Reversible mechanochromism and sunlight-induced photoisomerization in a multi-stimuli responsive phenothiazine-oxadiazoborole system

Combining mechanochromic and photoisomerization properties within a single, synthetically accessible molecular framework represents a compelling strategy for advancing the design of multifunctional stimuli-responsive materials. Herein, we present a D-A molecule (PTZ-BOAHY), integrating a phenothiazine (PTZ) donor and an oxadiazoborole (BOAHY) acceptor unit. The E/Z isomeric forms of PTZ-BOAHY were separated using column chromatography and were characterized using NMR, HRMS, single crystal XRD and various spectroscopic techniques. The E and Z isomers were fluorescent in both solid and solution states and exhibit a large Stokes shift, pronounced solvatochromism and aggregation-induced quenching. The Z isomer shows reversible chromic responses to grinding with a distinct red-shift of 52 nm in emission wavelength. Photoisomerization of the Z to the E isomer occurs in solution upon sunlight exposure with a visible colour change from red to green. Cyclic voltammetry measurements and density functional theory (DFT) calculations were employed to probe the electronic properties of the molecules. Additionally, thermal measurements showed that the isomers exhibit good thermal stability up to 250 o C. • A new multi-stimuli responsive material was synthesized that exhibit reversible mechanochromism and photoisomerization • Two geometric isomers were separable using column chromatography. • Both isomers show strong fluorescence, with a large Stokes shift, distinct solvatochromism, and aggregation-induced quenching. • The Z isomer displays a reversible mechanofluorochromic response. • Upon exposure to sunlight, Z -isomer undergoes photoisomerization to the E form, along with a visible color change from red to green.