Radiationless decay via ESIPT of the first excited singlet of the hemiporphyrazine

It is proposed that free base hemiporphyrazine, a phthalocyanine analogue where two opposite isoindoles are replaced by pyridines, should exhibit excited state intramolecular proton transfer (ESIPT). This conjecture is based on MNDO/CI computations, according to which the most stable positions of the two inner hydrogens are inverted upon excitation to the first excited singlet. The computed barriers for hydrogen migration are large both for the synchronous and the asynchronous path, but they are in line with those previously computed by the same method for the ground state migration of the inner hydrogens of free base porphines, a process which is known by NMR measurements to be fast at room temperature.