Henri Bouas-Laurent

Photodimerization of anthracenes in fluid solution:structural aspects

Owing to their versatile photophysical and photochemical properties, anthracene and its derivatives are being employed in many systems, for instance as energy migration probes in polymers, triplet sensitizers, molecular fluorosensors, electron acceptor or donor chromophores in artificial photosynthesis, photochromic substrates in 3D memory materials, etc. One remarkable feature is their ability to photodimerize under UV irradiation which induces considerable changes in their physical properties. This account reports the preparative and structural aspects of this very useful and protean photocycloaddition.

Photodimerization of anthracenes in fluid solutions: (part 2) mechanistic aspects of the photocycloaddition and of the photochemical and thermal cleavage

One of the classics in photochemistry, the photodimerization of anthracenes can be considered as a paradigm of the photocycloaddition of non saturated hydrocarbons. The historical steps of the mechanistic studies are reviewed: based on fluorescence quenching, cyclization quantum yields measurement, the influence of dioxygen and solvents, they support a singlet state pathway; the dimerization rate constants are found to be generally high for reactions occurring within a few nanoseconds unless they are slowed down or inhibited by steric strain. In several cases, excimers have been demonstrated to be intermediates and it is shown that excimer fluorescence and cyclization are competitive processes. Another intermediate known as pericyclic minimum (or conical intersection) is postulated to form a sort of floppy cycloadduct where the reacting centres are at mutual distances shorter than in excimers and longer than in dimers. For intermolecular dimerizations, the triplet state is also reactive but through triplet–triplet annihilation in dilute solutions. Intramolecular photocycloadditions have also been carefully examined, for the role of multiple excimer formation, regioselectivity (9,10∶1′,4′ and 9,10∶1′,2′ cyclization) and solvent polarity. The triplet state reactivity is shown to lead to 4π + 2π or 4π + 4π cycloadducts, depending on geometric factors. In the latter case when intersystem crossing is favoured by the substituents, cyclization quantum yields as high as 0.65–0.72 have been observed. Photodissociation quantum yields are generally high and the reactions are partly adiabatic, leading to excimer and monomer fluorescence, but the major part follows another pathway not fully elucidated by flash photolysis. Thermodynamic and kinetic parameters for the thermal cleavage are given; they reveal a large gamut of stability for the photocycloadducts.

A novel small molecular luminescent gelling agent for alcohols

2,3-Bis-n-decyloxyanthracene can form thermoreversible gels with alcohols at very low concentrations; electronic absorption spectra and fluorescence emission studies suggest that the network of the gel consists of head-to-tail assemblies of the aromatic component coexisting with the alcohol, the organization of the system essentially relying upon dipolar forces and van der Waals interactions.

Photodegradation of lignin: a photophysical and photochemical study of a non-phenolic α-carbonyl β-O-4 lignin model dimer, 3,4-dimethoxy-α-(2′-methoxypenoxy)acetophenone

Abstract Photoyellowing of bleached high yield wood pulps induced by lignin was studied using the non-phenolic α-carbonyl β- O -4 lignin model compound 3,4-dimethoxy-α-(2′-methoxyphenoxy)acetophenone ( 1 ). Its photochemical reactivity was examined in both liquid (benzene and water-ethanol mixture) and solid (adsorbed on lignin free paper fibres) media; the influence of oxygen on the photolysis was considered. Efficient photodecomposition of 1 (φ R ≈ 0.3 in solution) yields monomers, dimers and a complex mixture of oligomers including coloured materials. The structures of the photoproducts are in accordance with a β-CO bond cleavage as the primary photochemical step, leading to phenacyl and guaiacoxy radicals. Laser flash photolysis experiments enables us to detect two transient absorptions attributed to two different triplet states of 1 : one with a fast decay (τ = 50 ns) from conformers where the β-phenyl ring and the carbonyl group interact, and another one with a slower decay (τ = 830 ns) attributed to conformers with no interchromophoric interactions. Whatever the experimental conditions (degassed or aerated, liquid or solid media), α-carbolyl β-1 structures isolated as photoproducts of compound 1 appear to also play a role in the formation of coloured oligomeric materials.

DIFFERENT SYNTHETIC ROUTES TOWARDS EFFICIENT ORGANOGELATORS : 2,3-SUBSTITUTED ANTHRACENES

Abstract Three synthetic approaches towards 2,3-substituted anthracenes are reported and discussed in terms of selectivity and viability. This allowed us to introduce a variety of substituents as sidearms. Promising results have been found using a tandem Diels-Alder aromatization reaction using 2,3-dimethoxybutadiene 9 as a key intermediate. However, for multigram preparations the Friedel-Crafts approach is preferred.

Synthesis, cation binding, and photophysical properties of macrobicyclic anthraceno-cryptands

The new cryptands, (1a) and (1b), possess remarkable spectroscopic and photophysical properties which are markedly affected by cation binding and by protonation.

