Structures and Reactivity of Salts of <i>E</i> -4-Pyridylvinyl Benzothiazole: A Photochemical and Photophysical Study

Abstract

Stimuli-responsive crystals have gained significant attention as a new class of next-generation smart materials for flexible electronics, actuators, and soft robotics. However, tuning both their ground and excited states remains a challenge. Here, we introduce a series of novel pyridinium salts [(4-PVBTH)(NO3)] (1), [(4-PVBTH)(BF4)·H2O] (2), and [(4-PVBTH)(CF3CO2)] (3) derived from E-4-pyridylvinyl benzothiazole (4-PVBT). Different counteranions form different types of hydrogen bonding and other intermolecular interactions as revealed by SCXRD and Hirshfeld surface analysis. Crystals of 3 exhibited a photosalient behavior, caused by a [2 + 2] solid-state photocycloaddition reaction in a head-to-tail (HT) manner under UV light, resulting in the formation of the rctt-isomer of the dimer, 1,3-bis(benzothiazole)-2,4-bis(4’-pyridyl) cyclobutane (PBTCB), for the first time. N-Protonation led to altered FMOs facilitating intramolecular charge transfer (ICT), resulting in large Stokes shifts and bathochromic shifts in emission. Experimental data were corroborated by density functional theory (DFT) calculations. Our work emphasizes the effect of N-quaternization, by fusing a crystal engineering approach to photochemistry, on broadening the scope of accessing novel functional organic molecular materials.