High-density assembly of gold nanoparticles on multiwalled carbon nanotubes using 1-pyrenemethylamine as interlinker

In this article, we describe the formation of carbon nanotube (CNT)-gold nanoparticle composites in aqueous solution using 1-pyrenemethylamine (Py-CH2NH2) as the interlinker. The alkylamine substituent of 1-pyrenemethylamine binds to a gold nanoparticle, while the pyrene chromophore is noncovalently attached to the sidewall of a carbon nanotube via pi-pi stacking interaction. Using this strategy, gold nanoparticles with diameters of 2-4 nm can be densely assembled on the sidewalls of multiwalled carbon nanotubes. The formation of functionalized gold nanoparticles and CNT-Au nanoparticle composites was followed by UV-vis absorption and luminescence spectroscopy. After functionalization of gold nanoparticles with 1-pyrenemethylamine, the distinct absorption vibronic structure of the pyrene chromophore was greatly perturbed and its absorbance value was decreased. There was also a corresponding red shift of the surface plasmon resonance (SPR) absorption band of the gold nanoparticles after surface modification from 508 to 556 nm due to interparticle plasmon coupling. Further reduction of the pyrene chromophore absorbance was observed upon formation of the CNT-Au nanoparticle composites. The photoluminescence of 1-pyrenemethylamine was largely quenched after attaching to gold nanoparticles; formation of the CNT-Au nanoparticle composites further lowered its emission intensity. The pyrene fluoroprobe also sensed a relatively nonpolar environment after its attachment to the nanotube surface. The present approach to forming high-density deposition of gold nanoparticles on the surface of multiwalled carbon nanotubes can be extended to other molecules with similar structures such as N-(1-naphthyl)ethylenediamine and phenethylamine, demonstrating the generality of this strategy for making CNT-Au nanostructure composites.     

Opening a spiropyran ring by way of an exciplex intermediate

A molecular dyad has been synthesized in which the main chromophore is a 1,4-diethynylated benzene residue terminated with pyrene moieties, this latter unit acting as a single chromophore. A spiropyran group has been condensed to the central phenylene ring so as to position a weak electron donor close to the pyrene unit. Illumination of the pyrene-based chromophore leads to formation of a fluorescent exciplex in polar solvents but pyrene-like fluorescence is observed in nonpolar solvents. The exciplex has a lifetime of a few nanoseconds and undergoes intersystem crossing to the pyrene-like triplet state with low efficiency. Attaching a 4-nitrobenzene group to the open end of the spiropyran unit creates a new route for decay of the exciplex whereby the triplet state of the spiropyran is formed. Nonradiative decay of this latter species results in ring opening to form the corresponding merocyanine species. Rate constants for the various steps have been obtained from time-resolved fluorescence spectroscopy carried out over a modest temperature range. Under visible light illumination, the merocyanine form reverts to the original spiropyran geometry so that the cycle is closed. Energy transfer from the pyrene chromophore to the merocyanine unit leads to an increased rate of ring closure and serves to push the steady-state composition in favor of the spiropyran form.     

Study of the absorption and emission spectroscopy of "A-B" type photosensitive compounds including two-photon chromophore and benzophenone moiety

"A-B" type photosensitive compounds including two-photon chromophore and benzophenone moiety have been designed, synthesized and characterized. The UV-vis absorption and fluorescence emission of the compounds have been extensively studied in various solvents. The results show that the absorption of "A-B" type compounds displays obvious double absorption bands, one of which at short-wavelength is related to the benzophenone moiety, the other at long-wavelength is mainly contributed by chromophore. The emission of "A-B" type compounds at 500-700nm shows an "unexpected" blue-shift comparing with that of the sole chromophore. The photosensitive compounds with amino group display strong emission in apolar solvents and have a low fluorescence quantum yields in polar solvents. In contrast, the compounds without amino group exhibit strong fluorescence emission in polar solvents, and low fluorescence quantum yields in apolar solvents. The fluorescence quantum yields of "A-B" type compounds are higher than those of the sole chromophore. The discoveries suggest that charge redistribution induced by the introduction of benzophenone moiety plays a key role on the absorption and emission spectroscopy.     

