This is the accepted version of the article: Torres-Pierna H., Ruiz-Molina D., Roscini C.. Highly transparent photochromic films with a tunable and fast solution-like response. Materials Horizons, (2020). 7. : 2749 - . 10.1039/d0mh01073a. Available at: https://dx.doi.org/10.1039/d0mh01073a PleaseMaterials Horizons do not adjust margins Page 2 of 11 View Article Online DOI: 10.1039/D0MH01073A ARTICLE Highly Transparent Photochromic Films with Tunable and Fast Solution-like Response Published on 14 August 2020. Downloaded by Universitat Autonoma de Barcelona on 8/14/2020 1:41:05 PM. Materials Horizons Accepted Manuscript Received 00th January 20xx, Héctor Torres-Pierna,a,b Daniel Ruiz-Molina,b Claudio Roscinib,* Accepted 00th January 20xx The increasing interest towards photochromic films and their practical applications are driving researchers to the continuous DOI: 10.1039/x0xx00000x design and synthesis of novel organic photochromic dyes with optimized performances in polymeric matrices. However, whereas their photochromic properties could be readily rationalized in organic liquid solutions, these could not be directly extrapolated to polymers as their performance changes unpredictably upon integration into a solid matrix. This leads to a time-consuming synthetic re-tuning of the dye chemical structure and/or the polymeric medium. To avoid this, herein we report an efficient, straightforward and universal strategy to embed commercial T-Type organic photochromic dyes of different nature in a polymeric material without compromising their optimum solution absorption and isomerization kinetics. Our approach is based on trapping emulsified nanodroplets of a hydrophobic solution enclosing the dye into a hydrophilic polymeric matrix. The material is prepared through one single process using commercially available materials, without further modification of the components (the dye and the polymer matrix) nor requiring previous encapsulation steps. The films, which manifest true solution liquid-like and finely tunable photochromic behavior, are also highly transparent, recyclable, scalable, and show enhanced fatigue resistance, making them highly suitable for different smart glass applications. In liquid solutions, the photochromic response (e.g. color type Introduction and intensity of the activated form, fading rate, etc.) can be easily correlated to the dye chemical structure, the solvent The reversible light-induced color change of photochromic polarity and viscosity, allowing for straightforward and often materials is of great interest for the fabrication of light- predictable fine tuning of the optical properties.14–16 responsive devices, such as dynamic optical filters, rewritable Unfortunately, this does not apply to solid solutions, i.e. displays, optical memories, smart biomaterials, etc.1–10 Among photochromic polymeric films, which are far more relevant for the different possibilities, those materials based on organic practical applications. In these, the photochromism is much photochromic dyes (mainly spirooxazine and naphthopyran more difficult to control, with kinetic responses dramatically derivatives)7,11 are increasingly attracting the attention of the slowed down or even inhibited.6,13,17 This is due to the steric scientific and industrial sectors due to their broad commercial hindrance that the solid matrix opposes to the conformational availability, relative good fatigue resistance and easy tunability changes of the photochromic molecules involved during the of the optical properties.12,13 Specifically, most relevant photoinduced and thermal back isomerization (also known as industrial applications are typically based on T-type organic matrix effect).14,18–20 This originates that the response of these photochromic dyes: under UV radiation, these dyes reversibly dyes in commercial products, such as ophthalmic lenses, is far interconvert from a thermodynamically stable colorless closed from being the best possible or prevents spreading out of these form to a more energetic photoactivated colored merocyanine dyes to many other sectors. (MC)-like isomer, which reverts back to the initial ring-closed state in the dark through spontaneous thermal relaxation.11 To accomplish as much as possible matrix-independent This photochromism allows to obtain self-responding photochromic responses, researchers have explored two main photoprotective coatings, which use is currently limited to strategies over the years. The first approach consists in the photochromatic ophthalmic lenses.12,13 synthesis of new photochromic molecules with minimal conformational changes along the photochromic interconversion to minimize the matrix effect. This has been mainly achieved with imidazole-based molecules,21–33 prepared a. Futurechromes S. L., Carrer Còrsega 516, 08025 Barcelona, Spain. b. CatalanInstitute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, by Abe et