Solid Materials with Tunable Reverse Photochromism

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 reverse photochromic properties (i.e. colours and colouration rates). To finely control the concentration ratio of these species and, as such, tailor the optical performance of the photochromes, we exploited their differential interaction with surrounding 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 rewritable multicoloured devices.

Highly transparent photochromic films with a tunable and fast solution-like response

“…This has been accomplished by i) functionalizing the photochromic molecules with flexible tails, [46][47][48][49][50] ii) embedding the dyes in different media such as low glass transition and/or plasticized polymers, 51,52 hybrid organic/inorganic materials, [53][54][55] hydrogels, 56 or organogels, 57 and iii) encapsulating the molecules in the cages of hybrid mesostructured (nano)materials, [58][59][60][61][62] metal-organic frameworks, 63,64 and supramolecular assemblies. [65][66][67] The encapsulation was also explored by our group, which reported tunable photochromic properties (fast or reverse photochromism) in solid materials by embedding core-shell capsules into polymeric films, 20,68,69 a strategy that was also successfully extended to the fabrication of liquid-like thermochromic 70 and white-emitting 71 materials. However, despite all these advances, the implementation of these materials in commercial goods still faces real challenges as most of them are: i) too specific requiring determined cage dimensions, capsules with suitably designed shell material, plasticizers and/or chemically modified environments; 18,72 ii) difficult to scale up, as multiple steps and time-consuming synthetic procedures are required for the dyes functionalization, micro/nanocontainers fabrication and/or the film formation; and more importantly iii) opaque, since many of these approaches lead to non-optically transparent films.…”

Shape Memory Polyurethane Microcapsules with Active Deformation

“…The microcapsules were stretched by embedding them in a polymer film, which is elongated while heated above its glass-transition temperature ( T g ). PVA was chosen as the polymer matrix because (i) it is a thermoplastic film-forming polymer which can trap microcapsules using the casting method; − (ii) it can be stretched when heated above its T g ; (iii) it has a T g around 70 °C (Figure S6), that is, it can be heated at higher temperature than the T m of PU, allowing to reach the melting point of the PU shell polymer and therefore the thermally induced capsule deformation upon film stretching; and (iv) it is water-soluble, which means it allows to recover and characterize the deformed microcapsules upon film dissolution. The microcapsules were dispersed into the PVA solution, which was then cast onto a Petri dish.…”

Tunable Thermofluorochromic Sensors Based on Conjugated Polymers

“…On the other hand, the direct mixing of MEH/PCM solutions with matrix polymers would alter the final composition and, as a result, their emission switching properties. For all this, we then structured the MEH/PCM mixtures into solid lipid microparticles (SLMs) that can be easily dispersed within inert polymer films, following a similar approach already used in our group with core-shell microcapsules, [10,[87][88][89] liquid nanodroplets, [90] and solid lipid nanoparticles [91] to preserve the optical properties of macroscopic solutions in solid composites.…”