Reduction of TCNQ in Mixed Monolayers with Cationic Amphiphiles at the Air/Water Interface

Abstract

Investigations of the molecular organization characteristics of octadecyltetracyanoquinodimethane (C18TCNQ) in mixed monolayers with cationic amphiphiles such as dioctadecyldimethylammonium bromide (DOMA) has revealed that the TCNQ moiety is spontaneously reduced at the air/water interface. The reduced TCNQ molecules react further with oxygen resulting in the formation of decay products. No such reduction is observed either in pure C18TCNQ monolayers or in mixed monolayers with lipids having neutral or negatively charged head groups. The reduction process depends on the molecular composition (molar ratio of DOMA and C18TCNQ in the mixed monolayer), subphase pH (e.g. no reduction at pH 2), and the nature (e.g. no reduction in the presence of ClO 4 - ) and concentration of co-counterions in the subphase. The reduction process and the molecular organization characteristics have been studied using surface pressure- and surface potential-area isotherms and surface enhanced UV-vis reflection spectroscopy. The results are discussed in terms of interfacial pH, dielectric constant, and the redox potential of electroactive species at the air/water interface