Vibronic relaxation energies of acene-related molecules upon excitation or ionization.

With the perspective of future vibronic studies, ab initio calculations of the energies of vibrational relaxation that follows the processes of singlet or triplet excitation, and positive or negative ionization, are reported for four series of compounds of potential interest in the context of photovoltaic applications, notably those rooted in singlet exciton fission. The commonly used method of evaluating the energy of vibrational relaxation following ionization of an excited molecule is examined and found to be dubious, especially when used within approaches based on the concept of an effective progression-forming mode. In this regard, methodological consistency in computing the relaxation energies and Franck-Condon parameters for different excitation processes is found to be of paramount importance.

Partial atomic multipoles for internally consistent microelectrostatic calculations.

An extension of the extant microelectrostatic methodologies, based on the concept of distributed generalized polarizability matrix derived from the Coupled Perturbed Hartree-Fock (CPHF) equations, is proposed for self-consistent calculation of charge carrier and charge-transfer (CT) state electrostatic energies in molecular solids, including the doped, defected and disordered ones. The CPHF equations are solved only once and the generalized molecular polarizability they yield enables low cost iterations that mutually adjust the molecular electronic distributions and the local electric field in which the molecules are immersed. The approach offers a precise picture of molecular charge densities, accounting for atomic partial multipoles up to order 2, which allows one to reproduce the recently reported large charge-quadrupole contributions to CT state energies in low-symmetry local environments.

Dopant-Catalyzed Singlet Exciton Fission.

In acene-based molecular crystals, singlet exciton fission occurs through superexchange mediated by two virtual charge-transfer states. Hence, it is sensitive to their energies, which depend on the local environment. The crucial point is the balance between the charge-quadrupole interactions within the pair of molecules directly involved in the process and those with the surrounding crystal matrix, which are governed by local symmetry and may be influenced by breaking this symmetry.

Locally Broken Crystal Symmetry Facilitates Singlet Exciton Fission.

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.

Quasiperiodic energy dependence of exciton relaxation kinetics in the sexithiophene crystal.

Femtosecond kinetics of fluorescence rise in the sexithiophene crystal is studied on a microscopic model of intraband relaxation, where exciton energy is assumed to be dissipated by phonon-accompanied scattering, with the rates calculated earlier. The temporal evolution of the exciton population is described by a set of kinetic equations, solved numerically to yield the population buildup at the band bottom. Not only the time scale but also the shape of the rise curves is found to be unusually sensitive to excitation energy, exhibiting unique quasiperiodic dependence thereon, which is rationalized in terms of the underlying model.

Lowest singlet exciton in pentacene: modern calculations versus classic experiments.

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%.

Is dipole moment a valid descriptor of excited state's charge-transfer character?

In the ongoing discussion on excited states of the pentacene crystal, dipole moment values have been recently invoked to gauge the CT admixture to excited states of Frenkel parentage in a model cluster. In the present paper, a simple dimer model is used to show that, in general, the dipole moment is not a valid measure of the CT contribution. This finding eliminates some apparent disagreement between the computational results published by different research groups.

Intermediate vibronic coupling in charge transfer states: comprehensive calculation of electronic excitations in sexithiophene crystal.

A comprehensive theory of linear vibronic coupling in a coupled manifold of Frenkel and charge-transfer states in an infinite molecular crystal is presented and applied for sexithiophene. The approach, valid in the intermediate-coupling regime, includes up to three-particle terms of the Philpott expansion, with the vibronic wavefunctions represented in the Lang-Firsov basis. As a stringent test, the scheme is used to reproduce the complete set of available sexithiophene absorption and electroabsorption spectra within a unified theoretical framework.

Intermediate vibronic coupling in sexithiophene single crystals. II. Three-particle contributions.

Validity of the approach recently proposed to describe intermediate-to-strong linear vibronic coupling in an infinite molecular crystal is tested by assessing the importance of the (previously neglected) three-particle excitations. The Hamiltonian, denoted in the Lang-Firsov representation and including the three-particle terms, is numerically diagonalized to yield the eigenstates and ultimately the absorption spectrum. In addition, the wave functions are analyzed to probe the size and nature of the phonon cloud surrounding the exciton.

Intermediate vibronic coupling in sexithiophene single crystals.

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.

Aggregates of naphthalene chromophores in poly(vinylalcohol)-graft-poly(vinylnaphthalene) pseudomicelles.

A novel amphiphilic grafted copolymer, poly(vinylalcohol)-graft-poly(vinylnaphthalene), was synthesized and studied spectroscopically. The unusual photophysical properties of its aqueous solutions were observed for the first time and are attributed to the aggregation of the naphthalene chromophores in the bulk of polymer pseudomicelles. The enormous red shift of the fluorescence spectra is interpreted in terms of the confinement effect.