H-Bond Acceptor Parameters for Anions.

UV/vis absorption titrations have been used to investigate the formation of H-bonded complexes between anionic H-bond acceptors (HBAs) and neutral H-bond donors (HBDs) in organic solvents. Complexes formed by three different HBDs with 15 different anions were studied in chloroform and in acetonitrile. The data were used to determine self-consistent HBA parameters (ß) for chloride, bromide, iodide, phosphate diester, acetate, benzoate, perrhenate, nitrate, triflimide, perchlorate, hexafluorophosphate, hydrogen sulfate, methyl sulfonate, triflate, and perfluorobutyl sulfonate. The results demonstrate the transferability of H-bond parameters for anions between different solvents and different HBD partners, allowing reliable prediction of anion recognition properties in other scenarios. Carboxylates are the strongest HBAs studied, with ß parameters (≈ 15) that are significantly higher than those of neutral organic HBAs, and the non-coordinating anion hexafluorophosphate is the weakest acceptor, with a ß parameter comparable to that of pyridine. The effects of ion pairing with the counter-cation were found to be negligible, provided small polar cations were avoided in the less polar solvent (chloroform). There is no correlation between the H-bonding properties of the anions and the pKa values of the conjugate acids.

Investigation of free base, Mg, Sn, and Zn substituted porphyrin LB films as gas sensors for organic analytes.

The visible absorption spectra of various substituted porphyrin compounds both in chloroform solution and as Langmuir-Blodgett (LB) solid-state films have been investigated. The porphyrin compounds examined were the Zn, Sn, Mg, and free base derivatives of 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine (EHO). Changes in the absorption spectra of these materials induced by their exposure to various organic compounds are reported with a view toward determining whether this is a useful approach toward an optical gas sensor.

Transmission of binding information across lipid bilayers.

A synthetic transmembrane receptor that is capable of transmitting binding information across a lipid bilayer membrane is reported. The binding event is based on aggregation of the receptor triggered by copper(II) complexation to ethylenediamine functionalities. By labelling the receptor with fluorescent dansyl groups, the copper(II) binding event could be monitored by measuring the extent of fluorescence quenching. Comparing the receptor with a control receptor lacking the transmembrane linkage revealed that the transmembrane receptor binds copper(II) ions more tightly than the non-spanning control receptor at low copper(II) concentrations. Since the intrinsic binding to copper(II) is the same for both receptors, this effect was attributed to synergy between the connected interior and exterior binding sides of the transmembrane receptor. Thus, this is the first reported artificial signalling event in which binding of a messenger on one side of the membrane leads to a cooperative binding event on the opposite side of the membrane, resembling biological signalling systems and helping us to get a better understanding of the requirements for more effective artificial signalling systems.

Toward development of water soluble dye derivatized nitrosyl compounds for photochemical delivery of NO.

This report describes the synthesis, spectroscopy, and photochemistry of a new fluorescein-derivatized iron sulfur nitrosyl compound, the Roussin's red salt ester bis-((mu-S,mu-S')-fluorescein-2-thioethyl-ester)-tetranitrosyldiiron (Fluor-RSE). Under continuous photolysis Fluor-RSE decomposes with moderate quantum yields (0.0036 +/- 0.0005 at lambda(irr) = 436 nm) with the corresponding release of most of the NO carried by the Fe2S2NO4 cluster. Large changes in the optical absorptivity occur upon photolysis of the Fluor-RSE, and these changes have been attributed to the different protic forms available to the fluorescein chromophore as it is separated from the cluster. Steady-state luminescence experiments have shown that the fluorescence of Fluor-RSE is about 85% quenched relative to the model compound ethyl fluorescein (Fluor-Et). Thus, it is clear that excitation of the fluorescein chromophore antennae is followed by energy transfer to the Fe/S/NO cluster at a rate at least comparable to fluorescence. However, the effect of the iron-sulfur core on the fluorescent lifetimes from fluorescein chromophore is much smaller. A single-exponential decay (tau = 3.3 ns) was seen for Fluor-RSE that is only modestly shorter than that for Fluor-Et (tau = 4.5 ns), and this is the effect of the smaller radiative rate constant (k(r)) for the former. These systems further demonstrate that attachment of a pendant dye chromophore as an antenna significantly improves the effective rate for photochemical NO generation from the Roussin's red salt esters at longer excitation wavelengths.