Triplet State Of Benzophenone Research Articles

Water-Triggered Photoinduced Electron Transfer in Acetonitrile-Water Binary Solvent. Solvent Microstructure-Tuned Reactivity of Hydrophobic Solutes.

The solvent-composition dependence of quenching triplet states of benzophenone (3BP) by anisole in acetonitrile–water (ACN–H2O) mixtures was investigated by laser flash photolysis over the water mole fraction (xw) increasing from 0 to 0.92. Single exponential decay of 3BP was observed over the whole composition range. The quenching rate constant consistently increased with the water content but increased far more rapidly with xw > 0.7. The water-triggered electron-transfer (ET) mechanism was confirmed by a steeply growing quantum yield of the benzophenone ketyl radical anion, escaping back-ET when the partial water volume exceeded the acetonitrile one. The water-content influence on the 3BP quenching rate was described by a kinetic model accounting for the microheterogeneous structure of the ACN–H2O mixtures and the very different solubility of the reactants in the solvent components. According to the model, the ET mechanism occurs at a rate constant of 1.46 × 109 M–1 s–1 and is presumably assisted by the ACN–H2O hydrogen-bonding interaction.

Photoinduced intermolecular charge transfer processes on pure and modified silica surfaces

This article briefly reviews some of our work over the past two decades devoted to the photoinduced electron-transfer processes and formation of bimolecular complexes–exiplexes between adsorbed donor pyrene and anthracene and acceptor N,N-dimethylaniline molecules on silica and silica–titania surfaces. Quenching of the fluorescence of acenes by N,N-dimethylaniline on silica surfaces is found to be diffusion controlled and results from exciplex formation at room temperature. The effect of silica–titania on the processes of photoinduced electron-transfer resulted vise-versa in increases of the fluorescence intensity from the adsorbed acenes. This phenomenon is explained by the selective adsorption of N,N-dimethylaniline on the acid “titanium” centers and a shift of the acenes to weaker adsorption sites located on the silica support. The excited molecular pair effectively yields reduced titanium ions due to an electron transfer process. This study demonstrates the important role that adsorption of organic molecules plays in photocatalytic processes on the surface of semiconductor composites. Sol–gel produced porous silica modified with benzophenone molecules leads to full photoreduction of silver and gold ions. Formation of stable Ag and Au nanoparticles embedded in porous silica films was carried out by irradiation of silver (gold) ions-modified silica in benzophenone–water–isopropanol solution. The triplet state of benzophenone takes part in the formation of the reducing agents, such as ketyl radicals and anion radicals of benzophenone and isopropanol. These systems can be used as bactericide materials, for extraction of metal ions from solution, for photocatalytic reduction of toxic transition metals and other applications.

Tuning Excited‐State Reactivity by Proton‐Coupled Electron Transfer

AbstractThe reactivity, and even reaction pathway, of excited states can be tuned by proton‐coupled electron transfer (PCET). The triplet state of benzophenone functionalized with a Brønsted acid (3*BP‐COOH) showed a more powerful oxidation capability over the simple triplet state of benzophenone (3*BP). 3*BP‐COOH could remove an electron from benzene at the rate of 8.0×105 m−1 s−1, in contrast to the reactivity of 3*BP which was inactive towards benzene oxidation. The origin of this great enhancement on the ability of the excited states to remove electrons from substrates is attributed to the intramolecular Brønsted acid, which enables the reductive quenching of 3*BP by concerted electron–proton transfer.

Tuning Excited-State Reactivity by Proton-Coupled Electron Transfer.

The reactivity, and even reaction pathway, of excited states can be tuned by proton-coupled electron transfer (PCET). The triplet state of benzophenone functionalized with a Brønsted acid (3 *BP-COOH) showed a more powerful oxidation capability over the simple triplet state of benzophenone (3 *BP). 3 *BP-COOH could remove an electron from benzene at the rate of 8.0×105 m-1 s-1 , in contrast to the reactivity of 3 *BP which was inactive towards benzene oxidation. The origin of this great enhancement on the ability of the excited states to remove electrons from substrates is attributed to the intramolecular Brønsted acid, which enables the reductive quenching of 3 *BP by concerted electron-proton transfer.

Reactivity of Benzophenone Ketyl Free Radicals in an Elongated Elastomer Film.

