The six-membered-ring azomethine N-((E)-{5-[(E)-(pyridin-3-ylimino)methyl]thiophen-2-yl}methylidene)pyridin-3-amine.

The title compound, C16H12N4S, forms a three-dimensional layered network structure via intermolecular hydrogen bonding and π-stacking. The azomethine molecule adopts the thermodynamically stable E regioisomer and the pyridine substituents are antiperiplanar. The mean planes of the pyridine rings and the azomethine group to which they are connected are twisted by 27.27 (5) and 33.60 (5)°. The electrochemical energy gap of 2.3 eV based on the HOMO-LUMO energy difference is in agreement with the spectroscopically derived value.

Low-cost synthesis and physical characterization of thieno[3,4-c]pyrrole-4,6-dione-based polymers.

The improved synthesis of thieno[3,4-c]pyrrole-4,6-dione (TPD) monomers, including Gewald thiophene ring formation, a Sandmeyer-type reaction, and neat condensation with an amine, is presented. This protocol enables faster, cheaper, and more efficient preparation of TPD units in comparison to traditional methods. Furthermore, a series of TPD homo- and pseudohomopolymers bearing various alkyl chains was synthesized via a direct heteroarylation polymerization (DHAP) procedure. UV-visible absorption and powder X-ray diffraction measurements revealed the relationship between the ratio of branched to linear alkyl chains and the optoelectronic properties of the polymers as well as their packing in the solid state.

Diethyl 2,5-bis-[(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)methyl-idene-amino]-thio-phene-3,4-dicarboxyl-ate acetone monosolvate.

The unique 3,4-ethyl-ene-dioxy-thio-phene (EDOT) unit of the title compound, C(24)H(22)N(2)O(8)S(3)·C(3)H(6)O, is twisted by 1.9 (3)° relative to the central thio-phene ring. The three heterocyclic units are anti-periplanar. In the crystal, inversion dimers linked by pairs of C-H⋯O hydrogen bonds connect the heterocycles. π-π interactions occur between the central thiophene and the imine bond of the molecule [distance between the ring centroid of the ring and the azomethine bond = 3.413 (3) Å.

Diethyl 2,5-bis-[(1E)-(1H-pyrrol-2-yl-methyl-idene)amino]-thio-phene-3,4-dicarboxyl-ate.

In the crystal structure of the title compound, C(20)H(20)N(4)O(4)S, the azomethine group adopt E conformations. The pyrrole units are twisted by 10.31 (4) and 18.90 (5)° with respect to the central thio-phene ring. The three-dimensional network is close packed and involves N-H⋯O, N-H⋯N, C-H⋯N and C-H⋯O hydrogen bonding.

Conjugated 4-methoxybipyrrole thiophene azomethines: synthesis, opto-electronic properties, and crystallographic characterization.

In the search of functional materials with improved electrochromic properties, thiophenes and asymmetric bipyrroles have been conjugated with azomethine units. 4-Methoxy-2,2'-bipyrroles 3-6 were first synthesized by a general route from 4-hydroxyproline and converted subsequently to dialdehydes 8-15, which underwent condensations with different aminothiophenes to provide azomethine conjugates 14-18 and 20-22. The crystallization and X-ray analysis of 20 showed the heterocycles and azomethine bonds were all co-planar with the heterocycles adopting an anti-parallel arrangement. These configurations result in extended conjugation and enhanced opto-electronic properties of the azomethines. Oxidation potential (E(pa)) was tailored by modification of the substitution pattern of the terminal thiophenes and central pyrroles of the azomethines. The combined low E(pa) and extended azomethine degree of conjugation resulted in stark color transitions occurring between their neutral and oxidized states. Reversible color formation was induced both electrochemically and by doping/de-doping with trifluoroacetic acid/triethylamine.

Diethyl 2-amino-5-[(E)-(furan-2-yl-methyl-idene)amino]-thio-phene-3,4-di-carboxyl-ate.

