Synthesis and photoluminescence properties of π-extended fluorene derivatives: the first example of a fluorescent solvatochromic nitro-group-containing dye with a high fluorescence quantum yield

Functional π-extended fluorene derivatives, 2,7-di(p-substituted-phenyl)fluorenes containing different functional groups such as hydrogen, trimethylsilyl (TMS), methoxycarbonyl, cyano, and nitro groups, were synthesized. Except for the nitro group, the resulting compounds exhibited extremely high fluorescence quantum yields (Φ_F >0.85 in chloroform). The diphenylfluorene containing nitro groups have higher fluorescence quantum yield (Φ_F = 0.31 in N,N’-dimethyl-formamide) than other nitro-group-containing fluorophores which were previously reported (Φ_F <0.1). Furthermore, this compound exhibited large Stokes’ shift with green to orange emission and unique on-off behavior of the emission by solvents.     

Selective fluorescence turn-on of a prefluorescent azomethine with Zn

A prefluorescent conjugated azomethine (4) was prepared by condensing 8-hydroxyquinoline-5-carbaldehyde with 2-amine thiophene. The fluorescence of the azomethine was quenched in organic solvents including dichloromethane, methanol, DMSO, and DMF. However, the fluorescence of 4 was selectively revived in the presence of zinc and an absolute quantum yield Φ_fl = 0.15 was measured.     

Electrochemical and Solvent-Mediated Visible-to-Near-Infrared Spectroscopic Switching of Benzoselenadiazole Fluorophores

A series of electronic push–pull, pull–pull, and push fluorophores has been prepared from a benzoselenadiazole core so that their spectroscopic, electrochemical, spectro-electrochemical, and spectro-electrofluorescence properties could be examined. The emission wavelengths and fluorescence quantum yields (Φ_fl) of the N,N-dimethyl fluorophores were contingent on the solvent polarity and they ranged from 615 to 850 nm in aprotic solvents. The positive solvatochromism and the quenched Φ_fl in polar solvents were consistent with an intramolecular charge-transfer state (ICT). Meanwhile, a locally excited state (LE) was assigned in nonpolar solvents from the blue-shifted emission and high Φ_fl. The N,N-dimethylamine fluorophores examined could be both electrochemically oxidized and reduced, whereas the symmetric dinitro pull–pull derivative could be only reversibly reduced. Courtesy of their electrochemical reversibility, the fluorophores could reversibly change color from yellow to blue with an applied potential in addition to switching off their emission. The absorption of the electrochemically generated intermediates of the N,N-dimethyl derivatives spanned 500 nm over the visible and the NIR regions. The colors could be switched for upwards of two hours with applied potential, illustrating their potential use as electroactive materials in electrochromic devices.     

Hexaphenylbenzene end-capped tri(p-phenylenevinylenes): synthesis and properties

Three new hexaphenylbenzene end-capped tri(p-phenylenevinylenes) were synthesized by the Horner–Wadsworth–Emmons reaction in good yields (86–76%). All compounds have bright fluorescent emissions in the blue to blue–green region in solution (λ_max = 445–492 nm in tetrahydrofuran) and also high emission efficiency (Φ_fl = 0.51–0.72 in tetrahydrofuran). They exhibit good electrochemical reversibility, high thermal stability, and have high HOMO value.     

Synthesis and optical properties of conjugated N,N-dimethyl and thienyl end-capped 2,5-(arylethynyl)thiophene oligomer structures

