Rhenium diimine complex states

Understandably, there is an enormous richness in the photophysical and photochemical behavior of the excited states present in diimine rhenium tricarbonyl complexes. Indeed, this plethora of molecular photophysical characteristics has led to a wide range of interesting and important applications, including their use as catalysts [21-25], sensors [26-33], probes for photo-polymerization [10, 34, 35], optical switches [36 15], light-emitting materials [46-52], nonlinear optical materials [53-56], binding or photocleavage of DNA [57-61], and radiopharmaceuticals [62-66], Under the purview of this article our focus will be to cover photophysical and photochemical properties and hence other aspects, such as synthetic, catalytic, pharmaceutical, etc., will not be discussed. 

These bands show negative solvatochromism as revealed by band shifts to lower energy in less polar solvents [5, 7, 8, 12], The direction of the solvent dependence is associated with a reduced (and reversed) molecular dipole in their MLCT excited states. Emissions from these complexes are typically broad and structureless, and they also often exhibit a rigidochromic effect [7-12], Tables 1 and 2 summarize the luminescence characteristics and environmental effects on absorption and emission maxima for rhenium(I) tricarbonyl diimine complexes. 

Fig. 2. The schematic energy diagram of the electronic states of the rhenium(I) diimine complexes.

As the lowest excited state of many Re(I) diimine complexes is the emissive MLCT state, it can be tuned by chemical modification of the diimine ligand and the identity of the monodentate ligands. Figure 3 shows the UVA is absorption and emission spectra of a typical rhenium(I) diimine complex, /ac-Re(bpy) (COaCl (bpy = 2,2 -bipyridine) (la), measured in CH3CN. 

It has been also reported that Re—R bond homolysis can be promoted by the photoexcitation of mononuclear rhenium(I) complexes with a Re—alkyl bond, /ac-[Re(CO)3(a-diimine)R] (R = alkyl ligand). This type of the reaction also proceeds via a state, which is produced by the relaxation from the MLCT excited state. In the cases of R = ethyl (Et) and benzyl (Bz), the homolysis reaction proceeds efficiently. However, the reaction efficiency is much lower in the case R = methyl (Me) because the transition from MLCT to requires extra energy (Scheme 1). Using transient absorption spectroscopy, the state was... 

Another important type of LLCT state arising in diimine rhenium(I) tricarbonyl complexes is found in IRe (CO)3(diimine) complexes. When I replaces Cl or Br, the lowest excited states changes from being MLCT in nature to that of XLCT (halide-to-ligand charge transfer) in character. Figure 2 clearly shows that the lowest energy band of the transient absorption... 

Z)-Stilbene]Cr(CO)3 showed three optically accessible excited states. One of these excited states caused (E)-(Z) isomerization and the other resulted in a slow release of carbon monooxide. " Tetranuclear rhenium(I) tricarbonyl diimine complexes (20) with a stilbene-like bridging ligand showed efficient photoswitching properties. The accurate quantum yields of the photoisomerization of the mononuclear rhenium(I) tricarbonyl diimine complex could be determined readily by the H NMR technique in eombination with absorption spectroscopy. ... 

Villaume S, Daniel C (2005) Emission spectroscopy of metal-to-ligand-charge-transfer states of HRe(CO)3(H-DAB), model system for alpha-diimine rhenium tricarbonyl complexes. Comptes Rendus Chimie 8 1453-1460... 

These complexes also usually exhibit substantial photostability under visible light irradiation and, due to their relatively long-lived triplet excited-state characteristics, the emission lifetimes are easily quenched by bimolecular electron- and/or energy-transfer processes in solution [6, 76], The electronic structures of MLCT excited molecules of diimine rhenium(I) tricarbonyl complexes can be viewed as a charge-separated species, [LRen(CO)3(diimine ")], with an essentially oxidized... 

When more conjugated diimine or pyridine ligands are used, the excited states of rhenium(I) carbonyl complexes can have substantial IL character. While the MLCT emission is often broad, with a lifetime in the submicrosecond to microsecond timescale, the IL emission usually has noticeable structural features, even in fluid solutions at ambient temperature. The emission lifetime is usually very long. A simple and widely applicable approach is to evaluate the ratio of the emission quantum yield and the emission lifetime (the product of the intersystem crossing efficiency and radiative decay-rate constant). Experimental values of... 

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