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In this article, a theoretical study of the electronic and spectroscopic properties of well-defined DNA hairpins is presented. The excited states in the hairpins are described in terms of an exciton Hamiltonan model, and the structural dynamics of the DNA model systems is explicitly taken into account by molecular dynamics simulations. The results show that the model reproduces the experimentally observed absorption and circular dichroism spectra accurately in most cases. It is shown that structural disorder leads to excited states that are largely localized on a single base pair, even for regular DNA sequences consisting only of AT base pairs. Variations in the base pair sequence have a significant effect on the appearance of the spectra but also on the degree of delocalization of the excited state.

Using a tight-binding model of charge transport in systems with static and dynamic disorder, we present a theoretical study of the positive charge transfer in molecular assemblies that involve a hole donor and an acceptor connected by fluorene and phenyl bridges. Two parameters that determine the rate of charge transfer within the proposed model are the charge transfer integral between neighboring units and the site energies. Fluctuations in the values of the charge transfer integral and the energy landscape for hole transport were calculated by taking into account variations of the dihedral angle between neighboring units and electrostatic interaction of positive charge moving along the bridge and the negative charge that remains on the hole donor. Analysis of the dynamics of hole transfer and the distribution of the positive charge during this process allows the conclusion that the rapid fall of the hole transfer rate coefficient observed in experiments with short bridges (three and four structural units for systems with fluorene and phenyl bridges, respectively) can be attributed to the electrostatic interaction. This interaction is responsible for the formation of the effective barrier between donor and acceptor with the height that increases as the number of structural bridge units remains less than 3 (fluorene bridge) or 4 (phenyl bridge). For longer bridges, however, the effective barrier changes only weakly and now the charge transport is mostly dominated by the fluctuation-assisted incoherent hole migration along the bridge. The latter mechanism exhibits much weaker dependence of the rate coefficient on the bridge length in agreement with the available experimental results.

We have studied the motion of charge carriers along isolated phenylene-vinylene (PV) chains using a combination of experimental and theoretical methods. The conductive properties of positive charges along PV chains in dilute solution were studied by using the pulse-radiolysis time-resolved microwave conductivity (TRMC) technique. This technique enables the measurement of high-frequency (tens of GHz) charge carrier mobilities along isolated PV chains without the use of electrodes. The charge carrier mobility along PV chains with finite and infinite length was studied theoretically by charge transport simulations with parameters from density functional theory (DFT) calculations. The high-frequency charge carrier mobility is found to depend strongly on the conjugation length of the PV chains and is found to increase both with increasing length of the PV chain and with increasing conjugation fraction. The experimental results are in good agreement with the calculated results. On the basis of this combined experimental and theoretical study an intrachain charge carrier mobility of a few tens of cm2/Vs is expected for an infinitely long PV chain without conjugation breaks.

A quantum chemical study of the electronic structure of negatively charged phenylenevinylene (PV) oligomers and methoxy-substituted derivatives is presented. The geometries of the PV oligomers were optimized using density functional theory. The geometry deformations are found to be delocalized along the entire oligomer chain without indication of polaron formation. The optical absorption spectra of the negatively charged PVs were calculated using both time-dependent density functional theory (TDDFT) and the singly excited configuration interaction method with an intermediate neglect of differential overlap reference wave function (INDO/s-CIS). The available experimental optical absorption energies are reproduced by the calculations. Introduction of methoxy substituents reduces the transition energies, while this does not have a strong effect on the charge distribution along the chain. DFT calculations yield a more delocalized excess negative charge than that of INDO/s-CIS calculations.

Pulse radiolysis of oxygen-saturated dilute solutions of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene]s in benzene showed charge-transfer from the solvent radical cation to the polymer, yielding positive charge carriers (radical-cations or "holes") on the isolated chains. The charge-transfer reactions are diffusion controlled with (time-dependent) rate coefficients that are determined by the conformation adopted by the polymer chains. The radical cation of the fully conjugated polymer exhibited an absorption maximum at 1.32 eV. The absorption maximum shifted to higher energies with decreasing conjugated fraction, in a way consistent with thermal equilibration of the positive charge over the conjugated segments of different length. The mobility of the positive charge along the fully conjugated MEH-PPV chains was determined to be 0.42 cm 2 V - 1 s - 1 . The introduction of conjugated breaks decreased the charge mobility strongly, which is attributed to the disruption of the π conjugation and/or the conformational disorder caused by the flexibility of the chains at the sites of conjugation breaks.