Switchable encapsulated luminescent pigments

Abstract

The invention relates to an encapsulated luminescent pigment comprising a luminescent substance and a diarylethene as a molecular switch, wherein the luminescent substance and the diarylethene are dissolved and / or dispersed as separate molecules in a polymer particle. The luminescent substance has an emission spectrum, and the diarylethene has a first absorption spectrum in a first switched state and a second absorption spectrum in a second switched state, the first absorption spectrum of the diarylethene not significantly overlapping with the emission spectrum of the luminescent substance and the second absorption spectrum of the diarylethene significantly overlapping with the emission spectrum of the luminescent substance such that the luminescence of the luminescent substance is substantially more intense in the first switched state than in the second switched state of the diarylethene. The invention further relates to a switchable luminescent printing ink, comprising an encapsulated luminescent pigment as described herein, and to a value document, preferably a banknote, comprising an imprint which has 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 diarylethene as a molecular switch, wherein the luminescent substance and the diarylethene are dissolved and / or dispersed as separate molecules in a polymer particle, the luminescent substance has an emission spectrum, the diarylethene 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 diarylethene does not significantly overlap with the emission spectrum of the luminescent substance and the second absorption spectrum of the diarylethene significantly overlaps with the emission spectrum of the luminescent substance, such that in the first switching state significantly more intense luminescence of the luminescent substance occurs than in the second switching state of the diarylethene.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.

[0008] - Combination of several separately excitable luminescent substances. However, this requires the use of multiple excitation wavelengths.

[0009] - Combination of a luminescent substance with a photochemically switchable absorber. Photochemically switchable absorbers (hereinafter also referred to as "switches") are particularly well-known 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 spectrum 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 emission spectrum of an additionally present luminescent substance, and the other of the two different absorption spectra of the two switching states does overlap with the emission 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 substance 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. CN 1 14 907 312 A describes a switchable luminescent substance based on a diarylethene that is excited in the UV range and switches at 405 nm. Here, too, the diarylethene itself emits light. No other luminescent substances are present.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] Consequently, previously described security printing inks with switchable luminescence using diarylethenes fall into two groups.

[0017] The first group comprises systems with a close molecular connection between the luminescent center and the photochromic center, such as diarylethenes with a luminescent substituent, systems in which the 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 coating, and nanoparticles with a luminescent substance and a diarylethene as a coating. 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.

[0018] 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 makes 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.

[0019] However, switchable luminescent printing inks would be desirable, which would have at least some, ideally all, of the following properties:

[0020] Luminescence excitation and switching at 365 nm, i.e. a wavelength commonly used in UV lamps at checkout counters for security checks;

[0021] 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;

[0022] Compatibility with various printing inks; and ease of manufacture.

[0023] 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. A further objective of the present invention is to provide capsule luminescent pigments that can be used to produce such improved luminescent printing inks. A further objective of the present invention is to provide securities, preferably banknotes, which have improved luminescent properties.

[0024] exhibit security features with the advantageous properties described above.

[0025] Summary

[0026] According to a first aspect, the present invention relates to a capsule luminescence pigment comprising a luminescent substance and a diarylethene as a molecular switch, wherein the luminescent substance and the diarylethene are dissolved and / or dispersed as separate molecules in a polymer particle, the luminescent substance has an emission spectrum, the diarylethene 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 diarylethene does not significantly overlap with the emission spectrum of the luminescent substance and the second absorption spectrum of the diarylethene significantly overlaps with the emission spectrum of the luminescent substance, such that in the first switching state significantly more intense luminescence of the luminescent substance occurs than in the second switching state of the diarylethene.

[0027] 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.

[0028] 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.

[0029] Figures

[0030] Figure 1 shows the absorption spectra of a diarylethene switch (DAE 1) before and after switching (SZ1 and SZ2) and the emission spectrum (LY Em) of a luminescent material (LS2) in the range of 245 to 700 nm.

[0031] Figure 2 shows the absorption spectra of a diarylethene switch (DAE 1) before and after switching (SZ1 and SZ2) and the emission spectrum of a luminescent material (LS2) in the range of 390 to 690 nm. Figure 3a shows a monochrome printed pattern comprising a switchable luminescent printing ink according to the invention and a non-switchable luminescent printing ink.

[0032] Figure 3b shows a monochrome print pattern comprising a switchable luminescent printing ink according to the invention and a non-switchable luminescent printing ink.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] Figure 6A shows a visually recognizable pattern for a switchable mélier fiber with color change before a switch is activated.

[0037] Figure 6B shows a visually recognizable pattern for a switchable variegated fiber with color change after the activation of a switch.

[0038] Figure 7 shows the results of a fatigue resistance test over 50 switching cycles for the diarylethene switch DAI (European Blue Wool Scale 0).

[0039] Figure 8 shows the results of a fatigue resistance test over 50 switching cycles for the diarylethene switch DAI (European Blue Wool Scale 3).

[0040] Figure 9 shows the results of a fatigue resistance test over 50 switching cycles for the diarylethene switch DA2 (European Blue Wool Scale 0).

