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[Display omitted] • Arylphosphines undergo photodecomposition forming the oxides in aerated solution. • The quantum yields for the photooxidation exceed unity. • Quenching of triplet arylphosphines by dissolved oxygen implies the formation of singlet oxygen. • The mechanism for the self-photooxidation mediated by phosphadioxirane is suggested. Arylphosphines (ArP) undergo UV-photodecomposition with quantum yields of 2 in aerated acetonitrile to form the corresponding oxides (ArPO). Laser flash photolysis studies showed that triplet ArP was quenched by the dissolved oxygen. Based on these results, it was concluded that singlet oxygen produced by triplet energy transfer from triplet ArP is involved as the primary oxidant in the formation of ArPO upon photolysis of ArP. We discuss the mechanism for forming ArPO upon photolysis of ArP with the over-unity quantum yield by considering phosphadioxiranes of ArP as the secondary oxidant. [ABSTRACT FROM AUTHOR]

We prepared amide-heterocycle (HC) compounds having various aromatic π-electron systems (Ar), such as phenyl, naphthyl, furyl, thienyl and phenanthryl moieties, and converted them as ligands to difluoroboronated complexes, Ar@HCs. Blue fluorescence from Ar@HCs was observed in solution and the solid state, and the fluorescence quantum yields (Φf) and lifetimes (τf) were determined. The Φf values in CHCl3 were as small as 0.1 except for the phenanthrene derivatives (0.4–0.6). Observation of the triplet–triplet absorption upon laser flash photolysis of Ar@HCs in solution indicated that the fluorescence process competes with intersystem crossing to the triplet state. Blue fluorescence in the solid state was observed with the Φf values of 0.3–0.7. Based on the crystallographic data, the relationship between the crystal structures and emission features of Ar@HCs in the solid state is discussed. [ABSTRACT FROM AUTHOR]

Photoinduced ω-bond cleavage was investigated in seven newly synthesized aromatic ketones having five-membered rings (furan and thiophene) and/or phenyl group connected to the phenylthiyl moiety with C S bond. Following direct laser excitation of these ketones in acetonitrile, the transient absorption spectra that appeared were shown to be due to the corresponding triplet states. No chemical reactions occurred upon direct excitation. However, triplet sensitization of these aromatic ketones using xanthone revealed no formation of their triplet states. Instead absorption spectra of the phenylthiyl radical (PTR) appeared as a result of C S bond cleavage. These observations indicate that higher triplet states were reactive leading to C S bond dissociation. The C S bond dissociation mechanism upon triplet sensitization for the studied carbonyl compounds was discussed considering energy diagrams of the higher excited states. [ABSTRACT FROM AUTHOR]

Photochemical processes of 4-tert-butyl-4′-methoxydibenzoylmethane (Avobenzone, AB), 4-phenylbenzoylbenzoyl-, 4-phenylbenzoyl-2′-furanyl- and 4-phenylbenzoyl-2′-thenoylmethanes (PB@Ph, PB@F and PB@T, respectively) substituted with Br and Cl at the C2 position were studied by stationary and laser flash photolyses in solution. The absorption spectral features showed that the molecular structures of the halogenated diketones are in the keto forms while those of halogen-free diketones are in the enol forms. The excited singlet and triplet state energies were determined from the absorption and emission spectra. From the absorption spectral changes upon steady state photolysis of brominated diketones in ethanol, the corresponding halogen-free diketones were formed due to Br elimination being the major photochemical process. The determined quantum yields for the formation of the halogen-free diketones were independent of the amount of dissolved oxygen, indicating that the elimination process is an event in the excited singlet (S1) states. In contrast, from the observed absorption spectra obtained upon photolysis of chlorinated AB and PB@Ph, it was inferred that Norrish type I is the major photochemical reaction in the S1 states in acetonitrile. Chlorinated PB@F and PB@T were found to undergo Cl elimination in the S1 states in cyclohexane to form the corresponding halogen-free diketones. Laser photolysis studies of brominated AB in acetonitrile and ethanol provided a transient absorption spectrum ascribable to the Avobenzone radical (ABR) produced by debromination as the initial intermediate, followed by the AB formation in ethanol. The quenching rate constant of ABR by ethanol and the quantum yield of the AB formation via ABR were determined. These observations provided evidence that H-atom abstraction of ABR from ethanol is responsible for the AB formation. Conversely, laser flash photolysis of brominated and chlorinated PB@Ph, PB@F and PB@T demonstrated the formation of the triplet–triplet absorption spectra. No chemical reactions were found to occur in the triplet (T1) states. Two-color two-laser photolysis studies were carried out on the T1 state of chlorinated PB@Ph, PB@F and PB@T, resulting in the formation of the corresponding halogen-free diketones. These observations confirmed the occurrence of Cl elimination in the highly excited triplet (Tn, n≥ 2) states. Based on the computed bond dissociation energies for the C–halogen and C–C bonds, switching mechanisms of dehalogenation and α-cleavage were discussed. [ABSTRACT FROM AUTHOR]

We synthesized 1,3-diketones having the ferrocene moiety (FcDKPhs) and the corresponding difluoroboronated complexes (Fc@Phs), and investigated the photophysical and photochemical properties in solution by steady-state and laser flash photolyses. FcDKPhs and Fc@Phs had absorption bands with large absorptivities in the UV-region and ones with small absorptivities in the Vis-region, but provided neither fluorescence nor phosphorescence. FcDKPhs decomposed upon exposure to UV–vis light while Fc@Phs were stable. No transient signals obtained upon laser flash photolysis of FcDKPhs and Fc@Phs demonstrated absence of intersystem crossing to the triplet states. Fc@Phs efficiently convert absorbed photon energy to heat. [ABSTRACT FROM AUTHOR]

