Anomalously selective quenching of S2 fluorescence from upper excited state of zinc 5-(1'-pyrenyl)-10,15,20-triphenylporphyrin derivatives through intramolecular charge transfer state.

Zinc 5-(1'-pyrenyl)-10,15,20-tris(p-methoxyphenyl)porphyrin (ZnP-Py) and zinc 5-(1'-pyrenyl)-10,15,20-tris(pentafluorophenyl)porphyrin (ZnFP-Py), in which a pyrenyl group is directly connected at the meso-positions of their corresponding porphyrins, were synthesized and investigated on the photophysical properties. While the S1 fluorescence of the porphyrin moieties was not quenched by the pyrenyl group at all, the S2 fluorescence was remarkably quenched. The anomalously selective quenching of the S2 fluorescence was dependent on the solvent polarity, and the quenching was enhanced in the polar solvent. The S2 fluorescence quenching is considered to take place through an intramolecular charge transfer state. Interestingly, the charge transfer state relaxes to the S1 state through the rapid charge recombination with high quantum yield. The charge transfer rate of ZnFP-Py, which was calculated from the S2 fluorescence quenching, was larger than that of ZnP-Py, possibly due to the differences in the free-energy changes of the electron transfer and the electron densities at their meso-positions. This study has suggested that ultrafast electron transfer from the upper excited state and charge recombination to S1 can occur in a directly connected electron donor-acceptor system.