From Monomers to π Stacks, from Nonconductive to Conductive: Syntheses, Characterization, and Crystal Structures of Benzidine Radical Cations

• Published in: Chemistry : a European journal 2012-09, Vol.18 (37), p.11828-11836
• Main Authors: Chen, Xiaoyu, Ma, Binbin, Wang, Xingyong, Yao, Shengxin, Ni, Lichen, Zhou, Zhaoyi, Li, Yizhi, Huang, Wei, Ma, Jing, Zuo, Jinglin, Wang, Xinping
• Format: Article
• Language: English
• Subjects: benzidine; Benzidines - chemistry; Cations; Cations - chemistry; Chains; conducting materials; Crystallography, X-Ray; Derivatives; Dimers; Electric Conductivity; Free radicals; Free Radicals - chemistry; Models, Molecular; Molecular Structure; Organometallic Compounds - chemical synthesis; Organometallic Compounds - chemistry; pi interactions; radical ions; Radicals; Single crystals; Stacks; X-ray diffraction
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201103972

Salts that contain radical cations of benzidine (BZ), 3,3′,5,5′‐tetramethylbenzidine (TMB), 2,2′,6,6′‐tetraisopropylbenzidine (TPB), and 4,4′‐terphenyldiamine (DATP) have been isolated with weakly coordinating anions [Al(ORF)4]− (ORF=OC(CF3)3) or SbF6−. They were prepared by reaction of the respective silver(I) salts with stoichiometric amounts of benzidine or its alkyl‐substituted derivatives in CH2Cl2. The salts were characterized by UV absorption and EPR spectroscopy as well as by their single‐crystal X‐ray structures. Variable‐temperature UV/Vis absorption spectra of BZ.+[Al(ORF)4]− and TMB.+[Al(ORF)4]− in acetonitrile indicate an equilibrium between monomeric free radical cations and a radical‐cation dimer. In contrast, the absorption spectrum of TPB.+SbF6− in acetonitrile indicates that the oxidation of TPB only resulted in a monomeric radical cation. Single‐crystal X‐ray diffraction studies show that in the solid state BZ and its methylation derivative (TMB) form radical‐cation π dimers upon oxidation, whereas that modified with isopropyl groups (TPB) becomes a monomeric free radical cation. By increasing the chain length, π stacks of π dimers are obtained for the radical cation of DATP. The single‐crystal conductivity measurements show that monomerized or π‐dimerized radicals (BZ.+, TMB.+, and TPB.+) are nonconductive, whereas the π‐stacked radical (DATP.+) is conductive. A conduction mechanism between chains through π stacks is proposed. Totally radical: Salts that contain radical cations of benzidine (BZ), 3,3′,5,5′‐tetramethylbenzidine, 2,2′,6,6′‐tetraisopropylbenzidine, and 4,4′‐terphenyldiamine (DATP) have been isolated and structurally characterized. Among these newly synthesized radical cations, only DATP.+ is conductive. A conduction mechnism between chains through π stacks is proposed (see figure).