[0041] Figure 10 shows the absorption spectra of a diarylethene switch (DAE 4) before and after switching, as well as the emission spectrum of a luminescent material (LS2) in the range of 245 to 700 nm. Figure 11 shows the absorption spectra of a diarylethene switch (DAE 4) before and after switching, as well as the emission spectrum of a luminescent material (LS2) in the range of 452 to 636 nm.

[0042] Detailed description of the invention

[0043] The present invention is based on the finding that a high switching depth, combined with advantageous stability and printing properties, can be achieved by encapsulating a luminescent substance and a diarylethene switch together in a capsule luminescent pigment. This is surprising because the switching effect relies on Förster resonant energy transfer (FRET) between the luminescent substance and the diarylethene switch, as described above. Since FRET is highly dependent on the distance between the molecules involved, it was not expected that simply encapsulating the mixed molecules in micrometer-sized polymer spheres would reproducibly result in a sufficiently small distance and thus a sufficiently high switching depth.

[0044] According to a first aspect, the present invention relates to a capsule luminescent pigment comprising a luminescent substance and a diarylethene as a molecular switch, wherein the luminescent substance and the diarylethene are dissolved and / or dispersed as separate molecules in a polymer particle, the luminescent substance has an emission spectrum, the diarylethene 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 diarylethene does not significantly overlap with the emission spectrum of the luminescent substance and the second absorption spectrum of the diarylethene significantly overlaps with the emission 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.

[0045] To ensure luminescence switching, the emission spectrum of the luminescent substance must overlap with the absorption spectrum of the diarylethene switch, but only in its second switching state, not in the first. In the first switching state, the luminescence is unaffected by the diarylethene switch; in the second switching state, however, the luminescence is reabsorbed by the diarylethene switch, the energy is transferred non-radiatively via FRET, and / or the excitation light is absorbed, thus significantly attenuating the luminescence intensity. A non-significant overlap of the first absorption spectrum of the diarylethene (i.e.,(of the diarylethene before the circuit) with the emission spectrum of the luminescent material means that the first absorption spectrum of the diarylethene, normalized to 1 at the wavelength of highest absorption in the range of 245 to 700 nm, has at most one absorption in the wavelength range of an emission band in the emission spectrum of the luminescent material, which (i.e., the absorption) at its highest point is less than 5%, preferably less than 3%, more preferably less than 2%, even more preferably less than 1%, and particularly preferably less than 0.5%, of the maximum emission of the luminescent material in its emission spectrum normalized to 1 at the wavelength of highest emission in the range of 245 to 700 nm.

[0046] An emission band of the luminescent material is defined as a wavelength range in which the emission corresponds to at least 10%, for example at least 20%, of the maximum emission of the luminescent material. The (local) absorption maximum of the diarylethene switch after switching occurs in this relevant emission band of the luminescent material 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 emission maximum of the luminescent material is absolutely more than 50%, preferably more than 70%, even 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 second absorption spectra are each normalized to the value 1 at the wavelength of highest absorption in the range of 245 to 700 nm.

[0047] A significant overlap of the second absorption spectrum of the diarylethene (i.e., the diarylethene after the circuit) with the emission spectrum of the luminescent material means that the second absorption spectrum (in the range of 245 to 700 nm) of the diarylethene, normalized to 1 at the wavelength of highest absorption, exhibits absorption in the same wavelength range of an emission band in the emission spectrum of the luminescent material, wherein the absorption in this emission band at its highest point is at least 5%, preferably at least 8%, more preferably at least 10%, even more preferably at least 12%, and particularly preferably at least 15%, of the maximum emission of the luminescent material in its emission spectrum (in the range of 245 to 700 nm), normalized to 1 at the wavelength of highest emission.

[0048] 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 emission 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.

[0049] Figure 1 illustrates this. The graph labeled "DAE SZ 1" (SZ = switching state) shows the first absorption spectrum of a diarylethene (DAE), i.e., a photochromic before switching. The absorption spectrum is normalized to 1 at the wavelength of highest absorption. From approximately 360 nm and above, the absorption spectrum shows no absorption by the unswitched diarylethene. The graph labeled "DAE SZ 2" is the second absorption spectrum of the diarylethene after switching. Here, too, the absorption spectrum is normalized to 1 at the wavelength of highest absorption. An absorption band is visible in the range of approximately 380 to 620 nm. The graph labeled "LY Em" is the emission spectrum of the luminescent material. The emission band is located in the range of approximately 450 to 640 nm.Figure 1 shows that the absorption spectrum of the unswitched diarylethene switch does not overlap with the emission spectrum of the luminescent substance, since no absorption of the unswitched diarylethene switch is observed in the relevant range for the luminescence of the luminescent substance, from 450 to 640 nm. In contrast, the absorption spectrum of the switched diarylethene switch overlaps significantly with the emission spectrum of the luminescent substance due to the formation of an absorption band at approximately 380 to 620 nm.

[0050] Figure 2 is a section of the wavelength range from 390 to 690 nm from Figure 1, 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 emission of the luminescent material. This is desirable for efficient FRET energy transfer. Secondly, Figure 2 shows that the absolute difference between the maxima of the absorption bands of the unswitched and switched diarylethene switch in the relevant range from 390 to 690 nm is at least 95%. This is desirable, among other things, for achieving a sufficient switching depth.

