Pascal Le Poul

Reactivity of carbanions of Fischer-type carbene complexes with pyrylium salts. Synthesis and characterization of new γ-methylenepyran carbene complexes via an addition–oxidation–deprotonation process

The condensation reaction between γ-unsubstituted pyrylium salts and carbanions of Fischer-type carbene complexes allowed the synthesis of new γ-methylenepyran carbene complexes. NMR data, single crystal X-ray analysis, and DFT calculations suggest that these complexes have a partial pyrylium character due to the π electron-accepting ability of the carbenic fragment and to the electron-releasing properties of the methylenepyran part.

Dipolar NLO Chromophores Bearing Diazine Rings as π-Conjugated Linkers

The synthesis of a series of push-pull derivatives bearing triphenylamine electron-donating group, cyclopenta[c]thiophen-4,6-dione electron acceptor and various π-linkers including (hetero)aromatic fragments is reported. All target chromophores with systematically varied π-linker structure were further investigated by electrochemistry, absorption measurements, and EFISH experiments in conjunction with DFT calculations. Based on electrochemical and photophysical measurements, when a polarizable 2,5-thienylene moiety is embedded into the chromophore π-backbone the highest intramolecular charge transfer (ICT) is observed. Benzene, pyrimidine, and pyridazine derivatives exhibit lower polarizability and extent of the ICT across these π-linkers. The elongation of the π-conjugated system via additional ethenylene linker results in a significant reduction of the HOMO-LUMO gap and an enhancement of the NLO response. Whereas it does not significantly influence electrochemical and linear optical properties, the orientation of the pyrimidine ring seems to be a key parameter on the μβ value due to significant variation of the dipolar moment (μ) value. In 2a and 2c, pyrimidine is oriented to behave as an acceptor and thus generate dipolar molecule with μ above 5 D, whereas in 2b and 2d ground state dipole moment is significantly reduced. This study seems to indicate a high aromaticity of pyrimidine and pyridazine derivatives, close to the benzene analogues and significantly higher than thiophene analogues.

Diferrocenylpyrylium salts and electron rich bispyran from oxidative coupling of ferrocenylpyran. Example of redox systems switched by proton transfer

Electro or chemical oxidation of ferrocenylmethylenepyran gave an ethanediferrocenylbispyrylium salt through the dimerization of a ferrocenylpyran radical-cation (C–C bond making). Electro or chemical reduction gave back the ferrocenylmethylenepyran (C–C bond breaking). This electrochemical reverse system constitutes an example of C–C bond making–breaking process in a metallocenyl series with rather high stability. DFT calculations and electrochemical studies were carried out in order to determine the electronic structure of the radical cation intermediate, the role of the ferrocenyl groups and the mechanism of the C–C bond making and C–C bond breaking processes. Reversible deprotonation of the ethanediferrocenylbispyrylium salt afforded an extended diferrocenylbismethylenepyran, which was subsequently reversibly oxidized to an ethenediferrocenylbispyrylium salt. X-Ray crystallographic data of diferrocenylbismethylenepyran and ethenediferrocenylbispyrylium salt allowed to determine the molecular movements, which come with the electron transfer (ET). A comparison with the behavior of the corresponding isoelectronic bisdithiafulvenes (extended TTF) and bisdithiolium salts was made.

Reversible Redox Switching of Chromophoric Phenylmethylenepyrans by Carbon–Carbon Bond Making/Breaking

Electrochromic organic systems that can undergo substantial variation of their optical properties upon electron stimulus are of high interest for the development of functional materials. In particular, devices based on radical dimerization are appropriate because of the effectiveness and speed of carbon-carbon bond making/breaking. Phenylmethylenepyrans are organic chromophores which are well suited for such purposes since their oxidation leads to the reversible formation of bispyrylium species by radical dimerization. In this paper, we show that the redox and spectroscopic properties of phenylmethylenepyrans can be modulated by adequate variation of the substituting group on the para position of the phenyl moiety, as supported by DFT calculations. This redox switching is reversible over several cycles and is accompanied by a significant modification of the UV-vis spectrum of the chromophore, as shown by time-resolved spectroelectrochemistry in thin-layer conditions.

The synthesis of flexible tetrapyridylethanes from pyridylpyrylium dications

New tetrapyridylethanes can be prepared as new tetradentate pyridine ligands via the aminolysis reaction of pyrylium dications, using an original oxidation process of phenylpyridyl methylenepyrans. Crystal structure analysis, and spectroscopic and electrochemical studies show the possibility for using these new ligands in coordination chemistry.

2,4-Distyryl- and 2,4,6-Tristyrylpyrimidines: Synthesis and Photophysical Properties

The synthesis of a series of 20 new 2,4,6-tristyrylpyrimidines and three new 2,4-distyrylpyrimidines by means of combination of Knoevenagel condensation and Suzuki-Miyaura cross-coupling reaction is reported. This methodology enables us to obtain chromophores with identical or different substituent on each arm. The photophysical properties of the compounds are described. Optical properties and time-dependent density functional theory calculations indicate that photophysical properties of target compounds are mainly affected by the nature of the electron-donating group in C4/C6 positions, except when the C2 substituent is a significantly stronger electron-donating group. However, the C2 substituent has a strong influence on emission quantum yield: addition of a strong electron-donating group tends to decrease the fluorescence quantum yield, whereas a moderate electron-withdrawing group results in a significant increase of fluorescence quantum yield.