Acidic pH Modulated Photoswitching of Sulfur-bridged Seven-membered Cyclic Azopyridines

Azoarene molecular photoswitches with bistability are a family of widely employed structure-tuning units for photopharmacology and smart material construction. Notably, medium-ring azobenzenes, especially seven-membered dibenzo[b,f][1,4,5]thiadiazepines (DBTD), are characterized as fast-responsive T-type molecular photoswitches with particular features for light-energy conversion to ring-strain energy. The burgeon of azoheteroarenes with enhanced bistability and solubility have considerably broadened the horizon of their utilization. Herein, we present a novel class of seven-membered cyclic azoheteroarenes, benzo[b]pyrido[f][1,4,5]thiadiazepines (BPTD) and dipyrido[2,3-b:3',2'-f][1,4,5]thiadiazepine (DPTD). The integration of pyrido-heteroarenes in BPTD and DPTD enables pH-modulated T-type photoswitching performance spanning from pH = -0.33 to 7.0, distinguishing them from DBTD. Importantly, benzo[b]pyrido[3,4-f][1,4,5]thiadiazepine (3-BPTD) exhibits slightly enhanced photoswitching amplitude (photostationary distribution of E-isomers) as well as decent photo- and thermal stability in highly acidic environments. These features make them promising T-type photoswitches for potential acid-resistant light-energy converters and acid-endurable fast-responsive smart materials.