The photophysical properties of a series of homochelated complexes of the type Cu(NN)2+ are reported where (NN) denotes a heteroaromatic ligand. For the (NN) ligands 2,9-dimethyl-1,10-phenanthroline, 2,9-dimethyl-4,7-diphenyl-l,10phenanthroline, and 4,4,,6,6'-tetramethyl-2,2'-bipyridine, the emission quantum yields are 2.1 X 10"4, 2.5 X 104, and 0.5 X 104, respectively, in CH2C12 solution at 25 °C. Under these conditions the lifetimes are 90, 80, and 18 ns, respectively, for the same series of compounds. In each case the emission intensity is found to decrease as the solution temperature is lowered. This is interpreted in terms of two emitting states where the population of the upper level, which has a more favorable radiative rate constant, requires thermal energy. Detailed temperature studies of the 2,9-dimethyl-1,10-phenanthroline complex have been carried out, and the results have been analyzed by assuming that the two excited states are thermally equilibrated. We conclude that the lowest energy emitting state exhibits a radiative rate constant of ca. 103 s"\ typical of a triplet state. The other state lies some 1800 cm"1 to higher energy and exhibits a radiative rate constant of ca. 107 s"1. This state is ascribed singlet multiplicity. Tentatively, both states are assumed to derive from the lowest energy charge-transfer configuration of the complex.

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Supplementary Material Available: Tables of coordinates and isotropic thermal parameters for hydrogen atoms and anisotropic thermal parameters (2 pages); listings of observed and calculated structure factors (14 pages). Ordering information is given on any current masthead page.

The title compound, [ C ~( d p p ) ~]+ (dpp = 2,9-diphenyl-I ,I 0-phenanthroline), exhibits long-lived emission at room temperature in solution, and, in contrast with copper(1) complexes of less bulky ligands, the emission persists in polar media; a water soluble derivative of [Cu(dpp),]+ is luminescent a t room temperature in aqueous solution and undergoes efficient redox photochemistry in the presence of electron acceptors.

A chemically modified electrode has been developed as a detector for the sensitive and selective determination of thiol-containing compounds following capillary electrophoresis separation. Electrodes were constructed by entrapment of the coenzyme pyrroloquinoline quinone (PQQ) into a polypyrrole (PPy) matrix on a 245-microm graphite electrode during electropolymerization of pyrrole in the presence of PQQ. PQQ serves as an efficient biocatalyst to mediate the oxidation of thiols at a substantially reduced overpotential relative to an unmodified electrode. Furthermore, this design takes advantage of the pH-dependent reversible electrochemical properties of PQQ, which facilitates optimization of separation and detection conditions. The PQQ/PPy-modified electrode was incorporated as an end-column detector, and a separation of homocysteine, cysteine, N-acetylcysteine, and glutathione was developed. Detection limits for these four thiols were determined to be 11, 23, 104, and 134 nM, respectively, with mass detection limits ranging from 0.29 to 3.48 fmol. The PQQ/PPy electrode was also found to be very reproducible in run-to-run, day-to-day, and electrode-to-electrode comparisons. The utility of this electrode was demonstrated for the detection of cysteine in dietary supplements and human urine, resulting in excellent agreement with reported values.

A modified electrode sensor for the detection of thiols is described. The sensor was constructed by incorporation of the coenzyme pyrroloquinoline quinone (PQQ) into a polypyrrole (PPy) film on a glassy carbon electrode substrate by the electropolymerization of pyrrole in the presence of PQQ. The electrochemical properties of entrapped PQQ in the PPy film were influenced by the applied potential during electropolymerization and by film thickness, both of which were optimized to yield a stable and reproducible response for entrapped PQQ. The PQQ/ PPy sensor was utilized for the amperometric detection of cysteine, homocysteine, penicillamine, N-acetylcysteine, and glutathione. The response for each thiol in pH 8.42 borate buffer was found to be linear with detection limits (S/N = 3) ranging from 13.2 microM for glutathione to 63.7 nM for cysteine with sensitivities of 0.023 nA/microM and 4.71 nA/microM, respectively. The response and detection limits were found to be sensitive to the nature of the thiol and the solution pH. Furthermore, in the presence of dopamine, ascorbic acid, or uric acid, the pH-dependent redox potential of the PQQ catalyst allows tuning of the detection potential to enhance the selectivity for thiols over these potential electroactive interferences.

The extremely low sample volumes required for capillary electrophoresis and the high sensitivity and selectivity of electrochemical detection make capillary electrophoresis/electrochemistry (CEEC) a very useful method for bioanalysis. In this paper, two types of dual-electrode detectors for CEEC are described. The first employs a ring-disk microelectrode placed in a wall-jet configuration and is used for the selective detection of substances undergoing chemically reversible oxidations. Collection efficiencies obtained for catecholamines with this configuration were between 25 and 35%. The second electrode design consists of two adjacent carbon fibers embedded in an epoxy matrix and is analogous to the parallel dual-electrode configuration used in liquid chromatography/electrochemistry. This configuration can be used to confirm peak identity and purity by operating the electrodes at two different potentials. Alternatively, it is possible to perform simultaneous oxidative and reductive electrochemical detection.

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