Long-lived photoinduced charge separation due to the inverted region effect in 1,6-Bis(phenylethynyl)pyrene-phenothiazine dyad

Abstract

Photoinduced electron-transfer processes in a 1,6-bis(phenylethynyl)pyrene-phenothaiazine dyad, BPEP-PT, are examined using various techniques. The BPEP singlet excited state is quenched by electron transfer from PT leading to formation of BPEP radical anion and PT radical cation. Rate constants and quantum yields of the PET processes were determined from steady-state and time-resolved fluorescence experiments, and spectroscopic identification of the radical ion products was achieved using picosecond and nanosecond flash photolysis experiments. The charge-separated (CS) state was found to be long-lived but decayed to the BPEP triplet state under the influence of external heavy atom effect. The energy level diagram constructed on the basis of experimental data revealed the existence of the local triplet state below the CS state, yet the CS state did not exhibit any tendency to decay to this level. This showed that hyperfine interaction (HFI) or spin-orbit charge-transfer intersystem crossing (SOCT-ISC) mechanisms are not effective in inducing intersystem crossing in the CS state. It is suggested that absence of SOCT-ISC in the CS state may be a consequence of the total absence of ISC in the parent BPEP chromophore. The only option available to the CS state is a spin-allowed transition to the ground state, and this process is slow because of inverted region effects.