Pulse radiolytic one-electron reduction of 2-hydroxy- and 2,6-dihydroxy-9,10-anthraquinones

The semiquinone free radicals produced by one-electron reduction of 2-hydroxy-9,10-anthraquinone (2HAQ) and 2,6-dihydroxy-9,10-anthraquinone (26DHAQ) in aqueous formate solution, water-isopropyl alcohol-acetone mixed solvent and isopropyl alcohol have been studied using the pulse radiolysis technique. The absorption characteristics, kinetic parameters of formation and decay, acid-base behaviour and redox characteristics of the semiquinones have been investigated and compared with the corresponding characteristics of a few intramolecularly hydrogen-bonded anthrasemiquinone derivatives. The non-hydrogen-bonded semiquinones show two pKa values (4.7 and 10.7 for 2HAQ and 5.4 and 8.7 for 26DHAQ, respectively) within the pH range 1-14, whereas other intramolecularly hydrogen-bonded semiquinones show only one pKa. The one-electron reduction potential (E'7) values for 2HAQ (-440 mV) and 26DHAQ (-400 mV) are more negative than those of the intramolecularly hydrogen-bonded systems

The fluorescence lifetimes of isomeric tyrosines

Fluorescence characteristics of isomeric ortho-, meta- and para-tyrosine have been determined. Differences in fluorescence lifetimes, especially at pH 1, provide a possible method for detection of ortho-tyrosine. A modified mathematical expression provides a photophysical method of detecting and estimating ortho-tyrosine in a mixture with para-tyrosine and phenylalanine. A possible application of this method is discussed

Pulse radiolytic one-electron oxidation of some dihydroxy-substituted anthraquinones

The spectroscopic characteristics and the kinetic parameters associated with the transients formed on one-electron oxidation of quinizarin (1,4-dihydroxy-9, 10-anthraquinone), quinizarin 2- and 6-sulfonates, 1,5-dihydroxy-9, 10-anthraquinone and 1,8-dihydroxy-9, 10-anthraquinone have been studied by pulse radiolysis and kinetic spectrophotometric techniques, using OH•, O•-, N3•, Br and •CH2CHO as the oxidising radicals. The pKa and the disproportionation equilibria of the semi-oxidised quinones have been studied for the water-soluble sulfonates. In contrast to the complex decay of the semi-oxidised naphthazarin (5,8-dihydroxy-1,4-naphtho-quinone), the semi-oxidised anthraquinone derivatives decay by simple second-order kinetics. The pKa values of the latter are also much higher (ca. 8) compared to the former (ca. <4). The differences observed are attributed to the loss in symmetry in the free radical structures of the semi-oxidised anthraquinone derivatives

One-electron reactions of 1,5- and 1,8-dihydroxy-9, 10-anthraquinones. A pulse radiolysis study

Absorption characteristics of the semiquinone free radicals formed by the one-electron reduction (using e-aq, CO•-2 and CH3OHCH3 as the reductant) as well as the oxidation (using OH•, O•- and N•3 as the oxidants) of 1,5-dihydroxy-9, 10-anthraquinone (1,5-DHAQ) and 1,8-dihydroxy-9, 10-anthraquinone (1,8-DHAQ) have been studied by pulse radiolysis in pure isopropyl alcohol and in aqueous solutions containing various appropriate additives. The first acid dissociation constants for the reduced semiquinones were measured as pKa= 3.65 and 3.95 for 1,5-DHAQ and 1,8-DHAQ, respectively. Second-order rate constants for various formation and decay reactions have been determined. The one-electron reduction potentials (vs. NHE) were determined at pH 11, as E111=-350 mV (for 1,5-DHAQ) and E111=-377 mV (for 1,8-DHAQ), respectively. Differences with 1,4-dihydroxy-9, 10-anthraquinone (quinizarin) are discussed

Interaction of the excited singlet state of disubstituted anthraquinones with aromatic hydrocarbons: a fluorescence-quenching study

The dynamic nature of the quenching of the fluorescence of 1,4-amino- and hydroxy-substituted 9,10-anthraquinone, with benzene, alkylbenzenes and other aryl hydrocarbons as quenchers, indicates formation of exciplexes, where charge transfer (CT) occurs from the S0 (quencher) to the S1 (fluorophore), as kq decreases with increasing ionisation potential of the quenchers, Exciplex emission spectra and kinetics have been established

