From monomers to π stacks, from nonconductive to conductive: Syntheses, characterization, and crystal structures of benzidine radical cations
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(CF 3)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.