Corinne Moustrou

Photomodulable Materials. Synthesis and Properties of Photochromic 3H-Naphtho[2,1-b]pyrans Linked to Thiophene Units via an Acetylenic Junction

The synthesis of 3H-naphtho[2,1-b]pyrans linked to mono-, di-, or terthiophene via an acetylenic junction is described (Schemes 2 and 3). The synthetic approaches involve successive Sonogashira coupling reactions. The photochromic properties in solution of these novel materials were investigated under continuous irradiation.

New thermally irreversible and fluorescent photochromic diarylethenes.

A photokinetic investigation is here carried out on four newly synthesized diarylethenes with the aim to test their performance as photoreversible chromogenic and light emitting materials. The pentatomic ring, which fixes these diarylethenes in the cis conformation, contains a Si atom or a PO group. The 1,2 positions at the ethenic bond are symmetrically substituted with thienyl or benzothienyl groups. The results are compared with those for the structurally related and widely studied 1,2-bis(2-methyl-benzo[b]thiophen-3-yl)perfluorocyclopentene (BTF6), investigated here under the same experimental conditions. Spectra of the colorless and colored forms and photoreaction quantum yields were determined; temperature, excitation wavelength and viscosity effects were explored. Compounds containing benzothienyl substituents were found to be good bistable photochromes, with high photochemical yields of both the cyclization and cycloreversion reactions, and to display appreciable fluorescence emission from the colored forms, which is a rare and desirable property for photochromes. In contrast, the molecules not bearing the benzene condensed rings were found unsuitable as photochromes because of side degradation processes occurring in competition with cyclization.

Light-triggered molecular devices based on photochromic oligothiophene substituted chromenes

An original concept for the realization of a molecular photoswitch is proposed, based on photochrochromic oligothiophene-substituted chromenes. When optically excited, these compounds undergo a structural change passing from a neutral state (closed form) to a strongly polarized one (open form). This photochromism process is also accompanied by a large increase in the electrical conductivity.

Mn(III)-mediated radical lactonisation of allylic esters of acetoacetic and malonic acids

Mn(OAc)3 oxidises allyl acetoacetate and allyl malonate to 3-oxa bicyclo (3.1.0) 2-hexanone derivatives whereas under similar experimental conditions cinnamyl and crotyl esters lead to monocyclic γ-lactones derivatives and bis-lactones.

Comprehensive Photokinetic and NMR Study of a Biphotochromic Supermolecule Involving Two Naphthopyrans Linked to a Central Thiophene Unit Through Acetylenic Bonds

A photophysical and photochemical study of a biphotochromic compound where two naphthopyran units are linked by an acetylene–thiophene–acetylene bridge has been carried out in toluene. Both fluorescence and intersystem crossing to the triplet manifold were found to compete with the photocoloration process. Two photoproducts (transoid‐trans and transoid‐cis stereoisomers), absorbing at ∼480 nm and corresponding to the opening of a single photochromic unit, were detected by spectrophotometric analysis after short irradiation time in diluted solution and identified by 1H–nuclear magnetic resonance (NMR) spectroscopy. After prolonged irradiation at 228 K of highly concentrated solutions (up to 3 × 10−3 mol dm−3), two additional isomers, absorbing at ∼550 nm, were formed. Their NMR spectra indicate the opening of both photochromic units. An interesting effect of selective vibronic excitation was found, showing that the photoreaction is favored at excited vibronic levels to the detriment of the radiative relaxation.

Transient absorption studies of the photochromic behavior of 3H-naphtho[2,1-b]pyrans linked to thiophene oligomers via an acetylenic junction.

The photophysical and photochemical properties of four 3,3-diphenyl-3H-naphtho[2,1- b]pyrans substituted, via an acetylenic junction, to (thiophene) n oligomers (n = 0-3 units) were investigated by transient absorption in the femtosecond to microsecond time domain and by stationary absorption and fluorescence. The decay of the initially produced excited S1(pi pi*) state is found to occur via three competing processes: fluorescence, intersystem crossing, and a ring-opening reaction leading to a colored merocyanine product, with relative yields varying drastically with n. Whereas ultrafast (sub-picosecond) reaction dynamics and high product quantum yield are observed for n = 0 and 1, the reaction is considerably slowed down on going to the n = 2 (105 ps) compound and does not occur for n = 3. A reaction scheme that accounts for this behavior is proposed and the effect of the oligothiophenic chain length on the photoinduced properties is discussed. It is suggested that increasing the chain length from 1 to 3 thiophene units stabilizes the S1(pi pi*) state by pi conjugation and induces an excited-state potential barrier along the reaction pathway.

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.

Molecular switch devices realised by photochromic oligothiophenes

In this report we describe a new chromene bearing at their three position terthiophene and quaterthiophene. These chromene-substituted oligothiophenes have been prepared by palladium-catalysed coupling reaction. When optically excited, these molecules undergo a structural change passing from a neutral state (closed form) to a strongly polarised one (open form). This photochromic process is also accompanied by a large increase in the electrical conductivity.

Spectroscopic properties of thiophene linked [2H]-chromenes

Abstract The influence of an oligothiophenic substituent upon the spectroscopic behaviour of chromenes is discussed. Fluorescence emission is observed for several compounds. The photochromism/fluorescence ratio depends on the polythiophenic chain length and on the substitution site on the chromene.

Synthesis and Photochromic Behaviour of Novel 2H-1-Benzopyrans (=2H-Chromenes) Derived from Carbazololes

The synthesis and photochromic properties of new 2,2-diphenyl-2H-1-benzopyrans, fused to an indole moiety, are described. All compounds exhibit photochromic behaviour in solution at room temperature. The heteroanellation effects are variable and depend on the position and geometry of the fused indole moiety. A general bathochromic shift in the spectra of the open forms is observed. The presence of a N-methyl group prevents the broadening of the absorption spectra and promotes the instability of some photoinduced forms of compounds with the indole moiety fused at the 5,6 positions of the 2H-1-benzopyran skeleton. The enhanced photocolouration efficiency in the near-UV and the kinetics of thermal bleaching indicate that the novel compounds with an indole moiety fused at the 6,7 positions, particularly those with a linked thiophene moiety, are very interesting molecules for applications in the field of variable optical absorption systems.