Electrochemical Synthesis of Optically Active Polyaniline Films** By Wenguang Li and Hsing-Lin Wang* 1. Introduction Recently, there has been increased interest in synthesizing optically and electrically active polymers, mainly because of their potential applications in chiral separations, [1,2] surface- modified electrodes, [3] as well as chemical [4,5] and biological sensors. [6,7] Polyaniline (PANI) and its derivatives comprise a set of electroactive polymers that have been the subject of nu- merous studies. Chiral PANI is usually prepared by either co- dissolving PANI and a chiral acid in a common solvent [8±10] or by polymerizing aniline in the presence of a chiral acid. [11] The electrostaticinteractionbetweenanionicchiralacidandcation- ic aniline/PANI promotes the formation of a camphor sulfonic acid (CSA)/aniline complexÐthe precursor of chiral PANI. The asymmetrical (helical) PANI conformation and chain packing are most responsible for its optical activity. The solva- tion of the chiral acid and aniline/PANI in a solvent-based chemical synthesis of chiral PANI can weaken the electrostatic interaction between them. [12] For this obvious reason, most at- temptsusingsolvent-basedchemicalsynthesistopreparechiral PANI only yield products with low chirality. Previously, Wal- lace and co-workers reported the polymerization of aniline in the presence of optically active CSA to form chiral PANI and PANI nanocomposites. [13,14] Although not being produced in largequantity,thesepioneeringworkspresentafacilesynthesis ofchiralPANI. In our previous work, we reported the chemical synthesis of chiral PANI nanofibers with very high optical activity. [15] The highopticalactivitywasachievedbyoligomer-assistedpolymer- ization of aniline in concentrated CSA solution. The enhanced interactionbetweenCSAandaniline/PANIisachievedbymini- mizingtheamountofsolventusedforthereaction.Anilineolig- omersinthiscaseactasªseedsºtostartthepolymerizationdue to their lower oxidation potentials. [16] In this solvent-based syn- thesis, ammonium persulfate was found to be among the best oxidants for preparing chiral PANI. Ammonium persulfate is a strongoxidant,capableofoxidizingbothanilinemonomersand oligomers. Therefore, initiation of the polymer chains starts from both the aniline monomers and oligomers. Although we are confident that the majority of the polymer chains originate fromananilineoligomer,itisverypossiblethatinhomogeneity (morethanonekindofPANIchain)mayexistinthesample.In addition, persulfate and sulfate ions are introduced in the solu- tion, and these ions will compete with chiral CSA for the cat- ionic sites on the aniline monomers and actively polymerizing PANI;thiscompetinginteractionwillinhibitformationofasso- ciative precursor of chiral PANI. All the above factors contrib- utetolesseningthechiralityofPANI.Thesedrawbacksmaybe alleviated somewhat by controlling the rate of addition of am- monium persulfate into the solution. To entirely avoid any ad- verse effects caused by ammonium persulfate or any other oxi- dants,wehavedevelopedanelectrochemicalmethodtoprepare chiral PANI. The electrochemical approach has several advan- tages over the chemical route. By initiating the polymerization reaction at a lower oxidation potential, we can selectively oxi- dize specific oligomer species instead of aniline according to theirredoxpotentials.Therefore,wecancontrolthepolymeriza- tionreactionbyensuringallchiralPANIsareinitiatedfromthe aniline oligomer. We can then correlate the rate of polymeriza- tionandinitiatorswiththeopticalactivityofthePANIthinfilms to better understand the underlying mechanisms and the effect ofanilineoligomersonthefilmproperties.Theelectrochemical methodalsoallowscontrolofthemorphologyandopticalactiv- ityofthePANIthinfilmsbytuningtheoxidationpotentials. Adv. Funct. Mater. 2005, 15, 1793±1798 DOI: 10.1002/adfm.200500177  2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1793 ± [*] Dr. H.-L. Wang,Dr. W. Li MSJ586, Bioscience Division, Los Alamos National Laboratory Los Alamos, NM 87545 (USA) E-mail: hwang@lanl.gov [**] This work is supported by the Cross Enterprise Technology Develop- ment Program of the National Aeronautics and Space Administration (NASA) and by Basic Energy Sciences, Office of Science (DOE). We thank Dr. Yonghao Zhao and Dr. Yuntian Zhou for their assistance in taking the X-ray diffraction measurements. Polyaniline(PANI)thinfilmswithhighopticalactivityarepreparedelectrochemicallyonindiumtinoxide(ITO)substratesby polymerizing aniline in the presence of aniline oligomers and optically active camphor sulfonic acid (CSA). These optimized PANI chiral thin films have an anisotropic factor of 0.03 and an estimated molar ellipticity of 8.810 5 degcm 2 dmol ±1 , ~ 25% higher than our previously synthesized chiral PANI nanofibers that were prepared using the chemical method. The thin film preparedwithoutanilineoligomersisbasicallyachiral.Theelectrochemicalmethoddoesnotrequireanoxidizingagentandal- lows control of the rate of polymerization reaction by varying the oxidation potential. The electrochemical polymerization also ensures that the initiation occurs solely with the oligomers. Varying the experimental parameters, such as oxidation potential and the choice of oligomers, allows tuning of optical activity, adsorption properties, crystallinity, and morphology of the PANI thinfilms. 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