Stilbazolium Salts Research Articles

Synthesis and characterization of organic nanocrystals of stilbazolium salts by solvothermal technique

It has been demonstrated that the properties of nanocrystals change considerably with respect to their bulk counterparts. In past two decades, most research work focuses on developing semiconducting nanocrystals, metals and metal-oxides nanocrystals, and Carbon-based nanocrystals. Organic nanocrystals find little attention as it is challenging to synthesize them due to their poor thermal and mechanical properties when compared to the inorganic materials. Therefore, in this article, we report the successful synthesis of organic nanomaterials. We have selected Ionic organic crystal called 4-N, N-dimethylamino-4-N-methyl-stilbazolium tosylate (DAST) and its derivatives because of their Terahertz generation capabilities. Organic nanocrystals of 4-N, N-dimethylamino-4-N-methyl-stilbazolium tosylate (DAST), 4-N, N-dimethylamino-4-N-methyl-stilbazolium 2-napthalenesulfonate (DSNS) and 4-N, N-dimethylamino-4-N-methyl stilbazolium 4-styrenesulfonate (DSSS) are prepared by solvothermal method. The structure and composition of the prepared samples are studied by employing powder XRD and EDX analyses. The effect of nanocrystallization on the optical absorption properties is investigated. The morphology and size of the organic nanocrystals are examined by SEM and TEM analyses.

Dicationic stilbazolium salts: Structural, thermal, optical, and ionic conduction properties

Two series of new dicationic stilbazolium salts containing tosylate and triflimide counterions, respectively, were synthesized and characterized by spectroscopic techniques and elemental analysis. Despite of their promesogenic structures, none of these salts are mesomorphous, and instead they solely exhibit crystalline polymorphism, as deduced from differential scanning calorimetry, polarizing optical microscopy and variable temperature X-ray measurements. These salts were also found to be highly thermally stable with decomposition temperatures occurring well above 300 °C, and up to 367 °C for the triflimide salts, as determined by thermogravimetric analysis. UV–Vis absorption and photoluminescent properties were examined in both solution and in the solid state. They exhibited higher absolute quantum yields in the powdered state than in solution. The dielectric response was evaluated by impedance spectroscopy, revealing notable values of short-range conductivity in triflimide salts through amorphous regions. Our work demonstrates the potential of these new stilbazolium salts as advanced materials in optoelectronic devices, with performances that can be tailored by molecular design of the spacer’s flexibility and choice of counter anions.

Pagoda[5]arene with Large and Rigid Cavity for the Formation of 1∶2 Host–Guest Complexes and Acid/Base-Responsive Crystalline Vapochromic Properties

Pagoda[5]arene (P5), which is composed of five 2,6-dimethoxylanthracene (2,6-DMA) subunits bridged by methylene groups at 1,5 positions, was conveniently synthesized in 43% by trifluoroacetic acid ...

Acid-base strength and acidochromism of some dimethylamino-azinium iodides. An integrated experimental and theoretical study.

The effects of pH on the spectral properties of stilbazolium salts bearing dimethylamino substituents, namely, trans isomers of the iodides of the dipolar E-[2-(4-dimethylamino)styryl]-1-methylpyridinium, its branched quadrupolar analogue E,E-[2,6-di-(p-dimethylamino)styryl]-1-methylpyridinium, and three analogues, chosen to investigate the effects of the stronger quinolinium acceptor, the longer butadiene π bridge, or both, were investigated through a joint experimental and computational approach. A noticeable acidochromism of the absorption spectra (interesting for applications) was observed, with the basic and protonated species giving intensely colored and transparent solutions, respectively. The acid–base equilibrium constants for the protonation of the dimethylamino group in the ground state (pKa) were experimentally derived. Theoretical calculations according to the thermodynamic Born-Haber cycle provided pKa values in good agreement with the experimental values. The very low fluorescence yield did not allow a direct investigation of the changes in the acid-base properties in the excited state (pKa*) by fluorimetric titrations. Their values were derived by quantum-mechanical calculations and estimated experimentally on the basis of the Förster cycle.