[0051] In particular, switching the diarylethene switch results in a reduction of the luminescence intensity of the luminescent material by at least 10%, for example, at least 20%, at least 30%, or at least 40%. Preferably, the reduction in luminescence intensity is at least 45%, for example, 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. The wavelength at which the luminescent material can be excited is not inherently limited.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 the luminescence of the luminescent material 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 300–400 nm, 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 range in which the diarylethene can be switched 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 diarylethene 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 luminescent material. For example, the switching of the diarylethene 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.

[0053] According to a further advantageous embodiment, it is therefore preferred that in the wavelength range of 310-400 nm the excitation spectrum of the luminescent substance and the absorption spectrum of the diarylethene overlap in the first switching state.

[0054] 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, quantitatively by 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 50% is desirable. Accordingly, the capsule luminescent pigments used in the system according to the invention preferably have a residual intensity of at least 50%.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%.

[0055] 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 45%, such as at least 50%, at least 60%, or at least 70%, is achieved within a period of less than 30 s, preferably less than 20 s, and particularly preferably less than 10 s.

[0056] The diarylethene used as a molecular switch has a first absorption spectrum in a first switching state and a second absorption spectrum in a second switching state. Thus, the diarylethene is limited in principle only by the fact that it has two switching states, i.e., it is a photochromic substance, with the absorption spectra of the two switching states differing. In principle, any suitable diarylethene can be used in the capsule luminescent pigments according to the invention. A person skilled in the art can select a suitable diarylethene for a specific application by considering the desired switching wavelength of the diarylethene switch and / or the emission maximum of the luminescent substance (which, as described above, should significantly overlap with the absorption spectrum of the second switching state of the diarylethene) 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 most 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 H 11- 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:

[0057] The 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. It is further preferred that the diarylethene switch remains in its state in the dark 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 diarylethene 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 diarylethene switch preferably switches automatically back from its second switching state to its initial state (first switching state) under daylight at an 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.

[0058] 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 / 014 658 A2, US 2015 / 0 132 575 Al and EP 2 195 395 Bl.

[0059] The luminescent substance and the diarylethene are dissolved and / or dispersed as separate molecules in a polymer particle in the capsule luminescent pigments according to the invention. Thus, there is no direct chemical bond between the luminescent substance and the diarylethene; rather, they exist as separate molecules. The luminescent substance and 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 distinct from one another, i.e., they cannot be the same molecule.

[0060] Encapsulation means that the luminescent substance and the diarylethene 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.

[0061] 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 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 diarylethene switch in addition to the luminescent substance during the core material production step.

[0062] 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.

[0063] The proportion of the luminescent substance in the core of the capsule luminescent pigment according to the invention can

[0064] The proportion of the luminescent material may 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 material consists of several different luminescent materials, the proportions of the respective individual luminescent materials may 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.

[0065] The proportion of diarylethene in the core of the capsule luminescence pigment according to the invention can be 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.

[0066] 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. 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.

[0067] 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.

[0068] 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 / 178325 A2, EP 2 888 112 Bl, DE 10 2012 010534 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.

[0069] 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.

[0070] 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.

[0071] 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 / 028 825 A2. The substrate can be impregnated with polymers of any kind and is preferably impregnated with polyvinyl alcohol.

[0072] 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 2634309 Al and WO 2004 / 072 378 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.

[0073] The security document according to the invention further comprises a print made of a first, switchable luminescent printing ink, as described in the second aspect of the invention. The print can be either a full-surface or at least partially surface-covering print layer made of a first, switchable printing ink or a mixture of printing inks containing a first, switchable printing ink, applied to the security document, or alternatively, a print pattern obtained by applying a first, switchable printing ink. Parts or sections of a security document, such as a melange fiber, can also be printed with the switchable printing ink. Naturally, a print can also contain several print patterns made of different printing inks applied side by side or (partially) on top of each other, wherein at least one of the printing inks is the first switchable printing ink.In addition to at least one printed pattern using a first, switchable printing color, a further printing layer may also be applied to the entire surface or at least partially across the surface of the document. Printing with more than one printing color is preferred, wherein at least one of the printing colors is the first switchable printing color. The printing may, for example, be an underprint or an intaglio print and may be applied 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.

[0074] Printing a security 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 irradiated with a suitable wavelength. The diarylethene 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 luminescence of the luminescent substance, which is excited simultaneously, preferably at the same wavelength as the switch, is reduced by FRET. For visual inspection, this results in a gradual decay of the luminescent dye's luminescence 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.

[0075] 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.

[0076] Figures 3-6 show exemplary applications of the switchable luminescent printing ink according to the invention in printing patterns and melange fibers.

[0077] 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. Figures 4A and 4B show a possible print pattern with color change, comprising a switchable luminescent ink according to the invention and a non-switchable luminescent 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 start 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), while the first luminescent printing ink does change its color (state B; Figure 4B). In particular, the first, switchable luminescent printing ink changes color because one or more of the pigments in the mixture darken, thus altering the color composition.

[0078] Figure 5 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 luminescence color. The second, non-switchable luminescent ink does not change its intensity upon irradiation, for example, UV irradiation, while the first luminescent ink darkens.

