Marek Mac

Photochemical reaction mechanisms of 2-nitrobenzyl compounds: 2-Nitrobenzyl alcohols form 2-nitroso hydrates by dual proton transfer

Irradiation of 2-nitrobenzyl alcohol (1, R = H) and 1-(2-nitrophenyl)ethanol (1, R = Me) in various solvents yields 2-nitroso benzaldehyde (4, R = H) and 2-nitroso acetophenone (4 R = Me), respectively, with quantum yields of about 60%. The mechanism of this reaction, known since 1918, was investigated using laser flash photolysis, time-resolved infrared spectroscopy (TRIR), and 18O-labeling experiments. The primary aci-nitro photoproducts 2 react by two competing paths. The balance between the two depends on the reaction medium. Reaction via hydrated nitroso compounds 3 formed by proton transfer prevails in aprotic solvents and in aqueous acid and base. In water, pH 3-8, the classical mechanism of cyclization to benzisoxazolidine intermediates 5, followed by ring opening to carbonyl hydrates 6, predominates. The transient intermediates 3 and 6 were identified by TRIR. Potential energy surfaces for these reactions were mapped by density functional calculations.

The photo-Favorskii reaction of p-hydroxyphenacyl compounds is initiated by water-assisted, adiabatic extrusion of a triplet biradical.

The p-hydroxyphenacyl group 1 is an effective photoremovable protecting group, because it undergoes an unusual photo-Favorskii rearrangement concomitant with the fast release (<1 ns) of its substrates in aqueous solution. The reaction mechanism of the diethyl phosphate derivative 1a was studied by picosecond pump−probe spectroscopy, nanosecond laser flash photolysis, and step−scan FTIR techniques. The primary photoproduct is a triplet biradical, 33, with a lifetime of about 0.6 ns. The release of diethyl phosphate determines the lifetime of the triplet state T1(1a), τ(T1) = 60 ps in wholly aqueous solution. Formation of a new photoproduct, p-hydroxybenzyl alcohol (6), was observed at moderate water concentrations in acetonitrile. It is formed by CO elimination from the elusive spirodione intermediate (4), followed by hydration of the resulting p-quinone methide (5). Computational studies show that CO elimination from the spirodione is a very facile process.

Deriving intrinsic electron-transfer rates from nonlinear Stern—Volmer dependencies for fluorescence quenching of aromatic molecules by inorganic anions in acetonitrile

The fluorescence quenching of aromatic molecules by inorganic anions in acetonitrile was investigated by stationary fluorescence measurements. The dependence of the quenching rate constants, kq on the free energy of electron transfer, ΔG0et, obeys the well known Rehm—Weller expression which indicates that electron transfer is responsible for fluorescence quenching. Nonlinear Stern—Volmer behaviour was observed in many cases especially at very negative ΔG0et (< −0.7 eV). Distance-averaged electron-transfer rate constants, 〈ket〉, were determined from the concentration dependence of the fluorescence quenching rates using a procedure developed by Keizer. In three cases these values can be compared with those obtained by analysis of the transient effect seen directly in picosecond time-resolved fluorescence decay curves.

Mechanisms of fluorescence quenching of aromatic molecules by potassium iodide and potassium bromide in methanol–ethanol solutions

Fluorescence quenching of 1-cyanonaphthalene, 2-cyanonaphthalene, 1-methoxynaphthalene, 2-methoxynaphthalene, and fluoranthene by iodide and bromide ions in methanol–ethanol solutions was measured between 293 and 333 K. The dependences of the quenching rate coefficients on the Gibbs free energy (ΔG) of the electron-transfer reaction and temperature are analysed on the basis of: (i) Rips–Jortner and Onuchic theories of electron-transfer reactions in fluid solutions, (ii) numerical evaluation of diffusion limitation of the rate coefficient. The results show clearly that the mechanisms of the fluorescence quenching of aromatic molecules by halide ions are different for negative ΔG(electron-transfer mechanism) and positive ΔG(heavy atom effect). For ΔG < – 0.25 eV, the activation energies of the quenching process show dependences that can be modelled using the Onuchic formula for the electron-transfer rate and the diffusion model for molecular transport.

Salt effects on the reactions of radical ion pairs formed by electron transfer quenching of triplet 2-methyl-1,4-naphthoquinone by amines. Optical flash photolysis and step-scan FTIR investigations

Electron transfer quenching of triplet 2-methyl-1,4-naphthoquinone by amines in acetonitrile was investigated by means of nanosecond flash photolysis and step-scan infrared spectroscopy. With 2,5-di-tert-butylaniline as the electron donor, proton transfer following the primary electron transfer process forms neutral radicals, which were identified by comparing the resulting transient IR absorption spectra with those obtained in the presence of phenol as the quencher. Addition of lithium perchlorate promotes the formation of free radical ions. The salt effect on the reactions of the nascent radical ion pairs formed by electron transfer is discussed in terms of Eigen theory. The quinone radical anion strongly associates with lithium cations. The salt retards the diffusional recombination of the free radical ions, as predicted by the Debye-Smoluchowski equation. The observed, predictable salt effects provide a useful diagnostic tool for elucidation of the reaction mechanism.

