[2.2]Paracyclophane-Bridged Mixed-Valence Compounds:  Application of a Generalized Mulliken−Hush Three-Level Model

Abstract

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.

Computational and spectroscopic studies of organic mixed-valence compounds: where is the charge?

“…We find 5-7 to be localized (class II) in both solvents, in agreement with experimental observation. 5,7,8,10,[44][45][46][47][48][49][50][51][52] This is indicated by the ET barriers, the dipole moments, and the View Article Online asymmetry of the C Ar -N distances. As expected from the increasing donor capacity of the substituents on the central phenyl ring (H for 5, CH 3 for 6, and OCH 3 for 7), the amount of symmetry breaking tends to decrease from 5 to 7 in a given solvent (albeit 5 and 6 behave very similarly and only 7 is notably less localized).…”

“…This is confirmed by experimental observations (see below). 5,7,8,10,[44][45][46][47][48][49][50][51][52] Cation 8 exhibits the rather large central anthracene group and thus is a special case (see below). 8 Cations 5-7 move towards the border from the class II side.…”

“…Typical redox centers are e.g. triarylamines, [5][6][7][8][9][10] perchlorotriphenylmethyl radicals, [11][12][13] hydrazines, 14,15 dimethoxybenzenes, 16,17 or quinones. 18,19 ET may proceed between the redox centers via the bridge, thermally or optically induced.…”

“…29 For a test series of four MV bis-triarylamine radical cations we could show that a computational evaluation of ground-and excitedstate properties by this protocol allows a relatively fine bracketing of the localized/delocalized nature, the structure, and the spectroscopic parameters of such systems. In this contribution we will extend the validation of this protocol to a series of bistriarylamine radical cations with degenerate redox centers which have been well characterized experimentally 5,7,8,10,[44][45][46][47][48][49][50][51][52][53][54][55][56] and which shall serve as test cases to demonstrate how to apply DFT electronic structure calculations to discriminate between class II and class III MV systems. Furthermore we will apply the same computational protocol 29 to triarylamineperchlorotriphenylmethyl radical MV systems with nondegenerate redox centers.…”

Charge Delocalization in a Homologous Series of α,α′-Bis(dianisylamino)-Substituted Thiophene Monocations

“…[36][37]50 Indeed, these absorptions fall into the spectral range in which intervalence absorptions of bis(triarylamine) radical cations are commonly observed. 16,20,[34][35][51][52][53][54][55][56][57][58][59][60][61] An important difference to the intervalence absorptions of coordination compounds 40,[62][63][64][65] are the comparatively large extinction coefficients of these bands, but this is a (favorable) peculiarity of organic mixed-valence compounds which has been noted many times before. Table 2.…”