This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in ACS Applied Materials and Interfaces, copyright © American Chemical Society after peer review. To access the final edited and published work see: https://dx.doi.org/10.1021/acsami.8b22335. Alex Julià López,# Daniel Ruiz-Molina,# Jordi Hernando,*,‡ Claudio Roscini.*,# # Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology (BIST), Campus UAB, Bellaterra, Barcelona 08193, Spain ‡Universitat Autònoma de Barcelona, Edifici C/n, Campus UAB, Cerdanyola del Vallès, 08193, Spain reverse/negative photochromism, spiropyrans, merocyanine forms, microcapsules, rewritable devices ABSTRACT: Herein we report on a novel, straightforward and universal strategy to achieve solid materials with highly tunable reverse photochromism. This was accomplished by means of commercially available spiropyran dyes, which can produce different types of stable merocyanine states (i.e. non-protonated and protonated forms) displaying distinct re- verse photochromic properties (i.e. colours and colouration rates). To finely control the concentration ratio of these spe- cies and, as such, tailor the optical performance of the photochromes, we exploited their differential interaction with sur- rounding media of distinctive nature (i.e. non-volatile protic and aprotic polar solvents). In this way, solutions displaying different photochromic responses were prepared for individual spiropyrans without requiring chemical derivatization, an approach that can be generalized to other spiro dyes with distinct acid-base properties. To transfer this behaviour to the solid state, core-shell capsules of these solutions were prepared, which were then used as ink materials for the fabrication of flexible polymeric films with unprecedented tunability of their photochromic properties that can be employed as re- writable multi-coloured devices. 1. INTRODUCTION and largely used T-type photochromes such as spirooxa- During the last decades both the industrial and the ac- zines and spiropyrans.4,5 Normally, these compounds ademic sectors have shown growing interest towards or- show direct photochromism since their thermodinamical- ganic photochromes for the preparation of colour-tunable ly stable form is the closed, colourless spiro state, which functional materials.1-3 Aiming to address specific applica- isomerizes to the coloured, unstable merocyanine (MC) tions, an increasing number of strategies are being devel- when irradiated.1,3 However, several methodologies have oped to finely tailor the photochromic performance of been proposed to alter the relative stability of both iso- these systems (e.g. switching colours, rates and wave- mers and attain reverse photochromism,6-17 where stabili- lengths). This is the case of T-type photochromes, the ca- zation of the MC form is accomplished via: a) chemical pacity of which to undergo UV-induced colouration and modification of the dyes22-26 resulting in a dramatic varia- subsequent spontaneous colour bleaching in the dark (i.e. tion of their electronic properties,27 intramolecular hy- direct photochromism) is currently employed for the fab- drogen-bond formation,23 or complexation with metals;28- 36 rication of photoprotective coatings on industrial scale b) dispersion of the dyes in selected media,37-40 which (e.g. in ophthalmic lenses and smart windows).3 However, either stabilize the more polar zwitterionic MC structure a wealth of research is devoted to additionally accomplish (e.g. via hydrogen-bond interactions) or convert it into reverse photochromism with these compounds (i.e. pho- the protonated form MCH+ through acid-base reactions.41- 43 toinduced colour fading followed by thermal coloration after irradiation),4,5 which would not only allow operation All these strategies have been explored to obtain re- with less energetic and harmless visible light, but also verse photochromism with spirocompounds in liquid me- grant access to novel applications, such as anti- dia (e.g. protic solvents,37-38 ionic liquids,44-45 solutions of counterfeiting technologies, rewritable displays and opti- acids46-47 or metal ions28-36) as well as in bulk or mi- cal data storage systems, among others.6-10 In spite of this, cro/nanostructured solid systems, which are eventually