminescent substance occurs in the first switching state than in the second switching state.

[0049] To ensure switching of the luminescence, the excitation spectrum of the luminescent substance must overlap with the absorption spectrum of the switch, but only in its second switching state, not in the first. In the first switching state, the luminescence is unaffected by the switch; in the second switching state, however, the excitation energy is reabsorbed by the switch, the energy is transferred non-radiatively, and / or the excitation light is absorbed, thus significantly reducing the luminescence intensity.

[0050] A non-significant overlap of the first absorption spectrum of the switch (i.e., the switch before the circuit) with the excitation spectrum of the luminescent material means that the first absorption spectrum of the switch, 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 spectrum of the luminescent material, which (i.e., the absorption) at its highest point is 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 luminescent material in its excitation spectrum normalized to 1 at the wavelength of highest excitation in the range of 245 to 700 nm.

[0051] An excitation band of the 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 luminescent material. The (local) absorption maximum of the switch after switching occurs in this relevant excitation band of the luminescent material in the second switching state. 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 luminescent material can be more than 40% in absolute terms, 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. The first and second absorption spectra are each normalized to the value 1 at the wavelength of 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 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 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 excitation band of the 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.

[0052] A significant overlap of the second absorption spectrum of the switch (i.e., the switch downstream of the circuit) with the excitation spectrum of the luminescent material means that the second absorption spectrum of the switch, normalized to 1 at the wavelength of highest absorption in the range of 235 to 400 nm, exhibits an absorption in the wavelength range of an excitation band in the excitation spectrum of the luminescent material. This 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 luminescent material in its excitation spectrum normalized to 1 at the wavelength of highest excitation in the range of 245 to 700 nm. The excitation band of the luminescent material is defined as above.

[0053] 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 diarylethene with the excitation spectrum of the luminescent substance, the first and second absorption spectra are measured under the same conditions, for example under the same settings on the measuring instrument, at the same concentrations, etc.

[0054] 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 350 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.

[0055] 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 from the luminescent material to the switch.

[0056] In particular, when the switch is activated, the luminescence intensity of the luminescent material decreases by at least 10%, for example, by at least 20%, at least 30%, or at least 40%. Preferably, the reduction in luminescence intensity is at least 45%, for example, by 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.

[0057] The wavelength at which the luminescent material can be excited is not inherently limited. However, for the use of the capsule pigments according to the invention in security applications, such as for valuable documents, it is advantageous to select the excitation wavelength such that luminescence of the luminescent material perceptible to the human eye is ensured. This will generally be the case with shorter excitation wavelengths. Excitation wavelengths in the UV range are preferred, for example, in the range of 300–400 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.

[0058] The wavelength range in which the switch can be turned from the first to the second switching state is not inherently limited, and in principle any suitable wavelength can be chosen. However, it is advantageous and therefore preferred if the switching of the switch from the first to the second switching state can occur in the same wavelength range, or preferably even at exactly the same wavelength, as the excitation of the luminescent material. For example, the switching of the switch from the first to the second switching state and the excitation of the luminescent material can occur in the range of 300–400 nm, preferably in the range of 350–380 nm, and particularly preferably at an excitation wavelength of 365 nm.Simultaneous excitation of the luminescent substance and switching of the switch is advantageous because only one light / radiation source is then required, which significantly simplifies methods for verifying the authenticity of valuable documents based on the capsule luminescent pigments according to the invention.

[0059] The light intensities required for switching are preferably in the UV range, comparable to the excitation intensity of the first luminescent substance. This allows both effects to be triggered simultaneously with the UV lamps typically used at checkout counters, and the re-switching intensity in the visible range to be at the level of typical room lighting. The intensity of the light source primarily affects 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 used to describe and quantify the luminescence properties of a luminescent substance 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 perceptibility of a luminescence change of different capsule luminescence pigments relative to each other with 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 20%. 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 20%, for example at most 15%, more preferably at most 10%, and particularly preferably at most 5%.

