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We investigated the spectroscopic properties of a series of four bistriarylamine donor-pi-bridge-donor D-pi-D compounds (dimers), composed of two asymmetric triarylamine chromophores (monomers). UV/vis, fluorescence, and transient absorption spectra were recorded and compared with those of the corresponding D-pi monomers. Bilinear Lippert-Mataga plots indicate a major molecular reorganization of the excited state in polar media for all compounds. The excited states of the dimers are described as mixed-valence states that show, depending on the chemical nature of the pi bridge, a varying amount of interactions (couplings). We found that superradiant emission, that is, an enhancement of the fluorescence rate in the dimer, is observed only in the case of weak and medium coupling. Whether the first excited-state potential energy surface of the dimers is described by single minimum or a double minimum potential depends on the solvent polarity and the electronic coupling. In the latter case, the dimer relaxes in a symmetry broken CT state with partial positive charge at the triarylamine donor and negative charge at the pi bridge. The [2.2]paracyclophane bridged dimer is an example of a weakly coupled system because the spectroscopic behavior is very similar to the corresponding p-xylene monomer. In contrast, anthracene as well as p-xylene bridges mediate a stronger coupling and reveal a significant cooperative influence on the optical properties.

In this paper, we present the absorption properties of a series of bis-triarylamino-[2.2]paracyclophane diradical dications. The localized pi-pi and the charge-transfer (CT) transitions of these dications are explained and analyzed by an exciton coupling model that also considers the photophysical properties of the "monomeric" triarylamine radical cations. Together with AM1-CISD-calculated transition moments, experimental transition moments and transition energies of the bis-triarylamine dications were used to calculate electronic couplings by a generalized Mulliken-Hush (GMH) approach. These couplings are a measure for interactions of the excited mixed-valence CT states. The modification of the diabatic states reveals similarities of the GMH three-level model and the exciton coupling model. Comparison of the two models shows that the transition moment between the excited mixed-valence states mu(ab) of the dimer equals the dipole moment difference Delta of the ground and the excited bridge state of the corresponding monomer.

A series of [2.2]paracylophane-bridged bis-triarylamine mixed-valence (MV) radical cations were analyzed by a generalized Mulliken-Hush (GMH) three-level model which takes two transitions into account: the intervalence charge transfer (IV-CT) band which is assigned to an optically induced hole transfer (HT) from one triarylamine unit to the second one and a second band associated with a triarylamine radical cation to bridge (in particular, the [2.2]paracyclophane bridge) hole transfer. From the GMH analysis, we conclude that the [2.2]paracyclophane moiety is not the limiting factor which governs the intramolecular charge transfer. AM1-CISD calculations reveal that both through-bond as well as through-space interactions of the [2.2]paracyclophane bridge play an important role for hole transfer processes. These electronic interactions are of course smaller than direct pi-conjugation, but from the order of magnitude of the couplings of the [2.2]paracyclophane MV species, we assume that this bridge is able to mediate significant through-space and through-bond interactions and that the cyclophane bridge acts more like an unsaturated spacer rather than a saturated one. From the exponential dependence of the electronic coupling V between the two triarylamine localized states on the distance r between the two redox centers, we infer that the hole transfer occurs via a superexchange mechanism. Our analysis reveals that even significantly longer pi-conjugated bridges should still mediate significant electronic interactions because the decay constant beta of a series of pi-conjugated MV species is small.