Photophysical studies of porphyrins and metalloporphyrins: Accurate measurements of fluorescence spectra and fluorescence quantum yields for soret band excitation of zinc tetraphenylporphyrin

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 x 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 lambda(ex) > 430 nm and the blue at lambda(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.