Excited Triplet State Of Benzophenone Research Articles

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.

Effect of SiO2 and zeolite surfaces on the excited triplet state of benzophenone, BT; a spectroscopic and kinetic study.

Laser flash photolysis has been used to study the triplet excited state of benzophenone B(T), on various surfaces, SiO(2), zeolites NaY, KY, NaX and KX, and in rigid media at room temperature, polyethylene and polymethylmethacrylate. The studies point to similarities of the spectroscopy and kinetics of B(T) in fluid solution, in a solid matrix (polymers) and on a SiO(2) surface. However, stark differences are observed for B(T) in zeolites where the absorption spectrum mimics that of the protonated ketone, and the reactivities of B(T) with C(6)H(12) and CH(3)OH are an order of magnitude smaller than those in liquid C(6)H(12) and CH(3)OH. Inclusion of ammonia, which blocks acidic sites in the zeolite, produces a triplet spectrum which is similar to that in polar solution. The reactivity of the triplet with ammonia in a zeolite is also comparable to that observed for this reaction in polar solution. These data are discussed in terms of the interaction of benzophenone with acidic sites in the zeolites, and to restrictions placed on the reactants in the zeolite cages. The blocking of the zeolite acidic sites by ammonia produces spectral and kinetic data (reactivity with NH(3)) of the triplet that are comparable to those observed in solution. This is one of the few cases where zeolites inhibit rather than promote reactions of a solute adsorbed in them.

Photoinitiated grafting of maleic anhydride onto polypropylene

AbstractThe photoinitiated grafting of maleic anhydride (MAH) onto polypropylene with the use of benzophenone (BP) as the initiator has been investigated. In comparison with the process of thermally initiated grafting with peroxide as the initiator, photoinitiated grafting affords a higher grafting efficiency. The efficient photografting sensitized by BP can be explained by two possible mechanistic processes: the sensitization of the formation of the excited triplet state of MAH by BP and electron transfer followed by proton transfer between MAH and the benzopinacol radical, which may operate together. In the former case, the generated MAH excited triplet state abstracts a hydrogen from the polymer substrate to initiate grafting. A rate constant of 3.6 × 109 M −1 s −1 has been determined by laser flash photolysis for the process of quenching the excited triplet state of BP with ground‐state MAH. In comparison, the rate constant for the quenching of the excited triplet state of BP by hydrogen abstraction has been determined to be 4.1 × 105 M −1 s −1. In a study of photografting using a model compound, 2,4‐dimethylpentane, as a small‐molecule analogue of polypropylene, the loss of BP was significantly reduced upon the addition of MAH, and this is consistent with the proposed mechanistic processes. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1953–1962, 2004

Excited Donor–Acceptor Complexes in the Polymerization of Vinyl Monomers

Butyl methacrylate was found to affect the composition of radical intermediates formed in the photoreduction of benzophenone with triethylamine. In the presence of the monomer, the yield of free radicals decreased and the yield of complexes of the geminate radical pair increased. This was explained by the formation of excited ternary complexes resulted from the interaction of the excited triplet state of benzophenone with the ground-state complex of butyl methacrylate and triethylamine. The substituent effect in benzophenone on the stability of the radical complex was studied. The reaction rate constant for the decay of the radical complex was correlated with the Hammett σ0c constant that determines the mesomeric effect of the substituent.

Novel Photocycloaddition of Aromatic Ketones to Cyclic Oligosilanes

Abstract Photolysis of aromatic ketones (2,2,2-trifluoroacetophenone, acetophenone, and benzophenone) in the presence of cyclic oligosilanes afforded two types of cycloadducts depending on the nature of the ketone employed. The quenching of the excited triplet state of benzophenone by oligosilanes was investigated by laser flash photolysis.

Photoinitiators and photoinitiation, 12. The aromatic ketone/t‐amine Type‐II photoinitiating system. Identification of the initiating species

AbstractThe system benzophenone/N,N‐dimethylaniline (DMA) representing a typical Type‐II photoinitiating system is irradiated in the presence of 1,1‐di(p‐tolyl)ethylene (DTE) as a non‐polymerising model substrate. In the presence of DMA the excited triplet state of benzophenone is not quenched by DTE and N‐[3,3‐di(p‐tolyl)propyl]‐N‐methylaniline is isolated as the major product, indicating the DMA derived radical as the main initiating species in such Type‐II systems. The mechanism of the reaction and its implications are discussed.

Absolute magnitude of spin polarization in the radical-triplet pair mechanism: CIDEP generation by level crossings in a triplet-doublet interaction

Chemically induced dynamic electron polarization (CIDEP) generated through the interaction of the excited triplet state of benzophenone with the 2,2,6,6,-tetramethyl-1-piperidinyloxyl (TEMPO) radical was investigated by time-resolved ESR spectroscopy in benzene solution. We carefully examined what factors control the CIDEP intensities. By comparing the CIDEP intensity of TEMPO obtained in the triplet benzophenone-TEMPO system with the intensity in the C 60-TEMPO system, the absolute magnitude of net emissive polarization was determined to be −6.9 in the unit of Boltzmann polarization. The emissive polarization is attributed to state mixing between a quartet and a doublet in the radical-triplet pair induced by the zero-field splitting interaction of the counter triplet molecule. Our result is quantitatively explained by the theory that the net CIDEP is generated predominantly in regions where the quartet and doublet levels cross. This indicates that the quenching of the excited triplet benzophenone by TEMPO in benzene can proceed via an electron-exchange interaction.

