Luminescence, from *Cr

Luminescence from Cr + complexes, both in the solid state and in solution, is a widespread phenomenon. The great majority belong to type a) in Figure 1, where the luminescent state is 2E and the optical transitions are sharp. The well-known ruby emission is a prototype for this situation. In a weaker ligand field the situation b) in Figure 1 is approached, the T2 state becomes competitive with E as the luminescent state. The T emission, corresponding to a spin-allowed d-d transition, is vibronically broadened. Pure 2 luminescence from Cr + has been observed in halide and oxide coordinations (J ). Intermediate situations with both 2E and 2 emissions ar also known. 

Glynn T, Imbusch C, Walker G (1991) Luminescence from Cr centers in forsterite Mg2Si04. J Lumin 48 49 541-544... 

Under lamp excitation tourmaline is practically non-luminescent, while under X-ray excitation it exhibits impurity luminescence from Fe centered at 700-750nm and Mn + centered at 560-570nm (Kusnetsov and Tarashchan 1988). The natural tourmaline in our study consisted of four samples. The laser-induced time-resolved technique enables us to detect Cr + emission centers (Fig. 4.58). 

Lindholm and Adamson144 report that photochemically, coordinated NH3 is preferentially lost from Cr(NH3)5NCS2 + while thermally NCS" is replaced. A study of the temperature dependence of Cr(NCS)e3- phosphorescence was reported.145 A general discussion of the luminescence behavior of coordination complexes has been given146 and a careful study of the Cr(C204)33 luminescence spectra has been reported.147... 

Emission from transition metal complexes obey Kasha s rule and originate from the lowest excited state which are (i) 3(n, n ) state in [Rh (phen)3] (C104)3 in water-methanol glass (ii) d, iz ) state in [Ru (bpy)J Clj in ethanol-methanol glass, and (iii) d d) state, in solid [RhClj (phen)J Cl. Their characteristics differ in details and are given in Figure 8.16-Sometimes weak fluorescence is also observed, from Cr + complexes. K [Co(CN)t] is highly luminescent. The 4>p and 7p are temperature... 

Figure 9a shows the emission spectra of co-doped [Rh1 yCry(bpy)3] [NaAl1 xCrx(ox)3]C104 at 1.5 K for x=l% and y between 0 and 2.5%. Despite the fact that at the excitation wavelength of 568 nm only the [Cr(ox)3]3-chromophores are excited, quite strong luminescence from the 2E state of [Cr(bpy)3]3+ is observed [27]. As schematically shown in Fig. 10, this luminescence is the result of excitation energy transfer from the 2E state of... 

The fast rise in the [Cr(bpy)3]3+ luminescence can be attributed to energy transfer from those [Cr(ox)3]3- chromophores which happen to have an acceptor complex within the shell of nearest neighbour sites, in which case the probability of an energy transfer process is close to unity. The slow component can be attributed to energy transfer from [Cr(ox)3]3- chromophores which do not have an acceptor in the nearest neighbour shell, in which case the probability for an energy transfer process to occur within the natural lifetime of the 2E state of [Cr(ox)3]3- is less than unity. The fact that there is a more than three orders of magnitude difference between the rate constants of the two processes indicates that the nature of the interaction must be different, irrespective of the actual nature of the interaction ... 

The quenching of the CC luminescence from the copper(l) clusters Cu4I4py4 by a series of Cr(acac-x)3 and other electron-transfer and energy-transfer acceptors demonstrated a similar pattern of pressure effects [52]. 

Luminescence from block metal complexes was first observed for Cr(III) complexes, and is illustrated by [Cr OC(NH2)2 6] - In a typical fluorescence experiment, the absorption spectrum is initially recorded. The sample is then irradiated with light corresponding to A, ax in the... 

Luminescence from discrete transition metal complexes was first observed for classical Werner complexes of Cr many years ago and early studies of metal complex luminescence provided the basis for characterizing the spin multiplicity and relaxation processes of excited states of complexes in solution and the solid state. This early work has been thoroughly documented in a number of books, monographs, and reviews on inorganic photochemistry. " ... 

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-... 

The Cr " luminescence properties in natural chrysoberyl minerals have been studied as a function of the Cr content as well as impurities such as Fe and V. A competition was foxmd between Cr and V for very low Cr concentration with the vanishing of Cr " emission from Cr " ions located in inversion site. The Fe " ions substitute in mirror site efficiently with a strong impact on the Cr " lifetime of mirror site (Ollier et al. 2015). 

Those lines have been previously ascribed to Eg luminescence of Cr " " in an intermediate crystal field site (Tolstoy and Shinfue 1960 Wojtowicz 1991). Nevertheless, several contradictions prevent us from accepting such an interpretation. Excitation spectra of two types of enussion lines are very similar to those pubhshed earlier and determined as a and b environments (Platonov et al. 1998). Crystal field parameters calculated based on their excitation and polarized absorption spectra gave Dq = 1720 cm and B = 730 cm for a and Dq= 1600 cm and B = 570 cm for b environments. As was already mentioned, it was concluded that these great differences in the crystal field parameters cannot be explained by a distribution of Cr between two or more of the four crystallographicaUy different octahedral sites in the kyanite structure. The presence of a corundum precursor in kyanite was confirmed by our experiments. Nevertheless, those lines have long decay times typical for Cr in strong crystal field. 

Those considerations stimulated us to look for different interpretations, such as the possibility that a trace component different from Cr is responsible for this emission, such as Mn and Nevertheless, in orange Mn-containing kyanite, such luminescence from Mn lines was not detected (Gaft et al. 2011). Another potential cause of luminescence is V, which is present in the studied samples in quantities sufficient for detectable luminescence. For a definite conclusion, synthetic kyanite samples artificially activated by Cr and V are needed. 

Thus possibilities different from Cr " " emission may be considered. A well known luminescence center in minerals is Fe in tetrahedral coordination which is characterized by a broad red-lR band, but its transitions are spin forbidden and their decay times are usually in the milliseconds range. Ti " " in octahedral sites with a broad spin-allowed luminescence band and a correspondingly short decay time of several microseconds may be much a more probable candidate. For example, Ti ... 

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