Lowest Singlet Exciton in Pentacene: Modern Calculations versus Classic Experiments

Abstract

Based on simple model calculations, the expected magnitude of the field-induced shift observable in electroabsorption is estimated for three alternative assignments proposed in the literature for the lowest singlet excitation of the pentacene crystal (pure Frenkel exciton, pure charge-transfer exciton, or a mixture of both). The results are compared with the corresponding experimental value, which is also known from the literature. The latter turns out to be compatible only with the mixed parentage of the pertinent state, which contains the charge-transfer contribution in the range from 25 to 70%. The conclusion is discussed in the context of the present controversies concerning the existing experimental and theoretical evidence on this subject.

Charge Transfer–Mediated Singlet Fission

“…Additionally, CT-Frenkel exciton coupling is required to accurately reproduce the magnitude of Davydov splitting observed in linear absorption spectra (19,27), and the excitonic dispersion measured with inelastic electron scattering (28,29). Extraction of CT character from experimental measurements is sensitive to the employed theoretical model (30). Consequently, reasonable CT mixing proportions for pentacene range from 25% to 75% (30).…”

“…Extraction of CT character from experimental measurements is sensitive to the employed theoretical model (30). Consequently, reasonable CT mixing proportions for pentacene range from 25% to 75% (30). Importantly, not all molecules have significant CT character in their excited states.…”

Excitonic couplings between molecular crystal pairs by a multistate approximation

“…A particularly frequent choice among these diabatization schemes is the Generalized Mulliken-Hush (GMH) method, 21,22 which is based on its predecessor method (the Mulliken-Hush model) and identifies the transformation that diagonalizes the adiabatic dipole moment matrix. When the transformation is applied to the adiabatic to describe the nature of the lowest-energy excitons in these oligoacene crystals is still highly debated [41][42][43][44][45][46] and, therefore, a diabatization scheme insensitive to the level of theory would be also demanded.…”

Pentacene Excitons in Strong Electric Fields

“…Its lowest energy excitons have been the subject of various experimental and theoretical studies . The discussion about the character of the lowest energy singlet exciton has been intense in recent years and despite a number of illuminating studies the issue has not been resolved . Specifically, questions remain, whether the exciton is of the Frenkel type, in which the electron and the hole are localized on the same molecule, and/or of the charge‐transfer (CT) type, for which the two charges are on different molecules (see Figure ).…”

“…Most theoretical papers converge on the idea that the lowest excited singlet exciton is of CT type, while the lowest excited triplet is of Frenkel type. The possibility of admixtures between CT and Frenkel excitons has also been discussed . The fact that the lowest excited singlet may predominantly be a CT exciton is particularly interesting because of its known propensity to fission into triplet excitons which may then be long lived and may play a role in improving solar conversion efficiency.…”