Orientational dynamics of anthracene in a cyclodextrin functionalized layered solid

Cyclodextrin cavities have been intercalated in a layered metal hydroxide to create hydrophobic nanopockets within the galleries of the layered solid. Anthracene molecules have been included in the anchored cavities by partitioning from a polar solvent. The excitation-emission fluorescence spectra of the included anthracene show a total absence of Stokes shift. The orientational dynamics of the isolated, solvent-free anthracene molecules in the anchored cyclodextrin cavities have been probed by fluorescence anisotropy decay measurements. The results have been compared with those for anthracene included in cyclodextrin cavities in aqueous solutions.     

Inclusion of poly-aromatic hydrocarbon (PAH) molecules in a functionalized layered double hydroxide

The internal surface of an Mg-Al layered double hydroxide has been functionalized by anchoring carboxy-methyl derivatized β-cyclodextrin cavities to the gallery walls. Neutral polyaromatic hydrocarbon (PAH) molecules have been included within the functionalized solid by driving the hydrophobic aromatic molecules from a polar solvent into the less polar interior of the anchored cyclodextrin cavities by a partitioning process. The optical (absorption and emission) properties of the PAH molecules included within the functionalized Mg-Al layered double hydroxide solid are similar to that of dilute solutions of the PAH in non-polar solvents. The unique feature of these hybrid materials is that they are thermally stable over a wide temperature range with their emission properties practically unaltered. Dedicated to Prof J Gopalakrishnan on his 62nd birthday.     

Insertion of iodine in a functionalized inorganic layered solid

The galleries of a Mg-Al layered double hydroxide have been functionalized by intercalation of (carboxymethyl)-beta-cyclodextrin cavities. The functionalized inorganic solid can adsorb iodine molecules from vapor as well as nonaqueous and aqueous solutions. The adsorbed iodine complexes with the grafted cyclodextrin cavities, disassociating heterolytically to form polyiodide species. Electronic and Raman spectra provide conclusive evidence for the existence of linear symmetric triodide, I(3)(-), and pentaiodide, I(5)(-), species within the cyclodextrin cavities confined in the layered double hydroxide.     

ENERGY TRANSFER FROM MICELLAR SODIUM PHENYLUNDECANOATE TO SOLUBILIZED NAPHTHALENE

Abstract— Excitation energy transfer from the phenyl groups of surface active phenylundecanoate ions to naphthalene molecules has been studied under conditions such that the naphthalene molecules have been solubilized by micelles of phenylundecanoate. From measurements of the naphthalene fluorescence intensity in solutions of varying surfactant concentration the critical micelle concentration has been determined as 0·0091 M. The product of the micellar aggregation number and the efficiency of energy transfer has been obtained as 75 from measurements of both the sensitized naphthalene fluorescence and the quenching of the phenylgroup fluorescence. In the evaluation of the experimental data it has been assumed that the partition of naphthalene between the micelles and the aqueous phase may be treated as a distribution equilibrium, and that the solubilized naphthalene molecules are partitioned among the micelles according to a Poisson distribution. With this model, the naphthalene fluorescence intensities may be accounted for over the whole range of surfactant concentrations.
At high naphthalene concentrations, emission from naphthalene excimers has been observed. The possibility of self-quenching via excimer formation is considered.
The experimental results point to a quantum efficiency near unity for the transfer of excitation energy from the phenyl groups of the surfactant ions that form a micelle to a single solubilized naphthalene molecule. The high efficiency suggests that the naphthalene molecule and the phenyl groups are present inside the micelles.     

多足化合物的分子内激基缔合物形成及pH对其荧光的影响

研究了含萘脲基多足化合物溶液的稳态和瞬态光物理行为.由于分子内不同足间脲基的相互作用干扰了萘基的π-π叠合,使分子内萘基的激基缔合物生成受到影响.实验表明:由于三足化合物存在着给电子叔胺基团,因此当萘基被激发时、可因分子内的光诱导电子转移而导致荧光猝灭.正因如此,在三足化合物归一化后的稳态光谱中激基缔合物的发光强度很弱.用皮秒级单光子记数技术测得该化合物的瞬态荧光为三指数衰变过程.其中最长寿命的物种,即属于生成激基缔合物后再分解为萘激发态的部分仅占总量的4%,与稳态的结果相一致.工作表明对这类可用作荧光化学敏感器的三足化合物,如利用其激基缔合物的强度变化为其识别外来物种的敏感部位并不适合.相反,如引入的外来物种能影响化合物的分子内光诱导电子转移,进而影响萘基的发光强度,则是一较好的判别外来物种是否已进入主体的标志.     