[0060] 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, a visually detectable switching depth of at least 10%, such as at least 20%, at least 30%, or at least 40%, preferably at least 45%, such as at least 50%, at least 60%, or at least 70%, is achieved under UV light of the specified intensity and at a distance of approximately 20 cm within a period of less than 30 s, preferably less than 20 s, and particularly preferably less than 10 s.

[0061] The photochemically switchable absorber used as a molecular switch, i.e., the switch 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 switch 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.

[0062] In principle, any suitable photochemically switchable absorber can be used in the capsule luminescent pigments according to the invention. A person skilled in the art can select a suitable photochemically switchable absorber for a specific application by taking into account the desired switching wavelength of the switch and / or the excitation maximum of the luminescent material (which, as described above, should significantly overlap with the absorption spectrum of the second switching state of the switch) and / or the desired wavelength for the (re-)switching from the second to the first state. For example, the photochemically switchable absorber can be a diarylethene, preferably a diheteroarylethene, even more 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:

[0063] The switch preferably switches back from the second to the first state when illuminated by visible light, for example, in the wavelength range of 400–700 nm. It is further preferred that the switch remains in its state in darkness at room temperature, i.e., neither switching from the first to the second state nor from the second to the first state occurs. Preferably, the switch can be switched from the second to the first state by the prescribed average illuminance of 500–750 lux at office workplaces (DIN EN 12464-1). The switch preferably switches automatically back from its second switching state to its initial state under daylight 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 illuminance is achieved.

[0064] The luminescent substance itself is not limited in principle, and any suitable luminescent substance can be used in the capsule luminescent pigments according to the invention. For optimal applicability as a security feature, the luminescent substance is preferably selected such that it can be excited between 300 and 400 nm, preferably between 350 and 380 nm, and particularly preferably at 365 nm. Furthermore, it is preferred that the luminescent substance emits visible light. The luminescent substance need not be a pure chemical substance, 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 agents are 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, oxinates, aldazines, anthranilic acid derivatives, salicyclic acid derivatives, and mixtures thereof. Preferably, the luminescent agent is selected from the group consisting of perylenes, benzoxazinones, oxinates, benzthiazine, aldazines, anthranilic acid derivatives, salicyclic acid derivatives, oxazines, oxazoles, anthrones, and mixtures thereof. Suitable luminescent materials are described, for example, in WO 2006 / 014658 A2, US 2015 / 0132575 Al and EP 2195395 Bl.

[0065] The luminescent substance and the switch are dissolved and / or dispersed as separate molecules in a polymer particle within the capsule luminescent pigments according to the invention. Thus, there is no direct chemical bond between the luminescent substance and the switch; rather, they exist as separate molecules. The luminescent substance and the switch 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 switch and the luminescent pigment are distinct from one another, i.e., they cannot be the same molecule.

[0066] Encapsulation means that the luminescent material and the switch are together enclosed in a polymer 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. These external influences include, but are not limited to, humidity, aqueous environments, perspiration, greases, 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 WO2017 / 080 653 Al (polymethyl methacrylate (PMMA) / melamine-formaldehyde resin (MF) coating) or WO2017 / 080 656 Al (polyurethane (PUR) / MF coating). The processes described therein for producing core-shell particles containing a luminescent substance in the core can be modified to produce the capsule luminescent pigment according to the invention by adding the switch in addition to the luminescent substance during the core material production step.

[0067] For example, the capsule luminescence pigment according to the invention 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 luminescence pigment according to the invention contains 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.The capsule luminescence pigment according to the invention particularly preferably contains as a core a thermoplastic polymer selected from polymethyl methacrylate or polystyrene and as a shell a melamine-formaldehyde resin.

[0068] The proportion of the luminescent substance in the core of the capsule luminescent pigment according to the invention can be 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 substance consists of several different luminescent substances, the proportions of the respective individual luminescent substances 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.

