The electronic structure at organic/organic interfaces plays a key role, among others, in defining the quantum efficiency of organics-based photovoltaic cells. Here, we perform quantum-chemical and microelectrostatic calculations on molecular aggregates of various sizes and shapes to characterize the interfacial dipole moment at pentacene/C 60 heterojunctions. The results show that the interfacial dipole mostly originates in polarization effects due to the asymmetry in the multipolar expansion of the electronic density distribution between the interacting molecules, rather than in a charge transfer from donor to acceptor. The local dipole is found to fluctuate in sign and magnitude over the interface and appears as a sensitive probe of the relative arrangements of the pentacene and C 60 molecules (and of the resulting local electrical fields sensed by the molecular units).

Five diamines with thiophene-based bridges--(E)-1,2-bis{5-[bis(4-butoxyphenyl)amino]-2-thienyl}ethylene (1), 5,5'-bis[bis(4-methoxyphenyl)amino]-2,2'-bithiophene (2), 2,6-bis[bis(4-butoxyphenyl)amino]dithieno[3,2-b:2',3'-d]thiophene (3), N-(4-tert-butylphenyl)-2,6-bis[bis(4-methoxyphenyl)amino]dithieno[3,2-b:2',3'-d]pyrrole (4 a) and N-tert-butyl-2,6-bis[bis(4-methoxyphenyl)amino]dithieno[3,2-b:2',3'-d]pyrrole (4 b)--have been synthesised. The syntheses make use of the palladium(0)-catalysed coupling of brominated thiophene species with diarylamines, in some cases accelerated by microwave irradiation. The molecules all undergo facile oxidation, 4 b being the most readily oxidised at about -0.4 V versus ferrocenium/ferrocene, and solutions of the corresponding radical cations were generated by addition of tris(4-bromophenyl)aminium hexachloroantimonate to the neutral species. The near-IR spectra of the radical cations show absorptions characteristic of symmetrical delocalised species (that is, class III mixed-valence species); analysis of these absorptions in the framework of Hush theory indicates strong coupling between the two amine redox centres, stronger than that observed in species with phenylene-based bridging groups of comparable length. The strong coupling can be attributed to high-lying orbitals of the thiophene-based bridging units. ESR spectroscopy indicates that the coupling constant to the amino nitrogen atoms is somewhat reduced relative to that in a stilbene-bridged analogue. The neutral species and the corresponding radical cations have been studied with the aid of density functional theory and time-dependent density functional theory. The DFT-calculated ESR parameters are in good agreement with experiment, while calculated spin densities suggest increased bridge character to the oxidation in these species relative to that in comparable species with phenylene-based bridges.

The electron spin resonance spectra of the radical cations of 4,4'-bis[di(4-methoxyphenyl)amino]tolane, E-4,4'-bis[di(4-methoxyphenyl)amino]stilbene, and E,E-1,4-bis{4-[di(4-methoxyphenyl)amino]styryl}benzene in dichloromethane exhibit five lines over a wide temperature range due to equivalent coupling to two 14N nuclei, indicating either delocalization between both nitrogen atoms or rapid intramolecular electron transfer on the electron spin resonance time scale. In contrast, those of the radical cations of 1,4-bis{4-[di(4-methoxyphenyl)amino]phenylethynyl}benzene and E,E-1,4-bis{4-[di(4-n-butoxyphenyl)amino]styryl}-2,5-dicyanobenzene exhibit line shapes that vary strongly with temperature, displaying five lines at room temperature and only three lines at ca. 190 K, indicative of slow electron transfer on the electron spin resonance time scale at low temperatures. The rates of intramolecular electron transfer in the latter compounds were obtained by simulation of the electron spin resonance spectra and display an Arrhenius temperature dependence. The activation barriers obtained from Arrhenius plots are significantly less than anticipated from Hush analyses of the intervalence bands when the diabatic electron-transfer distance, R, is equated to the N[symbol: see text]N distance. Comparison of optical and electron spin resonance data suggests that R is in fact only ca. 40% of the N[symbol: see text]N distance, while the Arrhenius prefactor indicates that the electron transfer falls in the adiabatic regime.

The electronic and optical properties of 2,6-dialkyl and 2,6-bis(5-alkyl-2-thienyl) derivatives of the fused-ring systems 4H-cyclopenta[2,1-b:3,4-b']bithiophene, 4,4-di-n-hexyl-4H-cyclopenta[2,1-b:3,4-b']bithiophene, 4H-cyclopenta[2,1-b:3,4-b']bithiophene-4-one, 4-alkyl and 4-aryldithieno[3,2-b:2',3'-d]pyrrole, 4-phenyldithieno[3,2-b:2',3'-d]phosphole, 4-phenyldithieno[3,2-b:2',3'-d]phosphole 4-oxide, dithieno[3,2-b:2',3'-d]thiophene, dithieno[3,2-b:2',3'-d]thiophene 4-oxide, and dithieno[3,2-b:2',3'-d]thiophene 4,4-dioxide have been compared to those of the analogous unbridged 5,5'-substituted 2,2'-bithiophene derivatives using electrochemistry, UV-visible absorption and emission spectroscopy, and DFT and TD-DFT calculations. The planarization in the fused-ring compounds means that the methylene-bridged cyclopentabithiophenes are more readily oxidized than their unbridged bithiophene analogues. In each case, the bridging group (X) lies on a nodal plane of the HOMO; accordingly, within each series of fused-ring compounds, electrochemical oxidation potentials and calculated ionization potentials depend primarily on the inductive donor/acceptor strength of the bridging group. On the other hand, significant LUMO coefficients can be found on X groups with π-donor or acceptor properties; accordingly, the electrochemical reduction potentials, calculated electron affinities, and the energies of the HOMO→LUMO optical transitions depend on both inductive and mesomeric donor and acceptor strengths. In particular, within the 2,6-bis(5-alkyl-2-thienyl) series, increasingly electron-withdrawing bridging groups lead to a bathochromic shift and weakening of the low-energy absorption band relative to that of methylene- or π-donor-bridged examples and also to a loss of vibronic structure, with the compound that has the strongest π-accepting bridge of those examined (X = CO) showing a particularly low-energy and weak band. The fluorescence of acceptor-bridged compounds exhibits greater Stokes shifts and a loss of vibronic structure relative to those of methylene- or π-donor-bridged analogues, with the carbonyl-bridged derivative showing no observable fluorescence. These results can be related to increasing localization of the LUMO on the core and toward the bridging group, leading to increased charge-transfer character for the first excited state. The radical cations of some examples have been generated by chemical oxidation and investigated using visible-NIR and ESR spectroscopy and DFT and TD-DFT calculations. The absorption spectra of the radical cations of the 2,6-bis(5-alkyl-2-thienyl) compounds are generally similar to those previously reported for quaterthiophene derivatives, while the hyperfine couplings obtained from ESR spectra are consistent with delocalization of the unpaired electron over both the core and terminal thienyl rings of the π system.