Photochromic Properties of Thiophene and Thiazole-Substituted Bicyclic Aziridine in Solution and Solid State

Bicyclic aziridine (1,3-diazabicyclo[3.1.0]-hex-3-enes) has derivatives (DABH) known for their photochromic properties. A photochromic compound undergoes reversible isomerization between two states upon light irradiation. In the transition from the closed-ring to open-ring form, formal charges are generated in the more conjugated open-ring state. Thiophene and thiazole, known for their electron-rich nature and high charge mobility, were conjugated to a DABH substituent. We report the synthesis of DABH conjugated to thiophene (Th-DABH) and thiazole (Tz- DABH), both of which exhibit photochromic behaviors in the solution and the solid state. Optical absorbance measurements revealed a redshift in the maximum absorption wavelength λ_max from 437 to 440 nm in solution to 618 to 632 nm in the solid state. Notably, the photogenerated state demonstrated significantly extended lifetimes in the solid state, over 2 and 3 orders of magnitude longer than in the methanol solution (Th-DABH 176 times, Tz-DABH 2750 times). Both compounds exhibited reversible switching over multiple cycles in the solid state. Structural analysis via single-crystal X-ray diffraction and ab initio calculations provided insights into the observed photochromic behavior. These findings suggest that integrating DABH-based photochromic materials into optoelectronic devices or organic field-effect transistors could enable stimuli-responsive materials whose electronic properties can be modulated by an external light source.