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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The electronic structure and optical properties of charged oligofluorenes were studied experimentally and theoretically. Measurements of the optical absorption spectra of charged oligofluorenes in dilute solutions have been performed by using the pulse radiolysis technique. In addition, optical absorption spectra of radical cations and anions in a solid matrix were measured after γ-irradiation at 77 K. The optical absorption spectra were measured in the range of 440−2100 nm (0.6−2.8 eV) and compared with results from time-dependent density functional theory (TDDFT) calculations. The calculated charge induced deformations and charge distribution do not indicate the occurrence of polaronic effects. The potential energy profiles for rotation around the inter-unit bond show that oligofluorenes are nonplanar in their neutral state, while they tend to more planar structures in their charged state. The optical absorption spectra of charged oligofluorenes are dependent on the angle between neighboring units. TDDFT absorption energies shift to lower values with increasing chain length, which suggests that the charge delocalizes along the oligomer chain. [ABSTRACT FROM AUTHOR]