functional solid devices based on reverse T-type photo- more relevant for practical applications (e.g. polymers,26 chromes are very scarce and only started to be reported polar silica,6,48-52 metal-organic framework,53 gold nano- recently (e.g. in multicoloured light-responsive rewritable particles,54 hydrogels,55-56 liquid-filled capsules57). Never- devices).11-17 theless, the materials so far prepared in this way do not Although new families of dyes can be prepared to di- provide flexible tunability of their photochromic respons- rectly exhibit reverse photochromism (e.g. imidazole- es (i.e. colour in the dark and isomerization kinetics) based compounds18-20 and donor-acceptor Stenhouse ad- since their properties are very dependent on the particu- ducts21), much attention is dedicated to achieve this be- lar dyes and host matrices selected and, in some haviour by means of well-known, commercially available Scheme 1: Strategy used in this work to obtain tunable reverse photochromism with spiropyrans: a) modulation of the col- + oured merocyanine forms (non-protonated MC vs. protonated MCH forms); b) encapsulation of the designed dye mixtures and integration of the core-shell capsules in solid films. cases, even specific chemical modification of these spe- Importantly, these compounds differ on the basicity of cies is needed. Therefore, the development of a simple the phenolate group of their MC isomers due to the dis- approach to design and achieve tunable reverse photo- tinct substituents that they present. As such, they could chromism in solid materials from readily available differently interact with nonanoic acid (NA), a weakly- commercial dyes without requiring chemical derivatiza- acid non-volatile medium that we already proved to sta- tion would be highly desirable, thus paving the way for bilize the open form of spiropyrans.57 the broad use of this technology in real applications. When colourless SP-1-SP-5 were dissolved in NA (c = To reach this objective, in this work we propose a 14-18 mM), different colours were effectively observed to novel methodology that relies on: a) the embedment of develop in the dark, which we ascribed to stabilization readily accessible spiropyran dyes in rationally designed of their merocyanine isomer as MC, MCH+ or an equilib- non-volatile media (with hydrogen-bonding and/or rium mixture of both (Figure 1a). On one hand, brown- acidic properties), to fine control the equilibrium be- yellow NA solutions characteristic of the protonated tween the differently coloured merocyanine forms (typi- MCH+ form41-43 were obtained for SP-3 and SP-5 cally, non-protonated, low-energy visible light absorbing (λabs,maxSP-3/NA = 430 nm, λabs,maxSP-5/NA = 400 nm), for MC and protonated, high-energy visible light absorbing which larger basicity is expected upon ring-opening due MCH+ forms41-43) and their relative concentration ratio; to the absence of an electron-withdrawing group (EWG) b) the encapsulation of these dye-containing mixtures in at the para position of the phenolate ring. On the other core-shell polymer capsules to preserve the photo- hand, SP-2 produced a violet solution in NA (λabs,maxSP- 2/NA chromism in the solid state, as recently demonstrated by = 564 nm) with very little absorption in the region us;57 and c) the integration of these capsules in polymer- around 430 nm, indicating selective formation of the ic matrices, to achieve solid materials with high and non-protonated MC state as a consequence of the less straightforward tunability of the reverse photochromic basic character of its o-methoxy-p-nitrophenolate performances (Scheme 1). group.41-43 Finally, mixtures of the MC and MCH+ forms were obtained for NA solutions of SP-1 and SP-4, which showed different spectral contributions from both spe- 2. RESULTS and DISCUSSION cies owing to their distinct acid-base properties: while 2.1. Tunable reverse photochromism of spiropy- major formation of MC was observed for SP-1 also bear- rans in nonanoic acid ing a low-basicity p-nitrophenolate group upon isomeri- In a first step, the capacity to tune the reverse photo- zation (red colour, λabs,maxSP-1/NA= 528 nm with a pro- chromic behaviour of spiropyrans (SP) by modulating nounced shoulder at around 430 nm), a much larger