Hexyl-Derivatized Poly(3,4-ethylenedioxyselenophene): Novel Highly Stable Organic Electrochromic Material with High Contrast Ratio, High Coloration Efﬁciency, and Low-Switching Voltage By Mao Li, Asit Patra, Yana Sheynin, and Michael Bendikov* Currently, there is signiﬁcant interest in the design of new electrochromic (EC) materials exhibiting a high contrast ratio (a large change in percent transmittance between the colored and bleached states), high coloration efﬁciency (efﬁcient color changes in response to applied charge) and long-term stabi- lity. [1–4] EC materials could be employed in the fabrication of efﬁcient displays and EC windows if they could be made to exhibit a large change in transmittance upon application of a small charge. While considerable research was performed on inorganic EC materials, better performance has been reported recently using conducting polymers, mostly based on poly(3,4- ethylenedioxythiophene) (PEDOT) and its derivatives. [3–10] Con- ducting polymers offer better processability and better color tunability than their inorganic and molecular counterparts. [1e,5b,9,11] PEDOT ﬁlm can be quite rapidly switched between the absorptive (neutral) and transmissive (doped) states at low switching potentials. The neutral PEDOT ﬁlm shows a band gap of around 1.6 eV and a light sky blue appearance as a doped ﬁlm. [5,12] The coloration efﬁciency (CE) of PEDOT is 137 cm 2 C 1 at full switch, 183 cm 2 C 1 at 95% of full switch, 206 cm 2 C 1 at 85% of full switch, [8] and the dibutyl derivative of poly(3,4-propylenedioxythiophene) (PProDOT(Bu) 2 ) [13] obtained by Reynolds’ group achieves a CE of 1365 cm 2 C 1 , which is currently the highest CE value reported for an electropolymerized EC ﬁlm measured in liquid electrolyte. The stability of the electrochromic materials is another crucially important feature for practical application. Reynolds’ group studied PEDOT, PEDOT-C 8 , and PEDOT-C 14 ﬁlms and found that the ﬁlms retained 65%, 50%, and 62% of their electroactivity after 6000, 9000, and 16000 switching cycles, respectively. [6d] However, the contrast ratios were less than 65%. [6d] Kumar’s group obtained a dibenzyl poly(3,4-propylenedioxythiophene) (PProDOT-Bz 2 ) that has the highest known contrast ratio (89%), a CE of 575 cm 2 C 1 and high stability after 5000 switching cycles. [3] The poly(1,4-bis(2-(3 0 ,4 0 -ethylenedioxy)thienyl)-2-methoxy-5-2 00 - ethylhexyloxybenzene) (P(BEDOT-MEHB)) reported by Wudl’s group has a CE of 680 cm 2 C 1 and also exhibits high stability after 5000 switching cycles, though it has a contrast ratio of only 57%. [4,14] Catellani et al. reported that poly(3-butylthiophene) can be cycled 20000 times without signiﬁcant modiﬁcation of its optical properties, however, its contrast ratio was low and its coloration efﬁciency was not reported. [15] Recently, our group developed a synthetic method for poly(3,4-ethylenedioxyselenophene) (PEDOS), [16] which contains a novel building block for electron-rich p-conjugated polymers. PEDOS has a smaller band gap (1.4 eV) than PEDOT (1.6 eV) and its HOCO (highest occupied crystal orbital) lies 0.1 eV lower than that of PEDOT. [17] O O Se PEDOS-C 6 n-C 6 H 13 O O Se PEDOS n n Here, we report on novel polyselenophenes as EC materials. We ﬁnd that polyselenophenes are excellent electrochromic materials in terms of their contrast ratios and CEs, while retaining stability and switching time characteristics comparable to those of PEDOT derivatives. In particular, here we ﬁnd that poly(hexyl-3,4-ethylenedioxyselenophene) (PEDOS-C 6 ) has one of the highest reported contrast ratios and CEs while maintaining excellent electrochemical stability and a fast switching time. In order to evaluate the EC properties of polyselenophenes, the PEDOS ﬁlms were electrodeposited onto indium-tin-oxide (ITO) coated glass having dimensions of 3.2 cm  0.7 cm at a constant potential of 0.93 V versus Ag/AgCl and passing charge of 0.03 C to form a ﬁlm with a thickness of 100 nm. PEDOS ﬁlm obtained under these conditions has a band gap of ca. 1.42 eV (873 nm, see Fig. S1) as determined by the onset of the p-to-p* transition from the UV-vis-NIR spectrum. [16,18] It also exhibits a high contrast ratio of 55% (see Fig. 1a) at 666 nm (l max ) and a CE of 212 cm 2 C 1 . [19–21] The measured bleached and colored times are 0.4 s and 0.6 s, respectively (at 95% of the maximum value of the contrast ratio). Both the contrast ratio and the coloration efﬁciency of PEDOS are better than those of PEDOT, [8] which has a contrast ratio of 54% and a CE of 137 cm 2 C 1 at full switching. Introduction of alkyl chains onto the polymer backbone signiﬁcantly improves the EC properties of PEDOT derivatives, as shown by Reynolds et al. [6d,g,7–9] Inspired by their work, we have synthesized hexyl-3,4-ethylenedioxyselenophene (EDOS-C 6 ). For spectroelectrochemical measurement, EDOS-C 6 was electro- COMMUNICATION www.advmat.de [*] Dr. M. Bendikov, Dr. M. Li, Dr. A. Patra, Dr. Y. Sheynin Department of Organic Chemistry, Weizmann Institute of Science Rehovot 76100 (Israel) E-mail: michael.bendikov@weizmann.ac.il DOI: 10.1002/adma.200802259 Adv. Mater. 2009, 21, 1707–1711 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1707