Research Article www.advopticalmat.de Multimodal Fluorescence Switching Materials: One Dye to Have Them All Jaume Ramon Otaegui, Aleix Carrascull-Marín, Daniel Ruiz-Molina, Jordi Hernando,* and Claudio Roscini* single-color off-on and on-off fluorescent Off/on, on/off, and on1/on2 fluorescence switching systems find application switches interconvert between emis- in a variety of areas, for each of which a particular dye or dye-switch tether sive and nonemissive states. Among must be specifically designed and synthesized. Herein it is demonstrated them, off-on systems are preferred for optical sensing,[11,23,24] as higher sensi- that such tight requirement can be avoided by using easily prepared mixtures tivities can be reached by detecting tiny of readily available emitters and phase change materials (PCMs). By proper fluorescence signals above a dark back- selection of the PCM and, if needed, additives, thermo- and photothermore- ground. On the contrary, on-off switches sponsive materials showing all classes of emission switching modes can be are exploited in super-resolution fluo- prepared from a single dye and without chemical modifications. This strategy rescence microscopy, which typically requires reversibly extinguishing the can be generalized to distinct emitters and, thanks to the facile and versa- emission of fluorophores either in time tile printability of dye–PCM mixtures, it can be used for the fabrication of (in coordinate-stochastic methods)[25,26] fluorescent patterns showing complex (photo)thermal responses with direct or space (in coordinate-targeted applicability in sensing and anti-counterfeiting. methods).[27,28] On the other hand, dual- color fluorescent switches are designed to toggle between two differently colored emissive states (on1-on2),[17] a behavior 1. Introduction that offers several advantages for sensing and anti-coun- terfeiting applications (e.g., reduced interference from the Switchable fluorescent materials are receiving much atten- autofluorescence of the medium,[22] quantitative ratiometric tion from both the academic and industrial sectors due to measurements,[22,29] and multiple readout modes for higher their broad applicability in a wide range of areas,[1–6] which security levels[18,20]). span from optical memories[7,8] and logic gates[9] to (bio) In all these systems fluorescence modulation is normally sensing[10–14] and bioimaging probes,[15–17] and anti-counter- accomplished through switchable molecules that, when feiting technologies.[18–21] Different fluorescent switching subjected to an external stimulus, either show intrinsic modes are available for these materials, each of which is interconversion between distinct emissive states or differ- best suited for particular applications.[22] On the one hand, ently interact with nearby fluorescent dyes.[22,30] Therefore, in order to vary the switching scheme, different inherently fluorescent switching or multicomponent molecules must J. R. Otaegui, A. Carrascull-Marín, C. Roscini be designed and synthesized on a case-by-case basis, which Nanostructured Functional Materials hampers their general implementation. Thus, the develop- Catalan Institute of Nanoscience and Nanotechnology (ICN2) ment of a universal approach to accomplish multiple emis- CSIC and BIST sion switching behaviors using the same dye has become a Campus UAB, Bellaterra, Barcelona 08193, Spain long-sought mission. E-mail: claudio.roscini@icn2.cat Herein we hypothesize that this challenge can be reached J. R. Otaegui, D. Ruiz-Molina, J. Hernando Departament de Química reversing the underlying design criterion, that is, by modula- Universitat Autònoma de Barcelona tion of the environment or matrix, while fixing a fluorescent dye Edifici C/n, Campus UAB, Cerdanyola del Vallès 08193, Spain with multiple emissive responses. To demonstrate this premise, E-mail: jordi.hernando@uab.cat we chose phase change materials (PCMs) as matrices, whose The ORCID identification number(s) for the author(s) of this article (photo)thermally-triggered solid-to-liquid transition has been can be found under https://doi.org/10.1002/adom.202200083. recently proven to modulate the fluorescence of the dissolved © 2022 The Authors. Advanced Optical Materials published by Wiley-VCH emitters thanks to changes of medium rigidity or dye aggre- GmbH. This is an open access article under the terms of the Creative gation state.