Self-Assembled Hexagonal Nanoarrays of Novel Macrocyclic Oligothiophene-Diacetylenes** By Elena Mena-Osteritz* and Peter Bäuerle Dedicated to Professor Siegfried Hünig on the occasion of his 80th birthday Monodisperse linear p-conjugated oligomers of defined length and structure, considered as “molecular wires”, [1] semi- conducting or metallic nanowires, [2] or carbon nanotubes [3] exceed the current resolution limits of nanolithography (‡100 ) and therefore represent ideal functional units for the mediation of electron transport and photonic processes. [4] They will play an essential role as key components in future molecu- lar scale electronics [5] and the first model nanodevices have recently been demonstrated. [6] However, for the realization of “nanoelectronics” the programmed assembly of molecular ma- terials in nanoscale architectures [7] as well as surface patterning with nanometer-sized objects [8] will be a crucial step. Well-defined p-conjugated macrocycles are of interest as modular building blocks for the assembly of new materials [9] and supramolecular architectures, not only because of their higher symmetry. Due to their toroidal structure, they could represent intriguing “molecular circuits” which would addition- ally include sites for recognition and selective complexation. In this respect, we were recently able to synthesize the first series of fully conjugated macrocyclic structures: cyclo(oligothio- phene–diacetylenes) and cyclo[n]thiophenes. [10] Herein, we de- scribe the most interesting adsorption and self-assembly proper- ties of these macrocyclic oligothiophene–diacetylenes revealed by in-situ scanning tunneling microscopy (STM) at the solu- tion/HOPG (highly oriented pyrolythic graphite) interface. STM offers an excellent way to directly investigate ordered p-conjugated structures, and their dynamics in situ in great detail. [11] A wealth of information is provided concerning their supramolecular organization and their molecular parameters due to sub-molecular resolution. Physisorption and self-orga- nization of the molecules in certain adsorbate patterns is influenced by the atomic structure of the substrate, by mole- cule/substrate interactions, and by the strength and type of intermolecular interactions. Typically, for one-dimensional “conjugated wires”, [12] in particular substituted oligo- [13] and polythiophenes, [14] spontaneous ordering and self-assembly in lamellar structures formed by the interdigitation of longer alkyl side chains is observed whereas epitaxial effects play a minor role. STM investigations on a two-dimesional (2D) crystal of a disc-shaped alkylated hexabenzocoronene reveal self-assembly of the “discs” in a way that the six-fold symme- try of the molecule is reduced to a two-fold symmetry in the adsorbate lattice. [15] The macrocyclic oligothiophene–diacetylenes 1 and 2 were prepared by oxidative coupling of 5,5²-diethynyl-3,3²,4,4²-tet- rabutylterthiophene under pseudo high-dilution conditions. As main products cyclotrimer 1 and cyclotetramer 2 were iso- lated and purified by preparative HPLC. [10] The STM measurements were carried out at ambient tem- perature with a low-current STM (Rochester Hills, MI (RHK)) equipped with a RHK STM 1000 control system. Mechanically cut Pt/Ir tips were used. A freshly cleaved sur- face of HOPG was first carefully characterized, then solutions of the macrocycles in 1,2,4-trichlorobenzene were brought onto the substrate. In-situ STM imaging of the self-organized monolayers was performed at the solution/HOPG interface. All of the images presented were obtained at quasi-constant height in the variable current mode, without using a voltage pulse in order to induce the ordering. The bias voltages were typically about –430 to –700 mV and the set point currents under 1.0 nA. Ab initio and semiempirical calculations were performed with Gaussian 94 (Gaussian Inc., Pittsburg, PA) and Hyperchem (Hypercube Inc., FL) software. In contrast to the parent linear compounds which have the identical substitution pattern, [16] we find spontaneous adsorp- tion and ordering of the cyclotrimeric terthiophene–diacety- lene 1 at the solution–HOPG interface probably due to the higher trigonal symmetry of the molecule. The molecular image usually appears immediately after bringing the solution of the macrocycle onto the (001) face of HOPG. In Figure 1 (left) a representative and characteristic STM image of a well- ordered and very stable 2D crystalline monolayer is shown. The STM images are typical for large areas (up to 1  1 lm 2 ) and reveal excellent long-range ordering of molecularly resolved individual macrocycles. A perfect “honeycomb” pat- tern in which the toroids are arranged with an optimal packing density (1.11  10 5 molecules/lm 2 ; 2.84  10 –8 g/cm 2 ) in a hex- agonal symmetry (6mm) and consequently only one persistent domain is observed. The periodicities of the 2D layer in the three main directions were determined to be 2.46, 2.52, and 2.53 nm, which result in lattice constants of a = 2.46 nm, b = 2.52 nm, a = 62 (Table 1). The arrangement and dimensions Adv. Mater. 2001, 13, No. 4, February 19 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim,2001 0935-9648/01/0402-0243 $ 17.50+.50/0 243 COMMUNICATIONS – [*] Dr. E. Mena-Osteritz, Prof. P. Bäuerle Department Organic Chemistry II (Organic Materials and Combinatorial Chemistry), University of Ulm Albert-Einstein-Allee 11, D-89081 Ulm (Germany) E-mail: peter.baeuerle@chemie.uni-ulm.de [**] This work was financially supported by the Fonds der Chemischen Industrie. We thank Dipl.-Chem. Gerda Fuhrmann (Dep. Org. Chem. II, Univ. Ulm) for her help with synthesis, Dr. Reiko Azumi (NIMC, Tsukuba, Japan) for her help with crystallization experiments, and Prof. T. Debaerdemaeker (Di- vision of X-ray and Electron Diffraction, Univ. Ulm) for his continuous and very fruitful cooperation with X-ray structure analyses.