al., or preventing the formation of the naphthopyrans Campus UAB, Bellaterra, 08193 Barcelona, Spain. (transoid-trans)34–43/spirooxazines (trans-merocyanines)44,45 E-mail: claudio.roscini@icn2.cat long-lived isomeric open form, responsible for the slowdown of Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x the ring-closure reaction (oxazines, substituted Please do not adjust margins Page 3 of 11 PleaseMaterials do not adjust margins Horizons ARTICLE Journal Name naphthopyrans). The second strategy is to facilitate the dye applications herein we designed and developed a novel strategy View Article Online isomerization by providing softer (less steric hindrance) or void based on a polymeric matrix entrapping oil nanodroplets, DOI: which 10.1039/D0MH01073A (free-volume) matrix environments. This has been contain the photochromic dye. Whereas the continuous accomplished by i) functionalizing the photochromic molecules polymer matrix provides the mechanical properties to the with flexible tails,46–50 ii) embedding the dyes in different media material, the hydrophobic droplets constitute the soft medium such as low glass transition and/or plasticized polymers,51,52 that allows the dye molecules to interconvert as fast as in a hybrid organic/inorganic materials,53–55 hydrogels,56 or liquid solution. Similar materials design has been successfully organogels,57 and iii) encapsulating the molecules in the cages used to yield functional coatings and films of relevance in of hybrid mesostructured (nano)materials,58–62 metal-organic commercial electronic papers,73 photoprotective packaging,74 frameworks,63,64 and supramolecular assemblies.65–67 The liquid crystal displays for smart windows,75 or scratch-and-sniff Published on 14 August 2020. Downloaded by Universitat Autonoma de Barcelona on 8/14/2020 1:41:05 PM. encapsulation was also explored by our group, which reported coatings,76 inter alia, obtained by trapping microdroplets of Materials Horizons Accepted Manuscript tunable photochromic properties (fast or reverse different materials in polymer matrices. However, in our case, photochromism) in solid materials by embedding core-shell the droplet size has to be reduced up to the nanoscale to capsules into polymeric films,20,68,69 a strategy that was also minimize the light scattering and guarantee the high successfully extended to the fabrication of liquid-like transparency of the film, which is a key requirement in smart thermochromic70 and white-emitting71 materials. However, glasses.77 despite all these advances, the implementation of these The advantages of this strategy are remarkable. It materials in commercial goods still faces real challenges as most represents a straightforward and universal solution for the of them are: i) too specific requiring determined cage preparation of highly transparent solid films with tunable and dimensions, capsules with suitably designed shell material, fast liquid solution-like photochromic response. Films can be plasticizers and/or chemically modified environments;18,72 ii) prepared in one-pot step through a reaction-free and scalable difficult to scale up, as multiple steps and time-consuming processes, using readily available starting materials. On top of synthetic procedures are required for the dyes that, the photochromic performance can be fine-tuned by functionalization, micro/nanocontainers fabrication and/or the selecting the initial components of the mixture, without film formation; and more importantly iii) opaque, since many of requiring chemical functionalization or modification neither of these approaches lead to non-optically transparent films. the photochromes nor the embedding matrix. Therefore, there is still an urgent need to develop photochromic solid films that can be easily handled and incorporated in commercial products that require tunable photochromic Results and discussion responses and high transparency. Proof of concept of transparent fast-responsive photochromic films To overcome these important issues that are limiting the A graphic representation of the experimental approach broadening of solid photochromic materials to various followed for the preparation of the films is shown in Scheme 1. Scheme 1. Nanoemulsification and nanodroplets entrapment: schematic representation of (top) the o/w nanoemulsion formation and (bottom) nanodroplets entrapment (nanoemulsion entrapment) in photochromic films upon casting the emulsion on a surface and solvent evaporation. A schematic representation of the oil nanodroplets containing the photochrome molecules within the polymeric matrix is shown in the zoomed areas. 