Photophysical Properties and Coordination Interaction of d 0 Metallocenes with Unsaturated Hydrocarbons

Abstract

One of the most promising approaches to an understanding of the nature and properties of frontier orbitals of organometallic π complexes of Group IVB metalsmajor components of catalysts of a large number of organic syntheses-involves generation and study of electronically excited and ground redox states of molecules [1-6]. However, fundamental processes, such as energy and electron transfer, and properties of electronically excited states of organometallic compounds have been little studied partly due to the complexity of their synthesis and of manipulations with solutions of these compounds (especially with catalytic concentrations), which are extremely sensitive to moisture and oxygen traces and to minor impurities [7].In this paper, we report on direct photophysical experiments that demonstrate for the first time the triplet nature of long-lived ligand-to-metal charge transfer (LMCT) excited states based on Group IVB metallocenes. A strong specific interaction and competitive coordination of substrates of Ziegler-Natta catalystsunsaturated hydrocarbons (UHCs)-with metallocene precursors of active centers have been observed. Our approach is based on the method of radiationless T − T energy transfer from organometallic complexes of d 0 metals to UHCs and on the shift of optical spectra.It should be noted that solutions of metallocenes Cp 2 M ( IV ) Cl 2 (M = Zr, Hf; C = 10 -5 -10 -4 mol L -1 ) in unsaturated and saturated hydrocarbons were prepared and the hydrocarbons used additionally purified from trace impurities and moisture immediately prior to photophysical experiments. Energy transfer between UHC molecules and aromatic or organometallic donors at low concentrations of the donor ( c D ≤ 10 -4 M) and acceptor ( c Ä ~ 0.1-0.5 M) and cooperative effects were studied in glassy solutions of individual UHCs and in binary glassy solutions (UHC acceptor-MCH (methylcyclohexane), 1 : 1 vol/vol) at 77 K.Group IVB metal ions in most compounds have a closed electronic configuration d 0 and act as rather hard acids. However, as follows from the photoproperties of the representative triad of π complexes Cp 2 M ( IV ) Cl 2 , their radiative lifetimes decrease manyfold in the series Ti 4+ → Zr 4+ → Hf 4+ (table). It is worth noting that this is a consequence of a considerable increase in spinorbit coupling typical for transition metal ions with open electronic subshells, i.e., for soft acids.Previously [2,4,5], we assumed that these longlived states (their experimental lifetimes are among the largest known for metal-containing molecules) are triplet. In this case, we should observe their luminescence quenching in the presence of a triplet quencher of UHCs with large S -T splitting energies. Note that the properties of excited, especially triplet, states of UHCs have not hitherto been systematically studied; therefore, they represent a fundamental independent problem [8,9]. Under the conditions of the series (in MCH glasses), the lowest triplet level of UHCs with one or several unconjugated C=C bonds (3,3-dimethyl-1-butene...

Multinuclear Cytotoxic Metallodrugs: Physicochemical Characterization and Biological Properties of Novel Heteronuclear Gold–Titanium Complexes

“…In fact, the bent metallocenes of the IVb family (Ti, Zr and Hf) are known to exhibit rich photophysical properties . Several theoretical works addressed the electronic structure of such species, and it was quickly established that the nature of the luminescent excited state was a nonlocalized triplet ligand-to-metal charge-transfer (LMCT). , Subsequently, numerous electrochemical, , photochemical, and photophysical ,− studies were reported, including rare examples of luminescence arising from species in solutions. , The Ph 2 P-containing mononuclear complex 1 in 2MeTHF at 77 K exhibits a strong luminescence centered at 659 nm (Figure ). However, this luminescence is absent at 298 K, which is common to most complexes investigated so far in the literature.…”

Photonics of metallocene complexes based on metals of the titanium subgroup

“…The observed luminescence is a forbidden triplet-sin glet transition, i.e., phosphorescence in nature, as fol lows from our experiments on triplet-triplet energy transfer to unsaturated hydrocarbons (olefines, diens, etc.) [10,[13][14][15]. This tendency towards changing the spectral and luminescent properties is preserved in the related structurally modified Ti, Zr, and Hf complexes (upon a change in the ligand sphere architecture).…”

Radius of energy transfer as an indicator of the interaction of aromatic amines with an olefin

“…The need of determining the ener gies, Е T , of α and β olefins originated from studies of photosensitized cis-trans isomerization of the latter 20, 21 and from the use of olefins as non phosphorescing accep tors of triplet excitation. [22][23][24][25] The energies of the lowest lying triplet levels of some α and β olefins were mea sured using nonradiative T-T energy transfer. 22 Olefins and dienes are characterized by high energy singlet states and large S 1 -T 1 splitting; 26,27 they are not involved in the redox and electron transfer reactions under the experi mental conditions, thus being convenient triplet accep tors.…”