Azothiophene-based molecular switches: influence of substituent position and solvent environment on photophysical behavior

Stimuli-responsive materials have attracted much attention in the design of 'smart' materials that can undergo reversible changes in their physical and/or chemical characteristics upon exposure to an external stimulus such as light. Notable examples include azobenzenes, spiropyrans, dithienylethenes and stilbenes. We have previously shown that azothiophene esters also undergo efficient <i>E</i> ↔ <i>Z</i> photoisomerization under both ultraviolet and visible light sources. In this contribution a series of five novel molecular photoswitches, comprising two 3,3'-disubstituted azothiophenes and three hemi-azothiophenes was synthesized and characterized. The effect of substitution position and aromatic ring bulk on photoswitching behavior was explored. Photophysical performance is rationalized through X-ray crystallography, solvent-parameter modelling, and tautomerism studies, providing comprehensive insight into how molecular structure and environment govern the switching efficiency of azothiophene-based dyes. Increasing conjugation through bulkier aromatic substitution leads to a bathochromic shift in the absorption maximum (<i>λ</i>_max), enabling visible-light-activated isomerization. We show that these azothiophenes have varying photoswitch ability, with diacid derivatives exhibiting superior solvatochromic behavior including an example of a clear pH-dependent color change, suggesting potential application for these molecules as pH sensors or indicators. The data presented suggests a strategy for the rational design of tunable azothiophene photoswitches, with potential application in photoresponsive materials and optical sensing.