Spectroscopic study of NH-tautomerism in novel cycloketo-tetraphenylporphyrins.

The NH-tautomerism is a fundamental property of all metal-free porphyrins. In the present study it was investigated for three novel nonsymmetrical cycloketo-tetraphenylporphyrins in isotropic solutions. The combination of steady-state absorption and fluorescence techniques with decay-associated fluorescence spectroscopy was demonstrated to be a powerful tool to bring into light the photophysical properties of individual NH-tautomers even at room temperature. For all investigated free-base porphyrins, the equilibrium concentration of one tautomer, named T1, which has a lower frequency of the S(0,0)-->S(1,0) transition, was found to be higher than the concentration of the other tautomer, named T2, even at room temperature. Lowering the temperature enriches the population of tautomer T1 and it becomes dominant in absorption and fluorescence. The fluorescence decay time of tautomer T1 was found to be approximately three times shorter compared to that of tautomer T2, whereas both tautomers have similar fluorescence quantum yields. In contrast only one conformation exists for zinc cycloketo-porphyrin, which was used as reference compound.