Stéphane Dufresne

Tunable spectroscopic and electrochemical properties of conjugated push–push, push–pull and pull–pull thiopheno azomethines

Novel azomethines consisting uniquely of thiophene units were examined. The highly conjugated compounds were prepared by condensing air stable aminothiophenes with 2-thiophene aldehydes, which were substituted with various electronic groups. The resulting azomethines are highly conjugated and are both reductively and hydrolytically resistant. Various electron donating and accepting groups placed in the 2-position of 5-thiophene carboxaldehyde lead to electronically delocalized push–push, pull–pull, and push–pullazomethines. These electronic groups affect both the HOMO and the LUMO levels, which influence the absorption and emission spectra. Colors spanning the entire visible spectrum ranging from yellow to blue are possible with these nitrogen containing conjugated compounds. Excited state deactivation of the singlet excited state occurs predominately by internal conversion while only a small amount of energy is dissipated by intersystem crossing to the triplet state and by fluorescence. The ensuing fluorescence and phosphorescence of the thiopheno azomethines are similar to those of their thiophene analogues currently used in functional devices, but with the advantage of a low triplet state and tunable HOMO–LUMO energy levels extending from 3.0 to 1.9 eV. Quasi-reversible electrochemical radical cation formation is possible while the oxidation potential is dependent on the nature of the electronic group appended to the thiophene. The crystallographic data of the electronic push–push system show the azomethine bonds are planar and linear and they adopt the E isomer.

EDOT-containing azomethine: an easily prepared electrochromically active material with tuneable colours

An azomethine derived from EDOT segments was easily prepared and exhibited reversible oxidation. The spectroelectrochemical behaviour of this EDOT containing azomethine was observed as intense colour changes for the oxidized products relative to the neutral form. Multiple reversible colours states were observed with the anodically and hydrolytically stable azomethine upon electrochemical and chemical oxidation.

Towards materials with reversible oxidation and tuneable colours using heterocyclic conjugated azomethines

The spectroelectrochemical behaviour of heterocyclic azomethines prepared by condensing complementary aldehydes and amines was observed as intense colour changes of the oxidized products relative to the neutral form. The tuneable colours and reversible oxidation were contingent on the heterocycles.

Photophysical, electrochemical, and crystallographic investigation of conjugated fluoreno azomethines and their precursors

The photophysical investigation of amino- and aldehyde-substituted fluorenes revealed that these compounds are not only highly fluorescent, but dissipation of their singlet excited energy occurs by a combination of nonradiative means involving intersystem crossing (ISC) and internal conversion (IC). Quantification of the triplet state formed by ISC was possible by laser-flash photolysis (LFP). The efficiency by which this manifold was populated varied between 10 and 40% depending on the fluorene substitution. Condensation of these aldehyde and amine precursors yielded conjugated thiopheno azomethines with robust covalent bonds. Fluorescence of the azomethinefluorene derivatives was reduced relative to their precursors while the degree of IC remained unchanged. Deactivation of the singlet excited state occurred predominately by ISC and the resulting triplet state was rapidly and efficiently quenched by energy transfer by the azomethine linkage. Cyclic voltammetry of the fluoreno azomethines showed both oxidation and reduction processes, and the measured redox potentials and the band-gaps are lower than a bisfluorene analogue. The fluoreno azomethine LUMO energy levels are sufficiently low, making them compatible with common cathodes, therefore eliminating the use of an electron-injection layer.

Conjugated fluorene-thiophenes prepared from azomethine connections. Part I. The effect of electronic and aryl groups on the spectroscopic and electrochemical properties.

The spectroscopic investigation of new fluoreno-thiophene azomethines revealed that these compounds are fluorescent. However, they exhibit reduced fluorescence compared to native fluorene owing to competitive deactivation of the singlet excited state by nonradiative means involving both internal conversion and intersystem crossing. The absorption and emission wavelengths can be tuned and the HOMO-LUMO energy gap modulated from 2.0 to 3.2 eV by incorporating various electronic groups, number of azomethine bonds, and the fluorene-thiophene sequence. Electrochemical investigation confirmed that both oxidation and reduction occur resulting in irreversible radical ion formation.

