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, Φ Z→E =0.71) with millisecond relaxation half-life (t 1/2 =40 ms). Photo-energy transduction efficiency was also exceptionally high with 29.1 % converted into ring-strain energy mainly loaded on azo π-bond.