Electroactivity of a starburst hole-transport material in Langmuir–Blodgett films. Solid state effects and intervalence charge transfer

Abstract

Here we report on the electroactivity properties of Langmuir-Blodgett (LB) films of the hole-transport molecule 4,4',4''-tris[3-methylphenyl(phenyl)amino] triphenylamine (m-MTDATA). Fairly stable Langmuir films at the air-water interface are accomplished, despite the non-amphiphilic character of the molecule. The reflection-absorption infrared spectroscopy (RAIRS) and Fourier transform infrared (FT-IR) analysis revealed that the molecules arrange with no neat preferential orientation, in agreement with the amorphous glassy nature of this starburst molecule. However, there is a tendency of the molecules to organize in a more planar conformation due to the intermolecular stacking induced by the LB technique. On the other hand, the fundamental electrochemistry (by cyclic voltammetry, CV) of the films is also analyzed. The CV studies of both solution and films reveal that both the solid state and the electrolyte's anions clearly affect the m-MTDATA's electroactivity, exhibiting a unique and broad redox process instead of the two reversible oxidations observed in solution. The oxidization mechanism is discussed. Finally, the spectroelectrochemistry studies evidence that the oxidization of the films leads to new absorption bands, among which the emerging bands in the NIR region ascribed to intervalence charge transfer (IVCT) between the generated aminyl radical cations should be pointed out.

Solid-state electropolymerization and doping of triphenylamine as a route for electroactive thin films

“…Changing the electrolyte in the working solution to NaCl leads to important changes in the CV (Fig. 35 More generally, a high sensitivity of the voltammetric profile to the electrolyte being employed has been evidenced in previous reports on the electrochemical behavior of poly-TPA. The less negative hydration Gibbs energy 34 of perchlorate (À237 kJ mol À1 ) versus chloride (À330 kJ mol À1 ) explains its easier ingress into the film.…”

“…A similar behavior was described recently for the electrochemical behavior of a thin solid film of a starburst hole transporting material based on TPA units. 35 More generally, a high sensitivity of the voltammetric profile to the electrolyte being employed has been evidenced in previous reports on the electrochemical behavior of poly-TPA. 31 Thin films of TTA were also scanned from À0.4 to 1.0 V at 20 mV s À1 (50 cycles) in a 0.5 M NaClO 4 aqueous solution.…”