End-capped (N,N-dimethylaminophenyl) and 2′-thienylethynyl 2,5-thiophene oligomer structures were synthesized by heterocoupling between the terminal acetylenes such as: p-(N,N-dimethylaminophenyl)ethyne (3) [or 1-(p-(N,N-dimethylaminophenyl)-2-p-(ethynylphenyl)ethyne, 4]; p-(β-ethenyl-2′-thienyl)phenylethyne (E-9) [or p-(β-ethynyl-2′-thienyl)phenylethyne, 11], and 2,5-diiodothiophene, catalyzed by the Cl₂Pd(PPh₃)₂/CuI system, in good to excellent yields. The 2,5-di[(3′,5′-di(trimethylsilylethynyl)phenyl]_x-1-ethynyl]thiophene oligomers were prepared by heterocoupling between 3′,5′-di[(trimethylsilylethynyl)phenyl]_x-1-ethyne (n = 0-2) terminal acetylenes and 2,5-diiodothiophene, in excellent yields. The terminal acetylenes were efficiently prepared by a specific protection-deprotection methodology. All the ethynylphenyl compounds obtained show fluorescence radiation emission, with a bathochromic shift of the wavelength that increases with the chain conjugation.     

Synthesis of B-trisubstituted borazines via the rhodium-catalyzed hydroboration of alkenes with N,N′,N″-trimethyl or N,N′,N″-triethylborazine

Hydroboration of terminal and internal alkenes with N,N′,N″-trimethyl- and N,N′,N″-triethylborazine was carried out at 50 °C in the presence of a rhodium(I) catalyst. Addition of dppb or DPEphos (1 equiv.) to RhH(CO)(PPh₃)₃ gave the best catalyst for hydroboration of ethylene at 50 °C, resulting in a quantitative yield of B,B′,B″-triethyl-N,N′,N″-trimethylborazine. On the other hand, a complex prepared from (t-Bu)₃P (4 equiv.) and [Rh(coe)₂Cl]₂ gave the best yield for hydroboration of terminal or internal alkenes.     

N,N-polymethylene-bisadenine complexes with d metal ions

The reaction of N⁹,N^9′-(tri or tetramethylene)-bisadenines (Ade₂C_x; x = 3 or 4) in HCl 2 M at 50 °C with MCl₂ · 2H₂O [M = Zn(II), Cd(II)] yields outer sphere compounds like the previously described [(H-Ade)₂C₃][ZnCl₄] · H₂O (3) and [(H-Ade)₂C₃]₂[Cd₂Cl₈(H₂O)₂] · 4H₂O (4) for Ade₂C₃ and the new {[(H-Ade)₂C₄][Cd₂Cl₆(H₂O)₂] · 2H₂O}_n (5) for Ade₂C₄. On the other hand, only in case of Zn(II) complexes by changing [HCl] to 0.1 M, the inner sphere compounds [H-(Ade)₂C₃(ZnCl₃)] (6) and [H-(Ade)₂C₄(ZnCl₃)] · 1.5H₂O (7) are obtained. X-ray diffraction study of compound 6, which represents the first inner sphere complex with a N⁹,N^9′-bisadenine, shows a zwitterionic form with one adenine ring protonated at N(1) while the other ring is coordinated via N(7) to a ZnCl₃ moiety as in other alkyl-adenine derivatives. In addition, with Ade₂C₄, is also possible to obtain another inner sphere complex: [(H-Ade)₂C₄(ZnCl₃)₂] · 3H₂O (8).     

Complexes derived from the general copper(II)/maleamic acid/N,N′,N′′-chelate reaction systems: Synthetic,reactivity, structural and spectroscopic studies

The synthetic investigation of the Cu^II/maleamate(−1) ion (HL⁻)/N,N′,N′′-chelate general reaction system has allowed access to compounds [Cu₂(HL)₂(bppy)₂](ClO₄)₂·H₂O (1·H₂O), [Cu(HL)(bppy)(ClO₄)] (2) and [Cu(HL)(terpy)(H₂O)](ClO₄) (4) (bppy = 2,6-bis(pyrazol-1-yl)pyridine, terpy = 2,2′;6′,2′′-terpyridine). In the absence of externally added hydroxides, compound [Cu₂(L′)₂(bppy)₂](ClO₄)₂ (3) was obtained from MeOH solutions; L′⁻ is the monomethyl maleate(−1) ligand which is formed in situ via the Cu^II-assisted HL⁻ → L′⁻ transformation. In the case of tptz-containing (tptz = 2,4,6-tris(2-pyridyl)-1,3,5-triazine) reaction systems, the Cu^II-assisted hydrolysis of tptz to pyridine-2-carboxamide (L1) afforded complex [Cu(L1)₂(NO₃)₂] (5). The crystal structures of 1–5 are stabilized by intermolecular hydrogen bonding and π–π stacking interactions. Characteristic IR bands of the complexes are discussed in terms of the known structures and the coordination modes of the ligands.     