In the crystal structure of the title compound, C(15)H(16)N(2)O(5)S, the azomethine adopts the E configuration. The two heterocyclic rings adopt an anti-periplanar orientation. The mean planes of the thio-phene and furan rings are twisted by 2.51 (4)°. The crystal structure exhibits inter-molecular N-H⋯O hydrogen bonding. π-π stacking is also observed, the centroid-to-centroid distance being 3.770 (4) Å.

Diethyl 2-amino-5-[(E)-(1-methyl-1H-pyrrol-2-yl)methylideneamino]thiophene-3,4-dicarboxylate.

The structure of the title compound, C(16)H(19)N(3)O(4)S, shows the planes described by the thio-phene and the pyrroles are twisted by 17.06 (4)°. Additionally, the structure shows the azomethine bond adopts the E configuration, while the pyrrole is disordered as a heterocycle flip [occupancy ratio 0.729 (5):0.271 (5)]. The three-dimensional network is well packed and involves N-H⋯O hydrogen bonding and π-π stacking [centroid-centroid distance = 4.294 (8) Å].

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 fluorene's 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 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.

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.

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.

Diethyl 2,5-bis-[(E)-2-furylmethyl-ene-amino]thio-phene-3,4-dicarboxyl-ate.

The title compound, C(20)H(18)N(2)O(6)S, crystallizes as two independent mol-ecules that are disposed about a pseudo-inversion center (1/2, 1/4, 1/8). The mean planes of the two terminal furyl rings are twisted with respect to the central thio-phene ring by 7.33 (4) and 21.74 (5)° in one mol-ecule, and by 6.91 (4) and 39.80 (6)° in the other.

Diethyl 2-[(1-methyl-1H-pyrrol-2-yl)methyl-ene-amino]-5-(2-thienylmethyl-ene-amino)thio-phene-3,4-dicarboxyl-ate.

Both imine bonds of the title compound, C(21)H(21)N(3)O(4)S(2), were found to be in the E configuration. The terminal pyrrole and thio-phene rings are twisted by 2.5 (3) and 2.3 (2)°, respectively, from the mean plane of the central thio-phene to which they are attached. The structure is disordered by exchange of the terminal heterocyclic rings; the site occupancy factors are ca 0.8 and 0.2. The crystal packing involves some π-π stacking [3.449 (4) Šbetween pyrrole and terminal thio-phene rings].

Convergent preparation and photophysical characterization of dimaleimide dansyl fluorogens: elucidation of the maleimide fluorescence quenching mechanism.

Dimaleimide fluorogens are being developed for application to fluorescent protein labeling. In this method, fluorophores bearing two maleimide quenching groups do not fluoresce until both maleimide groups have undergone thiol addition reactions with the Cys residues of the target protein sequence [J. Am. Chem. Soc. 2005, 127, 559-566]. In this work, a new convergent synthetic route was developed that would allow any fluorophore to be attached via a linker to a dimaleimide moiety in a modular fashion. Series of dimaleimide and dansyl derivatives were thus prepared conveniently and used to elucidate the mechanism of maleimide quenching. Intersystem crossing was ruled out as a potential quenching pathway, based on the absence of a detectable triplet intermediate by laser flash photolysis. Stern-Volmer rate constants were measured with exogenous dimaleimide quenchers and found to be close to the diffusion-controlled limits, consistent with electron transfer being thermodynamically favorable. The thermodynamic feasibility of the photoinduced electron transfer (PET) quenching mechanism was verified by cyclic voltammetry. The redox potentials measured for dansyl and maleimide confirm that electron transfer from the dansyl excited state to a pendant maleimide group is exergonic and is responsible for fluorescence quenching of the fluorogens studied herein. Taking this PET quenching mechanism into account, future fluorogenic protein labeling agents will be designed with spacers of variable length and rigidity to probe the structure-property PET efficiency relationship.

Preparation, photophysics, and electrochemistry of segmented comonomers consisting of thiophene and pyrimidine units: new monomers for hybrid copolymers.

