MLCT Character Research Articles

Characterization of the Lowest Excited States of [Rh(bpy-h(8))(n)(bpy-d(8))(3-n)](3+) by Highly Resolved Emission and Excitation Spectra.

Highly resolved emission, excitation, and resonantly line-narrowed spectra, as well as emission decay properties of [Rh(bpy-h(8))(n)(bpy-d(8))(3-n)](3+) (n = 0, 2, 3; bpy = 2,2'-bipyridine) doped into [Zn(bpy-h(8))(3)](ClO(4))(2) are presented for the first time. [Rh(bpy-h(8))(3)](3+) and [Rh(bpy-d(8))(3)](3+) exhibit one low-lying triplet T(1) at 22 757 +/- 1 and 22 818 +/- 1 cm(-1), respectively (blue shift 61 cm(-1)), while [Rh(bpy-h(8))(2)(bpy-d(8))](3+) has two low-lying triplets at 22 757 +/- 1 and 22 818 +/- 1 cm(-1). The well-resolved vibrational satellite structures show, that the equilibrium positions of the triplet and the singlet ground S(0) state are not very different and that the force constants in T(1) are mostly slightly smaller than in S(0). Moreover, the vibrational satellite structure is strongly dominated by vibrational ligand modes, which demonstrates the pipi character of the corresponding transition. However, the occurrence of several very weak vibrational modes of metal-ligand character displays a small influence of the metal ion. This is supported by the emission decay behavior. [Rh(bpy-h(8))(2)(bpy-d(8))](3+) exhibits an emission which is clearly assignable to the protonated ligand(s), even when the deuterated ligand is selectively excited. Obviously, an efficient intramolecular energy transfer from the deuterated to the protonated ligand(s) occurs, presumably mediated by the small Rh(3+) d-admixture. A so-called "dual emission" is not observed. Moreover, a series of spectroscopic properties of the lowest excited state of [Rh(bpy)(3)](3+) (energies of electronic origins, emission decay times, zero-field splittings, structures of vibrational satellites, etc.) is compared to properties of bpy, [Pt(bpy)(2)](2+), [Ru(bpy)(3)](2+), and [Os(bpy)(3)](2+). This comparison displays in a systematic way the increasing importance of the metal d and/or MLCT character for the lowest excited states and thus provides guidelines for an experimentally based classification. In particular, the lowest excited states of [Rh(bpy)(3)](3+) may be ascribed as being mainly of (3)pipi character confined to one ligand in contrast to the situation found for [Ru(bpy)(3)](2+) where these states are covalently delocalized over the whole complex.

A Tale of Two Complexes, [PtMen(RNCHCHNR)] (n = 2 and n = 4, R = Cyclohexyl): Why do PtII and PtIV Complexes Exhibit Virtually Identical Redox Behavior and Colors?

AbstractIn spite of their very similar cyclic voltammograms, absorption spectra, and solvatochromic behavior, the two 1,4‐diazabutadiene title complexes exhibit markedly different photoreactivities and underlying electronic structures, as evident from absorption and EPR spectra of the persistent anion radical forms. The lowest excited state of the nonphotoreactive PtII system [(CyNCHCHNCy)‐PtMe2] has MLCT (metal‐to‐ligand charge‐transfer, 5d → π*) character, and the EPR spectrum of the corresponding anion radical at 〈g〉 = 2.016 exhibits sizable metal/ligand orbital mixing. On the other hand, the structurally characterized PtIV complex [(CyNCHCHNCy)‐PtMe4] (C2/c; a = 2021.6(2), b = 805.3(1), c = 1254.2(1) pm; β = 111.05(1)°; V = 1905.7(4) × 106 pm3; Z = 4) has a lowlying photoreactive LLCT (ligand‐to‐ligand charge‐transfer, σPtC → π*) excited state in which the axial PtC bonds are activated, as already suggested by the longer PtC(ax) bonds (214.0(8) pm) relative to PtC(eq) in the ground state (204.5(5) pm). The anion radical of the PtIV complex has lost the long‐wavelength absorption band in the visible; it shows a well‐resolved EPR spectrum at 〈g〉 = 1.9945 with π‐ligand and 195Pt hyperfine structure and a small g anisotropy. A qualitative MO scheme is presented to account for the similar frontier‐orbital energy differences despite dissimilar underlying electronic structures.

