J. Phys. Chem. 1995, 99, 8690-8697
8690
STM Investigation of 2D Crystals of Anthrone Derivatives on Graphite: Analysis of Molecular Structure and Dynamics A. Stabel: R. Heinz,? J. P. Rabe,*$*and G. Wegner' Max-Planck-Institut fiir Polymerforschung, Posfach 3148, 0-55021 Mainz, Germany, and Institut fur Physik, Humboldt- Universitiit zu Berlin, Invalidenstrasse I IO, D-I 0099 Berlin, Germany
F. C. De Schryver,@g" D. Corens;' W. Dehaen,"and C. Suling" KU Leuven Department of Chemistry, Celestijnenlaan 200F, B-3001 Heverlee, Belgium Received: July 27, 1994; In Final Form: February 4, 199.5@
The influence of the functional modification (>CH*, 'CO, >CN2) of alkylated anthrone derivatives of the structure of their two-dimensional crystals on the basal plane of graphite are studied by STM. In situ calibration against the hexagonal graphite lattice, analysis of Moire patterns and the study of domain boundaries are used to determine models for the observed structures. For 2-hexadecylanthraquinonetwo modifications with significantly different two-dimensional crystal structures have been observed as coexisting crystallites, which is a kinetic phenomenon. One of the structural modifications can only be explained by a coalescence of two space-filling models (oscillating model).
TABLE 1: Unit Cell Data of 2-Hexadecylanthrone (1) (Structure I)
Introduction
Physisorbed monolayers of alkylated molecular species have been intensively investigated by STM.'-2 However, still little systematic work is available on the influence of a small chemical modification of a molecule on its two-dimensional crystalline structure. Also only little is known about molecular structure and dynamics at domain boundaries of two-dimensional molecular crystals (2D crystal^).^-^ More recent examples include work on grain boundaries between two coexisting phases of 4'-n-decyl-4-cyanobiphenyl ( 1OCB)' and 4'-n-octyl-4-cyanobiphenyl (8CB),8 respectively, which were physisorbed on highly oriented pyrolytic graphite (HOPG). Parks et aL9 investigated in detail the coexistence of two-dimensional crystalline and onedimensional stacking disordered phases of a liquid crystal at the melt-HOPG interface. In the present paper we present the details of the structural analysis of 2D crystals on a subnanometer scale. In contrast to X-ray diffraction analysis of 2D crystals,1° the STM can be used to investigate phenomena on the time scale of seconds and the length scale of nanometers. We report on the influence of the functional modification (>CH2, >CO, >CN2) of alkylated anthrone derivatives upon the 2D crystalline structure and on various types of domain boundaries including one between two different crystal structures. Two of the compounds studied here are furthermore starting material and product, respectively, of a photoreaction that was recently carried out in a very thin film and was followed in situ by STM." The structural data of their 2D crystals were used as parameters to monitor the photoreaction. Experimental Section
Compounds 1, 2, and 3 (Figure 1) were synthesized and purified ('99%) as previously reported." Solutions (10 mM) of compounds 1, 2, and 3 in phenyloctane were applied to the ~
~
Max-Planck-Institut. 3 Humboldt-Universitat. 9 Humboldt Forschungspreistrager at the Max-Planck-Institut fur Polymerforschung . II KU Leuven. Abstract published in Advance ACS Abstracts, April 15, 1995. @
unit cell
2-hexadecylanthrone (1) (structure I)
molecules per unit cell L(alky1 chain, graphite axis) L(lamel1a axis, graphite axis) interchain distance interlamella distances (AL)
740 f 50 pm 3500 f 100 pm 135 5" 2 0 If 2" 15 f 2" 470 5 pm 2500 f 100 pm
a b
Y
* *
basal plane of highly oriented pyrolytic graphite (HOPG quality ZYB, Advanced Ceramics, Cleveland, OH). Since 2-hexadecyldiazoanthrone decomposes readily upon irradiation (A,,, = 418 nm), it was imaged in the absence of light. The physisorbed two-dimensional crystals were investigated in situ at the solidliquid interface using a homebuilt STM.'* The tunneling tips were electrochemically etched (2 N KOH 6 N NaCN) from a 0.25 mm P a r (80:20) wire. The STM images were obtained in constant height mode under ambient conditions (scan rate, 0.3 Hdimage; tip bias, positive). They were recorded on videotape in real time and digitalized later. In the following, data are displayed as recorded without digital image processing.
+
Results and Discussion
1. Structure of 2D Crystals. 1.I. 2-Hexadecylunthrone. Physisorption of the 2-hexadecylanthrone (1) from phenyloctane leads to 2D crystals (structure I). In the STM images (Figure 2a,b) the bright rows (high tunneling current) are attributed to the aromatic moiety of the molecule^^^^^-^^ and the darker rows (low tunneling current) to the alkyl chains. This is consistent with theoretical calculations on the contrast in STM images of organic monolayers at the graphite surface.I6 Its validity was demonstrated for a wide variety of organic adsorbates on graphite.I7 In situ calibration against the underlying hexagonal graphite lattice, analysis of MoirC patterns, and the study of domain boundaries are employed to determine the unit cell parameters of the 2-hexadecylanthrone, which are summarized in Table 1. An illustrative molecular model of the adsorbate is
0022-3654/95/2099-8690$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 21, 1995 8691
2D Crystals of Anthrone Derivatives on Graphite
0 0 mc 1 6H33
0
1
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1 6H33
