Aging Effects on Molecular Orientation, Structure, and Morphology in Langmuir−Blodgett Films of 2-Dodecyl-, 2-Pentadecyl-, and 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane Studied by Infrared and Ultraviolet−Visible Spectroscopy and Atomic Force Microscopy

Abstract

Time-dependent changes in molecular orientation, structure, and morphology in one-layer Langmuir−Blodgett (LB) films of 2-dodecyl-, 2-pentadecyl-, and 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (dodecyl-TCNQ, pentadecyl-TCNQ, and octadecyl-TCNQ) have been investigated by means of infrared and ultraviolet and visible (UV−vis) spectroscopy and atomic force microscopy (AFM). The intensities of bands at 2920 and 2850 cm-1 due to CH2 antisymmetric and symmetric stretching modes of the alkyl chain and those of bands at 2222, 1547, and 1529 cm-1 assigned to C⋮N, CC, and CC stretching modes of the TCNQ chromophore increase significantly with time in the infrared reflection−absorption (RA) spectrum of a one-layer LB film of pentadecyl-TCNQ after the film deposition on a gold-evaporated glass slide. These observations indicate that the alkyl chain becomes more tilted with respect to the surface normal during the time course and the TCNQ plane becomes more perpendicular with respect to the substrate surface. The time-dependent changes are also observed for one-layer LB films of dodecyl-TCNQ, but the changes are much smaller than those for the films of pentadecyl-TCNQ. One-layer LB films of octadecyl-TCNQ do not show appreciable infrared spectral changes with time. A UV−vis spectrum in the 350−450 nm region of the one-layer LB film of pentadecyl-TCNQ on a CaF2 plate consists of two bands due to the stacked and aggregation forms of the TCNQ chromophore. As a function of time, the intensity of the band due to the aggregation form increases while that of the band due to the stacked form decreases, indicating that some of the stacked TCNQ chromophores change into the aggregation with the change in the molecular orientation. The increase in baseline is also noted in the UV−vis spectrum. The baseline change may be due to the partition of the large domains of the one-layer film into small domains with the size of ∼1 μm. Time-dependent measurements of the AFM image of a one-layer LB film of pentadecyl-TCNQ on mica have revealed that the thickness of the film changes from 4.6 to 8.2 nm and that the flat and uniformed structure of the film changes into the condensed and accumulated structure during the time course from 15 to 180 min after the film deposition. Aged one-layer LB films of dodecyl- and octadecyl-TCNQ are similar to the aged and fresh one-layer LB films of pentadecyl-TCNQ, respectively, in terms of both the film morphology and thickness. On the basis of the results of infrared and UV−vis spectroscopy and AFM, we propose a model for the time-dependent morphological and structural changes in the one-layer LB films of the alkyl-TCNQ. The differences in the aging effects among the one-layer LB films of the three kinds of alkyl-TCNQ may be concerned with the differences in the strength of the hydrophobic interaction between the interdigitated alkyl chains and in the degree of three-dimensional microcrystal growth of the one-layer LB films.