Structural, spectral and thermal studies of substituted N-(2-pyridyl)-N′-phenylthioureas

N-2-(3-picolyl)-N′-phenylthiourea, 3PicTuPh, monoclinic, P2₁/n, a=7.617(2) b=7.197(5), c=22.889(5) Å, β=94.63(4)°, V=1250.7(1) Å³ and Z=4; N-2-(4-picolyl)-N′-phenylthiourea, 4PicTuPh, triclinic, P-1, a=7.3960(5), b=7.9660(12), c=21.600(3) Å, α=86.401(4), β=84.899(8), γ=77.769(8)°, V=1237.5(3) Å³ and Z=4; N-2-(5-picolyl)-N′-phenylthiourea, 5PicTuPh, monoclinic, P2₁/c, a=14.201(1), b=4.905(3), c=17.689(3) Å, β=91.38(1)°, V=1231.8(7) Å³ and Z=4; N-2-(6-picolyl)-N′-phenylthiourea, 6PicTuPh, monoclinic, C2/c2, a=14.713(1), b=9.367(1), c=18.227(1) Å, β=92.88(1)°, V=2515.5(1) Å³ and Z=8 and N-2-(4,6-lutidyl)-N′-phenylthiourea, 4,6LutTuPh, monoclinic, C2/c, a=11.107(2), b=11.793(2), c=20.084(4) Å, β=96.10(3)°, V=2616(1) Å³ and Z=8. Intramolecular hydrogen bonding between N′H and the pyridyl nitrogen and intermolecular hydrogen bonding involving the thione sulfur are affected by substitution of the pyridine ring, as is the planarity of the molecule. ¹H NMR studies in CDCl₃ show the NH′ hydrogen resonance considerably downfield from other resonances in the spectrum for each thiourea.     

Synthesis, characterization and Schlenk equilibrium studies of methylmagnesium compounds with O- and N-donor ligands - the unexpected behavior of [MgMeBr(pmdta)] (pmdta = N,N,N′,N″,N″-pentamethyldiethylenetriamine)

