Near-infrared II photochromic behavior triggered by green light in an in situ protonated dithienylethene functionalized by quinoxalinone moieties†

Exploiting the near-infrared (NIR) photochromic dithienylethenes (DTEs) triggered by visible light is urgently needed for various biological scenarios. However, all the NIR photochromic DTEs reported so far are located in the first NIR window (NIR-I, 700–900 nm), which usually shows shallower penetration in biological tissues due to autofluorescence and photon scattering compared to NIR light in the second window (NIR-II, 1000–1700 nm). Herein, we present a novel quinoxalinone-functionalized DTE derivative (QDTE) with acceptor (A)–DTE (D)–acceptor (A) structural features, in which electron-withdrawing quinoxalinone groups ensure visible light-driven NIR I photochromism. Besides, the facile protonation of the quinoxalinone moieties favors the formation of the more electron-deficient A′–D–A′-type DTE (QDTE-2H, where A′ is a stronger electron-withdrawing unit) for a unique NIR II photochromism by reducing the HOMO–LUMO energy gap of a closed isomer after protonation. As expected, the resulting QDTE displays a blue light-controlled NIR I photochromic performance in various solvents. Furthermore, an unprecedented green light-triggered NIR II photochromism for the in situ protonated QDTE-2H is successfully implemented in CHCl₃ and toluene in the presence of trifluoroacetic acid (TFA), representing the first case of NIR II photochromic DTE. By virtue of these properties, QDTE has been successfully applied in dual information encryption, demonstrating its versatility in functional materials.