[0079] 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 diarylethene 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 upon 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 altering the color composition. This can result in a color change to different colors in the final state. Examples:

[0080] Methods

[0081] 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. Subsequently, the proofs were 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 fatigue resistance test described below and again assessed for switching time, switching depth, and residual intensity of the luminescent substance in the first state.

[0082] 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 fatigue resistance and chemical resistance, a test procedure analogous to WO 2017 / 080 654A1 was used. Both the luminescent material and the diarylethene switch were required to pass the BWS test.The luminescent substance is measured, particularly in the first switching state of the diarylethene, and exhibits a residual luminescence intensity of at least 50% of the original luminescence intensity at "BWS3". The diarylethene switch should be stable with respect to its switching depth, i.e., the switching depth at "BWS3" is at least 50% of the original switching depth.

[0083] Diarylethene syntheses 3,3'-(Perfluorocyclopent-l-ene-l,2-diyl)bis(2-methyl-6-nitrobenzo[b]thiophene) (DAE 2)

[0084] DAE 1 DAE 2 69%

[0085] 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%).

[0086] 4-Methyl-2-phenylthiophen

[0087] 3-Methylthiophene (2.95 mL, 3.00 g, 30.6 mmol, 1.0 eq.) was dissolved in 40 mL of dry THF, cooled to 0°C, and n-BuLi (2.5 M in H, 12.3 mL, 30.6 mmol, 1.0 eq.) was added dropwise. The reaction mixture was heated to 35°C, stirred at this temperature for 1 hour, and then cooled back to 0°C. Trimethyl borate (5.1 mL, 4.76 g, 45.8 mmol, 1.5 eq.) was then added dropwise, the reaction mixture was heated to room temperature, and stirred for 1 hour. After the addition of 32 mL of Na₂CO₃... ( The reaction mixture was heated to 40°C for 6 hours using aq) (20 wt%), iodobenzene (3.5 mL, 6.38 g, 31.3 mmol, 1.0 eq.), 50 mL THF, and Pd(PPIi3)4 (0.44 g, 0.38 mmol, 1.2 mol%). After cooling to room temperature, the reaction was diluted with 100 mL H2O, the phases were separated, and the aqueous phase was extracted with Et2O (3 x 50 mL). The combined organic phases were treated with NaCl( aq) (1 x 100 mL) washed, dried over Na₂SO₄, filtered, and the solvent removed under vacuum. The product was purified by column chromatography (SiO₂, H, UV) and isolated as a colorless oil (4.19 g, 24.1 mmol, 77%). 3-Methyl-2-(perfluorocyclopent-l-en-l-yl)-5-phenylthiophene (Innt-1) lnt-1

[0088] 54%

[0089] 4-Methyl-2-phenylthiophene (2.00 g, 11.5 mmol, 1.0 eq.) was dissolved in dry THF (20 mL), cooled to -78°C, n-BuLi (2.5 M in H, 5.1 mL, 12.8 mmol, 1.1 eq.) was added dropwise, and the reaction mixture was stirred for 1 hour at -78°C. This solution was added dropwise, while still cold, to a solution of octafluorocyclopentene (2.5 mL, 3.95 g, 18.6 mmol, 1.6 eq.) in dry THF (10 mL), also cooled to -78°C. The reaction mixture was stirred for 1 hour at -78°C, warmed to room temperature, and stirred overnight at room temperature. The reaction was stopped by adding HCl (1 M, 30 mL), the phases were separated, and the aqueous phase was extracted with EtjO (3 x 20 mL). The combined organic phases were treated with NaCl( aq One 40 mL sample was washed, dried over NajSCU, filtered, and the solvent was removed under vacuum. The crude product was purified by column chromatography (SiOz, H, UV), yielding a pale yellow oil (2.28 g, 6.21 mmol, 54%).

[0090] 3-(3,3,4,4,5,5-Hexafluoro-2-(3-methyl-5-phenylthiophen-2-yl)cyclopent-l-en-l-yl)-2-methylbenzo[b]thiophene (DAE 3)

[0091] 3-Bromo-2-methylbenzo[b]thiophene (0.50 g, 2.20 mmol, 1.0 eq.) was dissolved in dry THF (10 mL), cooled to -78°C and n-BuLi (2.5 M in H, 0.97 mL, 2.43 mmol, 1.1 eq) was added dropwise.

[0092] The reaction solution was then stirred for 1 hour, and a solution of intermediate 1 (0.81 g, 2.20 mmol, 1.0 eq.) in dry THF (5 mL) was added dropwise. The reaction mixture was stirred for 1 hour at -78°C, warmed to room temperature, and stirred overnight at room temperature. The reaction was stopped by the addition of HCl (1 M, 10 mL), the phases were separated, and the aqueous phase was extracted with EtjO (3 x 10 mL). The combined organic phases were treated with NaCl( aqThe sample (1 x 20 mL) was washed, dried over NajSC, filtered, and the solvent was removed under vacuum. The crude product was purified by column chromatography (SiOz, H, UV), and the product (0.34 g, 0.69 mmol, 31%) was obtained as a colorless solid.

