Dynamics of the excitonic coupling in organic crystals

We show that the excitonic coupling in molecular crystals undergoes a very large fluctuation at room temperature as a result of the combined thermal motions of the nuclei. This observation dramatically affects the description of exciton transport in organic crystals and any other phenomenon (like singlet fission or exciton dissociation) that originates from an exciton in a molecular crystal or thin film. This unexpected result is due to the predominance of the short-range excitonic coupling mechanisms (exchange, overlap, and charge-transfer mediated) over the Coulombic excitonic coupling for molecules in van der Waals contact. To quantify this effect we develop a procedure to evaluate accurately the short-range excitonic coupling (via a diabatization scheme) along a molecular dynamics trajectory of the representative molecular crystals of anthracene and tetracene

Developing accurate molecular mechanics force fields for conjugated molecular systems

A rapid method to parameterize the intramolecular component of classical force fields for complex conjugated molecules is proposed. The method is based on a procedure of force matching with a reference electronic structure calculation. It is particularly suitable for those applications where molecular dynamics simulations are used to generate structures that are therefore analysed by electronic structure methods, because it is possible to build force fields that are consistent with electronic structure calculations that follow classical simulations. Such applications are commonly encountered in organic electronics, spectroscopy of complex systems and photobiology (e.g. photosynthetic systems). We illustrate the method by parameterizing the force fields of a molecule used in molecular semiconductors (2,2-dicyanovinyl-capped S,N-heteropentacene or DCV-SN5), a polymeric semiconductor (thieno[3,2-b]thiophene-diketopyrrolopyrrole TT-DPP) and a chromophore embedded in a protein environment (15,16-dihydrobiliverdin or DBV) where several hundreds of parameters need to be optimized in parallel

Very large π-conjugation despite strong nonplanarity : a path for designing new semiconducting polymers

When two π-conjugated fragments are connected by a bond between two sp2 carbon atoms, a torsion around this bond is expected to break the overall π-conjugation. We show that for specially selected monomers, the π-conjugation is insensitive to torsions around a C–C bond up to about 60°. We provide a number of examples for this very unexpected phenomenon and a simple explanation. We propose that this feature can be incorporated into conjugated polymers to generate semiconducting materials that are extremely insensitive to structural disorder

Modulating the exciton dissociation rate by up to more than two orders of magnitude by controlling the alignment of LUMO + 1 in organic photovoltaics

Efficient organic solar cells require a high yield of exciton dissociation. Herein we investigate the possibility of having more than one charge-transfer (CT) state below the first optically bright Frenkel exciton state (FE) for common molecular donor (D)/acceptor (A) pairs and the role of the second-lowest CT state (CT2) in the exciton dissociation process. This situation, previously explored only for fullerene acceptors, is shown to be rather common for other D/A pairs. By considering a phenomenological model of a large aggregate, we reveal that the position of CT2 can remarkably modulate the exciton dissociation rate by up to more than two orders of magnitude. Thus, controlling the alignment of CT2 is suggested as a promising rule for designing new D/A heterojunctions

Modeling charge transport in high-mobility molecular semiconductors: Balancing electronic structure and quantum dynamics methods with the help of experiments

Computing the charge mobility of molecular semiconductors requires a balanced set of approximations covering both the electronic structure of the Hamiltonian parameters and the modeling of the charge dynamics. For problems of such complexity, it is hard to make progress without independently validating each layer of approximation. In this perspective, we survey how all terms of the model Hamiltonian can be computed and validated by independent experiments and discuss whether some common approximations made to build the model Hamiltonian are valid. We then consider the range of quantum dynamics approaches used to model the charge carrier dynamics stressing the strong and weak points of each method on the basis of the available computational results. Finally, we discuss non-trivial aspects and novel opportunities related to the comparison of theoretical predictions with recent experimental data

A computational study of the competing reaction mechanisms of the photo-catalytic reduction of CO2 on anatase(101)

We perform a computational study of three different reaction mechanisms for the photo-catalytic reduction of CO2 on the TiO2 anatase(101) surface known as (i) the carbene, (ii) the formaldehyde and (iii) the glyoxal pathways. We define a set of approximations that allows testing a number of mechanistic hypotheses and design experiments to validate them. We find that the energetically most favourable reaction mechanism among those proposed in the literature is the formaldehyde path, and the rate-limiting step is likely to be the formation of CH3 radicals from dissociation of CH3OH. We show that an intermediate that supports this mechanism is OCH2OH. We also find that formaldehyde would be an energetically favorable intermediate forming from CO and HCO, intermediates that are proposed in the early stage of the carbene and glyoxal pathways respectively. Some possible variants of mechanisms and methods to ease the formation of CH3 radicals are also discussed

Vibronic enhancement of excitation energy transport : interplay between local and non-local exciton-phonon interaction

It has been reported in recent years that vibronic resonance between vibrational energy of the intramolecular nuclear mode and excitation-energy difference is crucial to enhance excitation energy transport in light harvesting proteins. Here we investigate how vibronic enhancement induced by vibronic resonance is influenced by the details of local and non-local exciton-phonon interaction. We study a heterodimer model with parameters relevant to the light-harvesting proteins with the surrogate Hamiltonian quantum dynamics method in a vibronic basis. In addition, the impact of field-driven excitation on the efficiency of population transfer is compared with the instantaneous excitation, and the effect of multi-mode vibronic coupling is presented in comparison with the coupling to a single effective vibrational mode. We find that vibronic enhancement of site population transfer is strongly suppressed with the increase of non-local exciton-phonon interaction and increasing the number of strongly coupled high-frequency vibrational modes leads to further decrease in vibronic enhancement. Our results indicate that vibronic enhancement is present but may be much smaller than previously thought and therefore care needs to be taken when interpreting its role in excitation energy transport. Our results also suggest that non-local exciton-phonon coupling, which is related to the fluctuation of the excitonic coupling, may be as important as local exciton-phonon coupling and should be included in any quantum dynamics model

Predicting with confidence the efficiency of new dyes in dye sensitized solar cells

We ask whether it is possible to predict the efficiency of a new dye in dye sensitized solar cells (DSSCs) on the basis of the known performance of existing dyes in the same type of device. We evaluate a number of computable predictors of the efficiency for a large set of dyes whose experimental efficiency is known. We have then used statistical regression methods to establish the relation between the predictors and the efficiency. Our predictions are associated with a rigorously determined confidence level. For a new dye of the same family we are able to predict the probability that its efficiency in a DSSC is larger than a certain threshold. This method is useful for accelerating the discovery of new dyes and establishing more rigorously the existence of specific correlations between structure and properties. Within the properties considered we find that the dye efficiency correlates more strongly with its oxidation potential and reorganization energy