# Ovalene Photophysics Revisited

## TL;DR

This paper clarifies the light-related behavior of ovalene, a type of nanographene, by resolving conflicting data and showing how its optical properties compare to pyrene.

## Contribution

The study resolves conflicting spectroscopic results for ovalene and provides a revised understanding of its excited-state dynamics.

## Key findings

- The S1 ← S0 transition in ovalene is forbidden, while the S2 ← S0 transition is allowed.
- The S2–S1 energy gap is significantly larger (∼1200 cm–1) than previously reported (∼400 cm–1).
- Fluorescence lifetime decreases and quantum yield increases with rising temperature due to thermal pre-equilibrium between S1 and S2 states.

## 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 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.