Vibrational satellite structures and properties of electronic states of transition metal complexes

Pd(2- thpy) 2, Pt(2- thpy) 2, [ Ru( bpy-h 8) 3] 2+, [ Ru( bpy-h 8) 2( bpy-d 8)] 2+, [ Ru( bpy-d 8) 3] 2+, [ Os( bpy-h 8) 3] 2+, [ Os( bpy-h 8) 2( bpy-d 8)] 2+, and [ Os( bpy-d 8) 3] 2+ were isolated in crystalline matrices in order to obtain highly resolved emission spectra. A detailed analysis of the electronic origins and of the vibrational satellite structures reveals trends, which allow the various compounds to be ordered with respect to increasing MLCT character. Interestingly, an increasing MLCT character is connected with decreasing shifts of the equilibrium positions of the potential hypersurfaces and with decreasing changes of force constants upon excitation. Presumably, this behavior is a consequence of an increasing covalency in the compounds investigated. Moreover, especially the resolved vibrational satellite structures of [ Ru( bpy) 3] 2+ and [ Os( bpy) 3] 2+ allow the conclusion that the electronic excitation in the lowest excited states is delocalized over the metal and the three ( bpy)-ligands.

Bonding properties of 3,5-di-tert-butyl-1,2-benzosemiquinone radical-anionic ligand: resonance Raman spectra of Re(CO) 4(DBSQ) and Re(CO) 3(PPH 3)(DBSQ) complexes

Absorption spectra in the visible region and resonance Raman spectra of the spin-doublet Re(CO) 4− n (PPh 3) n (DBSQ) ( n=0,1) complexes were measured in order to obtain information on the nature of their electronic transitions and excited states. Qualitative Raman excitation profiles measured across the visible absorption band of both complexes point to the presence of two electronic transitions. Based on the rR data, both transitions are identified as d π→π*SOMO of the DBSQ ligand. These transitions have essentially MLCT character, with only very small amount of mixing between the metal and ligand-SOMO orbitals. The spectral assignment is also supported by the shift of the spectral band going from the tetracarbonyl to the tricarbonyl-phosphine complex and by the solvatochromism observed for the latter one. From these spectral data, the DBSQ radical-anion emerges as a ligand lacking any pronounced π-bonding properties toward the Re central atom. At best, it can be viewed as a very weak π-acceptor. This bonding picture is also supported by the EPR and IR data.

Spectroscopic properties of Pt(II) and Pd(II) complexes with aromatic terdendate (C″>N^C) cyclometallating ligands

Emission spectra, emission lifetime and electrochemical behavior of the complexes Pt(H-dppy) 2, Pt(dppy)S(Et) 2, Pt(dppy)py, Pt(dppy)pyr, (dppy)Pt-pyr-Pt(dppy), and Pd(dppy)py (where dppy 2− is the C − deprotonated form of 2,6-di-phenylpyridine and py and pyr are pyridine and pyrazine) have been studied. The structured luminescence spectra observed at 77 K have been assigned to transitions having i) MLCT character in the case of Pt(H-dppy) 2, ii) mainly MLCT character with some LC contribution for the four tridentate trans Pt(II) complexes and iii) almost pure LC character for the Pd(II) complex. The luminescence emission involves in all cases the dppy ligand even though the electrochemical results suggest a LUMO orbital centred on the pyrazine ligand in the Pt(dppy)pyr complex and on the metal in the Pd(dppy)py complex.

Relationship between the emission spectra and resonance Raman excitation profiles of W(CO)4(.alpha.-diimine) complexes

The photophysical properties of several W(CO)/sub 4/(..cap alpha..-diimine) complexes at 293 and 80 K are reported. For complexes of the type W(CO)/sub 4/L (L = substituted 1,10-phenanthroline or 2,2'-bipyridine) three emissions are found while the lowest energy emission is not observed at room temperature for complexes of the type W(CO)/sub 4/L (L = substituted 1,4-diaza-1,3-butadiene or pyridine-2-carbaldehyde imine). The highest energy emission in all these complexes is assigned to be LF in character; the other ones appear to have MLCT character. The occurrence of the lowest energy emission, which is related to the z-polarized b/sub 2/..-->..b/sub 2//sup */ transition, depends on the MLCT character of this transition. This relationship is deduced from the resonance Raman excitation profiles and the luminescence excitation spectra of these complexes. Complexes with strong MLCT character for the z-polarized b/sub 2/ ..-->.. b/sub 2//sup */ transition show a low-energy emission, while this is not the case for complexes with much less MLCT character for this transition. These results are explained by the efficiency of the radiationless processes, which is large when metal-ligand stretching modes act as promoting modes. 20 references, 6 figures, 2 tables.

Steric and electronic effects on the quantum yield of photosubstitution of CO in W(CO) 4(α-diimine) complexes

Photolysis of the complexes W(CO) 4(α-diimine) in the presence of a nucleophilic ligands cause photosubstitution of CO. The quantum yields are low and depend on the basicity, cone angle and concentration of the phosphine ligands used, indicating an associative mechanism. Apart from steric effects electronic effects determine the quantum yield of this reaction. These electronic effects are connected with the MLCT character of the lowest excited state of these complexes which strongly decreases from W(CO) 4 (1,10-phenanthroline) to W(CO) 4(1,4-diaza-1,3-butadiene) according to the Resonance Raman Spectra.