Jean-Claude Micheau

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.

Continuous irradiation and flash-photolysis studies of new[3H]naphtho[2,1-b]pyrans linked by covalent bonds to oligothiophene units. Effect of thiophene substituents on the photochromism

The influence of oligothiophene substituents on five new [3H]naphtho[2,1-b]pyrans has been investigated using flash photolysis and continuous irradiation techniques. Photochromic properties strongly depend on the number and position of the thienyl units. Thienyl substitution increases the UV/visible absorbance of the lower energy transition of both the naphthopyran and merocyanine isomers. The rate constants of the thermal fading processes also increase with the number of thiophenic entities. Although several minor photoisomers, whose relative amounts depend on the irradiation conditions could be involved, it has been assumed that under continuous monochromatic irradiation, the evolution of the photochromic reaction can be described by the formation of only one photoisomer. Under these conditions, quantum yields of photocolouration and molar absorption coefficients of the non-isolable photomerocyanine isomer were evaluated by photokinetic analysis. Flash-photolytic colourability can be interpreted using continuous irradiation parameters. A linear correlation has been established between experimental and calculated colourability. This result shows that similar photochromic reactions leading to similar photoisomers are triggered either by polychromatic flash photolysis or by continuous monochromatic irradiation.

Photochromism and solvatochromism of push???pull or pull???push spiroindolinenaphthoxazinesElectronic supplementary information (ESI) available: Analysis of products 12, 13, 14 and 17 absorbance of solutions of 2. See http://www.rsc.org/suppdata/b2/b204603b/

The photochromic and solvatochromic behaviour of 17 variously substituted spiroindoline naphthoxazines has been investigated in cyclohexane, toluene, acetonitrile and methanol in fluid solution. Specific parameters such as the wavelengths and the molar absorption coefficients of the closed spiro- and open merocyanine forms, together with the photocoloration and photobleaching quantum yields, rate constants and activation energies of thermal fading have been determined under continuous monochromatic irradiation. It has been shown that most of these parameters are closely related to the electron distribution on the highly conjugated photomerocyanines and on their quinoidal or zwitterionic character. Depending on the acceptor or donor properties and on the position of their substituents, three classes of compounds have been detected.

The Origin of Biomolecular Chirality

Life is characterized by broken mirror symmetry (Palyi et al., 1999). On the molecular level, proteins are composed almost exclusively of L-amino acids while nucleic acids only contain D-sugars. Without this chiral asymmetry, prebiotic molecular complexity leading to the formation of biologically active polymers could probably not have evolved (Joyce et al., 1984; Avetisov and Goldanskii, 1991). Nevertheless, more than 1½ century after Pasteur’s discovery, the origin of biomolecular chiral asymmetry is still a mystery. Meanwhile, it is accepted that homochirality has already appeared early during chemical evolution and that a homochiral molecular environment was rather a pre-condition than a consequence of life (Keszthelyi, 1995; Avalos et al., 2000). Parity violation (MacDermott, 1993) and other chiral factors such as circularly polarized light are omnipresent and can lead under favorable conditions to enantio-meric enrichment. However, this enhancement usually remains tiny and can be annihilated by long-term racemization processes.

Kinetic analysis of self-replicating peptides: possibility of chiral amplification in open systems.

A simplified kinetic model scheme is presented that addresses the main reactions of two recently reported peptide self-replicators. Experimentally observed differences in the autocatalytic efficiency between these two systems - caused by variations in the peptide sequences - and the possible effect of chiral amplification under heterochiral reaction conditions were evaluated. Our numerical simulations indicated that differences in the catalytic performance are exclusively due to pronounced variations in the rate parameters that control the reversible and hydrophobic interactions in the reaction system but neither to alterations in the underlying reaction network nor to changes in the stoichiometry of the involved aggregation processes. Model predictions further demonstrated the possible existence of chiral amplification if peptide self-replication is performed under heterochiral reaction conditions. Pointing into the direction of a possible cause for biomolecular homochirality, it was found that in open flow reactors, keeping the system under non-equilibrium conditions, a remarkable amplification of enantiomeric excess could be achieved. According to our modeling, this is due to a chiroselective autocatalytic effect and a meso-type separation process both of which are assumed to be intrinsic for the underlying dynamics of heterochiral peptide self-replication.

