Photophysics of Soret-Excited Tetrapyrroles in Solution. IV. Radiationless Decay and Triplet−Triplet Annihilation Investigated Using Tetraphenylporphinato Sn(IV)

Abstract

The S(2) population decay rates and triplet-triplet annihilation efficiencies of Sn(IV)Cl(2)TPP have been measured in fluid solutions using its weak S(2)-S(0) fluorescence as a metric. A detailed description of the excited-state photophysics of Sn(IV)Cl(2)TPP has allowed comparisons to be made between this rigid, D(4h) axially coordinated molecule and axially uncoordinated tetrapyrroles of greater flexibility and lower symmetry. S(2)-S(1) internal conversion is the major S(2) population decay path for Sn(IV)Cl(2)TPP as it is for the S(2) states of all other d(0) and d(10) metalated tetrapyrroles. The S(2) state of Sn(IV)Cl(2)TPP exhibits S(2)-S(1) relaxation rates that follow the energy gap law of radiationless transition theory and are only slightly faster than those exhibited by MgTPP and the weak coupling limit. Differences in S(2)-S(1) radiationless decay rates among the series MTPP (M = Mg, Zn, Cd, SnCl(2)) cannot be traced to differences in the displacements of the S(2) and S(1) potential surfaces. Instead, the most likely source of the large differences in S(2)-S(1) radiationless decay rates between CdTPP and Sn(IV)Cl(2)TPP is the lower symmetry of the former (near C(4v)), which permits a much larger number of vibrations to participate in S(2)-S(1) vibronic coupling. Triplet-triplet annihilation of the type 2T(1) --> S(2) + S(0) has been observed in Sn(IV)Cl(2)TPP for the first time, but is of substantially lower efficiency than seen in ZnTPP in noncoordinating solvents because of its shorter triplet lifetime and the shielding effects of its axial Cl ligands, which tend to block the short-range interaction needed for Dexter energy transfer.