Chromium complexes luminescence

The excited-state redox reaction, equation (8.12), is thermodynamically favorable (E° > 0) while ground-state reaction, equation (8.13) is not (E° < 0). Therefore, a mixture of [Cr(phen)3]3+ and such a substrate will only undergo a redox reaction after the chromium complex has been excited. This is the process of photo-induced electron transfer light initiates an electron-transfer reaction. This experiment will explore how substrates such as DNA may be oxidized by the excited-state [Cr(phen)3]3+ complex. Because the electron-transfer reaction competes kinetically with luminescence, the presence of such a suitable substrate leads to a decrease in the intensity of luminescence. For this reason, the substrate is termed a quencher. 

For each solution in the Cr + G series, Table 8.2 (1) measure the UV-vis spectrum to obtain the absorbance A at Aex, and (2) measure an emission spectrum and obtain the integrated emission intensity, I. Also, prepare a solution having [G] the same as the last in your series (Table 8.2), but with no chromium complex. This will serve as a control sample. Save your sample solutions for time-resolved luminescence quenching if you are conducting these experiments (see below). Save all UV-vis and luminescence spectra files that you acquire—they may be useful for data analysis. 

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

Luminescence of Chromium(III) Complexes in Rigid Glass Solutions... 

Chromium is unusual in that it forms a stable hexakis O-bonded urea complex. The complex was first prepared as the chloride salt by Pfeiffer804 and a crystal structure of the complex salt [Cr OC(NH2)2 6][Cr(CN)6]-2DMSO>2EtOH has recently been reported.805 Coordination at chromium(IH) is octahedral r(Cr—O) is in the range 1.96-1.98 A. The reduction of the perchlorate salt of this complex to a chromium(II) species has been studied polarographically.806 Detailed studies of the luminescence spectra of several salts of the chromium urea complex have been reported.807,808... 

Luminescence spectrocopy is potentially a powerful technique for studying chromium(III) complexes and a series of fluoro and aqua complexes have been studied.1066 The luminescence correlates well with ligand field strength and, at. liquid air temperatures, the lifetime of the doublet state from which phosphorescence originates is 2 x 1CT7 s 1. 

Quenching of the ( CT)[Ru(bipy)3] by [Cr(bipy)]3 has been studied. This is via electron transfer to the Cr complex and a rapid back reaction. The ruthenium complex will also quench the 727 nm emission of the metal-centred doublet excited state of the chromium species, by a similar mechanism. Evidently both ligand- and metal-centred excited states can be quenched by bimolecular redox processes. A number of Ru complexes, e.g. [Ru(bipy)3] and [Ru(phen)3] also have their luminescence quenched by electron transfer to Fe or paraquat. Both the initial quenching reactions and back reactions are close to the diffusion-controlled limit. These mechanisms involve initial oxidation of Ru to Ru [equation (1)]. However, the triplet excited state is more active than the ground state towards reductants as well as... 

This work lead to an interest in the luminescence of lower symmetry chromium ammine complexes. In these complexes, the 2Eg state is split into two components by the lower symmetry ligand field. The tetragonal ligand field parameters for many of these compounds were well known or easily available from optical spectroscopy, but the splitting of the 2Eg as measured by the absorption and emission spectroscopy... 

Complexes of Cr and Cr are produced in the photoprocess leading to the hardening of chromated poly(vinyl alcohol). Methods have also been reported for the determination of chromium by a luminescence method and by a photochemical titration procedure. ... 

There is some controversy concerning the existence of the a isomers of these tris complexes, Israily reported a purple complex to be or-[Cr(gly)3] however, subsequent workers have shown that this substance most probably was the dihydroxy dimer [Cr2(gly)4(OH)2]. Careful chromatography, on potato starch, of solutions from chromium(III)/glycine reactions yielded red and purple fractions,the electronic spectra of which were consistent with jS and a isomers respectively. Solutions of the a complex were unstable even in the dark and cold. Hoggard has recently claimed the preparation of the a isomer of the glycine complex by a fractional crystallization. The complex was anhydrous, unlike its cobalt(III) analogue. X-Ray powder methods could hence not be used to confirm the identity of the complex the luminescence spectra were held to be consistent with meridional coordination. There have been a number of studies of the physical properties of /S-[Cr(gly)3], summarized in Table 99. 

Figure 10 Selectively excited luminescence spectra for a bimetallic chromium(III) complex, [(Bispicam)-Cr (0H)2(S04)Cr (Bispicam)]S206 3H20, where Bispicam denotes 7V,7V -bis(2-pyridylmethyl)-amine. The traditional, unselectively excited luminescence spectrum is shown as trace (a). Traces (b) to (e) denote selectively excited luminescence spectra from specific subsets of molecules (reproduced by permission of Elsevier Science from Chem. Phys. Lett. 1987, 133, 429-432).

Luminescence is observed from coordination complexes of first row transition metals less often than from complexes of second or third row metals. This is a result of the presence of low-energy ligand field (LF) states into which the excitation is funneled that can rapidly deactivate through either non-radiative pathways to the ground state or photo-reaction pathways to products. Recent reports relating to the luminescence of titanium," chromium", and zinc complexes provide examples of new emissive first row coordination compounds. 

Luminescence from tf-block metal complexes was first observed for chromium(III) complexes. This is illustrated in Fig. 20.26a with the absorption and emission spectra of [Cr OC(NH2)2 e] + (OC(NH2)2 = urea). In a typical fluor-... 

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