Herein, we reinvestigate the photophysics of ovalene, a prototypical nanographene for which conflicting spectroscopic results have been reported. Owing to its structural similarity and its identical D2h point-group symmetry, ovalene can essentially be viewed as a larger pyrene. We show that its optical transitions can be understood using the same model that is invoked to explain the excited states of pyrene. Absorption and (polarized)-emission measurements reveal that the S1 ← S0 (1B3u ← 1Ag) transition is forbidden, whereas the first prominent absorption band can be assigned to the allowed S2 ← S0 (1B2u ← 1Ag) transition, in contrast to recent reassignments. Temperature and time-dependent spectroscopic measurements show that the S1 and S2 states quickly establish a thermal pre-equilibrium, giving rise to thermally activated S2 → S0 emission at room-temperature. As a result, the fluorescence lifetime of ovalene decreases with increasing temperature while its fluorescence quantum yield increases. Contrary to the frequently cited small energy gap of ∼400 cm-1, our measurements reveal a significantly larger S2-S1 gap of approximately 1200 cm-1.
Wega et al. (Wed,) studied this question.