Relaxation Properties of Porphyrin, Diprotonated Porphyrin, and Isoelectronic Tetraoxaporphyrin Dication in the S2 State

The fluorescence spectra of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and isoelectronictetraoxaporphyrin dication (TOxP2+) have been measured in solution at room temperature. The S2 f S0fluorescence has been observed, much more intense for TOxP2+ than for H4P2+ and H2P. In the TOxP2+ case,the S2 f S0 fluorescence spectrum is remarkably sharp and shows an excellent mirror symmetry with respectto S0 f S2 absorption. On the contrary, the spectra of H4P2+ and H2P are shifted and more extended withrespect to the absorption counterparts. The differences have been attributed primarily to the change of theequilibrium geometry upon excitation, larger in H2P and H4P2+ than in TOxP2+ and in the case of H4P2+ tothe nonplanar conformation of the macrocycle. Also the S1 f S0 spectra of H2P, H4P2+, and TOxP2+ havebeen measured and more qualitatively discussed. The S1 and S2 fluorescence decays have been observed forH4P2+ and TOxP2+ exciting with ultrashort pulses. The S2 lifetime of TOxP2+ is of the order of the temporalresolution of our experimental apparatus, whereas that of H4P2+ is shorter. The S2 f S0 quantum yield ofTOxP2+ has been estimated to be 0.035, 3 orders of magnitude higher than that of H4P2+. It is proposed onthe basis of ab initio model calculations that excited states of the H4P2+(CF3COO-)2 complex with chargetransfercharacter are responsible of the increased extension of the S2 f S0 spectrum with respect to that ofH2P.