Conjugated Fluorenes Prepared From Azomethines Connections-II: The Effect of Alternating Fluorenones and Fluorenes on the Spectroscopic and Electrochemical Properties

The photophysics and electrochemistry of fluorene and fluorenone azomethine derivatives were examined in order to understand the deactivation pathways responsible for the quenched fluorescence of these compounds, which should otherwise be fluorescent. Steady-state fluorescence showed that the fluorene singlet excited state is quenched both by fluorenone (1) and a model aliphatic azomethine compound (14) with k(q) approximately 10(10) M(-1) s(-1). The quencher concentration required to deactivate 95% of the excited singlets formed was 8.4 mM for fluorenone and 34 mM for 14. Intramolecular photoinduced electron transfer (PET) from fluorene to both 1 and 14 was found as the principle deactivation mode of the fluorenes excited state. The high degree of conjugation of the azomethines promotes intersystem crossing to the triplet manifold by narrowing the singlet-triplet energy gap, which is also in part responsible for the reduced fluorescence observed for the fluorenone azomethine derivatives 5-11. Fluorescence quenching by PET was corroborated from the electrochemical and spectroscopic data by applying the Rehm-Weller equation. Meanwhile, the inherent fluorene fluorescence can be restored by protonating the azomethine, resulting from suppressed PET. Although PET is exergonically favorable (-242 kJ/mol for 1 and -96 kJ/mol for 14), intersystem crossing still occurs. The resulting fluorene triplet state is efficiently quenched by both 1 (k(q) = 7 x 10(9) M(-1) s(-1)) and 14 (k(q) = 5 x 10(9) M(-1) s(-1)) confirming that the absence of triplet signal by laser flash photolysis is a result of rapid intramolecular energy transfer to the two quencher sites. A concentration-dependent second emission was observed for the fluorenone containing azomethines assigned to the formation of excimers.

Unsymmetric Pyrrole, Thiophene, and Furan-Conjugated Comonomers Prepared Using Azomethine Connections : Potential New Monomers for Alternating Homocoupled Products

Unsymmetric comonomers consisting of thiophene, pyrrole, and furan heterocycles were prepared using azomethine bonds. Photophysical investigation of the novel pi-donor-donor-donor segmented compounds revealed that their singlet excited state is only partially deactivated by internal conversion unlike their all-thiophene azomethine analogues. Temperature-dependent steady-state and time-resolved emission studies demonstrated that the unsymmetric compounds deactivated efficiently their singlet excited state by intersystem crossing to populate the triplet manifold. This lower energy state is rapidly deactivated by nonradiative self-quenching. The comonomers and their anodically prepared conjugated homocoupled products are both electrochemically active, resulting in new compounds that can be mutually oxidized and reduced. Meanwhile, the oxidation potentials of the coupled products are shifted by up to 400 mV to more cathodic potentials relative to their corresponding comonomers, confirming their increased degree of conjugation.

Unsymmetric electronic push-pull bipyrroles - synthesis, spectroelectrochemical, and photophysical investigation.

The electrochromic and photophysical behaviors of unsymmetric bipyrroles were observed as intense color changes of the oxidized products relative to the neutral form. The color of the oxidized form and the oxidation potential were both tunable contingent on electronic group and substitution.

Chemical doping of EDOT azomethine derivatives: insight into the oxidative and hydrolytic stability

A series of EDOT (3,4-ethylenedioxythiophene) containing azomethines were prepared for investigating their opto-electronic properties. These properties were compared to those of their thiophene azomethine counterparts and it was found that incorporating the EDOT moiety resulted in a 30 nm bathochromic shift in the absorbance. Meanwhile, the oxidation potential (Epa) could be reduced by 100 mV by incorporating the electron rich moiety. Cyclic voltammetry revealed a one-electron oxidation process, resulting in a radical cation. This intermediate was stable when the azomethines contained amines in the 2,2′-positions, evidenced by the reversible oxidation in cyclic voltammetry. In contrast, the radical cation was irreversible when the 2,2′-positions were unsubstituted. It was found that the resulting radical cation was coupled by standard anodic polymerisation to form a polymer that was physisorbed onto the ITO electrode. The resulting polymer was mauve in colour with a λmax of 515 nm and a degree of polymerisation of ca. 5. This was spectroscopically determined relative to an EDOT polyazomethine derivative and a soluble thiophene polyazomethine. The stability of the EDOT azomethine derivatives towards electrochemical and chemical oxidation was also spectroscopically investigated. It was found that the resulting radical cation exhibited a ca. 100 nm bathochromic shift in absorbance relative to the neutral form and reversible colour switching between the neutral and oxidized states was possible. Chemical doping with FeCl3 generated a stable dication. High contrast colours between the neutral and oxidized states of the azomethines were observed. Multiple oxidation/neutralisation cycles were possible without detectable colour deterioration, demonstrating the chemical robustness of the conjugated azomethines towards oxidative decomposition and hydrolysis.

Diethyl 2-amino-5-[(E)-(furan-2-yl­methyl­idene)amino]­thio­phene-3,4-di­carboxyl­ate

In the crystal structure of the title compound, C15H16N2O5S, the azomethine adopts the E configuration. The two heterocyclic rings adopt an antiperiplanar orientation. The mean planes of the thiophene and furan rings are twisted by 2.51 (4)°. The crystal structure exhibits intermolecular N—H⋯O hydrogen bonding. π–π stacking is also observed, the centroid-to-centroid distance being 3.770 (4) Å.