In this paper a combined experimental and quantum chemical study of the geometry and opto-electronic properties of unsubstituted and dialkoxy-sustituted phenylene-vinylene oligomers ͑PV's͒ is presented. The optical absorption spectra for PV cations with different chain lengths and substitution patterns were measured using pulse radiolysis with time-resolved spectrophotometric detection from 1380 to 500 nm ͑0.9 to 2.5 eV͒. The geometries of the PV's studied were optimized using density functional theory ͑DFT͒ for both the neutral and singly charged molecule. The spectra for the PV radical cations were then calculated using singly excited configuration interaction with an intermediate neglect of differential overlap reference wave function method together with the DFT geometry. The agreement between experimental and theoretical absorption energies is excellent; most of the calculated radical cation absorption energies are within 0.15 eV of the experimental values. The pattern of dialkoxy-substitution is found to have a large effect on the optical absorption spectrum of the cation. Using the calculated charge distribution it is shown that the degree of delocalization of the charge correlates with the energy of the lowest absorption band. If alkoxy side chains are present on some of the rings the positive charge tends to localize at those sites.

We have studied the high-frequency (34 GHz) mobility of positive charge carriers on isolated laddertype polymer chains in dilute solution. We find that the high-frequency mobility is limited by the chain ends on chains as long as 35 monomers. The intrachain motion of charge carriers can be described by onedimensional diffusion between infinitely high reflecting barriers, representing the chain ends. Our data indicate that the intrachain mobility for ladder-type polymer chains is close to 600 cm 2 =V s. With this high mobility the ladder-type polymer is a promising candidate for future use as an interconnecting wire in molecular electronics. DOI: 10.1103/PhysRevLett.96.146601 PACS numbers: 72.20.Ee, 72.80.Le The optoelectronic properties of conjugated polymers are of interest due to their applications in devices, such as organic light-emitting diodes, photovoltaic cells, and field-effect transistors [1][2][3]. Using solution-processable polymers as semiconducting compounds opens the way to flexible electronics and enables cheap and easy production of devices [4,5]. Moreover, the ongoing miniaturization of electronic components rapidly approaches the fundamental limit of photolithography. Therefore it is of interest to study the ultimate size limit for organic electronics. Single molecules are the smallest components that are likely to be functional in integrated circuits [6 -8]. Conjugated polymers can be used as interconnecting molecular wires between molecular scale electronic components.In all applications, the mobility of charge carriers is a key factor determining the performance of the electronic devices. Recently, charge carrier mobilities as high as 15 to 35 cm 2 =V s have been found for organic oligoacene crystals [9,10]. In these materials the calculated bandwidth is of the order of a few tenths of an electron volt [11,12]. For perfectly ordered conjugated polymer chains the bandwidth is of the order of a few electron volts [13]. As a consequence the mobility of charge carriers moving along ordered conjugated polymer chains is expected to be considerably higher than the mobilities found for oligoacene crystals, i.e., on the order of a few hundred cm 2 =V s. The values found experimentally for the device mobility in conjugated polymers however, range from 10 ÿ7 to 10 ÿ1 cm 2 =V s, for derivatives of poly(para-phenylenvinylene), poly(thienylenevinylene), and poly(thiophene) [3,14,15], which is several orders of magnitude lower than expected.This discrepancy is likely to be due to a combination of device-specific properties and intrinsic material properties. Contact effects are the most important of the devicespecific properties, and determine the performance of organic optoelectronic devices to a large extent [16,17].Intrinsic properties of organic materials that limit the charge carrier mobility are mostly related to disorder in the material. Two examples are interchain transport and charge transport over grain boundaries (boundary junctions between different relatively ordered domains within the sam...

In this paper, time-dependent density functional theory ͑TDDFT͒ calculations of excited state polarizabilities of conjugated molecules are presented. The increase in polarizability upon excitation was obtained by evaluating the dependence of the excitation energy on an applied static electric field. The excitation energy was found to vary quadratically with the field strength. The excess polarizabilities obtained for singlet excited states are in reasonable agreement with the experimental results for the shorter oligomers, particularly if the experimental uncertainties are considered. For longer oligomers the excess polarizability is considerably overestimated, similar to DFT calculations of ground state polarizabilities. Excess polarizabilities of triplet states were found to be smaller than those for the corresponding singlet state, which agrees with experimental results that are available for triplet polarizabilities. Negative polarizabilities are obtained for the lowest singlet A g states of longer oligomers. The polarizability of the lowest B u and A g excited states of the conjugated molecules studied here are determined mainly by the interaction between these two states. Upon application of a static electric field a quadratic Stark effect is observed in which the lower B u state has a positive excess polarizability and the upper A g state exhibits a decrease in polarizability upon excitation. All results are explained in terms of a sum-over-states description for the polarizability.

The effect of the chain length of conjugated polymers on the real and imaginary components of the frequency dependent mobility of charge carriers is studied. A theoretical model for one-dimensional diffusive motion of charges between reflecting chain ends is presented. It is shown that the model can be used to describe the measured ratio of the imaginary and real components of the microwave (30 GHz) mobility of charge carriers moving along ladder-type polymer chains with different lengths. From comparison of theoretical and experimental data it is inferred that an intra-chain mobility of 30 cm 2 /Vs can be achieved for ladder-type polymer chains, which are sufficiently long so that chain ends do not limit the mobility.

Poly(thienylenevinylene)s (PTVs) are p-conjugated polymers with great potential for use in electronic devices. In thin films, they have been shown to be good p-type conductors (positive charge mobility ca. 10 ±3 cm 2 V ±1 s ±1 ) but no n-type conduction has been reported. In order to investigate the mobility and dynamics of the charge carriers on PTVs at the molecular level, we used pulses of ionizing radiation to generate charges on PTV molecules in dilute solution, i.e., isolated from intermolecular interactions between polymer chains. The mobility of the charge carriers and the kinetics of their decay were monitored by time-resolved microwave conductivity (TRMC) measurements at a frequency of 34 GHz. It was found that both electrons and holes have a high mobility along the isolated polymer chains (0.23 ± 0.04 and 0.38 ± 0.02 cm 2 V ±1 s ±1 , respectively). The electrons were observed to become trapped at defects or impurities within 4 ls. No trapping was observed COMMUNICATIONS 718