[11,31–40] As for the dye, we must select molecular Commons Attribution-NonCommercial License, which permits use, emitters that, under controlled modification of the surrounding distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. environment, are capable of undergoing both single- and two- color fluorescence switching through different photochemical DOI: 10.1002/adom.202200083 mechanisms. For instance, this is the case of the perylene Adv. Optical Mater. 2022, 2200083 2200083 (1 of 6) © 2022 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.advopticalmat.de diimide derivative N,N′-bis(1-hexylheptyl)perylene-3,4,9,10- 2. Results and Discussion bis(dicarboximide) (PDI, Figure S1, Supporting Information), a bright and stable lime green fluorophore in diluted organic To validate our strategy, we first aimed to obtain dual-color on1/ solutions whose emission can be i) switched off by photoin- on2 switching using PDI-PCM mixtures. For this, we employed duced electron transfer (PET) in the presence of close-by elec- a 0.1 wt.% solution of PDI in eicosane (EC, TmEC = 37 °C[44]), a tron donors,[41,42] and ii) turned into red excimer luminescence 20-carbon paraffin that has already been used to interconvert upon aggregation in concentrated mixtures[41] or polymer between the monomer and aggregation-induced excimer fluo- media.[43] Therefore, the use of PCMs to modulate the behavior rescence of polycyclic aromatic dyes upon thermally-induced of this class of dyes offers a unique and straightforward solid-liquid phase transition.[32,34] This is due to the fact that approach toward photo- and thermofluorochromic materials these dyes dissolve well in the liquid paraffin, while they aggre- that exhibit multiple switching modes. gate in the solid phase because of lower solubility (Figure 1a Figure 1. a) Schematic representation of the distinct fluorescence switching behaviors accomplished by varying the nature of the PCM and/or the incor- poration of additives: on1-on2 modulation by interconverting between excimer and monomer dye emission; off-on modulation by adding a PET quencher as an additive; on-off modulation by using the PCM matrix as a PET quencher. b) Emission spectra of PDI-loaded PCM mixtures in the solid (blue) and liquid (red) state (λexc = 445 nm): PDI@EC (on1-on2, cPDI = 0.1 wt.%), PDI-DBA@EC (off-on, cPDI = 0.1 wt.%, cDBA = 0.005 wt.%), and PDI@EC-OAMe (on-off, cPDI = 0.5 wt.%, 7:3 EC:OAMe). c) Emission spectra of DCA-loaded PCM mixtures in the solid (blue) and liquid (red) state (λexc = 405 nm): DCA@EC (on1-on2, cDCA = 0.007 wt.%), DCA-DMA@EC (off-on, cDCA = 0.007 wt.%, cDMA = 0.17 wt.%) and DCA@OA (on-off, cDCA = 0.007 wt.%). Adv. Optical Mater. 2022, 2200083 2200083 (2 of 6) © 2022 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.advopticalmat.de and Figure S2a, Supporting Information). As anticipated, the All the three emission switching modes registered for solid PDI@EC mixture displayed the typical red and broad PDI@EC, PDI-DBA@EC, and PDI@EC-OAMe mixtures were excimer-like emission of this dye with rather high quantum found to be fully reversible, as the initial luminescence of the yield (on1, λmax = 640 nm, Φf = 0.40), which turned into the solid samples was retrieved upon cooling their melts and even bright, lime green and structured fluorescence characteristic after applying several heating-cooling cycles (Figure S5, Sup- of PDI monomers upon thermally melting the sample (on2, porting Information). In addition, their behavior was demon- λmax = 528 nm, Φf = 0.89) (Figure 1b). Such emission meas- strated to be generalizable to other emitters that also sustain ured in the liquid state is slightly altered and less intense than multiple fluorescence modulation mechanisms (e.g., excimer expected for diluted PDI solutions (Φf ≈ 1 in most organic sol- formation upon aggregation and PET-induced quenching). vents[41]), which we attribute to self-absorption processes occur- For this, we chose the dye 1,6-dicyanoanthracene (DCA, ring at the higher dye concentrations employed. Figure S1, Supporting Information). After some optimization Off/on thermofluorochromism was accomplished by dis- work required because of the different chemical nature of DCA, solving PDI in EC (0.1 wt.