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According to recent ab initio calculations, the energy gap between the two oppositely polarized charge-transfer (CT) states of a model pentacene dimer is anomalously large, attaining 0.8 eV. Here we introduce the self-consistent charge field approach to evaluate the electrostatic stabilization energies of the pertinent states in the pentacene crystal represented by a dedicated multiscale model system containing the model dimer as its core. We demonstrate that, contrary to common wisdom, the lower of the two CT states is barely affected by the crystalline environment whereas the upper one undergoes a large red shift. Effectively, embedding of the dimer in the crystal bulk reduces the pertinent splitting by an order of magnitude, because most of the intradimer charge−quadrupole interactions are compensated by similar interactions with surrounding molecules. This resolves the apparent contradiction between the ab initio result obtained for the dimer and the splitting of about 0.04 eV resulting from microelectrostatic calculations for the crystal. ■ INTRODUCTIONThe discovery of spontaneous singlet exciton fission (SF) in solid pentacene prompted widespread interest in the electronic properties of this system, in the recent years triggering a surge of theoretical papers that strive to unravel the underlying physical mechanisms. 1−4 Various methodologies have been invoked for this purpose.No matter how vibronic coupling is treated and how the dynamic aspects of the problem are described, 1−3 the process is ultimately controlled by the energetics of the electronic states involved. Yet, as the small energy gaps between the relevant eigenstates are inevitably sensitive to the details of the wave function, the requisite accuracy is just on the verge of the predictive power of the most sophisticated quantum chemistry tools available to date. By these standards, the pentacene molecule is large. Moreover, in order to describe the fission process, it is necessary to take into account at least the two molecules on which the two emerging triplets are located, which makes a dimer the minimum model system to be considered in this context. At the ab initio level, this creates a numerical problem of formidable complexity, practically ruling out inclusion of any elements of the crystal environment in which the pertinent pair of molecules is in reality embedded.In a number of papers 1−3 focusing on diverse aspects of the fission phenomenon, various versions of the dimer model were adopted. In most cases, parametrization of the model Hamiltonian was in principle based on quantum chemistry calculations but fine-tuned on intuitive grounds to account for simplifications of the model and for potential intrinsic errors of the applied quantum chemistry methods.The paper by Zeng et al. 4 stands out from this collection, representing a consistent ab initio approach at a highly advanced level. However, in view of the accuracy it offers, even minor energy shifts resulting from the simplifications of the underlying model may significantly influen...

A phenomenological wave-function-based model of vibrationally coherent absorption modulation is proposed and applied to reproduce the triplet−triplet absorption spectra of bis-triisopropylsilylethynyl (TIPS)pentacene, with the objective of testing whether the singlet fission process in that system spontaneously generates coherent vibrational packets, as recently suggested for TIPStetracene. The model is parameterized by a complete set of Franck−Condon parameters obtained from methodologically consistent density functional theory calculations for all relevant normal modes in all relevant electronic states. The results strongly depend on inhomogeneous broadening of absorption bands, which is explicitly included. They very well agree with the recently published experimental coherence spectra of the pertinent system, validating our underlying principal assumption that the singlet fission process, which generates the observed triplet states, is neutral with respect to vibrational coherences. Experimental confirmation of this interpretational posit demonstrates that in the pentacene derivative, apparently in contrast to the case of its tetracene analogue, fission is not a source of vibrational coherence. Our finding suggests that although the singlet fission process may possibly in individual cases induce vibrational coherence, this feature is not a constitutive characteristic of the fission phenomenon.

A two-dimensional analog of the Merrifield model of the coupling between the Frenkel and charge transfer (CT) excitations of a molecular crystal is applied for the calculation of the electroabsorption (EA) spectra of polyacene crystals. The approach is essentially nonempirical, with most of the necessary input data estimated either from theoretical calculations or from independent experiments. Good quantitative reproduction of the experimental EA spectra is achieved, both in their absolute amplitude and intensity pattern. The large amplitude of the Frenkel exciton part of the spectra is successfully accounted for without the necessity to invoke anomalously large molecular polarizabilities. Some basic assumptions of previous analyses are shown to be invalid and future prospects of the new approach are discussed.

Photovoltaic yield is normally limited to at most two charge carriers per photon. In solid pentacene this limit may be potentially bypassed owing to singlet exciton fission into a pair of triplets. The process occurs via a superexchange mechanism mediated by charge-transfer (CT) configurations and is sensitive to their energies. As demonstrated recently, these strongly depend on the local environment of the two molecules on which the charges reside. Using a multiscale model, here we show that in the crystal bulk approximate local symmetry affects CT state energetics in a way unfavorable for fission, so that at the places where this symmetry is broken the fission probability is enhanced by up to an order of magnitude. These fission-favorable locations entail the vicinity of vacancies, specific impurities, and interfaces, such as crystallite boundaries. Hence, photovoltaic yield might be substantially increased by using nanoscopically disordered pentacene rather than highly ordered specimens.

A new approach is proposed to describe intermediate-to-strong linear vibronic coupling in an infinite molecular crystal. The Hamiltonian, transformed to the Lang-Firsov representation, is approximated by disregarding the terms involving more-than-two-particle excitations and block-diagonalized by the Fourier transformation. The spectroscopically relevant block corresponding to zero wave vector is further simplified by introducing a cutoff in the off-diagonal matrix elements and reduced to a manageable size by truncating the basis set, which enables one to diagonalize it numerically. The parametrization, based on independent experiments or theoretical estimates, is aimed to represent the sexithiophene crystal. The results, compared to those obtained for a finite cluster with equivalent material parameters, highlight the favorable convergence properties of the infinite-crystal approach.

An extended two-dimensional analogue of the Merrifield model of the mixing between Frenkel and charge-transfer excitons is used to calculate the electro-absorption spectrum of the α-sexithiophene single crystal. The model reflects the symmetry of the crystal and takes into account all the major interactions between the molecules. The input parameters are estimated from independent quantum-chemical and micro-electrostatic calculations. The simulated spectrum is in very good agreement with experiment, both in shape and in absolute amplitude. The results demonstrate that the eigenstates of the crystal between 2.55 and 2.85 eV are primarily of charge-transfer parentage, so that charge-transfer contributions dominate the electro-absorption spectrum in that region. This first successful reproduction of the electro-absorption spectrum of a single crystal is a stringent test of the theoretical description that confirms its validity.

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