Rhenium complexes photophysical studies

Most photosensitizers, however, are reasonably photostable compounds, and their optical properties have been studied in depth. In particular, there has been much interest in ruthenium-based photosensitizers such as [Ru(bpy)3]2+ and [Ru(phen)3]2+, due to their stability and absorption of visible light. Detailed information on their optical properties, including ground and excited state information in relation to photosensitization, has been reviewed by Creutz et al. [16]. Similarly, the photochemistry and photophysics of rhenium complexes, as discussed here, have been reviewed in detail by Kirgan et al. [7]. 

The CO stretching vibrational bands of rhenium carbonyl complexes are useful spectroscopic probes for studying their electronic excited states. As these IR absorption bands reside in the frequency region where few absorption bands exist, their absorption cross section are large and the band frequencies are greatly influenced by the amount of electron density on the central rhe-nium(I). Therefore, transient IR absorption (TR-IR) measurements should be one of the ultimate tools for the study of the photophysics and photochemistry of the rhenium carbonyl complexes. 

Sun. S.-S. Lees, A.J. Self-assembly triangular and square rhenium(I) tricarbonyl complexes A comprehensive study of their preparation, electrochemistry, photophysics, photochemistry, and host-guest properties. J. Am. Chem. Soc. 2000, 722 (37). 8956-8967. 

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. 

A series of luminescent rhenium(I) triynyl complexes, Re(CO)3L(-C = C-C = C-C = CR) (L = Me2bpy or Bu 2bpy, R = Ph or SiMe3), have been prepared and their photophysical and electrochemical properties studied. Surprisingly, lengthening the carbon chain resulted in a blue shift in the emission energy of the complexes, rather than the expected red shift. 

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