%), though in the presence of we observed that, by dissolving this dye in pristine eicosane the electron donor N,N-dibutylaniline as an additive (DBA, (DCA@EC, 0.007 wt.%), in eicosane together with N,N-dimeth- 0.005 wt.%). Because of its aromatic core, DBA should aggre- ylaniline (DMA, 0.17 wt.%) (DCA-DMA@EC, 0.007 wt.%), and gate together with PDI molecules in solid EC, and thus pro- in octadecylamine (OA) (DCA@OA, 0.007 wt.%), the resulting mote efficient emission quenching by PET (Figure 1a). In fact, mixtures exhibited on1/on2, off/on and on/off emission modu- nearly no fluorescence was measured for the PDI-DBA@EC lation schemes, respectively (Figure 1c). Noticeably, both the mixture in its solid-state (off, Φf = 0.035). In contrast, when monomer and excimer emissions of DCA in these samples are the sample was heated above TmEC, the intense PDI monomer blue-shifted with respect to those made from PDI, thus allowing emission was registered owing to the dissolution of the het- the three switching modes to occur in different spectral regions. eroaggregates of PDI and DBA molecules, which barely Prompted by these successful results, we aimed to exploit interact in the liquid PCM at the low concentrations used (on, our PCM-based strategy for the fabrication of different solid λmax = 528 nm, Φf = 0.81) (Figure 1b and Figures S2b–S3, Sup- materials of interest for practical applications. First, we pre- porting Information). pared thermally-responsive smart papers by soaking cel- Finally, to realize thermoresponsive on/off fluorescence lulose substrates with the PDI- and DCA-loaded mixtures switching, the use of a different PCM that not only triggers described above. The soaked papers fully preserved all the selective PDI aggregation but is also sufficiently electron- optical switching modes of the corresponding bulk mixtures donating as to induce PET-based emission quenching was (Figure 2 and Figure S6, Supporting Information). However, it tested. Under such circumstances, the red excimer fluores- must be noted that larger amounts of the quenching amines cence from the large PDI homoaggregates formed in the solid- DBA and DMA had to be used to reproduce the bulk off/on state is expected to be mainly preserved, as only the minor fluorescence behavior in the impregnated substrates (camine ≈ fraction of dyes located at (or close to) the outer surface of the 0.20–0.25 wt.%). We attribute this result to the migration of the aggregates should undergo PET processes with the enclosing amine molecules towards the cellulose fibers, thus decreasing PCM matrix. However, redissolution of the aggregates upon their concentration in the PCM phase. Actually, this process thermally melting the mixture should make all PDI emitters be was found to occur even after preparation of the samples, as surrounded by the large excess of PCM molecules, which must the amine quenching effect in the solid-state of the material completely quench the fluorescence of the sample in its liquid strongly decreased after storage at room temperature for several state by PET (Figure 1a). days and irrespective of the surrounding atmosphere (air or To test this hypothesis, we first dissolved PDI in pure inert atmosphere), which discarded the loss of the amine due to N-methyloctadecylamine (OAMe, 0.1 wt.%, TmOAMe = oxidation. By contrast, when slowing down molecular diffusion 52.9 °C[44]). Although the resulting PDI@OAMe mixture by storing the substrates at 4 °C, their emission off/on modula- exhibited on/off thermofluorochromism, a rather poor emis- tion was preserved after at least 1 week. sion efficiency was registered for the solid-state of the sample In addition, when the cellulose papers soaked with dye-PCM (Φf = 0.03), probably due to the small size of the PDI aggre- mixtures were also impregnated with an aqueous dispersion of gates formed (i.e., the short distances between the inner dyes gold nanoshells (diameter = 96 ± 34 nm, Figure S7, Supporting and the aggregates’ surface) and/or the large amount of