Molecular mechanism of the triplet states formation in bodipy-phenoxazine photosensitizer dyads confirmed by ab initio prediction of the spin polarization

Efficient formations of excited triplet states on the metal-free photosensitizer dyads, bodipy-phenoxazine (BDP-PXZ) and tetramethylbodipy-phenoxazine (TMBDP-PXZ), were investigated by 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. The mechanism was corroborated by comparing the spin polarization ratio of the triplet spin-sublevels measured by the 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 molecular structures’ effect provided by this theoretical study would be a compass to molecular design for metal-free triplet photosensitizers.