[0069] The proportion of the switch in the core of the capsule luminescent pigment according to the invention can be 1 to 20.

[0070] 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.

[0071] The capsule luminescent pigments according to the invention can have a diameter of 0.05 to 1000 pm, for example, 0.05 to 500 pm, 0.05 to 200 pm, or 0.05 to 100 pm, preferably 0.1 to 50 pm, for example, 0.1 to 20 pm, more preferably 0.2 to 10 pm or 0.5 to 5 pm, and particularly preferably 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. Determining 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.

[0072] According to a second aspect, the present invention relates to a switchable luminescent printing ink comprising a capsule luminescent pigment according to the first aspect of the present invention. The capsule luminescent pigment contained in the switchable luminescent printing ink according to the invention can therefore be any capsule luminescent pigment described above.

[0073] 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.

[0074] According to a third aspect, the present invention relates to a security document comprising a print with a first switchable luminescent printing ink according to the second aspect of the present invention. The security document can, in principle, be any type of security document, for example, a security document, a banknote, or an identity document, and is preferably a banknote or an identity document, and particularly preferably a banknote.

[0075] 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 made of 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.

[0076] 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 varnishes are particularly preferred. For example, a sizing layer may contain polyvinyl alcohol and / or polyurethane. Examples of suitable primer compositions are compositions based on acrylates, polyester acrylates, urethane acrylates, polyester polyurethanes, and acrylonitrile styrene polyurethanes. Water-based dispersions, in particular water-based dispersions of aliphatic components, are especially preferred. The security document according to the invention further comprises printing with a first switchable luminescent printing ink according to the second aspect of the invention.A printed surface can be either a layer of ink applied to the entire surface or at least partially across the surface of the document, consisting of a first, switchable ink or a mixture of inks containing a first, switchable ink, or alternatively, a printed pattern obtained by applying a first, switchable ink. Parts or sections of a document, such as a melange fiber, can also be printed with the switchable ink. A printed surface can, of course, also contain several printed patterns made of different inks, applied side by side or (partially) on top of each other, with at least one of the inks being the first, switchable ink. Likewise, in addition to at least one printed pattern made of a first, switchable ink, there can also be another printed layer applied to the entire surface or at least partially across the surface of the document.Printing with more than one ink is preferred, wherein at least one of the inks is the first switchable ink. The printing can be, for example, a background print or an intaglio print and can be applied, for example, directly to the substrate or to at least one, preferably both, of the two optionally opposing layers applied to the substrate or to any further layers of the document.

[0077] Printing a valuable document with a first, switchable luminescent ink according to the second aspect of the present invention enables a simple security check of the document by visual inspection when the document is irradiated with a suitable wavelength. The switch contained in the first, switchable luminescent ink, as described above, switches from the first to the second switching state when irradiated with a suitable wavelength. In the second switching state, the excitation of the luminescent substance, which is simultaneously excited, preferably at the same wavelength as the switch, is reduced by energy transfer. For visual inspection, this results in a gradual decrease in the luminescence of the luminescent ink when the radiation source is switched on. This is noticeable as a darkening of the ink during visual inspection.After the radiation source is switched off, the switch resets, allowing further appropriate safety checks to be carried out.

[0078] Preferably, the security document according to the invention further comprises printing with a second, non-switchable luminescent ink, which has the same color appearance and preferably the same luminescence intensity as the first, switchable luminescent ink. For example, the second, non-switchable luminescent ink can contain the same luminescent substance as the first, switchable luminescent ink, but without a molecular switch. 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 first, switchable luminescent ink) and some areas retain their luminescence unchanged (i.e., the areas printed with the second, non-switchable luminescent ink). The security document according to the invention can, of course, also include printing with further inks.For example, printing with additional, switchable luminescent inks, different from the first switchable luminescent ink, is conceivable. By printing with such additional, for example, two, three, four, or five different switchable luminescent inks, complex patterns can be created that only become visible under irradiation, such as UV light. Similarly, in complex color mixtures that produce luminescence in a specific color, the visually perceptible luminescence under irradiation, such as UV light, can be altered by selectively switching off individual components of the color mixture.

