Modulation of the Photophysical, Photochemical, and Electrochemical Properties of Re(I) Diimine Complexes by Interligand Interactions.

The photophysical and photochemical properties of transition metal complexes have attracted considerable attention because of their recent applications as photocatalysts in artificial photosynthesis and organic synthesis, as light emitters in electroluminescent (EL) devices, and as dyes in solar cells. The general control methods cannot be always used to obtain transition metal complexes with photochemical properties that are suitable for the above-mentioned applications. In the fields of solar energy conversion, strong metal-to-ligand charge-transfer (MLCT) absorption of redox photosensitizers and/or photocatalysts in the visible region with long wavelength is essential. However, the usual methods, i.e., introduction of electron-withdrawing groups into the electron-accepting ligand and/or weak-field ligands into the central metal, have several drawbacks, including shorter excited-state lifetime, lower emission efficiency, and lower oxidation and reduction power. Herein we describe a new method to control the photophysical, photochemical, and electrochemical properties of Re(I) diimine carbonyl complexes that have been widely used in various fields such as photocatalysts for CO reduction and emitters in EL devices and sensors. This method involves the introduction of interligand interactions (π-π and CH-π interactions) into the Re(I) complexes; the aromatic diimine ligand coordinating to the Re center approaches the aryl groups on the phosphine ligand or ligands at the cis position, which "compulsorily" induces a weak interaction between these aromatic groups. As a result of this interligand interaction, the Re complexes with the aromatic diimine ligand and the arylphosphine ligand(s) exhibit red-shifted MLCT absorption but afford blue-shifted emission from the triplet metal-to-ligand charge-transfer (MLCT) excited state. This increases the oxidation power and lifetime of the MLCT excited state. These unique property changes are favorable, particularly for redox photosensitizers. The interligand interaction is strongly expressed by the ring-shaped multinuclear Re(I) complexes (Re-rings). In the case of Re-rings with high steric hindrance due to a small inner cavity, the lifetime of the MLCT excited state is up to 8 μs and the emission quantum yield is up to 70%. These properties cannot be obtained by the corresponding mononuclear Re(I) complexes, which generally exhibit shorter lifetimes (<1 μs) and lower emission quantum yields (<10%). Some of the Re-rings could be successfully applied as efficient photosensitizers in photocatalytic systems for CO reduction; the highest quantum yields for CO reduction were achieved by using photocatalytic systems composed of Re-rings as the photosensitizers and Re(I) (82%), Ru(II) (58%), and Mn(I) (48%) complexes as catalysts. This interligand interaction potentially provides unique and useful methods for controlling the photophysical, photochemical, and electrochemical functions of various metal complexes, paving the way to create new functions for metal complexes.