Polythiophene derivatives by step-growth polymerization via photoinduced electron transfer reactionsw Binnur Aydogan, Ali Senol Gundogan, Turan Ozturk and Yusuf Yagci* Received (in Cambridge, UK) 23rd July 2009, Accepted 17th August 2009 First published as an Advance Article on the web 7th September 2009 DOI: 10.1039/b914953h A novel method for the step-growth polymerization of diphenyl- dithienothiophene in the presence of diphenyliodonium salt by photoinduced electron transfer is described. Conjugated polymers have attracted great interest 1 due to their wide range of potential applications, including light emitting diodes, 2 batteries, 3 electrochromic devices, 4 sensors, 5 for electromagnetic shielding, 6 and corrosion inhibition. 7 Polythiophenes are one of the most extensively studied families of conjugated polymers because of their characteristic electronic and optical properties. 2,3,6–12 Usually, polythiophenes are obtained by chemical or electrochemical polymerization processes, which provide films with different morphologies and consequently slightly different physical and chemical properties. Photopolymerization is considered to be a useful candidate for the production of these materials in thin film forms. Various strategies have been employed to polymerize thiophene and derivatives photochemically. 13–18 For example, Davis and co-workers 13 photopolymerized an acetic acid derivative of thiophene in aqueous solution using potassium dichromate as catalyst. It was suggested that the polymerization proceeds via a charge transfer reaction of a complex between the thiophene derivative and Cr(VI) in the excited state (Scheme 1). Another successful approach that has been employed is the photolysis of dithienothiophene derivatives and oligo- thiophenes in the presence of electron acceptors such as dinitrobenzene and carbon tetrachloride. 14–16 In this case, the polymerization intermediate radical cation of the monomer was efficiently generated by photoinduced electron transfer from the excited thiophene derivative to the electron acceptor. Recently, we reported photoinitiated polymerization of thiophene by using onium salts such as diaryliodonium and triarylsulfonium salts. 17,18 Although these salts are known to be photoinitiators for UV-induced cationic polymerizations, 19–23 they were also found to be effective in facilitating the polymerization of thiophene. Detailed investigations 18 involving laser flash photolysis and EPR studies revealed that the polymerization mechanism involves electron transfer from photochemically generated phenyliodinium radical cations to thiophene (Scheme 2). More recently, we have reported 24 the use of a polynuclear thiophene derivative, namely 3,5-diphenyldithieno[3,2-b:2,3-d]- thiophene (DDT), 25,26 as an electron-transfer photosensitizer for diaryliodonium salt cationic photoinitiators. DDT provides good sensitivity in the near UV region (350–400 nm). Several types of cationically polymerizable monofunctional monomers, such as cyclohexene oxide, n-butyl vinyl ether, styrene and N-vinylcarbazole, and bifunctional monomers such as 3,4-epoxycyclohexyl-3 0 ,4 0 -epoxycyclohexene carboxylate were readily polymerized in bulk or dichloromethane solutions upon irradiation at l 4 350 nm in the presence of DDT and diphenyliodonium hexafluorophosphate. A mechanism based on electron transfer between excited DDT and iodonium ion was proposed (Scheme S1, ESIw). A wide range of polynuclear aromatic compounds are known to undergo similar electron transfer reactions. 27–32 Polymerizations were initiated either by DDT radical cations or the protons formed via hydrogen abstraction or coupling reactions. By virtue of the thiophene-type radical cation formation and the crucial role of these species in the electropolymerization, the described photoinduced process has the potential to form polymeric molecules. As part of our continuing interest in applying photoinitiating systems for polymer synthesis, we now report the photo- chemical synthesis of polydiphenyldithienothiophene (PDDT). Thus, step-growth polymerization of DDT was triggered by generating radical cations not by electrochemical means but by the photochemical process described. Since the diphenyliodonium ion does not absorb at irradiation wavelengths l 4 350 nm, all the light is absorbed by DDT (Fig. S1, ESIw). Experimentally, DDT was polymerized quite effectively when the solution containing DDT and diphenyliodonium hexafluorophosphate in dichloromethane was irradiated for about 30 min. As can be seen from Fig. 1, the polymerization was accompanied by a darkening of the solution and a new absorption band at Scheme 1 Photochemical polymerization of thiophene carboxylic acid in aqueous solution catalyzed by potassium dichromate. Depatrment of Chemistry, Istanbul Technical University, Maslak, Istanbul, 34469, Turkey. E-mail: yusuf@itu.edu.tr; Fax: +90 212 285 6386; Tel: +90 212 285 3241 w Electronic supplementary information (ESI) available: Experimental section including materials, polymerization procedure and characteri- zation. Photoinitiated cationic polymerization mechanism based on electron transfer (Scheme S1), optical absorption spectra (Fig. S1), GPC profile (Fig. S2), Zimm plot (Fig. S3), 1 H NMR spectra (Fig. S4) and FT-IR spectra (Fig. S5). See DOI: 10.1039/b914953h 6300 | Chem. Commun., 2009, 6300–6302 This journal is  c The Royal Society of Chemistry 2009 COMMUNICATION www.rsc.org/chemcomm | ChemComm