Probing the effects of cholesterol on pyrene-functionalized interfacial adlayers

We have synthesized a novel set of pyrene-functionalized, covalently bound surface adlayers with and without cholesterol derivatives coadded to the adlayer. We have deposited these adlayers on quartz, oxidized silicon wafers, and indium-doped tin oxide coated substrates. The addition of tethered cholesterol to the adlayer creates a hydrophobic, likely disordered, microenvironment in which the surface-bound pyrene resides. X-ray photoelectron spectroscopy measurements demonstrate the covalent attachment of both cholesterol and pyrene in our adlayers. The presence of the cholesterol moieties gives rise to a reduction in film thickness, as measured ellipsometrically, and contact angle data indicate significant surface heterogeneity. Steady-state fluorescence data show that the presence of cholesterol moieties reduces the extent of pyrene excimer formation and provides a less polar environment for the chromophore. Fluorescence lifetime measurements on surface-bound pyrene were biexponential, consistent with multiple local environments, regardless of whether tethered cholesterol was present or not. Cyclic voltammetry reveals competition between the pyrene and cholesterol moieties for binding to available surface sites on the epoxide-terminated surface-binding layer we use.     

Bisimidazoacridones: Effect of Molecular Environment on Conformation and Photophysical Properties

Bisimidazoacridones (BIA) are highly selective antineoplastic and antiviral agents. Ultraviolet-visible spectroscopy and steady-state and time-resolved fluorescence spectroscopy studies were carried out to probe the behavior of BIA in aqueous and nonaqueous (organic solvents, colloid micelles) solutions. Three ranges of fluorescence lifetimes were revealed: approximately 0.2-0.5 ns (presumably reflecting the chromophore-chromophore interaction), approximately 1-5 ns (interpreted as linker-perturbed chromophore decay) and approximately 6-12 ns (nonperturbed chromophore decay). The pre-exponential and steady-state contributions of these components to the decay signal as well as the data on steady-state fluorescence intensities, wavelength maxima and bandwidths showed that the BIA conformations in solution were sensitive to the environment and influenced strongly by their propensity to minimize hydrophobic interactions. In water, the molecules tend to adopt condensed conformations that bring the two imidazoacridone moieties into close proximity (resulting in intramolecular fluorescence energy transfer), while in nonaqueous systems the conformations become more relaxed. The transfer from a polar to more lipophilic environment of macromolecules is suggested to be the main driving force for binding of BIA to biomacromolecules, such as nucleic acids.     

SPECTRAL CHARACTERISTICS OF FREE AND LINKED PYRENE-TYPE CHROMOPHORES IN SOLUTION,POLYMER MATRICES,AND INTERPENETRATING NETWORKS

Monomers, 1-pyrenylmethyl methacrylate (PyMMA), 1-pyrenylbutyl methacrylate (PyBMA), 4-(1-pyrenyl)methoxymethyl styrene (PyMMS) and allyl-(1-pyrenylmehtyl) ether (PyMAE), with pyrene as chromophore, were prepared. Their spectral properties (absorption, emission and emission decay) in solution, and doped or bonded in polymer matrices and complex polymer systems as interpenetrating polymer networks (IPN), were investigated. Spectral properties of pyrene-containing monomers doped in polystyrene (PS), polymethylmethacrylate (PMMA), polyvinylchloride (PVC), polyethylene (PE) and grafted on PE as well as copolymerized in buthylmethacrylate-co-styrene copolymer (BMA-co-S) have been compared. Absorption and emission spectra of pyrene type probes in solution and in IPN matrices exhibit typical absorption of the pyrene moiety. For IPN with grafted probes on PE, the absorption is slightly shifted to red wavelength. For monomers, PyMMA, PyBMA and PyMAE grafted to PE, the shape of the emission spectrum depends on the wavelength of excitation. The ratio of intensity of the vibrational band, I₁/I₃, (I₁ peak at 377 nm and I₃ peak at 388 nm) has been a quite useful indicator of polarity of IPN. The relative quantum yields of fluorescence in IPN matrices are lower in comparison to polymer matrices of PE, PS, PMMA for all probes under study. The fluorescence life-times for bound and free probes have been in the range 100–200 ns, which is substiantialy shorter than for the parent pyrene chromophore under the same or similar condition. Grafted probes on PE alone, or as a part of IPN, exhibit substantially shorter life-time around 10 ns and decay is rather complex.     

