Photo‐switchable Fluorescence in Hydrogen‐Bonded Liquid Crystals

Abstract A series of hydrogen‐bonded liquid crystals showing switchable fluorescence is reported. The fluorescence behavior results from the unique combination of hydrogen bonding, liquid crystallinity, and photobasicity. Thus, the molecular mobility in the mesophase is essential for the reversible photo‐initiated proton transfer switching on the fluorescence of the assemblies. The application potential of the materials for photo‐patterning was demonstrated.

Abstract: As eries of hydrogen-bondedl iquid crystals showing switchable fluorescencei sr eported.T he fluorescence behavior results from the unique combination of hydrogen bonding, liquid crystallinity,a nd photobasicity. Thus, the molecular mobility in the mesophase is essential for the reversible photo-initiated proton transfer switching on the fluorescenceo ft he assemblies. The application potential of the materials for photo-patterning was demonstrated.
Organicp hotoluminescent materials with stimuli-responsive properties have recently gained much attention due to their potentialf or application in,f or example, biological sensing, [1] light-emitting diodes, [2] logic gates, [3] and anti-counterfeiting. [4] In addition, gaining control over the luminescence by external stimuli,f or example, by temperature or irradiationw ith light is highly valuable, as it provides access to responsive functional materials. With respect to the design of such luminescentm aterials,aself-assembly approach based on noncovalent bonds providesm any advantages as it facilitates the fabrication, processing and recycling of materials. [5] The functional materials are obtainedb ys imple mixing of pre-tailored building blocks in an appropriate solvent at room temperature. Furthermore, the dynamic nature of noncovalenti nteractionsp rovides access to materials that respond reversibly to externals timuli or damages (self-healing/-repair). [6] In the late 1980s, Kato, [7] FrØchet, [7b, 8] and Lehn [9] introduced an ew design concept for liquid crystalsb ys elf-assembly of benzoica cid derivatives or based on the complementarity of between benzoic acid groups and pyridyl derivatives. [6a, b, 10] Later,B ruce and co-workersd emonstrated that phenols are also suitable protond onors for the formation of hydrogenbondeda ssemblies with liquid-crystallinep roperties. [11] Althoughm any of these examples employ substituted stilbazole derivatives, only little is reported about the luminescenceb ehavior. [12] In 2016, we introducedamodulara pproachf or systematic studies on the design of hydrogen-bonded liquid crystals with tailor-made properties. [13][14][15] The initial studies have granted insights into the structure-property relationshipso ft he assemblies, for example, the impact of the substitution pattern of the hydrogen-bond donors, [16] the fluorination degree, [13b] and the linking group in the hydrogen-bond accepting moieties was systematically investigated. [14] Af undamentalu nderstand-ing of the structure-property relationship is crucial for the development of new functional materials. For instance, our findings helped us to stabilize liquid-crystalline blue phases (BP I) [15] or to manipulate the opticalp ropertieso fp hotonic crystals by infiltration of the porousm aterials with hydrogen-bonded liquid crystals. [17] Recently,w eu sed the modularm ethodology to create luminescent liquid crystalst hrough their aggregation-induced emissionb ehavior. [18] The presentw ork employs the modularc oncept to yield photo-switchable, luminescentm aterials. Therefore, as eries of hydroxybenzoic acids (4-(4HBA), 3-(3HBA)a nd 2-hydroxybenzoic acid (2HBA); Scheme 1) were combined with alkoxy-stilbazole (St8)t of orm hydrogen-bonded assemblies with liquidcrystalline properties. The modularity of our approacha llows efficient fine-tuning of the hydrogen-bonding capability and the acid-base equilibrium to obtain assemblies with balanced intermolecular forces enabling photoswitchable fluorescence behavior.
In total, six hydrogen-bonded liquid crystalsw ere obtained by mixing solutionso ft he HBAs with St8 in 1:1a nd 1:2r atios in acetone. After removal of the solvent under reduced pressure, the materials were obtained and analyzed with respectt o their liquid-crystalline behavior.T he formation of the hydrogen-bonded assemblies was proven by FTIR spectroscopy (for detailss ee the Supporting Information). Taking the 4HBAbased assemblies as representative examples, the OH signal shifts from 3364 to 3116 cm À1 for the 1:1m ixture and disappears for the 1:2mixture. In addition, the C=Oband undergoes as hift from 1669 to 1656 cm À1 for the 1:1a nd to 1687 cm À1 for the 1:2c omplex. An additional proofo ft he formation of the assemblies is given by the occurrence of liquid-crystalline properties of the assemblies. None of the startingm aterials showedm esomorphic properties upon self-assembly and for-mation of the hydrogen-bonded materials;h owever,l iquid crystallinity was observed.