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.

Nonlinear Effects in Asymmetric Synthesis: A Practical Tool for the Discrimination between Monomer and Dimer Catalysis

Nonlinear effects in asymmetric synthesis are identified by a nonlinear relationship between the enantiomeric excess of the product versus that of the chiral catalyst. Such information is not always sufficient to reveal if the active catalyst acts in its monomeric or dimeric form. Numerical simulations of two kinetic models including monomer and dimer catalysis with a Michaelis–Menten‐type reaction mechanism suggest that the nonlinear effects are mainly sensitive to an analogue of the Michaelis–Menten constant as well as to the diastereomeric energy difference between homochiral and heterochiral catalyst dimers. A practical monomer–dimer discrimination table is presented showing that the influence of the initial catalyst enantiomeric excess and the catalytic charge on the product enantiomeric excess and on the reaction half‐time together with the assessment of the kinetic order of the prochiral substrate are sufficient to assess whether the active catalyst acts in its monomeric or dimeric form.

Quantitative investigations of thermal and photoinduced J- and H-aggregation of hydrophobic spirooxazines in binary solvent through UV/vis spectroscopy

The aggregation of two hydrophobic spirooxazine dyes has been investigated in water–acetonitrile binary solvent and in presence of sodium dodecylsulfate (SDS). For the thermochromic molecule 1, aggregation is spontaneous and occurs when a large amount of water is added to an acetonitrile solution of the dye. There exists an optimum value of the water–acetonitrile molar ratio giving rise to a sharper H-aggregate visible absorption spectrum. Kinetic analysis of the H-aggregate formation has shown that there is an increase in the sticking probability of the monomers, as the aggregates grow. A subtle interplay between H- and J- aggregation was witnessed by adding small amounts of SDS. This change is so sensitive that this system could be the basis of a new chemosensory technique for SDS assessment. Excess of surfactant induces the collapse of the aggregates and the re-dissolution of the dye under the form of a merocyanine–SDS ion-pair. For the spirooxazine 2, aggregation must be photo-induced. Comparison of the spectral evolutions of the two dyes in the presence of SDS has shown that the opening of the closed spirooxazines paves the way to the merocyanine–SDS ion-pair formation. Reversibility of the aggregation has been suggested to occur when the aggregate size is still small. Finally, reversibility has been checked experimentally by precipitate re-dissolution and spirooxazine recovery. Possible structures and reactivity of the H- and J-aggregates have been deduced.

Proton Catalysis in the Redox Responsivity of a Mini‐Sized Photochromic Diarylethene

A thermally irreversible dithienylethene (DTE) photochrom can be turned into a thermally reversible one in presence of Cu(II) triflate. A ring opening (DTEC closed→DTEO open) occurs through the formation of a copper-containing fast transient intermediate. Stopped-flow experiments monitored at 410 and 780 nm have allowed to show that the stoichiometry of this intermediate is DTE/Cu=1:1. At longer monitoring times (i.e., several seconds after mixing), the intermediate undergoes a slow decay while the residual DTEC closed form opens. A joint detailed kinetic and electrochemical analysis has unveiled a proton catalysis scenario in which electron transfer between DTEC and Cu(II), ligand exchange, protonation-deprotonation equilibria of the cation radicals and ring opening are embedded into two main reaction cycles. At the beginning of the reaction, Cu(II) is reduced into Cu(I) and DTE is degraded without ring opening. Then, as the reaction progresses, the triflic acid released from the Cu(II) reduction switches-on a propagation cycle during which ring opens without any more Cu(II) consumption. Cyclic voltammetry, spectro-electrochemical measurements, delayed photocoloration experiments in presence of Cu(II) and acid-base additions have confirmed the main features of the proton catalysis.