荧光淬灭法研究疏水缔合共聚物(PAM-b-PPOEA)_n自组装聚集数

运用荧光淬灭技术,包括稳态荧光淬灭法(SSFQ)和时间分辨荧光淬灭法(TRFQ),研究了疏水缔合水溶性丙烯酰胺2苯氧基丙烯酸酯多嵌段共聚物[P(AM POEA)]在水溶液中自组装的聚集数.这类聚合物在水溶液中易形成胶束状聚集体,探针芘分子和淬灭剂二苯酮增溶于疏水微区,荧光测定结果很好地符合Poisson淬灭模型.实验结果表明聚合物链结构、聚合物浓度和无机盐对聚集体的尺寸具有重要影响.聚合物自组装聚集数NA随疏水单体含量的增加和疏水嵌段长度的减小而增大,同时也随聚合物浓度和NaCl浓度增加而增大.另外对聚合物链结构、聚集数和溶液粘度的相互关系进行了讨论.     

Characterization of the ground state pyrene complex in ethylene-propylene copolymer solutions

Pyrene-labeled functionalized ethylene-propylene (EP) copolymer was prepared by grafting 1-pyrenebutyrylhydrazine onto EP copolymer through maleic anhydride pendants. The EP copolymer contained 60 mol % ethylene; its weight-average molecular weight (M^w) was 148,000. The pyrene-labeled amide functionalized EP copolymer, PA-EP(60/40), was made to simulate the amine functionalized EP copolymers that are commonly used as dispersant additives in motor oils. UV absorption spectra, fluorescence emission and excitation spectra, and fluorescence decay profiles of the pyrene were studied to determine the copolymer conformation and dynamics in methylcyclohexane and tetrahydrofuran (THF). The pyrene fluorescence characteristics of PA-EP(60/40) were highly dependent on the solvent. The dependence of fluorescence emission intensity on the excitation wavelength was large in methylcyclohexane and moderate in THF. A frequency shift of about 2 nm was observed between the excitation spectrum obtained with the emission line at 377 nm and that at 550 nm in the methylcyclohexane solutions, but no shift was found in the corresponding tetrahydrofuran solutions. The ratios of the preexponential factors (a₂₁/a₂₂) of the excimer decays obtained in both methylcyclohexane and THF solutions were different from ?1.0. However, the deviation of the excimer formation process from the Birks scheme is small in THF but large in methylcyclohexane. In addition, the Huggins constants obtained from intrinsic viscosity measurements of the PA-EP(60/40) copolymer solutions suggest that copolymer aggregation occurs in methylcyclohexane but not in THF. H-bonding between two pyrene-containing pendants is apparently the main driving force for the formation of the ground state pyrene complex. THF is found to be effective in inhibiting the H-bonding formation. © 1995 John Wiley & Sons, Inc.     

"Extra stabilisation" of a pyrene based molecular couple by γ-cyclodextrin in the excited electronic state

Two thiosemicarbazide and semicarbazide functionalized pyrene labeled Schiff base compounds have been synthesized. The pyrene moieties in the compounds result in formation of π-π coupled complexes in aqueous medium in the excited electronic state. Added γ-cyclodextrin allows incorporation of the pyrene head inside its less polar core and promotes hydrogen bonding of the thio and oxo groups of the compounds with its rim hydroxyl groups to "stabilise" the monomers. This is confirmed by FT-IR and absorption spectroscopy. The stabilised monomers lead to formation of stabilised excimers as monitored by steady state and time-resolved fluorescence spectroscopy through varying the experimental conditions. The proposed model for the stabilisation of the Schiff base monomers has been evidenced by comparing with the fluorescence spectroscopic changes of two control compounds. The present work reports a step ahead toward proposing a simple host-guest method to extra stabilise the pyrene based excimers that can be biologically utilised.     