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins PleaseMaterials Horizons do not adjust margins Page 4 of 11 View Article Online DOI: 10.1039/D0MH01073A ARTICLE First, an oil-in-water (o/w) nanoemulsion of hydrophobic casting of the resulting o/w nanoemulsion onto a polystyrene nanodroplets (< 100 nm) containing the photochromic dye is Petri plate and posterior evaporation of the water gave a obtained in the presence of a hydrophilic polymer that acts as flexible and transparent free-standing PhI@M812@PVA film of Published on 14 August 2020. Downloaded by Universitat Autonoma de Barcelona on 8/14/2020 1:41:05 PM. Materials Horizons Accepted Manuscript both droplet stabilizer and film-forming material. Afterwards,  150 m thickness (Figures 1c-d). SEM analysis of a cross the formation of the nanostructured film takes place upon section of a cryo-fractured film showed the presence of porous water evaporation, resulting in the physical trapping of the structure (Figure 1d). Worth to mention, similar nanoemulsion photochromic nanodroplets within the matrix as a separated entrapment films were obtained using the higher molecular phase without droplet coalescence, which guarantees the weight PVA 40-88 (PhI@M812@PVA-40), in which holes (of formation of a highly transparent film. diameter 50-200 nm), that correspond to the cavities left over by the oil droplets, can be differentiated (Figures S2a-b). For the first proof-of-concept the following three Contrariwise, nanodroplets-free plain PVA 4-88 and PVA 40-88 components were selected: as the dye we used the films did not show these nanocavities, neither when the initial commercially available T-type spirooxazine 1,3-dihydro-1,3,3- water solution was subjected to sonication, which might be trimethylspiro[2H-indole-2,3’-[3H]naphth[2,1-b][1,4]oxazine] argued to produce air nanobubbles (Figure S2c). (also known as Photorome I, PhI, see Supporting Information). Differential scanning calorimetry (DSC) of the In solution and under UV irradiation, PhI photoisomerizes from PhI@M812@PVA films (Figure S3a) showed an exothermic the spiro (SP) colorless form (max = 322 nm in toluene) to the peak at -31 oC, associated to the solidification of M812 oil,81 and blue-colored MC-like isomer (max = 593 nm in toluene), while a glass transition at Tg = 71.6 oC, only a few degrees lower than fades completely in less than 10 s back to the colorless isomer that of the plain PVA 4-88 film (Tg = 73.6 oC) studied under the in the dark (Scheme S1).20 The oil of choice was Miglyol® 812 same experimental conditions (Figure S3b). A further increase (M812), a coconut oil derivative (triglicerydes of capric/caprylic of the temperature also showed heat-flow variations in the acids) that is colorless (to prevent color distortions of the range 140 oC and 200 oC, related to the melting of the PVA.82 photoactivated state), lipophilic (to permit the o/w emulsification and good separation from a hydrophilic polymer), of high boiling point (to guarantee long term stability of the film) and dissolves PhI without compromising its fast optical response.20 Finally, a water solution of polyvinyl alcohol (PVA 4-88, i.e. low hydrolysis degree, low molecular weights) was used as aqueous phase. PVA is a good o/w emulsion stabilizer and has film-forming properties, both characteristics required for the formation of the polymer matrix entrapping the emulsified oil nanodroplets. It also exhibits high transparency in the visible and lowest-energy UV spectral regions, avoiding any interference with the photochromism of the dyes. Finally, PVA is well-known to have a low intrinsic oxygen permeability,78,79 which should improve the dye fatigue resistance under irradiation (vide infra). After mixing the aqueous PVA solution (20 wt.%) with the dye-containing oil mixture, ultrasounds (US) were used to obtain the o/w nanoemulsion (see Supporting Information for more details). The opacity of the emulsion gradually decreases with time, reaching a significant transparency after 20 min and a bluish coloration distinctive of the Tyndall effect (Figure 1a), produced by the dispersed nanodroplets that only scatter the Figure 1. Nanoemulsion and photochromic film: a) images of a vial containing the o/w shortest wavelengths (blue frequencies) of the incident visible nanoemulsion at different sonication times; b) Z-average size for each one of the light.80 This qualitative observation was corroborated by sonication times; c) image of the o/w nanoemulsion deposited on a Petri plate (left dynamic light scattering (DLS) analysis, which showed an image) and of the resulting film obtained after the water evaporation (right); d) cross- asymptotic decrease of the average droplets diameter down to section SEM image of the resulting PhI@M812@PVA self-standing film and a zoom of approximately 90 nm with time (Figure 1b and S1). Finally, the section showing the porous structure. Please do not adjust margins Page 5 of 11 PleaseMaterials do not adjust margins Horizons ARTICLE Journal Name The comparable oil solidification and the specific PVA transition Interestingly, the normalized spectra of the photoinduced View Article Online temperatures found for the film and the isolated components, PhI open form in the film matches quiteDOI: well10.1039/D0MH01073A with the one in a together with the SEM image shown in Figure 1d, seem to bulk M812 solution (λmaxPhI@M812 = 601 nm, Figure S5). This confirm that the polymer matrix and the oil nanodroplets underscores two important facts. First, the methodology used remain mostly as two separated phases. for the film preparation preserves the integrity of the dye. Thermogravimetric analysis (TGA) of both plain PVA and Secondly, the lack of a significant solvatochromic spectral shift PhI@M812@PVA films showed the presence of ~ 6 wt.