Jean-Pierre Desvergne

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.

Rational design of new acid-sensitive organogelators

2,3-Di-n-alkoxyphenazines were shown to act as acid-sensitive organogelators at ambient temperature in acetonitrile; the protonated yellow species formed in the presence of acid displays stronger aggregative properties and higher temperature resistance than the colourless neutral phenazine; the protonation/deprotonation process is reversible.

(Thio)Amidoindoles and (Thio)Amidobenzimidazoles: An Investigation of Their Hydrogen-Bonding and Organocatalytic Properties in the Ring-Opening Polymerization of Lactide

The mechanism of the ring-opening polymerization (ROP) of lactide catalyzed by two partner hydrogen-bonding organocatalysts was explored. New amidoindoles 4 a,c, thioamidoindoles 4 b,d, amidobenzimidazoles 5 a,c, and thioamidobenzimidazoles 5 b,c were synthesized and used as activators of the monomer. In the solid state and in solution, compounds 4 and 5 showed a propensity for self-association, which was evaluated. (Thio)Amides 4 and 5 do catalyze the ROP of lactide in the presence of a cocatalyst, tertiary amine 3 a or 3 b, which activates the growing polymer chain through hydrogen-bonding. Reactions were conducted in 2-24 h at 20 degrees C; conversion yields ranged between 22 and 100 %. A detailed study of the intermolecular interactions undertaken between the participating species showed that, as expected, simultaneous weak hydrogen bonds do exist to activate the reagents. Moreover, interactions have been revealed between the partner catalysts 4/5+3. ROP catalyzed by these partner activators is thus governed by multiple dynamic equilibria. The latter should be judiciously adjusted to fine-tune the catalytic properties of (thio)amides and organocatalysts, more generally.

Ring-Opening Polymerization of L-Lactide Efficiently Triggered by an Amido-Indole. X-ray Structure of a Complex between L-Lactide and the Hydrogen-Bonding Organocatalyst

N-(3,5-Bis(trifluoromethyl)phenyl)-1H-indole-2-carboxamide 1e is an efficient hydrogen-bonding organocatalyst for the ring-opening polymerization of l-lactide. This new catalytic species does control the dispersity (1.08) and molecular weight (3460 g/mol vs 3064 in theory) of the poly(l-lactides) prepared in 2 h. (1)H NMR analysis showed that compound 1e complexes l-lactide in CDCl(3) through the two available NH groups (amide and indole). In particular, the catalytic species appeared to be mainly the H-bonding donor amide (1e in extended conformation, alone or dimer (1e)(2)) and, to a lesser extend, the dual H-bonding amido-indole (1e in its the pinched conformation). The first X-ray structure of the complex between a H-bonding organocatalyst and l-lactide also revealed a tight H-bonded network between the dimer (1e)(2) and l-lactide.

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.

Kinetic study of photophysical and photochemical processes in α,ω-(bis-9-anthryl)-n-alkanes (ethane to decane) at room temperature

Abstract Fluorescence and photocyclomerization quantum-yield measurements and a transient kinetic analysis were performed for a series of α,ω-(bis-9-anthryl)- n -alkanes (A-C 2 -A to A-C 10 -A and A-mXy-A) in methylcyclohexane and ethanol. Of note is an irregular periodicity of intramolecular excimer fluorescence yields and the rate parameters for excimer collapse to photocyclomer.

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.

A new example of small molecular non-hydrogen bonding gelators for organic solvents

Abstract Two novel low molecular weight gelling agents 1 and 2 for organic sovents are presented. These molecules, which do not contain hydrogen bonding groups, can gelify reversibly a great variety of solvents.