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The photophysical behavior of three Soret-excited diamagnetic meso-substituted tetraphenylmetalloporphyrins, MgTPP, ZnTPP, and CdTPP, have been examined in a wide variety of solvents using both steady-state and femtosecond fluorescence upconversion methods. The S 2 population of MgTPP decays to S 1 on the time scale of a few picoseconds with unit S 2-S 1 internal conversion efficiency, and the decay rates conform to the weak coupling case of radiationless transition theory. The energy gap law parameters characterizing the coupling of the S 2 and S 1 states of MgTPP have been obtained. The most important accepting vibrational modes in the S 1 state are multiple in-plane C-C and C-N stretches in the 1200-1500 cm (-1) range. Net S 2-S 1 decay is the dominant decay path for ZnTPP and CdTPP as well, but the process occurs at rates that exceed (in the case of CdTPP, they vastly exceed) those predicted by weak interstate coupling. Alternate mechanisms for the radiationless decay of the S 2 states of ZnTPP and CdTPP have been explored. Large spin-orbit coupling constants and the presence of multiple, near-equiergic triplet states suggest that S 2-T n intersystem crossing might occur at rates competitive with internal conversion. However, the measured efficiencies of S 2-S 1 internal conversion show that, at most, only a few percent of the S 2 population of ZnTPP and no more than about 30% of the S 2 population of CdTPP can decay by a "dark" path such as intersystem crossing.

For the test system zinc tetraphenylporphyrin in ethanol, the S 2 -S 0 and S 1 -S 0 absorption and emission spectra and fluorescence quantum yields have been measured as a function of excitation wavelength within the Soret and Q-bands under conditions where self-absorption of emission and solute aggregation are either eliminated or properly compensated. Under these conditions, the smallest S 2 -S 0 Stokes shift yet measured, 115 cm -1 , and the largest S 2 -S 0 absolute fluorescence quantum yield yet measured, 1.84 × 10 -3 , are obtained. Accurate measurements of the relative quantum yields of S 2 -S 0 to S 1 -S 0 emission as a function of excitation wavelength reveal that a fast radiationless process that bypasses S 1 is operative among states accessed at excitation energies that span the Soret band. The data can be interpreted using the evidence of Yu, Baskin, and Zewail that a second excited state, S 2 ′, contributes a small fraction of the Soret band's integrated molar absorptivity and is responsible for an increasing fraction of photon absorption on both the red edge of the Soret band at λ ex > 430 nm and the blue at λ ex < 409 nm. The lifetimes of the S 2 and S 1 states have also been measured under similar conditions; the values obtained confirm previous measurements.