% of of the PhI@M812@PVA (λmaxPhI@M812@PVA = 603 nm) film respect water (mostly removed before reaching 200 oC), that possibly to the initial M812 solution of PhI (λmaxPhI@M812 = 601 nm), remained trapped during the film formation due to the indicates that there is a negligible diffusion/mixing of the dye hygroscopic character of the PVA (also confirmed by the DSC and/or M812 to the hydrophilic PVA matrix along the film Published on 14 August 2020. Downloaded by Universitat Autonoma de Barcelona on 8/14/2020 1:41:05 PM. exothermic peak at around 0 oC, Figure S3a), and thermal Materials Horizons Accepted Manuscript formation. degradation of the film components only above 350 oC (Figure To investigate the discoloration kinetics in the dark, time- S3c). resolved UV-vis measurements were carried out at room In the absence of irradiation, the colorless PhI@M812@PVA temperature and at the absorption maximum of the MC isomer film exhibits high transmittance values in the high energy visible spectral range (%T400-450 nm ~ 89% respect to air reference and (λmaxMC = 603 nm) after reaching very quickly the comparable to PVA plain films, Figures S4a-S4b). This indicates photostationary state (< 5 s, Figure S6a). The film spontaneously that the trapped nanodroplets are small enough to induce only discolored through a first-order monoexponential decay a negligible light scattering, even for higher frequencies. In the function (kPhI@M812@PVA = 0.56 s-1, R2 = 0.995) with characteristic dark, only an absorption band around λmax = 319 nm, typical of fading kinetics of homogeneous liquid solutions (kPhI@M812 = 0.56 the PhI spiro form can be found (Figure S4c). Upon UV s-1, Figures 2b and S6b-c). The matching of normalized fading irradiation (λexc = 365 nm), the film interconverts very fast kinetics of the PhI@M812@PVA film and of the bulk M812 (within a few seconds) to blue, associated to the appearance of solution, provides a further evidence of the negligible diffusion the absorption band at λmax = 603 nm (Absmax = 0.06), of the dye from the oil phase to the PVA matrix. The film characteristic of the open MC form (Figures 2a and S4c). recovered the colorless state in less than 10 seconds (Figure 2c and the Video S1) and the measured half (t1/2) and three- quarters (t3/4) fading times were just 1.4 and 2.7 s, respectively. Reproducible photochromic properties were obtained when nanoemulsion-based films were prepared with other hydrophilic matrices of different molecular weight (i.e. PVA 40- 88) or nature (i.e. hydroxylethyl cellulose, HEC) respect to PVA 4-88 polymer. Remarkably, both PhI@M812@PVA-40 and PhI@M812@HEC films showed the same photoactivated species (λmax  600 nm, Figures S7a-b) as PhI@M812@PVA and, most importantly, very fast fading responses (t3/4PhI@M812@PVA-40 = 2.1 s; t3/4PhI@M812@HEC = 3.1 s, Figure 7c), indicating that this strategy guarantees the photochromic dye to remain dissolved in the oil phase and, thus, the liquid-like response of the film. However, PVA-based films (i.e PhI@M812@PVA and PhI@M812@PVA-40) were much more transparent than that PhI@M812@HEC film, which suggests that PVA, regardless the molecular weight, is a more suitable polymer for the stabilization of the oil nanodroplets during the nanoemulsion formation and/or the film drying and minimize the scattering associated to the oil-droplets. Homogenous PVA-based polymer-dispersed photochromic organogels had been reported in the past,57 but the combination with such a fast liquid solution-like discoloration process and high transparency Figure 2. Photochromic performances of the films: a) absorption spectra of the (%T ~ 90%, in the non-activated state) is unprecedented. PhI@M812@PVA film before and after UV irradiation and image of a film being For comparison purposes, a nanoemulsion-free film was irradiated (inset); b) fading kinetics of PhI@M812@PVA, PhI@PMMA, PhI@PC, prepared by directly dispersing PhI dye in the PVA matrix. Due PhI@PVA films and PhI@M812 liquid solution, after reaching the photostationary state; to the low solubility of the dye in the hydrophilic polymer, the c) snapshots of the fading of PhI@M812@PVA and PhI@PC films at different periods of time as indicated in the figure, where the difference in the discoloration kinetics between obtained PhI@PVA film was slightly opaque and presented both samples is clearly reflected; d) coloration/fading cycles of PhI@M812@PVA film; e) higher scattering at shorter wavelengths (Figure S8). More absorption variation of the photoactivated PhI@M812@PVA and PhI@PC films, before transparent photochromic films of poly(methyl methacrylate) and after 20 h of continuous irradiation with UV (365 nm, 2.5 mW cm-2). 