Abstract
Our goal in this review is three-fold. First, we provide an overview of a number of quantum-chemical methods that can abstract charge-transfer (CT) information on the excited-state species of organic conjugated materials, which can then be exploited for the understanding and design of organic photodiodes and solar cells at the molecular level. We stress that the Composite-Molecule (CM) model is useful for evaluating the electronic excited states and excitonic couplings of the organic molecules in the solid state. We start from a simple polyene dimer as an example to illustrate how interchain separation and chain size affect the intercahin interaction and the role of the charge transfer interaction in the excited state of the polyene dimers. With the basic knowledge from analysis of the polyene system, we then study more practical organic materials such as oligophenylenevinylenes (OPVn), oligothiophenes (OTn), and oligophenylenes (OPn). Finally, we apply this method to address the delocalization pathway (through-bond and/or through-space) in the lowest excited state for cyclophanes by combining the charge-transfer contributions calculated on the cyclophanes and the corresponding hypothetical molecules with tethers removed. This review represents a step forward in the understanding of the nature of the charge-transfer interactions in the excited state of organic functional materials.
Highlights
Exploring new organic semiconductor materials and understanding the relationship between the molecular structure and the properties are still major challenges
It is believed that photoexcitation of an organic material may generate charge-transfer (CT) excitons in the strong interchain interaction limit which has been described as spatially indirect excitons or bound polaron pairs
These results indicate the necessity of further investigation the chromophore-chromophore interactions with short interchain separation, and how the charge transfer interactions affects the electronic structure of the aggregate in the organic solid
Summary
Exploring new organic semiconductor materials and understanding the relationship between the molecular structure and the properties are still major challenges. The studies on the admixture of the charge transfer transitions in the excited states are increasingly being examined for many molecular systems [32,33,34,35] These results indicate the necessity of further investigation the chromophore-chromophore interactions with short interchain separation, and how the charge transfer interactions affects the electronic structure of the aggregate in the organic solid. Note that the possibility of formation of these species is dependent on the proximity of the neighboring molecules and their relative orientation [43,44,45] In this contribution, we review some of our recently quantumchemical studies and discuss the importance of the CT exciton in the excited states of the polyene system as well as some organic materials, including oligophenylenevinylenes (OPVn), oligothiophenes (OTn), and oligophenylenes (OPn) in Section 3 [46]. We have investigated the interaction between two 1Bu states of those organic dimers
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