Structural Characterization of Mixed-Stack Charge Transfer Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane and 5,10-Dimethyl-5,10-dihydrophenazine Prepared by the Langmuir−Blodgett Technique and Donor Doping. 3. Thickness Dependence of Thermal Behavior Investigated by Ultraviolet−Visible−Near-Infrared and Infrared Spectroscopies and X-ray Diffraction

Thermal behavior of mixed-stack charge transfer (CT) films prepared by 5,10-dimethyl-5,10-dihydrophenazine ((Me)2P) doping of Langmuir−Blodgett (LB) films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ) has been investigated by using X-ray diffraction and ultraviolet−visible−near-infrared (UV−vis−NIR) and infrared (IR) spectroscopies. Temperature-dependent changes in the UV−vis−NIR and IR spectra reveal that donors (D, (Me)2P) dedope from the CT complex, resulting in a restoration of acceptors (A, octadecyl-TCNQ) to their neutral state in the temperature range 70−90 °C. The dedope temperature was determined to be 80, 84, 86, and 88 °C for the 1-, 3-, 7-, and 11-layer CT films, respectively, by monitoring an ag C⋮N stretching band of the TCNQ chromophore in the temperature-dependent IR spectra. From room temperature to 60 °C, the molecular arrangement and orientation of D and A in the well-ordered multilayered CT complex seem to be thermally stable because the X-ray diffraction pattern and UV−vis−NIR and IR spectra are nearly unchanged. The dedoping begins in the temperature range 60−70 °C, but it proceeds rather slowly for the multilayered CT films. Dramatic structural changes take place in the region of 70−90 °C, where the X-ray diffraction pattern and characteristic absorption bands of the CT complex in the UV−vis−NIR and IR spectra disappear. The thickness-dependent thermal behavior may be attributed to the longitudinal interactions between the sheetlike multilayered microcrystals as well as to the interaction between the first layer and a CaF2 substrate.