Photophysical properties of perylenebisimide derivatives studied by time-resolved transient optical and electron paramagnetic resonance spectroscopy

We studied the intersystem crossing (ISC) of the neutral, monoanionic, and dianionic forms of two perylenebisimide PBI derivatives with twisted π-conjugation framework in the molecular structure: one is substituted with four chloride atoms (PBI-Cl), whereas the other has a fused dimeric molecular structure (DiPBI). PBI-Cl is highly fluorescent, and no ISC was observed. However, the DiPBI shows efficient ISC (triplet state lifetime τT = 3.6 μs), supported by theoretical computation with the Herzberg-Teller effect taken into account. Time-resolved electron paramagnetic resonance spectra of the triplet state of DiPBI show that the zero field splitting (ZFS) D parameter is 778 MHz, indicating that the electron spin density is delocalized, because the ZFS D is much smaller than the triplet state of the unsubstituted PBI (D = 1303 MHz). No ISC was observed for the dianion [DiPBI]2-, whereas for [PBI-Cl]2, ISC was observed. This result infers that twisting of the π-conjugation framework of a molecular structure does not necessarily induce ISC, and excited state energy level matching of S1/Tn states may play the decisive role in ISC. These studies are helpful to fully exploit the super strong reductivity of photoexcited dianions in photocatalytic redox organic reactions due to the enhanced intermolecular electron transfer based on the long-lived triplet excited state of the dianions.