Formation, Chemical and Optical Properties of 1,2,5‐Triphenylpentadienyl Cation Fixed in a Rigid Dibenzobarrelene Skeleton

Abstract

1,2,5‐Triphenylpentadienyl cation fused with a dibenzobarrelene skeleton was synthesized. The pentadienyl cation was persistent in the solution and characterized by 1H and 13C NMR spectroscopies. The cation abstracts a hydride from Et2O at the 5‐position to give the 5‐H derivative together with acetaldehyde and ethene. The reaction with H2O provided the 5‐OH derivative and not the 1‐OH precursor. The hydride reduction of the pentadienyl cation with silanes or NaBH4 occurred at the 3‐ and 5‐positions, and the ratio was dependent on the bulkiness of reducing reagents. The cation in dichloromethane exhibited long‐wavelength absorptions at 678, 523, and 443 nm and an emission maximum at 729 nm with the emission quantum yield of 0.175, which are longer and larger than those of the referenced 1,5‐diphenylpentadienyl cation. TD‐DFT calculations show that the absorptions are due to the π‐π* excitations on 1,2,5‐triphenylpentadienyl and dibenzobarrelene benzene rings.