[0079] Figures 3-6 show exemplary applications of the switchable luminescent printing ink according to the invention in printing patterns and melange fibers.

[0080] Figures 3a and 3b show two possible monochrome print patterns, comprising 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 printed onto a substrate (hatched area). The first and second luminescent inks have the same luminescence color. However, the second, non-switchable luminescent ink does not change its intensity when irradiated, for example, by UV radiation, while the first, switchable luminescent ink darkens.

[0081] Figures 4A and 4B show a possible color-changing print pattern comprising a switchable luminescent printing ink according to the invention and a non-switchable luminescent printing ink. A first, switchable luminescent printing ink (light hatched areas) and a second, non-switchable luminescent printing ink (white areas) are printed onto a substrate (dark hatched area). At the beginning of irradiation, the first and second luminescent printing inks have the same luminescent color, which is created by mixing different luminescent pigments (state A, Figure 4A). The second luminescent printing ink does not change its intensity or color upon irradiation (e.g., UV irradiation), whereas the first luminescent printing ink changes its color (state B, Figure 4B).In particular, a color change occurs in the first, switchable luminescent printing ink, as one or more of the pigments in the mixture become dark and therefore the color composition changes.

[0082] Figure 5 shows a possible pattern 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. A first, switchable luminescent printing ink (black areas) and a second, non-switchable luminescent printing ink (white areas) are incorporated into a melange fiber. The first and second luminescent printing inks have the same luminescence color. The second, non-switchable luminescent printing ink does not change its intensity upon irradiation, for example, UV irradiation, while the first luminescent printing ink darkens.

[0083] Figures 6A and 6B show a possible visually recognizable pattern for a switchable luminescent fiber with color change before (Figure 6A) or after (Figure 6B) the activation of a switch. A first, switchable luminescent printing ink (light hatched areas), a second, non-switchable luminescent printing ink (white areas), and a third, switchable luminescent printing ink (dark hatched area) 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 created by mixing different luminescent pigments (State A; Figure 6A). The second luminescent printing ink does not change its intensity or color under UV irradiation, while the first and third luminescent printing inks change their color (State B; Figure 6B).In particular, a color change occurs between the first and third switchable luminescent printing inks, as one or more of the pigments in the mixture darken, thus changing the color composition. This can result in a color change to different colors in the final state.

[0084] Examples:

[0085] Methods

[0086] 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 below and again assessed for switching time, switching depth, and residual intensity of the luminescent substance in its initial state.

[0087] 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 / 080 654 Al was used. Both the luminescent material and the switch were required to pass the BWS test.The luminescent material is measured, particularly in the first switching state of the switch, and at "BWS3" still exhibits a residual luminescence intensity of at least 30% of the original luminescence intensity. The switch 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.

[0088] Syntheses of diarylethene switches

[0089] 3,3'-(Perfluorocyclopent-l-ene-l,2-diyl)bis(2-methyl-6-nitrobenzo[b]thiophene) (DAE 2)

[0090] 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 iaqj 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%).

[0091] 2,2'-(Perfluorocyclopent-l-ene-l,2-diyl)bis(3-methylbenzo[b]thiophene) (DAE 3)

