Chemistry

ar (M.R. Féliz) 1 Member of CIC. 2 Member of CONICET. 0010-8545/00/$ -see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 1 0 -8 5 4 5 ( 9 9 ) 0 0 1 8 6 -1 66 L.S. Villata et al. / Coordination Chemistry Re6iews 196 (2000) 65-84

The permeability of liposomes to water, glucose, Ca 2. and alkaline cations was monitored by recording the change in absorbance at 450 nm using a rapid reaction stopped-flow spectrophotometer. Liposomes were prepared with egg phosphatidylcholine and concentrations of dolichol ranging from 0.1% to 9% (w/w). Net permeability of phosphatidylcholine bilayers to alkaline cations was induced by the incorporation of dolichol. This effect was not observed in the case of non-charged solutes like glucose or in that of alkaline earth cations such as calcium.

Solvent and temperature effects on the photophysical properties of the polymer {[(vpy) 2 -vpyRe(CO) 3 2,2Ј-bipyridine] CF 3 SO 3 } n∼200 and the related monomer CF 3 SO 3 [pyRe(CO) 3 2,2Ј-bipyridine] were investigated in solution by flash photolysis at 337, 351 and 355 nm and by steady state irradiations. MLCT excited states in the polymer undergo a more efficient annihilation than in the monomer. Differences between the polymer and the monomer photophysical behavior are rationalized in terms of solvent and thermal effects on the transition between rigid rod and coil structures of the Re(I)-polymer.

The UV−vis spectroscopy and photochemical properties of {(vpy-[Re(CO) 3 (2,2′-bpy)]) m (vpy-[Re(CO) 3 (phen)]) n (vpy) p }-(CF 3 SO 3 ) m+n }, vpy ) 4-vinylpyridine, m ) 131, n ) 131 or m ) 200, n ) 150, and m + n + p ) 600, were investigated in solution phase. The polymers exist in solution as aggregates of polymer strands with radii as large as ∼10 2 nm. Given the size of the poly-vpy backbone, the aggregates must contain a large number of strands. The luminescence spectrum exhibits a strong resemblance to the emission spectrum of {(vpy-[Re(CO) 3 (phen)]) 200 -(vpy) 400 }(CF 3 SO 3 ) 200 . The existence of Re(I) chromophores in diverse environments was shown by the intrinsic kinetics of the luminescence, the decay kinetics of the MLCT excited states observed by time resolved-absorption spectroscopy, and the quenching of the luminescence by various quenchers. Redox reactions of the MLCT excited states with the quenchers were responsible for the luminescence quenching. While static quenching resulted when Cu(II) and Fe(III) EDTA complexes were the quenchers, a dynamic quenching resulted with Fe(CN) 6 4or 2,2′,2′′triethanolamine, TEOA. The photochemical and photophysical properties of the mixed-pendant polymers have been discussed in terms of arrays of MLCT excited states whose energies are determined by the diverse environments of the Re(I) chromophores. Conversions (with and without radiation) of the upper-energy MLCT excited states to the ground state and lower-energy MLCT excited states and the latter excited state to the ground state account for the experimental results.

A series of new rhenium(I) complexes of the type [Re I (CO) 3 (L)(L')] n + (L = 2,2'-biquinoline (bqui) or 3,3'-(ethane-1,2-diyl)-2,2'-biquinoline (CH 2 CN 2 )bqui); L' = CF 3 SO À 3 , pyridine (py), or 4-substituted pyridine (HOpy, Bzpy, or NCpy); n = 0 or 1) were prepared and characterized by FT-IR, 1 H-NMR, UV/VIS and emission spectroscopy, luminescence lifetimes, and cyclic voltammetry. The pseudo-octahedral facial configuration was established by X-ray single-crystal diffraction analysis of two complexes and by a FT-IR study of all complexes. The [Re(CO) 3 (bqui)(Bzpy)](CF 3 SO 3 ) complex crystallizes in the form of two mirror isomers arising from the conformational mobility of the biquinoline ligand. A correlation between the metal-to-ligand charge-transfer (MLCT) emission maxima and the s + Hammett parameter was established for the complexes of the bqui series, while such correlation was not observed for the complexes of the (CH 2 CH 2 )bqui series. No correlation between oxidation potentials and the Hammett parameters was established. The results were rationalized in function of the effect of the 4-substituted pyridine ligand on the octahedral distortion and conformational characteristic of the complexes. The 1 H-NMR data confirmed these results.