4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins PleaseMaterials Horizons do not adjust margins Page 6 of 11 Journal Name ARTICLE (PMMA) and poly(bisphenol A carbonate) (PC), of relevance for All these films developed intense coloration under ambient View Article Online several applications, were prepared by directly embedding the solar irradiation (Absmax = 0.3 – 1), reproducing the spectral DOI: 10.1039/D0MH01073A dye in these polymers. Despite the photochromic response in features and t1/2 and t3/4 fading times (in the dark) of the dye in these films was preserved (Figure S9), decay studies showed a M812 solution (Figures S13-S14,Table S2). The discrepancies that the fading kinetics of PhI are one or two orders of of t1/2 and t3/4 fading times of the M812 solutions and the magnitude slower (t3/4PVA = 349 s, t3/4PMMA = 65 s, t3/4PC = 127 s) dye@M812@PVA films observed for some dyes were ascribed than that observed for PhI@M812@PVA film (Figures 2b-c, to temperature variations during the measurements and/or to S6b-c, S8, Video S1, and Table S1). Moreover, these interactions (e.g. hydrogen-bonding interaction) that these PhI@PMMA PhI@PC and PhI@PVA films faded following could establish with the matrix at the oil/PVA interface. Nevertheless, in all cases these films discolored much faster multiexponential decay functions due to the matrix Published on 14 August 2020. Downloaded by Universitat Autonoma de Barcelona on 8/14/2020 1:41:05 PM. than the corresponding dye@PMMA films (which show Materials Horizons Accepted Manuscript inhomogeneity and the presence of distinct microenvironments comparable spectral features, Figures S11 and S13), reaching created by the polymer around the embedded dyes. fading times up to two orders of magnitude shorter (Figures Finally, the radiation-induced coloration and spontaneous S14-S15, Videos S2-S3, Table S2). Actually, based on the intrinsic fading cycle of the PhI@M812@PVA film was repeated at least response of the dyes in M812 (t1/2 and t3/4 values), it is possible for 20 times in a row without any detectable dye degradation to make the following dye classification: very fast (PhI, Misty, (Figure 2d). Additional fatigue resistance experiments of the PP), medium (AG, CL, RY, MD) and slow (BR, GR), which is film by continuous irradiation for 20 h at exc = 365 nm showed preserved in the dye@M812@PVA films. This order cannot be no significant variation of the photoactivated states, whereas extrapolated for the PMMA films prepared with the same dyes. remarkable changes were detected in a PhI@PC film under the This is due to fact that the media in which the dyes are same experimental conditions (Figures 2e and S10). Such good embedded are different, being i.e. the M812 oil for the solution fatigue resistance showed by the photochrome in the and the dye@M812@PVA film and a rigid matrix for the PMMA PhI@M812@PVA film, even in the absence of additional film. Moreover, the matrix effect given by the rigid PMMA stabilizers (e. g. UV absorbers, oxygen and radical scavengers),83 matrix and the hindering caused by the polymeric chains on the was correlated to the inertness of the dissolving oil and to the embedded dyes, make the photochromic response much more intrinsic high impermeability of PVA to oxygen,78,79 which is dependent on the volume changes involved in the known to be one of the causes of the photochromic materials interconversion, compared to what happens in the liquid oil degradation.11,84,85 (Table S3). These results suggest that the nanoemulsion entrapment technique is a valid approach to produce transparent photochromic films with liquid-like and predictable Tuning of the spectral and fading kinetics properties. behavior (based on data in liquid solution) with any commercial The versatility and flexibility of our approach was or newly synthesized T-Type photochromic dyes, provided demonstrated by developing novel dye@solvent@PVA films these are soluble in the oil medium. where the dye and the oil used for the emulsification have been Oil modification. Given the solvatochromism and the ring- systematically tuned to control the optical response of the films. closure rate dependency of the open form of these dyes with Dye modification. For these experiments, M812 was the solvent polarity and viscosity,15,87,88 the oil of choice is also maintained as the oil of choice for the nanodroplets, while expected to confer an easy way to fine tune the photochromic different commercially available spirooxazines and properties of the films. To explore this