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.