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 mixedstack charge transfer (CT) complex of 7,8, 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 CtN 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 seven-layer film. Both the UV-vis-NIR and IR spectra show that the one-layer CT complex film undergoes a progressive thermal process. In contrast, the seven-and 11-layer films are stable up to 120°C and show rather abrupt changes near their dedope temperatures. Furthermore, pre-dedope phenomena are observed for the multilayer CT complex films but not for the one-layer CT film. This dependence of the thermal behaviors of the CT films on the number of layers may be attributed to the differences in the film thickness, the longitudinal interactions between the one-dimensional needle-like microcrystals of the CT complex, and the effects of the interaction between the first layer and a CaF 2 substrate.

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. 2. Morphology and Annealing Effects of the Films Investigated by Atomic Force Microscopy and Ultraviolet−Visible−Near Infrared and Infrared Spectroscopies

“…The following conclusions were reached in the preceding paper: 1 (1) Donor molecules (TMB) sublimate from the CT complexes at the dedope temperature, resulting in the restoration of neutral octadecyl-TCNQ; (2) temperaturedependent changes in characteristic absorption bands of the CT complex films (i.e. the bands due to the CT excitation between octadecyl-TCNQ and TMB and the intramolecular excitation of chromophores in octadecyl-TCNQ molecules) depend upon the number of layers.…”

“…The doping method for forming the mixed-stack CT complex LB films was the same as that in the preceding paper. 1,4 In the AFM experiments, a NanoScopeIII (Digital Instruments) was used to investigate the morphology of the CT complex LB films which had been built up onto silicon substrates (n-Si(100) surface) in order to achieve a high resolution. We used a silicon nitride tip on a cantilever with a spring constant of 0.06 N/m.…”

“…For example, the frequencies of the CH 2 antisymmetric and symmetric stretching bands are significantly different between them. 1,2 This implies a different stacking pattern of the hydrocarbon chains of octadecyl-TCNQ. It is well-known that appearances of the CH 2 antisymmetric and symmetric stretching bands at lower wavenumbers (ca 2918 and 2848 cm -1 , respectively) indicate a highly ordered and well-organized arrangement of the hydrocarbon chains, while their appearances at higher wavenumbers (ca 2926 and 2856 cm -1 ) are indicative of the increase in conformational disorder in the hydrocarbon chains.…”

Global Phase Angle Description of Generalized Two-Dimensional Correlation Spectroscopy: 2. Its Application to Temperature-Dependent Infrared Spectra of a Langmuir—Blodgett Film of 2-Dodecyl-7,7,8,8-Tetracyanoquinodimethane

“…Temperature-dependent IR spectral changes have been investigated extensively for LB lms of functional dyes because they provide information about the therm al behavior of alkyl chains and chromophoric parts. 3,4,[6][7][8][9] Papers on the CH stretching band region generally focus on the temperature-dependent variations of CH 2 stretching bands. 12,13 M eanwhile, CH 3 stretching bands have seldom been investigated, largely owing to their weakness in intensity compared with the CH 2 stretching bands.…”

Electrode Coating With Ultrathin Films Containing Electroactive Molecules for Biosensor Applications

“…HF structures were realized by means of concurrent use of SAM and LB procedures in order to combine high molecular order and architectural control of the LB technology [13] with chemical, mechanical, and thermal stability of SAMs [14], [15]. Three redox mediators for NADH were selected, i.e., ferrocene (Fc), ferrocene carboxylic acid (Fc-COOH) and 3,3'-5, 5'-tetramethylbenzidine (TMB), on the basis of their redox features [2], [16], [17]. We chose dipalmitoylphosphatidic acid (DPPA) as lipid matrix for LB fabrication, whereas octadecylmercaptane (ODM) and octylmercaptane (OM) were used for the preparation of SAMs on gold surfaces [18]- [21].…”