possibility, the naphthopyrans dyes of distinct colors in their photoactivated commercially available PP spironaphthoxazine was dissolved in state, were used. Some of the resulting dye@M812@PVA films various high boiling point oils of different polarity and viscosity presented a residual color related to the intrinsic low (see Table S4 for the full list). When the nanoemulsion absorption properties of the dye in its dark state. All films entrapment process was carried out with these solutions highly colored (or enhanced their coloration) upon irradiation with a transparent PP@oil@PVA films with %T500nm > 85%§ were UV radiation source (λexc = 365 nm, 2.7 mW/cm2), solar obtained, except for the 3-phenyl bicyclohexyl (3PhbCH) and simulator (~ 0.44 W/m2) or ambient sunlight. Films obtained hexylbenzene (HxBz) oils that provided opaque films (Figure from the spirooxazines Palatinate Purple (PP),86 Aqua Green S16, Table S4). Upon UV irradiation, all films turned blue (AG), Claret (CL) or naphthopyrans Berry Red (BR)14 and Rush (Figures S17) with shifts in the absorption maxima (λmax) up to Yellow (RY)12 displayed the instantaneous appearance of well- 18 nm between different oils (Figure S18a and Table S4). Even defined absorption bands with λmax covering different visible more dramatic was the difference in the fading kinetics with t1/2 spectral regions, and therefore different colors (Figures 3a-b and t3/4 values that varied up to 7 times (Figures 3d and Table and S11). Those obtained from Misty Grey (Misty), Midnight S4). Moreover, it was found a relationship between λmax and Grey (MD) and Graphite (GR) yielded neutral grey colors with t1/2/t3/4 values: PP@oil@PVA films absorbing at longer broad absorptions in the whole visible spectral region (Figures wavelengths (in the photoinduced state) were also the ones 3a and 3c)‡ Finally, neutral color was also achieved by with the shortest t1/2 and t3/4 values (fastest to fade), and vice entrapping a mixture of complementary colored dyes in their versa (Figures 3e and S18b). photoactivated state, such as AG and BR (Figures 3c and S12). This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5 Please do not adjust margins Page 7 of 11 PleaseMaterials Horizons do not adjust margins View Article Online DOI: 10.1039/D0MH01073A ARTICLE Published on 14 August 2020. Downloaded by Universitat Autonoma de Barcelona on 8/14/2020 1:41:05 PM. Figure 3. Tunability of the films photochromic response: a) dye@M812@PVA films both in the dark (upper figures) and their photoinduced color state under ambient sunlight irradiation (bottom figures); b-c) absorption spectra of dye@M812@PVA films in their photoinduced color state; d) fading kinetics and e) trend of t3/4 vs max of photoinduced colored PP@oil@PVA films. Dye names abbreviations: RY, rush yellow; BR, berry red; CL, claret; PP, palatinate purple; AG, aqua green; Misty, misty grey; MD, midnight grey; GR, graphite. Materials Horizons Accepted Manuscript Oil names abbreviations: M812, Miglyol® 812; M829, Miglyol® 829; DEP, diethyl phthalate; DOP, dioctly phthalate; DOT, dioctyl terephthalate; DINOP, diisononyl phthalate; SiAR20 and SiAP1000, silicon polyphenyl-methylsiloxane oils (~ 20 mPa.s and ~ 1000 mPa.s, respectively); HD, hexadecane; HxBz, hexylbenzene; 3PhbCH, 3-phenyl-1,1’-bicyclohexyl. This type of correlation is typically reported for liquid Large area films and derived prototypes solutions of T-Types photochromic dyes.14,15,87,88 These results After demonstrating the viability and the versatility of the reveal that by using a fixed dye, a fine tuning of the nanoemulsion entrapment method for the achievement of fast- photochromic properties is accomplished by simply varying the responsive and tunable photochromic films, we aimed to use oil component of the film, without requiring the modification of them to prepare product prototypes. This involved two steps: the matrix rigidity or the use of plasticizers. first the fabrication of photochromic films with suitably large As expected, the photochromic properties resulted to be dimensions and second their implementation in the prototypes. independent on the matrix used: the absorption spectrum and Large area films. Before fabricating films large enough for the fading rate of the film prepared from a M812 solution of PP the prototypes, it was necessary to scale up the preparation of and PVA 40-88 matrix material (PP@M812@PVA-40) overlap the nanoemulsions obtained by sonication ( 10 g). For this we with those of PP@M812@PVA (Figure S19). used a high-pressure-homogenization equipment, where up to Please do not adjust margins PleaseMaterials Horizons do not adjust margins Page 8 of 11 Journal Name ARTICLE 250 g of the mixture is reprocessed several times yielding a different responses are required. Some of theView applications Article Online nanoemulsion with similar droplets size distribution as for that where these new films are