[0092] 3-Methylbenzo[fa]thiophene (2.25 mL, 2.50 g, 16.9 mmol, 1.0 eq.) was dissolved in 50 mL of dry THF (20 mL), cooled to -40°C, and n-BuLi (2.5 mmol in H, 7.50 mL, 18.8 mmol, 1.1 eq.) was added dropwise. The reaction solution was then stirred at -40°C for 1 hour, and octafluorocyclopentene (1.15 mL, 1.82 g, 8.57 mmol, 0.5 eq.) was added dropwise. The reaction mixture was stirred at -40°C for 1 hour, warmed to room temperature, and stirred overnight at room temperature. The reaction was stopped by adding 20 mL of H₂O and 20 mL of NaCl( aq The reaction was stopped with 40 mL of Et₂O, the phases were separated, and the aqueous phase was extracted with Et₂O (3 x 20 mL). The combined organic phases were washed with NaCl(aq) (1 x 40 mL), dried over Na₂SO₄, filtered, and the solvent was removed under vacuum. The product was purified by column chromatography (SiC₂, H / EtOAc) and isolated as a pale yellow solid (3.05 g, 6.50 mmol, 77%).

[0093] Example 1:

[0094] Example a) Blue, switchable luminescent pigment at 365 nm with a polymethyl methacrylate core and a melamine-formaldehyde shell

[0095] The 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[fa]thiophene) (DAE 2, C23H12F6N2O4S2) was used as the diarylethene switch. The luminescent substance is present at 0.5 wt% and the diarylethene switch at 7.5 wt%.

[0096] The resulting pigment luminesces blue under illumination at 365 nm and switches, in particular, upon irradiation with light of this wavelength. The excitation band of the luminescent material, as defined above, lies in the range of 350–410 nm. The absorption maximum of the diarylethene switch in its second state, within the spectral range relevant for excitation of the luminescent material, is located at 372 nm and thus lies within the defined excitation band. Additionally, 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 (see also Figure 1).

[0097] Due to this overlap between the absorption spectrum of the diarylethene switch after activation and the excitation spectrum of the luminescent material—i.e., enhanced absorption in the second switching state of the diarylethene—a very good switching depth of 53% for BWSO (see also Figure 7) and 19% for BWS3 (see also Figure 8) is achieved. The switching depth is thus significantly above the threshold of 10%, and the difference in luminescence intensity is clearly visible. The residual intensity of the luminescent material in the first switching state for BWS3 is 44%, exceeding the desired residual intensity of 30%. In particular, the pigment exhibits exceptional fatigue resistance with a residual switching depth of 36% for BWS3 and is therefore ideally suited for use as a safety feature.

[0098] Table 1 summarizes the experimentally obtained data for example a) .

[0099] Table 1.

[0100] [1]for the excitation of relevant luminescent material,

[0101] [2] Residual intensity of the luminescent substance at BWS3

[0102] Example lb): Blue, switchable luminescent pigment at 365 nm with a polymethyl methacrylate core and a melamine-formaldehyde shell

[0103] The switchable capsule luminescent pigment was prepared according to Example 2 of WO 2017 / 080 653 Al and contains CD311 (Honeywell) (LS2) as the luminescent agent dissolved in the core. 3,3'-(Perfluorocyclopent-l-ene-l,2-diyl)bis(2-methyl-6-nitrobenzo[b]thiophene) (DAE 2, C23H12F6N2O4S2) was used as the diarylethene switch. The luminescent agent is present at a weight of 1% and the diarylethene switch at a weight of 7.5%. The resulting pigment luminesces blue under illumination at 365 nm and switches, in particular, under irradiation with light of wavelength 365 nm. The excitation band of the luminescent substance, as defined above, lies in the range of 280–415 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 band.Additionally, 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.

[0104] Due to this overlap between the absorption spectrum of the diarylethene switch after activation and the excitation spectrum of the luminescent material—i.e., enhanced absorption in the second switching state of the diarylethene—a very good switching depth of 47% for BWSO and 18% for BWS3 is achieved. This switching depth is thus significantly above the threshold of 10%, and the difference in luminescence intensity is clearly visible. The residual intensity of the luminescent material in the first switching state at BWS3 is 31%, exceeding the desired residual intensity of 30%. In particular, with a residual switching depth of 38% at BWS3, the pigment exhibits exceptional fatigue resistance and is therefore ideally suited for use as a security feature.