[0093] 2-(2-(3,5-Dimethylthiophen-2-yl)-3,3,4,4,5,5-hexafluorocyclopent-l-en-l-yl)-3-methyl-5-phenylthiophene (DAE 4)

[0094] 74%

[0095] 2,4-Dimethylthiophene (0.32 g, 2.82 mmol, 1.0 eq.) was dissolved in dry THF (20 mL), cooled to 0°C, and n-BuLi (2.5 M in H, 1.2 mL, 3.00 mmol, 1.1 eq.) was added dropwise. The reaction solution was then stirred for 30 min, and a solution of intermediate 1 (1.01 g, 2.73 mmol, 1.0 eq.) in dry THF (5 mL) was added dropwise. The reaction mixture was stirred at 0°C for 1 hour, warmed to room temperature, and stirred overnight at room temperature. The reaction was stopped by the addition of HCl (1 M, 20 mL), the phases were separated, and the aqueous phase was extracted with EtjO (3 x 20 mL). The combined organic phases were treated with NaCl( aq (1 x 40 mL) was washed, dried over NajSCU, filtered, and the solvent was removed under vacuum. The crude product was purified by column chromatography (SiOz, H, UV) and the product (0.93 g, 2.02 mmol, 74%) was obtained as a colorless solid.

[0096] Example 1: Green, UVC-switchable (100-280 nm) luminescent pigment with a polymethyl methacrylate core and a melamine-formaldehyde shell

[0097] The switchable capsule luminescent pigment was prepared according to Example 2 of WO 2017 / 080653 Al and contains, as the luminescent agent dissolved in the core, a mixture of diisobutyl 4,10-dicyanoperylene-3,9-dicarboxylate (LSI, C32H26N2O4) and 2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole) (LS2, C26H26N2O2S). The emission spectrum of LS2 is shown in Figure 1. 1,2-Bis[2-methylbenzo[b]thiophen-3-yl]-3,3,4,4,5,5-hexafluoro-1-cyclopentene (DAE 1, C23H14F6S2) was used as the diarylethene switch. The absorption spectra of DAE 1 in the unswitched and switched states are also shown in Figure 1. Each of the individual luminescent substances is used at a weight of 0.5%, and the diarylethene switch at 5%. The resulting pigment luminesces green under illumination at 365 nm and UVC light (100-280 nm) and switches under irradiation with light in the UVC wavelength range.The emission band of the luminescent material, as defined above, lies in the range of 452–636 nm. The absorption maximum of the diarylethene switch in the second state is located at 523 nm and thus lies within the defined emission band. Additionally, the difference between the absorption curves of the first and second switching states at the emission maximum is more than 50% (see Figures 1 and 2; where “DAE SZ1” represents diarylethene DAE 1 in the first switching state; “DAE SZ2” represents diarylethene DAE 1 in the second switching state; and “LY Em” represents the emission spectrum of the luminescent material (LS2)).

[0098] Due to the significant overlap of the absorption and emission spectra of the diarylethene switch and the luminescent material, a good switching depth of 36% for BWSO (see Figure 7) and 38% for BWS3 (see Figure 8) is achieved. This switching depth is thus well 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 75%, significantly above the desired residual intensity of 50%. In particular, the pigment exhibits exceptional fatigue resistance at full residual switching depth for BWS3 and is therefore ideally suited for use as a safety feature.

[0099] Table 1 summarizes the data obtained experimentally for Example 1.

[0100] Table 1.

[0101] [1] Luminescent substance relevant to the emission color,

[0102] [2]Residual intensity of the luminescent substance at BWS3

[0103] Example 2: Different colored, switchable luminescent pigments in the UVC range (100-280 nm) with a polymethyl methacrylate core and a melamine-formaldehyde shell

[0104] 2a) green switchable luminescence

[0105] 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. 1,2-Bis[2-methylbenzo[b]thiophen-3-yl]-3,3,4,4,5,5-hexafluoro-l-cyclopentene (DAE 1, C23H14F6S2) was used as the diarylethene switch. The luminescent agent is present at a weight of 0.5% and the diarylethene switch at a weight of 5%. The resulting pigment luminesces green under illumination at 365 nm and UVC light (100-280 nm) and switches under irradiation with light in the UVC wavelength range. The emission band of the luminescent substance, as defined above, lies in the range of 483-627 nm. The absorption maximum of the diarylethene switch in the second state is located at 523 nm and thus lies within the defined emission band.Additionally, the difference between the absorption curves of the first and second switching states at the emission maximum is more than 50%.

[0106] Due to the significant overlap of the absorption and emission spectra of the diarylethene switch and the luminescent substance, a good switching depth of 13% for BWSO and 14% for BWS3 is achieved. This switching depth is thus well above the threshold of 10%, and the difference in luminescence intensity is clearly visible. The residual intensity of the luminescent substance in the first switching state for BWS3 is 75%, significantly higher than the desired residual intensity of 50%. In particular, the pigment exhibits exceptional fatigue resistance at full residual switching depth for BWS3 and is therefore ideally suited for use as a safety feature.