expected to have a great impact are DOI: 10.1039/D0MH01073A obtained through ultrasonication (Figure S20). Thanks to the photochromic glasses, helmet face shield and antiglare mirrors, nanoemulsions kinetic stability,85 the resulting mixture turned where both transparency and fast fading response are key out to be stable for several months. This long-term stability features. For the fabrication of photochromic sunglasses (Figure allowed to obtain films with fairly reproducible transparency 5a), the dye@M812@PVA films, prepared with the fast fading (%T450nm > 85%, Figure S21) from nanoemulsions stored for up PP or Misty dyes, were layered between two curved glasses, to three months at room temperature. which were then cut with the appropriate dimension and shape The nanoemulsion mixture resulting from each batch is (see supporting information for more details). For the helmet enough as to allow for the fabrication of at least 30 identical 8.5 face shield (Figure 5b), a neutral grey@M812@PVA film was Published on 14 August 2020. Downloaded by Universitat Autonoma de Barcelona on 8/14/2020 1:41:05 PM. cm diameter photochromic films ( 64 cm2), as those shown in prepared using the Dr. Blade applicator and adhered in the Materials Horizons Accepted Manuscript Figure 3. The same volume can be used to prepare films of much internal surface of the face shield. The mirror was coated with larger surface areas (up to 22 X 40 cm2) and similar a neutral grey@M812@PVA film (Figure 5c), prepared by direct transparencies (%T450nm ~ 85%) through casting or a Dr. Blade casting the nanoemulsion on the top surface, and letting the applicator (Figure S22). water evaporate. The photochromic glasses and the helmet face shield colored under UV sources or solar light radiation and maintained the liquid solution-like fast photochromic response, recovering the initial state, in the dark, in less than 1 min. Noticeably, despite the films for non-flat photochromic glasses required to be thermoconformed, at 60 oC for 1 min, to provide the curved shape of the lenses, the final prototypes retained quite well the optical transparency (%T > 80%), the spectral properties (maxPP@M812@PVA = 600 nm; maxMisty@M812@PVA = 489 and 588 nm) and the fast fading response (t3/4PP@M812@PVA = 4.0 s, t3/4Misty@M812@PVA = 7.5 s) of the initial films (Figure S25, Videos S4). The mirror reduced the reflectance of visible light from 92% to 56% upon exposition to sunlight (Figure S26), suggesting a potential applicability for vehicle external rear-mirrors. Finally, the prototype of a rewritable device (Figure 5d), of interest for commercial applications, was also obtained by laminating a PP@M812@PVA film between two glasses. Upon irradiation of the device with a 405 nm laser pointer or a phone torch light (PP has residual absorption in the visible region) it is Figure 4. Scaled up coatings or self-standing films: images of a) glass coated with a possible to produce instant and high contrast handwritings or GR@M812@PVA film prepared through nanoemulsion entrapment process, in the dark drawings, which very quickly erase once the irradiation is and under ambient sunlight irradiation, b) pealed out flexible 15x15 cm2 stopped (Video S5). CL@M812@PVA photochromic film and c) the laminated photochromic glass using the CL@M812@PVA film, under ambient sunlight irradiation. Another area of potential relevance for these photochromic films is construction. Windows glasses coated with smart optical These large area films (90-200 m in thickness) retain the filters that guarantee a dynamic filtering of the visible light characteristic photochromic properties and can be fabricated as during very sunny days, are of interest in the building industry coating, directly adhered to the substrate (Figure 4a) or pealed for improving comfort and reducing the amount of heat out yielding flexible and self-standing films (Figure 4b) that entering in the building. Smart windows prototypes were could be used in laminates between glasses (Figure 4c) or other prepared by layering the dye@M812@PVA films between substrates. glasses and installing the laminates in a house model. The Moreover, these films revealed to be completely recyclable. windows colored under sunlight and recovered their colorless Indeed, given the water solubility of PVA, the film (e.g. state in the dark (Figure 6a). To investigate their effectiveness Misty@M812@PVA film) could be re-dissolved in warm water, in contrasting the heat increase during sunlight exposition, upon stirring, and reformed several times, through casting. The photochromic windows were prepared by coating the glasses recycling process could be successfully carried out with or with the GR@M812@PVA film, through casting method and without introducing the homogenization step (e.g. sonication) letting the water to evaporate. The same film was applied onto before the re-casting on the Petri plate. In each case