Intramolecular hole-transfer in protonated anthracene

Abstract

Excitation spectra of protonated and deuteronated anthracene are obtained by triple-resonance dissociation spectroscopy. Very cold cations, protonated/deuteronated exclusively at the 9-position, are generated from two-colour two-photon threshold ionisation of 9-dihydroanthracenyl radicals (C₁₄H₁₁). The excitation spectra reveal rich structure, not resolved in previous studies, that is assigned based on anharmonic and Herzberg–Teller coupling calculations. This work reveals that the excitation of protonated anthracene induces a symmetry-breaking intramolecular charge-transfer process along a Marcus–Hush coordinate, where the positively charged hole hops from the central bridging sp² carbon, onto one of the aromatic rings. Signatures of this charge-transfer event are observed in the excitation spectrum, through active Herzberg–Teller progressions.