Inclusion of ferrocene in a cyclodextrin-functionalized layered metal hydroxide: a new organometallic--organic-LDH nanohybrid

Cyclodextrin cavities have been grafted into a layered metal hydroxide to create hydrophobic nanopockets within the galleries of the inorganic solid. Neutral ferrocene molecules can be included within the grafted cavities by partitioning from a polar solvent to generate a new organometallic-organic-inorganic hybrid. The included ferrocene has been characterized by electronic and Raman spectroscopy. The capability of the cyclodextrin-functionalized solid to separate hydrophobic and hydrophilic derivatives of ferrocene is demonstrated.     

Steric-controlled excimer formation in naphthalene analogues of chalcone

Naphthalene analogues of chalcone (typical vinylarenes) are well known for their intramolecular charge transfer (CT) process. The all four possible isomers NC1, NC2, NC3 and NC4 were observed to give structure-less broad CT bands, whereas, excimer formation at higher concentration was reported only for NC4 [Res. Chem. Intermed. 25 (1999) 903]. However, conventional GMMX calculation data reveals that the naphthalene portion of all the isomers are planar, hence, excimer emission is expected from all of them, i.e., if NC4 can form an excimer, then the rests are also capable of doing it. In this paper we have actually succeeded in resolving the excimer peaks for all these four isomers by optimization of concentration, though the extent of excimer formation was observed to be the maximum for NC4. These differential tendencies of excimer formation can be explained by the change in extent of intermolecular stacking interaction of the naphthalene moieties. Variation occurs here due to steric perturbation arising from the specific orientation of the near resident non-planar aroyl component with respect to the naphthalene moiety.     

Singlet energy migration and excimer formation in polymers of 2-tert-butyl-6-vinylnaphthalene

The homopolymer of 2-tert-butyl-6-vinylnaphthalene (BVN) and its copolymers with styrene were prepared to examine the effects of the bulky tert-butyl groups on singlet energy migration and excimer formation among the naphthalene chromophores. The intensity of naphthalene excimer emission relative to that of monomer emission was found to depend linearly on the concentration of the potential excimer forming site, i.e., the BVN dyad fraction of the polymer. The rate of fluorescence quenching by biacetyl, on the other hand, increased only slightly with the increase in the BVN content. These results are consistent with the slow energy hopping model and suggest that the neighboring naphthalene chromophores are virtually isolated from one another owing to the unfavorable interactions of the tert-butyl groups.     

Excimer formation of fluoradene. Evidence for aggregation in alcohols

The absorption of fluorescence spectra of fluoradene in several solvents have been measured. In addition to the expected fluorescence band, a broad structureless band at 440 nm is observed in hexane which increases in intensity with increasing fluoradene concentration. In alcoholic solutions and in polycrystalline solid, only the broad, red-shifted band is observed. This broad band is attributed to excimer emission. It is shown that in alcoholic solutions that rate of excimer formation via diffusion is unacceptably large and it is suggested that fluoradene is aggregated in these solvents.     

Excited singlet yield in T-T annihilation, a comparative study of naphthalene and anthracene in solution

A method based on the study of both triplet absorption and delayed fluorescence (DF) decays after flash excitation has been developed to determine DF yields. Values of the excited singlet yield in the T-T annihilation process have been obtained for naphthalene and anthracene in solution at room temperature by application of this method. Triplet extinction coefficients and annihilation rate constants for these compounds were also determined. Excited singlet generation is the predominant annihilation channel for naphthalene whereas it is only of minor importance for anthracene. The difference in DF yields is ascribed to differences between these aromatics with respect to the efficiencies of the annihilation channels leading respectively to excited singlet and triplet states. The higher rate of excited singlet formation for naphthalene is qualitatively accounted for by considering the possibility of radiationless transitions from the bimolecular states formed in T-T interaction towards an upper excimer singlet state.     

