Intramolecular Transport of Charge Carriers in Trimeric Aniline upon a Three-Step Acid Doping Process Qi Zhang, † Abdelahad Khajo, ‡ Tsunehiro Sai, † Ian de Albuquerque, † Richard S. Magliozzo, ‡ and Kalle Levon* ,† † Department of Chemical and Biological Sciences, Polytechnic Institute of New York University, Six MetroTech Center, Brooklyn, New York 11201, United States ‡ Department of Chemistry, Brooklyn College and the Graduate Center of the City University of New York, 2900 Bedford Avenue, Brooklyn, New York 11210, United States ABSTRACT: The “acid doping” of a methyl-capped aniline trimer, N-[4- (dimethylamino)phenyl]-N-(4-{[4-(dimethylamino)phenyl]imino}-2,5- cyclohex- adien-1-ylidene)-amine (TANI), was performed stoichiometrically to study the nature of charge carriers induced by the acid protonation process. The redox centers in TANI were found to undergo a reversible three-step protonation with 1 equiv, 2 equiv and a large molar excess of dodecylbenzenesulfonic acid (DBSA) in chloroform, as evidenced by three different chromophores (doping levels I, II and III) observed using UV-vis-NIR. Acidity of the dopants and solvent polarity were controlling factors. As revealed by electron paramagnetic resonance spectroscopy (EPR), the doping levels I, II, and III achieved by doping 0.1 mM TANI/ chloroform solutions with different amounts of DBSA exhibited relative spin densities of 1:1.2:2.2. Since the expected maximum spin population of TANI through acid doping is two spins per molecule, the reduced paramagnetism given by the doubly protonated TANI (doping level II) indicated partially coupled unpaired spins. The third protonation step (doping level III) produced almost double the unpaired spin concentration compared to the lower doping levels and a multiline EPR spectrum likely comprising two overlapping signals of similar overall line width. The hyperfine couplings contributing to the splittings in this signal were estimated by simulation incorporating 6-H and 1-N nuclei most likely from one highly localized unpaired spin on a terminal dimethylamino group, with an underlying apparent singlet arising from a delocalized unpaired spin; the diradical proposed as the species exhibiting the multiplet EPR signal is isolated by the bridging ammonium cation created by the third doping step. The phenomena suggested the stepwise evolution of partly formed diamagnetic bipolarons from polaron interactions at doping level II and the transformation to the more isolated unsymmetrical system we label “two polarons on a chain” in a triplet state at doping level III. The results provide the characterization of novel doping behaviors for a trimeric aniline molecule in organic solution. ■ INTRODUCTION Polyaniline (PANI), as one of the most studied conducting polymers, has attracted considerable attention during the past 20 years due to its adjustable conductivity, good stability and mechanical flexibility. It has been widely used in sensors, solar cells, rechargeable batteries, light emitting diodes and electro- chromic materials. 1-6 Unlike most conducting polymers depending on oxidative and reductive doping, PANI changes into its conducting form, known as the emeraldine salt form (ES), through the so-called nonredox or acid doping of its insulating emeraldine base form (EB). 7 Two types of charge carriers created in that process have been described: the delocalized paramagnetic singly charged polaron with spin = 1/ 2, and the localized doubly charged diamagnetic bipolaron. The bipolaron structure is widely accepted as being formed by the energetically favorable coalescence of two polarons into a nonparamagnetic dication. 8 In such a bipolaron structure, the two unpaired spins are coupled and form a unique deformation segment. It was postulated that in relatively longer chains, the “two polarons on a chain” structure is predominant because of reduced coupling between two sites, while the bipolaron structure (S = 0) tends to form in oligomers with shorter length of the conjugated system, evidenced by the loss of EPR signals. 9,10 Oligomeric PANI analogues have been synthesized with a variety of well-defined conjugational lengths for the past decade, mostly because of their good solubility in common solvents, ease of crystallization, and chemical manipula- tion. 11-15 While oligomers of the thiophene family have frequently been used to study the electronic properties evolved upon oxidation in conjugated structures both experimentally and theoretically, the systematic study of stoichiometric acid- base doping-dedoping of aniline oligomers has rarely been conducted. 16-18 For example, Janssen et al. synthesized and Received: March 21, 2012 Revised: June 19, 2012 Article pubs.acs.org/JPCA © XXXX American Chemical Society A dx.doi.org/10.1021/jp3027258 | J. Phys. Chem. A XXXX, XXX, XXX-XXX