Fluorescence quenching of derivatives of anthracene by organic electron donors and acceptors in acetonitrile. Electron and proton transfer mechanism

Abstract Fluorescence quenching of anthracene derivatives by organic electron donors (amines) and acceptors was investigated using stationary fluorescence measurements. The dependence of log(kq) on ΔGet shows Rehm-Weller-type behavior. The formation of anion radicals of anthracene, bianthryl, and 9-cyanoanthracene was detected by flash photolysis in systems containing aromatic amines (aniline, 2-bromoaniline, 4-bromoaniline, N,N-dimethylaniline, 4-bromo-N,N-dimethylaniline, N,N-diethylaniline, and 1,4-diazabicyclo[2.2.2]octane). The radical yields decreased and triplet yields increased when bromo derivatives of amines were used as donor quenchers, indicating the heavy-atom effect on spin conversion within radical pairs. The importance of the heavy-atom effect decreased when the energy gap between the charge transfer and molecular triplet states was small. The formation of separated radicals decreased when primary amines were used as quenchers which indicated the existence of an additional path of deactivation of the radical pair. The behavior of amines as quenchers of bianthryl and anthracene is compared with that of inorganic anion quenchers.

New Fluorescent Sensors Based on 1H-pyrazolo(3,4-b) quinoline Skeleton

Novel fluorescing dyes 1,3,4-triphenyl-6-(1,4,7,10-tetraoxa-13-aza-cyclopentadec-13-ylmethyl)-1H-pyrazolo[3,4-b]quinoline (K1) and 2-[(2-hydroxyethyl)-(1,3,4-triphenyl-1H-pyrazolo[3,4-b]quinolin-6-ylmethyl)-amino]ethanol (L1) have been synthesized and investigated by the means of steady state and time-resolved fluorescence techniques. These compounds act as sensors for the fluorescence detection of small inorganic cations (lithium, sodium, barium, magnesium and calcium) in solvents of different polarities (THF and acetonitrile). The mechanism, which allows application of these compounds as sensors, is an electron transfer from the electro-donative part of molecule to the acceptor part (fluorophore), which is retarded upon complexation of the electro-donative part by inorganic cations. We found that crown ether-containing compound is very sensitive to the addition of any investigated ions but amino alcohol-containing one exhibits better selectivity to the addition of two-valued cations. Two kinds of the complexes (LM+ and L2M+) were found in the investigated systems. In addition, the dyes may be used as fluorescence indicators in solvents of lower polarity like tetrahydrofuran.

Investigations of the heavy atom effect occurring in bianthryl and 10,10′-dibromobianthryl. Fluorescence, cyclovoltamperometric and actinometric studies

A preliminary study of photophysical and photochemical properties of 9,9′-bianthryl (BA) and 10,10′-dibromo,9,9′-bianthryl (DBrBA) is presented. Dual fluorescence occurring in bianthryl (BA) has been investigated in solvents containing heavy atoms, such as chlorine, bromine and iodine. The presence of heavy atoms reduces strongly the fluorescence lifetimes and the fluorescence quantum yields of BA, the effect is strongest in ethyl iodide, i.e. in the solvent containing the heaviest atom, iodine. On the other hand, it has been found that introduction of two heavy atoms (bromine) into the bianthryl molecule modifies noticeably its fluorescence properties, which indicates existance of an internal heavy atom effect. Again, a reduction of the fluorescence lifetime and the fluorescence quantum yield, compared to the parent molecule, has been observed. On the basis of these observations the mechanism of 1CT ⇒ 3LE intersystem crossing has been discussed. In BA the rate determining process is a reversible spin inversion within the radical pair. In DBrBA where the spin flip is accelerated by the presence of heavy atoms (bromines) the spin-allowed electron transfer 3CT ⇒ 3LE becomes important in the overall intersystem crossing process. It has been found that DBrBA undergoes a photoreduction in the presence of aromatic donors, such as amines, leading finally to BA. Similar reaction seems to be observed in electrochemical measurement. The reduction of DBrBA originates from the primary electron transfer process either photoinduced or electrochemical leading to the free anion radicals of DBrBA.

Temperature dependence of bianthryl dual fluorescence

The fluorescence of bianthryl in ethyl acetate and acetone has been investigated in dependence on the temperature. The variation of temperature changes the ratio of the locally excited ( 1 LE) and the charge transfer ( 1 CT) fluorescence quantum yields, which parallels the dielectric properties of the solvent. Thermodynamic functions ðDGCT; DSCTÞ of the excited state charge transfer process of bianthryl in ethyl acetate and acetone have been determined from equilibrium constants ð 1 LE () 1 CTÞ accessible from the fluorescence data. Charge-transfer induced intersystem crossing occurring in BA is slightly temperature dependent in both solvents, which indicates that the ISC rate constant is mainly governed by the spin inversion in the anthracene subunits of bianthryl. 2002 Published by Elsevier Science B.V.

Quenching of exciplex fluorescence by lithium perchlorate in acetonitrile

Abstract Fluorescence quenching of exciplexes, formed by diffusion from excited electron acceptors such as 9-cyanoanthracene and 9,10-dicyanoanthracene, and electron donors such as biphenyl, durene and naphthalene, by lithium perchlorate, in acetonitrile has been measured using fluorescence techniques. It has been found that lithium perchlorate is an efficient quencher of bimolecular charge transfer forms such as exciplexes. The quenching efficiency depends strongly on the charge transfer contribution in the overall exciplex. The Stern-Volmer plots constructed from the exciplex lifetimes or exciplex quantum yields possess negative curvatures. This may be explained on the basis of the partial dissociation of lithium perchlorate in acetonitrile.