G. V. Loukova

Phosphorescent ligand-to-metal charge-transfer excited states in the group IVB metallocene triad

Abstract Long-lived ligand-to-metal charge transfer (LMCT) excited states based on a zirconium complex (ca. 2000 μs) and a hafnium complex (ca. 300 μs) are first characterised with use of the Cp 2 Zr(IV)Cl 2 and Cp 2 Hf(IV)Cl 2 . The tendency in the observed photophysics (absorption and phosphorescence spectra, lifetimes, and quantum yields) in the homologous group IVB metallocenes Cp 2 M(IV)Cl 2 is discussed in view of the nature of the unique metal (Ti, Zr or Hf)–ligand π-bonding character.

The first experimental approach to probing frontier orbitals and HOMO-LUMO gap in bent metallocenes

Abstract Combination of optical and electrochemical methods provided first experimental estimate of both HOMO and LUMO relative energies and HOMO–LUMO gap in organometallic π-complexes. Energies of redox and optical HOMO-to-LUMO electron transitions in (π-L)2MX2 (Ti, Zr, Hf) linearly correlate viz. ELMCT∝ΔEredox,Eem(0–0)∝ΔEredox. Replacement of Cl− with Me− σ-ligands increases the HOMO–LUMO gap, predominantly, by destabilization introduced in the LUMO via Me-ligands; that is experimental justification for a hypothesis by Harrigan et al. [J. Organomet. Chem. 81 (1974) 79]. Effect of introduction of larger π-ligands and a bridging group is progressive reduction of frontier orbitals energy gap.

Electrochemical versus optical insight in frontier orbitals of Ti(IV), Zr(IV), and Hf(IV) bent metallocenes

Abstract The linear relationship between the redox and optical HOMO-to-LUMO electron transitions are reported for complex molecules with π-ligands for the first time. Linear correlations between electrochemical (redox) gaps and absorption charge-transfer energies have been noted for Ti(IV), Zr(IV), and Hf(IV) bis(cyclopentadienyl) dichlorides. A similar correlation was also observed for the fourth ionization potentials of the corresponding Group IVB metals. The correlation between absorption charge-transfer energies and redox gaps was justified for a series of ansa - and unbridged zirconocene dichlorides and dimethyls with variable sandwich ligands.

Simulation of highly emissive states based on ligand-to-metal charge transfer

Properties of the frontier orbitals of a d0-organometallic complex with promising photoluminescent and photosensor characteristics have been systematically studied by modern quantum-chemical methods. It has been demonstrated that the lowest electronically excited states are related to charge transfer from high-lying ligand-centered molecular orbitals to the predominantly metal-centered lowest unoccupied molecular orbital. Such an approach makes it possible to predict complexes with promising spectral-luminescent properties, including catalytic precursors of early transition metals.

Frontier orbitals and ligand-to-metal charge transfer electronic transitions in d 0 -metal complexes

Properties of frontier orbitals and low-energy electronic transitions in a d0-organometallic complex have been studied by TDDFT and DFT methods using B3LYP hybrid functional and 3-21G*, 6-31G**, SDD, CEP-121G, and DGDZVP basis sets. It has been shown that the electronic transition between frontier orbitals in the excitation and absorption spectra is associated with charge transfer mainly from π-type ligands to a central metal d0-ion. The good agreement of the data (the shape and band position of the spectra of electronic absorption and excitation, energy of electronic transitions, and strength of the harmonic oscillator) of quantum-chemical and photophysical studies is demonstrated.

Photophysical, electronic, and catalytic properties of tetracyclopentadienylzirconium

