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 pi-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, phi<INF>Delta</INF> = 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 (Delta E<INF>S<INF>1</INF>/T<INF>1</INF></INF> = 0.61 eV). This postulate is tested by critical examination of a distorted Bodipy having an anthryl unit at the meso-position, for which phi<INF>Delta</INF> is increased to 40%. The improved ISC yield is rationalized by the presence of a T<INF>2</INF> state, localized on the anthryl unit, with energy close to that of the S<INF>1</INF> state. The electron spin polarization phase pattern of the triplet state is (e, e, e, a, a, a), with the T<INF>z</INF> sublevel of the T<INF>1</INF> 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 pi-conjugation framework does not necessarily induce ISC, but S<INF>1</INF>/T<INF>n</INF> energy matching may be a generic feature for increasing ISC for a new-generation of heavy atom-free triplet photosensitizers.