Abstract
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 S(1) ← S(0) ((1)B(3u) ← (1)A(g)) transition is forbidden, whereas the first prominent absorption band can be assigned to the allowed S(2) ← S(0) ((1)B(2u) ← (1)A(g)) transition, in contrast to recent reassignments. Temperature and time-dependent spectroscopic measurements show that the S(1) and S(2) states quickly establish a thermal pre-equilibrium, giving rise to thermally activated S(2) → S(0) 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 S(2)-S(1) gap of approximately 1200 cm(-1).