LMCT

It seems to be surprising that only a few examples of luminescent LMCT states are known. This lack could have two explanations. Many complexes with prominent LMCT states have other excited states such as MC states at lower energies. These provide a facile access for radiationless deactivation of LMCT states. In other cases LMCT states are quite reactive and photoreactions compete successfully with the luminescence. 

Molecular d oxo complexes of M04 which have low-energy LMCT states have not yet been observed to emit. However, some d oxometallates are known to luminesce in the solid state [76]. The organometallic d oxide CH3Re03 emits ( = 640 nm) from the lowest LMCT state in low 

Strong luminescence of the uranyl cation has been investigated in all details [35]. 

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Mn(acac)3 in the above mechanism undergoes an intramolecular photooxidation-reduction reaction arising from the ligand to metal charge transfer process (LMCT). 

Fig. 3 a UV-Vis DRS spectra of dehydrated TS-1 catalyst reporting the typical 208 nm (48000cm i) LMCT hand, see Fig. 2h also reported are the four excitation laser lines used in this Raman study near-lR (dotted), visible (full), near-UV (dashed) and far-UV (dot-dashed), b Raman spectra of dehydrated TS-1 obtained with four different lasers emitting at 7 = 1064,422,325, and 244 nm (dotted, full, dashed, and dot-dashed lines, respectively). Raman spectra have been vertically shifted for clarity. Although the intensity of each spectrum depends upon different factors, the evolution of the 7(1125)//(960) ratio by changing the laser source is remarkable. The inset reports the Raman spectrum collected with the 244 nm laser in its full scale, in order to appreciate the intensity of the 1125 cm enhanced mode. Adapted from [48] with permission. Copyright (2003) by The Owner Societies 2003... 

Bordiga et al. [48,52] explained the experimental evidence reported in Fig. 6 in terms of symmetry once the [Ti(H20)204] or [Ti(NH3)204] complexes are formed, the T -like symmetry of Ti(IV) species is destroyed in such a way that the symmetry of the vibrational modes is no longer the same as that of the LMCT, and the Raman resonance is quenched. 

Yeom and Frei [96] showed that irradiation at 266 nm of TS-1 loaded with CO and CH3OH gas at 173 K gave methyl formate as the main product. The photoreaction was monitored in situ by FT-IR spectroscopy and was attributed to reduction of CO at LMCT-excited framework Ti centers (see Sect. 3.2) under concurrent oxidation of methanol. Infrared product analysis based on experiments with isotopically labeled molecules revealed that carbon monoxide is incorporated into the ester as a carbonyl moiety. The authors proposed that CO is photoreduced by transient Ti + to HCO radical in the primary redox step. This finding opens up the possibility for synthetic chemistry of carbon monoxide in transition metal materials by photoactivation of framework metal centers. 

Consider first blue sapphire Al203 Ti(III), Fe(III) (Fig. 2). In the absence of Fe(III) the absorption spectrum is easy to interpret. The weak band with a maximum at about 500 nm is due to the t2 —> e crystal-field transition on Ti(III) (3d ), the strong band at 2<280nm is due to a Ti(III)-0( — II) LMCT transition. The absorption band in the region around 700 nm in the case of the codoped crystal cannot be due to Fe(III). It has been ascribed to MMCT, i.e. to a transition within an iron-titanium pair ... 

An interesting case is the optical absorption of M(II)-doped MgTi205 [33]. The spectra of interest are given in Fig. 3. The undoped MgTi205 shows a strong optical absorption which starts at about 320 nm. This is due to the 0( - II)-Ti(IV) LMCT transition. The spectra of MgTijOj doped with Mn(II), Fe(II), Co(II) and Ni(II) show considerable additional absorption in the visible. Only Co(II) and Ni(II) are expected to show spin-allowed crystal-field transitions in this spectral range [14]. These are in fact observed (see Fig. 3) ... 

The term MMCT transition is in our opinion more suitable for classifieation than for characterizing its nature. The same is true for the seemingly more simple LMCT and MLCT transitions. To illustrate this we have tabulated in... 

However, for the late-transition-metal compounds the gap is related to the electronegativity of the anion and seems to be of the LMCT type. Therefore it is assumed that another excited state, viz. plays a role here. Here... 

L denotes a hole on the anion, or better in the anion valence band. This is a LMCT state. Its energy is assumed to be A. 

Calculations [104] show that for L7 > A (the heavier transition metal ions) the gap is of the charge-transfer type, whereas for 1/ < A (the lighter transition metal ions) the gap is of the d-d type. In our nomenclature this may be translated as MMCT LMCT. In the charge-transfer semiconductors the holes are light (anion valence band) and the electrons are heavy (d bands). Examples are CuClj, CuBrj, CuO, NiClj, NiBrj and Nil2. 

There is no essential difference between quenching via a MMCT state or a LMCT state. The latter occurs, for example, in Eu(III) if the LMCT state is either at low energy or if this state shows a large offset in the configurational coordinate diagram [23, 35]. The latter occurs in glasses [123], certain cryptates [124] and lanthanum compounds [125]. 

As a matter of fact low-lying MMCT states can also influence radiative transition probabilities. The long decay time of the VO4 luminescence is considerably shortened by the presence of Bi " [27] due to a Bi(IV)-V(IV) MMCT state (see also above). Such effects are very well-known for LMCT states in case of transition-metal ions and lanthanide ions [6]. They will not be discussed here any further. 

The effects of metal-to-metal electronic coupling are observed for several complexes where a ferrocene moiety is bound to transition-metal complexes with a -conjugated chain, 68 (167), 69 (168), and 70 (169). Tertiary amine-ferrocene conjugated molecules, 71, show two-step le oxidation, and their monocationic forms exhibit strong LMCT bands at 600-700 nm (170). 

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