in- their acid-base properties was investigated in bulk solu- tensity of the MCH+ absorption band was found for SP-4 tion. For this study we chose five commercially available due to the presence of a weaker, bromine EWG that in- SPs (see supporting information, Scheme S1), holding sufficiently stabilizes the non-protonated MC state (yel- different chemical and electronic properties: 1’,3’,3’- low colour, λabs,max1SP-4/NA = 437 nm and λabs,max2SP-4/NA = trimethyl-6-nitrospiro[1(2H)-benzopyran-2,2’-indoline 550 nm). (SP-1), 1’,3’-dihydro-8-methoxy-1’,3’,3’-trimethyl-6- Notably, irradiation with visible light (laser pointer at nitrospiro[2H-1-benzopyran-2,2’-(2H)-indole] (SP-2), 532 nm and 470 mW/cm2, white-light emitting torch 1’,3’,3’-trimethylspiro[1(2H)-benzopyran-2,2’-indoline] (SP-3), (6-bromo-1’,3’,3’-trimethylspiro[1(2H)- benzopyran-2,2’-indoline] (SP-4) and 8-methoxy-1’,3’,3’- trimethylspiro[1(2H)-benzopyran-2,2'-indoline] (SP-5). On the other hand, when SP-1 was dissolved in a non- acidic protic oil such as dihydroxy-terminated poly(dimethylsiloxane-co-diphenylsiloxane) (PDMS- OH), it was selectively stabilized as its violet MC form through hydrogen bonds (λmaxSP-1/PDMS-OH = 551 nm, Fig- ure 2a) and no absorption contribution from MCH+ at around 430 nm was detected. In both cases, reverse pho- tochromism was preserved since the two SP-1 solutions bleached when subjected to visible light irradiation and recoloured in the dark (Figures 2b-d). Noticeably, dif- ferent back-isomerization rate constants were measured for these samples and that in NA (kSP-1/NA-DPA = 0.0044 min-1, kSP-1/PDMS-OH = 0.037 min-1), which indicates that Figure 1: a) Digital images and absorption spectra of NA not only the colour but also the switching kinetics of solutions of SP-1-SP-5. b) Digital images and absorption photochromes can be modulated with the strategy in- spectra of SP-2 and SP-3 in NA after irradiation with visible troduced in this work (Figure S3). light (dashed line) and during recolouration in the dark. Once demonstrated colour-tunable reverse photo- chromsim for SP-1, we explored the generality of this with exc> 400 nm and 20 mW/cm2) provoked colour behaviour for other spiropyrans. In particular, our atten- fading of all the NA solutions of SP-1-SP-5, a process tion focused on dyes producing merocyanine states with that was reverted in the dark according to a first-order different basicity than SP-1, since this is a crucial pa- kinetics characteristic of photochrome back- rameter controlling the equilibrium between their MC isomerization in homogeneous liquid systems (kSP-1/NA = and MCH+ forms. 0.013 min-1, kSP-2/NA = 0.70 min-1, kSP-3/NA = 0.52 min-1, kSP- 4/NA = 0.51 min-1, kSP-5/NA = 6.1 min-1, Figure 1b and Fig- ures S1-2). This, together with the different colours ob- tained for each of these solutions, demonstrate that tunable T-type reverse photochromism can be attained by simply dissolving commercial SP dyes in NA, without requiring further chemical derivatization or the use of additives. 2.2. Colour tuning for individual spiropyran pho- tochromes On the basis of the results obtained in NA for differ- ent types of spiropyrans, tunable reverse photochrom- ism could be reached for individual dyes if the equilibri- um between their non-protonated and protonated mer- ocyanine forms could be altered upon modification of the surrounding medium. To validate this hypothesis, we chose SP-1 as a benchmark system since it produces a reddish mixture of both MC and MCH+ in NA, and we aimed to tune its colour in the dark by varying the rela- tive concentration of these species using different sol- vents. In this study, well-designed, non-volatile solvents were used because they are more suitable for the fabri- cation of solid photochromic materials based on core- Figure 2: a) Absorption spectra of SP-1 in NA-DPA, NA shell capsules. and PDMS-OH; Absorption spectra of SP-1 in b) NA, c) NA- According to the SP-1 behaviour in NA, predominant DPA and d) PDMS-OH in the dark (green line), after irradi- formation of the yellow MCH+ or the violet MC forms of ation with visible light (red line) and during thermal recol- this dye should require increasing or decreasing the ouration (black lines); Absorption spectra of e) SP-3 and f) acidity