The effect of polymer film elongation leading to the thinning of the film up to three times on the decay of transients was studied. Kinetics of benzophenone (B) triplet state (3)B* and ketyl free radical BH• in soft rubber poly(ethylene-co-butylene) (abbreviated as E) was investigated by nanosecond laser flash photolysis. We monitored decay kinetics of the triplet state of (3)B* and of BH• decay in the polymer cage and decay of BH• in the polymer bulk. The fast exponential decay of (3)B*(lifetime τT ≈ 200 ns) is accompanied by hydrogen atom abstraction from E with the formation of BH• and a polymer free radical R•. The decay of BH• in the polymer cage occurs during τc ≈ 1 μs. Cage recombination, in turn, was followed by a cross-termination of BH• in the polymer bulk (τb ≈ 100 μs under our conditions) and is characterized by a rate constant kb ≈ 10(8) M(-1) s(-1). We studied changes of rates of transients decay upon elongation (thinning) of E. Decay of (3)B* is practically independent of elongation of the film. Recombination of BH• in the solvent bulk occurs with a two times lower kb than in a nonelongated E. The decrease in kb is ascribed mainly to a lower fractional polymer free volume Vf in elongated E compared with that in nonelongated E. Dependencies of log10 kb versus [Formula: see text], where [Formula: see text] is the thickness of the film, turned out to be linear with a negative slope. At the same [Formula: see text] recombination proceeds slower in the elongated elastomer compared with the nonelongated elastomer. Cage effect increases twice as well due to a lower rate of radicals escape from the polymer cage in the elongated film. We observed relatively large effects of external magnetic field (B = 0.2T) on the kinetics of cage recombination and recombination in the polymer bulk. Magnetic field effect on recombination rates in the cage and in the solvent bulk does not depend on elongation.

Laser flash photolysis of benzophenone in thin silicone films

The decay kinetics of benzophenone (BP) triplet state in the thin (∼200nm) films of poly(dimethylsiloxane) have been studied with a nanosecond laser flash photolysis technique. Decay kinetics of a triplet state of BP (3BP) is dispersive kinetics, and it can be well described by the kinetic law with Gaussian distribution of the activation free energy. The average rate constants of 3BP decay kav are 1500 and 70s−1 for air saturated and deoxygenated films, respectively. Width of distribution are 1.2 and 4.3 (dimensionless) and correspond to the first and second kav, respectively presented above. The rate of hydrogen abstraction of H-atom from Si–CH3 groups by 3BP is low which results in long-lived 3BP in silicone films.

Unusual photobehavior of benzophenone triplets in hexafluoroisopropanol. Inversion of the triplet character of benzophenone

► Inversion of the triplet state of benzophenone in hexafluoroisopropanol. ► Unique photobehavior of benzophenone triplets in hexafluoroisopropanol. ► Simultaneous detection of the two types of triplets in benzophenone. ► Solvent controlled changes in the triplet state character of benzophenone. In the present work a systematic study of the photophysical and photochemical behavior of unsubstituted benzophenone (BP) in acetonitrile–hexafluoroisopropanol (ACN–HFIP) mixtures was undertaken. Using laser flash photolysis, steady-state absorption and emission techniques, our results indicate that the lowest triplet excited state of BP can possess both n,π * and π,π * character, contrary to the previous assumption that the triplet of BP has pure n,π * character. In ACN, the BP triplet has an n,π * configuration with its typical triplet–triplet absorption spectrum having a maximum at 520 nm. However, in HFIP this band with λ max = 520 nm is accompanied by a new band with a maximum at 380 nm, which is attributed to a triplet of π,π * character. Furthermore, changes in the character of the triplet state of BP were also detected by the measurement of the triplet quenching rate constants by 2-propanol (good hydrogen donor) in ACN ( k q = 3.4 × 10 6 M −1 s −1 ) and HFIP ( k q = 8.6 × 10 4 M −1 s −1 ). In contrast to 2-propanol, the reactivity of an electronically excited BP molecule toward anisole was greatly enhanced by changes in the nature of the solvent. The quenching rate constant was three orders of magnitude larger in HFIP than in ACN. In addition, the quenching mechanism itself appears to change on going from charge transfer in pure ACN to efficient full electron transfer in ACN–HFIP mixtures having a large (>60%, v/v) HFIP content.