An efficient coupling route to novel pi-conjugated comonomers consisting of pyrimidine, thiophene, and bithiophene units was developed. The novel pi-donor-acceptor-donor and pi-donor-acceptor-acceptor-donor conjugated compounds were prepared by Suzuki heterocoupling and Ni(0)-mediated Ullman homocoupling reactions. Photophysical investigation of these alternating pi-donor and acceptor compounds indicated that the deactivation of their singlet excited state proceeds predominately by fluorescence and results in high fluorescence quantum yields. Intersystem crossing to the triplet state was also present in ca. 10%. Quantification of the triplet manifold by laser flash photolysis further revealed that bithiophene produced its triplet state in only 31%. Cyclic voltammetry studies showed that the comonomers undergo both oxidation and reduction leading to their radical cations and radical anions, respectively. The radical cations are highly reactive and undergo anodic polymerization resulting in mutual p- and n-type dopable polymers. The extended conjugation resulting from polymer formation was confirmed by both absorbance and fluorescence spectroscopy and by GPC. Ruthenium binding with the conjugated homocoupled ligand was also found resulting in a hybrid alternating copolymer with significantly different spectroscopic and electrochemical properties relative to its metal-free counterpart.

A fluoresceinophane: 1(9),4,16-trioxa-1(2,10)-anthracena-2(1,2)-benzenacyclohexadecaphane-1(7),3(1(9)H)-dione.

A novel macrocycle containing fluorescein, the highly fluorescent title compound, C(31)H(32)O(5), has a xanthene core and a benzyl unit that are planar. The latter is rotated by 72.99 (3) degrees from the xanthene mean plane. The C(11) alkyl tether and the xanthene group adopt a cage-like structure and the xanthene adopts a quinoid-type configuration. The compound crystallizes as a racemic mixture with one molecule of each isomer per unit cell. Even though the planes described by the xanthene and the benzene rings of different molecules are separated by 3.341 (4) and 3.73 (1) A, respectively, there is insufficient overlap between the aryl units to promote pi-stacking.

Photophysical, crystallographic, and electrochemical characterization of symmetric and unsymmetric self-assembled conjugated thiopheno azomethines.

Novel conjugated azomethines consisting uniquely of thiophene units are presented. The highly conjugated compounds were synthesized by simple condensation of a stable diamino thiophene (2) with its complementary thiophene aldehydes. These interesting nitrogen-containing thiophene units exhibit variable reactivity leading to controlled aldehyde addition. Because of the different amino reactivity, a one-pot synthesis of unsymmetric and symmetric conjugated azomethines with varying number of thiophene units was possible by judicious choice of solvent and careful control of reagent stoichiometry. The resulting covalent conjugated connections are both reductively and hydrolytically resistant. The thermodynamically E isomer is formed uniquely for all of the azomethines synthesized, which is confirmed by crystallographic studies. These also demonstrated that the azomethine bonds and the thiophene units are highly planar and linear. The fluorescence and phosphorescence of the thiopheno azomethines measured are similar to those of thiophene analogues currently used in functional devices, but with the advantage of low triplet formation and band-gaps as low as 1.9 eV. The time-resolved and steady-state temperature-dependent photophysics revealed the thiopheno azomethines do not populate extensively their triplet manifold by intersystem crossing. Rather, their excited-state energy is dissipated predominantly by nonradiative means of internal conversion. Quasi-reversible electrochemical radical cation formation of the thiophene units was found. These compounds further undergo electrochemically induced oxidative cross-coupling, resulting in conjugated products that also exhibit reversible radical cation formation.

(E)-5-(2-Thienylmethyl-eneamino)quinolin-8-ol.

Two mol-ecules of the title compound, C(14)H(10)N(2)OS, are hydrogen bonded about a center of inversion. In the mol-ecule, the two aromatic rings are twisted by 37.27 (5)° with respect to one another. The azomethine bond is in the E configuration.

Easy one-pot synthesis of energy transfer cassettes.

Novel cassettes suitable for energy transfer are presented involving simple and efficient condensation methods. The new rigid cassettes obtained by Schiff base formation are capable of undergoing energy transfer from a donor to an acceptor core both through space and through bonds.