Complexation of C₆₀ on a Cyclothiophene Monolayer Template

Abstract

The design and synthesis of molecules that form self-assembled monolayers on substrates is fairly well understood and developed. [1][2][3] Nevertheless, the controlled hierarchical construction of hybrid multilayers [4,5] comprising molecularscale (electronic) functionalities is a major challenge, and the first systems have been demonstrated only recently. [6,7] Here, we present the direct observation of self-assembled two-dimensional (2D) crystals of a novel class of organic semiconductors [8,9] on graphite by means of scanning tunneling mi- We have studied the long-range and short-range ordering of the conjugated macrocycle C[12]T (Scheme 1) [3,8] in self-assembled 2D crystals at the solution/highly oriented pyrolytic graphite (HOPG) interface by means of in-situ STM. Physisorption occurs from a 1,2,4-trichlorobenzene solution onto the (001) face of HOPG. In Figure 1a, a representative STM image of the cyclothiophene monolayer is shown. Longrange order of closely packed macrocycles is typically found over areas larger than 1 lm. The intrinsic high symmetry of the macrocycles (C 6v ) (Scheme 1) is preserved in the C[12]T adsorbate. The hexagonal arrangement of the molecules provides a unit cell of the 2D crystal defined by the parameters a = 2.3 ± 0.05 nm, b = 2.3 ± 0.07 nm, and c = 119 ± 3°. Six rotational domains are expected due to the two possible enantiomorphic arrangements of the macrocycles for each of the three crystallographic axes of the underlying HOPG.[10] However, only three domains have been observed in our experiments, and only two of them persist after dynamical reorganization at the surface. This fact can be explained by the weak molecule-substrate interaction and by the minimization of the interfacial energy, and will be accurately described in a forthcoming paper. Moreover, in these STM images, submolecular resolution has been achieved; the conjugated p-system of the individual rings appear in bright color, corresponding to higher tunneling currents. The interior cavity and the insulating alkyl side chains give a lower tunneling current and appear darker in the images. This image contrast is well known from previous investigations on related semiconducting linear oligo-and polyalkylthiophenes. [10][11][12] The close packing of the macrocycles on the surface brings the ring-shaped p-systems together to distances as small as 0.4 nm. Semiempirical calculations on the macrocycles indicate that the energetically most-favorable geometry has an almost planar p-system with the alkyl side chains bent in the same direction, out of the thiophene plane (Fig. 1b, inset). The "spider-like" conformation of the macrocycle on the surface allows intermolecular interactions between the alkyl side chains of adjacent molecules. These van der Waals' forces are typically the driving force for the self-organization of alkylated conjugated systems.[10] The unit-cell parameters for a calculated closest packed monolayer (a = 2.28 nm, b = 2.28 nm, c = 120°) are in excellent agreement with the experimental data (vide infr...

Scanning tunneling microscopy of the formation, transformation, and property of oligothiophene self-organizations on graphite and gold surfaces

“…The statistic results reveal that the left and right edge for 4T-tm-8T are at Ϫ1.05 Ϯ 0.2 eV and 1.12 Ϯ 0.2 eV, respectively, and for 4T-tm-4T the left and right edge are at Ϫ1.40 Ϯ 0.2 eV and 1.44 Ϯ 0.2 eV, respectively. The results show that the centers of energy gaps in both 4T-tm-8T and 4T-tm-4T are slightly shifted to positive regions, consistent with the previous report that oligothiophene is a p-type material in air (7). The experimental energy gap is 2.17 Ϯ 0.4 eV for 4T-tm-8T and 2.84 Ϯ 0.4 eV for 4T-tm-4T.…”

“…phase transition ͉ self-assembly ͉ molecular structure T hiophene derivatives, including oligothiophene (1-5), cyclothiophene (6)(7)(8), and polythiophene (9), are currently receiving considerable attention because of their well defined chemical structures, improved solubilities, and various electronic properties (10,11). They are promising materials in electronic and optical devices such as Schottky diodes (12), organic lightemitting diodes (OLEDs) (13,14), field-effect transistors (15), and organic thin film transistors (11,16).…”

Two-Dimensional Nanotemplates as Surface Cues for the Controlled Assembly of Organic Molecules

“…In this context an interesting host-guest system is the couple cyclo [12]thiophene (c [12]T) and C 60 fullerene, respectively a ringshaped p-type and a spherical n-type organic semiconductor [101]. Cyclothiophenes are a well-known class of macrocycles [102], whose self-assembly has been extensively studied by STM [103,104].…”

Self-Assembly of Nanoporous Chiral Networks with Varying Symmetry from Sexiphenyl-dicarbonitrile on Ag(111)

“…3 Thus, a series of bottom-up fabrication schemes 4 has been developed to realize two-dimensional (2D) layers presenting well-defined open spaces, using hydrogen bonding, [5][6][7][8][9] metal-directed assembly, [10][11][12][13][14] the organization of flexible species, [15][16][17][18] or covalent chemical reactions. 19,20 Nanoporous networks featuring 2D chirality represent a chemically and topologically interesting subclass in this area.…”