Enhancement of intramolecular excimer formation and photodimerization of anthrylmethyl α,ω-alkanedioates via hydrophobic interactions

The fluorescence spectra and photodimerization of anthrylmethyl a,w-alkanedioates (A-Mn-A) both in organic and in aqueous organic mixed solvents have been studied.In aqueous organic mixed solvents strong intramolecular excimer emission is detected and the quantum yield for the intramolecular photodimerization is significantly greater than those in organic solvents.These observations suggest that hydrophobic interactions force A-Mn-A molecule to self-coil.The ratio of the head-to-head to head-to-tail products in the intramolecular photodimers of A-Mn-A depends on the length of the linking chain.This work presents a successful example of application of hydrophobic interactions to enhancement of large-ring formation.     

Time-resolved fluorescence of α-(9-anthryl)-ω-(1-naphthyl)-oligosilanes: intramolecular electronic energy and charge transfer through π-π and σ-π interactions

The photophysical properties of a series of α,ω-diaryloligosilanes: (9-anthryl)-(SiMe₂)_n-(1-naphthyl) (1-4, and 6), n=1, 2, 3, 4, and 6, were investigated. For n?2, the intramolecular exciplex emission was observed in cyclohexane. For the stationary and the time-resolved fluorescence (TR-FL) measurements, the excitation of the naphthyl moiety showed a very efficient intramolecular energy transfer to the anthryl moiety, thus only fluorescence from the anthryl moiety was observed. In the case of 4 and 6, a charge-transfer (CT) emission was observed in acetonitrile. Then TR-FL of 6 revealed that the intramolecular-energy transfer from the naphthyl to anthryl moieties is faster than the CT.     

Electrochemistry, electrogenerated chemiluminescence, and excimer formation dynamics of intramolecular π-stacked 9-naphthylanthracene derivatives and organic nanoparticles

We report the electrochemical characterization and the observation of excimer emission from a series of 9-naphthylanthracene-based dimer- and trimer-bridged high steric hindrance aromatic groups during photoluminescence (PL) measurements in the solid state and in solution electrogenerated chemiluminescence (ECL) measurements. Cyclic voltammetry of 4,4'-bis(9-(1-naphthyl)anthracen-10-yl)biphenyl (4A) and 1,3,5-tris(9-(1-naphthyl)anthracen-10-yl)benzene (4C) showed two or three reversible, closely spaced one-electron transfers on oxidation in dichloromethane. The ECL emission spectra of 4A and 4C resulting from the annihilation reaction in benzonitrile showed two bands: one at the same wavelength as the PL peak in the solution state, and a broad band at longer wavelength. With a coreactant, such as peroxydisulfate, ECL spectra showed a single peak that was less broad in shape. PL measurement in the solid state and measurement of representative time traces of PL intensity, lifetimes, and picosecond time-correlated single-photon counting confirmed excimer emission at long wavelength. A reprecipitation method was used to prepare well-dispersed organic nanoparticles (NPs) of 4A in both aqueous and acetonitrile solutions. The smallest stable size of NPs produced was ~15 ± 6 nm, as analyzed by transmission electron microscopy. These organic NPs produced stable and weak ECL emission from the annihilation reaction in both aqueous and MeCN solutions. With a coreactant, such as peroxydisulfate, the ECL signal on reduction was sufficiently strong to obtain an ECL spectrum.     

Cholic acid-based high sensitivity fluorescent sensor for α,ω-dicarboxylate: an intramolecular excimer emission quenched by complexation

Fluorescent receptors (1 and 2) bearing two binding units on C3 and C24 and two signal display units on C7 and C12 of cholic acid, respectively, were designed and synthesized. Both 1 and 2 emit a much weaker fluorescence than that of the control compound 3 lacking of the binding units reflecting that a PET process originated from the C-3 thiourea group to the plural pyrenyl pendant groups is operative. Addition of terminal dicarboxylate anions to the CH₃CN solution of 1 or 2 enhances the PET process, which leads to a significant and highly sensitive fluorescence response, resulting in an almost complete quenching of the excimer emission of the signal units. To maximize the interaction of the host and the guest, carboxylates of more than five carbon chains could penetrate through the space between the two pyrenyl pendants of the host, triggering a considerable conformational change of the fluorophores. As a result, an enhancement of the monomer emission at the expense of the excimer emission will take place. The binding properties and mechanism of 1 and 2 to dicarboxylates in CH₃CN were manifested by the combined fluorescence, UV-vis, and ¹H NMR spectroscopic method.     

