Aza-aromatic spiropyrans: An efficient strategy to achieve rapid solid state photochromic response by the synergistic effect of enhanced π-electron delocalization ability and free volume

Abstract

Spiropyran is a well-established photochromic material. However, its intrinsic structural limitations render it challenging to exhibit ideal photochromic properties in its pure solid state. Typically, the photochromic properties in its solid state can only be achieved by introducing large steric hindrance groups, which may impede their further development. In this article, we proposed a novel design strategy by embedding several electron-deficient aza-aromatic rings to construct serial donor-acceptor (D-A) conjugated spiropyran derivatives (SP-3Py, SP-Py, and SP-Md). This approach not only provides sufficient free space for effective photoisomerization in the solid state but also effectively improves their π-electron delocalization ability via the electron-deficient nitrogen heterocyclic properties, which can significantly enhance their photochromic rate. The three aza-aromatic spiropyran derivatives exhibited rapid solid-state photoresponsivity (5–10 s) and excellent photochromic performance through the above synergistic strategies. Moreover, the D-A building block of the aza-aromatic spiropyrans can markedly tune their color-switching range. Following irradiation, all the aza-aromatic spiropyrans exhibited a blue color, potentially expanding their application fields. Our investigation indicates that introducing aza-aromatic rings to construct D-A-type spiropyrans can significantly enhance their solid-state photochromic performance, even without strong electron-deficient moieties or large steric hindrance groups, providing a concise and efficient strategy for the design of high-performance solid-state photochromic spiropyran derivatives.