Electronic couplings in organic mixed-valence compounds:: The contribution of photoelectron spectroscopy

We show that the electronic coupling in strongly coupled organic mixed-valence systems can be effectively probed by means of gas-phase ultraviolet photoelectron spectroscopy (UPS). Taking six diamines as examples, the UPS estimates for the electronic couplings H-ab are compared with the corresponding values determined from the intervalence charge-transfer absorption bands and from electronic structure calculations. Similar trends are observed for the H-ab values estimated from UPS and optical spectra; this provides support for the applicability of Hush theory to strongly coupled organic mixed-valence systems. The UPS electronic couplings are found to be somewhat smaller than those from optical spectroscopy, which is attributed to the role of vibronic coupling to symmetrical modes; when corrected for this vibronic coupling, the UPS H-ab estimates confirm that triarylamine-based mixed-valence systems are close to the class-II/class-III borderline.