MLCT transition

A CT transition which is very similar to the -> MMCT transition has been observed by Vogler et al. [55] for complexes [M(2,2 -bipyridyl)X3] with X = Cl, Br, I and M = Sb, Bi. These authors report MLCT transitions involving the promotion of an electron from the lone pair to the n orbital of the bipyridyl ligand. For example, for M = Sb and X = Br they observe an orange color for the complex due to an absorption band with a maximum at 435 nm. In the complexes considered by us the transition is to an antibonding n orbital (with pronounced d character) on the filled-shell transition-metal complex ion. 

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... 

Recently, a photoisomerization reaction of azoferrocene was found to proceed in polar solvents such as benzonitrile and DMSO through both a 7t it transition of the azo-group with a UV light (365 nm) and the MLCT transition with a green light (546 nm) (Fig. 6) (Scheme 1) (153). The quantum yields of the photo-isomerization reaction at 365 nm and 546 nm were estimated to be 0.002 and 0.03, respectively. The transformation into the cis form causes the higher field shift of Cp protons in the 1H-NMR spectrum and an appearance of u(N = N) at 1552 cm-1. The cis form is greatly stabilized in polar media, and dilution of the polar solution of cis-25 with less polar solvents resulted in a prompt recovery of the trans form. 

The electronic adsorption spectra for the complexes [Ir(OH)6]", where n = 0-2, have been resolved and peak maxima locations, molar extinction coefficients, oscillator strengths, and band half-widths calculated.44 Bands have been assigned in the main part to be one-electron MLCT transitions. Spectrophotometrically determined rate constants for the OH reduction of the IrVI and Irv complexes at 25 °C in 3M NaOH are (2.59 0.09) x 10—3 s—1 and (1.53 0.05) x 10 4 s 1 respectively. The activation energy for the reduction, Irv—>IrIV, is nAkcalmoC1. Cyclic voltammetry and potentiostatic coulometry of [Ir(OEI )r,]2 in 3M NaOH on a Pt electrode show that during the electro-oxidation compounds of Irv and IrVI are formed.45... 

The electronic absorption and MCD spectra of [Pt Bu )] can be explained in terms of 5d > 17ru MLCT transitions.75 76 The 5dz orbital is not strongly involved in the P — M a bonding. There is some M — P back bonding in these systems, but it does not add a lot to the stabilization. Thus, there is minimal 5d orbital involvement in the bonding and it mostly involves the 6,v and 6pz orbitals.75,76... 

The first reduction in the cobalt-based polymer is metal-centered, resulting in the appearance of a new MLCT transition, with the second reduction being ligand-centered. For the nickel-based polymer, in contrast, both redox processes are ligand-based. 

Okubo et al. have used Stark spectroscopy to study the unusual trinuclear complex salt (76) which has pseudo D3h symmetry and contains tetrahedral Cu1 centers and a symmetrical hexa-azatriphenylene-derived radical anionic ligand.139 The x responses of PMMA thin films doped with (76) are enhanced by an intense, broad MLCT transition which has a maximal absorbance in the NIR region.139... 

The trarax-dichloro and dithiocyanate complexes show MLCT transitions in the entire visible and near IR region. The lowest energy MLCT transition band of the trara-dichloro complexes is around 700 nm in DMF solution, and the complexes show weak and broad emission signals above 950 nm. The absorption and emission maxima of the Zrarax-dithiocyanate complexes are blue shifted compared to its trarax-dichloro analogues due to the strong -k acceptor property of the NCS- ligands compared to Cl-, which is consistent with the electrochemical properties of these complexes. 

After a while, sodium ions from the salt swap for the iron ion at the centre of the haem ring. There is no longer a couple (one component is lost), and consequently no scope for an MLCT transition, so the red colour of the blood fades. 

Several arylplatinum complexes of Tl+ have been reported (Table 15). The anionic /rreacts with T1N03 to produce a chain polymer 120, in which successive Pt atoms are bridged to each other by two Tl+ ions (Scheme 28).112 The compound is an acetone solvate, and there also appear to be weak Tl F interactions in the solid-state structure. Upon irradiation (A = 441 nm), the complex exhibits an intense emission with a maximum at 678 nm. The luminescence is attributed to MLCT transitions. 

Metal-to-ligand charge transfer (MLCT) transitions between the nonbonding metal-centred MOs and antibonding ligand-centred MOs. Such transitions are found where a metal is easily oxidised and the ligand is easily reduced. 

Figure 2.16 Structure of the octahedral ruthenium (II) trisbipyridyl complex. The orange colour of this complex results from metal-to-ligand charge-transfer (MLCT) transitions...

The large molar extinction coefficients of the spin-allowed CT transitions make them much easier to pump optically. The intense 454-nm visible band of Ru(bpy)32+is an MLCT transition. 

For the metal, A increases on descending a column in the periodic table. In addition, CT state energies are affected by the ease ofoxidation/reduction of the ligands and metal ion. For MLCT transitions, more easily reduced ligands and more easily oxidized metals lower the MLCT states. 

Chemical modifications can be used to tune the state energies and enhance properties. CO ligands greatly stabilize the t levels, which results in both an increased A and a higher-energy MLCT transition. For example, the primary MLCT bands of Os(phen)32+ and [Os(phen)2Cl(CO)l+are at 430 and 365 nm, respectively. The emissions are similarly shifted from 710 to 646 nm. In keeping with the expectations of the energy gap law, the r of[Os(phen)3l2+ and [Os(phen)2Cl(CO)J+ are 74 and 234 nsec, respectively,(21)... 

Complexes with less extended aromaticity such as Ru(bpy/phen)2HAT [73-76] (HAT = 1,4,5,8,9,12-hexaazatriphenylene, Fig, 2) and Ru(bpy)2PPZ [77-80] (PPZ = 4,7-phenanthrolino-[6,5-b] pyrazine. Fig. 2) exhibit also characteristics most relevant to intercalation. We can mention (1) a very slow mobility of the HAT complex along the DNA double helix [81], (2) a good protection of the complex versus reagents that remain in the bulk solution [73,79], and (3) a clear hypochromic effect on the MLCT transition in the presence of DNA [73, 75, 79,80]. 

Ley and Schanze have also examined the luminescence properties of the polymers Pq, Pio> P25> and P50 in solution at 298 K, and in a 2-methyltetrahydro-furan solvent glass at 77 K. These spectroscopic studies reveal that fluorescence from the 71,71" exciton state is observed at Amax=443 nm, 2.80 eV in the polymers P0-P50 at 298 and 77 K, but the intensity and lifetime of the fluorescence is quenched as the mole fraction of Re in the polymers is increased. This indicates that the metal chromophore quenches the 71,71" state. The quenching is inefficient even when the mole fraction is large, suggesting that interchain diffusion of the 71,71" exciton is slow compared to its lifetime [70]. Phosphorescence from the 71,71" state of the conjugated polymer backbone is observed at > max=b43 nm, 1.93 eV in P10-P50 at 77 K, and emission at Amax=690 nm, 1.8 eV is assigned to the d7i(Re) 7i oiy MLCT transition. 

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