Evidence for strong electron correlations in graphene molecular fragments: Theory and experiments on two-photon absorptions

Historically, the occurrence of the lowest two-photon state below the optical one-photon state in linear polyenes, polyacetylenes and polydiacetylenes provided the strongest evidence for strong electron correlations in these linear $\pi$-conjugated systems. We demonstrate similar behavior in several molecular fragments of graphene with $D_{6h}$ symmetry, theoretically and experimentally. Theoretically, we have calculated one versus two-photon absorptions in coronene, two different hexabenzocoronenes and circumcoronene, within the Pariser-Parr-Pople $\pi$-electron Hamiltonian using high order configuration interaction. Experimentally, we have performed z-scan measurements using a white light super-continuum source on coronene and hexa-peri-hexabenzocoronene to determine frequency-dependent two-photon absorption coefficients, for comparison to the ground state absorptions. Excellent agreement between experiment and theory in our work gives strong evidence for significant electron correlations between the $\pi$-electrons in the graphene molecular fragments. We particularly benchmark high order electron-hole excitations in graphene fragments as a key element behind the agreement between theory and experiment in this work.