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 (Δ E 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.