In the followingd iscussion, we will focus on the 1:2a ssemblies because they performed betterw ith respectt ol iquid crystallinity and photo-switchable fluorescence.T hese assemblies showed enantiotropic liquid-crystalline behavior.F or the 3HBA(St8) 2 and 4HBA(St8) 2 assemblies, the characteristic Schlieren texture of anematic phase was observed under apolarized optical light microscope (POM;F igure 1). The 2HBA(St8) 2 assemblies,h owever, tend to form focal-conic textures indicative for as mectic phase. We attribute the change in the nature of the mesophase to the formation of 1:1a ssembly,a so ne of the OH groups is blockedb yi ntramolecular hydrogen bonding to the carboxylic group. [19] This is in line with the observedi nhomogeneous melting/cooling behavior and the obtained IR data. The POMr esultsw ere supported by differentialscanning calorimetry (DSC, for detailssee the Supporting Information). The temperature range of the mesophases decreasedi nt he order 3HBA > 2HBA > 4HBA from DT = 70.2 to 54.3 and to 44.3 8C, respectively,upon cooling.
As already mentioned, the most interesting property of the obtained assemblies is their photoswitchable fluorescence. The properties of supramolecular functional entities rely on aw ellorchestrated interplay of variousi ntermolecularf orces. Subtle changes in the molecular structure can result in tremendous differences in the properties of the functional assemblies. In the present series, the substitution pattern of the hydrogen bond-donating HBAs is varied;t his has an impact on the shape of the resulting assemblies. Moreover, the acidity of the HBAs varies, and this controlst he strength of the noncovalent bonds and the protonation state in the assemblies.
After removalo ft he acetone, the 1:2a ssemblies were obtained as colorless or slightly yellow crystalline solids. No visible fluorescencew as observed under UV light. However,a fter one heating/cooling cycle, the assemblies differed in their fluorescence behavior.T he 2HBA(St8) 2 assembly showed visible blue light fluorescence, whereas 3HBA(St8) 2 and 4HBA(St8) 2 appeared only slightly fluorescent (Figure 2). The occurrence of the fluorescence can be attributed to the intermolecular proton transfer between the HBAs and the stilbazole, which is supported by IR investigations ( Figure S4 in the Supporting Information). The differences in the fluorescenceb ehavior indicates the crucial impacto ft he substitution pattern in the hydrogen bond donating units and can be attributed to the differences in the acidity of the HBAs. The acidity of the hydrogen bond donating moieties decreases in the order 2HBA (pK a = 2.75) > 3HBA (pK a = 3.90) > 4HBA (pK a = 4.61). [20] Thus the protonation equilibrium in the 2HBA(St8) 2 is shifted to the protonated stilbazole and the deprotonated 2HBA,c ausing the fluorescence. [21] In contrast, the assemblies based on 3HBA and 4HBA appear non-or only slightly fluorescent.H owever,b ringing these assemblies into their mesophases, followed by irradiation with light of l = 405 nm turns on the fluorescence; this can be attributed to ap hoto-initiated proton transfer.D oty et al. reported that the photo-excitation of trans-methoxy stilbazole yields ap K a shiftf rom 4.93 in the ground state to 13.02 in the excited state. The strongi ncrease in basicity shifts the equilibrias ignificantly to the protonated, fluorescents tilbazole species. [22] In order to evaluate the fluorescence behavior,s pectra of the assemblies in their fluorescent state were recorded. Where-  as 2HBA(St8) 2 emits at l em = 463 nm (l ex = 428 nm), the 3HBA(St8) 2 and 4HBA(St8) 2 assemblies emit at l em = 522 nm (l ex = 455 nm), and l em = 516 nm (l ex = 417 nm), respectively.A s ac omparison, the fluorescences pectrum of St8·HCl was recorded showing similarb ehavior to those of the 3HBA and 4HBA-based assemblies (l ex = 413 nm l em = 508 nm, for details see Figure S25). The significant hypsochromic shifto ft he 2HBA system with respect to the other investigated assemblies can be attributed to the fact that hydrogen-bonded complexes of 2HBA emitting around 450 nm. [23] Subsequently,t he photo-switchability of the fluorescence was investigated in detail.A s3HBA(St8) 2 performed bestw ith respectt ol iquid-crystalline properties and photo-switchable fluorescence, we will focus