MgMe₂ (1) was found to react with 1,4-diazabicyclo[2.2.2]octane (dabco) in tetrahydrofuran (thf) yielding a binuclear complex [{MgMe₂(thf)}₂(μ-dabco)] (2). Furthermore, from reactions of MgMeBr with diglyme (diethylene glycol dimethyl ether), NEt₃, and tmeda (N,N,N′,N′-tetramethylethylenediamine) in etheral solvents compounds MgMeBr(L), (L = diglyme (5); NEt₃ (6); tmeda (7)) were obtained as highly air- and moisture-sensitive white powders. From a thf solution of 7 crystals of [MgMeBr(thf)(tmeda)] (8) were obtained. Reactions of MgMeBr with pmdta (N,N,N′,N″,N″-pentamethyldiethylenetriamine) in thf resulted in formation of [MgMeBr(pmdta)] (9) in nearly quantitative yield. On the other hand, the same reaction in diethyl ether gave MgMeBr(pmdta) · MgBr₂(pmdta) (10) and [{MgMe₂(pmdta)}₇{MgMeBr(pmdta)}] (11) in 24% and 2% yield, respectively, as well as [MgMe₂(pmdta)] (12) as colorless needle-like crystals in about 26% yield. The synthesized methylmagnesium compounds were characterized by microanalysis and ¹H and ¹³C NMR spectroscopy. The coordination-induced shifts of the ¹H and ¹³C nuclei of the ligands are small; the largest ones were found in the tmeda and pmdta complexes. Single-crystal X-ray diffraction analyses revealed in 2 a tetrahedral environment of the Mg atoms with a bridging dabco ligand and in 8 a trigonal-bipyramidal coordination of the Mg atom. The single-crystal X-ray diffraction analyses of [MgMe₂(pmdta)] (12) and [MgBr₂(pmdta)] (13) showed them to be monomeric with five-coordinate Mg atoms. The square-pyramidal coordination polyhedra are built up of three N and two C atoms in 12 and three N and two Br atoms in 13. The apical positions are occupied by methyl and bromo ligands, respectively. Temperature-dependent ¹H NMR spectroscopic measurements (from 27 to −80 °C) of methylmagnesium bromide complexes MgMeBr(L) (L = thf (4); diglyme (5); NEt₃ (6); tmeda (7)) in thf-d₈ solutions indicated that the deeper the temperature the more the Schlenk equilibria are shifted to the dimethylmagnesium/dibromomagnesium species. Furthermore, at −80 °C the dimethylmagnesium compounds are predominant in the solutions of Grignard compounds 4-6 whereas in the case of the tmeda complex7 the equilibrium constant was roughly estimated to be 0.25. In contrast, [MgMeBr(pmdta)] (9) in thf-d₈ revealed no dismutation into [MgMe₂(pmdta)] (12) and [MgBr₂(pmdta)] (13) even up to −100 °C. In accordance with this unexpected behavior, 1:1 mixtures of 12 and 13 were found to react in thf at room temperature yielding quantitatively the corresponding Grignard compound 9. Moreover, the structures of [MgMeBr(pmdta)] (9c), [MgMe₂(pmdta)] (12c), and [MgBr₂(pmdta)] (13c) were calculated on the DFT level of theory. The calculated structures 12c and 13c are in a good agreement with the experimentally observed structures 12 and 13. The equilibrium constant of the Schlenk equilibrium (2 9c ? 12c + 13c) was calculated to be K_gas = 2.0 × 10⁻³ (298 K) in the gas phase. Considering the solvent effects of both thf and diethyl ether using a polarized continuum model (PCM) the corresponding equilibrium constants were calculated to be K_thf = 1.2 × 10⁻³ and K_ether = 3.2 × 10⁻³ (298 K), respectively.     

Spectroscopic and structural characterization of Na2[(VO)2(ttha)]·8H2O (ttha = triethylenetetraamine-N,N,N′,N″,N′″,N′″-hexaacetate): Interpreting the results with density functional theory

Na₂[(V^IVO)₂(ttha)]·8 H₂O (ttha = triethylenetetraamine–N,N,N′,N″,N′″,N′″–hexaacetate ion), prepared by treating [VO(H₂O)₅][(VO)₂(ttha)]·4 H₂O with Na₆(ttha), has been characterized by single crystal X-ray diffraction, infrared spectroscopy, UV–Vis absorption spectroscopy, electron spin resonance spectroscopy, and modeled by density functional theory (DFT). The X-ray structure revealed a distorted octahedral geometry around each vanadium center. The electronic absorption spectrum of [(VO)₂(ttha)]²⁻ (aq) features absorptions at ca. 200 nm (ε > 13900 L mol⁻¹ cm⁻¹), 255 nm (ε = 3480 L mol⁻¹ cm⁻¹), 586 nm (ε = 33 L mol⁻¹ cm⁻¹), and 770 nm (ε = 38 L mol⁻¹ cm⁻¹). The time-dependent density functional theory (TDDFT) calculated electronic absorption spectrum was remarkably similar to the actual spectrum, and TDDFT predicts absorption peaks at 297, 330, 458, 656, and 798 nm. TDDFT assigned the peak at 798 nm to be the α spin HOMO → LUMO transition. Hence, the peak at 770 nm in the actual spectrum is most likely the α spin HOMO → LUMO transition. Moreover, the TDDFT calculations revealed that the α spin HOMO and LUMO are partly comprised of d orbitals on both vanadium centers, and the first derivative electron spin resonance spectrum also suggests that the two unpaired electrons in [(VO)₂(ttha)]²⁻ are localized near the vanadium centers.     

