Electron Paramagnetic Resonance Spectroscopy of Bis(triarylamine) Paracyclophanes as Model Compounds for the Intermolecular Charge-Transfer in Solid State Materials for Optoelectronic Applications

A set of seven bis(triarylamine) mixed-valence radical cations with different bridging moieties were investigated by temperature-dependent electron paramagnetic resonance (EPR) spectroscopy in methylene chloride and ortho-dichlorobenzene to evaluate the thermal electron transfer rate constants. The bridges used comprise [2,2]paracyclophane and [3,3]paracyclophane as well as fully conjugated phenylene spacers. The cyclophanes serve as model structures for studying the intermolecular electron transfer in solid state materials. The activation barriers derived by EPR measurements are compared with those estimated by the two-state Marcus-Hush analysis as well as by its extension to three states. Both methods gave good agreement with the EPR data, the three-state method being slightly better than the two-state method. On the basis of the choice of the different bridging groups, our study shows that the bridge can have a significant influence on the internal reorganization energy. [2,2]Paracyclophane and [3,3]paracyclophane bridge units show practically the same electronic coupling and thermal barrier. Conjugated bridges have thermal rates about I order of magnitude larger than the radical cations with broken conjugation. These two aspects show that in solid state materials triarylamines drawn close to their van der Waals radii may exhibit efficient coupling and rate constants by only I order of magnitude smaller than fully conjugated materials.