Thermal Behavior of Mixed-Stack Charge Transfer Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and 3,3‘,5,5‘-Tetramethylbenzidine Prepared by the Langmuir−Blodgett Technique and Donor Doping. 1. Dependence of Thermal Behavior on the Number of Layers Studied by Ultraviolet−Visible−Near Infrared and Infrared Spectroscopies

Abstract

The dependence of thermal behaviors on the number of layers in Langmuir−Blodgett (LB) films of the mixed-stack charge transfer (CT) complex of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ) doped by 3,3‘,5,5‘-tetramethylbenzidine (TMB) has been investigated by using ultraviolet−visible−near infrared (UV−vis−NIR) and infrared (IR) spectroscopies. The appearance of a broad CT absorption band in the NIR region reveals that the mixed-stack CT films are formed by donor doping. The degree of the charge transfer determined by a shift of the C⋮N stretching band of the TCNQ chromophore verifies that the CT complex films are in a quasi-neutral state. Their temperature-dependent UV−vis−NIR and IR spectra show that donor molecules (TMB) dedope from the CT complexes in the LB films, resulting in the restoration of acceptor molecules (octadecyl-TCNQ) to their neutral states at their respective dedope temperatures. The dedope temperature of the CT complex films increases with the number of layers up to a ...