[0105] Table 2 summarizes the experimentally obtained data for example lb).

[0106] Table 2.

[0107] [1] for the excitation of relevant luminescent material,

[0108] [2] Residual intensity of the luminescent substance at BWS3

[0109] Example 2: Green, switchable luminescent pigment at 365 nm with a polymethyl methacrylate core and a melamine-formaldehyde shell

[0110] The 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 (LS3, C24H16N2O4S) as the luminescent agent dissolved in the core. 3,3'-(perfluorocyclopent-l-ene-l,2-diyl)bis(2-methyl-6-nitrobenzo[b]thiophene) (DAE 2, C23H12F6N2O4S2) was used as the diarylethene switch. 0.5 wt% of the luminescent agent and 5 wt% of the diarylethene switch are used. The resulting pigment luminesces green under illumination at 365 nm and switches, in particular, under irradiation with light of wavelength 365 nm. The excitation band of the luminescent substance, as defined above, 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 the excitation of the luminescent substance, is located at 372 nm and thus lies within the defined excitation band.Additionally, 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.

[0111] Due to this overlap between the absorption spectrum of the diarylethene switch after activation and the excitation spectrum of the luminescent material—i.e., enhanced absorption in the second switching state of the diarylethene—a good switching depth of 31% for BWSO and 22% for BWS3 is achieved. This switching depth is thus significantly above the threshold of 10%, and the difference in luminescence intensity is clearly visible. The residual intensity of the luminescent material in the first switching state for BWS3, at 73%, is well above the desired residual intensity of 30%. In particular, with a residual switching depth of 71% for BWS3, the pigment exhibits exceptional fatigue resistance and is therefore ideally suited for use as a security feature.

[0112] Table 3 summarizes the data obtained experimentally for Example 2.

[0113] Table 3.

[0114] [1] for the excitation of relevant luminescent material,

[0115] [2] Residual intensity of the luminescent substance at BWS3

[0116] Comparison example 1: blue luminescence, switchable at 365 nm, not encapsulated together

[0117] Comparative example 1a) Diarylethene switch and luminescent material, each encapsulated (polymethyl methacrylate core and melamine-formaldehyde shell), in superimposed printed

[0118] layers

[0119] A luminescent capsule pigment LP1 was produced according to Example 2 of WO 2017 / 080653 Al, containing 2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole) (LSI, C26H26N2O2S) as the luminescent substance dissolved in the core. 0.5 wt% of the luminescent substance was used. A printing ink Fl was produced from LP1. For this purpose, the pigment was 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 wt%.

[0120] Additionally, a switching capsule pigment SP1 was produced according to Example 2 of WO 2017 / 080653 Al, which contains 3,3'-(perfluorocyclopent-l-ene-l,2-diyl)bis(2-methyl-6-nitrobenzo[b]thiophene) (DAE 2, C23H12F6N2O4S2) as a diarylethene switch dissolved in the core. 7.5 wt% of the diarylethene switch was used. A second printing ink, F2, was produced from SP1. For this, the pigment was incorporated into a solvent-based varnish (Pröll GmbH) using a Retsch MM400 vibratory mill (manufactured in 2018). The degree of pigmentation was 1 wt%.

[0121] The printing ink Fl was applied with a printing thickness of 1 g / m². 2Printed on security paper, the proofs were dried at 60 °C for 2 hours. They were then coated with F2 ink at varying thicknesses, specifically 12, 24, and 50 pm. The thinnest F2 layer (12 pm) corresponds to the ratio of luminescent material to diarylethene switch used in Example 1. The thickest F2 layer (50 pm) contains more than twice the amount of diarylethene switch compared to Example 1. The proofs were then visually assessed for their switching duration and depth. The switching depth was quantitatively determined using a commercially available fluorescence spectrometer.