Molar absorption coefficient of pyrene aggregates in water

The molar absorption coefficient of pyrene aggregates, epsilon(E0), was determined for a series of pyrene-labeled poly(N,N-dimethylacrylamide)s (Py-PDMA) having different pyrene contents. Aqueous solutions of Py-PDMA having pyrene contents ranging from 263 to 645 mumol of pyrene per gram of polymer were studied by UV-vis absorbance and time-resolved fluorescence spectroscopy. The global analysis of the monomer and excimer fluorescence decays with the fluorescence blob model yielded the fractions of the overall absorption contributed by all the pyrene species present in solution. The combined knowledge of the fractions obtained from the global analysis of the time-resolved fluorescence decays, the total absorption of the Py-PDMA solution obtained from UV-vis spectroscopy, and the total pyrene concentration in the solution obtained from the known pyrene content of each Py-PDMA sample led to the determination of the molar absorption coefficient of pyrene aggregates. Regardless of the pyrene content of the Py-PDMA samples and hence the level of association of the pyrene pendants in solution, all Py-PDMA samples yielded similar epsilon(E0) values over the range of wavelengths studied, namely, from 325 to 350 nm. The averaged epsilon(E0) was found to be red-shifted relative to unassociated pyrenes by 3 nm as well as having a value at the 0-0 peak of 21 000 M(-1).cm(-1) reduced from 34 700 M(-1).cm(-1) for unassociated pyrenes. The determination of epsilon(E0) enabled the first determination of the absolute fraction of associated pyrenes for aqueous solutions of a series of pyrene-labeled water-soluble polymers. The procedure outlined in this study is applicable to any pyrene-labeled water-soluble polymer and provides a new means to study quantitatively the effect of the hydrophilic-to-lipophilic balance on the hydrophobic associations generated by hydrophobically modified water-soluble polymers. As an application, the average number of pyrenes involved in a pyrene aggregate generated by Py-PDMA in water is determined.     

UV-vis and fluorescence spectroscopic detection of anions by the conformational restriction of 2,2′-binaphthalene derivatives bearing thiourea groups through a methylene spacer

Novel fluorescence receptors, 2 and 3 based on 2,2′-binaphthalene possessing thiourea moieties via a methylene spacer have been synthesized. Hydrogen bonds of NH groups of thiourea moieties with acetate anion were confirmed by ¹H NMR study in MeCN-d₃. These receptors showed characteristic UV-vis spectral changes through isosbestic points on complexation with anions inspite of lacking conjugation between the chromophore and the binding sites in polar organic solvent such as acetonitrile. The UV-vis spectral changes arise from the conformational restriction of the 2,2′-binaphthyl skeleton on the complexation. The receptors exhibit high selectivities for AcO⁻ and F⁻. The fluorescence intensity of the receptors decreases with the increasing amount of the AcO⁻, however, addition of F⁻ induces a different change in its fluorescence spectrum, in which shorter emission of the receptors decreases with the increase in F⁻ concentration, while the longer emission of the receptors increases through an isoemissive point in MeCN. The results suggest that favorable dual-wavelength ratiometric fluorescence measurement can be conducted by the receptors for F⁻.     

Study of the semidilute solutions of poly(N,N-dimethylacrylamide) by fluorescence and its implications to the kinetics of coil-to-globule transitions

The large scale motions of poly(N,N-dimethylacrylamide) chains randomly labeled with pyrene (Py-PDMA) were monitored by steady-state and time-resolved fluorescence in semidilute solutions of naked PDMA in acetone and DMF for polymer concentrations ranging from 0 to 550 g/L. Although increasing the polymer concentration of the solution led to a decrease of the mobility of the chromophore attached onto the PDMA backbone, this reduction was rather modest when compared to the large increase of the macroscopic viscosity. This result indicated that locally, the monomer constituting the chains experienced freedom of movement despite the high solution viscosity. The restricted mobility of the chromophore was characterized by the number of monomers occupying the volume probed by the excited chromophore during its lifetime, referred to as a fluorescence "blob". The number of monomers constituting a fluorescence blob, N(F)(-)(blob), and the volume of a fluorescence blob, V(F)(-)(blob), were found to decrease as the polymer concentration of the solution increased, reflecting the decreased mobility experienced by the chromophore. In DMF, the radius of an F-blob was found to scale as N(nu)(F)(-)blob, where nu equaled 0.66 +/- 0.03, very close to the expected value of the Flory exponent of 0.6 for a polymer in a good solvent. The combined knowledge of how N(F)(-)(blob) varies with the fluorescence lifetime of the chromophore and the coil density of the polymer was used to propose a new means of studying coil-to-globule transitions with potential implications for predicting the rate of protein folding.     

