Dominique Lavabre

Photochemical Behavior of a New Long-chain UV Absorber†, Derived from 4-tert-Butyl-4′-Methoxydibenzoylmethane¶

A new W filter, the 1‐(4‐tert‐butylphenyl)‐2‐decanyl‐3‐(4′‐methoxyphenyl)‐propane‐1,3‐dione, called C10‐DBM, was prepared by grafting a 10‐carbon aliphatic chain to the acarbonyl position of 4‐tert‐butyl‐4′‐methoxydibenzoylmethane (BM‐DBM), a well‐known and often used UV filter. The UV‐A absorption efficiency of organic solutions containing the new filter was tested and compared with identical solutions containing BM‐DBM with or without irradiation (xenon lamp). The originality of this new filter is that its UV‐A absorbance appeared during irradiation of the molecule. Although the molar absorption coefficient of C10‐DBM in the UV‐A domain was lower than that of BM‐DBM, the solutions absorption exhibited a much more photostable behavior under irradiation. In this study, we first demonstrated that C10‐DBM was a precursor of BM‐DBM (enol isomer) by means of high performance liquid chromatography followed by mass spectrometry. Indeed, we showed that the UV‐A absorption of C10‐DBM solutions appearing during the irradiation of the molecule was due to a Norrish‐II reaction (β‐cleavage), which induced the release of the BM‐DBM enol form and l‐decene. Then, we established a kinetic model for the photochemistry of C10‐DBM and fitted the variation of W absorption spectra to confirm the proposed mechanism.

Kinetic modelling of the photochromism and photodegradation of a spiro[indoline-naphthoxazine]

The photochromism of the 1,3,3-trimethylspiro[indoline-naphthoxazine] A has been studied under continuous monochromatic irradiation (313 and 365 nm) in liquid toluene solution at 278 K. The values of the main photochromic parameters (quantum yields and absorption coefficients) have been determined by kinetic modelling of absorbance (Abs) versus time curves recorded under continuous irradiation. We showed that the quantum yields of both photocoloration and reversible photochemical bleaching were wavelength-dependent. Under 365 nm irradiation, photodegradation processes that are not sensitive to the presence of oxygen were observed. The degradation occurs either from the direct irradiation of the closed spiro form A or from the thermal decomposition of the open photomerocyanine form B. Under our experimental conditions, photochemical decomposition of the open photomerocyanine form B was negligible.

Kinetic Insight into Specific Features of the Autocatalytic Soai Reaction

The addition of diisopropylzinc to prochiral pyrimidine carbaldehydes (Soai reaction) is the onlyknown example of spontaneous asymmetric synthesis in organic chemistry. It serves as a model systemfor the spontaneous occurrence of chiral asymmetry from achiral initial conditions. This review describesthe possible kinetic origin of specific experimental features of this reaction. It is shown that generickinetic models, including enantioselective autocatalysis and mutual inhibition between the enantiomers,are adequate to describe a variety of the astonishing properties of the Soai reaction. Namely, theseare the unprecedented strong chiral amplification, extreme sensitivity to the presence of very small amountsof chiral initiator, mirror-symmetry breaking when starting from achiral conditions, and the reversal ofenantioselectivity of chiral catalysts by addition of achiral additives. The described kinetic approachallows a closer insight into the nonlinear dynamics of the Soai reaction. It also reveals that a numberof open questions concerning the detailed reaction mechanism are still to be solved and that further experimentalstudies are required.

Single run benzophenone-benzhydrol actinometry in an exponential dilution photochemical microreactor

Abstract The photoreduction of benzophenone by benzhydrol was investigated in a continuous flow stirred photochemical microreactor. In this system photoreduction can be carried out in combination with the exponential dilution of benzhydrol. The rate of consumption of benzophenone in the photochemical reaction is matched to its rate of input to the microreactor. The effect of benzhydrol concentration on the quantum yield of the photoreduction can be obtained from kinetic analysis of the slow transient behaviour during dilution; also the system can be used for actinometry. In contrast to the classical method in which the concentration of benzhydrol and the irradiation time must be varied incrementally, the reaction parameters vary continuously giving accurate results in a single run. The method is general and can be employed in other areas of photochemistry when a study of concentration effect is needed.

