Novel Method for Preparing a Mixed-Stack Charge-Transfer Film of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and 3,3‘,5,5‘-Tetramethylbenzidine: The Use of a Mixed Langmuir−Blodgett Film of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and Stearic Acid as Starting Material for the Fabrication of the Charge-Transfer Film

Abstract

The present paper reports a novel method for preparing a mixed-stack charge-transfer (CT) film having 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C18TCNQ) as an electron acceptor and the possibility of the control of molecular orientation in the CT film. A seven-layer mixed Langmuir−Blodgett (LB) film of C18TCNQ and stearic acid is deposited onto a CaF2 plate or a gold-evaporated glass slide at the surface pressure of 15 mN m-1. Dipping the mixed LB film into a petroleum ether solution of 3,3‘,5,5‘-tetramethylbenzidine (TMB) results in the formation of a mixed-stack CT film of TMB·C18TCNQ. Upon the formation of the CT film, the dilution molecules, namely stearic acid, are removed completely from the film. The appearance of a broad absorption band in the near-infrared region and the frequency shift of the CN stretching band (2208 cm-1) verify that the CT complex film of TMB·C18TCNQ is formed by the TMB doping. IR spectra of the CT films do not show any bands assignable to stearic acid, suggesting that stearic acid molecules are removed from the thin films after the dipping. The band at 3496 cm-1 due to the antisymmetric stretching mode of the NH2 group does not appear in the IR transmission spectrum. Thus, it seems that the chromophore plane of TMB component is nearly perpendicular to the substrate surface with its long axis lying on the substrate in the CT films. The molecular structure and orientation in the CT films of TMB·C18TCNQ prepared by the present method and those fabricated by dipping the LB films of C18TCNQ alone into the TMB solution have been compared. The comparison shows that the orientation of the chromophore planes of both donor and acceptor components and that of the alkyl chain of C18TCNQ in the CT films are controlled mainly by the doping process.