Solid materials with tunable reverse photochromism

Herein, we report 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., nonprotonated and protonated forms) displaying distinct reverse photochromic properties (i.e., colors and coloration 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., nonvolatile 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 behavior 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 multicolored devices.
This work was supported by projects MAT2015-70615-R and CTQ2015-65439-R from MINECO/FEDER and 2017SGR00465 from Generalitat de Catalunya. ICN2 acknowledges support from the Severo Ochoa (MINECO, Grant No. SEV-2017-0706) and the CERCA programmes (Generalitat de Catalunya).