Novel photoluminescent hemi-disclike liquid crystalline Zn(II) complexes of [N2O2] donor 4-alkoxy substituted salicyldimine Schiff base with aromatic spacer

A series of novel hemi-disclike four coordinated distorted square planar Zn(II) Schiff base complexes containing 4-substituted alkoxy chains on the side aromatic ring [Zn (4−C_nH_2n+1O)₂ salophen], n = 14, 16, 18 and salophen = N,N′-4-methyl phenylene bis (salicylideneiminato), have been prepared and their mesogenic, photophysical properties were investigated. The phase behavior of these compounds were characterized by differential scanning calorimetry, polarized optical microscopy and variable temperature PXRD study. The ligands are non-mesogenic but the complexes exhibited an unprecedented 2D-hexagonal columnar mesophase (Col_h) in the temperature 175–185 °C range. In the mesophase (Col_h), the molecules self assemble in a columnar stack in antiparallel fashion. All λ_max of the UV–Vis absorption and photoluminescence band occurred at ca. 291–425 and 504–524 nm, respectively. The ligands are non-emissive, but on coordination with Zn(II), the complexes show intense green emission at room temperature in dichloromethane solution (∼505 nm, Φ = 20%) as well as in solid (∼522 nm, Φ = 9%) at 360 nm excitation. The DFT calculations were performed using Dmol3 program at BLYP/DNP level to obtain the stable electronic structure of the complex. A small LUMO-HOMO band gap (∼2.1 eV), presumably suggests a rather strong electronic correlation among the molecules along the column.     

Coordination compounds of N,N′-olefin functionalized imidazolin-2-ylidenes

N-Heterocyclic carbene ligands (NHC) were metalated with Pd(OAc)₂ or [Ni(CH₃CN)₆](BF₄)₂ by in situ deprotonation of imidazolium salts to give the N-olefin functionalized biscarbene complexes [MX₂(NHC)₂] 3-7 (3: M = Pd, X = Br, NHC = 1,3-di(3-butenyl)imidazolin-2-ylidene; 4: M = Pd, X = Br, NHC = 1,3-di(4-pentenyl)imidazolin-2-ylidene; 5: M = Pd, X = I, NHC = 1,3-diallylimidazolin-2-ylidene; 6: M = Ni, X = I, NHC = 1,3-diallylimidazolin-2-ylidene; 7: M = Ni, X = I, NHC = 1-methyl-3-allylimidazolin-2-ylidene). Molecular structure determinations for 4-7 revealed that square-planar complexes with cis (5) or trans (4, 6, 7) coordination geometry at the metal center had been obtained. Reaction of nickelocene with imidazolium bromides afforded the η⁵-cyclopentadienyl (η⁵-Cp) monocarbene nickel complexes [NiBr(η⁵-Cp)(NHC)] 8 and 9 (8: NHC = 1-methyl-3-allylimidazolin-2-ylidene; 9: NHC = 1,3-diallylimidazolin-2-ylidene). The bromine abstraction in complexes 8 and 9 with silver tetrafluoroborate gave complexes [NiBr(η⁵-Cp)(η³-NHC)] 10 and 11. The X-ray structure analysis of 10 and 11 showed a trigonal-pyramidal coordination geometry at the nickel(II) center and coordination of one N-allyl substituent.     

Electrochemical and electrochromic properties of novel aromatic poly(amine-amide)s derived from N,N′-bis(4-carboxyphenyl)-N,N′-diphenyl-1,4-phenylenediamine

