Laser flash photolysis studies on radical formation by H-atom abstraction of triplet vitamin K3 and by H-atom transfer via the triplet exciplex between triplet methyldihydroxynaphthalene and benzophenone

Nanosecond laser photolysis techniques were incorporated to obtain (1) the absorption spectra and coefficients of triplet vitamin K₃ (2-methyl-1,4-naphthoquinone, MNQ) and its ketyl radical (2-methylnaphthosemiquinone, 2MNQH^*) in acetonitrile (ACN) as well as to reveal (2) the mechanisms for hydrogen atom abstraction of triplet MNQ (³MNQ^*) from phenol which proceeded in a diffusion process with an efficiency of unity. On the other hand, the hydroxymethylnaphthoxy radical was produced with the benzophenone ketyl radical (BPK) by the hydrogen atom transfer from triplet 2-methyl-1,4-dihydroxynaphthalene (MDHNp) sensitized by triplet benzophenone to benzophenone (BP) via the triplet exciplex. The question to be addressed was, which was produced in the MDHNp-BP system, the 2-methyl or 3-methylnaphthosemiquinone radical? Comparing the absorption spectrum and coefficient of the radical produced via the triplet exciplex with those of the 2MNQH^* obtained by H-atom abstraction of ³MNQ^*, the radical formed with BPK was revealed to be 2MNQH^*. The reasons for the preferable formation of 2MNQH^* are discussed for H-atom abstraction as well as the transfer reactions.     

Laser flash photolysis studies on the triplet state of aromatic hydrocarbons in the vapour phase

Triplet—triplet (T₁ → T_n) absorption spectra under low resolution and decay times have been measured for a variety of simple fluorine-substituted benzenes in the vapour phase. Assignments for the observed transitions were made on the basis of CNDO calculations and comparison with the corresponding ³B_1u → ³E_2g⁻ transition in benzene. Decay times measured were in the region 200 – 250 ns, orders of magnitude shorter than the corresponding decay time for benzene itself. Reasons for this shortening are briefly discussed.     

Intermolecular hydrogen atom abstraction from the fluoranil triplet excited state: a Raman study

Intermolecular hydrogen abstraction reaction mechanisms in photoexcited ketones have traditionally been studied using time resolved absorption spectroscopy. Another approach is presented involving time resolved resonance Raman spectroscopy to study such reactions, using the fluoranil/isopropanol system as an example. It has been shown that vibrational spectra can be recorded starting from the triplet excited state to the product state (radical anion) via the intermediate state, which is the ketyl radical. Thus, it is demonstrated that following the reaction evolution in terms of structural (vibrational modes) details would prove to be useful not only for mechanistic investigation but also for structure-reactivity correlations in photoexcited systems.     

Laser flash photolysis study of triphenylimidazole

Laser flash photolysis of the photocyclization of triphenylimidazole (TPI) in ethyl alcohol at 308 nm. indicates that the dihydrophenanthroimidazole (DHPI) intermediate is produced rapidly, has a lifetime of 0.25 ms, and returns predominantly back to triphenylimidazole. Analysis of the decay channels for this intermediate indicates two rate constants: (1) k1 = 3.3 x 10(3) s(-1), associated with reversion back to triphenylimidazole and (2) k2 = 0.67 x 10(2) s(-1), which is associated with the conversion of the dihydrophenanthroimidazole to the photoproduct, 2-phenyl-9,10-phenanthroimidazole. The photoproduct is readily observed as an increasing component in the biexponential fluorescence decay data. Fluorescence lifetimes for triphenylimidazole and 2-phenyl-9,10-phenanthroimidazole (PPI) in ethyl alcohol were determined to be 1.76 and 8.21 ns, respectively, with no additional components in the fluorescence decay as the photochemistry proceeds. An additional transient absorption observed in the 450 nm. region, with a lifetime of 0.7 micros, decaying faster than the dihydrophenanthroimidazole intermediate, is assigned to the triplet state of triphenylimidazole.     

