S2 Fluorescence from [26]Hexaphyrin Dianion.

S2 fluorescence from meso-hexakis(pentafluorophenyl)-substituted [26]hexaphyrin dianion was observed as the first example of expanded porphyrins despite its large molecular size and small HOMO-LUMO gap. The population kinetics among S2, S1, and S0 states have been studied by using femtosecond time-resolved absorption and fluorescence spectroscopies. Broad-band fluorescence upconversion spectroscopy allowed for simultaneous observation of S2 fluorescence decay in the visible region and S1 fluorescence rise in the NIR region, both with a time constant of 0.22 ps. The transient absorption spectroscopy revealed the presence of a direct decay path from the S2 state to the S0 state. The observation of S2 fluorescence from highly conjugated molecular systems is quite rare, and S2 fluorescence beyond 700 nm is also quite rare.