Biscrown fluoroionophores: has the dye conformation an influence upon the formation of sandwich complexes in the ketocyanine series?

The effect of the sensor conformation upon cation recognition, and more precisely upon the formation of 2∶1 (host-guest) sandwich complexes, was studied with three new fluoroionophores. These are symmetrical ketocyanine dyes substituted with two crown ethers. The first member of this series, 2,5-bis{[4-(N-aza-15-crown-5)-phenyl]methylene}cyclopentanone (1) has a planar and rigid structure. In compound 2, 2,6-bis{[4-(N-aza-15-crown-5)-phenyl]methylene}cyclohexanone, the two phenyl groups both are twisted by 40° with respect to the core of the molecule. Compound 3, 1,5-bis[4-(N-aza-15-crown-5)phenyl]penta-1,4-dien-3-one, displays possibilities of rotation about the carbonyl carbon quasi-single bond and was previously reported to exist in solution as a mixture of two conformers. For the sake of comparison, a monoazacrown dye of the cyclopentanone series (4) was also investigated. The behaviour of these dyes in acetonitrile in the presence of Ca2+ and Ba2+ perchlorates was studied by infrared spectroscopy, mass spectrometry, NMR and UV/vis absorption spectroscopy. The absorption data were fitted by taking into account three different stoichiometries and the corresponding association constants were calculated. Very stable 2∶1 sandwich complexes were observed for all three symmetrical dyes, meaning that small twists and conformational variations of the sensors hardly affect the formation of such structures. The monocrown dye also leads to sandwich complexes, but the corresponding association constants were slightly weaker than for biscrown dyes.

Intermolecular quenching of excited singlet states by ferrocenyl derivatives: study with ketocyanine dyes

Abstract The fluorescence quenching of 1,5-bis[4-(diethylamino)phenyl]penta-1,4-dien-3-one (1), 1-[4-(diethylamino)phenyl]-5-phenylpenta-1,4-dien-3-one (2) and chalcone 3 (1-[4-(diethylamino)phenyl]but-1-en-3-one) was studied in acetonitrile using ferrocene and three commercially available derivatives, 1,1′-diacetylferrocene, acetylferrocene and 1,1′-dimethylferrocene. The spectroscopic and electrochemical characteristics of all the compounds were investigated. The feasibility of the quenching process was calculated, showing in particular that electron transfer was thermodynamically possible in every case. The steady-state fluorescence of the three organic dyes was then measured in the presence and in the absence of the ferrocenyl derivatives and the data were corrected for inner-filter effects. 1,1′-Diacetylferrocene and acetylferrocene proved to be the most efficient quenchers, and among the three dyes investigated, chalcone 3 was the most sensitive to the presence of the ferrocenyl derivatives. However, the fluorescence decay times of the ketocyanines were not affected by the presence of the ferrocene derivatives, indicating that quenching was not diffusion-controlled but rather the consequence of the formation of a ground state complex.

KINETIC MODELING OF THE PHOTOOXIDATION OF DIMETHYLDISULFIDE IN THE LIQUID PHASE

A reaction mechanism for the photooxidation of dimethyldisulfide (DMDS) in aqueous acetonitrile has been established by kinetic modeling the UV absorbance vs. time curves under continuous irradiation. The model, built according to the known solution reactivity of oxysulfur radicals [1], consists of 22 steps involving 6 radical and 10 nonradical species. The first steps of the mechanism are the homolytic cleavage of the DMDS S—S bond with formation of methanethiyl radicals (CH3S·) followed by addition of these radicals to molecular oxygen. There are photoequilibria between thiyl (CH3S·), sulfinyl (CH3S·), and sulfonyl (CH3SO2·) radicals and the corresponding molecular species (methyl methanethiosulfinate CH3S(O)SCH3 or MMTSI, methyl methanethiosulfonate CH3S(O)2SCH3 or MMTS and meth-anesulfinic acid CH3S(O)OH or MSIA) which appear as long lived intermediates. Reactions of sulfonyl radicals with oxygen lead to methanesulfonic acid (CH3S(O)2OH) or MSA. Cleavage of sulfonyl radicals gives SO2 and CH3·, the parent compounds of sulfuric (H2SO4) and methanoic (HCOOH) acids. The predictive power of the model was tested at higher initial concentration of DMDS in anhydrous and aqueous acetonitrile. In these conditions, the proposed mechanism gives a semiquantitative description of the course of the reaction and reproduces the kinetic behavior of the long lived intermediates.

