title: The nature of singlet excitons in oligoacene molecular crystals.
abstract: A theory for polarized absorption in crystalline oligoacenes is presented, which includes Frenkel exciton coupling, the coupling between Frenkel and charge-transfer (CT) excitons, and the coupling of all neutral and ionic excited states to the dominant ring-breathing vibrational mode. For tetracene, spectra calculated using all Frenkel couplings among the five lowest energy molecular singlet states predict a Davydov splitting (DS) of the lowest energy (0-0) vibronic band of only -32 cm(-1), far smaller than the measured value of 631 cm(-1) and of the wrong sign-a negative sign indicating that the polarizations of the lower and upper Davydov components are reversed from experiment. Inclusion of Frenkel-CT coupling dramatically improves the agreement with experiment, yielding a 0-0 DS of 601 cm(-1) and a nearly quantitative reproduction of the relative spectral intensities of the 0-n vibronic components. Our analysis also shows that CT mixing increases with the size of the oligoacenes. We discuss the implications of these results on exciton dissociation and transport.
authors.affiliation: Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, USA.
authors.forename: H
authors.initials: H
authors.lastname: Yamagata
authors.forename: J
authors.initials: J
authors.lastname: Norton
authors.forename: E
authors.initials: E
authors.lastname: Hontz
authors.forename: Y
authors.initials: Y
authors.lastname: Olivier
authors.forename: D
authors.initials: D
authors.lastname: Beljonne
authors.forename: J L
authors.initials: JL
authors.lastname: Brédas
authors.forename: R J
authors.initials: RJ
authors.lastname: Silbey
authors.forename: F C
authors.initials: FC
authors.lastname: Spano