Synthesis and characterization of some α-naphthyl selenium/tellurium derivatives: X-ray crystal structure of benzyl-1-naphthyl selenide and diphenylmethyl-1-naphthyl selenide

A large number of α-naphthyl selenium and tellurium compounds (1-14) have been prepared through two different methods. The first method involves the alkylation of sodium 1-naphthylselenolate/tellurolate, generated in situ using hydrazine hydrate as reducing agent while the second method involves the reaction of in situ generated α-naphthylseleno/telluromagnesium bromide with an appropriate electrophile. The synthesized alkyl-1-naphthyl selenides/tellurides and some α,ω-bis(1-naphthylseleno)alkanes have been characterized with the help of elemental analysis and using various spectroscopic techniques viz., NMR (¹H, ¹³C, ⁷⁷Se and ¹²⁵Te), IR, UV/vis spectroscopy and mass spectrometry (only in few representative cases). Interpretation of ¹H, ¹³C NMR spectra and assignment of individual resonances for tris(1-naphthylseleno)methane have been done with the help of [¹H-¹H] and [¹H-¹³C] correlation spectroscopy (COSY). X-ray crystallographic results and molecular geometry of benzyl-1-naphthyl selenide, 2 and diphenylmethyl-1-naphthyl selenide, 3 have also been illustrated.     

Quenching of exciplex and charge-transfer complex fluorescence of poly(N-vinylcarbazole) – benzanthrone system

When benzanthrone (Bt), a weak electron acceptor, is doped into poly(N-vinylcarbazole) (PVCz) solution or film, an excited carbazole chromophore (D*) interacts with Bt to form a new exciplex state, which gives a broad fluorescence band (λ_max = 440 nm) in solution and a new state, which gives broad fluorescence (λ_max = 550 nm) in the film. In order to elucidate the origin of these new states, we have studied the results of experiments for absorption, concentration dependence of the excimer and exciplex fluorescence quenching, both in solution and in the film, and electric field-induced fluorescence quenching in the film. Taking into account that (i) the new state formation in the PVCz film containing small amounts of Bt enhances the photocurrent in the absorption region, where the photon energy is insufficient to excite polymer molecules directly into the conduction state, (ii) the 550 nm fluorescence of the PVCz - Bt system in film is only partly quenched by electric field, (iii) the appearance of structureless tail in the fluorescence excitation spectrum, the charge transfer interaction model of the PVCz - Bt system in film is proposed.     

The rate of intramolecular end-to-end photodimerisation of α,ω-dianthrylpolystyrene in solution

To study the effect of molecular weight on the rate of the intramacromolecular end-to-end reaction of a flexible polymer, the intramolecular photodimerisation of α,ω-dianthrylpolystyrene has been carried out by successive illumination with a 337 nm pulsed nitrogen laser of a dilute solution of butanone at 30°C. The extent of dimerisation against the number of pulses (measured fluorometrically) was converted, based on several assumptions, to the apparent diffusion-controlled rate constants for polymer cyclisation. The rate constants for polymer cyclisation decrease in proportion to DP^−0·3 for the range of DP = 110–2600. The implications of this result in the chain dynamics of polymers are briefly discussed.     

Synthesis,structure, and properties of anhydrous organic-inorganic proton-exchange membranes based on sulfonated derivatives of octahedral oligosilsesquioxanes and α,ω-di(triethoxysilyl) oligo(oxyethylene urethane urea)

A method of obtaining new organic-inorganic nanostructured proton-exchange membranes operating via the anhydrous proton-conduction mechanism is proposed. An oligo(ethylene oxide) component serves as a proton-conducting phase in these membranes, and the sulfo derivatives of octahedral oligosilsesquioxanes of the acidic and acid-base types are used as proton-donor dopants. These compounds are synthesized via the reaction of octaaminopropyl oligosilsesquioxane with the cyclic anhydride of 2-sulfobenzoic acid at various ratios and contain sulfo groups solely or sulfo and amine groups in the organic frame. The combination of these compounds taken at concentrations of 20 and 50 wt % with α,ω-di(triethoxysilyl) oligo(oxyethylene urethane urea) and phenyltriethoxysilane via the sol-gel method gives rise to hybrid organic-inorganic proton-exchange membranes. The synthesized dopants are distributed in the oligoether component, but the nature of dopant distribution depends on their structure and concentration and has a significant impact on the structure of the resulting amorphous membranes (according to DSC, SAXS, and AFM data). The synthesized membranes are thermally stable up to 219°C. Their conductivity is provided by the segmental mobility of oligooxyethylene fragments (the Grotthuss mechanism) and, regardless of the dopant structure, is primarily determined by the number of charge carriers and the membrane structure. The temperature dependence of the conductivity is described by the Vogel-Fulcher-Tammann equation. The maximum values of the ionic conductivity are attained at 120°C under anhydrous conditions and dopant concentration of 50%: 1.03 × 10^?4 for ampholytic oligosilsesquioxane and 7.43 ×10^?5 S/cm for fully sulfonated oligosilsesquioxane as a dopant.     