[0107] 2b) yellow switchable luminescence

[0108] The switchable capsule luminescent pigment was prepared according to Example 2 of WO 2017 / 080 653 Al and contains, as the luminescent agent dissolved in the core, a mixture of 2,9-bis(2,6-diisopropylphenyl)anthra[2,1,9-def:6,5,10-d'e'f']diisoquinoline-1,3,8,10(2H,9H)-tetraone (LS4, C48H42N2O4) and 2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole) (LS2, C26H26N2O2S). 1,2-bis[2-methylbenzo[b]thiophen-3-yl]-3,3,4,4,5,5-hexafluoro-1-cyclopentene (DAE 1, C23H14F6S2) was used as the diarylethene switch. Each of the individual luminescent substances is used at a weight of 0.5%, and the diarylethene switch at a weight of 5%.

[0109] The resulting pigment luminesces yellow under illumination at 365 nm and UVC light (100-280 nm) and switches under irradiation with light in the UVC wavelength range. The emission band of the luminescent material, as defined above, lies in the range of 521-652 nm. The absorption maximum of the diarylethene switch in the second state is located at 523 nm and thus lies within the defined emission band. Additionally, the difference between the absorption curves of the first and second switching states at the emission maximum is more than 50%. Due to the significant overlap of the absorption and emission spectra of the diarylethene switch and the luminescent material, a good switching depth of 34% for BWSO and 53% for BWS3 is achieved. The switching depth is therefore well above the threshold of 10%, and the difference in luminescence intensity is clearly visible.The residual intensity of the luminescent substance in the first switching state at BWS3 is 81%, significantly higher than the desired residual intensity of 50%. In particular, the pigment exhibits exceptional fatigue resistance at full residual switching depth at BWS3 and is therefore ideally suited for use as a safety feature.

[0110] 2c) Pink switchable luminescence

[0111] The switchable capsule luminescent pigment was prepared according to Example 2 of WO 2017 / 080 653 Al and contains, as the luminescent agent dissolved in the core, a mixture of Lumigen F Rosa 285 (LS5, BASF) and 2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole) (LS2, C26H26N2O2S). The diarylethene switch used was 3,3'-(perfluorocyclopent-1-ene-1,2-diyl)bis(2-methyl-6-nitrobenzo[b]thiophene) (DAE 2, C23H12F6N2O4S2). Each of the individual luminescent agents is used at a weight of 0.5%, and the diarylethene switch at 5%.

[0112] The resulting pigment luminesces pink under illumination at 365 nm and UVC light (100-280 nm) and switches under irradiation with light of wavelength 365 nm. The emission band of the luminescent substance, as defined above, lies in the range of 534-721 nm. The absorption maximum of the diarylethene switch in the second state is located at 559 nm and thus lies within the defined emission band. Additionally, the difference between the absorption curves of the first and second switching states at the emission maximum is more than 50%.

[0113] Due to the significant overlap of the absorption and emission spectra of the diarylethene switch and the luminescent material, a good switching depth of 80% for BWSO and 58% for BWS3 is achieved. This switching depth is thus well 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 66%, is significantly higher than the desired residual intensity of 50%. In particular, the pigment exhibits exceptional fatigue resistance with a residual switching depth of 73% for BWS3 and is therefore ideally suited for use as a safety feature.

[0114] Table 2 summarizes the experimentally obtained data for Example 2. Table 2.

[0115] [1] Luminescent substance relevant to the emission color,

[0116] [2] Residual intensity of the luminescent substance at BWS3

[0117] Overall, Example 2 shows that, in accordance with this invention, a multitude of switchable luminescent colors can be achieved. An improved switching depth in BWS 3 is related to the partial decomposition of the luminescent substance. In particular, the amount of diarylethene switch increases compared to the luminescent substance, resulting in a greater switching depth, since the switching depth depends, among other things, on the ratio of the luminescent substance to the diarylethene switch.

[0118] Example 3: green, switchable luminescent pigment at 365 nm with a polymethyl methacrylate core and a melamine-formaldehyde shell

[0119] The switchable capsule luminescent pigment was prepared according to Example 2 of WO 2017 / 080 653 Al and contains, as the luminescent agent dissolved in the core, a mixture of diisobutyl 4,10-dicyanoperylene-3,9-dicarboxylate (LSI, C32H26N2O4) and 2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole) (LS2, C26H26N2O2S). The diarylethene switch used was 3,3'-(perfluorocyclopent-1-ene-1,2-diyl)bis(2-methyl-6-nitrobenzo[b]thiophene) (DAE 2, C23H12F6N2O4S2). Each of the individual luminescent agents is used at a weight of 0.5%, and the diarylethene switch at 5%.

[0120] The resulting pigment luminesces green under illumination at 365 nm and switches, in particular, under irradiation with light of this wavelength. The emission band of the luminescent material, as defined above, lies in the range of 452–636 nm. The absorption maximum of the diarylethene switch in the second state is located at 559 nm and thus lies within the defined emission band. Additionally, the difference between the absorption curves of the first and second switching states at the emission maximum is more than 50%. Due to the significant overlap of the absorption and emission spectra of the diarylethene switch and the luminescent material, a very good switching depth of 70% for BWSO and 50% for BWS3 is achieved (see Figure 9). The switching depth is therefore well above the threshold of 10%, and the difference in luminescence intensity is clearly visible.The residual intensity of the luminescent substance in the first switching state at BWS3 is 65%, significantly higher than the desired residual intensity of 50%. In particular, the pigment exhibits exceptional fatigue resistance with a residual switching depth of 71% at BWS3, making it ideally suited for use as a safety feature.

