Supramolecular self-assembled multilayers of terpyridine-functionalized perylene bisimide metal complexes.

A terpyridine-functionalized perylene bisimide chromophore (TPBI) has been used as a building block in the stepwise, layer-by-layer fabrication of self-assembled Fe-TPBI multilayers on gold, with the assembled supramolecular chains oriented approximately perpendicular to the gold surface. Time-resolved spectroscopy measurements seem to indicate that the energy absorbed by the multilayer is promptly dissipated to the gold surface by ultrafast processes.

Photoinduced electron/energy transfer across molecular bridges in binuclear metal complexes.

Molecular bridges that efficiently move charge between remote donor and acceptor sites can be thought of as molecular wires. Insight into the properties of molecular wires can be obtained by studying photoinduced electron transfer in covalently linked donor--bridge--acceptor systems. This article summarizes some of the recent progress in the study of such systems involving transition metal complexes as donor and acceptor units. Specific classes of molecular bridges are considered, namely, polyphenylene, and polyquinoxaline bridges. Basic questions are discussed, such as the transfer mechanisms, the associated distance and bridge structure dependence, and the interplay between energy and electron transfer.

Solvent switching of intramolecular energy transfer in bichromophoric systems: photophysics of (2,2'-bipyridine)tetracyanoruthenate(II)/pyrenyl complexes.

The supramolecular systems [Ru(Pyr(n)bpy)(CN)(4)](2-) (n = 1, 2), where one and two pyrenyl units are linked via two-methylene bridges to the [Ru(bpy)(CN)(4)](2-) chromophore, have been synthesized. The photophysical properties of these systems, which contain a highly solvatochromic metal complex moiety, have been investigated in water, methanol, and acetonitrile. In all solvents, prompt and efficient singlet-singlet energy transfer takes places from the pyrene to the inorganic moiety. Energy transfer at the triplet level, on the other hand, is dramatically solvent dependent. In water, the metal-to-ligand charge transfer (MLCT) emission of the Ru-based chromophore is completely quenched, and rapid (200 ps for n = 1) irreversible triplet energy transfer to the pyrene units is detected in ultrafast spectroscopy. In acetonitrile, the MLCT emission is practically unaffected by the presence of the pyrenyl chromophore, implying the absence of any intercomponent triplet energy transfer. In methanol, triplet energy transfer leads to an equilibrium between the excited chromophores, with considerable elongation of the MLCT lifetime. The investigation of the [Ru(Pyr(n)bpy)(CN)(4)](2-) systems in methanol provided a very detailed and self-consistent picture: (i) The initially formed MLCT state relaxes toward equilibrium in 0.5-1.3 ns (n = 1, 2), as monitored both by ultrafast transient absorption and by time-correlated single photon counting. (ii) The two excited chromophores decay with a common lifetime of 260-450 ns (n = 1, 2), as determined from the decay of MLCT emission (slow component) and of the pyrene triplet absorption. (iii) These equilibrium lifetimes are fully consistent with the excited-state partition of 12-6% MLCT (n = 1-2), independently measured from preexponential factors of the emission decay. Altogether, the results demonstrate how site-specific solvent effects can be used to control the direction of intercomponent energy flow in bichromophoric systems.

Intramolecular energy transfer processes in binuclear Re-Os complexes.

A series of bimetallic complexes of general formula [(phen)Re(CO)(3)LOs(trpy)(bpy)](3+) (bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, trpy = 2,2':6',2' '-terpyridine, and L = 4,4'-bipyridine (4,4'-bpy), trans-1,2-bis(4-pyridyl)ethylene (t-bpe), or 1,2-bis(4-pyridyl)ethane (bpa)) and the model mononuclear species [(phen)Re(CO)(3)L](+) and [Os(trpy)(bpy)L](2+) have been synthesized and their photophysical and photochemical properties studied. In the binuclear species an efficient Re(I)-Os(II) energy transfer is observed, which is analyzed in terms of Förster theory. In the binuclear [(phen)Re(CO)(3)(t-bpe)Os(trpy)(bpy)](3+) complex, the trans to cis isomerization of the coordinated t-bpe ligand, characteristic of the [(phen)Re(CO)(3)(t-bpe)](+) subunit, is inhibited by competitive intramolecular energy transfer.

Atmospheric degradation of HFCs and HCFCs: chemistry and noxiousness.

Hydrohaloalkanes are potential alternatives to the fully halogenated compounds (CFCs) that are believed to be responsible for depletion of the stratospheric ozone. In order to establish whether hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) are actually more environmentally acceptable, it is extremely important to know, in addition to ODP and HGWP, their atmospheric degradation mechanism, tropospheric lifetime, and toxicity and noxiousness of their atmospheric degradation products. The primary atmospheric sink for hydrohaloalkanes is the reaction with hydroxyl radicals (OH), so that laboratory measurements or a reliable estimation of the rate constants (kOH) of these reactions is essential in order to assess their atmospheric lifetimes. The previsional models for kOH developed in our laboratory allow the estimation of the atmospheric lifetimes of as many as 449 halocarbons (i.e., all the halocarbons containing F and/or Cl with one, two, and three carbon atoms). The harmful character of some degradation products halogenated and oxygenated is also discussed.

Modelling physico-chemical properties of halogenated benzenes: QSAR optimisation through variables selection.

Abstract This paper describes the development of multivariate QSAR models for halogenated benzenes. Four physico-chemical properties have been modelled: Boiling Point (BP), Melting Point (MP), Flash Point (FP) and Density (D). For the multivariate characterisation of this class of compounds 90 topological indices derived from the chemical formula were used. The models were calibrated on training sets selected by a D-optimal design and were optimised by a statistical procedure of selection of the most informative variables. The predictive capacity of the resulting models measured in terms of the parameter SDEP (Standard Deviation of the Errors of Predictions) were as follows; MP: ± 21 ±C (with MP ranging from -48° to 181°C); BP: ±13°C (75° to 286°C); D: ±0.14 (1.02 to 2.52 g/cm(3)); FP: ±17°C (-12 to 126°C). These data were confirmed "experimentally", i.e., on external validation sets of compounds, the experimental values of the standard deviation of prediction errors were in fact as follows; MP: ±30°C; BP: ±16°C D: ±0.18 g/cm(3); FP: ±16°C. The developed models therefore provide a valuable tool for estimating physico-chemical properties relevant to environmental chemistry.