Synthesis and nonlinear optical properties in the near-IR range of stilbazolium dyes

A series of stilbazolium salts based on donor-π-acceptor (D-π-A) structure have been synthesized and fully characterized. Photophysical properties including linear absorption, one-photon excited fluorescence (OPEF), two-photon absorption (2PA) properties were systematically investigated. The results suggest that increasing electron-releasing character of the terminal group leads to a more pronounced donor-to-acceptor intramolecular charge transfer (ICT). In addition, the dyes possess the largest 2PA cross sections in the near infrared region (NIR) and display maximum two-photon absorption cross sections within the narrow wavelength range from 950 to 970nm and BL3 exhibits a large nonlinear refractive index coefficient and possesses very large values of the real part of the cubic hyperpolarizability χ(3) at 960nm. Furthermore, the initial density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations provide reasonable explanations for their absorption spectra, meanwhile we used the Lippert–Mataga equation to evaluate the dipole moment changes of the dyes with photoexcitation, the results are corresponding with linear and nonlinear optical properties of the dyes.

Engineering of Acentric Stilbazolium Salts with Large Second-Order Optical Nonlinearity and Enhanced Environmental Stability

A series of organic nonlinear optical (NLO) materials based on the stilbazolium derivatives of 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) were synthesized, and the single crystals were grown from the solutions. Single-crystal structure determinations and Kurtz powder tests reveal that replacement of the p-toluenesulfonate anion in the DAST crystal, forming 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium 4-aminotoluene-3-sulfonate (1) and 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium p-chlorobenzenesulfonate (2), constructs new acentric crystals with quite a large powder second-harmonic generation (SHG) of more than 3 orders of magnitude larger than that of urea around 2.0 μm, whereas the other two complexes show their centrosymmetric structures without any SHG effects. A possible estimation of the molecular first hyperpolarizabilities βijk performed on 2 further confirms its origin of its enhanced NLO merits. Furthermore, both 1 and 2 exhibit even higher environmental stabilities, includi...

Influence of pH on photophysical properties of (E)-1-(4-chlorobenzyl)-4-(4-hydroxystyryl)pyridinium chloride

The protonation/deprotonation equilibrium was investigated for N-p-chlorobenzyl-substituted (E)-4'-hydroxy stilbazolium halide, namely (E)-1-(4-chlorobenzyl)-4-(4-hydroxystyryl)pyridinium chloride (EPC). Absorption, emission and synchronous scanning spectra were used to explain the observed phenomena. The excited state lifetimes of the protonated and deprotonated forms of EPC were measured and discussed. Absorption spectra were used to determine its pK(a) value in the ground state. We conclude that the protonation/deprotonation equilibrium is not attained in the first excited state of EPC, for kinetic reasons. The quinoid and benzenoid structures of EPC in the ground and excited state are discussed in acidic and basic range of pH. Aqueous solutions of EPC were yellow at pH < 7 and red at pH > 7, and addition of alcohols (methanol or 2-propanol) enhanced this change. Therefore, quaternary stilbazolium salts were investigated for application as acid-base indicators.

Physical properties and inclusion interactions of new stilbazolium salts: experimental versus theoretical study

The experimental and theoretical spectroscopic and spectrometric elucidation in solid-state and gas-phase on the interacting ionic species of applied oriented synthetic derivatives on the base of the stilbazolium salts as molecular template was reported. The correlation between the molecular structure, and vibrational properties within THz-regime (10-0.3 THz) was performed. The collective vibrations, and gas-phase stabilized ionic species were comprehensive studied by the Raman spectroscopy and matrix-assisted laser desorption/ionization mass spectrometry, using the embedded organic dyes in host matrixes. The performed solid-state quantum chemical calculations contributed to further understanding of the nature of the guest–host interacting systems as well as to explain the observed optical phenomena within the THz-region.

N-Aryl stilbazolium dyes as sensitizers for solar cells

Eight new N-arylstilbazolium chromophores with electron donating –NR 2 (R = Me or Ph) substituents have been synthesized via Knoevenagel condensations and isolated as their PF 6 − salts. These compounds have been characterized by using various techniques including 1H NMR and IR spectroscopies and electrospray mass spectrometry. UV–vis absorption spectra recorded in acetonitrile are dominated by intense, low energy π → π* intramolecular charge-transfer (ICT) bands, and replacing Me with Ph increases the ICT energies. Cyclic voltammetric studies show irreversible reduction processes, together with oxidation waves that are irreversible for R = Me, but reversible for R = Ph. Single crystal X-ray structures have been determined for three of the methyl ester-substituted stilbazolium salts and for the Cl − salts of their picolinium precursors. Time-dependent density functional theory calculations afford reasonable predictions of ICT energies, but greater rigour is necessary for –NPh 2 derivatives. The four new acid-functionalized dyes give moderate sensitization efficiencies ( ca. 0.2%) when using TiO 2-based photoanodes, with relatively higher values for R = Ph vs Me, while larger efficiencies (up to 0.8%) are achieved with ZnO substrates.

