Assessing the Potential of Peropyrene as a Singlet Fission Material: Photophysical Properties in Solution and the Solid State

The photophysical behavior of the polycyclic aromatic hydrocarbon peropyrene is studied both in dilute solution and in the solid state, with the goal of evaluating this molecule as a singlet fission (SF) material. In solution, the fluorescence quantum yield is consistently in the range 0.90–0.95, while the fluorescence lifetime changes from 3.2 to 5.5 ns. Analysis of the solvent dependence of the radiative rate provides evidence that the bright 1Bu singlet state mixes with a second, optically dark state. The presence of a dark state slightly above the 1Bu state in energy is confirmed using two-photon fluorescence excitation spectroscopy. The crystal structure of solid peropyrene consists of a herringbone arrangement of π-stacked molecular pairs, similar to the α-polymorph of perylene. There are two emitting species, centered at approximately 550 and 650 nm, both of which are formed within the 15 ps time resolution of the experiment, and which relax independently via biexponential decays. We find no evidence for rapid SF in the peropyrene crystals, most likely due to the large shift of the singlet state to lower energy where it no longer fulfills the energy condition for SF. These results demonstrate how both energetics and crystal packing influence the ability of a molecule to function as a SF material.