[0122] In all three cases, the switching depth is significantly below the threshold of 10% (see Table 4 below), and the difference in luminescence intensity is not visually noticeable. Therefore, using separately encapsulated luminescent material and diarylethene switches applied in two layers is not possible to achieve switchable luminescence.

[0123] Table 4 summarizes the experimentally obtained data for comparison example aa.

[0124] Table 4.

[0125] [1] for the excitation of relevant luminescent material, [2] Layer thickness of the squeegeed DAE layer

[0126] Comparative example lb) Diarylethene switch and luminescent material, each individually encapsulated, (polymethyl methacrylate core and melamine-formaldehyde shell) mixed in a printing ink

[0127] A luminescent capsule pigment LP2 was produced 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. 0.5 wt% of the luminescent substance was used.

[0128] Additionally, a switching capsule pigment SP2 was prepared according to Example 2 of W02017080653A1, which contains 3,3'-(perfluorocyclopent-l-ene-l,2-diyl)bis(2-methyl-6-nitrobenzo[fa]thiophene) (DAE 2, C23H12F6N2O4S2) as a diarylethene switch dissolved in the core. 7.5 wt% of the diarylethene switch was used.

[0129] A printing ink, F3, was produced from LP2 and SP2. For this purpose, the pigments were incorporated into an offset printing ink (Sicpa Holding SA) using an Engelsmann JEL 25 / 53 ink mixing machine (manufactured in 2013). The pigmentation level was 7.5% by weight for each pigment. The F3 printing ink was applied at a printing weight of 2 g / m².2 Printed on security paper, and the proofs dried at 60 °C for 2 hours. The ratio of the two pigments LP2 and SP2 in combination with a double proof weight of 2 g / m² 2 This corresponds to the ratios and amounts of the luminescent material and the diarylethene switch used in Example 1. The impressions were then visually assessed for their switching duration and depth. The switching depth was quantitatively determined using a commercially available fluorescence spectrometer.

[0130] The switching depth is 6%, below the threshold of 10% (see Table 5 below). Therefore, any difference in luminescence intensity is not visually noticeable. Consequently, the use of separately encapsulated luminescent material and diarylethene switches mixed in a printing ink as a security feature is not possible.

[0131] Table 5 summarizes the experimentally obtained data for comparison example lb.

[0132] Table 5.

[0133] [1] For the excitation of relevant luminescent substances, comparative example 2: Blue luminescent pigment with a polymethyl methacrylate core and a melamine-formaldehyde shell, which cannot be switched sufficiently at 365 nm.

[0134] The 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. 2,2'-(perfluorocyclopent-1-ene-1,2-diyl)bis(3-methylbenzo[b]thiophene) (DAE 3, C23H14F6S2) was used as the diarylethene switch. The luminescent substance is present at 0.5 wt% and the diarylethene switch at 5 wt%.

[0135] The resulting pigment luminesces blue under illumination at 365 nm and switches, in particular, upon irradiation with light of this wavelength. The excitation band of the relevant luminescent material, as defined above, lies in the range of 370–410 nm. The absorption maxima of the diarylethene switch in its second state are located at 265, 304, and 445 nm and are therefore not within the defined excitation band (see Table 6 below, see Figure 10). Furthermore, the absorption of the second switching state of the diarylethene at the excitation wavelength is less than 100% of the absorption of the first switching state of the diarylethene (see Figure 9). In particular, there is no significant overlap between the absorption spectrum of the diarylethene switch after switching and the excitation spectrum of the luminescent material.

[0136] Due to the low overlap between the absorption spectrum of the diarylethene switch after activation and the excitation spectrum of the luminescent substance, only a very shallow switching depth of 5% is achieved for BWS0 and BWS3. This switching depth is therefore below the threshold of 10%. Consequently, the difference in luminescence intensity before and after activation of the diarylethene switch is not visually noticeable. The pigment exhibits insufficient switching capability and is therefore unsuitable for use as a security feature.