177 Metallocenes play a special role in synthesis of polyolefins and activation of small chemically inert molecules (N2, CO, CO2, etc.) [1]. Metallocene catal ysis of unsaturated hydrocarbon polymerization con stitutes an area of fundamental and applied impor tance [2–5]. The high performance of metallocene catalysts is observed, for example, in the case of heter ogenized systems based on Cp4Zr/MAO/SiO2 (up to 10 kg PE/g (catalyst) h; MAO is methylalumoxane) [6]. The molecular weight characteristics of the prod ucts can be controlled by introducing chain transfer agents, alkylated main group metals (Et2Zn, Me3Al, i Bu3Al, Me4Sn), into the catalytic system [7]. Emitting states of molecules resulting from ligand to metal charge transfer (LMCT) are the least studied but potentially the most promising excited states of metal containing molecules. It has recently been demonstrated that some titanium group metallocene complexes have unique highly emitting, long lived LMCT states [8–12]. The present study, deals with a new class of lumi nescent complexes with tetraaromatic ligands, as exemplified by d0 zirconocene Cp4Zr. The geometric and electronic characteristics of complexes of this type have been studied by the density functional theory (DFT) methods, and original soluble and heterog enized catalytic systems for (co)polymerization of ethylene with higher α olefins affording low and medium density polymers have been developed. Tris(η5 cyclopentadienyl)(η1 cyclopentadienyl)zir conium(IV) was obtained as described in [13], addi tionally purified by recrystallization from ether, and vacuum dried for photophysical studies by known pro cedures [9–11]. It is worth noting that the simplicity of the synthesis of (η5 Cp)3Zr(η 1 Cp), in contrast to the syntheses of other metallocenes, as well as the lack of flammable reagents (alkali metals) and products (hydrogen) coupled with the high catalytic activity of soluble catalysts based on Cp4Zr and MAO (up to 2200 kg PE/g (Zr) h), is favorable for the development of a new high performance process of producing high , medium , and low density polyethylenes. Quantum chemical calculations were performed by the DFT B3LYP/6 311G** method (for Zr, the 3 21G** basis set was used) using the Gaussian 03 program package [14]. All discussed structures corre spond to minima on the potential energy surface. The absorption spectra of zirconocene (η5 Cp)3Zr(η 1 Cp) in solutions (Fig. 1, curve 1) show, in the near UV region, a low energy absorption band (λmax ~280 nm) and a shoulder corresponding to the electronic transition between the frontier orbitals PHYSICAL CHEMISTRY

Effect of ether coordination on the luminescent properties of a d 0 metal complex

In recent years, considerable attention has beenattracted by bent metallocenes (especially complexesof Group IV transition metals), which have found wideuse in catalytic and stoichiometric reactions [1, 2].However, the fundamental problem of the structure–properties relationship for these complexes and theirfunctions in multicomponent systems has not yet beensolved. Advanced electronic spectroscopy methodsallow high-accuracy determination of the electronicand stereochemical features of complex molecules andtheir interaction with other compounds and can be usedto study bent metallocenes [3]. Using an intensely

Nature of ligand-to-metal charge transfer states in the metallocene series M(η 5 :η 1 -CpCMe 2 CB 10 H 10 C) 2 (M = Ti, Zr, Hf)

aInstitute of Problems of Chemical Physics, Russian Academy of Sciences, 1 prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation. Fax: (496) 522 5636. E mail: gloukova@mail.ru bSouthern Scientific Center, Russian Academy of Sciences, 41 ul. Chekhova, 344006 Rostov on Don, Russian Federation cSouthern Federal University, 105/42 ul. B. Sadovaya, 344006 Rostov on Don, Russian Federation

Unusual effect of carborane ligands on the electronic properties of d 0-metal complexes

Modern quantum-chemical and photophysical methods have been used to study the structure of the frontier molecular orbitals and the nature of ligand-to-metal charge transfer (LMCT) transitions of structurally complex d0-metallocenes. It has been shown that such metal complexes with carboranyl ligands have emissive LMCT states with preferential charge transfer from aromatic π-ligands to the metal and a large electric dipole moment. The electronic excitation and absorption spectra were simulated for the first time, and dipole moments of metal complexes containing metal–carbon σ- and π-bonds were estimated, which is of fundamental importance for the development of molecular photonics.

Triplet energy transfer from d0 organometallic compounds to unsaturated hydrocarbons and quantum-chemical calculations: Revealing of short-lived intermediates

and M is Zr or Hf)with extremely long lifetimes (τ on the order of milliseconds at 77 K) produced as a result of ligandtometal charge transfer.Based on the results of spectral–luminescent studies [4, 5], in particular, revealing the existence of linearcorrelation between the critical radius of the sphere ofelectronexchange resonance energy transfer and thechain length of the energy acceptor olefin (Fig. 1), wehave assumed that there is innersphere coordinationof the olefin (diene) to the