pubs.acs.org/cm r XXXX American Chemical Society Chem. Mater. XXXX, XXX, 000–000 A DOI:10.1021/cm900843b Polythieno[3,4-b]thiophene as an Optically Transparent Ion-Storage Layer Michael A. Invernale, Venkataramanan Seshadri, Donna Marie D. Mamangun, Yujie Ding, James Filloramo, and Gregory A Sotzing* University of Connecticut, Department of Chemistry and the Polymer Program, 97 N. Eagleville Road, Storrs, Connecticut 06269-3136 Received March 26, 2009. Revised Manuscript Received May 4, 2009 Ion storage layers have been employed in the construction of electrochromic devices to enhance device lifetimes through balanced ion shuttling. This has led to a search for a material which has a high charge capacity as well as optical transparency. Poly(thieno[3,4-b]thiophene) (PT34bT) exhibits high transparency in the visible region in both its neutral and oxidized states, in addition to having a high charge capacity, making it an ideal candidate for an ion storage layer. Herein we report devices fabricated by electrodeposition of several common chromophores, such as PEDOT, PDiBz- ProDOT, and PProDOT-Me 2 . Devices were made with and without a balanced layer of PT34bT on the counter electrode and were probed for coloration and contrast. It was found that the addition of the ion storage layer did not alter the color of any of the devices and resulted in a minimal, predicted loss of contrast corresponding to the thickness of the ion storage layer. Introduction The switching of conjugated polymers between an insulating and a conducting state is usually accompanied by a change in their absorption/transmittance and ion diffusion, forming the basis of their applications in elec- trochromics. In general, switching of conjugated poly- mers to their various oxidation states can also lead to multiple color transitions owing to their broad spectra. 1 Both anodically and cathodically coloring conjugated polymers that switch between a colored state and a trans- missive, colorless state are well-known. 2-5 Examples of green to colorless and black to colorless materials have been demonstrated, as well as various IR attenuation devices. 6-10 All of these electrochromic (EC) cells are basically redox cells; that is, an oxidation process in one of the electrodes must be compensated by a reduction in the auxiliary electrode. Thus, a charge compensating layer on the second, auxiliary electrode is indispensable for prolong- ing the lifetime of the device. A dual polymer approach using an anodically coloring polymer and a cathodically coloring polymer has been demonstrated earlier. 11 In these devices, the anodically and cathodically coloring polymers are complementarily coloring in nature and hence have a synergistic effect in the bleached and colored states. The most commonly used anodically coloring polymers, which are bleached in their neutral state and colored in their oxidized state, are high band gap polymers. For electro- chromic applications, the λ max of the neutral high band gap polymers is expected to lay completely within the UV region; in most cases the low energy absorption of the neutral polymer tails into the visible region yielding a yellow color in the bleached state. Yellow is known to distort the perception of color by the human eye, which would be undesirable for applications such as windows or sunglasses. This can be detrimental for some applications, including variable transmittance smart windows and dis- plays. Thus, any ion storage layer which presents any yellowing effects is undesirable. Dual polymer complemen- tary coloring electrochromic windows are effective in vari- able transmission devices since they synergistically bleach. However, for full color polymeric displays, there is a strong need for one of the conjugated polymers not to exhibit any coloring effect in the visible region in any of the oxidation states. In essence, the second polymer must act as an ion- storage layer alone. Reynolds and co-workers reported that the use of PProDOP-NPrS (Figure 1), a high band gap polymer *Corresponding author. E-mail: sotzing@mail.ims.uconn.edu. (1) Sotzing, G. A.; Reddinger, J. L.; Katritzky, A. R.; Soloducho, J.; Musgrave, R.; Reynolds, J. R. Chem. Mater. 1997, 9, 1578–1587. (2) Unur, E.; Jung, J. H.; Mortimer, R. J.; Reynolds, J. R. Chem. Mater. 2008, 20(6), 2328–2334. (3) Tehrani, P.; Hennerdal, L. O.; Dyer, A. L.; Reynolds, J. R.; Berggren, M. J. Mater. Chem. 2009, 19, 1799–1802. 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