低苯乙烯含量的丁二烯-苯乙烯共聚物的激基缔合物荧光研究

本文报导一种低苯乙烯含量(5％,Wt.)的丁二烯-苯乙烯共聚物,经~1H NMR证明其分子链上不存在相邻苯乙烯单元。荧光光谱结果表明,它在良溶剂(二氯乙烷)的稀溶液状态下只呈现单分子荧光。从良溶剂-θ溶剂(二氯乙烷-甲醇体系)及稀溶液-浓溶液-固体之荧光光谱的变化,证明形成了链内非近邻生色团之间和链间生色团之间的激基缔合物(Excimer)。从固体与θ溶剂的激基缔合物荧光的比较,还可区分链内非近邻和链间苯环形成激基缔合物对荧光强度的贡献。这一结果对阐明高分子链内非近邻和链间激基缔合作用提供了新的证据,并有助于了解高聚物本体中相互穿透、相互缠结的无规线团的形态。     

Dynamics and energetics of the self-assembly of a hydrophobically modified polyelectrolyte: Naphthalene-labeled poly(acrylic acid)

Steady-state and time-resolved fluorescence studies were performed on aqueous solutions of poly(acrylic acid) hydrophobically modified with two very different levels of naphthalene (Np). It is demonstrated that unique information on association phenomena involving hydrophobe-modifed polymers can be obtained from an extended fluorescence study by using data for a less-modified polymer as a reference. For the more highly modified polymer, the presence of excited-state (as well as ground-state) dimers in addition to monomer emission due to locally excited naphthalene gives evidence for hydrophobic association between naphthalene groups. This association becomes, as expected, much less important at higher pH due to the electrostatic repulsion between different chain segments. However, it is noted that even at high pH there is a significant self-association. The coexistence of static and dynamic quenching phenomena of the Np monomer label was also revealed in the time-resolved fluorescence data. The data are compatible with the existence of two types of monomers and one excimer and suggest that the essential contribution to the monomer emission comes from isolated chromophores, whereas excimer formation arises from both a dynamic route (excited Np chromophores able to produce a dynamic excimer) and a static route (excitation of ground-state Np dimers). At room temperature, the dissociative reaction, excimer-to-monomer, can be neglected, and thus the rate constant for excimer formation and decay could be obtained with and without considering the influence of preformed dimers. Temperature has shown to induce different behavior in the polymer photophysics. In the case of the less-labeled polymer, the decays were found to be single-exponential with the fluorescence lifetime decreasing with increasing temperature. From the temperature dependence of the steady-state fluorescence data, the activation energy for excimer formation and the binding energy of the excimer were evaluated at different pH values, through the Stevens-Ban-type plots of the excimer-to-monomer intensity ratio. With the time-resolved data, measured in the temperature range of 5-60 degrees C, it was possible to extract the intrinsic activation energies for excimer formation. The thermodynamic driving force for the intrapolymeric association was found to be dependent on a balance between hydrophobic and electrostatic interactions, which are dependent on the pH, temperature, and hydrophobic content of the polymer.     

Structure of a cyclodextrin functionalized anionic clay: XRD analysis, spectroscopy, and computer simulations

Carboxy-methyl beta-cyclodextrin (CMCD) cavities have been intercalated within the galleries of anionic clay, Mg-Al layered double hydroxide (LDH). The cyclodextrin functionalized LDH has been reported to adsorb neutral and nonpolar guest molecules. X-ray diffraction, IR, and Raman vibrational spectroscopy and (13)C CPMAS NMR have been used to characterize the confined CMCD molecules, whereas molecular dynamics simulations have been used to probe the interlayer arrangement and orientation of the intercalated species. Spectroscopic measurements as well as MD simulations show that there is no significant change in the geometry of the CMCD cavity on intercalation. Within the galleries of the anionic clay, the CMCD anions are arranged as bilayers with the carboxy methyl substituents, located at the narrower opening of the bucket-like cyclodextrin toroid, anchored to the LDH sheet. This arrangement leaves the wider opening of the CMCD anion facing away from the layers allowing the interior of the cyclodextrin cavity to be accessible to guest molecules. Finally, the hydrophobicity of the anchored cyclodextrin cavity has been characterized using fluorescence from pyrene included within it.     

