title: Multiphonon relaxation slows singlet fission in crystalline hexacene.
abstract: Singlet fission, the conversion of a singlet excitation into two triplet excitations, is a viable route to improved solar-cell efficiency. Despite active efforts to understand the singlet fission mechanism, which would aid in the rational design of new materials, a comprehensive understanding of mechanistic principles is still lacking. Here, we present the first study of singlet fission in crystalline hexacene which, together with tetracene and pentacene, enables the elucidation of mechanistic trends. We characterize the static and transient optical absorption and combine our findings with a theoretical analysis of the relevant electronic couplings and rates. We find a singlet fission time scale of 530 fs, which is orders of magnitude faster than tetracene (10-100 ps) but significantly slower than pentacene (80-110 fs). We interpret this increased time scale as a multiphonon relaxation effect originating from a large exothermicity and present a microscopic theory that quantitatively reproduces the rates in the acene family.
authors.affiliation: Energy Frontier Research Center, ¶Department of Chemistry, and §Departments of Physics and Electrical Engineering, Columbia University , New York, New York 10027, United States.
authors.forename: Erik
authors.initials: E
authors.lastname: Busby
authors.forename: Timothy C
authors.initials: TC
authors.lastname: Berkelbach
authors.forename: Bharat
authors.initials: B
authors.lastname: Kumar
authors.forename: Alexey
authors.initials: A
authors.lastname: Chernikov
authors.forename: Yu
authors.initials: Y
authors.lastname: Zhong
authors.forename: Htay
authors.initials: H
authors.lastname: Hlaing
authors.forename: X-Y
authors.initials: XY
authors.lastname: Zhu
authors.forename: Tony F
authors.initials: TF
authors.lastname: Heinz
authors.forename: Mark S
authors.initials: MS
authors.lastname: Hybertsen
authors.forename: Matthew Y
authors.initials: MY
authors.lastname: Sfeir
authors.forename: David R
authors.initials: DR
authors.lastname: Reichman
authors.forename: Colin
authors.initials: C
authors.lastname: Nuckolls
authors.forename: Omer
authors.initials: O
authors.lastname: Yaffe