Thephotothermal tautomerization processes between enol and keto formsof 4-tert-butyl-4′-methoxydibenzoylmethane(trade name, Avobenzone) in acetonitrile have been studied by steady-stateand laser flash photolysis. The keto form is produced upon photolysisof the enol in only acetonitrile with a quantum yield of 0.014. Themolar absorptivity of the keto form was determined. Phototautomerizationfrom the keto to the enol form was not seen. Laser flash photolysisof the keto form recognized the formation of the triplet state. Inthe dark, the keto form underwent thermal tautomerization to the enolwith a lifetime of 5.1 h at 295 K. The enolization rate in acetonitrilewas not accelerated by the presence of alcohols and/or water but increasedwith increasing temperature and followed the Arrhenius expression.The activation energy and the frequency factor were determined forthe enolization process from the keto to the enol form. On the basisof the energy states of the tautomers and isomers as estimated byDFT calculations, a schematic energy diagram was determined for thephotothermal tautomerization processes in acetonitrile. [ABSTRACT FROM AUTHOR]

Abstract: A photochemical formation process of avobenzone (AB; 4-tert-butyl-4′-methoxydibenzoylmethane) from 1,1-(4-tert-butybenzoyl)(4′-methoxybenzoyl)butane (PrAB) is studied by steady-state and laser flash photolysis in solution. The quantum yield of the formation via the triplet state of PrAB is determined to be 0.23 in degassed acetonitrile at 295K. The Arrhenius plots of the decay rate of triplet PrAB show that photoelimination proceeds with an activation energy of 6.0kcalmol−1 and the frequency factor of 4.6×1010 s−1. [Copyright &y& Elsevier]

1-Benzoyl-2-phenylacetylene (BPA) having a moderate triplet energy (65 kcal mol −1 ) was prepared, and steady state and laser photolysis techniques were conducted to characterize the photochemical features of the triplet state and the BPA ketyl radical that was formed via H-abstraction of the triplet n,π* from alcohol and phenol. The photophysical and photochemical properties of BPA similar to those of benzophenone may encourage photochemists to use it as an n , π* triplet sensitizer scheduled for triplet-energy-tuned experiments upon laser flash photolysis in solution. [ABSTRACT FROM AUTHOR]

Photochemical properties of α-cleavage of the C−S bond in excited states of p-biphenyl thioacetate and p-biphenyl thiobenzoate (Me-SBP and Ph-SBP) in solution are investigated using steady-state and laser flash photolyses in comparison with those of S-phenyl thiobenzoate, where the photo-Fries rearrangement was reported to be absent. Although Me-SBP and Ph-SBP decompose upon 254 nm photolysis in acetonitrile irrespective of the amount of the dissolved oxygen, no definite photoproducts due to the photo-Fries rearrangement were found. Laser flash photolysis (266 nm) of these molecules reveals the occurrence of the C−S bond cleavage in the excited state based on the observation of the formation of the biphenylylthiyl radical (BTR) in the transient absorption. Quantum yields (Φrad) of the BTR formation were determined to be 0.20 and 0.15 for Me-SBP and Ph-SBP, respectively. Triplet sensitization of Ph-SBP using xanthone (XT) as a sensitizer shows that the lowest triplet (T1) state of Ph-SBP is dissociative for the C−S bond with an efficiency of ≥0.56. In contrast, triplet sensitization of Me-SBP using acetone as a sensitizer demonstrates the efficient formation of triplet Me-SBP, and the molar absorption coefficient of the triplet−triplet absorption was determined. No photochemical reactions are found in the T1state of Me-SBP. Upon 355 nm laser flash photolysis of the T1state of Me-SBP, the formation of BTR is confirmed in the transient absorption. This observation indicates the C−S bond cleavage in a highly excited triplet (Tn) state of Me-SBP. The quantum yield (Φdec) of the α-cleavage in the Tnstate of Me-SBP was determined to be 0.69. Photochemical features of Me-SBP and Ph-SBP are discussed from the viewpoint of the spin-multiplicity of the bond dissociative states. [ABSTRACT FROM AUTHOR]

Abstract: C–O bond dissociation of naphthoxymethyl compounds NpO–CH2R (Np=α- or β-naphthalene moiety, R=Ph or Me) in the higher triplet excited state (T n , n ⩾2) was investigated using two-color two-laser flash photolysis. The C–O bond dissociation was found to occur with quantum yields (Φ) of 0.06 and 0.07 for α- and β-NpO–CH2Ph(T n ), respectively. With the use of reported data on C–O bond cleavage in NpCH2–OPh(T n ) and NpCH2–OMe(T n ), it was concluded that Φ is related to the T n -state lifetime and the energy barrier between the T n state and dissociative potential surfaces. [Copyright &y& Elsevier]

Abstract: Photophysical and photochemical properties of corannulene (CA) and tetrabromocorannulene (TBCA) in solution were investigated as follows. Spectra, quantum yields and lifetimes of fluorescence in solution at 295K, and spectra and lifetimes of phosphorescence in a rigid matrix at 77K were determined. Phosphorescence at 295K was observed from TBCA. Intersystem crossing yields were determined to be as large as 0.9 by optoacoustic techniques. Triplet–triplet absorption spectra were obtained by laser flash photolysis. Absorption spectra of the radical cation and anion of CA were obtained by pulse radiolysis techniques. [Copyright &y& Elsevier]