The molecular mechanism of the triplet state formation in bodipy-phenoxazine photosensitizer dyads confirmed by ab initio prediction of the spin polarization

Abstract

Efficient formation of excited triplet states on metal-free photosensitizer dyads, bodipy-phenoxazine (BDP-PXZ) and tetramethylbodipy-phenoxazine (TMBDP-PXZ), was investigated using ab initio calculations. We revealed the reason why two different triplet transient species, ³CT and ³BDP, can co-exist only for BDP-PXZ as observed in the previous study with the TR-EPR measurements. It was found that the state mixing of ³CT enables the transition from ¹CT to ³CT and ³BDP states only for BDP-PXZ. This mixing effect is commonly seen in the singlet states of twisted intermolecular charge transfer molecules, though the key factor that determines the mixing of the excited states of the dyes was found to be the electron-donating ability of the substituents rather than their steric hindrance. This mechanism was corroborated by comparing the spin polarization ratio of the triplet spin-sublevels measured by TR-EPR with the theoretical predictions. The spin polarization ratio of the triplets should contain information about the transition via intersystem crossing, e.g. the twisted angle of two chromophores of the dyad, and thus it can be a powerful tool to analyze the molecular mechanism of photochemical processes at the electronic structure level. These insights on the molecular structures’ effect provided by this theoretical study would be a compass to molecular design of metal-free triplet photosensitizers.