Self-Assembly of TPPS 4 Porphyrin Molecules into Nanorods Investigated by TEM, AFM, STM and UV/VIS Spectroscopy V.Snitka, R.Rodaite,V.Mizariene Research Center for Microsystems and Nanotechnology, Kaunas University of Technology Studentu 65, Kaunas, Lithuania, vsnitka@ktu.lt ABSTRACT In this paper we discuss the use of scanning probe microscopy techniques to obtain insight into structural and physicochemical properties of single conjugated (macro)molecules and of their molecular architectures as well as to induce conformational transitions in supramolecular objects with a precision on the molecular scale. The aggregates of the tetrakis (4 -sulfonatophenyl) porphyrin (TPPS 4 ) growth from solution on solid crystalline hydrophilic and hydrophobic substrates were investigated. The spatial structure of TPPS 4 deposited on the surface was imaged using Transmission Electron Microscopy (TEM), Scanning Atomic Force Microscopy (AFM). Scanning Tunneling Microscopy (STM) visualized porphyrin rings structure with molecular resolution. The aggregation of TPPS 4 was investigated by UV-VIS spectroscopy. The TEM investigation revealed a 3-dimensional rod and wheel like molecular order of the TPPS 4 porphyrin assemblies, while a corresponding shorter and mono-disperse oligomer exhibited 2D ribbon-like structure. The TPPS 4 porphyrin molecules self-assemble into micrometer-long nanorods and fibers on substrate surface. The investigations revealed a typical thickness of two or four molecular layers and a width of tenths of molecule diameter. We propose here TPPS 4 ribbons as polymolecular architectures, which are building blocks of TPPS 4 rods and built from the homo- associated porphyrin molecules as revealed by STM. The TPPS 4 nanoribbons can find an applications in a molecular- scale electronic and nanooptic devices. Keywords: porphyrin, self-assembly, nanorods, scanning probe microscopy 1 INTRODUCTION In the framework of the fabrication of molecular devices based on π-conjugated (macro)molecules, one of the major tasks is an understanding and controlling the physical-chemical properties of the materials over a wide range of length scales. In work in this direction, scanning probe microscopy have played a paramount role, since they allow one to explore organic surfaces on different scale lengths in various ambient [1,2]. Making use of these techniques it has been possible to approach the nanoworld in various ways which go far beyond pure imaging of a surface: manipulating single molecules at room temperature [3] opening new avenues towards the study of the conformational and nanomechanical properties of individual molecules [4], stimulating photochemical reactions with light and following the reaction in real-time at interfaces [5, 6], probing the electronic properties of single molecules by means of scanning tunneling spectroscopy (STS) [7, 8] and inducing with the scanning tunneling microscope (STM) tip the electroluminescence of a conjugated thin film [9] are just a few examples. In this paper we report on the first study of molecularly resolved images of a TPPS 4 porphyrin self-assembly and aggregation at the solid-liquid interface on the growth of this macromolecule into molecularly defined ribbons on the surface. 2 EXPERIMENTAL 2.1 Materials The tetra sodium salt of tetrakis-5,10,15,20(4- sulfonatophenyl) porphine was obtained from Porphyrin Products (Lugan, UT) and was used without further purification. The J-aggregate solutions were prepared by dissolving TPPS 4 in acidic aqueous medium (HCl was added to reach pH 1) at the concentration range (1⋅10 -4 – 2⋅10 -6 ) M. To stabilize the aggregates formation the solution was left at room temperature for aggregation for 10 days, before the thin films preparation. The J-aggregates of TPPS 4 formed after solution preparation was confirmed by absorption spectra. 2.2 Preparation of porphyrin films Highly oriented pyrolytic graphite (HOPG), glass, mica and silicon were chosen as supporting substrates. The thin films of TPPS 4 were prepared by drop casting solutions and allowing the solvent to evaporate at room temperature in the dust-free environment, dipping substrate into solution or spin-coating technique at 100 rpm. Then the sample was dried in ambient air. The TPPS 4 thin films were generally deposited from 1x10 -4 and 1x10 -5 M solution. 2.3 Scanning probe microscopy Muscovite mica slices for Atomic Force Microscopy measurements were cut into discs with a punch and die set in order to produce readily cleavable edges. The glass NSTI-Nanotech 2005, www.nsti.org, ISBN 0-9767985-0-6 Vol. 1, 2005 789