A series of novel triphenylamine-containing aromatic poly(amine-amide)s were prepared from the dicarboxylic acid, N,N′-bis(4-carboxyphenyl)-N,N′-diphenyl-1,4-phenylenediamine, and various diamines by direct phosphorylation polycondensation. All the poly(amine-amide)s were amorphous, soluble in a variety of organic solvents, and could be solution cast into transparent, tough, and flexible films with good mechanical properties. They had useful levels of thermal stability associated with relatively high glass-transition temperatures (195-280 °C). These polymers exhibited strong UV-Vis absorption bands at 330-346 nm and their photoluminescence showed maximum bands around 516-535 nm in NMP solution. The hole-transporting and electrochromic properties are examined by electrochemical and spectroelectrochemical methods. Cyclic voltammograms of the poly(amine-amide)s prepared by casting polymer solution onto an indium-tin oxide (ITO)-coated glass substrate exhibited two reversible oxidative redox couples at potential 0.73-0.78 V and 1.12-1.18 V, respectively vs Ag/AgCl in acetonitrile solution. All the poly(amine-amide)s exhibited excellent reversibility of electrochromic characteristics by continuous ten cyclic scans between 0.0 and 1.40 V, with a color change from original pale yellowish neutral form to the green and then to blue oxidized forms.     

Pyridine-cored V-shaped π-conjugated oligomers: synthesis and optical properties

A new series of V-shaped pyridine-cored π-conjugated oligomers are synthesized utilizing two-fold Heck/Suzuki coupling reactions. Optical properties of these compounds (λ_max=390–449 nm, Φ_fl=79–5%, in solutions) are discussed. They are shown to be thermally stable and soluble in common organic solvents. Stilbenoid oligomers exhibited much higher fluorescence quantum yields than tri- and tetra-phenylethylene substituted oligomers in solutions.     

Radical polymerization behavior of 3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate in water

The homogeneous polymerization of 3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate (MDAPS) with potassium peroxydisulfate (KPS) was kinetically in situ investigated in water by means of FT-near IR spectroscopy. The overall activation energy of the polymerization was calculated to be 16.0 kcal/mol. The initial polymerization rate (R_p) at 40 °C was expressed by R_p=k[KPS]^0.65[MDAPS]^1.0. The presence of alkaline metal salts was observed to accelerate the polymerization. The order of acceleration at 40 °C was CsCl > KCl > NaCl > LiCl when the chloride salts were used. NaCl showed higher acceleration effect than NaF. NaBr and NaI exhibited retardation and inhibition effect, respectively, because of reduction of KPS and its primary radical with bromide and iodide ions. The polymerization of MDAPS with KPS in water in the presence of NaCl at 2.0 mol/l gave R_p=k[KPS]^0.70[MDAPS]^1.4 at 40 °C. The overall activation energy of the polymerization in the presence of NaCl was estimated to be 11.6 kcal/mol being considerably lower value compared with that in its absence. The syndiotacticity of poly(MDAPS) tended to increase with rising temperature and decrease in the presence of NaCl.     

Preparation, structure and photoluminescence properties of Eu and Ce-doped SrYSi4N7

Undoped and Eu²⁺ or Ce³⁺-doped SrYSi₄N₇ were synthesized by solid-state reaction method at 1400-1660 °C under nitrogen/hydrogen atmosphere. The crystal structure was refined from the X-ray powder diffraction data by the Rietveld method. SrYSi₄N₇ and EuYSi₄N₇, being isotypic with the family of compounds MYbSi₄N₇ (M=Sr, Eu, Ba) and BaYSi₄N_7, crystallize with the hexagonal symmetry: space group P6₃mc (No. 186), Z=2, a=6.0160 (1) Å, c=9.7894 (1) Å, V=306.83(3) Å³; and a=6.0123 (1) Å, c=9.7869 (1) Å, V=306.37(1) Å³, respectively. Photoluminescence properties have been studied for Sr_1−xEu_xYSi₄N₇ (x=0-1) and SrY_1−xCe_xSi₄N₇ (x=0-0.03) at room temperature. Eu²⁺-doped SrYSi₄N₇ shows a broad yellow emission band peaking around 548-570 nm, while Ce³⁺-doped SrYSi₄N₇ exhibits a blue emission band with a maximum at about 450 nm. SrYSi₄N₇:Eu²⁺ can be very well excited by 390 nm radiation, which makes this material attractive as conversion phosphor for LED lighting applications.     

