Mixed-valence (MV) compounds are excellent model systems for the investigation of basic electron-transfer (ET) or charge-transfer (CT) phenomena. These issues are important in complex biophysical processes such as photosynthesis as well as in artificial electronic devices that are based on organic conjugated materials. Organic MV compounds are effective hole-transporting materials in organic light emitting diodes (OLEDs), solar cells, and photochromic windows. However, the importance of organic mixed-valence chemistry should not be seen in terms of the direct applicability of these species but the wealth of knowledge about ET phenomena that has been gained through their study. The great variety of organic redox centers and spacer moieties that may be combined in MV systems as well as the ongoing refinement of ET theories and methods of investigation prompted enormous interest in organic MV compounds in the last decades and show the huge potential of this class of compounds. The goal of this Review is to give an overview of the last decade in organic mixed valence chemistry and to elucidate its impact on modern functional materials chemistry.

In this paper we present the synthesis as well as a detailed study of the electrochemical and photophysical properties of a series of neutral organic mixed-valence (MV) compounds, 1-7, in which different amine donor centers are connected to perchlorinated triarylmethyl radical units by various spacers. We show that this new class of compounds are excellent model systems for the investigation of electron transfer due to their uncharged character and, consequently, their excellent solubility, particularly in nonpolar solvents. A detailed band shape analysis of the intervalence charge-transfer (IV-CT) bands in the context of Jortner's theory allowed the electron-transfer parameters (inner vibrational reorganization energy lambdav, outer solvent reorganization energy lambdao, and the difference in the free energy between the diabatic ground and excited states, DeltaG degrees degrees , as well as the averaged molecular vibrational mode v) to be extracted independently. In this way we were able to analyze the solvatochromic behavior of the IV-CT bands by evaluating the contribution of each parameter. By comparison of different compounds, we were also able to assign specific molecular moieties to changes in vv. For this class of molecules, we also demonstrate that the adiabatic dipole moment difference Deltamicroab and, consequently, the electronic coupling V12 can be evaluated directly from the absorption spectra by a new variant of the solvatochromic method. Furthermore, an investigation of the electrochemistry of compounds 1-7 by cyclic voltammetry as well as spectroelectrochemistry shows that, not only in the neutral MV compounds but also in their oxidized forms, a charge transfer can be optically induced but with exchanged donor-acceptor functionalities of the redox centers.

Triarylamine-perchlorotriphenylmethyl radicals (TARA-PCTM) may be viewed as open-shell mixed valence donor-acceptor compounds that exhibit strong charge-transfer (CT) bands in the visible to NIR spectral region. While open-shell molecules generally do not fluoresce at RT, we observed a surprisingly strong fluorescence from the highly polar excited CT state of the TARA-PCTM radicals in the visible and NIR spectral region. The fluorescence quantum yield is enhanced by a factor of up to 150 compared to the unsubstituted perchlorotriphenylmethyl radical. The enhancement depends on the donor strength of the TARA moiety which was tuned by small substituents (OMe, Me, Cl, CN, and NO 2 ) attached to the phenyl groups, thus forming a series of donor-acceptor species that mainly differ by the free energy difference of the excited CT state and the ground state. The reorganization parameters of the CT process were extracted by Bixon-Jortner fits to either the absorption or the fluorescence bands. The dynamics of the nonradiative back-electron transfer were investigated by time-resolved fluorescence and transient absorption spectroscopy in the ps to ns time regime. We observed a strong deviation of the back-electron transfer rate from the expected inverted Marcus behavior which might be due to anharmonic effects.

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 1 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 1 order of magnitude smaller than fully conjugated materials.