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

摘要

(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-Ill or class-ll/lll borderline mixed-valence species.In contrast,a broad,strongly solvatochromic IVCT band is observed for 2~+,indicating that this species is a class-ll mixed-valence species.The assignment of 1~+ and 2~+ as symmetric class-Ill and unsymmetric class-ll 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.