Polarized Absorption in Crystalline Pentacene: Theory vs Experiment

Abstract

The polarized absorption spectra of crystalline pentacene are obtained for excitation normal to the ab herringbone plane by measuring transmitted light in ultrathin crystals. The spectral line shapes for excitation polarized along b and orthogonal to b are analyzed theoretically using a Holstein-like Hamiltonian which includes both Frenkel and charge transfer (CT) excitons represented in a multiparticle basis set. The model agrees with prior estimates regarding the strong CT contribution (≈45%) of the exciton responsible for the b-polarized lower Davydov component.The polarization resolution allows one to also establish the nature of the upper Davydov component, which is found to contain far less CT content (≈15%), as well as the natures of the higher-energy vibronic excitons, which are found to consist of a complex mixture of Frenkel one-and two-particle states and CT excitons. Generally, the spectrum polarized along b displays J-aggregatelike vibronic signatures while the spectrum polarized orthogonal to b displays H-aggregate-like vibronic signatures. The assignment is entirely consistent with the calculated exciton band dispersions which agree well with the measured ones.

Vibronic exciton theory of singlet fission. I. Linear absorption and the anatomy of the correlated triplet pair state

“…Rather, they consist of some admixture of the two, additionally mixed with other states such as diabatic charge transfer states. 24,26,29,30 In this sense, the adiabatic states are labeled so as to distinguish between a predominant singlet or triplet composition.…”

“…I, while the electronic couplings are presented in the supplementary material. The dipoledipole couplings J m,m in crystalline pentacene are rather weak, and have therefore been neglected by Berkelbach et al 28,29 Here, we use values reported by Hestand et al, 30 which have been calculated through INDO/CCSD using the coordinates from Ref. 37, and screened (divided) by a directionally averaged optical dielectric constant = 3.5.…”

“…The model applied by Hestand et al 30 did not include triplet states, hence at this point our modeling and parameters start to differ from those used in that work. For the HOMO-LUMO couplings t HL m,m , not reported by Hestand et al, we adapt the couplings calculated by Berkelbach et al 28 Similarly to their work on crystalline pentacene, 29 we apply an overall rescaling of these couplings, while keeping the signs and relative differences among them unaltered.…”

“…24,26,30 Its HR factor associated with the diabatic singlet state is taken to be λ 2 s1 = 1.1, a value derived from fitting the vibronic peak areas of dissolved pentacene to a Poisson distribution. 30 The ionic HR factors (λ 2 a = 0.29 and λ 2 c = 0.39) 30 are based on calculations reported in Ref. 40.…”

“…A viable approach to bridge the gap between these calculations and measurements of singlet fission is through microscopic modeling. Such arXiv:1703.01173v2 [cond-mat.mtrl-sci] 11 May 2017 has been proven successful in several studies on fission materials, [24][25][26][27][28][29][30] however, applications did not address the fission dynamics, 24,26,30 or were limited to a perturbative treatment of electronic couplings 25 or the vibrational degrees of freedom. [27][28][29] The present article forms the first entry in a series of studies in which we present a microscopic vibronic exciton model aimed at unraveling the mechanistic principles underlying singlet fission, with a particular emphasis placed on the role of vibronic coupling.…”