Synthesis and photophysical properties of α,ω-bis(terpyridine)oligothiophenes

Four novel fully π-conjugated α,ω-bis(terpyridine)oligothiophenes characterized by NMR, IR, and HR-mass spectroscopy are presented and their electronic absorption/emission and redox properties are described based on both experiments and theoretical calculations. These compounds can be potentially utilized as building blocks for preparation of conjugated metallo-supramolecular polymers or dynamers and related functional materials.     

Synthesis,properties, and crystal structures of a new charge-transfer salt: Tris (dimethylammonium) hydrogen α-dodecamolybdosilicate

A new charge-transfer salt, [(CH₃)₂NH₂]₃[HSiMo₁₂O₄₀] (I), has been synthesized and characterized by elemental analysis, IR, UV-Vis, XPS, TG-DTA, and single-crystal X-ray diffraction. Crystal data: trigonal, R \(\bar 3\) m, a = 16.4365(8), c = 25.142(2) Å, V = 5882.3(7) ų, Z = 6. The X-ray crystallographic study shows that the title compound comprises three (CH₃)₂NH ₂ ⁺ cations and a heteropolyanion and is formed mainly by electrostatic interactions, hydrogen bonding being very weak. The title compound shows weak photochromic properties on UV irradiation. The TG-DTA curves suggest that I is stable up to 300°C     

Synthesis and characterization of α,ω-dichloro-polymethylphenylsilane

The synthesis of a functional polysilane, α,ω-dichloro-polymethylphenylsilane (α,ω-dichloro-PMPS), based on the Wurtz-type reductive coupling, has been considered. A study of some reaction parameters shows that both yield and molecular weight distribution of α,ω-dichloro-PMPS can be greatly influenced by the work-up conditions, reaction time, monomer concentration and temperature during the initiation of the polymerization. By varying the synthesis conditions, different ratios of high and low molecular weight fractions were obtained. Samples of different molecular weight were isolated by fractional precipitation of the crude polymers. The chlorine-end functionalization was confirmed by ²⁹Si NMR spectroscopy and the concentration of end-groups was quantitatively determined by elemental analysis of chlorine.     

Microenvironmental effects of cyclodextrins I: Enhancement of intramolecular excimer formation of polymethylene-bisβ-naphthoate via inclusion with cyclodextrins

The effects of α-, β- and γ-cyclodextrins (CDs) on the fluorescence spectra of a series of polymethylene-bis-β-naphthoates (Bn) have been studied. It is observed that β-CD and γ-CD enhance Bn intramolecular excimer fluorescence, indicating the formation of two-to-one guest host inclusion complexes. The possible conformation of these inclusion complexes is discussed.     

Studies on the solvent dependence of the carbamic acid formation from ω-(1-naphthyl)alkylamines and carbon dioxide

Carbamic acid formation from amine and carbon dioxide in a variety of solvents was investigated by measuring NMR (¹H, ¹³C, HMBC) and IR spectra in situ. Bubbling of CO₂ through solutions of naphthylalkylamines 1-3 in DMSO, DMF or pyridine (protophilic, highly dipolar, aprotic solvent) resulted in complete conversion of the amines to the corresponding carbamic acids 4-6. In dioxane (protophilic, dipolar, aprotic solvent), the carbamic acid and a small amount of the ammonium carbamate were formed. By contrast, in MeCN (protophobic, dipolar, aprotic solvent), in benzene or CHCl₃ (apolar, aprotic solvent), or in 2-PrOH or MeOH (dipolar, amphiprotic solvent), ammonium carbamates 7-9 rather than 4-6 were formed, although the ammonium bicarbonates/carbonates were competitively formed in MeOH. The ammonium carbamates precipitated in many cases and hence they could be separated. The selective generation of the undissociated carbamic acids in preference to the ammonium carbamates in protophilic, dipolar, aprotic solvents (DMSO, DMF, pyridine, and dioxane) is rationalized by considering the acid-base equilibria between the amines 1-3 and the carbamic acids 4-6 in nonaqueous media. The obtained selectivity is likely due to the larger pK_a values for 4-6 than the amines 1-3 in these solvents. Interestingly, the fluorescence intensities for 1-3 were dramatically enhanced (4-50 times) in DMSO or DMF upon introduction of CO₂, while they were not altered very much in dioxane, MeCN, benzene, CHCl₃, 2-PrOH, and MeOH, except small to medium increases (1.3-3 times) for 1 in dioxane, MeCN, 2-PrOH and MeOH. As a whole, the solvent effects observed in these fluorescence studies are consistent with those observed in the above NMR and IR studies. Finally, methoxycarbonylation of amine 3 into the methyl carbamate was successfully accomplished by using (trimethylsilyl)diazomethane in the presence of CO₂.     

