Dithieno[3,2‐b; 2′,3′‐f]phosphepinium‐Based Near‐Infrared Fluorophores: px–π* Conjugation Inherent to Seven‐Membered Phosphacycles

Abstract

AbstractPositively charged phosphorus‐containing heterocycles are characteristic core skeletons for functional molecules. While various phosphonium‐containing five‐ or six‐membered‐ring compounds have been reported, the seven‐membered‐ring phosphepinium have not been fully studied yet. In this study, dithieno[3,2‐b; 2′,3′‐f]phosphepinium ions containing electron‐donating aminophenyl groups were synthesized. An X‐ray crystallographic analysis of the resulting donor–acceptor–donor dyes revealed a bent conformation of the central seven‐membered ring. These compounds exhibit fluorescence in the near‐infrared region with a bathochromic shift of ca. 70 nm compared to a phosphepine oxide congener and a large Stokes shift. High fluorescence quantum yields were obtained even in polar solvents due to the suppression of the nonradiative decay process. A theoretical study revealed that the phosphepinium skeleton is highly electron‐accepting owing to the orbital interaction between a px orbital of the phosphonium moiety and a π* orbital of the 1,3,5‐hexatriene moiety. Due to the lower‐lying px orbital in the phosphonium moiety compared to that of the phosphine oxide and the bent conformation of the seven‐membered ring, the phosphepinium ring permits effective px–π* conjugation. A large structural relaxation with a contribution of a quinoidal resonance structure is suggested in the excited state, which should be responsible for the bright emission with a large Stokes shift.