[0121] Table 3 summarizes the data obtained experimentally for Example 3.

[0122] Table 3.

[0123] [1] Luminescent substance relevant to the emission color,

[0124] [2] Residual intensity of the luminescent substance at BWS3

[0125] Example 4: green, switchable luminescent pigment at 365 nm with a polyurea core and a melamine-formaldehyde shell

[0126] The switchable capsule luminescent pigment was prepared according to Example 2 of WO 2017 / 080 656 Al and contains, as the luminescent agent dissolved in the core, a mixture of diisobutyl 4,10-dicyanoperylene 3,9-dicarboxylate (LSI, C32H26N2O4) and 2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole) (LS2, C26H26N2O2S). The diarylethene switch used was 3-(3,3,4,4,5,5-hexafluoro-2-(3-methyl-5-phenylthiophen-2-yl)cyclopent-1-en-1-yl)-2-methylbenzo[b]thiophene (DAE 3, C25H16F6S2). Each of the individual luminescent substances is used at a weight of 0.5%, and the diarylethene switch at a weight of 5%.

[0127] The resulting pigment luminesces green under 365 nm illumination and switches, in particular, under irradiation with light of this wavelength. The emission band of the luminescent material, as defined above, lies in the range of 452–636 nm. The absorption maximum of the diarylethene switch in the second state is located at 480 nm and thus lies within the defined emission band. Additionally, the difference between the absorption curves of the first and second switching states at the emission maximum is more than 50%. Due to the significant overlap of the absorption and emission spectra of the diarylethene switch and the luminescent material, a switching depth of 13% is achieved for BWSO and BWS3. This switching depth is therefore 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 64%, significantly higher than the desired residual intensity of 50%.

[0128] In particular, the pigment exhibits exceptional fatigue resistance at full residual switching depth at BWS3 and is therefore well suited for use as a safety feature.

[0129] Table 4 summarizes the data obtained experimentally for Example 4.

[0130] Table 4.

[0131] [1] Luminescent substance relevant to the emission color,

[0132] [2] Residual intensity of the luminescent substance at BWS3

[0133] Comparison example 1: green, switchable luminescence in the UVC range (100-280 nm), not encapsulated together

[0134] Comparative example 1a) Diarylethene switch and luminescent material each encapsulated (polyurea core and melamine-formaldehyde shell) in superimposed printed layers

[0135] A luminescent capsule pigment LP1 was produced according to Example 2 of WO 2017 / 080 656 Al, containing diisobutyl-4,10-dicyanoperylene-3,9-dicarboxylate (LSI, C32H26N2O4) 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%.

[0136] Additionally, a switching capsule pigment SP1 was produced according to Example 2 of WO 2017 / 080 656 Al, which contains 1,2-bis[2-methylbenzo[b]thiophen-3-yl]-3,3,4,4,5,5-hexafluoro-l-cyclopentene (DAE 1, C23H14F6S2) as a diarylethene switch dissolved in the core. 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 pigmentation level was 1 wt%. The printing ink was applied at a pressure 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 layer of F2, at 12 pm, corresponds to the ratio of luminescent material to diarylethene switch used in Example 1. The thickest layer of F2, at 50 pm, corresponds to 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.

[0137] In all three cases, the switching depth is significantly below the threshold of 10%, and the difference in luminescence intensity is not visually noticeable. Therefore, using separately encapsulated luminescent material and diarylethene switches, applied in two layers, as a security feature to achieve switchable luminescence is not feasible.

[0138] Table 5 summarizes the experimentally obtained data for comparison example aa.

[0139] Table 5.

[0140] [1] Luminescent substance relevant to the emission color,

[0141] [2] Layer thickness of the squeegeed DAE layer

[0142] Comparative example lb) Diarylethene switch and luminescent substance each individually encapsulated (polymethyl methacrylate core and melamine-formaldehyde shell) mixed in a printing ink

[0143] A luminescent capsule pigment LP2 was prepared according to Example 2 of WO 2017 / 080 653 Al and contains, as the luminescent agent dissolved in the core, a mixture of diisobutyl 4,10-dicyanoperylene 3,9-dicarboxylate (LSI, C32H26N2O4) and 2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole) (LS2, C26H26N2O2S). Each of the individual luminescent agents was used in amounts of 0.5% by weight. Additionally, a switching capsule pigment SP2 was prepared according to Example 2 of WO 2017 / 080 653 Al, which contains 1,2-bis[2-methylbenzo[b]thiophen-3-yl]-3,3,4,4,5,5-hexafluoro-l-cyclopentene (DAE 1, C23H14F6S2) as a diarylethene switch dissolved in the core. 5 wt% of the diarylethene switch was used.

[0144] 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 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.

[0145] The switching depth of 2% is far below the threshold of 10%. A difference in luminescence intensity is not visually noticeable. Therefore, the use of separately encapsulated luminescent material and diarylethene switches mixed in a printing ink as a security feature is not possible.

[0146] Table 6 summarizes the experimentally obtained data for comparison example lb.

