Arnd Vogler

Photoreactivity of gold complexes

Abstract The light sensitivity of gold compounds has been known since 1737. More recent observations have led to a deeper understanding of the photochemistry of gold complexes, which are discussed on the basis of selected examples. Light sensitive Au(I) and Au(III) compounds are classified according to the nature of reactive excited states: metal-centered (MC), ligand-to-metal charge transfer (LMCT), metal-to-ligand charge transfer (MLCT), metal-to-metal charge transfer (MMCT) and intraligand (IL). Accordingly, some basic information on the electronic spectra of gold complexes including luminescence spectra is provided. The photochemistry is described in more detail. Essentially, our own observations are reported but some relevant observations of other groups are also included.

Luminescent Metal Complexes: Diversity of Excited States

The photoluminescence of metal complexes has attracted much recent interest since it can be utilized for a variety of applications such as optical sensors and LEDs. Moreover, the emission behavior provides a probe for the investigation of photoreactions including artificial photosynthesis. In this review, emitting compounds are classified according to the nature of their excited states: metal-centered, ligand-to-metal charge transfer, metal-to-ligand charge transfer, ligand-to-ligand charge transfer, metal-to-metal charge transfer, ligand-centered (or intraligand), and intraligand charge transfer excited states. Complexes of transition metals (dn with n=0 – 10), main group metals (s2), lanthanides and actinides (fn) are included in our discussion. However, this review does not cover the photoluminescence of metal complexes comprehensively, but illustrates this subject by selected examples. The viewpoint is that of a coordination chemist and not of a spectroscopist. Accordingly, molecular complexes which emit under ambient conditions are preferably chosen.

Excited state properties of transition metal phosphine complexes

Phosphine ligands determine the excited state properties of a variety of coordination compounds. Phosphines not only influence metal-centered excited states, but participate directly in charge transfer transitions owing to their electron donating and accepting ability. Moreover, intraligand excited states are accessible if the phosphine carries suitable substituents. This diversity is illustrated by selected examples. The excited state behavior is discussed on the basis of spectral (absorption and emission) and photochemical properties of appropriate phosphine complexes.

The photolysis of iron(III) chloride in chloroform

In air-saturated solutions, iron(III) chloride functions as a long wavelength sensitizer of the photolysis of chloroform. In the presence of oxygen, irradiation of chloroform solutions of iron(III) chloride at wavelengths above 340 nm leads to the formation of HFeCl4, followed by continued production of HCl and C2Cl6 as chlorine photodissociates from HFeCl4, while the Fe(II) species formed is reoxidized. When iron(III) chloride is photolyzed in the absence of oxygen, conversion to HFeCl4 and reduction to colloidal FeCl2 occur simultaneously, ending in complete reduction. Both CCl3OO and CCl3OOH appear to participate in the reoxidation of iron(II) chloride. # 2002 Elsevier Science B.V. All rights reserved.

Optical properties of boron, gallium and gold complexes with salen ligands. Emission from intraligand excited states under ambient conditions

The electronic spectra of [B(OMe2)]2salen, Ga[salen(t-bu)dac]Cl and [Au(salen)]PF6 with salen=N,N′-bis(salicylidene)ethylenediamine and salen(t-bu)dac=N,N′-bis(3,5-di-tert-butyl-salicylidene)-1,2-diaminocyclohexane are characterized by salen intraligand (IL) absorptions and emissions. While all three compounds show IL fluorescence the gold complex displays an additional IL phosphorescence which also appears under ambient conditions.

Optical properties of zinc(II) 5,7-diiodo-8-quinolinolate in solution. Phosphorescence under ambient conditions

Abstract The emission spectrum of Zn(DIQ) 2 with DIQ=5,7-diiodo-8-quinolinolate consists of an intraligand (IL) fluorescence at λ max =550 nm and a rather weak IL phosphorescence at λ max =804 nm. While the appearance of the phosphorescence under ambient conditions is certainly caused by the heavy atom effect of the iodine substituents their influence is smaller than might be expected.

Photoluminescence of 8-quinolinolatomethylmercury(II)

The organometallic complex CH 3 Hg II (oxinate) with oxine = 8-quinolinol or 8-hydroxyquinoline shows a longest-wavelength absorption at λ max = 375 nm (e = 1900) and a red phosphorescence at room temperature (λ max = 638 nm, o = 5 x 10 -3 at λ exc = 420nm in benzene). The emitting triplet is of the oxinate intraligand type. It follows that the heavy atom effect of mercury is fully transmitted to the oxinate ligand.

Electronic spectra and photoreactivity of cyclopentadienyl complexes

Abstract The cyclopentadienyl (Cp) ligand coordinates to high- and low-oxidation state transition metals as well as main group metals. Consequently, a variety of different excited states and photoreactions is possible. This diversity is illustrated by selected examples. Recent observations are emphasized. Spectral and photochemical properties are related to the following electronic transitions: metal-centered (e.g. Cp 2 NbCl 2 , TlCp), intraligand, ligand-to-metal charge transfer (e.g. CpReO 3 , CpTiCl 3 , Cp 2 TiS 5 , TlCp), metal-to-ligand charge transfer, ligand-to-ligand charge transfer (e.g. Cp 2 Zr-2,2′-biquinoline 2+ ), metal-to-metal charge transfer (e.g. Cp 2 Ti[NCRu(CN) 5 ] 2 6− ), outer sphere charge transfer (e.g. CoCp 2 + Mn(CO) 5 − , CpWH 2 ×9,10-phenanthrenequinone) and charge-transfer-to-solvent. The Cp ligand may be directly involved in these electronic transitions or act as a spectator ligand.

The photooxidation of tetrachloroplatinate(II) in chloroform

Abstract [PtCl 6 ] 2− is known to be photoreduced by UV light to [PtCl 4 ] 2− in chloroform, but at irradiation wavelengths of 254 nm or lower the reverse reaction, photooxidation of [PtCl 4 ] 2− to [PtCl 6 ] 2− , occurs. This is a solvent-initiated process, proceeding through absorption of light by CHCl 3 and subsequent bond homolysis. The quantum yield, based on the light absorbed by CHCl 3 , is 0.20. Tetrachloroplatinate(II) and hexachloroplatinate(IV) may be cycled back and forth several times by changing the irradiation wavelength.

Optical properties of tris(cyclopentadienyl)gadolinium. Luminescence from an interligand triplet under ambient conditions

Abstract The organometallic compound GdCp 3 (Cp=cyclopentadienyl) in ether solution shows a green luminescence ( λ max =523 nm, φ =0.2). It is suggested that this emission originates from a triplet of the Cp 3 3− moiety.