Fluorescence from upper excited states of porphyrins with one- and two-photon excitation

In this paper we report on the study of S-2 -> S-0 blue fluorescence and two-photon absorption (TPA) for a series of Zn-tetrabenzoporphyrins (Zn-TBP) with sequential substitution of phenyl groups in meso-positions. The blue fluorescence has been measured with both one- and two-photon excitation. The data on TPA showed that lowest g-parity state lies above B-state for all these compounds and blue fluorescence has been detected upon two-photon excitation at g -> g transition energies above that of Soret band. The data are in line with the suggested role of the g-parity low-lying state in the nonradiative deactivation of B state. More than two-fold decrease in the relative yield of blue fluorescence was found in going from Zn-monophenyl-TBP to Zn-tetraphenyl-TBP. The enhancement of the radiationless internal S-2 -> S-1 and S-1 -> S-0 conversion and the decrease in the quantum yield of blue fluorescence was proposed to explain by nonplanar distortions of tetrapyrrolic macrocycle in Zn-tetraphenyl-TBP. The obtained results indicate that the positioning of g-parity excited states above B states is a requirement but not the sufficient condition for presence of blue fluorescence.