LMCT absorption

Complex 6 exhibits an intense LMCT absorption band at 285 nm, which is shifted to longer wavelength and is more intense compared to the cis-isomer. Irradiation led to the disappearance of this band, indicating loss of azide. Photoactivation of the ira7zs-isomer initially resulted in the appearance of new Pt(IV) complexes (probably substitution of N3 for OH), as judged by 2D [1H, 15N] HSQC NMR, and after 60 min peaks for Pt(II) species appeared, including trans- [Pt(NH3)2(OH2)2]2+ (6r). 

Fig. 19. Diffuse reflectance difference spectrum of the LMCT absorption upon 355 nm photolysis of TS-l/TiOOH molecular sieve (20 min at 45 mW cm-2) [Reprinted from Lin and Frei (133) with permission. Copyright (2002) American Chemical Society].

Fabbrizzi and co-workers have demonstrated the use of bis-copper(II) cryptates to sense ambidentate anions [49]. On titrating molecule 71 with NaN3 in aqueous solution, the colour changed from pale blue to bright green and an anion-metal LMCT absorption appeared at 400 nm. X-ray diffraction studies have shown that the azide anion is held colinear with the two Cu(II) centres, coordinated through the two terminal sp2-hybridised nitrogen atoms. Stability constants for 71 with a variety of anions in aqueous solution were calculated and the selectivity of this anion sensor was found to be controlled by the bite distance between the two copper atoms (Fig. 1). 

The preferential formation of NH—S hydrogen bonding in less polar solvents is also supported by the results of the solvent dependence of the LMCT absorption maxima of [Fe4S4(Z-cys-Gly-Ala-OMe)4]2 or [Fe4S4(Z-cys-Gly-0Me)4]2. The former complex has maxima at 406 nm in DMF and 390 nm in dichloromethane, but the latter complex shows absorption maxima at 402 nm in DMF and 402 nm in dichloromethane. Absence of the effect of the hydrogen bonding in the complex with Z-Cys-Gly-OMe is evident. 

The model complex [Fe4S4(Z-cys-Ile-Ala-cys-Gly-Ala-cys-0Me)(Z-cys-Pro-Val-OMe)]2 was synthesized and had all amino acid residues within 5 A from the Fe4S42+ core in cluster I of P. aerogenes ferredoxin (51). The complex had a redox potential at —0.88 V versus SCE in DMF and —0.83 V versus SCE in dichloromethane. In this case only one isomer was detected by cyclic voltammetry. For these complexes, a lesser degree of solvent dependence was found in the LMCT absorption maxima and in the redox potentials. This is due to the fact that the [Fe4S4(SR)4]2 cluster is well shielded from solvent by the combined steric bulk of the Z-Cys-Ile-Ala-Cys-Gly-Ala-Cys-OMe... 

The synthetic [2Fe-2S] model complex of the 20-peptide complex exhibits two LMCT absorption maxima at 423 and 461 nm in DMF, maxima which are near to those of the native plant-type ferredoxin (423 and 466 nm) (69). Two redox couples for — 3/—2 were observed at — 0.64 V versus SCE and at —0.96 V versus SCE in DMF. One of them is very close to the value (—0.64 V versus SCE) of native ferredoxin. The 20-peptide complex containing invariant sequences Cys-A-B-C-D-Cys-X-Y-Cys and Leu-Thr-Cys-Val possesses all essential factors for a model of the active site except for the peptide conformation. The positive-shifted redox potential of the 20-peptide complex in DMF is undoubtedly due to the interactions between the Fe2S22+ core and adjacent amino-acid residues, giving rise to NH--S hydrogen bonding. 

Ligand-to-Metal Charge-Transfer (LMCT) Absorption Bands 257... 

The use of electrochemical data for the actual molecule can accommodate some of the effects of covalency. In general, the observations on LMCT absorptions in the ammine complexes suggest that nonlinear, or cross-term effects make only small contributions to the transition energies. Thus, the absorption maxima of the... 

Overall, the LMCT absorptions conform to expectation based on Eq. 9, but the effective values of Xp are smaller than those for IPCT absorptions. Nonlinear covalent effects in appear to be relatively small. 

As noted in Tables 3 and 4, the LMCT absorption bands of transition metal complexes tend to be broad (Avj/2 5 to 8 x 10 cm ) and the molar absorptivities are large (fimax 1 to 20 x 10 M cm ). Most of the bands are structureless in ambient solutions. 

The irradiation of the [MfNHj), ] complexes at corresponding to the high-energy LMCT absorptions leads to the same products seen for the lower-energy LF excitations ... 

Vanadium(V), which does not contain d electrons, obviously is restricted to intra-ligand and LMCT absorptions. Simple compounds such as vanadate are colourless, because the LMCT bands lie in the UV region. Decavanadate, and also vanadate-dependent haloper-oxidases (which contain vanadate additionally coordinated to a side-chain imidazole of the protein matrix), are yellow, because the LMCT tails from the UV into the violet range. More complex vanadium(V) complexes can be very colourful when the LMCT... 

Ley and Schanze have also reported on poly(aryl-ethynylene) polymers 33 containing bipyridine groups along the backbone, bound to the Re(I)(CO)3Cl chromophore, to various extents of loading [70]. In addition to the n-n polymer backbone absorption, the polymers display an LMCT absorption and fluorescence that is quenched (rather inefficiently) by energy transfer to the Re triplet manifold, which undergoes phosphorescence. 

Primary Processes. The most common primary photochemical processes Involving copper(II) complexes are ligand to metal charge transfer (LMCT) reactions (6,7), also referred to as charge transfer to metal (CTTM) reactions. LMCT reactions occur when light absorbed In an LMCT absorption band of the Cu(II) complex leads to reduction of the metal and oxidation of the ligand. 

Ligand-to-metal charge transfer may give rise to absorptions in the UV or visible region of the electronic spectrum. One of the most well-known examples is observed for KMn04. The deep purple colour of aqueous solutions of KMn04 arises from an intense LMCT absorption in the... 

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