Alkynyl photophysical properties

Salts with di(alkynyl)aurate(i) anions are very common reagents for the preparation of other gold(i) complexes. A large number of examples with R taken from all sorts of organic functionalities have been prepared and characterized, mainly in a search for new classes with potentially useful photophysical properties (below). 

The largest families stem from substituted py, bipy, phen, and DAB but virtually any kind of O, N, S or P donor can be used. Both the nature of the hgands and their substituents as well as the nature of X (halide, alkoxide, alkyl, alkynyl) have been used as handles for tuning the photophysical properties of the excited state and emission of these complexes. 

The photophysical properties of metal-alkyne materials are rich due to strong metal-ligand orbital overlap and unsaturated n system (16). Photoactive materials can be readily constructed from copper(l) (198-200), rhenium(l) (101, 186, 201-203) platinum(ll) alkynyls (13, 203-219), as well as various gold(l) derivatives (210, 220-226). 

Ziessel has described the heterotopic heterotrinuclear heptanuclear metallostars [Fe(42)3]" in which alkynyl substituted bpy units are connected via C—Pt bonds. These complexes have interesting photophysical properties and are likely to lead to novel antenna systems. 

Owing to the advances in the synthesis of organoplatinum(ii) alkynyl compounds as well as the intriguing physical and photophysical properties exhibited by these materials, the evaluation of their structure-property relationship is therefore attractive and feasible. The spectroscopic properties of mononuclear nickel(ii), palladium(ii) and platinum(ii) alkynyl complexes of the type trans-[M(C=CR)2L2] (R = alkyl, aryl L = phosphine, stibine) have been widely investigated [111-114]. The electronic absorption spectra of this class of complexes have been reported by Masai et al. [115], while the emission properties of a series of closely related platinum(ii) complexes were reported by Demas and coworkers [116]. The lowest... 

Besides the platinum and palladium diphosphine systems, platinum polypyridine complexes have also attracted attention in recent years [130-162). Their changes in spectroscopic and luminescence properties brought about by Pt Pt and n-n interactions are of particular interest [138-142). For instance, the square planar platinum(ii) diimine complex [Pt(bpy)Cl2) exhibits polymorphic behavior and rich photophysical properties [142-144). These, together with the recent growing development of platinum alkynyl systems [98, 122, 125, 145-147), mainly due to the... 

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. 

The luminescence properties of Au(l) alkynyl complexes were first reported by Che and co-woikers, in which the luminescence behavior of the dinuclear Au(I) alkynyl complex, [Au2(/t-dppe)(C=CPh)2], was described (Figure 5.6). Short intermolecular Au - Au contacts of 3.153(2) A were observed in the X-ray crystal structure. As discussed in the previous sections, Au -Au interactions also play an important role in the photophysical properties of Au(I) complexes. The 550-nm emission of the solid sample of [Au2(/t-dppe)(C=CPh)2] at room temperature was described as originating from a [(d5 ) (po) ] triplet excited state arising from the interactions between the two Au atoms. 

The copper(I) alkynyls displayed rich photochemistry and particularly strong photoreducing properties. The transient absorption difference spectrum of [Cu3(dppm)3(/X3-) -C=CPh)2]+ and the electron acceptor 4-(methoxycarbonyl)-A-methylpyridinium ion showed an intense characteristic pyridinyl radical absorption band at ca. 400 nm. An additional broad near-infrared absorption band was also observed and it was assigned as an intervalence-transfer transition of the mixed-valence transient species [Cu Cu Cu (dppm)3(/x3- -C=CPh)2] +. The interesting photophysical and photochemical properties of other copper(I) alkynyl complexes such as [Cu(BTA)(hfac)], 2 [Cui6(hfac)8(C=C Bu)8], and [Cn2o(hfac)8(CsCCH2Ph)i2] have also been studied. 

Au Au interactions also play a pivotal role in the photophysical characteristics of Au(i) alkynyl complexes. A red shift in the solid-state emission energies relative to that in solution is commonly observed for complexes with such interactions [87, 183, 188, 189]. In addition, the nonlinear optical (NLO) properties of a number of di- and trinuclear Au(i) alkynyl complexes with di- and tri-ethynylbenzene cores have been investigated by Humphrey and coworkers [191-194], These complexes may serve as potential NLO materials with reasonable first and second hyperpolarizabilities. Introduction of nitro group into these systems was suggested to enhance the optical nonlinearity. 

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