Photochromic Molecules as Building Blocks for Molecular Electronics

Abstract

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.

Influence of Multiple Protonation on the Initial Excitation in a Black Dye

“…By incorporation of Ru centers in supramolecular assemblies, devices capable of vectorial electron and energy transfer can be designed. The interest in such assemblies is twofold: (i) They play an important role in investigating the nature of electron- and energy-transfer processes − and (ii) they are valuable candidates for a wide variety of light-harvesting applications as, e.g., in photocatalysis − and as sensitizers in dye-sensitized solar cells (DSSCs). − Furthermore, Ru(II) polypyridine complexes can be employed as sensors − and in molecular wires. − In these applications the environmental conditions have to be monitored closely because the pH value of the solvent may influence the electronic and optical properties of the complexes by protonation/deprotonation of basic/acidic positions in the ligand sphere. This dependence can be exploited for pH sensing or switching, as environmental properties have a strong impact on the functionality by influencing, e.g., electron and energy transfer rates and redox potentials. ,− For example, in Ru complexes containing imidazole ligands the pH-dependent properties were studied: in imidazo[4,5- f ][1,10]phenanthroline coordinating complexes, the protonation state of the imidazole ring has been shown to modify the luminescence behavior, and solvent pH modulates the electron transfer in imidazo4,5- f ][1,10]phenanthroline-bridged supramolecular assemblies and across an electrode interface. ,,,− Another group of complexes with pH-dependent properties coordinates benzimidazole ligands: ,− ,,, in binuclear benzimidazole-bridged complexes metal–metal interactions can be switched o...…”