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The study of thin film molecular architectures is an increasingly important endeavor with respect to the development and characterization of materials ranging from liquid crystalline displays to receptor-based biosensors. Here we describe an apparatus capable of simultaneously acquiring broadband spectroscopic and electrochemical information on molecularly thin films deposited on a transparent electrode surface through a multiple internal reflection geometry. To demonstrate the capabilities of this system, the spectroelectrochemical behavior of a single, neutral copper phthalocyanine bilayer was evaluated.

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We report the first application of a potential-modulated spectroelectrochemical ATR (PM-ATR) instrument utilizing multiple internal reflections at an optically transparent electrode to study the charge-transfer kinetics and electrochromic response of adsorbed films. A sinusoidally modulated potential waveform was applied to an indium-tin oxide (ITO) electrode while simultaneously monitoring the optical reflectivity of thin (2-6 equivalent monolayers) copolymer films of poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3,4-ethylenedioxythiophene methanol) (PEDTM), previously characterized in our laboratory. At high modulation frequencies the measured response of the polymer film is selective toward the fastest electrochromic processes in the film, presumably those occurring within the first adsorbed monolayer. Quantitative determination of the electrochromic switching rate, derived from the frequency response of the attenuated reflectivity, shows a linear decrease in the rate, from 11 x 10(3) s(-1) to 3 x 10(3) s(-1), with increasing proportions of PEDTM in the copolymer, suggesting that interactions between the methanol substituent on EDTM and the ITO surface slow the switching process by limiting the rate of conformational change in the polymer film.

This work describes the electrochemical copolymerization and spectroelectrochemical characterization of 3,4-ethylenedioxythiophene (EDOT) with a commonly used EDOT derivative: 2,3-dihydrothieno[3,4-b]-1,4-dioxyn-2-yl methanol (EDTM), on indium-tin oxide (ITO) electrodes, as a function of the EDTM/EDOT comonomer feed ratio. The potential of initial polymerization and the degree of optical contrast between reduced and oxidized states increased steadily with increasing proportions of EDTM. Reactivity ratios were determined by spectroscopic characterization of the copolymer film and by monitoring the depletion of monomer from the starting solution by liquid chromatography, following the formation of relatively thick PEDOT/PEDTM films. Average reactivity ratios of 1.5 ( 0.2 and 0.4 ( 0.3 were obtained for EDOT and EDTM, respectively, demonstrating preferential deposition of EDOT on ITO electrode surfaces. Significant differences were noted at low and high degrees of conversion, indicating changes in copolymer composition with film thickness. These results have real significance for the characterization of electron-transfer rates for the first monolayer of PEDOT/ PEDTM on ITO, determined by a new mode of potential-modulated attenuated total reflectance spectroelectrochemistry. 1

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Charge transfer kinetics in immobilized, redox-active films have been studied using both cyclic voltammetry (CV) and electroreflectance (ER) techniques by other groups. 1,2 In these studies, rate constants obtained using ER techniques were higher than those determined from CV data, as observed here. This systematic difference can be explained by considering the following: Adsorption of a monolayer of molecules on a chemically and structurally heterogeneous electrode (e.g., indium-tin oxide) will generate a film that is chemically, structurally and energetically heterogeneous. This heterogeneity will be reflected in a distribution of thermodynamic formal potentials (E 0 ') and apparent rate constants for electron transfer. 3,4 In CV, the potential is scanned over a relatively large range (here it was -250 mV to 250 mV vs Ag/AgCl). At potentials well away from E 0 ' (i.e., at the extremes of the scan range), the large overpotential (E app -E 0 ') provides a sufficient driving force to oxidize and reduce all of the electroactive molecules in the film. In PM-ATR, a modulated potential (E ac ) is applied about the mid-point potential (E dc ) between the anodic and cathodic peaks (E dc ≈ E 0 '). The modulation range, E dc ± E ac , is typically less than 50% of the scan range used in CV (here E dc ± E ac was 44 mV). Thus in PM-ATR, only the subpopulation of electroactive molecules that are oxidized and reduced within E dc ± E ac are probed. This subpopulation should equilibrate more rapidly with the modulated electrode potential (i.e., have a higher k 0 ) relative to the remainder of the film, and thus the k 0 measured for this subpopulation by PM-ATR is expected to be greater than that measured for the entire film by CV.

We report laterally and vertically phase-separated thin film structures in conjugated polymer blends created by polymer molecular weight variation. We find that micrometer-scale lateral phase separation is critical in achieving high initial device efficiency of light-emitting diodes, whereas improved balance of charge carrier mobilities and film thickness uniformity are important in maintaining high efficiency at high voltages. The optoelectronic properties of these blend thin films and devices are strongly influenced by the polymer chain order/disorder and the interface state formed at polymer/polymer heterojunctions.