Extending Excited-state Lifetimes by Interchromophoric Triplet-state Equilibration in a Pyrene-Ru(II)diimine Dyad System

Abstract

The synthesis and the spectroscopic properties of a bichromophoric ruthenium trisbipyridyl-1,4-diethynylenebenzene-pyrene system (Ru-b-Py) and the corresponding pyrene ligand (b-Py) are reported. The ruthenium model systems Ru-b-OH, Ru-b-Ph are also presented. UV–Vis absorption and emission at room and low temperature and time-resolved spectroscopy are discussed. For the Ru-b-Py dyad, a mixing of the ³MLCT state of the ruthenium-based component and the triplet state of pyrene, ³Py, is observed. Time-resolved transient absorption studies performed on the Ru-b-Py and on the b-Py ligand show that the lowest energy absorption is due to the population of the triplet state localized on the pyrene-component. Time-resolved studies also evidenced a relatively slow forward triplet equilibration rate, in the order of 2×10⁵ s^-1 (5 μs), and an even slower back energy transfer rate, 3.3×10⁴ s^-1, still faster than the intrinsic decay time of the pyrene (200 μs).

Keywords:

• Supramolecular photochemistry
• Energy transfer
• Room temperature phosphorescence

Notes

The Future of Supramolecular ChemistryIn the last few years, supramolecular chemistry has become a key issue in many aspects of chemistry. A large community of researchers is assembling, covalently and non-covalently, simple “building blocks” to achieve large and complicated functional structures. New properties can in fact emerge when suitable molecules interact with each other, and the organization of “smart” components has led to the construction of nanosized architectures that can perform interesting functions upon an external input. More recently, the self-assembly principle moved from simple molecules in solution to surfaces in attempts to create molecular machines, switches, motors, wires, or in other words, photochemical or electrically driven devices. The next challenge will be the interface of such nanosystems with the macroworld or perhaps the creation of a complete molecular-based assembly able to give a readable and useful external signal. The control of the assemblies, in terms of modulation of the strength of interaction, size, shape, directionality, organization on surfaces, can certainly be called supramolecular. More recently, such self-assembly has involved not only simple molecules but also nanoclusters such as gold or semiconductive nanoparticles. Not only are these topics currently being pursued in our group, but also they represent how we see the future of supramolecular chemistry. Along this line, we intend to design and study the interactions and the functions of different types of nanosized objects, bridged by “molecular wires”, and triggered by light or electrons. Currently, we describe a simple molecular approach consisting of a pyrene-bridge-ruthenium system.Professor Dr Luisa De Cola was born in Messina, Italy, where she studied Chemistry and received her “Laurea” (110/110 cum laude) in 1983. She was a postdoctoral research fellow (NIH and Coulter Electronics) at the Virginia Commonwealth University, Richmond, USA (1984–1986) (Postdoctoral Adviser: Professor L. M. Vallarino). Then, she joined the National Research Council F.R.A.E. CNR of Bologna as Associate Researcher (1986–1989). In 1990, she became Assistant Professor at Dipartimento di Chimica “G. Ciamician”, in the group of Professor Vincenzo Balzani, University of Bologna (Italy). In 1998, she has been appointed Full Professor (chair of Molecular Photonic Materials) at University of Amsterdam (The Netherlands). She has experience in the design, synthesis, photochemical and photophysical properties of large (supra)molecular systems such as polynuclear compounds linked by saturated and conjugated bridging ligands, dendrimers, and molecular switches. In general, the purpose of the work has been the development of functional systems able to respond to an external stimulus such as light. She has industrial collaborations with Philips Research, and with Roche Diagnostics on (electro)luminescent materials.

Dr René M. Williams was born in 1965 in Amsterdam, where he also did his doctoral exam in 1990, on “Perylenebisimides, colorful compounds with a big potential”, synthesis and photophysical characterization of perylenebisimide-bridge-electron donor systems. In 1990, he started as a researcher at the “Chemiewinkel” of the UvA, within the framework of social service, working on the long-term effects of paper deacidification. He started his PhD in 1991 research in the group of Professor Dr J. W. Verhoeven in the Organic Chemistry department of the UvA, where, on 10 June 1996, he received his doctorate with the thesis entitled: “Fullerenes as electron accepting components in supramolecular and covalently linked electron transfer systems”. He than obtained a Marie-Curie fellowship, working at Max-Planck Institut für Strahlenchemie in Mülheim an der Ruhr, in the group of Professor Dr S. E. Braslavsky. He worked on “laser-induced optoacoustic spectroscopy” of cis–trans isomerization reactions of biological systems and photochromic compounds and, together with Silvia, wrote a chapter of a book: Photomovement—Molecular Basis. He did a second postdoc at Columbia University in the city of New York, in the group of Professor Dr N. J. Turro, working on an industrial project involving glass fibers: “Triplet sensitization effects on phosphine oxide photoinitiators used in photopolymerization”. Here, he mainly focused on time-resolved electron paramagnetic resonance (TR-EPR). Since 1 December 1999, he has been working as a Lecturer/Assistant Professor in the Molecular Photonic Materials group of Professor Dr Luisa De Cola in the Institute of Molecular Chemistry in Amsterdam. Here, he specializes on photoinduced processes such as electron and energy transfer in molecular, supramolecular and nano systems.

Professor Dr Frédéric Fages received his engineering degree in Physics & Chemistry from ENSCPB, Bordeaux, in 1983 and a PhD (Doctorat d'Etat) degree from the University Bordeaux 1 in 1988 with Henri Bouas-Laurent. He took up a position as Chargé de Recherche at CNRS in 1983, becoming Professor at the University Bordeaux 1 in 1998. The main focus of his research activity was the design of supramolecular systems with tunable fluorescence properties and the study of self-assembled gels from low-molecular-mass organogelators. In 1989, he was an Alexander-von-Humboldt postdoctoral fellow in the group of Kurt Schaffner and Alfred Holzwarth at Max-Planck-Institut für Strahlenchemie, Mülheim an der Ruhr, Germany, where he worked on light-harvesting antenna complexes for artificial photosynthesis. In September 2002, he joined the Laboratory of Organic Chemistry and Molecular Materials (UMR 6114 CNRS) at the Faculty of Sciences of Luminy, University Aix-Marseille II, where his research group works on the synthesis and study of conjugated nanostructured materials for optoelectronic applications.