Dibenzo[b,f][1,4,5]chalcogenadiazepine Photoswitches: Conversion of Excitation Energy into Ring Strain

Tetra‐ortho‐substituted, heteroaryl and cyclic azobenzenes have emerged as three key strategies on morphology design of photoswitch to diversify controllability. Cyclic azobenzene is of particular utilization in photo‐energy conversion due to rigid and ring‐strain structure. Despite the well‐recognized diazocine, the photo‐switching properties of seven‐membered cyclic azobenzenes (diazepines) have yet been exploited. Herein, we report a family of dibenzo[b,f][1,4,5]chalcogenadiazepines (DBChDs) and their T‐type photo‐switching nature with tunable relaxation rate. Based on experiments together with DFT calculations, we found that an unsymmetric 2‐bithiophenyl‐dibenzo[b,f][1,4,5]thiadiazepine exhibited an efficient response to 445 nm laser stimulation (quantum efficiency, ΦZ→E=0.71) with millisecond relaxation half‐life (t1/2=40 ms). Photo‐energy transduction efficiency was also exceptionally high with 29.1 % converted into ring‐strain energy mainly loaded on azo π‐bond. Chalcogen‐bridged seven‐membered cyclic azobenzenes, dibenzo[b,f][1,4,5]chalcogenadiazepine (DBChD), exhibit distinct photo‐switching characters with a high‐energy E‐configuration, especially for sulfur‐bridged DBTD with an energy transduction efficiency of 21 % (η) under 445 nm laser activation. Introducing oligothiophene π‐EDG on DBTD could further elevate the absorption λmax and coefficient ϵ, reaching η=29 %.