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 D_2h 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 (¹B_3u ← ¹A_g) transition is forbidden, whereas the first prominent absorption band can be assigned to the allowed S₂ ← S₀ (¹B_2u ← ¹A_g) 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.