Formation and dissociation of rhodamine 800 dimers in water: steady-state and ultrafast spectroscopic study

We investigated the fundamental photophysics and photochemistry of a cationic dye rhodamine 800 (R800) in water using steady-state and ultrafast time-resolved spectroscopies. In the ground state, the monomer and dimer coexist in equilibrium, which causes significant concentration dependence of UV-visible (vis) absorption spectra. We determined the equilibrium constant as well as the molar absorption spectra of the monomer and dimer from a global fitting analysis of the UV-vis spectra. The obtained pure dimer spectrum indicates that it is a nonparallel H-dimer. In contrast to the absorption spectra, the steady-state fluorescence spectra do not show any noticeable concentration dependence. The fluorescence lifetime was determined as 0.73 ns regardless of the concentration, and the fluorescence of R800 in water was solely attributed to the monomer. In femtosecond time-resolved absorption measurements, we observed the S(n) <-- S1 absorption bands of the monomer and the dimer, as well as the ground-state bleaching signals. It was found that the S1 dimer dissociates to produce the S1 monomer (and the S0 monomer) or relaxes to the S0 dimer with a time constant of as short as 3.0 ps, which brings about the absence of dimer fluorescence.     

Synthesis of oligo(p-phenylene vinylene)-porphyrin-oligo(p-phenylene vinylene) triads as antenna molecules for energy transfer

The oligo(p-phenylene vinylene)-porphyrin-oligo(p-phenylene vinylene) (P-OPVn, n=2, 4, where n is the number of phenyl rings) and the complex with Zn²⁺ based on P-OPVn were synthesized for investigating their photophysical properties via UV-vis, voltammetry, steady-state and time-resolved fluorescence spectra. In these molecules two OPV moieties as energy donors were linked to porphyrin center by virtue of Wittig reaction. The detailed studies of photophysical properties indicate that OPV group can act as an antenna unit for effective intramolecular energy transfer.     

PHOTOPHYSICS STUDIES OF ORGANIC COMPOUNDS——NANOSECOND LASER PHOTOLYSIS AND TRANSIENT ABSORPTION SPECTRA STUDIES OF PYRENE-AROMATIC AMINES

Nanosecond time-resolved transient absorption spectra of pyrene-triphenylamine, pyrenediphenylamine, pyrene-N, N-dibenzylaniline systems in various solvents have been investigated. In nonpolar solvent, pyrene-triphenylamine exciplex and pyrene-N, N-dibenzylaniline exciplex were observed directly. In acetonitrile, the pyrene anion radical, triphenylamine and diphenylamine cation radicals were detected. On the basis of the present experimental facts, the mechanism of fluorescence quenching processes of pyrene in polar and nonpolar solvents has been established.     

Strong two photon absorption and photophysical properties of symmetrical chromophores with electron accepting edge substituents

Two photon absorption (TPA) and photophysical properties of three new symmetrical chromophores with electron accepting phthalimide edge substituents have been studied. The three chromophores contain fluorene, alcoxy-substituted divinyl benzene, and carbazole moieties as central cores, respectively. The femtosecond time-resolved fluorescence upconversion spectroscopy and two photon excited fluorescence technique have been carried out. The effect of solvent polarity on TPA and on photophysics has also been determined. Ultrafast fluorescence dynamics, with decay times ranging from 1 to 13 ps, are revealed in polar solvents. This is attributed to the relaxation of the chromophores to the intramolecular charge transfer state. The chromophore bearing fluorene central core, being of the type A-pi-A, is the most efficient concerning TPA. Strong TPA, with a cross section value as high as 2100 GM at an excitation wavelength of 770 nm is found in acetophenone which is a solvent of intermediate polarity. The TPA spectra were also reproduced using a sum over states three-state model. A study of the TPA induced photobleaching of the fluorene molecule, doped in a solid poly(methyl-methacrylate) film, has shown that this material is very promising for efficient TPA optical data storage.     

