Remarkable Reactivities of the Xanthone Ketyl Radical in the Excited State Compared with That in the Ground State

Photosensitized xanthone-based oxidation of guanine and its repair: a laser flash photolysis study

The photosensitized oxidation of guanine (G) by the triplet state of xanthone (XT) and the repair for photo-damaged G(-H)(·) by ferulic acid (FCA) were investigated using the laser flash photolysis technique. The rate constants of the reaction of triplet state of XT with G and with FCA were determined as 4.5×10(9) and 8.0×10(9) L mol(-1) s(-1), respectively. Laser exposure was performed on the N(2)-saturated acetonitrile/water (v/v, 1:1) solution containing G, XT and FCA. The transient absorption spectra indicated that the triplet state of XT first reacted with G predominantly to form the oxidized radical G(-H)(·). The radical G(-H)(·) was rapidly repaired by FCA, and the rate constant for the repair reaction was determined as 1.1×10(9) L mol(-1) s(-1). These results demonstrated that non-enzymatic repair is a feasible method for repairing photosensitized DNA bases oxidation.

Photoinduced formation and characterization of electron-hole pairs in azaxanthylium-derivatized short single-walled carbon nanotubes

2-Azaxanthone, a nitrogenated derivative of the well-studied organic chromophore xanthone, has been covalently bound through 2-(ethylthio)ethylamido linkers to the carboxylic acid groups of short, soluble single-walled carbon nanotubes (CNTs) of 450 nm average length, and the resulting azaxanthylium-functionalized CNTs (AZX-CNT, 8.5 wt % AZX content) characterized by solution (1)H NMR, Raman and IR spectroscopy and thermogravimetric analysis. Comparison of the quenching of the triplet excited state of AZX (steady-state and time-resolved) and of the transient optical spectra of CNTs and AZX-CNT shows that the covalent linkage boosts the interaction between the azaxanthylium moiety and the short CNT units. The triplet excited state of the azaxanthylium derivative is quenched by CNT with and without covalent bonding, but when it is covalently bonded, the singular transient spectrum is compatible with the photogeneration of electron holes through electron transfer from CNT to excited azaxanthylium units.

Optical fiber-based single-shot picosecond transient absorption spectroscopy

A new type of single-shot transient absorption apparatus is described based on a bundle of optical fibers. The bundle contains 100 fibers of different lengths, each successively giving approximately 15 ps longer optical delay. Data are collected by imaging light from the exit of the bundle into a sample where it is overlapped with an electron pulse or laser excitation pulse, followed by imaging onto a charge coupled device (CCD) detector where the intensity of light from each fiber is measured simultaneously. Application to both ultrafast pump-probe spectroscopy and pulse radiolysis is demonstrated. For pulse radiolysis, the prototype bundle provides the ability to collect data with a time resolution limited only by the electron pulse width of 7-10 ps, over a total single-shot time window of approximately 1.5 ns. Tunable probe light is obtained from a titanium-sapphire laser and an optical parametric amplifier. Corrections are made to remove the fiber-to-fiber variations in signal magnitude due to the spatial overlap of the electron beam and probe image. High quality data can be collected over most of the sensitivity range of the CCD camera detectors. The single-shot instrument is valuable for measurement of samples that are only available in very limited quantities, are too viscous to flow, or are rigid. It is therefore excellent in applications, such as picosecond pulse radiolysis, where the thousands of pulses per kinetic trace typical in classical pump-probe experiments can damage the sample before useful results could be obtained.

Solvent Effect on the Deactivation Processes of Benzophenone Ketyl Radicals in the Excited State

The solvent effects on ketyl radicals of benzophenone derivatives (BPD) in the excited state (BPDH*(D1)) were investigated. Absorption and fluorescence spectra of BPDH*(D1) in various solvents were measured using nanosecond-picosecond two-color two-laser flash photolysis. The fluorescence peaks from BPDH*(D1) showed a shift due to the dipole-dipole interaction with the solvent molecules. The dipole moments (mu(e)) of BPDH*(D1) were estimated to be 7-10 D, indicating that BPDH*(D1) are highly polarized. It was revealed that the fluorescence lifetime (tau(f)) depends on mu(e) in acetonitrile because the stabilization by solvent molecules affects the tau(f) value in polar solvents, predominantly. On the contrary, the conformation of BPDH*(D1) plays an important role in cyclohexane because the efficiency of the unimolecular reaction from BPDH*(D1) depends on the conformation. The substituent effect on the electron transfer from BPDH*(D1) to their parent molecules was also discussed.

Properties of Excited Ketyl Radicals of Benzophenone Analogues Affected by the Size and Electronic Character of the Aromatic Ring Systems

The properties of benzophenone ketyl radical analogues with large aromatic ring systems, such as naphthylphenylketone (2), 4-benzoylbiphenyl (3), and bis(biphenyl-4-yl)methanone (4), were investigated in the excited state by using nanosecond-picosecond two-color two-laser flash photolysis. Fluorescence and transient absorption spectra of ketyl radicals of 2-4 in the excited state were observed for the first time. The fluorescence and properties of the excited ketyl radicals were significantly affected by the size and electronic properties of the aromatic ring systems. The reactivity of the ketyl radicals in the excited state with several quenchers was examined and they were found to show reactivity toward N,N-diethylaniline. In addition, for the benzophenone ketyl radical, a unique quenching process of the radical in the excited state by the ground-state parent molecule was found. The factors regulating the fluorescence lifetime of the ketyl radicals in the excited state are discussed quantitatively.

Dual Electron Transfer Pathways from 4,4‘-Dimethoxybenzophenone Ketyl Radical in the Excited State to Parent Molecule in the Ground State

Dual intermolecular electron transfer (ELT) pathways from 4,4'-dimethoxybenzophenone (1) ketyl radical (1H*) in the excited state [1H*(D1)] to the ground-state 4,4'-dimethoxybenzophenone [1(S0)] were found in 2-methyltetrahydrofuran (MTHF) by observing bis(4-methoxyphenyl)methanol cation (1H+) and 4,4'-dimethoxybenzophenone radical anion (1*-) during nanosecond-picosecond two-color two-laser flash photolysis. ELT pathway I involved the two-photon ionization of 1H* following the injection of electron to the solvent. The solvated electron was quickly trapped by 1(S0) to produce 1*-. ELT pathway II was a self-quenching-like ELT from 1H*(D1) to 1(S0) to give 1H+ and 1*-. From the fluorescence quenching of 1H*(D1), the ELT rate constant was determined to be 1.0 x 10(10) M(-1) s(-1), which is close to the diffusion-controlled rate constant of MTHF. The self-quenching-like ELT mechanism was discussed on the basis of Marcus' ELT theory.