A photochemical study of an O-methylated α-carbonyl β-1 lignin model dimer: 1,2-di(3′,4′-dimethoxyphenyl)ethanone (deoxyveratroin)

Abstract The photochemistry of 1,2-di(3′,4′-dimethoxyphenyl)ethanone (deoxyveratroin) ( 1 ) (an O-methylated α-carbonyl β-1 lignin model dimer) was examined. Its photochemical reactivity was studied in both liquid (benzene and ethanol) and solid (adsorbed on lignin-free fibres) states in degassed or aerated media. In degassed benzene and fibres, 1 is photochemically transformed into several identified compounds (monomeric to tetrameric units) and a coloured mixture of higher molecular weight fractions. The structures and titration of the photoproducts are well explained by Norrish type 1 cleavage and photoreduction of the carbonyl group. The presence of oxygen inhibits the reductive mechanism and leads to veratric acid by trapping the ketyl and desyl radicals. The observation of a low disappearance quantum yield of 1 in degassed ethanol is attributed to the presence of a low-lying π,π* triplet state as shown by phosphorescence.

Photoinduced formation of a cryptand from a coronand: an unexpected switch in cation binding affinity

Aza-crown ethers 2 and 3 with anthracene-containing pendant arms have been synthesised and characterised. Both compounds bind Group 1 metal cations in solution, forming complexes of 1:1 stoichiometry. The properties of compound 2 and its complexes have been studied by a range of techniques, including NMR, UV and fluorescence spectroscopy and X-ray crystallography. The pendant arms can adopt either a cis or a trans geometry, the cis geometry favoured with larger cations. The geometry of the complex affects the fluorescence properties of the system, with larger cations giving higher excimer/monomer ratios. Upon irradiation at lambda>300 nm, coronand 2 forms the cryptand 5 through a reversible intramolecular [4pi+4pi] cycloaddition reaction. The rates of the forward and reverse reactions of this photochromic process are cation dependent; in particular the rate of the thermal reverse reaction is decreased by smaller cations and increased by larger cations, especially Rb(+). The metal binding constants in methanol for 2 and 5 have been determined, revealing that the cryptand 5 binds Na(+) and Rb(+) more weakly than crown ether 2 by over two orders of magnitude.

Photochemical Degradation of Lignin through α C—O Bond Cleavage of Non Phenolic Benzyl Aryl Ether Units. A Study of the Photochemistry of a(2',4',6'-Trimethyl-Phenoxy)-3,4 Dimethoxy Toluene

The photochemical reactivity of ηοπ-phenolic benzyl aryl ether units of lignin has been examincd using degassed fluid media of the title reference compound 3. Whatever the solvent (protic, aprotic), 3 is efficiently photolyzed (φΛ 0.2-0.3) into a coloured oligomeric mixture and several (up to 8) idcntified compounds which incorporate one or two benzene rings; some of these have been isolated. The structurc of the latter strongly suggests that the primary step is a homolytic α C—O bond cleavage followed by in-cage and out-of-cage free radical reactions. i.e. H abstraction, radical-radical coupltng, disproportionation to methylenequinones, which could generate oxidizable phenolic derivatives and, finally, coloured oligomers. The results of this study show that, even in the case of a complete chemical group protection (phenolic groups s aliphatic ethers. carbonyl groups reduced to CH2), the U.V. part of the sunlight near 300 nm can initiale a benzylic cleavage responsible for a subsequent yellowing of the lignin materials.

Intramolecular Excimer Formation in Short- and Long-chainlength di(9-anthryl) Bichromophoric Compounds and Relation to Ground State Properties

The ground state conformational, UV absorption and fluorescence spectral as well as decay time data for a series of polymethylene- and one polyoxyethylene-linked di(9-anthracenes) in solution are compared, for chainlengths varying from n=0 (directly linked system A0A, 9,9′-bianthryl) to n=∞ (A∞, reference system 9-decylanthracene). 1H NMR spectroscopy of A2A to A11A shows that there is no clearly preferred ground state conformation of the anthracene moieties. The crystal structure of A2A was solved and the anti conformation of the anthracene units was established. The UV spectra show a redshift interpreted within the Forster exciton model (predominance of extended conformations). Fluorescence spectra for (CH2)n-bridged systems show no clear excimer band for short-chain systems but redshifted excimer emission for A4A to A9A. Loss of structure and fluorescence band shape changes as well as biexponential decay times allow the conclusion that excimer formation occurs in all chained systems. For a related polyoxyethylene-chained system, the excimer band is substantial, probably a consequence of the increased flexibility of the chain and moderate photoreactivity. Solvent polarity changes are strong for A0A [formation of a twisted intramolecular charge transfer (TICT) state] but also significant in the systems A1A and A2A. This is rationalized by the possiblity for competitive formation of an excimer-type (parallel but tilted chromophore arrangement) and a TICT-type (near perpendicular) conformation. For AnA (n>2) the solvent polarity was found to have virtually no influence.