[0147] Table 6.

[0148] [1] Luminescent substance relevant to the emission color

[0149] Comparative example lc) Diarylethene switch and luminescent substance not encapsulated mixed in a printing ink

[0150] A printing ink F4 was produced from the two luminescent substances diisobutyl-4,10-dicyanoperylene-3,9-dicarboxylate (LSI, C32H26N2O4) and 2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole) (LS2, C26H26N2O2S) and the diarylethene switch 1,2-bis[2-methylbenzo[b]thiophen-3-yl]-3,3,4,4,5,5-hexafluoro-1-cyclopentene (DAE 1, C23H14F6S2). For this purpose, the luminescent substances and the diarylethene switch were incorporated into an offset printing ink (Sicpa Holding SA) using an Engelsmann JEL 25 / 53 ink transfer machine (manufactured in 2013). The pigmentation level was 0.075 wt% for each of the luminescent substances and 0.75 wt% for the diarylethene switch. The F4 printing ink was applied at a pressure of 1 g / m². 2Printed on security paper, the proofs were dried at 60 °C for 2 h. The ratio of luminescent materials to diarylethene switch corresponds to the ratios and amounts of luminescent material and diarylethene switch used in Example 1. Subsequently, the proofs were visually assessed for their switching duration and depth. The switching depth was quantitatively determined using a commercially available fluorescence spectrometer.

[0151] The switching depth of 5% is below the threshold of 10%. Therefore, the F4 printing ink is not compliant with the invention, and any difference in luminescence intensity is not visually noticeable. Consequently, the unencapsulated mixture of the luminescent substance and the diarylethene switch in a printing ink is not suitable for use as a security feature.

[0152] Table 7 summarizes the data obtained experimentally for comparison example lc.

[0153] Table 7.

[0154] [1] Luminescent substance relevant to the emission color

[0155] Comparative example 2: green luminescent pigment with a polyurea core and a melamine-formaldehyde shell, which cannot be switched sufficiently at 365 nm.

[0156] The switchable capsule luminescent pigment was prepared according to Example 2 of WO 2017 / 080 656 Al and contains, as the luminescent agent dissolved in the core, a mixture of diisobutyl 4,10-dicyanoperylene 3,9-dicarboxylate (LSI, C32H26N2O4) and 2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole) (LS2, C26H26N2O2S). The diarylethene switch used was 2-(2-(3,5-dimethylthiophen-2-yl)-3,3,4,4,5,5-hexafluorocyclopent-1-en-1-yl)-3-methyl-5-phenylthiophene (DAE 4, C22H16F6S2). Each of the individual luminescent substances is used at a weight of 0.5%, and the diarylethene switch at a weight of 5%.

[0157] The resulting pigment luminesces green under illumination at 365 nm and switches, in particular, under irradiation with light of wavelength 365 nm. The emission band of the relevant luminescent material, as defined above, lies in the range of 452–636 nm. The absorption maximum of the diarylethene switch in the second state is located at 437 nm and is therefore not within the defined emission band. Furthermore, the difference between the absorption curves of the first and second switching states at the emission maximum is significantly less than 50%. In particular, there is no significant overlap between the absorption and emission spectra of the diarylethene switch and the luminescent material (see Figures 10 and 11; where “DAE SZ1” represents diarylethene DAE 4 in the first switching state; “DAE SZ2” represents diarylethene DAE 4 in the second switching state; and “LY Em” represents the emission spectrum of the luminescent material (LS2)).

[0158] Due to the low overlap of the absorption and emission spectra of the diarylethene switch and the luminescent material, only a very shallow switching depth of 6% is achieved for BWSO and BWS3. This switching depth is therefore below the threshold of 10%. The difference in luminescence intensity is not visually noticeable. Additionally, the residual intensity of the luminescent material in the first state for BWS3 is 48%, below the desired residual intensity of 50%. The pigment exhibits insufficient switching capability and fatigue resistance and is therefore unsuitable for use as a security feature.

[0159] Table 8 summarizes the experimentally obtained data for comparison example 2.

[0160] Table 8.

[0161] [1] Luminescent substance relevant to the emission color,

[0162] [2] Residual intensity of the luminescent substance at BWS3

Claims

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

2. Capsule luminescence pigment according to claim 1, wherein excitation of the luminescence agent takes place in the same wavelength range as the switching of the diarylethene from the first to the second switching state.

3. Capsule luminescence pigment according to claim 1 or 2, wherein in the wavelength range of 300-400 nm the excitation spectrum of the luminescence substance and the absorption spectrum of the diarylethene overlap in the first switching state.

4. Capsule luminescence pigment according to claim 2, wherein the excitation of the luminescence agent and the switching of the diarylethene 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 diarylethene 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, benzthiazine, aldazines, anthranilic acid derivatives, salicyclic acid derivatives, oxazines, oxazoles, anthrones, and mixtures thereof.

7. Capsule luminescence pigment according to any one of claims 1-6, wherein the diarylethene is a diheteroarylethene, 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 luminescence pigment according to any one of claims 1-9, wherein the proportion of the luminescence agent in the core is 0.1 to 10 wt% and / or the proportion of the diarylethene 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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