of the media. Indeed, addition of a little amount SP-4 in NA and TD-BA. of n-dodecylphosphonic acid (DPA) to a red solution of SP-1 in NA (DPA/NA molar ratio = 1/143) turned the For this reason, additional studies were conducted for sample yellow (λabs,maxSP-1/NA-DPA= 421 nm) and led to the SP-3 and SP-4, which represent high- and intermediate disappearance of the absorption band at λabs,max = 528 basicity cases in comparison to nitrosubstituted SP-1, nm, thus suggesting quantitative protonation of MC respectively. Actually, for both dyes, dissolution in weak (Figures 2a). acid NA gave rise to predominant formation of MCH+, in contrast to SP-1 (see Figure 1a). In order to favour stabi- that these materials are not mixed with the shell poly- lization of the non-protonated MC form of SP-3 and SP- mer and therefore are essentially confined to the core of 4, we employed a solid mixture made of tetradecanol the capsules (Figure S8).64 Consequently, this should (TD) and bisphenol A (BA) (TD/BA molar ratio = 18/1), guarantee preservation of the bulk photochromic prop- which is generally used in thermochromic materials to erties of the loaded dyes if they remain in the core- stabilize the open form of spirolactones through hydro- forming media. Indeed, the freeze-dried capsules of the gen-bond interactions.58-59 The dissolution of SP-3 and SP-1 mixtures showed yellow (SP-1/NA-DPA@PMMA), SP-4 in the TD-BA mixture provoked the development red (SP-1/NA@PMMA) and violet (SP-1/PDMS- of a pink/violet colour and the rise of absorption bands OH@PMMA) colours as well as reflectance-mode ab- at around 390 nm and 565 nm (Figures 2e-f), thereby sorption spectra (λabs,max= 425, 528 and 550 nm, respec- indicating MC formation. In both cases, reverse photo- tively) that nicely reproduced the optical properties of chromism was observed upon visible-light irradiation the bulk solutions (Figure 3). Similar results were ob- with relatively large back-isomerization rate constants tained for SP-3/NA@PMMA and SP-4/NA@PMMA in the dark despite the solid nature of the solvent used (yellow colour, abs,max = 434 and 440 nm, respectively) (kSP-3/TD-BA = 1.7 min-1 and kSP-4/TD-BA = 0.70 min-1, Figure and SP-3/TD-BA@PC and SP-4/TD-BA@PES (pink- S4). Interestingly, the growth of both absorption bands violet colour, λabs,max = 390 and 554 nm and λabs,max = 388 at 390 nm and 565 nm took place with very similar ki- and 546 nm, respectively), which demonstrates that en- netics in these experiments, which further suggests that capsulation of dye-solvent mixtures in polymer core- they derive from the same MC species rather than from shell capsules does not alter the stabilization mecha- a mixture of the protonated and non-protonated mero- nisms and efficiencies of the merocyanine state of the T- cyanine forms of SP-3 and SP-4 (Figure S5). type photochromes considered (Figure 3 and Table S1). In addition, irradiation with visible light provided in 2.3. Core-shell capsules with tunable reverse pho- all the cases colourless capsules, which recoloured in the tochromism dark with rather similar rates to the bulk systems (kSP- 1/NA-DPA@PMMA = 0.0038 min-1, kSP-1/NA@PMMA = 0.012 min-1, As shown above, full tunability of reverse photo- SP-1/PDMS-OH@PMMA k = 0.042 min-1, kSP-3/NA@PMMA = 0.17 min-1, chromism can be achieved in bulk solution for a wide SP-4/NA@PMMA k = 0.16 min-1, kSP-3/TD-BA@PC = 0.72 min-1, kSP- range of commercially available spiropyran dyes by sim- 4/TD-BA@PES = 0.19 min-1) (Figure 3 and Figures S9-11, Ta- ple solvent variation. However, direct transfer of this ble S2). For SP-3- and SP-4-containing capsules, howev- behaviour to the solid state is required for the fabrica- er, we systematically measured slower recolouration ki- tion of functional photochromic materials such as inks netics than in bulk mixtures, which was tentatively as- or paints. Exploiting the non-volatility of the solvents cribed to the formation of rigid photochromic subdo- employed, we aimed to achieve this objective by prepar- mains in the polymer shell where the isomerization pro- ing stable core-shell capsules, a strategy that we have cesses are slightly hampered. In spite of this, the fast already proven to allow preservation of bulk photo- colour recovery rates of