Subgap Two-Photon States in Polycyclic Aromatic Hydrocarbons: Evidence for Strong Electron Correlations

Abstract

Strong electron correlation effects in the photophysics of quasi-one-dimensional π-conjugated organic systems such as polyenes, polyacetylenes, polydiacetylenes, etc., have been extensively studied. Far less is known on correlation effects in two-dimensional π-conjugated systems. Here we present theoretical and experimental evidence for moderate repulsive electron−electron interactions in a number of finite polycyclic aromatic hydrocarbon molecules with D 6h symmetry. We show that the excited state orderings in these molecules are reversed relative to that expected within oneelectron and mean-field theories. Our results reflect similarities as well as differences in the role and magnitude of electron correlation effects in these two-dimensional molecules compared to those in polyenes.

Correlated electronic properties of some graphene nanoribbons: A DMRG study

“…In the past, the experimental demonstration that the lowest two-photon state occurs below the optical one-photon state in linear polyenes 29 , in contradiction to the predictions of tight-binding and Hartree-Fock theories 29 , provided the most convincing demonstration of the strong electron correlations in these systems. More recently, similar experimental results have also been obtained from nonlinear optical measurements of graphene nanofragments 30 . Accurate computational investigation of the relative energy orderings of one-and two-photon states also requires going beyond existing techniques.…”

Electron correlations and two-photon states in polycyclic aromatic hydrocarbon molecules: A peculiar role of geometry

“…In the case of coronene, we have previously shown from explicit calculations that the consequences of the deviations from idealized geometry on the energies of the one-and two-photon states, and of the lowest triplet, are negligible. 40 One-photon states for molecules with D 6h symmetry belong to the doubly degenerate 1 E + 1u symmetry subspace, while two-photon states belong to either 1 A − 1g or 1 E − 2g symmetry subspaces. As D 6h is not an Abelian group, we have adopted the D 2h nomenclature also for discussions of the excited states in the D 6h PAHs.…”

“…40, explicit comparisons of theoretical one-and two-photon absorption spectra for ovalene are made only against coronene as the representative of PAH molecules with D 6h symmetry. However, more detailed comparisons of spin-triplet and higher energy two-photon states, and in particular, the wavefunctions of the latter, are made against all the D 6h PAH molecules of Fig.…”

“…We have very recently presented joint theory-experiment results of investigations for a few select PAH molecules. 40 The molecules investigated theoretically in all cases had D 6h point group symmetry. The reason for choosing this particular set was because the two molecules investigated experimentally, coronene (C 24 H 12 ), which is commercially available, and hexa-perihexabenzocoronene (C 42 H 18 , hereafter HBC), which was synthesized by our experimental colleagues, both had D 6h symmetry.…”

“…At the same time, they are weaker in PAHs than in linear polyenes, since in PAHs the lowest two-photon states were found from our calculations to be predominantly (though not entirely) 1e-1h in character, in contrast to the 2 1 A − g in the polyenes, in which the 2e-2h contributions dominate over the 1e-1h contributions. 40 The weaker correlation effect in PAHs, relative to polyenes, is expected because of the larger one-electron bandwidth of PAHs in which each carbon (C) atom has three as opposed to two neighboring C-atoms.…”