Photophysics of soret-excited tetrapyrroles in solution. II. Effects of perdeuteration, substituent nature and position, and macrocycle structure and conformation in zinc(II) porphyrins

The steady-state absorption, fluorescence, and excitation spectra and upper excited-state temporal fluorescence decay profiles of 11 tetrapyrroles in several fluid solvents are presented and analyzed to ascertain the factors that control their S-2 population decay times. The S, lifetimes, which vary by more than 2 orders of magnitude, are controlled exclusively by their rates of radiationless decay. The only important electronic relaxation path is S-2-S-1 internal conversion, the efficiency of which is near 1.0 in all compounds studied (except CdTPP where it is 0.69). The rate of S, population rise equals the rate of S, population decay in all cases. Among the compounds studied, only MgTPP exhibits S-2-S-1 decay behavior that corresponds to the weak coupling limit of radiationless transition theory; all zinc metalloporphyrins exhibit intermediate to strong coupling. Perdeuteration of ZnTPP produces no significant change in the rate of S-2 decay or in the quantum yield of S-2-S-0 fluorescence, indicating that in-plane C-C and C-N vibrations are the accepting modes in S, with the largest Franck-Condon factors. The initial vibrational energy content of the S, states (0 < E-vib < 3500 cm I over the range of compounds) plays no significant role in determining their overall population decay rates in solution. The S, population decay rates of these tetrapyrroles are controlled by two factors: the Franck-Condon factor, which is inversely proportional to the exponent of the S-2-S-1 electronic energy spacing and the S-2-S-1 coupling energy. The S-2-S-1 electronic energy spacing is determined in solution by the difference in the polarizabilities of the S-2 and S-1 states and can be controlled by varying the polarizability of the solvent. The S-2-S-1 coupling energy is influenced by the nature, location, and effect of the substituents, with beta-alkyl substitution and reduction of symmetry in the tetrapyrrole-for example by loss of planarity-increasing the interstate coupling energy.