it was a standard glass slide and a commercial low-emitting insulating possible to re-obtain, after the first and the second recycle, glass unit (low-E IGU, Figure 6b and S27a), generally used to quite transparent films, reproducible spectral properties increase the energy saving in buildings. A solar simulator (1 (%T450nm, dark ~ 80%; %T450nm, UV ~ 63%) and fading response (t1/2 W/m2) was used to irradiate for 30 min, a holed cardboard box  3 s and t3/4  8 s, Figures S23-S24). through the coated photochromic glasses (simulating a Prototypes. Finally, we aimed to prepare various prototypes window), layered on top of the hole (see Supporting of products where transparent photochromic materials of Information for more details). This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7 Please do not adjust margins Page 9 of 11 PleaseMaterials do not adjust margins Horizons ARTICLE Journal Name View Article Online DOI: 10.1039/D0MH01073A a) Published on 14 August 2020. Downloaded by Universitat Autonoma de Barcelona on 8/14/2020 1:41:05 PM. Materials Horizons Accepted Manuscript b) sunlight indoor (0 s) indoor indoor indoor (30 s) c) d) Figure 5. Prototypes with fast fading response: a) two different sets of glasses obtained from thermoconformed photochromic dye@M812@PVA films, showing coloration under irradiation; b) fast discoloration kinetics (˂ 30 s) of a helmet face shield coated with our film; c) mirror coated with the dye@M812@PVA, film evidencing the different reflectance of visible light, in the dark and under UV irradiation or solar light; d) snapshot of the handwriting on the rewritable device prototype. a) The inside temperature increase, respect to the outside constant room temperature, was monitored with a hν thermometer (Figure S27). Irradiations through uncoated glasses were also carried out as control experiments. dark Interestingly, when the coated standard glass was used as window, it darkened and the temperature increment (T) inside the box upon irradiation reduced from 10 to 8 oC, respect to the same window without the photochromic film. The temperature b) increment was further reduced (to only 6.5 oC) when a low-E 10 IGU glass coated with GR@M812@PVA film was used (the 8 uncoated low-E IGU reduced the T to 8 oC), suggesting that these films could be also adapted to energy-saving applications  (oC) 6 4 (Figure 6b). Uncoated glass 2 Coated glass Uncoated Low-E IGU glass 0 0 10 Coated Low-E IGU glass 20 30 40 50 60 Conclusions Time (min) A novel simple, one-pot and reaction-free protocol, using Figure 6. Prototypes for smart windows application: a) house model containing readily available materials, has been shown effective for the laminated windows prepared with different dye@M812@PVA photochromic films, in the dark and under ambient solar irradiation; b) low-E IGU glass coated with fabrication of photochromic films with liquid solution-like GR@M812@PVA film under sunlight irradiation and temperature increment (T) caused response. This disruptive, but easy methodology, relies on the by solar simulator irradiation (1 W/m2) through glasses coated or not with the entrapment of oil nanodroplets containing the photochromic GR@M812@PVA film. dye within a polymer, without additional chemical modifications nor the use of plasticizers. This approach overcomes most of the challenges that are preventing 8 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins PleaseMaterials Horizons do not adjust margins Page 10 of 11 Journal Name ARTICLE photochromic materials to be used in different applications and 4 J. Zhang, J. Wang and H. Tian, Mater. Horizons, 2013, 1, 169. View Article Online that were only partially solved by the several previously 5 J. Boelke and S. Hecht, Adv. Opt. Mater., DOI:2019, 1900404. 10.1039/D0MH01073A 6 H. Nie, J. L. Self, A. S. Kuenstler, R. C. Hayward and J. Read de developed strategies. The resulting films exhibit high optical Alaniz, Adv. Opt. Mater., 2019, 1900224. transparency, fast fading kinetics, enhanced fatigue resistance 7 V. A. Barachevsky, J. Photochem. Photobiol. A Chem., 2018, and good colorability. Moreover, the photochromic response of 354, 61–69. the final films can be systematically fine-tuned by changing the 8 J. Li, H. K. Bisoyi, S. Lin, J. Guo and Q. Li, Angew. Chemie, 2019, dye and/or the oil of the droplets. Compared to previously 131, 16198–16202. 9 J. Li, H. K. Bisoyi, J. Tian, J. Guo and Q. Li, Adv. Mater., 2019, reported core-shell capsules strategies, the nanoemulsion 31, 1807751. entrapment approach also prescinds of the shell material, 10 S. Lin, H. Sun, J. Qiao, X. Ding and J. Guo, Adv. Opt. Mater., allowing the embedment of a larger volume of the dye solution. 2020, 8, 2000107. 11 J. C. Crano and R. J. Guglielmetti, Organic Photochromic and Published on 14 August 2020. Downloaded by Universitat Autonoma de Barcelona on 8/14/2020 1:41:05 PM. 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