these capsules allowed fatigue chromic57,60 and/or thermochromic61 properties in solid resistance studies to be performed, which confirmed matrices. photochrome stability after a few cycles of light-induced In view of this, polymer-coated capsules of solutions decolouration and recolouration in the dark (Figure of SP-1 in NA-DPA, NA and PDMS-OH, and of SP-3 and S12). SP-4 in NA and TD-BA were prepared through the reac- tion-free emulsion-solvent evaporation method (see the Experimental section and Supporting Information for 2.4. Photochromic films and rewritable devices the detailed procedure), which prevents dye degrada- Core-shell capsules displaying tunable reverse photo- tion by undesired chemical reactions.62 After optimiza- chromism were finally dispersed in polymeric matrices tion, poly(methylmethacrylate) (PMMA), polyethersul- to allow the fabrication of composite materials that fone (PES) and poly(bisphenol A carbonate) (PC) were could be of relevance for practical applications. For this, employed as shell polymers depending on the encapsu- capsule-containing, flexible poly(vinylalcohol) (PVA) lated mixture. SEM images showed the successful for- and polyacrylamide (PA) films were prepared by drop- mation of the capsules loaded with SP-1 (SP-1/NA- casting an aqueous suspension of these water-soluble DPA@PMMA, SP-1/NA@PMMA, SP-1/PDMS- polymers and of the capsules of choice in a Petri plate, OH@PMMA), SP-3 (SP-3/NA@PMMA, SP-3/TD- from which they could be easily peeled off after solvent BA@PC) and SP-4 solutions (SP-4/NA@PMMA, SP- evaporation. For the preparation of the films, the cap- 4/TD-BA@PES), the sizes of which ranged from 1-5 μm sules obtained from SP-1 and SP-4 in different media (for NA, NA-DPA) to 20-30 μm (PDMS-OH and TD-BA (NA, NA-DPA, PDMS-OH and TD-BA) were used, which systems, Figure 3). 1H-NMR spectroscopy proved the maintained their shape and structure once embedded in encapsulation of NA (47-52% wt.), PDMS-OH (54 % wt.) the polymer matrices (Figure S13). As expected, the re- and TD (47-57 wt. %, Figures S6-S7). For NA and TD, sulting polymer films were all coloured in the dark and similar melting points (Tm) were measured for the cap- preserved the optical properties and reverse photo- sules and in bulk by DSC (TmNA= 8-12.5 oC,63 TmNA-DPA = 4 chromic behaviour of the capsules dispersed (Figures o C and TmTD = 33.5-35.9 oC63), which further supports S14-15, Tables S1-S2). Notably, from each photochromic dye (i.e. SP-1 and SP-4), two or more differently col- oured films could be prepared, thus demonstrating the 3. CONCLUSIONS capability of the methodology introduced in this work to provide fine tunability of reverse photochromism for In summary, we reported in this work a novel, reaction- the fabrication of functional materials (Figure 4). free, universal and straightforward strategy to achieve tunable reverse photochromism in solid functional ma- terials. By exploiting the capacity of commercially avail- able spiropyrans to produce different stable merocya- nine forms with distinctive optical properties, we demonstrated that distinct colours and switching rates could be obtained for these photochromes without the need of chemical derivatization by simply varying the nature of the surrounding solvent. In addition, we showed that this behaviour could be directly transferred to solid matrices by preparing polymer capsules loaded with spiropyran solutions and eventually dispersing them in the final material of interest. As a result, poly- mer films with up to 3 different photochromic responses (i.e. colours and isomerization kinetics) could be gener- ated from the same commercial dye, providing unprece- dented tunability of the photochromic properties in the solid state. Considering that more colours could be ob- tained by combining capsules of different types, which might also display thermochromic, halochromic and/or fluorescence behaviour, functional materials could be prepared from spiropyran dyes exhibiting multicolour and multistimuli responses. Figure 3: SEM images and reverse photochromic behav- iour of: a) SP-1/NA-DPA@PMMA, b) SP-1/NA@PMMA, c) SP-1/PDMS-OH@PMMA, d) SP-4/NA@PMMA and e) SP- 4/TD-BA@PES microcapsules. For each sample, reflec- tance-mode absorption spectra are given in the