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, [ ( CH 3 – CH 2 ) 3 NH ] + ( 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 C 18 V 2 + : 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.