Laser flash photolysis study of carboethoxynitrene

[reaction: see text] Laser flash photolysis (LFP, 266 nm) of carboethoxyazide produces a mixture of the ethoxycarbonyl radical (lambda(max) = 333 nm, tau = 0.4 micros, CF(2)ClCFCl(2), ambient temperature) and triplet carboethoxynitrene (lambda(max) = 400 nm, tau = 1.5 micros, CF(2)ClCFCl(2), ambient temperature). The carbon-centered radical is selectively scavenged by oxygen allowing sole observation of the triplet nitrene. We deduce that the singlet nitrene has a lifetime between 2 and 10 ns in CF(2)ClCFCl(2) at ambient temperature.     

A flash photolysis study of the triplet state quenching of anthracene by oxygen in the presence of polystyrene

The rate constant for the quenching of triplet anthracene by molecular oxygen in the presence or absence of polystyrene was measured. The quenching ability is not affected by the presence of polymer.     

CIS-TRANS isomerization of styrenes via the triplet pathway:The perpendicular triplet state observed by laser flash photolysis

The kinetics of the cis-trans photoisomerization of 1-phenylcyclohexene via the triplet state, studied by either nanosecond pulse radiolysis or laser flash photolysis in the presence of sensitizers, reveal that the triplet species involved in the isomerization mechanism has a lifetime of 55 ns in fluid solution at room temperature. A transient absorption decaying with the same 55 ns lifetime, and therefore assigned to this triplet species, was abserved in the 320–345 nm region. Quite similar triplet-triplet absorptions were observed with 1-phenylcycloheptene, 1-phenylpropene and styrene itself From the experimental results and from considerations of the energy surfaces of the excited states of styrene, the observed triplet species is identified as the perpendicular (or “phantom”) triplet state of the styrene moiety.     

Solvent effects in hydrogen abstraction from cholesterol by benzophenone triplet excited state

Hydrogen abstraction from the C-7 position of cholesterol (Ch) by triplet excited benzophenone (BZP) exhibits remarkable solvent-dependence in product studies. Kinetic measurements on the intramolecular version of the process in dyads containing covalently linked Ch and BZP units reveal important solvent effects and significant stereodifferentiation.     

Deprotonation and dimerization of maleimide in the triplet state: a laser flash photolysis study with optical and conductometric detection

The photochemistry of maleimide in aqueous solution is governed by the coexistence of up to three different triplet states, the keto triplet (lambda(max)=250, 330 nm, lambda(min)=290 nm, pK(a)=4.4+/-0.1, tau=5 micros), the deprotonated or enolate triplet (lambda(max)=360, 260 nm, lambda(min)=320 nm, shoulder at 370-380 nm) and a dimer triplet. This biradical is formed by the addition of the keto triplet to the double bond of a ground state maleimide in competition with electron transfer, (k( (3)MI+MI)=2.6 x 10(9) dm(3) mol(-1) s(-1)). Its spectrum is identical to that of the maleimide H-adduct radical (lambda(max)=370-380 (broad), 255 nm (narrow), lambda(min)=290 nm) and its lifetime is 110 ns. While protolysis is confined to maleimide and aqueous solutions, the dimer triplet is also found in acetonitrile. Dimer triplet formation is also observed with N-ethylmaleimide. Time-resolved conductometry and buffer experiments were used to characterise excited state protolysis. Multi-wavelength "global analysis" of the time profiles allowed the separation of the transient spectra and study of the kinetics of the monomer and dimer triplets. The cyclobutane dimer yield (determined by GC) is independent of maleimide concentration. This indicates that the dimer triplet does not contribute significantly to the initiation of free-radical polymerisation. Time-dependent Hartree-Fock calculations agree with the experimental data and further confirm the proposed mechanisms.     