Synthesis and mass spectrometric study of α,ω-bis(heptamethylcyclotetrasiloxanyloxy)oligodimethylsilanes

α,ω-Bis(heptamethylcyclotetrasiloxanyloxy)oligodimethylsilanes were synthesized by the reaction of dichlorodimethylsilane and 1,3-dichlorohexamethyltrisilane with hydroxyheptamethylcyclotetrasiloxane. The peculiarities of fragmentation of the compounds obtained by electron impact mass spectrometry are discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1729–1733, September, 1999.     

Spectral and intramolecular charge transfer properties in terminal donor/acceptor-substituted all-trans-α,ω-diphenylpolyenes and α,ω-diphenylpolyynes

The absorption spectra and intramolecular charge transfer (CT) properties of terminal donor/acceptor-substituted all-trans-α,ω-diphenylpolyenes (DPE) and α,ω-diphenylpolyynes (DPY) molecules with different conjugated bridge length and substitution modes were investigated by using quantum chemical calculations. We calculated the ground state structures and energy of two series of terminal donor/acceptor DPE and DPY by DFT method. The dependence of conjugation length and substitution modes of the electronic absorption spectra was obtained by TDDFT calculation. The hybrid-GGA XC-functional PBE0 employed in this work was selected from several functionals by comparing the calculated electronic spectral data with experimental value. The CIS-based generalized Mulliken-Hush (GMH) approach was further used to calculate coupling values H(AD) of the CT process. The calculation shows that both the HOMO-LUMO energy gaps and average bond length alternations between unsaturated multiple (C≡C and C=C) and saturated single bonds (C-C) decrease regularly with the extension of conjugation. The effective conjugated length (ECL) of DPE and DPY with the same order MM > MP/PM > PP is found together with the regular red shift of the electronic absorption spectra with the extension of conjugation, resulting from the different π-electron delocalization and conjugation efficiency. The GMH analysis further suggests that the CT process in both DPE and DPY is predominated by the through-bond mechanism. The remarkable difference of the conjugated length dependence of squared CT coupling between substituted DPE and DPY is the result of the energetic matching degree of the frontier molecular orbitals between donor/acceptor and the conjugated bridge.     

Alkylation and aldol reactions of acyl derivatives of N-1-(1′-naphthyl)ethyl-O-tert-butylhydroxylamine: asymmetric synthesis of α-alkoxy-, α-substituted-β-alkoxy- and α,β-dialkoxyaldehydes

Treatment of a range of O-protected glycolate derivatives of the chiral auxiliary N-1-(1′-naphthyl)ethyl-O-tert-butylhydroxylamine with KHMDS in the presence of 18-crown-6 followed by addition of an alkyl halide generates α-substituted derivatives with very high levels of diastereoselectivity. Alternatively, reaction of the potassium enolate of a propanoate or O-protected glycolate derivative of N-1-(1′-naphthyl)ethyl-O-tert-butylhydroxylamine with a range of aldehydes gives syn-aldol products with high levels of diastereoselectivity. These adducts may be reductively cleaved with LiAlH₄ to give enantiopure α-alkoxy-, α-substituted-β-alkoxy- and α,β-dialkoxyaldehydes in good yield.     

Synthesis and Characterization of New α,ω-Bis(Maleimide-Ester) Substituted Siloxane Oligomers

Abstract

Organofunctional siloxane oligomers containing maleimide were synthesized by condensation reaction of α. ω-hydroxypropylsiloxane oligomers with n-[4-(chlorocarbonyl)phenyl]maleimide. A model compound [bis(maleimide-ester)disiloxane] was obtained to facilitate the characerization of the oligomers. Then, the heterogeneous catalyzed ring-opening cationic polymerization technique was applied to obtain an oligomer starting from the model compound and octamethylcyclotetrasiloxane.

The structures of the resulting oligomers were confirmed by elemental analysis, IR and ¹H-NMR spectroscopy. They were characterized by determining softening points, solubility and studying their thermal behavior by thermogravimetric (TGA and differential scanning calorimetry (DSC) measurements.