A fluorescence method for quantitative measurements of specific protein at powder surfaces

A new method using fluorescence labelled proteins was developed to determine the quantitative amount of specific protein at the powder surface. The method is based on steady-state fluorescence measurements of pyrene labelled proteins with oxygen gas phase quenching at the powder surface. The surface load of protein was measured for spray-dried dextran powders containing bovine serum albumin (BSA). The results show a patchwise surface load of about 1.3 mg m⁻² at a concentration of 0.33% (dry weight) BSA in the powder. The patchwise surface load stays constant with increased BSA concentration in the powder.     

Self-assembled fluorescent hexaazatriphenylenes that act as a light-harvesting antenna

In this paper we report the self-assembling nature of fluorescent hexaazatriphenylenes (HATs) 6a-d with six alkyl/alkoxy-chain-containing biphenyl groups and their application to light-harvesting antennae. In a nonpolar solvent and the film state, the HAT derivatives form one-dimensional aggregates with an H-type parallel stacking mode, which were analyzed by 1H NMR, UV-vis, and steady-state and time-resolved fluorescence spectroscopy. When HAT derivative 7 with six perylenediimide moieties is incorporated into the one-dimensional aggregates, an efficient energy transfer takes place from the self-assembled HAT moiety as a light-harvesting antenna to the perylenediimide moiety as an energy acceptor. Further, when HAT derivative 8 with six triphenylamino moieties is newly added to the light-harvesting system, an intermolecular electron transfer occurs subsequently between the electron-accepting perylenediimide molecule and the electron-donating triphenylamino molecule.     

Effect of side-chain length on the side-chain dynamics of alpha-helical poly(L-glutamic acid) as probed by a fluorescence blob model

Two series of pyrene-labeled poly(glutamic acid) (Py-PGA) were synthesized utilizing two different linkers for pyrene attachment, namely 1-pyrenemethylamine (PMA) and 1-pyrenebutylamine (PBA). Several Py-PGAs were synthesized for each series with pyrene contents ranging from 4 to 15 mol %. Py-PGA forms a rigid alpha-helix in DMF that effectively locks the backbone in place, thus enabling only side-chain or linker motions to be monitored by time-resolved fluorescence. Time-resolved fluorescence decays were acquired for the pyrene monomer of the Py-PGA constructs and the fluorescence blob model (FBM) was used to quantify the dynamics of the different linkers connecting pyrene to the backbone. Nitromethane was used to shorten the lifetime of the pyrene monomer, in effect controlling the probing time of the pyrene group, from 50 to 155 ns for PGA-PBA and from 50 to 215 ns for PGA-PMA. The FBM analysis of the fluorescence decays led to the conclusion that excimer formation around the rigid alpha-helix backbone takes place in a compact environment. The number of glutamic acid units within a blob, N blob, decreased only slightly with decreasing probing time for both Py-PGA constructs as a result of the compact distribution of the chromophores around the alpha-helix. The PGA alpha-helix was modeled using Hyperchem software and the ability of two pyrene groups to encounter was evaluated as they were separated by increasing numbers of amino acids along the alpha-helix. The number of amino acids required for two pyrenes to lose their ability to overlap and form excimer matched closely the N blob values retrieved using the FBM.     

From Graftable Biphotonic Chromophores to Water‐Soluble Organic Nanodots for Biophotonics: The Importance of Environmental Effects

The photophysical and two‐photon absorption (TPA) properties of biphotonic chromophores with one or two phenol pendant units were studied and compared with that of a model biphotonic quadrupolar chromophore. A water‐soluble dendritic structure was then synthesized by using the pendant moieties as starting points for the construction of dendritic branches. We show that the polarity of the environment significantly modulates both the fluorescence and the TPA responses of the different chromophoric derivatives. This extends to more subtle effects that involve phenol pendant moieties that were found to act as discrete solvating units and to modify both the photophysics and the TPA response of the chromophore. This demonstrates the high sensitivity of the TPA response of quadrupolar derivatives to minute alterations in the environment. Moreover, the dendritic branches were found to behave as a peculiar cybotactic environment that was able to tune the fluorescence and TPA response of the inner chromophore by creating a polar environment. This reveals a new direction for exploiting such effects by playing on the dendritic architecture (e.g., the nature and shape of the building blocks, the geometry and position of the chromophore) to modulate the TPA responses.