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

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journal contribution
posted on 2005-12-07, 00:00 authored by Stephen Barlow, Chad Risko, Sung-Jae Chung, Neil M. Tucker, Veaceslav Coropceanu, Simon C. Jones, Zerubba Levi, Jean-Luc Brédas, Seth R. Marder
(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 12+42+ 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.