Charge transfer complex formation at the air-water interface “in situ” studied by means of UV-vis reflection spectroscopy

Positively ionized monolayers of a viologen derivative, namely 1,1′-dioctadecyl-4,4′-bipyridilium (C18V2+), were prepared onto aqueous solutions of a tetracyanoquinodimethanide salt, [(CH3-CH2)3NH]+(TCNQ)2-, where the tetracyanoquinodimethanide anions are in a mixed valence state. UV-vis reflection spectroscopy has been used for proving in situ the incorporation of tetracyanoquinodimethanide anions into the viologen monolayer. The UV-vis spectra clearly show the presence of a band in the 550-850 nm region that is not visible in viologen monolayers fabricated onto a pure water subphase. Moreover, a charge-transfer interaction between the two molecules occurs at the air-water interface. A simultaneous quantitative analysis of the two active molecules in the UV-vis region has facilitated the investigation of the viologen tetracyanoquinodimethanide hybrid monolayers. An organization model at the air-water interface consistent with the reflection spectroscopy data has been proposed where a parallel arrangement of the viologen dications and (TCNQ)2- anions is assumed. The ratio of the two materials in the monolayer was also determined and resulted to be C18V2+:2(TCNQ)2-, i.e. stoichiometry of charges. This ratio decreases after a noticeable kink in the π-A isotherm, and has been interpreted as a reorganization of the molecules in the monolayer leading to an ejection of some (TCNQ)2- ions from the interface to the subphase, probably due to steric hindrance.