[0137] Table 6 summarizes the data obtained experimentally for comparison example 2.

[0138] Table 6.

[0139] [1] for the excitation of relevant luminescent material,

[0140] [2] Residual intensity of the luminescent substance at BWS3

Claims

REQUIREMENTS 1.Capsule luminescent pigment comprising a luminescent substance and a photochemically switchable absorber as a molecular switch, wherein the luminescent substance and the photochemically switchable absorber are dissolved and / or dispersed as separate molecules in a polymer particle; the luminescent substance has an excitation spectrum; the photochemically switchable absorber has a first absorption spectrum in a first switching state and a second absorption spectrum in a second switching state, the first absorption spectrum of the photochemically switchable absorber does not significantly overlap with the excitation spectrum of the luminescent substance, and the second absorption spectrum of the photochemically switchable absorber significantly overlaps with the excitation spectrum of the luminescent substance, such that in the first switching state of the photochemically switchable absorber, significantly more intense luminescence of the luminescent substance occurs than in the second switching state of the photochemically switchable absorber.

2. Capsule luminescence pigment according to claim 1, wherein excitation of the luminescence substance occurs at the same wavelength as switching the photochemically switchable absorber from the first to the second switching state.

3. Capsule luminescence pigment according to claim 1 or 2, wherein the second absorption spectrum of the photochemically switchable absorber in the second switching state at the wavelength of the highest excitation of the luminescence agent has at least 150% of the absorption of the first absorption spectrum of the photochemically switchable absorber in the first switching state at the same wavelength of the highest excitation of the luminescence agent.

4. Capsule luminescence pigment according to claim 2, wherein the excitation of the luminescence substance and the switching of the photochemically switchable absorber from the first to the second switching state takes place between 300 and 400 nm, preferably between 350 and 380 nm, and particularly preferably at 365 nm.

5. Capsule luminescent pigment according to one of claims 1-4, wherein the switching of the photochemically switchable absorber from the second to the first switching state is effected by visible light in the range of 400 - 700nm, preferably at an intensity of 500 - 750 lux.

6. Capsule luminescence pigment according to any one of claims 1-5, wherein the luminescence agent is 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, salicyclic acid derivatives and mixtures thereof, preferably selected from the group consisting of perylenes, benzoxazinones, oxinates, benzthiazones, aldazines, anthranilic acid derivatives, salicyclic acid derivatives, oxazines, oxazoles, anthrones, and mixtures thereof.

7. Capsule luminescence pigment according to one of claims 1-6, wherein the photochemically switchable absorber is a diarylethene, preferably a diheteroarylethene, more preferably a dithienylethene, and particularly preferably a fluorinated dithienylethene.

8. Capsule luminescent pigment according to any one of claims 1-7, 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.

9. Capsule luminescent pigment according to claim 8, wherein the polymer particle is a core-shell polymer particle comprising as its core a thermoplastic polymer selected from polymethyl methacrylate or polystyrene and / or as its shell a melamine-formaldehyde resin.

10. Capsule luminescent pigment according to any one of claims 1-9, wherein the proportion of the luminescent substance 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 one or more, preferably one, capsule luminescent pigments according to any one of claims 1-10.

12. A security document, preferably a banknote, comprising a printing with a first, switchable luminescent printing ink according to claim 11.

13. A security document, preferably a banknote, according to claim 12, further comprising a printing with a second, non-switchable luminescent printing ink, which has the same color impression and preferably the same intensity of luminescence as the first, switchable luminescent printing ink.

14. Security document, preferably banknote, according to claim 12 or 13, wherein the second, non-switchable luminescent printing ink contains the same luminescent substance as the first, switchable luminescent printing ink.

15. A security document, preferably a banknote, according to claim 13 or 14, further comprising a third, switchable luminescent printing ink according to claim 11, which is different from the first, switchable luminescent printing ink.

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