Laser photolysis study of the triplet states of fulvic acids in aqueous solutions

The spectral and kinetic characteristics of short-lived intermediates formed during the photolysis of aqueous and alkaline (0.1 mol l^?1 NaOH) solutions of fulvic acids (FA) were studied by the nanosecond laser photolysis technique. Laser photolysis of FA at 337 nm leads to the formation of short-lived triplet states of FA (³FA) with a quantum yield of about 1% and different relatively long-lived intermediates (with decay rate constants in deoxygenated solutions of 1.8 × 10³–2.1 × 10⁵ and 80–160 s^?1, respectively), which are characterized by absorption spectra with maximums at λ ≤ 400 nm. ³FA are quenched by atmospheric oxygen with rate constants of 5.4 × 10⁸–1.1 × 10⁹ l mol^?1 s^?1. Introduction of phenols into the solutions at concentrations up to 0.1 mol l^?1 has no appreciable effect on the decay kinetics of the detected intermediate products of FA photolysis.     

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.     

Laser photolysis study of the triplet states of phthalocyanines on the surface of silica nanoparticles in aqueous solutions

Aluminum and zinc phthalocyanines (AlPc and ZnPc, respectively) adsorbed on the surface of silica nanoparticles (60 nm in diameter) in aqueous solutions have been found to form H-aggregates, which possess characteristic absorption spectra with bands (having a maximum at 640 nm) in a shorter wavelength region with respect to the main Q-band of the monomer (having a maximum at 670 nm). For AlPc on the surface, J-aggregates of two types (long-wavelength bands with maximums at 740 and 770 nm) are also observed. Using nanosecond laser photolysis (with the excitation wavelength of 337 nm) in deoxygenated solutions of AlPc on the surface, the formation of the triplet electronically excited states of J-aggregates has been detected, which are characterized by a broad absorption spectrum in the region of 400–800 nm and a lifetime of 360 μs. No intermediate products have been detected during the photolysis of H-aggregates of ZnPc on the surface.     

Laser photolysis study of triplet exciplexes of nitronaphthalenes with tertiary aromatic amines

Conclusions Spectrokinetic characteristics were found for the triplet exciplexes of nitronaphthalenes with tertiary aromatic amines. The state with complete charge transfer makes the major contribution to the structure of the triplet exciplexes. Solvation of the triplet exciplexes by methanol molecules is accompanied by a decrease in the rate of reverse charge transfer.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2367–2370, October, 1987.     

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.     

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.     

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.     

A laser photolysis study of the decay kinetics of the triplet states and radicals of flavins in the bovine eye lens

The decay kinetics of the triplet states and radicals of riboflavin and flavin mononucleotide, which were introduced into the bovine eye lens as kinetic photochemical probes, was studied by nanosecond laser photolysis, as a function of dilution of the lens with water. Correlations were found between the kinetic parameters and the degree of ordering of crystallins in the aqueous solution of the lens.     

Higher triplet excited states of benzophenones and bimolecular triplet energy transfer measured by using nanosecond-picosecond two-color/two-laser flash photolysis

The lifetimes of benzophenone in the higher triplet excited state (BP(T(n))) and several BP derivatives in the T(n) states were measured directly to be tau(T(n))=37+/-7 ps and 20-33 ps, respectively, by using the nanosecond-picosecond (ns-ps) two-color/two-laser flash photolysis method. Based on the direct measurements of tau(T(n)) of BP(T(n)), the triplet energy transfer (TET) from BP(T(n)) to quenchers (Q), such as carbon tetrachloride (CCl4), benzene (Bz), and p-dichlorbenzene (DCB), was investigated. The fast TET from BP(T(n)) to Q can be attributed to the lifetime-dependent quenching process, according to the Ware theoretical model of the bimolecular energy transfer reaction. The contribution of the lifetime-dependent term on k(TET) was 27, 60, and 86% for CCl4, Bz, and DCB as the Q of BP(T(n)), respectively, indicating that the TET from BP(T(n)) to Q is influenced not only by tau(T(n)), but also by the size of Q.     

