Electron Transfer Kinetics of Organic Mixed-Valence Compounds

Triarylamines are very important hole transport components in optoelectronic devices. Based on previous cooperative work the applicants plan to use the now established methodology to investigate complex organic mixed valence compounds that are based on bis-triarylamine radical cations. Using dynamical ESR-spectroscopy we would like to investigate beside the rate constants of the intermolecular electron transfer, the thermodynamic activation parameters H# and S# obtainable from temperature dependent measurements and the activation volume V# by applying high pressure methods. By comparing the electron transfer parameters determined from optical spectra and dynamic ESR spectroscopy, several aspects shall be elucidated. First, solvent friction plays a significant role in the hole transfer process which shall therefore be investigated in detail by using various solvent with different physical properties and ionic liquids. Second, because orientational aspects between the triarylamine units will ultimately govern the hole transfer rates, new organic mixed valence compounds in which the triarylamine redox centres are oriented in a fixed relative position will be synthesised and investigated. Third, we will focus on the influence of localised bridge oxidation states on the hole transfer rate between two triarylamine redox centres. The elucidation of these three aspects will help to get a better insight into the hole transfer phenomena of triarylamines which then will ultimately allow to optimise their use in optoelectronic materials.

Electron transfer (ET) reactions are of fundamental importance in chemistry, biology and emerging fields of nanotechnology. Numerous investigations have been published on the kinetics of intermolecular electron transfer reaction in both, the ground state and the photo-excited state. In the elementary step of those reactions, no rupture or formation of covalent bonds occurs. In such freely diffusing donor and acceptor systems, the interpretation of the kinetic results is sometimes hindered by the occurrence of electron transfer over a range of reactant distances and orientations, because reactant diffusion and intrinsic electron transfer process determine the kinetics jointly. By linking the electron donor and acceptor covalently, a super-molecule is formed with a single conformation and quite important the influences of diffusion and variable reaction distances are eliminated. Suitable covalent spacers lead to a straight structural control of reaction distance, stereochemistry, orientation, etc. on the kinetics of the electron transfer reactions. Therefore, intra-molecular photo-induced electron transfer reactions of donor-acceptor linked compounds in solution have been studied extensively to clarify the contributions of inner-sphere energies caused by the changes in bond lengths and angles and the outer-sphere reorganization due to the rearrangement of polar solvent molecules. Most of these investigations focused on the design of new donor-spacer-acceptor molecules. Several papers report on the solvent dependence or on solvent dynamical effects, reports on the activation parameters obtained from temperature measurements of the rate constants are relatively rare, but only those measurements enable elucidation of the factors that determine the pre-exponential factor and the activation free energy. Different substituents and their various positions strongly influence the rate constants of the charge transfer reactions. The Austrian part of this common DACH-project was focused on dynamic ESR-spectroscopic measurements of the electron-transfer kinetics of the mixed-valence compounds synthesized by the Würzburg research group. Temperature and pressure dependent dynamic ESR-measurements were performed in different solvents to get the corresponding activation parameters, like activation energies and entropies as well as the activation volumes of such intra-molecular charge transfer reactions.