Bistable photochemical reactions

Abstract In photochemistry, under continuous irradiation, kinetic non-linearity results from Beer-Lamberts law. In monophotonic or photoequilibrated systems, non-linearity is too weak in order to exhibit dynamic instabilities. On the other hand, when there is a biphotonic process, as in the so-called ABC system or a process of auto-inhibition, as in the self-quenching model, screen effects (internal filter) are sufficient to give rise to kinetic instabilities such as bistability or oscillations. Starting from a simple bistable photochemical system, the kinetic scheme of a sustained photochemical oscillator operating in an isothermal reactor closed to flux of matter is designed. From the experimental point of view, the photochemistry of the triphenylimidazyl dimer (TPID) in chloroform is described as an example of a photochemical bistable system in a CSTR. The mechanism is interpreted quantitatively by application of the laws and concepts of photochemical dynamics. A series of experimental systems are reviewed and discussed in order to highlight the non-linear nature of their kinetics. Our objective is to pave the way for further experimental demonstrations of bistable or oscillating systems in photochemistry.

Unusual solvatochromic behaviour of the 4-hydroxy-7-nitrobenzofurazan conjugated anion in protic solvents

The absorption spectrum of 4-hydroxy-7-nitrobenzofurazan (NBDOH) and its conjugate anion has been recorded in 23 solvents. The data has been analysed according to the Taft and Kamlet treatment and compared with that of some parent compounds: 4-methoxy-, 4-propylamino- and 4-diethylamine-7-nitrobenzofurazan. The phenolate anion NBDO– has been shown to exhibit the solvatochromic behaviour characteristic of this NBD series in aprotic media, although in protic media hydrogen bonding had a drastic effect on the UV–VIS spectrum. Such a sensitivity to erotogenic solvents was not encountered for other phenoxide anions such as picrate. This difference of behaviour is discussed on the basis of electrostatic potentials obtained by MNDO calculations. Evidence is given of strong negative charges located on the C(5) and C(7) atoms of NBDO–, indicating that strong hydrogen bonding may take place with solvent molecules. However, this stabilization of the anion by hydrogen bonding does not seem to influence the dissociation equilibrium, the acidity of NBDOH being perfectly in line with that of other nitrophenols whatever the solvent.

Ester containing aggregates in the autocatalytic biphasic hydrolysis of ethyl alkanoate

Abstract Autocatalytic kinetics are observed during the biphasic alkaline hydrolysis of C-4, C-6 and C-8 (butanoate, hexanoate and octanaoate) ethyl esters. Ethanol and alkanoate anions are the products of the reaction. A kinetic model including spontaneous dissolution of the organic phase into the aqueous phase, aqueous hydrolysis of the ester, formation of ester containing aggregates and micellisation has been proposed. For C-4, taking into account salting-in and solvent effects is sufficient to reproduce the autocatalytic behaviour. For C-8, the major autocatalytic process is an ester containing aggregates (ECA) mediated phase transfer. For C-6, it appears that the role of ECA is double. Small sized ECA which are formed at the beginning of the reaction do not perform rapid phase transfer and trap surfactant molecules. Larger ECA which appear at the end of the reaction are the main phase transfer agent. Using an empirical formula to describe the size of the interface, the model is able to reproduce the effect of a variation of the volumic ratio of the two phases as well as the effect of stirring rate and non-emulsified experiments.