ChemInform Abstract: Development of Fluorescence Probes Based on Stilbazolium Salts (I) for Monitoring Free Radical Polymerization Processes.

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Isomorphism in 1-(2-halidobenzyl)-4-[(E)-2-(3-hydroxyphenyl)ethenyl]pyridinium halide hemihydrates (halide = Cl, Br)

The crystal structures of two (E)-stilbazolium salts, namely 1-(2-chlorobenzyl)-4-[(E)-2-(3-hydroxyphenyl)ethenyl]pyridinium chloride hemihydrate, C(20)H(17)ClNO(+) x Cl(-) x 0.5 H(2)O, (I), and 1-(2-bromobenzyl)-4-[(E)-2-(3-hydroxyphenyl)ethenyl]pyridinium bromide hemihydrate, C(20)H(17)BrNO(+) x Br(-) x 0.5 H(2)O, (II), are isomorphous; the isostructurality index is 99.3%. In both salts, the azastyryl fragments are almost planar, while the rings of the benzyl groups are almost perpendicular to the azastyryl planes. The building blocks of the structures are twofold symmetric hydrogen-bonded systems of two cations, two halide anions and one water molecule, which lies on a twofold axis. In the crystal structure, these blocks are connected by means of weaker interactions, viz. van der Waals, weak hydrogen bonding and stacking. This study illustrates the robustness of certain supramolecular motifs created by a spectrum of intermolecular interactions in generating these isomorphous crystal structures.

Crystal engineering principles applied to solution photochemistry: controlling the photodimerization of stilbazolium salts within γ-cyclodextrin and cucurbit[8]uril in water

Borrowing concepts from crystal engineering techniques we have been able to steer the photodimerization of stilbazolium salts included in gamma-cyclodextrin towards a desired dimer.

Molecular Engineering of Stilbazolium Derivatives for Second-Order Nonlinear Optics

Three series of ionic stilbazolium salts with different counteranions have been synthesized to investigate the effect of counteranion size to crystal packing. Single crystals of five new salts were successfully grown from solution by the slow cooling and slow evaporation techniques. X-ray studies revealed that three of them show the triclinic space group P1 and two crystallize in the monoclinic space group Cc and P21/c, respectively. Kurtz powder tests revealed that out of the eight new stilbazolium salts, six possess noncentrosymmetric structures and three show large powder second-harmonic generation efficiencies higher or similar to that of 4-N,N-dimethylamino-4‘-N‘-methyl-stilbazolium tosylate (DAST). The optimal crystalline packing for electro-optics with perfectly aligned chromophores was achieved in three of the investigated compounds. The reasons for considerably different macroscopic nonlinearities of these compounds have been explored in terms of intermolecular interactions.

Synthesis, Crystal Structure, and Second-Order Nonlinear Optical Properties of New Stilbazolium Salts

Two derivatives of the highly nonlinear optical crystal DAST (4-N,N-dimethylamino-4‘-N‘-methyl-stilbazolium tosylate) with m-trifluoromethylbenzene-sulfonate (DSMFS) and p-trifluoromethylbenzenesulfonate (DSPFS) replacing the p-toluenesulfonate of DAST have been synthesized and characterized. X-ray crystallographic analyses revealed that the crystal structure of DSPFS is monoclinic Cc and very similar to that of DAST, whereas DSMFS possesses a centrosymmetric structure with two molecules with different orientations on one site. A Kurtz powder test has shown that DSPFS exhibits a large second-order optical nonlinearity 3 orders of magnitude greater than that of the urea standard.

Developing of Fluorescence Probes Based on Stilbazolium Salts for Monitoring Free Radical Polymerization Processes. II

The series of 1-methyl-4-(4-aminostyryl)pyridinium perchlorates was investigated as fluorescent probes for the monitoring of the free radical polymerization progress. The study on the changes in fluorescence intensity and spectroscopic shifts of studied compounds were carried out during thermally initiated polymerization of methyl methacrylate. The purpose of these studies was to find a relationship between the structure of fluorophore and the changes in their fluorescence shape and intensity observed during the monomer conversion into polymer. The probes under the study during the course of polymerization increase their fluorescence intensity at least one order of magnitude. Such increase qualified the tested probes as good fluorescence probes.

