284 Macromolecules 1995,28, zyxwvu 284-287 Localization Effects in Asymmetrically Substituted Polythiophenes: Controlled Generation of Polarons, Dimerized Polarons, and Bipolarons Karim Fafd and Mario Leclerc* Dkpartement de Chimie, Universitd de Montreal, Montreal, QuLbec, zyxw H3C 357 Canada My Nguyen and Art Diaz IBM Almaden Research Center, San Jose, California 95120-6099 Received May zyxwvutsr 25, 1994; Revised Manuscript Received October 12, 1994@ ABSTRACT: The attachment to polythiophenes of substituents with different electronic properties produces polymeric materials with unusual electrical and optical properties. Controlled generation of polarons, dimerized polarons, and bipolarons was achieved through the formation of moieties of high and low band gap energy along the polythiophene backbone. The occurrence of such quantum wells decreased the recombination of the polarons in bipolarons as would be observed in symmetrically substituted polythiophenes. One of the most interesting consequences of this band-gap engineering is the ability of tuning the transport properties of these polymers. These novel characteristics make possible the formation of paramagnetic and diamagnetic conducting states through the control of the redox state. They may also enhance the polaron population available for singlet exciton conversion in electrolumi- nescent devices. Introduction The addition of relatively long and flexible alkyl side chains on polythiophene has led to the development of solution and melt processable conjugated po1ymers.l Moreover, the presence of these side chains can also modify the electrical, electrochemical, and optical prop- erties of the resulting materials. This synthetic ap- proach is extremely versatile and can be useful for the design of polythiophenes with tailored physical proper- ties.2 In this respect, it has been recently shown that the presence of strong electron-donating alkoxy sub- stituents dramatically decreases the oxidation potential and the band gap of the polymers and leads to stable and nearly transparent conducting (oxidized) poly- thiophene^.^ Other studies have revealed that it is possible to tune the oxidation potential of processable polythiophenes from zyxwvutsrq +0.05 to +1.3 V vs SCE by the adequate choice of the nature and the position of the substituent^.^,^ On the other hand, this strong dependence of the stituents should produce a conjugated polymer with a localized and specific HOMO-LUMO gap energy. This approach was recently developed through the synthesis of asymmetrically substituted poly(bithiophene)~.~ Pre- liminary studies on zyxw poly(4-bromo-4'-(octyloxy)-2,2'- bithi~phene)~ have revealed the presence of three redox processes which were related to the successive formation of radical cations (polarons), dimerized polarons, and dications (bipolarons). A high concentration of polarons is not usually observed in polythiophenes since the delocalization along the backbone allows the formation of the more stable bipolarons.2,8 Therefore, to shed some light about localization effects in asymmetrically substituted poly(bithiophene)s, a more complete characterization of the physical proper- ties of poly(4-bromo-4'-(octyloxy)-2,2'-bithiophene) has been carried out. This concept has also been extended to a poly(bithi0phene) substituted by two electron- donating groups of different strengths, namely, poly(4- butoxy-4'-decyl-2,2'-bithiophene). oxidation potential of the polythTophkes upon their substitution pattern could be utilized to create quantum wells along the polymer backbone, by analogy with energy wells (or quantum wells) are formed by sand- between two wide gap semiconductors (& NGds).' Quantum confinement of electrons at the semiconductor interface results in quasi-two-dimensional electron sys- tems in which the electronic motion is unbound within Experimental Section Materials. Poly(4-bromo-4'-(octyloxy)-2,2'-bithiophene) and ~ol~(4-butox~-4'-dec~l-2,2'-bithio~hene) were synthesized fol- lowing the procedures described in a previous p~blication.~ These polymers are completely soluble in chloroform and bration with polystyrene ~tandards,~ these polymers have a weight-average molecular weight of ca. 5000-~0000, Physical Measurements. In-situ conductivity measure- ments were carried out on a dual microelectrode (courtesv of inorganic semiconductors. In these systems, potential an low band gap layer (i'e'> tetrahydrofuran, From SEC measurements based on a Cali- the interface plane but quantized perpendicularly to it. The unique electronic properties of such systems have been widely investigated for more than two decades in materials such as GaAs-AlGaAs heterostructures. One- and zero-dimensional semiconductors (quantum wire and dots) have also been extensively investigated.6 Similarly, the polymerization of bithiophenes bearing both electron-donating and electron-withdrawing sub- * To whom correspondence should be addressed. @ Abstract published in Advance ACS Abstracts, December 1, 1994. 0024-9297/95/2228-0284$09.00/0 Dr. G. Zotti), and the experimental setup was derived from the literature.*O The working electrode was a two-band platinum electrode (0.3 cm x 0.01 cm for each band) with an interband spacing of 6 pm, coated by a thin polymer film of ca. 1 pm. The polymer films were held under various electrochemical potentials while a small-amplitude (ca. 10 mV) dc potential was applied between the bands. The current was recorded with a picoammeter (Keithley 485). Chemical oxida- tion of the polymers was performed by adding known quanti- ties of iron trichloride solution in chloroform. W-vis-near- IR spectra were recorded after homogeneization with a Cary 5 spectrophotometer. In-situ solid-state spectroelectrochem- istry ion ITO-coated glass), cyclic voltammetry (EG&G poten- 0 1995 American Chemical Society