dark (green line), after irradiation with visible light (red line) and dur- ing thermal recolouration (black lines). As a proof of this concept, we explored the use of films containing SP-1 and SP-4-loaded capsules as re- writable papers. Noticeably, by using just one single photochromic dye (e.g. SP-1), multicoloured films could be written and erased several times with different black masks or by hand-writing with a 532-laser pointer, ob- taining writings and draws with very good contrast and spatial resolution (Figure 5). Figure 4: PVA and PA films obtained from SP-1- and SP-4-loaded capsules with different core materials, which display colour-tunable reverse photochromism. Figure 5: Writing/erasing/rewriting of PVA (or PA) films of SP-1 and SP-4 capsules containing different media and providing different colours (hand-writing with laser pointer irradiation, 470 mW/cm2, or printed-like drawing with visible light torch, 20 mW/cm2, in the presence of a black mask). 4. EXPERIMENTAL SECTION through the Kubelka-Munk equation (F(R) = (1–R)²/2R Materials ).65 Time-dependent absorption measurements were performed using the spectrophotometer in scan mode. The polymers used for the preparation of the capsules, For all the photochromic materials, their thermal back- poly(methylmethacrylate) (PMMA, MW = 120000), isomerization was monitored by recording the absorp- poly(bisphenol-A carbonate) (PC, MW=45000), polyeth- tion (or reflectance) spectra at different time delays, in ersulfone (PES, MW = 22000)) and the films (poly(vinyl the dark after reaching their photostationary state. alcohol) (PVA, Mowiol® 4-88, 99% hydrolyzed, medium molecular weight), polyacrylamide (PA)), and the colour Differential Scanning Calorimetry (DSC): Measure- developers (bisphenol–A (BA, 99%)) and dihydroxy- ments were carried out in a TA Instruments Q20 differ- terminated poly(dimethylsiloxane-co-diphenylsiloxane) ential scanning calorimeter. Approximately between 4-7 (PDMS-OH) were purchased from Sigma Aldrich. Non- mg of a sample were deposited on a 0.5 cm-in-diameter anoic acid (NA, 97%), 1-tetradecanol (TD, 97%) and n- aluminum pan. An empty pan was used as a reference. dodecylphosphonic acid (DPA, 95%) were acquired The scanning rate was 10 K/min for both the heating from Alfa Aesar. Photochromes 1’,3’,3’-trimethyl-6- and cooling processes. The scanned temperature range nitrospiro[1(2H)-benzopyran-2,2’-indoline (SP-1), 1’,3’- depended on the sample. The melting/crystallization dihydro-8-methoxy-1’,3’,3’-trimethyl-6-nitrospiro[2H-1- point were calculated from the intersection between two benzopyran-2,2’-(2H)-indole] (SP-2), 1’,3’,3’- tangents (one is the slope of the endo- or exothermic trimethylspiro[1(2H)-benzopyran-2,2’-indoline] (SP-3), peak and the other is the straight line of the baseline). (6-bromo-1’,3’,3’-trimethylspiro[1(2H)-benzopyran-2,2’- Irradiation methods indoline] (SP-4) and 8-methoxy-1’,3’,3’- Visible light irradiations were carried out by using a trimethylspiro[1(2H)-benzopyran-2,2'-indoline] (SP-5) green laser pointer (JD850, 532 nm, 470 mW/cm2) or a were all obtained from TCI chemicals. CH2Cl2 was ac- non‐coherent flashlight torch (UltraFire CREE XMLT6, quired from Scharlau. All commercial solvents and 20 mW/cm2). chemicals were used without any further purification. Synthesis of core-shell capsules Distilled water was used in all syntheses. Deuterated solvents were acquired from Euriso-Top. All capsules were prepared through the solvent evapo- ration method.62 Typically the polymer (PMMA, PC, Characterization PES, 0.25-0.75 g) was dissolved in CH2Cl2 (5 mL) upon Electron microscopy (EM): Scanning electron micros- magnetic stirring, while the photochrome of interest copy (SEM) images were collected on a scanning elec- (SP-1-SP-5, 10-30 mg) was dissolved in the correspond- tron microscope FEI Quanta 650 FEG at acceleration ing colour developing mixture (0.5-1.0 g). During photo- voltages between 2-10 kV. Samples were mounted on chrome dissolution, colour development was observed. SEM metal stubs covered with aluminum or carbon tape Once all the photochrome was dissolved, the obtained and they were coated with a thin layer of platinum (~ 5 photochromic mixture was poured into the CH2Cl2 solu- nm). Microcapsules suspensions (100 µL) were deposited tion. The final organic mixture was subsequently added on the stub by drop casting, allowing solvent evapora- to a previously prepared PVA water solution (10 mL, 1 tion in air at room temperature. For solid powders, ap- wt. %). The mixture was homogenized (T18 Ultra- proximately 1 mg of capsules was deposited on the adhe- Turrax® IKA, 3000-5000 rpm) at room temperature for sive carbon tape. 10 minutes to induce the formation of the oil-in-water Proton nuclear magnetic resonance ( 1H-NMR): 20-30 emulsion. Successively, the whole emulsion was trans- mg of microcapsules were dissolved in CDCl3 and 1H- ferred into a round bottom flask and was heated at 44 oC NMR spectra were recorded using the following spec- under vacuum for 20 minutes, during which CH2Cl2 trometers: Bruker DPX250 (250 MHz for 1H-NMR) and evaporated and the polymer precipitated around the Bruker DPX400 (400 MHz for 1H-NMR). The spectra are non-volatile organic droplets. The obtained suspension given in chemical shifts, δ (ppm). The peaks are defined was diluted with water (30 mL) and centrifuged at 4000- as singlets (s), triplets (t) or multiplets (m). 10 µl of DMF 10000 rpm during 10 minutes. This process was repeated (δ = 2.88 (s, 3H, CH3), 2.97 (s, 3H, CH3), 7.96 (s, 1H, CH)) three times redispersing the capsules with water (10 were added to the samples, as an internal reference, for mL). After freeze-drying the resulting suspension in the quantitative determination of the microcapsules pay- dark, a coloured, fine solid powder was obtained. See load. the supporting information for the detailed amount of Absorption spectra: The absorption spectra of the sol- the components used in each capsule synthesis. id and liquid samples were obtained from a Cary 60 spectrophotometer. Liquid solutions were measured in transmittance mode. Solid samples were measured through diffuse reflectance mode, using an external in- tegrating sphere connected with a fiber optic. As a refer- Preparation of polymer films ence, the same material (i.e. TD, capsules, films) without A fraction of the obtained suspension of the capsules the dye, was used. The absorption spectra were estimat- (3 mL) was dispersed homogeneously with the vortex in ed from the measured diffuse reflectance spectra a previously prepared film-forming polymer solution (PVA or PA, 2-20 wt. %). This solution was slowly trans- poly(dimethylsiloxane-co-diphenylsiloxane); BA, bisphenol ferred to a plastic Petri plate (diameter of 5.5 cm) and A; SP-X, spiropyrans 1-5 (structures in supporting infor- + water was allowed to evaporate on a flat surface at room mation); MC, non-protonated merocyanine form, MCH , temperature during 24-48 h. The final film was easily protonated merocyanine; SP-X/YY@WW, capsules with removed from the Petri plate once it was dry. See the shell made of WW, containing a mixture of spiropyran SP- supporting information for the detailed amount of the X in the color developing mixture YY; SP-X/YY@WW@KK, components used in each film. capsules in films of polymer KK. (1) Bertleson, R. C.; Maeda, S.; Van Gemert, B. Organic pho- Supporting Information. The Supporting Information tochromic and thermochromic compounds, Vol. 1; Crano, include the list of materials used and respective providers, J. C.; Guglielmetti, R. J. Eds.; Plenum: New York, 1999. scheme of encapsulation process, chemical structures of (2) Irie, M. Photochromism: Memories and Switches- used spiropyrans, procedures for the syntheses of the cap- Introduction Chem. Rev. 2000, 100, 1683–1684. sules, preparation of the films, descriptions of the charac- (3) Corns, S. N.; Partington, S. M.; Towns, A. D. Industrial 1 terization techniques, H-NMR spectra of dissolved cap- organic photochromic dyes Color. Technol. 2009, 125, sules, DSC curves of capsules, kinetic curves for re- 249–261. colouration processes, further absorption spectra and digi- (4) Barachevsky, V. A. Advances in photonics of organic pho- tal images of bulk solutions, capsules and films, tables tochromism J. Photochem. Photobiol., A 2018, 354, 61–69. summarizing the absorption maxima and kinetic rates of (5) Aiken, S.; Edgar, R. J. L.; Gabbutt, C.; Heron, B. M. Nega- the investigated systems. 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