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Microsecond Time Scale (microsecond + time_scale)

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Chemistry100%

Selected Abstracts

Synthesis, characterization and oxygen-sensing properties of a novel luminescent Cu(I) complex

APPLIED ORGANOMETALLIC CHEMISTRY, Issue 10 2009
Linfang Shi
Abstract A novel luminescent copper(I) complex with formula [Cu(PPh3)2(PIP)]BF4 (PPh3 = triphenyl phosphine, PIP = 2-phenyl-1H -imidazo[4,5- f][1,10]phenanthroline) has been synthesized and characterized by 1H NMR, IR, elemental analysis and X-ray crystal structure analysis. In solid state, it displays broad band emission upon excitation at , = 420 nm with the emission maximum locates at 551 nm. Its excited-state lifetime is in the microsecond time scale (3.02 µs); as a result, its emission intensity is sensitive to oxygen concentration and shows oxygen-sensing properties after being encapsulated into mesoporous silica MCM-41. For the system with 60 mg/g loading level, a sensitivity (I0/I) of 4.35, a fluorescence quenching time (tQ) of 5 s and a recovery time (tR) of 36 s were achieved. Even after aging for 5 months, the sensitivities of the three loading level systems can be retained, ignoring the measurement error, which indicates that they possess long-term stability. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Metal Binding Properties of Fluorescent Analogues of Trichogin GA,IV: A Conformational Study by Time-Resolved Spectroscopy and Molecular Mechanics Investigations

CHEMBIOCHEM, Issue 1 2009
Mariano Venanzi Prof.
Abstract The metal ion binding properties of two fluorescent analogues of trichogin GA,IV, which is a natural undecapeptide showing significant antimicrobial activity, were studied by circular dichroism, time-resolved optical spectroscopy, and molecular mechanics calculations. Binding of CaII and GdIII to the peptides investigated was shown to promote a structural transition from highly helical conformations to folded structures characterized by formation of a loop that embedded the metal ion. Time-resolved spectroscopy revealed that peptide dynamics is also remarkably affected by ion binding: peptide-backbone motions slowed down to the microsecond time scale. Finally, molecular mechanics calculations emphasized the role of the central Gly5-Gly6 motif, which allowed for the twisting of the peptide segment that gave rise to the formation of the binding cavity. [source]

Semiconductor Behavior of a Metal-Organic Framework (MOF)

CHEMISTRY - A EUROPEAN JOURNAL, Issue 18 2007
Mercedes Alvaro Dr.
Abstract Upon light excitation MOF-5 behaves as a semiconductor and undergoes charge separation (electrons and holes) decaying in the microsecond time scale. The actual conduction band energy value was estimated to be 0.2,V versus NHE with a band gap of 3.4,eV. Photoinduced electron transfer processes to viologen generates the corresponding viologen radical cation, while holes of MOF-5 oxidizes N,N,N,,N, -tetramethyl- p -phenylenediamine. One application investigated for MOF-5 as a semiconductor has been the shape-selective photocatalyzed degradation of phenol in aqueous solutions. [source]

Photohydroxylation of 1,4-Benzoquinone in Aqueous Solution Revisited

CHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2004
Justus von Sonntag Dr.
Abstract In water, photolysis of 1,4-benzoquinone, Q gives rise to equal amounts of 2-hydroxy-1,4-benzoquinone HOQ and hydroquinone QH2 which are formed with a quantum yield of ,=0.42, independent of pH and Q concentration. By contrast, the rate of decay of the triplet (,max=282 and ,410 nm) which is the precursor of these products increases nonlinearly (k=(2,3.8)×106 s,1) with increasing Q concentration ((0.2,10) mM). The free-radical yield detected by laser flash photolysis after the decay of the triplet also increases with increasing Q concentration but follows a different functional form. These observations are explained by a rapid equilibrium of a monomeric triplet Q* and an exciplex Q2* (K=5500±1000,M,1). While Q* adds water and subsequent enolizes into 1,2,4-trihydroxybenzene Ph(OH)3, Q2* decays by electron transfer and water addition yielding benzosemiquinone .QH and . OH adduct radicals .QOH. The latter enolizes to the 2-hydroxy-1,4-semiquinone radical .Q(OH)H within the time scale of the triplet decay and is subsequently rapidly (microsecond time scale) oxidized by Q to HOQ with the concomitant formation of .QH. On the post-millisecond time scale, that is, when .QH has decayed, Ph(OH)3 is oxidized by Q yielding HOQ and QH2 as followed by laser flash photolysis with diode array detection. The rate of this pH- and Q concentration-dependent reaction was independently determined by stopped-flow. This shows that there are two pathways to photohydroxylation; a free-radical pathway at high and a non-radical one at low Q concentration. In agreement with this, the yield of Ph(OH)3 is most pronounced at low Q concentration. In the presence of phosphate buffer, Q* reacts with H2PO4, giving rise to an adduct which is subsequently oxidized by Q to 2-phosphato-1,4-benzoquinone QP. The current view that . OH is an intermediate in the photohydroxylation of Q has been overturned. This view had been based on the observation of the . OH adduct of DMPO when Q is photolyzed in the presence of this spin trap. It is now shown that Q*/Q2* oxidizes DMPO (k ,1×108,M,1,s,1) to its radical cation which subsequently reacts with water. Q*/Q2* react with alcohols by H abstraction (rates in units of M,1,s,1): methanol (4.2×107), ethanol (6.7×107), 2-propanol (13×107) and tertiary butyl alcohol (,0.2×107). DMSO (2.7×109) and O2 (,2×109) act as physical quenchers. [source]