Photoinitiated crosslinking of low-density polyethylene. 7. Initial radical reactions with model compounds studied by spin-trapping ESR spectroscopy

The initial radical reactions in the photoinitiated cross-linking of n-paraffins (lignocerane and dodecane) and tertiary (isooctane and 3-ethylhexane) and unsaturated (1-hexene, cis- and tans-3-hexene) hydrocarbons as LDPE model compounds in the presence of benzophenone (BP) as photoinitiator have been studied using 2,4,6-tert-butylnitrosobenzene (BNB) as a spin trap. Several kinds of initial radicals were identified. These photoinitiated radicals, which combine to form cross-links, are of tertiary, secondary and primary carbon types formed by hydrogen abstraction from the model compounds by the excited triplet state of BP

Photothermal beam deflection study of energy transfer from excited triplet states in fluid solution

Photothermal beam deflection spectroscopy was used to follow the decay of the lowest excited triplet state of benzophenone including energy transfer to biphenyl. Because of the simple step function time response of the beam deflection signal, we were able to extract two exponential functions from the heat accumulation waveform. From the results, we obtained a triplet—triplet energy transfer rate from benzophenone to biphenyl of k q = (4.6 ± 0.8) × 10 8 M −1 s −1. The triplet energy of biphenyl was also extracted from the results: E AT = 67 ± 0.5 kcal mol −1. From an analysis of the calorimetric amplitudes, we report the first photothermal observation of an endothermic energy transfer process from the triplet excited state of biphenyl to ground state benzophenone.

UV-excited transient raman spectra and the co stretching frequencies of the lowest excited triplet state of benzophenone

Transient resonance Raman spectra of T 1 benzophenone (BP) and its carbonyl- 18O-substituted analogue were measured with 355 nm excitation which is in resonance with a triplet-triplet (T n ←T 1) transition in the ultraviolet. The intensity of the CO stretch band was greatly enhanced under this resonance condition. Reliable values of the T 1 CO stretching frequency were obtained including the solvent shifts. It is concluded that the solvent dependence of the photochemical reactivity of BP is not primarily due to the solvent-induced structural changes in the T 1 state. The character of the two different T-T transitions (T n ←T 1 and T n ←T 1) is discussed in relation to the observed Raman spectral changes with different resonance conditions.

Time-resolved EPR studies of the benzophenone—diphenyl kethyl radical system. Possible evidence for quartet—doublet intersystem crossing

The photoreduction of benzophenone by several substrates has been studied by time-resolved EPR. When cyclohexadiene, MTHF, and decane are employed as hydrogen donors to the excited triplet state of benzophenone the diphenyl ketyl radical which is produced exhibits emissive electron spin polarization with rise times on the order of a few microseconds. It is suggested that the experimental observations can be explained by intersystem crossing to a quartet state in the excited ketyl radical.

Detection of deuterium quadrupole splittings by odendor in the excited triplet state of benzophenone

Using a fully deuterated sample of benzophenone, optically detected ENDOR transitions reveal line splittings due to deuterium quadrupole interaction in the excited triplet state. The line splittings were used to determine the ring-carbonyl-ring bond angle 2θ = 138(1.5)° and the phenyl-ring twist ansle ψ = 28.5(1.5)° of benzophenone in its metastable excited state.

Photo-oxidation of polymers—III: Molecular weight changes in the photolysis and photo-oxidation of polystyrene

Changes of molecular weight distribution resulting from vacuum photolysis and photo-oxidation of polystyrene have been determined by gel permeation chromatography. On irradiation at 253.7 nm, crosslinking is predominant in vacuo; in the presence of oxygen, crosslinking and main chain scission occur simultaneously because of light absorption by two or more different chromophores. Main chain scission is more important than crosslinking in the photolysis and photo-oxidation of polystyrene containing cumene hydroperoxide irradiated at 313 nm or containing benzophenone irradiated in the range 320–420 nm. In this last case, main chain scission and crosslinking are both strongly inhibited if the sample contains napthalene which acts as quencher of the excited triplet state of benzophenone. Only moderate inhibition is observed in the presence of 2,6-di- t-butyl-4 methylphenol.

Intermolecular Hydrogen Transfer Reactions in the Excited States of EDA-Complexes of Benzophenone with Aromatic Amines at Low Temperature

Abstract Electronic absorption and emission spectra of the mixtures of benzophenone and aromatic amines have been studied in rigid solutions. From absorption spectral measurements, it has been concluded that benzophenone as an acceptor forms the molecular complexes with various aromatic amines as donors. Emission spectra different from those of the component molecules were observed at longer wavelengths in the same systems. The maxima of the anomalous emission spectra of these systems were found to shift slightly to lower frequencies with the decrease in the ionization potentials of donor molecules. On account of their decay times, the emissions may be ascribed to the phosphorescence of benzophenone-aromatic amine complexes. Photolysis of benzophenone-aromatic amine complexes in rigid matrices always resulted in the appearance of the visible absorption spectrum of benzoketyl radical, together with ESR spectra, the analysis of which indicated that there is an interacting pair of radicals which is in the combined triplet (S=1) state. From the evaluation of the zero field splitting constants, the average distances between the interacting radicals were estimated. These results suggest that intermolecular hydrogen atom transfer from aromatic amines to benzophenone occurs, yielding the ketyl and aromatic amine radicals which lie close to each other. The reaction was found to proceed by way of the lowest excited triplet state of benzophenone.