The flash photolysis of di-p-oxo his(oxo-molybdenum(V)) complexes in aqueous solutions of perchloric acid in the presence of excess chloride induced both photolabilization and photo-oxidation of Cl-and/or H,O ligands. Photolabilization produces the formation of reaction intermediates identified as p-oxo-bis (oxo-molybdenum(V)) '+ species: MqO,(p-O)(H,O):+ (A) and M&O+-O)(C1)(HZO),

Ground and excited state properties of the newly prepared complexes Re(CO) 3 (2,2′-biquinoline)L S + , L S ) pyrazine or 4,4′-bipyridine, and Re(CO) 3 (2,2′-bipy)(2-pyrazinecarboxylate) were investigated by steady state and time-resolved spectroscopy. The X-ray structure of the latter compound showed that the spectator ligand 2-pyrazinecarboxylate is coordinated through the carboxylate group to Re(I). A component of the complexes' luminescence was associated with long-lived Re to ligand, 2,2′-biquinoline or 2,2′-bipy, charge transfer, while a fast component of the emission was associated with intraligand excited states. Quenching of the luminescence by CuCl 2 involved energy transfer via dynamic and static mechanisms. The complexes in their excited states were reduced by 2,2′,2′′-nitrilotriethanol with the formation of Re(I) ligand-radical species. Similar products were generated by the pulse radiolytic reduction of the complexes. The photochemical properties of the 2,2′-biquinoline complexes and Re(CO) 3 (1,10-phen)4-nitrobenzoate are compared, and the mechanisms of their photochemical reactions are discussed. (1) Abbreviations used in this work: 2,2′,2′′-nitrilotriethanol, TEOA; 4-nitrobenzoate, NO2Bz; pyrazine, pz; 2-carboxylate-1,4-diazine, pz-CO2; 2,2′-biquinoline, (2,2′-biq); metal-to-ligand charge-transfer excited state, MLCT; intraligand excited state, IL.

Luminescence lifetime measurements of Eu(fod) 3 solutions in carbon tetrachloride, benzene, and acetonitrile were performed at temperatures between 5 and 75°C. Ligand to metal charge transfer involving Eu(II) formation upon 300 nm steady state irradiations of Eu(fod) 3 solutions, besides the dependence of the difference in energy of the emitting 5 D 0 and the upper level support, photoinduced electron transfer as the main deactivation mechanism in the thermal quenching of the Eu(III) chelate.

The reduction of dipyridil[3,2-a:2′3′-c]phenazine, dppz, by pulse radiolytically generated esol or by the reaction of the dppz excited states with electron donors produces the radical dppzH • . The dimer radical, (dppz) 2 H • , exists in equilibrium with dppz with an association constant, K ) 10 3 M -1 . The rate constant for the reaction of dppzH • with dppz is k ) 4.3 × 10 6 M -1 s -1 . DFT calculations on the structures of dppzH • and the doubly reduced and doubly protonated dppzH 2 rendered a planar structure for the former species and a bent one for the latter.

Excited state properties of the newly prepared complexes R-CO 2 -Re(CO) 3 (2,2Ј-bipy), where R-CO 2 Ϫ = naphthalene-2-carboxylate, anthracene-9-carboxylate, pyrene-1-carboxylate and acetate, were investigated by steady state and time resolved spectroscopy. The X-ray structure revealed that the R containing ligand is coordinated through the carboxylate group to Re(). Results showed that two emitting excited states play a major role in the photophysics of the complexes. The fast component of the complexes luminescence was associated with the charge transfer excited state, MLCT, Re-to-2,2Ј-bipyridine, while experimental evidence led to the long lived component being attributed to a ligand-to-ligand charge transfer excited state. This emissive LLCT excited state can also evolve to a photodissociative state which ultimately produces the complex decarboxylation. On the other hand, the carboxylate bridge was shown to be able to impede connection between R and the Re(CO) 3 (2,2Ј-bipy) chromophore.