Ovalene is a large polycyclic aromatic hydrocarbon that shows spectroscopic features similar to those of its smaller analogue, pyrene. As in pyrene, the S1 ← S0 transition in ovalene is forbidden, resulting in a fluorescence lifetime of several hundred nanoseconds, whereas the S2 ← S0 transition is fully allowed. Unlike pyrene, however, ovalene exhibits a much smaller S2-S1 energy gap, which enables thermal population of its S2 state.
The figure above shows the steady-state emission spectrum of ovalene upon heating from room temperature to 150°C. Notably, anti-Kasha S2 → S0 emission is already observed at room temperature and increases continuously with increasing temperature. In contrast, the bands associated with the forbidden S1 → S0 emission decrease in intensity. Although our experiments indicate that the thermal population of the S2 state does not exceed ~1 %, anti-Kasha emission remains clearly observable due to the much larger radiative rate constant of the allowed S2 → S0 transition compared to the weak radiative decay of the forbidden S1 → S0 transition.
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Contact:
Eric Vauthey
Physical Chemistry Department - Sciences II - University of Geneva
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