Molecular Devices Based on Bridged Transition-Metal Complexes

The article from A. von Zelewsky, in this issue gives a broad survey about some scientific activities at the Institute of Inorganic Chemistry of the University of Fribourg. The present contribution will be rather focused on a detailed study of the photophysical and photochemical behavior of a molecular device that is capable to undergo energy- and/or electron-transfer processes

Photochromic Molecules as Building Blocks for Molecular Electronics

Energy and electron transfer processes can be easily induced by a photonic excitation of a donor metal complex ([Ru(bpy)3]2+), which is connected via a wire-type molecular fragment to an acceptor metal complex ([Os(bpy)3]2+). The rate constant for the transfer process can be determined by emission measurements of the two connected metal complexes. The system can be modified by incorporation of a switching unit or an interrupter into the wire, influencing the transfer process. Such a molecular device corresponds to an interrupter, mimic the same function applied in molecular electronics. We have used organic switches, which show photochromic properties. By irradiation with light of different wavelengths, the switch changes its functionality by a photochemical reaction from an OFF- to an ON-state and vice versa. The ON- respectively OFF-state is manifested by a color change but also in different conductivity properties for energy and electron transfer processes. Therefore, the mentioned molecular device can work as a simple interrupter, controlling the rate of the transfer processes

Gated photochromism of 1,2-diarylethenes

A dithienylethene derivative containing a cyclobutene-1,2-dione skeleton does not exhibit photochromic properties. However, when both ketone functions are protected with cyclic acetal groups, photochromic behavior is observed

On the Way towards New Materials: Metal Complexes as Building Blocks

Two related directions of materials research are described: i) The synthesis of photonic materials, based on transition-metal compounds mainly with ruthenium and osmium as the photoactive centers. The devices envisaged are antenna molecules, molecular wires, photon-activated switches, and molecules performing other logical functions. ii) The development of a strategy for the synthesis of complex coordination species suitable for molecular devices, based on modular synthons, where a special emphasis is put on the stereochemical aspect of the fragments employed. This strategy is based on a new family of chiral pyridine and bipyridine ligands that are useful for the construction of inert building blocks, as well as in self-assembly processes leading to highly organized systems

Photochemical investigation of a photochromic diarylethene compound that can be used as a wide range actinometer

The photochromic diarylethene derivative 1,2-bis(5-(4-ethynylphenyl)-2-methylthiophen-3-yl)perfluorocyclopentene (1) was submitted to photochemical, thermal stability and fatigue resistance studies in acetonitrile, also to evaluate its possible application as a new actinometer. This photochromic system covers a wide spectral absorption range, with intense bands in the UV and visible regions for the open-ring and closed-ring isomers, respectively. Very high ring-closure quantum yield values were obtained, in contrast with the low ring-opening quantum yields, which are nevertheless high enough to exploit 1 as an actinometer. The procedure required to determine the photon flux of an irradiation source with this fatigue resistant compound is indeed very simple

The use of the triptycene framework for observing O⋯C=O molecular interactions

The triptycene skeleton has been used to measure (1,5) interactions between aldehyde groups, placed at both sp³ centres, and hydroxy or methoxy groups, placed at the respective ortho position on a benzene ring; HO⋯CHO interactions of 2.621–2.624 Å and MeO⋯CHO interactions of 2.528–2.584 Å were observed with the O⋯C vector making angles of 105.3–133.7° with the carbonyl bond. The lack of a competing conjugation with the framework for the electrophilic group is a favourable factor compared to the use of peri-naphthalene systems

Luminescent acetylthiol derivative tripodal osmium(II) and iridium(III) complexes: Spectroscopy in solution and on surfaces

Luminescent Os(II) and Ir(III) complexes containing a tripodal-type structure terminalized with three thiol derivatives are described. The tripod is introduced through derivatization, with a rigid spacer, of a phenanthroline ligand coordinated to the metal ion, and the entire structure possesses axial geometry. The geometry of the complexes combined with the three anchoring sites, the thiol groups, allows the complexes to adopt an almost perpendicular arrangement to the surfaces and the formation of a well-packed monolayer on Au substrates. The photophysical and electrochemical behavior of the complexes is studied in solution and on surfaces. Furthermore, a self-assembled monolayer (SAM) of Os(II) complexes on an ultraflat Au surface is used to fabricate a metal-molecule-metal junction with Au and In Ga eutectic as electrodes. The Os(II) SAM in the tunneling junction exhibits rectification behavior which is opposite in direction to that which we have previously shown for Ru(II) SAM