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

Time-resolved resonance Raman identification and structural characterization of a light absorbing transient intermediate in the photoinduced reaction of benzophenone in 2-propanol

A nanosecond time-resolved resonance Raman (ns-TR3) spectroscopic study of the triplet state benzophenone reaction with the 2-propanol hydrogen-donor solvent and subsequent reactions is presented. The TR3 spectra show that the benzophenone triplet state (npi*) hydrogen-abstraction reaction with 2-propanol is very fast (about 10 to 20 ns) and forms a diphenylketyl radical and an associated 2-propanol radical partner. The temporal evolution of the TR3 spectra also indicates that recombination of these two radical species occurs with a time constant of about 1170 ns to produce a LAT (light absorbing transient) intermediate that is identified as the 2-[4-(hydroxylphenylmethylene)cyclohexa-2,5-dienyl]propan-2-ol (p-LAT) species. Comparison of the TR3 spectra with results obtained from density functional theory calculations for the species of interest was used to elucidate the identity, structure, properties, and major spectral features of the intermediates observed in the TR3 spectra. The structures and properties of the reaction intermediates observed (triplet benzophenone, diphenyl ketyl radical, and p-LAT) are briefly discussed.

Intermolecular Electron Transfer from Excited Benzophenone Ketyl Radical

The electron transfer from the benzophenone ketyl radical in the excited state (BPH(.-)(D(1))) to several quenchers (Qs) was investigated using nanosecond/picosecond two-color two-laser flash photolysis and nanosecond/nanosecond two-color two-laser flash photolysis. The electron transfer from BPH(.-)(D(1)) to Qs was confirmed by the transient absorption and fluorescence quenching measurements. The intermolecular electron-transfer rate constants were determined using the Stern-Volmer analysis. The driving force dependence of the electron-transfer rate was revealed.

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 and Reactivity of Xanthyl Radical in the Excited State

The properties and reactivity of the 9-xanthyl radical (X(*)) in the doublet excited state (X(*)(D(1))) were investigated using nanosecond-picosecond two-color two-laser flash photolysis. The absorption and fluorescence spectra of X(*)(D(1)) were observed for the first time. The reactivity of X(*)(D(1)) toward a series of halogen donors and electron acceptors in acetonitrile and 1,2-dichloroethane (DCE) was investigated. It is confirmed that X(*)(D(1)) has a halogen abstraction ability from a series of halogen donors. On the basis of the solvent effect on the quenching rate constants of X(*)(D(1)), an electron transfer from X(*)(D(1)) to CCl(4) was indicated.