sur- Information), they underwent localized on1/on2, off/on and rounding OAMe quenching sites. Accordingly, we decided to on/off fluorescence switching upon irradiation with a near- i) use mixtures of OAMe with a nonquenching PCM (EC) as a infrared (NIR) laser (λexc = 830 nm, Figure 2 and Figure S6, host, and ii) raise PDI concentration to increase dye aggregates’ Supporting Information). As previously reported by us,[31] this size. After evaluating the variation of PDI emission quantum effect results from the photothermal heating generated by the yield when dispersed in solid and melted EC-OAMe matrices gold nanoshells, which induces the melting of the dye-PCM of variable composition, the best results were obtained for a phase and, as a result, the modulation of emission selectively 7:3 EC-OAMe mass ratio and a 0.5 wt.% dye concentration in the illuminated area. Importantly, the same behavior could (Figures S2c and S4, Supporting Information). At these condi- be accomplished when replacing the gold nanoshells with solid tions PDI@EC-OAMe shows strong red excimer fluorescence gold nanoparticles (diameter = 18 ± 5 nm, Figure S8, Sup- in the solid-state (on, λmax = 642 nm, Φf = 0.19) and negligible porting Information) and using green excitation light to induce lime green monomer emission upon melting (off, Φf = 0.010) photothermal heating instead (λexc = 532 nm, Figure S9, Sup- (Figure 1b). porting Information). Therefore, this demonstrates that the Adv. Optical Mater. 2022, 2200083 2200083 (3 of 6) © 2022 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.advopticalmat.de photothermal heat diffusion. Thus, when the same experi- ment was performed for a cellulose paper impregnated with a PDI-octacosane mixture (OC, TmOC = 60 °C[44]), we were not only able to retrieve the on1-on2 fluorescence switching previ- ously observed for PDI@EC, but in this case, the local red-to- green emission change could be essentially restricted to the photo­excited area (Figure S10, Supporting Information). Con- sequently, wise selection of the sample components and illumi- nation conditions should enable high spatial resolution of the local fluorescence modulation attained in dye-PCM mixtures. Moreover, we also investigated the kinetics of the photo- activated fluorescence changes locally induced in soaked cel- lulose papers. It must be noted that such kinetics is not only dependent on the rate with which the PCM is photothermally melted in the illuminated area, but also on the time that it takes the local heat generated to diffuse to other regions of the sample and the surrounding environment until a thermo- stationary state is created. Overall, this means that a number of parameters should affect the actual switching kinetics observed; for example, the PCM melting temperature, the photo­ excitation power and photothermal nanoheater con- centration determining the heating rate, the dimensions of the heated volume, the room temperature, and the heat con- ductivity of the material. As a result, large variability of the emission switching times could be observed. For instance, for papers impregnated with low-Tm mixtures PDI@EC, PDI- DBA@EC, and PDI@EC-OAMe, emission switching could be appreciated almost immediately when photothermally heated with a non-focused NIR laser, though constant fluorescence modulation was not observed until illumination for tens of Figure 2. Emission spectra (left, λexc = 355 nm) and images (right, λexc = 365 nm) of cellulose papers soaked with a) PDI@EC, b) PDI-DBA@EC, seconds (Figure S11, Supporting Information). Actually, such and c) PDI@EC-OAMe at room temperature in the dark, when heated very fast activation of the fluorescence changes could be clearly above TmPCM and when locally irradiated with a NIR laser (λexc = 830 nm). observed by fluorescence microscopy for substrates soaked with both low-Tm PDI@EC and high-Tm PDI@OC mixtures conditions required to obtain local light-induced fluorescence (Figure S12, Supporting Information), which permits real-time modulation in dye-PCM mixtures can be easily tuned by var- handwriting of fluorescent patterns on the papers (Movie S1, ying the nature and excitation spectra of the photothermal Supporting Information). nanoheaters employed. Aside from continuously coated cellulose supports, we For the cellulose substrates impregnated with dye-PCM mix- also investigated the deposition of our PCM mixtures on dif- tures, we further analyzed the performance of the local emis- ferent substrates as stimuli-responsive fluorochromic pat- sion changes achieved optically. First, we investigated the spatial terns, as described for smart papers inkjet-printed with gold resolution with which these changes can be induced. With nanoshells[31] or nanostructured dye-loaded paraffin mixtures.