Laser flash photolysis of 1,2-diketopyracene and a theoretical study of the phenolic hydrogen abstraction by the triplet state of cyclic alpha-diketones

Laser flash photolysis (LFP) studies, atoms in molecules (AIM) studies, and density functional theory (DFT) calculations have been performed in order to study the mechanism of the hydrogen abstraction by alpha-diketones in the presence of phenols. Laser irradiation of a degassed solution of 1,2-diketopyracene in acetonitrile resulted in the formation of a readily detectable transient with absorption at 610 nm, but with very low absorptivity. This transient decays with a lifetime of around 2 micros. The quenching rate constant for substituted phenols, kq, ranged from 1.10x10(8) L mol-1 s-1 (4-cyanophenol) to 3.87x10(9) L mol-1 s-1 (4-hydroxyphenol). The Hammett plot for the reaction of the triplet of 1,2-diketopyracene with phenols gave a reaction constant rho=-0.9. DFT calculations (UB3LYP/6-311++G**//UB3LYP/6-31G*) of the triplet complex ketone-phenol revealed that hydrogen transfer has predominantly occurred and that the reaction with alpha-diketones are generally 7 kcal/mol less endothermic than the respective reactions of the monoketones. These results together with the geometries obtained from the DFT calculations, natural bond order (NBO) analysis, and AIM results indicate that hydrogen abstraction for alpha-diketones is facilitated by the electrophilicity of the ketone, instead of neighboring group participation by the second carbonyl group.     

A laser photolysis study of the triplet exciplexes of quinones with 4-phenylaniline

Conclusions The rate constants were measured for the annihilation of the triplet exciplexes of quinones with 4-phenylaniline in various solvents and the prototropic equilibrium constants in the primary exciplex were also determined. Hydrogen bonding between the radicals in the exciplex leads to an acceleration of radiationless deactivation of the exciplex to the ground state and retardation of the dissociation of the exciplexes into radical-ions. The solvation of the exciplexes of an alcohol is accompanied by a decrease in the rate of deactivation of the exciplexes to the ground state.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2587–2590, November, 1986.     

Theoretical study of low-lying triplet states of aniline

Multireference complete active space self-consistent-field CASSCF(10,12)/ANO and second-order perturbation theory MS-CASPT2 calculations were performed to determine the vertical low-lying singlet and triplet states of aniline. The sequence of the seven lower lying triplet states is T1(1(3)A'), T2(1(3)A' '), T3(2(3)A'), T4(3(3)A'), T5(2(3)A' '), T6(4(3)A'), and T7(3(3)A' '). The 3(3)A', 4(3)A', and 3(3)A' ' states are assigned as 3s, 3py, and 3pz Rydberg states, respectively, while other states correspond to pi <-- pi excitations. Both the T1 and T2 states are found to be below at the lowest-lying singlet S1 (1(1)A' ') state. Geometry, vibrational modes, and electron distribution of the lowest lying T1 state were determined using UB3LYP calculations. The vertical and adiabatic singlet-triplet energy gaps DeltaE(S0-T1) amount to 3.7 and 3.5 +/- 0.2 eV, respectively. In clear contrast with the S0 state, the triplet aniline is no longer aromatic, and its protonation occurs preferentially at the ring meta-carbon site, with a proton affinity PA = 243 +/- 3 kcal/mol.     

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.     

Laser photolysis study of spectral and kinetic characteristics of the triplet state of all-trans-retinal in aqueous solutions of retinal-binding proteins and liposomes

Spectral and kinetic characteristics of the triplet state of all-trans-retinal ( ³ ATR) in aqueous solutions of bovine serum albumin (BSA), interphotoreceptor retinal-binding protein (IRBP), and phosphatidylcholine (PC) liposomes have been studied by nanosecond flash photolysis. In the protein solutions, the decay kinetics of ³ATP obeys the two-exponential law (with the rate constants of 5 × 10⁵ and 5 × 10⁴ s^?1). The introduction of liposomes into the protein solutions increases the contribution of the fast component and slows it down. The corresponding concentration dependence indicates that ATR remains localized in the proteins at the PC/protein molar ratio up to 100. The introduction of oxygen into the protein solutions has no effect on the rate constant of the slow component, but accelerates the fast component and increases its contribution. The efficiency of ³ATP quenching by molecular oxygen in the protein solutions is much lower than in water and liposomes. The photoprotective role of IRBP in the retinoid cycle is discussed.     

Laser photolysis study of the spectral-kinetic characteristics of short-lived triplet exciplexes and the mechanism of atomic hydrogen transfer in the course of 2,6-diphenyl-1,4-benzoquinone triplet quenching with aromatic amines

The spectral-kinetic characteristics of short-lived triplet exciplexes arising in the quenching of 2,6-diphenyl-1,4-benzoquinone triplet with aromatic amines: N,N,N′,N′-tetramethyl-p-phenylenediamine, triphenylamine and diphenylamine have been studied by means of the nanosecond laser photolysis technique. The absorption spectra of triplet exciplexes exhibit distinct maxima characteristic of the absorption spectra of corresponding amine radical cations. The state with complete charge transfer gives the basic contribution to the exciplex structures. A detailed analysis is presented of the kinetic and thermodynamic deactivation characteristics of triplet exciplexes in low-polar solvents.