Liquid crystal behaviour of ionic allylpalladium complexes containing 2-pyrazolylpyridine as bidentate N,N′-ligand

Two types of Pd-complexes containing the new N,N′-ligands 2-[3-(4-alkyloxyphenyl)pyrazol-1-yl]pyridine (pz^Rpy; R = C₆H₄OC_nH_2n+1, n = 6 (hp), 10 (dp), 12 (ddp), 14 (tdp), 16 (hdp), 18 (odp)) (1-6), namely c-[Pd(Cl)₂(pz^Rpy)] (7-10) and c-[Pd(η³-C₃H₅)(pz^Rpy)]BF₄ (11-16), have been synthesised and characterised by different spectroscopic techniques. Those members of the second type containing the largest chains (R = ddp 13, tdp 14, hdp 15, odp 16) have been found to have liquid crystal properties showing smectic A mesophases. By contrast, neither the free ligands pz^Rpy nor their related c-[Pd(Cl)₂(pz^Rpy)] complexes exhibited mesomorphism. The new synthesised metallomesogens are mononuclear complexes with an unsymmetrical molecular shape as deduced from the X-ray structures of c-[Pd(η³-C₃H₅)(pz^Rpy)]BF₄ (R = hp, 11; dp, 12). Both compounds, which are isostructural, show a distorted square-planar environment on the palladium centres defined by the allyl and the bidentate pz^Rpy ligands. The crystal structure reveals that both the counteranion and the pz^Rpy ligand function as a source of hydrogen-bonding and intermolecular π?π contacts resulting in a ²D supramolecular assembly.     

Stereoselective synthesis of (E)-N,N-diethyl-3-(imidazo[1,2-a]pyrid-2-yl)-2-(phenylsulfonyl)propenamide

The title compound, gem-amidovinylsulfone 3, was synthesized stereoselectively by aldolic condensation of N,N-diethylphenylsulfonylacetamide 1 on imidazo[1,2-a]pyridine-2-carbaldehyde 2 adding Et₃N at the end. The X-ray crystal structure of 3 [C₂₀H₂₁N₃O₃S: M_r=383.5, monoclinic, P2₁, a=8.191(4) Å, b=21.132(2) Å, c=11.752(1) Å, β=96.40(2)°, V=2022(1) Å³, Z=4 (two molecules per asymmetric unit), D_calc=1.260 g cm⁻³, λ(Mo Kα)=0.71073 Å, μ=0.184 mm⁻¹, F(000)=808, T=293(2)K, R=0.059 for 5105 observed reflections with I≥2σ(I)] was determined, and confirmed the (E) configuration.     

Versatility of an intramolecularly hydrogen-bonded 4-(N,N-dimethylamino)benzoate group as a signaling subunit for anion recognition

The versatility of the 4-(N,N-dimethylamino)benzoate (DMAB) group embedded in host 1 as a signaling subunit for anion recognition was elucidated in terms of ¹H NMR, CD, and fluorescence studies. Host 1 showed 1:1 complexation with monovalent anions and stepwise 1:1 and 2:1 (host 1: anion) complexation with divalent phosphate anions. The binding constants between host 1 and anions were determined by means of ¹H NMR titrations in CD₃CN (HPO₄²⁻: log K_1:1=6.2, log K_2:1=4.9; H₂P₂O₇²⁻: log K_1:1=4.4, log K_2:1=1.8; AMP²⁻: log K_1:1>7, log K_2:1>5) and the affinity of host 1 toward divalent anions, HPO₄²⁻, H₂P₂O₇²⁻, and AMP²⁻, is stronger than that toward monovalent anions, NO₃⁻, BF₄⁻, ClO₄⁻, HSO₄⁻, and PF₆⁻. The CD exciton chirality studies of host 1 with divalent anions, HPO₄²⁻ and AMP²⁻, revealed that the two DMAB groups in the 2:1 complexes were arranged with negative chirality (counterclockwise). The dual fluorescence behavior of the DMAB group demonstrated not only the complexation stoichiometry but also the role(s) of the lipophilic countercation such as tetrabutylammonium and/or the hydrophilic residue in AMP during anion recognition.