Photosensitized oxidation of tryptophan and tyrosine by aromatic ketones: A laser flash photolysis study

Photochemical processes of benzophenone (BP) and xanthone (XT) with tryptophan (TrpH) and tyrosine (TyrOH) were studied using the laser flash photolysis technique. It has been observed that the triplet state of BP and XT reacted with TrpH and TyrOH by hydrogen transfer with the formation of BP and XT ketyl radicals and oxidized radicals of Trp· and TyrO·. The related rate constants of these reactions were determined in this paper. The free energy changes (ΔG) of these reactions suggested that the proposed hydrogen transfer mechanism was thermodynamically feasible. These results provide theoretical foundation for further studying structural effects on the photochemical behaviors of proteins with triplet state BP and XT.

Femtosecond Transient Absorption, Nanosecond Time-Resolved Resonance Raman, and Density Functional Theory Study of Fenofibric Acid in Acetonitrile and Isopropyl Alcohol Solvents

Hydrogen abstraction reaction of fenofibric acid (FA) in acetonitrile and isopropyl alcohol solvents was studied by femtosecond transient absorption (fs-TA) and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy experiments. The singlet excite state ((1)FA) (nπ*) with a maximum transient absorption at 352 nm observed in the fs-TA experiments undergoes efficient intersystem crossing (ISC) to convert into a nπ* triplet state FA ((3)FA) that exhibits two transient absorption bands at 345 and 542 nm. The nπ* (3)FA species does not decay obviously within 3000 ps. In the ns-TR(3) experiments, the nπ* (3)FA is also observed and completely decays by 120 ns. Compared with the triplet states of benzophenone (BP) and ketoprofen (KP), the nπ* (3)FA species seems to have a much higher hydrogen abstraction reactivity so that (3)FA decays fast and generates a FA ketyl radical like species. In isopropyl alcohol solvent, the nπ* (3)FA exhibits similar reactivity and promptly abstracts a hydrogen from the strong hydrogen donor isopropyl alcohol solvent to generate a ketyl radical intermediate. With the decay of the FA ketyl radical, no light absorption transient (LAT) intermediate is observed in isopropyl alcohol solvent although such a LAT species was observed after similar experiments for BP and KP. Comparison of the ns-TR(3) spectra for the species of interest with results from density functional theory calculations were used to elucidate the identity, structure, properties, and major spectral features of the intermediates observed in the ns-TR(3) spectra. This comparison provides insight into the structure and hydrogen abstraction reactivity of the triplet states of BP derivatives.

Time-Resolved and Pulse EPR Study of Triplet States of Alkylketones in β-Cyclodextrin

The triplet states of deoxybenzoin (DOB) and benzophenone (BP) molecules in randomly methylated β-cyclodextrin (CD) cavity are studied by time-resolved (TR) and pulse electron paramagnetic resonance (EPR). The observed TR EPR spectrum of DOB in β-CD at 30 K is close to the spectrum measured in polar solvent trifluoroethanol, revealing strong hydration by water molecules. At the same time, TR EPR spectrum of BP in β-CD corresponds to nonpolar surrounding of the CO-group. The electron spin relaxation times T1 and T2 of triplet BP at 30 K measured by pulse EPR are found to be different in β-CD compared to nonpolar toluene glass. The observed increase of T2 by up to a factor of four in β-CD is caused by the lower vibration amplitude of CO-bond of BP due to the confinement in β-CD. The influence of β-CD with covalently attached nitroxide on the triplet states of DOB and BP is principally different: the excited triplet states could not be observed by TR EPR due to the efficient quenching of the excited states by nitroxide.

Laser Flash Photolysis of Benzophenone in Polymer Films

With a nanosecond laser we studied flash photolysis of benzophenone (BP) dissolved in four different polymer films. We measured kinetics of decay of a triplet state of benzophenone (3)BP as well as kinetics of decay of benzophenone ketyl free radicals BPH(•). Polymer matrices have plenty of reactive C-H bonds, and the hydrogen abstraction by (3)BP leads to a formation of geminate pair which either recombines into molecular products or dissociates. Decay kinetics of (3)BP is well described by dispersive kinetics and in particular by the kinetic law suggested in Albery, W. J.; et al. J. Am. Chem. Soc. 1985, 107, 1854. We observed a broader distribution of rate constants in hard films. It was observed that the decay kinetics of transients radicals in the "hard" polymers is quite satisfactory described by the same law for dispersive kinetics. Kinetics of radicals decay in "soft" polymers is satisfactorily described as a diffusion-enhanced reaction. Effect of a hardness of polymer matrix on the measured kinetic parameters is discussed.

