Origin of intersystem crossing in highly distorted organic molecules: a case study with red light-absorbing ,,,-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 ,,,-boron-chelated Bodipy derivative possessing a severely distorted molecular structure. Surprisingly, this chromophore is highly fluorescent, showing inefficient ISC (singlet oxygen quantum yield, = 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 (Δ = 0.61 eV). This postulate is tested by critical examination of a distorted Bodipy having an anthryl unit at the -position, for which is increased to 40%. The improved ISC yield is rationalized by the presence of a T state, localized on the anthryl unit, with energy close to that of the S state. The electron spin polarization phase pattern of the triplet state is (, , , , , ), with the T sublevel of the T state overpopulated. The small zero-field splitting 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/T energy matching may be a generic feature for increasing ISC for a new-generation of heavy atom-free triplet photosensitizers.