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Photophysical properties and relaxational dynamics of porphyrin systems

The Raman spectra of tetraoxaporphyrin dication exciting with micro-Raman instrumentation at 785 nm and of diprotonated porphyrin in near-resonance conditions with the Soret band have been measured. Structural calculations by means of the density functional method have been performed on tetraoxaporphyrin dication and on diprotonated porphyrin as well as on the sulfur and carbon analogues of porphyrin. A detailed study of the stable conformations of these compounds is reported. The correlation between calculated frequencies has allowed to make a vibrational assignment for diprotonated porphyrin and tetraoxaporphyrin dication in excellent agreement with experiment. In order to study the correlation between conformational flexibility and photophysical properties in nonplanar porphyrins the fluorescence spectra of free base porphyrin, diprotonated porphyrin and tetraoxaporphyrin dication have been measured in solution at room temperature. In the case of tetraoxaporphyrin dication the S2 → S0 fluorescence spectrum is remarkably sharp and shows an excellent mirror symmetry with respect to S0 → S2 absorption. The spectra of diprotonated porphyrin and free base porphyrin are shifted and more extended with respect to the absorption counterparts. The differences have been attributed primarily to the change of the equilibrium geometry upon excitation and in the case of diprotonated porphyrin to the non planar conformation of the macrocycle. Also the S1 → S0 spectra of these tree molecules have been measured and more qualitatively discussed. The fluorescence lifetimes and quantum yields have been estimated. The relaxation dynamics of protoporphyrin IX and hemin has been studied. By probing with spectrally broad femtosecond pulses, we have observed transient absorption spectra at delay times up to 1.8 ns. We have determined the kinetic profiles corresponding with the band maxima due to excited-state absorption.

Raman spectroscopy; time-resolved fluorescence; transient absorption spectroscopy; planar and nonplanar porphyrins; molecular structures; stable conformations; excited states; relaxation properties; fluorescence lifetimes

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