Hole Transfer Processes in meta- and para-Conjugated Mixed Valence Compounds: Unforeseen Effects of Bridge Substituents and Solvent Dynamics

To address the question whether donor substituents can be utilized to accelerate the hole transfer (HT) between redox sites attached in para- or in meta-positions to a central benzene bridge, we investigated three series of mixed valence compounds based on triarylamine redox centers that are connected to a benzene bridge via alkyne spacers at para- and meta-positions. The electron density at the bridge was tuned by substituents with different electron donating or accepting character. By analyzing optical spectra and by DFT computations we show that the HT properties are independent of bridge substituents for one of the meta-series, while donor substituents can strongly decrease the intrinsic barrier in the case of the para-series. In stark contrast, temperature-dependent ESR measurements demonstrate a dramatic increase of both the apparent barrier and the rate of HT for strong donor substituents in the para-cases. This is caused by an unprecedented substituent-dependent change of the HT mechanism from that described by transition state theory to a regime controlled by solvent dynamics. For solvents with slow longitudinal relaxation (PhNO₂, oDCB), this adds an additional contribution to the intrinsic barrier via the dielectric relaxation process. Attaching the donor substituents to the bridge at positions where the molecular orbital coefficients are large accelerates the HT rate for meta-conjugated compounds just as for the para-series. This effect demonstrates that the para-meta paradigm no longer holds if appropriate substituents and substitution patterns are chosen, thereby considerably broadening the applicability of meta-topologies for optoelectronic applications.