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 (<b>DABH</b>) 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 <b>DABH</b> substituent. We report the synthesis of <b>DABH</b> conjugated to thiophene (<b>Th-DABH</b>) and thiazole (<b>Tz- DABH</b>), 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 (<b>Th-DABH</b> 176 times, <b>Tz-DABH</b> 2750 times). Both compounds exhibited reversible switching over multiple cycles in the solid state. Structural analysis via single-crystal X-ray diffraction and <i>ab initio</i> calculations provided insights into the observed photochromic behavior. These findings suggest that integrating <b>DABH</b>-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.