Photophysics of Gold Complexes

The discriminatory emission properties between two-coordinate d ° gold(I) complexes and their respective three-coordinate counterparts have been demonstrated in the literature [6, 10-13]. As discussed in the later sections, Che and coworkers have rationalized that the extraordinarily large Stokes shift of the visible emission of [Au2(diphosphine)2] from the [5da 6pa] transition is due to the exciplex formation ofthe excited state with solvent or counterions [6]. Fackler [14—16] reported the photophysical properties of monomeric [AUL3] complexes, which show visible luminescence with large Stokes shifts (typically lOOOOcm ), suggesting significant excited-state distortion. Gray et al. [10] examined the spectroscopic properties of 

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Luminescence has been observed from a large number of later transition element complexes and a rich array of excited states have been observed. Related sections from CCC (1987) include Chapter 16.5 on Pt, Rh, and Ir complexes, 36.3 on Mo halide clusters, 43 on Re complexes, 45.4 on Ru polypyridyls, 46.4 on Os polypyridyls, 48.6 on Rh complexes, and sections of Chapter 52 on Pt complexes. Several recent reviews have been published on polynuclear complexes, the photophysics of gold complexes,and platinum diimine complexes. Many other more narrowly focused review articles have been published on transition metal complex luminescence a significant number are published in the journal Coordination Chemistry Reviews and some of these reviews are cited in this chapter. 

This chapter provides an overview of spedroscopic investigations into the photophysics of luminescent mono- and polynuclear gold(I) and -(III) complexes. 

Forward, J.M. Fackler. J.P., Jr. Assefa, Z. Photophysical and Photochemical Properties of Gold(I) Complexes in Optoelectronic Properties of Inorganic Compounds Roundhill, D.M., Fackler, J.P., Jr.. Eds. Plenum Press New York, 1999. chapter 6. 

The idea of metal-metal bond establishment in the excited state of this complex has also stimulated the interest on the photophysical and photochemical investigations of related d -d complexes. In 1970, Dori and co-workers first reported the luminescence properties of phosphine complexes of d metal centers such as copper(I), silver(I), gold(I), nickel(0), palladium(0), and platinum(0) [12]. Later in 1985, Caspar reported the luminescence properties of polynuclear nickel(O), palladium(O), and platinum(O) complexes of phosphine, phosphite, and arsine [13]. Similar to the related d -d systems, the emissive states of [Pd2(dppm)3] and [Pd Cdpam),] have been suggested to be metal-centered (d-p) in nature modified by metal-metal interaction. 

The photophysical properties of another trinuclear silver(I) complex [Ag3([i3-dppnt)3] (38a) have also been studied [115]. The complex shows absorption bands at 238 and 367 nm in CH2CI2. The emission spectrum of the complex in CH3CN shows an emission band at 550 nm (T6 = 5.0 ps) at room temperature. The phosphorescent state has been assigned to be ligand-centered in nature. The X-ray crystal structure of the dinuclear gold(I) counterpart shows that a potassium ion is encapsulated in the macrocyclic cavity, forming the complex [Au2K(p3-dppnt)j + (38b) [115]. 

Data reported for compound 17a revealed that excitation at = 332 nm afforded emissions in the solid state at = 482 nm at room temperature and = 465 nm at 77 K, while an acetone solution produced = 429 following excitation at = 379 nm. As the data for compound 17b showed little difference in photophysical behavior, the authors concluded that this emission could not result from a phosphine-located excitation. At this time no other rationale was offered for the lack of luminescence from the parent closo species 16 other than a significant change in electronic structure. Nevertheless, the ligating ability of the phosphine arms in the nido compounds 17 was exploited to produce luminescent gold complexes, which is discussed in the next section. 

FIGURE 3.14 The structure of a cationic gold(II) complex, [ [Au(dppn)I])j]2+, with an nnsnpported Au -An" bond. Only one carhon atom of each phenyl ring is shown. (Data from synthesis, strnctnral characterization, and photophysics of dinuclear gold(II) complexes [ Au(dppn)Br 2](PFg)2 and [ AnfdppnllljKPFgfj with an unsupported Au -Au bond, V. W.-W. Yam, C.-K. Li, C.-L. Chan, and K.-K. Cheung, Irwrg. Chem., 2001,... 

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