Nanosecond laser flash photolysis of 5-hydroxy-1,4-naphthoquinone (Juglone)

The triplet properties of juglone, a monohydroxyquinone, were thoroughly investigated in cyclohexane, acetonitrile, methanol and isopropanol, by the technique of nanosecond laser flash photolysis. While the triplets absorb at about 285, 325 and 370 nm, the decay product, which absorbs at about 370 nm, is shown to be a neutral semiquinone free radical of juglone. The extinction coefficient for T-T absorption and the quantum yields of both triplet formation and semiquinone formation are estimated, and all the kinetic parameters associated with the triplet decay are evaluated. The effect of juglone concentration, laser intensity and the presence of triplet scavengers on the triplet properties were studied

T-T absorption and semiquinone formation in solutions of 5,8-dihydroxy-1,4-naphthoquinone (naphthazarin): a nanosecond laser flash photolysis study

The triplet properties of naphthazarin (5,8-dihydroxy-1,4-naphthoquinone) have been investigated in cyclohexane, methanol and isopropanol solutions using the technique of nanosecond laser flash photolysis. The T-T absorption maxima are at around 280, 335, 377, 410 and 440 nm, while the transients formed after the disappearance of the triplets in alcohols absorb at around 265, 365, 395 and 440 nm. The second transient has been shown to be a semiquinone free radical of naphthazarin, produced by abstraction of a hydrogen atom from the alcohols. The extinction coefficient at many wavelengths and the quantum yields of both triplet and semiquinone formation have been estimated. Kinetic parameters associated with the decay of both the triplet and the semiquinone have been evaluated. The effect of laser intensity and the presence of triplet scavengers on the triplet properties have been studied. A comparison has been made with the corresponding transients produced from juglone (5-hydroxy-1,4-naphthoquinone)

Single-molecule detection in exploring nanoenvironments: an overview

In the last one decade or so, a variety of optical experiments have been designed and performed that are capable of exploring down to the regime of single-molecule detection and measurements in all different environments, including solids, surfaces, and liquids. Single-molecule detection in condensed phases has many important chemical and biological applications. A few to list are: rapid DNA sequencing, DNA fragment sizing, medical diagnosis, forensic analysis, understanding of chemical dynamics and mechanisms, etc. Single-molecule spectroscopy allows us to observe the individual molecules hidden in a condensed phase sample, by using a tunable laser light. This technique has the ability to detect and monitor systems with an ultimate sensitivity level of ~1.66 × 10-24 moles (1/N0). Measurement at the single-molecule level can completely remove the complicacy associated with ensemble-averaged macroscopic measurements. It allows us to construct a frequency histogram of the distribution of values for a parameter of interest following a large number of measurements on many individual molecules. Such a distribution carries much more information than the average value of the parameter obtained from a macroscopic measurement. As there is no ensemble averaging involved, only measurements at the single-molecule level can give an appropriate test for microscopic dynamical theories. Using single-molecule spectroscopy one can, in principle, follow the temporal evolution of any complex reaction path. As the field is still emerging, with newer methodologies of detecting single molecules with improved signal-to-noise ratios, it is expected that many new physical and chemical phenomena will certainly be explored using this technique. In the present article, our endeavor is to give an overview of the different aspects of single-molecule detection, along with some of its important applications in the areas of bioscience and chemical physics

Triplet states and semiquinone free radicals of 1,5- and 1,8-disubstituted anthroquinones: a laser flash photolysis and pulse radiolysis study

The triplet state properties of 1,5-diamino-9,10-anthroquinone and 1,5- and 1,8-dihydroxy-9,10-anthraquinone in cyclohexane, acetonitrile and isopropyl alcohol were investigated using a nanosecond laser flash photolysis technique. The T-T absorption spectra, the triplet quantum yield, the triplet energy level and the triplet decay kinetics were estimated. In isopropyl alcohol, along with the triplet, the neutral semiquinone radicals are also formed and were characterized by comparing the long-lived transient spectra with the semiquinone spectra obtained by pulse radiolysis of these quinones in isopropyl alcohol. Higher quantum yields of the semiquinone radicals isopropyl alcohol, in comparison with the triplet quantum yield, are explained by assuming that both the excited singlet and the triplet states abstract hydrogen atoms from the solvent molecules to form the semiquinone radicals. A comparison of the photophysical properties of 1,4-disubstituted anthraquinones with the present series is discussed