title: Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy.
abstract: Singlet fission is the spin-allowed conversion of a spin-singlet exciton into a pair of spin-triplet excitons residing on neighbouring molecules. To rationalize this phenomenon, a multiexcitonic spin-zero triplet-pair state has been hypothesized as an intermediate in singlet fission. However, the nature of the intermediate states and the underlying mechanism of ultrafast fission have not been elucidated experimentally. Here, we study a series of pentacene derivatives using ultrafast two-dimensional electronic spectroscopy and unravel the origin of the states involved in fission. Our data reveal the crucial role of vibrational degrees of freedom coupled to electronic excitations that facilitate the mixing of multiexcitonic states with singlet excitons. The resulting manifold of vibronic states drives sub-100 fs fission with unity efficiency. Our results provide a framework for understanding singlet fission and show how the formation of vibronic manifolds with a high density of states facilitates fast and efficient electronic processes in molecular systems.
authors.affiliation: FOM Institute AMOLF, 1098 XG Amsterdam, The Netherlands.|Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: Artem A
authors.initials: AA
authors.lastname: Bakulin
authors.affiliation: Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: Sarah E
authors.initials: SE
authors.lastname: Morgan
authors.affiliation: Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: Tom B
authors.initials: TB
authors.lastname: Kehoe
authors.affiliation: Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: Mark W B
authors.identifier: http://orcid.org/0000-0002-1957-2979
authors.initials: MW
authors.lastname: Wilson
authors.affiliation: Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: Alex W
authors.initials: AW
authors.lastname: Chin
authors.affiliation: Department of Chemical Physics, Lund University, PO Box 124, 22100 Lund, Sweden.
authors.forename: Donatas
authors.initials: D
authors.lastname: Zigmantas
authors.affiliation: Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Olshausenstr. 40, D-24098 Kiel, Germany.
authors.forename: Dassia
authors.initials: D
authors.lastname: Egorova
authors.affiliation: Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
authors.forename: Akshay
authors.identifier: http://orcid.org/0000-0003-0320-2962
authors.initials: A
authors.lastname: Rao