Origin of intersystem crossing in highly distorted organic molecules: a case study with red light-absorbing N,N,O,O-boron-chelated Bodipys

To explore the relationship between the twisted π-conjugation framework of aromatic chromophores and the efficacy of intersystem crossing (ISC), we have studied a N,N,O,O-boron-chelated Bodipy derivative possessing a severely distorted molecular structure. Surprisingly, this chromophore is highly fluorescent, showing inefficient ISC (singlet oxygen quantum yield, Φ(Δ) = 12%). These features differ from those of helical aromatic hydrocarbons, where the twisted framework promotes ISC. We attribute the inefficient ISC to a large singlet-triplet energy gap (ΔE(S(1)/T(1)) = 0.61 eV). This postulate is tested by critical examination of a distorted Bodipy having an anthryl unit at the meso-position, for which Φ(Δ) is increased to 40%. The improved ISC yield is rationalized by the presence of a T(2) state, localized on the anthryl unit, with energy close to that of the S(1) state. The electron spin polarization phase pattern of the triplet state is (e, e, e, a, a, a), with the T(z) sublevel of the T(1) state overpopulated. The small zero-field splitting D parameter (−1470 MHz) indicates that the electron spin density is delocalized over the twisted framework. It is concluded that twisting of π-conjugation framework does not necessarily induce ISC, but S(1)/T(n) energy matching may be a generic feature for increasing ISC for a new-generation of heavy atom-free triplet photosensitizers.