Laser flash photolysis study of triplets of cyclobutanethiones

Five cyclobutanethiones with different chromophores at the 3-position were examined for triplet state behaviour in benzene using laser excitation into their low lying nπ^*1 band systems. A weak transient absorption attributable to the triplet state is observed in all these cases. Results concerning triplet lifetimes, intersystem crossing yields (S₁ → T₁), self-quenching kinetics and kinetics of energy transfer to all-trans-1,6-diphenyl-1,3,5-hexatriene and oxygen and quenching by di-t-butyl nitroxide (DTBN) are presented. Intersystem crossing yields estimated with reference to p,p′-dimethoxythiobenzophenone are roughly unity in all five cases. Self-quenching rates are found to be less than diffusion limited and this is attributed to steric crowding at the α positions (dimethyl group). The rates of oxygen and DTBN quenching compare well with those reported for several other thiones in the literature. No transients other than the triplet were detected in the above cyclobutane-thiones.     

Hydrogen atom abstraction and electron transfer of triplet 2,3-dihalo-1,4-naphthoquinones studied by the cidep technique and laser flash photolysis

Investigations using CIDEP techniques showed that an H-atom abstraction from phenol and the electron transfer from 1,2,4,5-tetramethoxybenzene to photoexcited 2,3-dibromo-1,4-naphthoquinone (DBNQ) and 2,3-dichloro-1,4-naphthoquinone (DCNQ) in the polar media originated from their triplet states. Nanosecond laser photolysis at 355 nm was carried out to determine the absorption spectra and coefficients of the corresponding triplet states, semiquinone, and anion radicals for a quantitative investigation of the mechanisms involved in the H-atom abstraction and the electron transfer. The steric hindrance of the substituted groups was indifferent to H-atom abstraction. The electronic structures of triplet DBNQ and DCNQ at 295 K were both revealed to be the mixed states of ³(n,π^*) with a ³(π,π^*) character.     

Laser photolysis study of the triplet states of nitroaromatic amides

The spectrokinetic indices of the triplet states of nitroaromatic amides were established. The quenching of triplet states by tertiary aromatic amines in apolar solvents leads to the formation of triplet exciplexes. A charge transfer state makes the major contribution to the structure of these exciplexes. Hydrogen atom transfer proceeds efficiently in the case of secondary aromatic amines, leading to the formation of the corresponding neutral radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1912–1915, August, 1989.     

Laser photolysis study of chloranil triplet exciplexes with stilbene

Absorption spectra and decay kinetics of the polar triplet exciplexes (contact radical-ion pairs) formed during quenching of the chloranil triplet state by trans- or cis-stilbenes in benzene with added acetonitrile and methanol, have been studied by laser flash photolysis. The exciplexes include cation-radicals of stilbene dimers, which are deactivated by reverse electron transfer within 10–50 nsec. The dynamics of the intercombination electron transfer and the exciplex dissociation into ion-radicals were determined. The isomerization of stilbene via triplet exciplex formation was not observed.N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow 117977. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 572–576, March, 1992.     

A combined laser flash photolysis,density functional theory and atoms in molecules study of the photochemical hydrogen abstraction by pyrene-4,5-dione

The photochemical hydrogen abstraction reaction of pyrene-4,5-dione (3) has been investigated by laser flash photolysis. Excitation (λ = 355 nm) of a degassed solution of 3 in acetonitrile resulted in the formation of a detectable transient with absorption maxima at 380 and 470 nm. This transient decays with a lifetime of around 4.8 μs and is quenched by oxygen. This transient is most probably a triplet state of 3. Addition of hydrogen donors, such as 2-propanol; 1,4-cyclohexadiene or 4-methoxyphenol led to the formation of a new transient with λ_max at 380, 500 nm and a broad absorption at 640 nm. This new transient slowly decays with second order kinetics and was assigned to the semiquinone radical obtained from the hydrogen abstraction reaction. Using DFT and AIM calculations the reactivity of 3 and 9,10-phenanthrenequinone (1) is best interpreted as a proton coupled electron transfer like mechanism for the hydrogen abstraction from 2-propanol.     