Supramolecular Photochemistry in β-Cyclodextrin Hosts: A TREPR, NMR, and CIDNP Investigation

A systematic investigation of the photochemistry and ensuing radical chemistry of three guest ketones encapsulated in randomly methylated beta-cyclodextrin (beta-CD) hosts is reported. Dibenzyl ketone (DBK), deoxybenzoin (DOB), and benzophenone (BP) triplet states are rapidly formed after photolysis at 308 nm. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy, steady-state NMR spectroscopy, and time-resolved chemically induced nuclear polarization (TR-CIDNP) experiments were performed on the ketone/CD complexes and on the ketones in free solution for comparison. The major reactivity pathways available from these excited states are either Norrish I alpha-cleavage or H-atom abstraction from the interior of the CD capsule or the solvent. The DOB triplet state undergoes both reactions, whereas the DBK triplet shows exclusively alpha-cleavage and the BP triplet shows exclusively H-atom abstraction. Radical pairs are observed in beta-CDs by TREPR, consisting of either DOB or BP ketyl radicals with sugar radicals from the CD interior. The TREPR spectra acquired in CDs are substantially broadened due to strong spin exchange. The electron spin polarization mechanism is mostly due to S-T(0) radical pair mechanism (RPM) in solution but changes to S-T(-) RPM in the CDs due to the large exchange interaction. The TR-CIDNP results confirm the reactivity patterns of all three ketones, and DOB shows strong nuclear spin polarization from a novel rearrangement product resulting from the alpha-cleavage reaction.

Remote Sensitized Photoisomerization via Through‐Bond Triplet–Triplet Energy Transfer Mediated by a Salt Bridge in a Supramolecular Dyad

A supramolecular dyad, BP-(amidinium-carboxylate)-NBD is constructed, in which benzophenone (BP) and norbornadiene (NBD) are connected via an amidinium-carboxylate salt bridge. The photophysical and photochemical properties of the assembled BP-(amidinium-carboxylate)-NBD dyad are examined. The phosphorescence of the BP chromophore is efficiently quenched by the NBD group in BP-(amidinium-carboxylate)-NBD via the salt bridge. Time-resolved spectroscopy measurements indicate that the lifetime of the BP triplet state in BP-(amidinium-carboxylate)-NBD is shortened due to the quenching by the NBD group. Selective excitation of the BP chromophore results in isomerization of the NBD group to quadricyclane (QC). All of these observations suggest that the triplet-triplet energy transfer occurs efficiently in the BP-(amidinium-carboxylate)-NBD salt bridge system. The triplet-triplet energy transfer process proceeds with efficiencies of approximately 0.87, 0.98 and the rate constants 1.8x10(3) s(-1), and 1.3x10(7) s(-1) at 77 K and room temperature, respectively. The mechanism for the triplet-triplet energy transfer is proposed to proceed via a "through-bond" electron exchange process, and the non-covalent bonds amidinium-carboxylate salt bridge can mediate the triplet-triplet energy transfer process effectively for photochemical conversion.

Photoinduced oxidation of sea salt halides by aromatic ketones: a source of halogenated radicals

Abstract. The interactions between triplet state benzophenone and halide anion species (Cl

The use of molecular probes for the characterization of dispersions of functionalized silica nanoparticles

Butoxylated silica nanoparticles (BSN) were prepared by esterification of the silanol groups of fumed silica nanoparticles with butanol and characterized by 13C and 29Si NMR and thermogravimetry. The molecular probes benzophenone (BP) and safranine-T were used to investigate the BSN suspensions in water:acetonitrile. Laser flash-photolysis experiments at λ exc = 266 nm performed with BSN suspended in acetonitrile:aqueous phosphate buffer supported previous results of our group obtained by time-resolved phosphorescence experiments and showed that only free and adsorbed excited triplet states of BP and diphenylketyl radicals contribute to the signals. The UV–vis spectroscopic and photophysical properties of safranine-T are strongly solvent-dependent. Thus, the analysis of the emission spectra and fluorescence lifetimes yielded information on the localization of this probe molecule in suspensions of BSN and of the bare silica nanoparticles. The values of the equilibrium constant for the adsorption of the ground-state safranine-T on the particles were found to be (9.2 ± 0.8) × 10 4, (7.2 ± 0.8) × 10 5, and (3.0 ± 0.1) × 10 4 for the BSN in 1:1 acetonitrile:water, SiO 2 in 1:1 acetonitrile:water, and SiO 2 in acetonitrile, respectively.