[32] this aim, we conducted fluorescence microscopy experiments Herein we instead fabricated thermofluorochromic patterns by where a soaked paper was excited with a focused NIR laser means of spray coating, a much more versatile technique that beam (λexc = 980 nm, spot = 1000 × 300 µm2) while monitoring allows depositing such thermofluorochromic mixtures onto its fluorescence over a larger field of view (Figure S10, Sup- substrates other than cellulose papers. However, this approach porting Information). For PDI@EC samples, photothermally- requires previous microstructuration of the PDI-loaded PCMs triggered red-to-green fluorescence switching was observed to as solid lipid microparticles (SLMs), which we achieved through occur within a region of the substrate that is much larger than an oil-in-water emulsion cooling method. In this method, the oil the excitation spot (>1400 × 1400 µm2, Figure S10, Supporting droplets were obtained by homogenizing the molten PDI-PCM Information). This is due to the thermal gradient created, mixture in a hot surfactant-containing aqueous phase, which which spreads the photothermal heat outside of the illumi- was then rapidly cooled down to obtain the desired SLMs (see nated area and promotes PCM melting in the surrounding the Supporting Information for further details). Optical micro­ region because of the low Tm of EC (TmEC = 37 °C[44]). Though scopy measurements corroborated the successful formation of this result could be slightly improved by decreasing the excita- microparticles with 15–21 µm in diameter (Figure S13, Sup- tion power and the concentration of photothermal nanoheaters, porting Information), while 1H NMR spectroscopy confirmed the best spatial resolution was accomplished when selecting the encapsulation of the amines required for accomplishing PCMs of higher melting points; that is, when disfavoring PCM off/on (DBA) and on/off (OAMe) fluorescence modulation melting in the areas surrounding the illuminated spot through (Figures S14 and S15, Supporting Information). Adv. Optical Mater. 2022, 2200083 2200083 (4 of 6) © 2022 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.advopticalmat.de Figure 3. a) Images of thermofluorochromic patterns prepared from PDI@EC_SLMs (top), PDI-DBA@EC_SLMs (middle), PDI@EC-OAMe_SLMs (bottom) spray-coated onto cellulose paper, glass and polystyrene substrates, respectively. b) Images of thermofluorochromic patterns prepared from PDI@EC (top), PDI-DBA@EC (middle), and PDI@EC-OAMe (bottom) printed onto cellulose with a custom-made plotter. c) Wax-printed patterns obtained depositing on/off (PDI-DBA@EC) and off/on (PDI@EC-OAMe) fluorescence switching dye-PCM mixtures in different (left) or the same (right) area of the cellulose paper. All images are taken under UV light (365 nm). The microparticle suspensions obtained for the three different patterns, a very appealing feature for high-security different PDI-loaded mixtures (PDI@EC_SLMs, PDI- anti-counterfeiting applications that normally is only possible DBA@EC_SLMs, PDI@EC-OAMe_SLMs) could be success- with the use of electronic digital devices. fully spray-coated onto different substrates (porous cellulose Taking advantage of the simplicity of the wax-printing pro- papers, flat glass, and a polystyrene film, Figure S16, Sup- cess, we also demonstrated the potential of our dye-PCM mix- porting Information), for which we used a customized spray- tures to be used as thermal sensors. For this, we printed three dryer and a pre-patterned poly(methyl methacrylate) mask off/on thermofluorochromic patterns that exhibit distinct acti- (Figure S17, Supporting Information). As shown in Figure 3a vation temperatures