Excited-State Dynamics of (Organophosphine)gold(I) Pyrenyl Isomers

Ultrafast dynamics of isomeric (tricyclohexylphosphine)gold(I) pyrenyl complexes have been measured in chloroform and cyclohexane at room temperature. Internal conversion from an upper excited singlet (S-n) to the S-1 state occurs in less than 200 fs after 340 nm excitation. Internal conversion in the singlet manifold is followed by 11-100 ps intersystem crossing to a receiver triplet state depending on the site of pyrene metalation. The receiver triplet state (T-n) then decays to the T-1 state on an ultrafast time scale, which decays back to the So state on a microseconds time scale in N-2-saturated conditions. Time-dependent density functional theory calculations on model complexes predict an accidental degeneracy of the S-1 and T-2 states of the 1-pyrenyl. No such degeneracy occurs for the 2-pyrenyl isomer. A small S-1-T-2 energy gap promotes the 10-fold increase in the intersystem crossing rate in the 1-pyrenyl complex.