Reactivity of aromatic amines with triplet 1,8-dihydroxyanthraquinone: a laser flash photolysis study

The property of the lowest excited triplet states of 1,8-dihydroxyanthraquinone (DHAQ) was investigated by using time-resolved laser flash photolysis at 355nm in organic solvents, i.e. acetonitrile and cyclohexane. The transient absorption spectra of the excited triplet DHAQ were obtained in acetonitrile, which have an absorption maximum at 480nm and two broad absorption bands around 350 and 650nm. 3DHAQ(*) is efficiently quenched by triphenylamine (TPA) via photoinduced electron transfer pathway, which was testified by the finding of TPA radical cation. In addition, aniline derivatives such as N,N-dimethylaniline (DMA), 3,5,N,N-tetramethylaniline (TMA), 4-dimethylaminobenzoic acid (DMABA) and dimethyl-p-toluidine (DMT) could also quench 3DHAQ(*) rapidly. Evidence for electron transfer interaction with anilines in acetonitrile was obtained from transient spectral characterization of formed radicals. Experimental k(q) values approach the diffusion-controlled rate limit, and decrease significantly from DMT (1.85x10(10)M-1s-1) to DMABA (1.95x10(9)M-1s-1). These k(q) values depend on the charge density on the "N" atom of anilines, which could be quantified by Hammett sigma constant.     

Laser flash photolysis of tert-butyl aroylperbenzoates: kinetics of the singlet and triplet states and the aroylphenyl radicals

tert-Butyl aroylperbenzoates (1-4) were studied by laser flash photolysis (LFP). LFP (380 nm, pulse width approximately 350 fs) of 2 and 3 allowed direct observation of their singlet states, which showed broad absorption (lambda(max) approximately 625 nm; tau approximately 20 and approximately 7.9 ps, respectively). The triplet state of each (lambda(max) approximately 530-560 nm) rapidly dissociates by O-O cleavage as indicated by the short triplet lifetimes (e.g., triplet lifetime of 3 approximately 0.74 ns). The approximately 550 nm absorption obtained from the 355 nm LFP (pulse width approximately 7 ns) of 1, 2, and 4 has been assigned to the corresponding aroylphenyl radicals. Two representative radicals (4-benzoylphenyl 5 and 3-(4'-methylbenzoyl)phenyl 6) investigated in detail showed solvent-dependent lifetimes. Absolute bimolecular rate constants of reactions of these radicals with various quenchers including double-bond-containing monomers have been observed to range from 7.56 x 10(7) to 1.68 x 10(9) M(-1) s(-1) in CCl(4) at room temperature. A possible structure of the aroylphenyl radicals and the transition responsible for the 550 nm absorption are discussed.     

Direct ab initio MD study on the hydrogen abstraction reaction of triplet state acetone from methanol molecule

Solvent re-orientation process of triplet acetone/methanol complex and intermolecular hydrogen atom abstraction reaction on the triplet state energy surface, (CH₃)₂C=O (T₁) + CH₃OH → (CH₃)₂C–OH + CH₂OH in gas phase, have been investigated by means of density functional theory (DFT) and direct ab initio molecular dynamics (MD) methods. The static DFT calculation of hydrogen abstraction reaction at the T₁ state showed that the transition state is 16.4 and 30.9 kcal/mol lower than the energy levels of S₁ and S₂ states, respectively, and 9.2 kcal/mol higher than the bottom of T₁ state. The product state, (CH₃)₂C–OH⋯CH₂OH, is 8.4 kcal/mol lower in energy than the level of T₁ state. The direct ab initio MD calculation showed that the product is rapidly formed within 150 fs and the separated products (CH₃)₂C–OH + CH₂OH were formed. The mechanism of reaction dynamics of the triplet acetone/methanol complex was discussed on the basis of theoretical results.