on the external surface of a disposal paper and Figure S18a, Supporting Information, all printed materials cup: TmEC = 37 °C[44] for the already described PDI-DBA@EC preserved the switching properties of the respective bulk mix- mixture and TmTC = 51 °C[44] and TmOC = 60 °C[44] for the anal- tures and maintained the pattern resolution even after various ogous samples with the higher-Tm PCMs tetracosane (PDI- heating/cooling cycles. DBA@TC) and octacosane (PDI-DBA@OC), respectively. As In addition, we also explored the patterning of dye-PCM shown in Figure S20, Supporting Information, the change in mixtures using wax-printing, a rather simple method that does the fluorescence emission from these patterns clearly report on not require prior PCM structuration. This was accomplished by the temperature of the liquid inside the cup when it varies from using a custom-made plotter where a pen was loaded with the 14 to 65 °C. desired PDI-PCM mixture, coated with metallic foil connected In summary, herein we demonstrate that, by dispersing a to a power supply to induce PCM melting by Joule heating, and selected dye in adequate PCMs, thermofluorochromic systems attached to a drawing robot with 3D motion that allows repro- are obtained that exhibit all the possible types of emission ducing pre-designed patterns (Figure S19, Supporting Informa- switching schemes (on1-on2, off-on, on-off) without the need for tion). In this way, complex marks could be printed onto paper, synthetic modification of the emitter. Moreover, thanks to the which retained the reversible thermofluorochromic switching versatility of the resulting dye-PCM mixtures, smart responsive behavior of the bulk samples and did not smear out upon papers were obtained by simply soaking them into the desired sequential heating/cooling cycles (Figure 3b, Figure S18b and mixture, and even more sophisticated thermally-responsive Movie S2, Supporting Information). On top of that, we could patterns were created with facile printing technologies. These also use wax-printing for sequentially depositing on/off (PDI- results pave the way for the fabrication of luminescent smart DBA@EC) and off/on (PDI@EC-OAMe) PDI-loaded PCM materials showing intricate stimuli-responsive behavior as mixtures on different or even the same region of cellulose sub- highly attractive platforms for the development of optical sen- strates (Figure 3c). In this way, materials were obtained that sors and anti-counterfeiting technologies at very reduced cost show dual-color thermofluorochromic transition between two and effort. Adv. Optical Mater. 2022, 2200083 2200083 (5 of 6) © 2022 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH www.advancedsciencenews.com www.advopticalmat.de Supporting Information [14] W. Ren, G. Lin, C. Clarke, J. Zhou, D. Jin, Adv. Mater. 2020, 32, 1901430. Supporting Information is available from the Wiley Online Library or [15] M. Sauer, Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 9433. from the author. [16] M. Li, J. Zhao, H. Chu, Y. Mi, Z. Zhou, Z. Di, M. Zhao, L. Li, Adv. Mater. 2018, 31, 1804745. [17] D. Kim, K. Jeong, J. E. Kwon, H. Park, S. Lee, S. Kim, S. Y. Park, Nat. Commun. 2019, 10, 3089. Acknowledgements [18] A. Abdollahi, H. Roghani-Mamaqani, B. Razavi, M. Salami-Kalajahi, This work was supported by grant RTI2018-098027-B-C21 and PID2019- ACS Nano 2020, 14, 14417. 106171RB-I00 funded by MCIN/AEI/10.13039/501100011033 and by [19] Q. Qi, C. Li, X. Liu, S. Jiang, Z. Xu, R. Lee, M. Zhu, B. Xu, W. Tian, ERDF – “A way of making Europe”. The ICN2 is funded by the CERCA J. Am. Chem. Soc. 2017, 139, 16036. program/Generalitat de Catalunya and supported by the Severo Ochoa [20] Y. Ma, Y. Dong, S. Liu, P. She, J. Lu, S. Liu, W. Huang, Q. Zhao, Adv. “Centers of Excellence” program, grant SEV-2017-0706 funded by MCIN/ Opt. Mater. 2020, 8, 1901687. AEI/10.13039/501100011033. U.A.B. thanks the support from Generalitat [21] A. Kishimura, T. Yamashita, K. Yamaguchi, T. Aida, Nat. Mater. de Catalunya (2017 SGR00465 project). J.R.O. thanks the Generalitat 2005, 4, 546. de Catalunya (AGAUR) for his predoctoral FI fellowship. The authors [22] D. Kim, S. Y. Park, Adv. Opt. 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