Ovalene Photophysics Revisited.

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 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 S ← S (B ← A) transition is forbidden, whereas the first prominent absorption band can be assigned to the allowed S ← S (B ← A) transition, in contrast to recent reassignments. Temperature and time-dependent spectroscopic measurements show that the S and S states quickly establish a thermal pre-equilibrium, giving rise to thermally activated S → S 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, our measurements reveal a significantly larger S-S gap of approximately 1200 cm.