Charge transfer interactions at the air-water interface and in Langmuir-Blodgett films

Abstract

The charge transfer (CT) interactions between an amphiphilic electron donor dioctadecyl- p-phenylenediamine (L2) and an amphiphilic electron acceptor octadecyltetracyanoquinodimethane (C 18TCNQ) have been investigated in monolayers at the air-water interface and in Langmuir-Blodgett (LB) films. Surface pressure-area π− A and surface potential-area Δ V− A isotherms together with surface-enhanced UV-visible reflection spectroscopy at the air-water interface have been used for the characterization of CT. The isotherms indicate that neither L2 nor C 18TCNQ forms well-defined monolayers. However, for the L2-C 18TCNQ mixed monolayers, CT interactions between the two components lead to the formation of stable monolayers as indicated by the π− A and Δ A− A isotherms. The absorption spectrum of L2 and C 18TCNQ in CH 3CN solution exhibits two radical-anion bands (750 nm and 850 nm) of TCNQ. At the air-water interface, the reflection spectrum of L2C 18TCNQ mixed monolayer does not exhibit the radical-anion band of TCNQ −. However, the dimer band of TCNQ − 2 at 650 nm is observed. LB films on hydrophobized quartz plates were prepared in such a way that donor and acceptor were located either in the mixed monolayer or in adjacent monolayers. The LB exhibit the radical-anion bands of TCNQ as well as the radical-anion dimer band at 680 nm.