Intervalence Transitions in the Mixed-Valence Monocations of Bis(triarylamines) Linked with Vinylene and Phenylene−Vinylene Bridges

Abstract

(E)-4,4'-Bis[bis(4-methoxyphenyl)amino]stilbene, 1, (E,E)-1,4-bis[4-[bis(4-methoxyphenyl)amino]styryl]benzene, 2, and two longer homologues, (E,E,E)-4,4'-bis[4-[bis(4-methoxyphenyl)amino]styryl]stilbene, 3, and (E,E,E,E)-1,4-bis(4-[4-[bis(4-methoxyphenyl)amino]styryl]styryl)benzene, 4, have been oxidized to their mono- and dications using tris(4-bromophenyl)aminium hexachloroantimonate. The intervalence charge-transfer (IVCT) band of 1(+) is narrow and asymmetric and exhibits only weak solvatochromism. Analysis of this band indicates that 1(+) is a class-III or class-II/III borderline mixed-valence species. In contrast, a broad, strongly solvatochromic IVCT band is observed for 2(+), indicating that this species is a class-II mixed-valence species. The assignment of 1(+ ) and 2(+) as symmetric class-III and unsymmetric class-II species, respectively, is also supported by AM1 calculations. Hush analysis of the IVCT bands of both 1(+) and 2(+) gives larger electronic couplings, V, than for their analogues in which the double bonds are replaced with triple bonds. The diabatic electron-transfer distance, R, in 1(+) can be estimated by comparison of the V estimated by Hush analysis and from the IVCT maximum; it is considerably less than the geometric N-N separation, a result supported by quantum-chemical estimates of R for 1(+)-4(+). In 3(+) and 4(+), the IVCT is largely obscured by an intense absorption similar to a band seen in the corresponding dications and to that observed in the monocation of a model compound, (E,E,E)-1-[bis(4-methoxyphenyl)amino]-4-[4-[4-(4-tert-butylstyryl)styryl]styryl]benzene, 5, containing only one nitrogen redox center; we attribute this band to a bridge-to-N(+) transition. The corresponding dications 1(2+)-4(2+) show a complementary trend in the coupling between redox centers: the shortest species is diamagnetic, while the dication with the longest bridge behaves as two essentially noninteracting radical centers.

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

“…Clark and coworkers 61 and others 8,47,[62][63][64] have applied semi-empirical CI calculations to a number of organic MV systems and obtained substantial insights. When applied within a semi-empirical framework, the CI covers mainly the non-dynamical correlation part, whereas it is assumed that the semi-empirical parameterization of the method accounts for the dynamical correlation part (note that semi-empirical MO methods use mainly minimal basis sets and thus could not provide the dynamical correlation explicitly with sufficient accuracy).…”

“…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.…”

“…8 Finally, 9 and 10 have relatively short bridges and are expected to be on the class III side. 5,44,47,49,[53][54][55][56] The radicals 11-17 are asymmetric neutral mixed-valence systems with two inequivalent redox centers. However, they have an electronic structure closely related to the bis-triarylamine radical cations and feature a number of interesting properties that make them suitable targets for further study.…”

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.…”