Reactivity of adrenaline toward alkoxyl radicals and carbonyl triplet states

The reactivities of adrenaline toward hydrogen abstraction by the tert-butoxyl radical and the excited triplet state of benzophenone have been examined in solution using laser flash photolysis techniques. The rate constants obtained in acetonitrile-rich solvents are 13 and 1.5 x 10(8) M(-1) s(-1) for benzophenone triplet and the tert-butoxyl radical, respectively. Adrenaline is a better hydrogen donor than phenol, but not as efficient as alpha-tocopherol.

Benzophenone−Phenylthioacetic Acid Tetraalkylammonium Salts as Effective Initiators of Free-Radical Photopolymerization of Vinyl Monomers, Mechanistic Studies

The mechanism of the photooxidation of tetraalkylammonium salts of phenylthioacetic acid, C6H5−S−CH2−COO-N+R4, was investigated in detail in order to understand why they turned out to be efficient co-initiators of free-radical polymerizations. The photosensitizer was benzophenone (BP), and the alkyl substituents (R) were n-butyl, n-propyl, or ethyl. The techniques used were steady-state and nanosecond flash photolysis. It was shown that electron transfer from the sulfur atom to the benzophenone triplet state was the primary photochemical step followed by a decarboxylation reaction leading to CO2, the C6H5−S−CH2• radical, and a [BP•-···N+R4] ion pair. The latter underwent a Hofmann elimination (unexpected in these mild experimental conditions) leading to alkene-1 and trialkyl amine. The quantum yields of all the observed transients and the stable products were determined. The mechanism of primary and secondary photochemical reactions was quantitatively described, and it was shown to be similar for all of t...

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.

Synthesis and Characterization of Butoxylated Silica Nanoparticles. Reaction with Benzophenone Triplet States

Butoxylated silica nanoparticles (BSN) were prepared by esterification of the silanol groups of fumed silica nanoparticles with butanol. These particles were characterized by FTIR, BET, TEM, and TOC. BSN suspensions in water:acetonitrile mixtures were used as quenchers of benzophenone (BP) phosphorescence in time-resolved experiments at the excitation wavelengths of 266 and 337 nm. The phosphorescence signals obtained in the presence of the nanoparticles were fitted to biexponential decays. Both decays were accelerated in the presence of increasing amounts of BSN. A model including the reversible adsorption of BP on BSN and supported by computer simulations accounts for the observed results.

Modification of photochemical pathways of sensitized oxidation of phenylthioacetic acid: Effects of solvent and tetrabutylammonium salt

The mechanisms of triplet-sensitized electron transfer from thioether-containing aromatic carboxylic acids were investigated using steady state and laser flash photolysis in acetonitrile and aqueous solutions. Benzophenone (BP) and 4-carboxybenzophenone were used as the triplet sensitizers, and phenylthioacetic acid and its tetrabutylammonium salt were used as the electron-donating quenchers. In aqueous solution, (BP −⋯>S +) radical pairs (formed in the electron-transfer quenching of the triplet state of BP) decayed mainly via a charge-separation reaction leading to the formation of ketyl radical anions (BP −) and sulfur-centered radical zwitterions (>S +). The later transients underwent fast decarboxylation leading to the formation of alkyl-type radicals. In acetonitrile solutions, however, proton transfer between the radical ions was observed with the formation of ketyl radicals (BPH ). It was shown that the carboxylic and methylene groups in the phenylthioacetic acid moiety within the radical-ion pair are responsible for the proton-transfer reactions. The presence of counter cations from the tetrabutylammonium salt of phenylthioacetic acid can significantly modify the decay pathways of the photochemically produced radical-ion pairs in acetonitrile. In this case, the reaction leads to the formation of [BP −⋯N +(C 4H 9) 4] transients that can undergo Hofmann elimination leading to the formation of an alkene and tributylamine. These results indicate that the photochemical pathways (primary and secondary reactions) for the sensitized oxidation of phenylthioacetic acid depend on its state of ionization (that can be modified by the solvent used), properties of solvent, and the presence of associated counter cations from the tetrabutylammonium salt of phenylthioacetic acid.