Preparation and properties of thin films of photocrosslinkable hydrophilic polymers

New hydrophilic photocrosslinkable polymers were prepared from water-soluble copolymers and photodimerizable chromophores. Water-soluble prepolymers were obtained by copolymerization of N-isopropylacrylamide (NIPAAm) or N,N-dimethylacrylamide (DMAAm) with acrylic acid (AAc). The polymers were then modified with hydrophobic or hydrophilic photoreactive groups, trans-4-hydroxystilbene and styrylpyridinium (stilbazolium) salts, in polymer analogous reactions. Thereby water-soluble photoreactive polymers with or without temperature-sensitive solubility were obtained. The glass transition temperatures of the polymers and the cloud points of their solutions in water were determined and found to depend on the content of the photoreactive groups. Thin films of the materials on quartz plates were prepared from solutions in different solvents such as butanone, chloroform, ethanol, tetrahydrofuran, and water, and the photocrosslinking properties of these films were studied. The nature of the chromophores as well as the solvents used for the film preparation were found to influence the photocrosslinking behaviour.

Development of fluorescence probes based on stilbazolium salts for monitoring free radical polymerization processes

A series of 1-methyl-2-(4-aminostyryl)pyridinium perchlorates and iodides were investigated as fluorescent probes for the monitoring of the progress of free radical polymerization. The study on the changes in the fluorescence intensity and spectroscopic shifts of studied compounds were carried out during thermally initiated polymerization of methyl methacrylate and during photoinitiated polymerization of a 2-ethyl-2-(hydroxymethyl)propane-1,3-diol triacrylate (TMPTA)–1-methylpyrrolidin-2-one (MP) mixture. The purpose of these studies was to find a relationship between the changes in the shape and intensity of probe fluorescence and degree of monomer conversion into polymer. The polymer formation was estimated gravimetrically or by the measurement of the degree of monomer double bond disappearance using FTIR spectroscopy.

Macroscopic non‐linearities of some stilbazolium derivatives and the calculatium derivatives and the calculation of their molecular hyperpolarisability

AbstractThe synthesis of some optically non‐linear stilbazolium salts is described. Since their solid state non‐linearities were small, MOPAC5 was used to compare their relative molecular non‐linearities.

Calculation of the microstructure of Langmuir-Blodgett films derived from stilbazolium salts

The atom-atom potential method was used to calculate the microstructure of Langmuir-Blodgett films derived from stilbazolium salts and previously obtained experimental results were analyzed. The structure, in which the positively charged stilbazolium heads are arranged perpendicularly to the plane of the film with bromide ions between them, was found to be the most stable. The hydrocarbon tails in this packing may be displaced relative to each other such that the total thickness of the bilayer is reduced. The calculated thickness of the bilayer (48 A) is in accord with the experimental value (49 A). The calculated surface area of the films per molecule is 1.5 times greater than the experimental value. We propose that partial creep of the molecules relative to each other occurs upon film formation on the water surface.

Singlet Mechanism for Trans → Cis Photoisomerization of Quaternary Salts of 4-Substituted 4′-Azastilbenes (R = CN, H, CH3, and OCH3) and their Quinolinium Analogues. VIII

The trans → cis photoisomerization of a series of substituted stilbazolium salts (At +X− , At +: trans-1-alkyl-4-[4-R-styryl]-pyridinium and -quinolinium, R = CN. H, CH3 and OCH3, X− = I and ClO4 −) was studied by laser flash photolysis and steady state irradiation measurements. The quantum yields of cis ⇄ trans photoisomerization (ϕc → t and ϕt → c ) and of fluorescence of the trans isomers (ϕf) were determined in several solvents at room temperature and at low temperatures in mixtures of either 2-methyltetrahydrofuran-dichloromethane or ethanolmethanol (E-M). In polar solvents at room temperature ϕt → c is substantial ( ≧ 0.3) and ϕf is small (10−3-10−2). Competition of fluorescence and an activated step in the trans → cis pathway is indicated by the effects of temperature on ϕf and ϕt → c (activation energy: 2 - 3 kcal/mol). A transient, observed only at low temperatures (lifetime τT &gt; 0.5 ms in E-M below -170°C), is assigned to the lowest triplet state with trans configuration. On the basis of the effects of temperature on ϕf, ϕt → c , and the triplet yield and those of quenchers on ϕf and ϕt → c , involvement of the triplet state in the twisting process at room temperature is excluded. Therefore, a singlet mechanism is suggested for the trans → cis photoisomerization of the stilbazolium salts examined. Significant reduction of ϕt → c for iodides in solvents of moderate polarity, where ion pairs are present, is accounted for by photoinduced electron transfer in competition to trans → cis photoisomerization.