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 × 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.