The Class II/III Transition in Triarylamine Redox Systems

Abstract

The mixed-valence character of a set of six bistriarylamine derivatives with varying π-electron spacers has been investigated. The distances of triarylamine redox centers vary from 0.5 nm (N,N,N‘,N‘-tetra-4-methoxyphenyl-p-phenylenediamine, 6 + ) to 2 nm (1,4-bis{4-[N,N-di(4-methoxyphenyl)amino]phenylethynyl}benzene, 1 + ). All radical cation species show rather strong intervalence charge-transfer (IV−CT) bands in the NIR as measured by UV/vis/NIR spectroelectrochemistry. Hush analysis was used to derive the electronic coupling V, which is very high and ranges from 500 to 3240 cm-1. A detailed band shape analysis revealed a cutoff at 2V at the low-energy side of the IV−CT bands when the coupling 2V approaches the band maximum λ. This results in very narrow IV−CT bands much smaller than the high-temperature limit, although even the phenylenediamine derivative 6 + belongs to the Robin/Day class II (localized redox centers) just at the border to class III (delocalized redox centers). Hush theory anticipates this cutoff because the smallest energy transition possible is at 2V. The ultrafast thermal electron-transfer rate constants were estimated from V and range from about 109 s-1 to 1012 s-1. The electrochemical properties have also been investigated: a linear correlation of redox potential splitting vs electronic coupling was found.

Oligo(p‐phenyleneethynylene) (OPE) Molecular Wires: Synthesis and Length Dependence of Photoinduced Charge Transfer in OPEs with Triarylamine and Diaryloxadiazole End Groups

“…[29,30] The symmetrical D--D compound 6 contains two oxidation waves only 120 mV apart, corresponding to sequential oxidations at the two (electronically interacting) triarylamine units. [31] Additional oxidation waves ascribed to the bridge unit are also observed at higher potentials (0.65-0.88 V) which shift cathodically from 2 to 5. This shows that higher oxidation states are stabilized by the longer, more delocalised bridges.…”

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.…”

“…[6][7][8][9][10][11][12][13][14] In a purely computational study, Lacroix and coworkers arrived at the conclusion that oligothiophene monocations may be described as organic mixed-valence species. 15 Indeed, the oneelectron oxidized or one-electron reduced forms of several oligo-p-phenylene and oligo-p-phenylene vinylene systems could be described well as organic mixed-valence compounds in various experimental studies, [16][17][18][19][20][21][22][23][24][25] but until now there has been comparatively little conceptually analogous work on thiophene systems. [26][27][28][29][30][31][32][33][34][35] A notable exception is the recent study of the oxidized forms of bis (4-(alkoxyphenyl)amino) derivatives of dithienylethene and bithiophene and the finding that a mixed-3 valence description of these cations is meaningful.…”

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

“…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.…”