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 S2−S0 and S1−S0 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 S2−S0 Stokes shift yet measured, 115 cm-1, and the largest S2−S0 absolute fluorescence quantum yield yet measured, 1.84 × 10-3, are obtained. Accurate measurements of the relative quantum yields of S2−S0 to S1−S0 emission as a function of excitation wavelength reveal that a fast radiationless process that bypasses S1 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, S2‘, contributes a small fraction of the Soret band's integrated molar absorptivity and is responsible for an increasing fractio...