Dithieno[3,2-b; 2',3'-f]phosphepinium-Based Near-Infrared Fluorophores: p_x-π* Conjugation Inherent to Seven-Membered Phosphacycles

Positively 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 p_x orbital of the phosphonium moiety and a π* orbital of the 1,3,5-hexatriene moiety. Due to the lower-lying p_x 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 p_x-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.