Electric field effects on absorption and fluorescence spectra of trimethylsilyl- and trimethylsilylethynyl-substituted compounds of pyrene in a PMMA film

External electric field effects on absorption and fluorescence spectra of 1,3,6,8-tetrakis(trimethylsilyl)pyrene and 1,3,6,8-tetrakis(trimethylsilylethynyl)pyrene (TMSPy and TMS(E)Py, respectively) have been examined in a poly(methyl methacrylate) (PMMA) film at various concentrations at various temperatures. TMS(E)Py preferentially forms an aggregate in a PMMA film, as the concentration increases, indicating that the acetylenic groups enhance the pi-pi interactions between pyrene molecules. The change in molecular polarizability following excitation has been determined both for the monomer and for the aggregate, based on the electroabsorption spectra. The change in molecular polarizability following emission has also been determined in both compounds, based on the electrofluorescence spectra. TMSPy exhibits two excimer fluorescence emissions at high concentrations which are ascribed to the partially overlapping excimer and the sandwich-type excimer, respectively, besides the monomer fluorescence emitted from the locally excited state. The sandwich-type excimer fluorescence as well as monomer fluorescence is quenched by an electric field, whereas the fluorescence of the partially overlapping excimer is enhanced by an electric field. Excimer fluorescence of TMS(E)Py, which arises from the sandwich-type excimer, is quenched by an electric field at any temperature. Only one species of the partially overlapping excimer is confirmed in TMSPy, while no partially overlapping excimer is confirmed in TMS(E)Py.     

Fluorescence from aromatic compounds isolated in the solid state by double intercalation using layered polymer crystals as the host solid

Poly(muconic acid)s, stereoregular polymer crystals obtained by topochemical polymerization using supramolecular control, function as the layered host solids for organic intercalation, in which alkylamines as the guest species are reversibly inserted into them through an acid-base interaction. We now report a double-intercalation method using alkylamine and pyrene as the guests to control the fluorescence property in the solid state. An aromatic compound can be separately introduced into the hydrophobic layers of the ammonium polymer crystals. The aromatic molecules, which are sandwiched between two alkyl layers, show fluorescence emission from the single molecule but not the excimer. This method can be applied to various organic photofunctional materials showing unique fluorescence properties.     

Excimer dissociation in poly(1-vinyl naphthalene)

Time-resolved fluorescence spectroscopy has been used to probe the formation and decay of excimeric sites in poly(1-vinyl naphthalene) in methylene chloride solution. Contrary to earlier reports, it has been found that reverse dissociation of excimers does occur appreciably at room temperature. Monomer fluorescence decay curves are thus represented by the sum of two exponential terms, and excimer curves by the difference in two exponentials. Lifetimes of 7.43 and 43.1 ns were obtained from the monomer and excimer fluorescence decay data.     

Covalently linked, water-dispersible, cyclodextrin: reduced-graphene oxide sheets

Reduced-graphene oxide (rGO) sheets have been functionalized by covalently linking β-cyclodextrin (β-CD) cavities to the sheets via an amide linkage. The functionalized β-CD:rGO sheets, in contrast to rGO, are dispersible over a wide range of pH values (2-13). Zeta potential measurements indicate that there is more than one factor responsible for the dispersibility. We show here that planar aromatic molecules adsorbed on the rGO sheet as well as nonplanar molecules included in the tethered β-CD cavities have their fluorescence effectively quenched by the β-CD:rGO sheets. The β-CD:rGO sheets combine the hydrophobicity associated with rGO along with the hydrophobicity of the cyclodextrin cavities in a single water-dispersible material.     

Resolution of ultrafast pyrene excimer emission rise times in zeolites X and Y

Pyrene has been a favorite photophysical probe molecule for zeolite research because of its ability to exhibit both monomer and excimer emission upon excitation. This study combines the use of ultrafast time-resolved fluorescence spectroscopy with steady-state fluorescence spectroscopy to study the excimer emission of pyrene incorporated within zeolites LiY, NaY, KY and NaX. The effects of sealing technique and coincorporated solvents are also explored. Pyrene excimer emission is resolvable with the use of an ultrafast streak camera under all conditions examined in this study with a rise-time range of 6.8 to 16.0 picoseconds. For each zeolite sample the addition of cosolvents decreases the rise time, with a greater decrease for polar solvents than for a nonpolar solvent. The presence of a detectable rise time for excimer emission indicates that pyrene excimer formation is a dynamic process when pyrene is embedded within the cavities of zeolite host materials.