our discussion on these materials. The 3HBA(St8) 2 assembly was brought into its nematic phase by heatingt o1 30 8C. Upon irradiation with light (l = 405 nm) under the POM, the assemblies start to crystallize, and green fluorescenceo ccurred (FigureS11). This result is in line with our previous observation and attributed to the crystallization of the stilbazolium salt upon photo-initiated proton transfer, which is supported by the changes in the IR spectra (Figure S4). The band related to the hydrogen bond at around 2600 cm À1 undergoes ab athochromics hift of around 200 cm À1 ;a dditionally the C=Os ignal at % 1690 cm À1 disappears. It should be noted that irradiation of the assembly in its crystalline state at room temperature did not affect the fluorescence, thereby provingt he relevance of the dynamics in the liquid-crystalline state for the performance of the material. In order to compare this behavior,f luorescence spectra of the 3HBA(St8) 2 assembly were recorded after different times of exposure to UV light ( Figure 3). The spectra clearlys how at remendousi ncrease in the fluorescencea fter irradiation of 10 s. Further irradiation did not result in significant changes of the fluorescencei ntensity. In addition, as lights hift in the maximum of emissionw avelength was detected (from 530 to 522 nm).
It should be noted that the irradiated material does not show liquid-crystalline behavior as supported by DSC data (Figure S22). However,r esetting the samples to their initial nonfluorescent state was possible by heatingt he samples to 150 8C. After approximately 2h the fluorescence completely vanished and the enantiotropic liquid-crystalline behavior was recovered ( Figure S22 and S24).
In order to prove the application potential of the photoswitchable fluorescenceathin film of 3HBA(St8) 2 wasprepared and used for photo-imprinting. Therefore, the film was partly coveredb yaphoto mask showing the acronym of the University of Duisburg-Essen( UDE), heatedt oi ts nematic phase at 130 8Ca nd irradiated with light of aw avelength of l = 405 nm for approximately 5s.T he sample was cooled to room temperature, and the mask was removed. Subsequentlyt he film was observed under UV light. Although the covered regions remained dark, the irradiated areaso ft he films appeared highly fluorescent,d ue to photo-initiated proton transfer (Figure 4b). By heating the sample for2ht o1 50 8Ct he image was erased, and the film could be used for as econd writing cycle.
In addition, we found that longere xposure times with 365 nm light yieldeda ni rreversible photo-pattern, due to the [2+ +2] cycloaddition of the exciteds tilbazole. [22,24] This behavior was used for inverse photo-patterning of the films. The whole sample was irradiated with 405 nm light for 5s to yield a green fluorescent film (Figure 4c). Partial covering of the films and subsequent irradiation with UV light (l = 365 nm) for 60 s yielded photo-patterned films again showing the UDE acronym. Theuncovered regions of the films lost their fluorescence due to [2+ +2] cycloaddition of the excited stilbazole, which  could not be reversed under the chosen conditions. However, the fluorescenceo ft he acronym could reversibly be switched on and off according to the previously described photo-initiated protont ransfer.
In conclusion, we have reported as upramolecular approach towardsh ydrogen-bonded assemblies with photoswitchable fluorescencet hat results from ac ombination of hydrogen bonding, liquid crystallinity,a nd photobasicity.T he hydrogen bondingb etween hydroxybenzoic acids and as tilbazole induces liquid crystallinity,w hich is crucial for the reversible, photoswitchable fluorescenceo ft hese materials. Irradiation of these materials in their mesophase with light (l = 405 nm) causes a photo-initiated protont ransfer from the hydrogen-bond donor to the acceptor and turns on the fluorescence. This process can be used for reversible photo-patterning of thin films by lithography. The study provest he relevance of ad eep understanding of the complex interplay of noncovalent forces in supramolecular materials, which allows the materials' properties to be controlled. We believe that the modularity of the chosen approach makesi tappealing for the designo favariety of new supramolecular materials with switchable luminescence.