Exciton-phonon coupling in molecular crystals: Synergy between two intramolecular vibrational modes in quaterthiophene single crystals

Abstract

Exciton-phonon (EP) coupling in molecular crystals is investigated in the case where two intramolecular vibrational modes are involved and a theoretical model is presented which applies when one of the modes is strongly coupled to crystal excitons. The model is used to simulate the low energy portion of the absorption spectra of quaterthiophene (4T) single crystals, for which we find it appropriate to consider a low energy vibrational mode at 161 cm(-1) and an effective strongly coupled high energy mode at 1470 cm(-1). Our numerical results demonstrate that the high energy mode renormalizes the excitonic band, thereby strongly affecting the environment seen by the low energy mode and the overall EP coupling regime. Numerical simulations also confirm the existence of the new coupling regimes "intermediate-I" and "strong-I" already introduced for oligothiophene aggregates [Spano et al., J. Chem. Phys. 127, 184703 (2007)], which arise as a consequence of the large effective mass of low energy excitons in 4T crystals. Comparison with experimental high resolution absorption spectra is also reported and shown to support the model predictions.

Optical Properties of Dibenzo[d,d′]thieno[3,2-b;4,5-b′]dithiophene Monocrystals: The Effect of Intermolecular Interactions

“…22,26,27 The sum in eq 1 was computed for an infinite three-dimensional crystal in two steps: first we used the point dipole-point dipole approximation and made use of the Ewald's procedure to obtain convergence of the sum and then we replaced point dipolepoint dipole interactions with the more accurate expression of eq 2 whenever the distance between the centers of mass of two molecules was less than a cutoff distance r max . In our calculations we used r max ) 100 Å, which was enough to achieve convergence of the energy bands.…”

Microscopic theory of polariton lasing via vibronically assisted scattering

“…A composite vibronic structure emerges, which can be understood in terms of a high-energy phonon (considered in this work) and of a low-energy phonon, which is not resolved at room temperature because of thermal broadening. Such a system requires the use of two-phonon states in order to exactly reproduce the spectra; 35 however, we ignore this complication because we are only interested in the phenomenological properties of the line which is responsible for lasing.…”

Optical Properties of Molecular Crystals: The Effect of Molecular Packing and Polymorphism

“…First we compute the microscopic mechanical exciton states and then we consider the propagation of light in an anisotropic medium, including long-range interactions through the macroscopic dielectric tensor. This theoretical approach takes full account of all the crystal symmetries and the numerical calculations provide a fairly accurate description of the dispersion of the purely excitonic bands, allowing a quantitative comparison with experiments (Raimondo et al 2006;Silvestri et al 2009;Spearman et al 2005;Tavazzi et al 2006aTavazzi et al , 2006b.…”

“…This transition is the strongest one with oscillator strength of 12.8 Debyes, which is quite high, compared to other conjugated molecules. For example, the lowest electronic transition of two of the most studied oligothiophenes, quaterthiophene and sexithiophene, have transition moments of about 10.6 and 10.4 Debyes, respectively (Petelenz & Andrzejak 2000;Silvestri et al 2009;Spano et al 2007). The components of the TPP transition at lowest energy with respect to the molecular inertial axes are listed in Table 6 and the results clearly indicate the strong polarization of the molecular transition at lowest energy along the long (L) molecular axis.…”

“…The ac polarized replicas in the spectral region between 2.7 and 3.6 eV are built on the lower b polarized excitonic band and obtain their oscillator strength through an intermolecular Herzberg-Teller coupling to the nearly free ac polarized exciton (Silvestri et al 2009;Spano 2003Spano , 2004Spano et al 2007;Tavazzi et al 2006a). On the contrary, the b polarized exciton is very weakly allowed and it is more strongly coupled to phonons, showing a vibronic progression.…”