Rhenium diimine complex properties

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. 

Photophysical Properties of Rhenium Carbonyl Diimine Complexes... 

The MLCT emission properties of many rhenium(I) carbonyl diimine complexes have been used to demonstrate the energy-gap law. For example, the complexes [Re(N-N)(CO)3(L)] (N N = bpy, phen, 5-Ph-phen L = substituted pyridines or quinoline) displayed MLCT (d7r(Re)- 7t (N-N)) phosphorescence at room temperature. The radiationless decay-rate constants depended on the emission energy as described in the energy-gap law (Inknr A-B fern) From the Hammett constant of the substituent, both the emission energy and hfetime of related complexes can be predicted. ... 

The optoelectronic properties of conjugated polymers containing the rhenium diimine unit [Re(CO)3(phen)Cl] have been studied. Charge-carrier mobility measurements showed that the presence of metal complexes could facilitate the charge-transport process, and the enhancement in carrier mobility was dependent on the metal content in the polymer. The use of transition metal complexes for both photovoltaic and electroluminescence applications was demonstrated. 

All these polymers, A (39), B (40), C (41,42), D (43), showed distinctive absorption and emission due to the MLCT transition of the rhenium(I) diimine complexes. Among these rhenium-containing polymers, polymer C has the most notable emission properties, with an emission quantum yield and a lifetime ( e = 0.132, Te = 2019ns in a deaerated CH2CI2 solution) comparable to those of the corresponding mononuclear complex (0e = O.181, Te = 2200 ns) (43). 

The MLCT emission properties of many rhenium (I) carbonyl diimine complexes have been nsed to demonstrate the energy-gap law. For example, the complexes [Re (N-N) (CO) 3(E)] (N-N = bpy, phen, 5-Ph-... 

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. ... 

Lo and co-workers reported the synthesis, characterisation, photophysical and electrochemical properties of luminescent rhenium(I) diimine indole complexes [Re(N"N)(CO)3(py-indole)]+ (36) [75,76]. Upon visible-fight irradiation, the complexes exhibited 3MLCT (dTt(Re) —> Tt (N N)) emission in fluid solutions at 298 K and in low-temperature glass. When these com-... 

Rhenium(I) diimine carbonyl complexes have been well investigated because of their functionalities, such as the intense emission properties, capabilities as a building block for multinu-clear complexes, and photocatalytic activities. 

In addition, the family of rhenium(I) diimine carbonyl complexes/he-[Re(LL)(CO)2(X)(Y)]" (LL = a-diimine) has imique properties and application as follows ... 

In this chapter, we focus on these imique photochemical properties of rhenium(I) diimine carbonyl complexes (Fig. 1), especially photochemical reactions and photocatalysis. 

Complexes of the general formula /ac-Re(a-diimine)(CO)3X and Re(a-diimine)(CO)2XX (where ot-diimine = bpy, phen, substituted bpy or phen, etc. and X, X = halide, solvent, alkyl, benzyl, monodentate phosphine, CO, etc.), have attracted interest since the mid-1970s [51-53]. Many of these complexes show emission from their lowest long-lived MLCT state at room temperatme in solution. Their catalytic properties for CO2 reduction have also been investigated. Electrolysis of a solution containing/uc-Re(bpy)(CO)3 Cl and 0.1 M BU4NPF6 in freshly distilled C02 saturated MeCN at —1.5 V (vs. SCE) produces both CO and C03 with cmrent efficiencies of 98 and 110 %, respectively [54]. Further, yhc-Re(bpy)(CO)3X (X = Cl, Br ) has been used successfully as a photocatalyst for CO2 reduction to CO with TEOA in DMF [55-58]. When X = Cl , a quantiun yield of 0.14 has been measured in the presence of excess Cl". A formato-rhenium complex,/ac-Re(bpy)(C0)3(02CH), has been isolated in the absence of excess Cl". 

EXCITED-STATE PROPERTIES OF DIIMINE RHENIUM(I) TRICARBONYL COMPLEXES INTERRELATIONSHIPS BETWEEN MLCT, LLCT, AND IL EXCITED STATES... 

This case study deals with different photophysical properties of a variety of diimine rhenium(I) tricarbonyl complexes. The exceptionally diverse photophysical behavior of these complexes is largely dependent on the nature of their lowest excited states. Varying the substituents on either the diimine ligands or ancillary ligands can easily change the relative order of these excited states and provides a way to tune the excited-state characteristics. A range of important applications is now becoming apparent, based on the richness of the photophysical and photochemical properties of diimine rhenium(I) tricarbonyl complexes. 

The robustness of the rhenium(i) diimine alkynyl systems and rich photophysical behavior have rendered them suitable as metalloligands for the synthesis of mixed-metal complexes. It is well-known that organometallic alkynes exhibit rich coordination chemistry with Cu(i), Ag(i) and Au(i) [214-218], however, photophysical properties of these r-coordinated compounds are rare. Recent work by Yam and coworkers has shown that luminescent mixed-metal alkynyl complexes could be synthesized by the metalloligand approach using the rhenium(i) diimine alkynyl complexes as the z -ligand. Reaction of the rhenium(i) diimine alkynyl complex [Re(bpy)(CO)3C=CPh] with [M(MeCN)4]PF6 in THF at room temperature in an inert atmosphere afforded mixed-metal Re(i)-Cu(i) or -Ag(i) alkynyl complexes (Scheme 10.31) [89]. Their photophysical properties have also been studied. These luminescent mixed-metal complexes were found to emit from their MLCT[d7i(Re) —> 7i (N N)] manifolds with emission bands blue-shifted relative to their mononuclear precursors (Table 10.5). This has been attributed to the stabilization of the dTi(Re) orbital as a consequence of the weaker t-donating ability of the alkynyl unit upon coordination to the d metal centers. 

In recent years, this section of this report has been dominated by the photochemical and photophysical properties of Re(CO)3(diimine)X complexes. Although the main deluge of research is over, there are still examples to be found. The photochemical/physical properties of monomer and polymer species with ReI(CO)3(phen) chromophores has been investigated by Wolcan and Ferraudi, and proton-controlled photoisomerisation of rhenium(I) tricarbonyl bipyridine linked to amine or azacrown ether groups by a styryl pyridine bridging ligand has been discovered by Perutz et alP... 

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