Nanoassemblies Designed from Semiconductor Quantum Dots and Molecular Arrays E. Zenkevich, ‡ F. Cichos, ² A. Shulga, ‡ E. P. Petrov, ²,§ T. Blaudeck, ² and C. von Borczyskowski* ,² Institute of Physics, Chemnitz UniVersity of Technology, 09107 Chemnitz, Germany and National Academy of Sciences, Institute of Molecular and Atomic Physics, Minsk 220072, Belarus ReceiVed: August 31, 2004; In Final Form: NoVember 17, 2004 The formation of nanoassemblies of CdSe/ZnS quantum dots (QD) and pyridyl-substituted free-base porphyrin (H 2 P) molecules has been spectroscopically identified by static and time-resolved techniques. The formation of nanoassemblies has been engineered by controlling the type and geometry of the H 2 P molecules. Pyridyl functionalization gives rise to a strong complex formation accompanied by QD photoluminescence (PL) quenching. For some of the systems, this quenching is partly related to fluorescence resonance energy transfer (FRET) from the QD to H 2 P and can be explained according to the Fo ¨rster model. The quantitative interpretation of PL quenching due to complexation reveals that (i) on average only about 1 / 5 of the H 2 P molecules at a given H 2 P/QD molar ratio are assembled on the QD and (ii) only a limited number of “vacancies” accessible for H 2 P attachment exist on the QD surface. 1. Introduction “Bottom-up” approaches for functionalized nanoassemblies are an intriguing field for both fundamental science and envisaged applications. During the last two decades, consider- able progress has been reported in two fields not initially related, that is, nanostructured semiconductor materials prepared by lithographic methods or conventional organometalic synthesis 1-4 and chemical approaches based on supramolecular chemistry. 5-9 In recent years, the merging of both fields has been, besides others, driven by the concept of molecular electronics. 10,11 Following these general trends, considerable progress has been made in the preparation and characterization of both colloidal semiconductor nanocrystals, such as CdSe, 12 and self-assembled molecular architectures. 13,14 Both kinds of nanoassemblies are realized in solution or at solid/liquid interfaces. Only a few attempts to interlink semiconductor nanocrystals and organic chromophores have been reported on a microscopic level. 15-19 Nanocrystals are subject to quantum size effects, and such quantum dots (QD) are often strongly emissive. 4,12 In assemblies of this type, for example, the phenomenon of photosensitized electron injection from organic subunits to semiconductor nanoparticles is used to activate charge separation even at optical excitations below the semiconductor band gap energy. 15-17 In addition, nonradiative energy transfer depending on absorption/ emission properties and intercenter distances between interacting nanoparticle-dye moieties strongly influences the dynamics and relaxation pathways in such complex systems. 18,19 The other driving force for the growing interest in nano- assemblies is, besides fundamental investigations, the use of QD as photoluminescence markers linked to biologically relevant molecules 19-22 and the creation of new optical and laser materials. 23,24 According to these attractive applications, the anchoring of organic molecules to wide band gap semiconductor colloidal nanocrystals is of considerable scientific and practical interest. The envisaged perspectives of such a combination are broad. Among other reasons, the tunability of the optical band gap of nanocrystals via size-dependent quantum confinement 4,12,16 (optical tunability) and the nearly unlimited possibilities of the chemical engineering of electronic properties of organic mol- ecules (chemical tunability) 5-7,13,14 are, with respect to new material properties, stimulating and promising arguments in concentrating on the architecture of organic-inorganic nano- assemblies. During the past decade, we have developed a concept to self assemble biomimetic porphyrin arrays that are tunable with respect to photoinduced energy and/or charge transfer. The so- called “key-hole” organization principle is based on the com- plexation of metal ions in the center of porphyrin macrocycles via suitable ligands such as pyridiyl rings 7,25,26 . We have shown extensively that the complexation is based essentially on a “Lego-type” key-hole principle effectively controllable via steric factors such as distance matching, optimization of relative orientations, and solvent composition. Recently, we have reported the first successful attempts to extend such concepts to semiconductor colloids and have anchored pyridyl-substituted tetrapyrrolic organic molecules on CdSe/ZnS core/shell semiconductor nanoparticles. 27,28 In addi- tion, it has been reported 16 that the interaction of CdSe/ZnS core/shell nanocrystals with specially designed diazaperylene molecules (having two anchoring nitrogens) manifests itself in a complete quenching of the nanocrystal emission accompanied by the appearance of a new, not yet identified electronic state. In this publication, we will report in detail for the first time on the extension of the above-outlined self-assembly principle with the final goal being to organize molecular arrays on semiconductor quantum dot surfaces in a systematic way. We intend to make use of both the tunability of the QD band gap via size variation and the chemical engineering of functionalized organic molecules. Although the main emphasis of this paper * Corresponding author. E-mail: borczyskowski@physik.tu-chemnitz.de ² Chemnitz University of Technology. ‡ National Academy of Sciences. § Present address: Institute of Biophysics/BioTec, Dresden University of Technology, 01307 Dresden, Germany. 8679 J. Phys. Chem. B 2005, 109, 8679-8692 10.1021/jp040595a CCC: $30.25 © 2005 American Chemical Society Published on Web 04/09/2005