Fluorescent Guest Research Articles

Spectral properties and supramolecular inclusion complex formation between 2-styrylbenzothiazolium dye and cyclodextrins

The effect of native and randomly methylated β-CDs on the absorption and steady-state fluorescence spectra of 2-(4-dimethylaminostyryl)-3-(2-hydroxyethyl)-benzothiazolium chloride (DHB) in aqueous buffer solutions with various pH values was studied. The inclusion with both CDs at pH 7.2 barely changed the UV spectra, whereas significant variations were produced in the emission spectra in all buffer solutions. In all cases the CDs increase guest fluorescence. The 1:1 stoichiometry of the inclusion complexes of the dye with both CDs was established according to the modified Benesi-Hildebrand method. Binding constant values were calculated using the iterative nonlinear least-squares regression approach. The pH of the solution and the type of the CD affected complex stability. The results indicate that native β-CD possesses better complexing ability towards DHB than randomly substituted β-CD and that the most stable inclusion complexes are formed in basic medium because of the structural changes in the guest molecule. In basic medium an attempt is made to interpret the proposed mechanism in terms of molecular rearrangement which take place as the dye penetrates the CD cavity.

Fluorescence of Syndiotactic Polystyrene/Trimethylbenzene Clathrate and Intercalate Co-Crystals

The self-assembling of fluorescent guest molecules, with the δ nanoporous host crystalline phase of syndiotactic polystyrene (s-PS) films, reduces chromophore diffusivity. Moreover, the chromophore...

A Comparative Study of Complexation of β-Cyclodextrin, Calix[4]arenesulfonate and Cucurbit[7]uril with Dye Guests: Fluorescence Behavior and Binding Ability

The complexation behaviors of acridine red (AR), neutral red (NR) and rhodamine B (RhB) dye guest molecules by three kinds of supramolecular hosts, including β-cyclodextrin (β-CD), calix[4]arene tetrasulfonate (C4AS) and cucurbit[7]uril (CB[7]), have been investigated by means of fluorescence spectra in aqueous citrate buffer solution (pH 6.0). The results obtained show that the three hosts, possessing different types of cavity, lead to various complexation-induced fluorescence of dye guests, and present different binding ability and molecular selectivity. The complexation stability constants decrease in the order of NR > AR > RhB for C4AS and CB[7] hosts, while in the order of RhB > AR > NR for β-CD host. Particularly, CB[7] displays the strongest binding ability with NR (K S = 33300 M− 1), and provides the molecular selectivity of 4.8 for NR/AR pairs. Although the binding ability of C4AS for present dye guests is weaker than CB[7], but the molecular selectivity of the two hosts are nearly equivalent. β-CD shows stronger binding ability with RhB (K S = 5880 M− 1) as comparison with CB[7] and C4AS. Furthermore, the solvent effects and salt effects during the course of complexation have also been investigated.

Solid-Supported Monolayers and Bilayers of Amphiphilic β-Cyclodextrins

This paper describes the adsorption and spreading of beta-cyclodextrin (CD) vesicles on hydrophobic and hydrophilic substrates, which involves a transition from bilayer vesicles to planar molecular monolayers or bilayers. On substrates that are patterned with self-assembled monolayers by microcontact printing (muCP), the CD vesicles preferentially adsorb on hydrophobic areas instead of hydrophilic (nonionic) areas, and on cationic areas instead of hydrophilic (nonionic) areas. Supported monolayers of amphiphilic cyclodextrins CD1 and CD2 were obtained by adsorption of CD vesicles to hydrophobic substrates, and supported bilayers of amphiphilic cyclodextrins CD1 and CD2 were prepared by adsorption of CD vesicles on cationic substrates. Contact angle goniometry, atomic force microscopy and confocal fluorescence microscopy (CFM) were used to analyze the supported CD layers. The fluidity of the supported CD layers was verified using fluorescence recovery after photobleaching experiments. The supported layers function as a supramolecular platform that can bind suitable guest molecules through inclusion in the CD host cavities. Additionally, the CD host layers were patterned with fluorescent guest molecules by supramolecular muCP on the supported CD layers. The host-guest interactions were investigated with CFM and fluorescence resonance energy transfer experiments.

Recent Applications of Host-Guest Inclusion in Fluorescence-Based Trace Analysis

Host-guest inclusion complexes represent the simplest example of a supramolecular system, in which a small “guest” molecule becomes included within the interior cavity of a larger, cage-like “host” molecule. This inclusion can have a number of significant effects on the physical and spectroscopic properties of the guest. One particularly useful effect is the significant enhancement of the guest fluorescence, which occurs for a wide range of guests, for a variety of reasons. If the guest is an analyte of interest, then this inclusion-induced fluorescence enhancement provides a simple, convenient means to significantly increase the sensitivity of its fluorescence-based trace analysis. This article will provide a comprehensive review of the recent application of host-guest inclusion to the spectrofluorimetric trace analysis of a wide range of guests of interest, including pharmaceutical and environmentally important compounds, in a variety of hosts, including cyclodextrins and calixarenes.

Surface Modification of Elastomeric Stamps for Microcontact Printing of Polar Inks

Chemical modification of the surface of a stamp used for microcontact printing (microCP) is interesting for controling the surface properties, such as the hydrophilicity. To print polar inks, plasma polymerization of allylamine (PPAA) was employed to render the surface of poly(dimethylsiloxane) (PDMS), polyolefin plastomers (POP), and Kraton elatomeric stamps hydrophilic for long periods of time. A thin PPAA film of about 5 nm was deposited on the stamps, which increased the hydrophilicity, and which remained stable for at least several months. These surface-modified stamps were used to transfer polar inks by microCP. The employed microCP schemes are as follows: (a) a second generation of dendritic ink having eight dialkyl sulfide end groups to fabricate patterns on gold substrates by positive microCP, (b) fluorescent guest molecules on beta-cyclodextrin (beta-CD) printboards on glass employing host-guest recognition, and (c) Lucifer Yellow ethylenediamine resulting in covalent patterning on an aldehyde-terminated glass surface. All experiments resulted in an excellent performance of all three PPAA-coated stamp materials to transfer the polar inks from the stamp surface to gold and glass substrates by microCP, even from aqueous solutions.

Assembly and Exchange of Resorcinarene Capsules Monitored by Fluorescence Resonance Energy Transfer

Resorcinarenes were functionalized with fluorophores and fluorescence resonance energy transfer (FRET) was employed as a tool to probe the dynamic behavior of hydrogen-bonded hexameric assemblies at nanomolar concentrations. The encapsulation of a fluorescent guest inside the capsule made it possible to observe FRET across the mechanical boundary of the hexameric assembly.

A highly color-stability white organic light-emitting diode by color conversion within hole injection layer

We demonstrated a novel-structure white organic light-emitting devices (WOLEDs) composed of a greenish blue fluorescent emitting layer and a red fluorescent dye-doped hole injection layer. The spectra of WOLEDs show no change at a wide range current density operation and long-term continuous operation. We concluded that carriers recombined inside the emitting layer in the WOLEDs to produce a greenish blue electroluminescence (EL), and a part of the EL was absorbed by the red fluorescent guest doped hole injection layer and converted into red photoluminescence (PL); the whole white emission from the device was a mixture of the greenish blue EL and red PL.

Photophysical properties and electroluminescent applications of donor–acceptor–donor functionalized red electroactive fluorescent materials

Four electroactive molecular materials with donor-π-acceptor-π-donor structures have been synthesized for investigation of structure–properties relationships, and used as dopant for electroluminescent applications. The optical and electrical properties of these materials are electronically manipulated by attaching different donors. The twisted intramolecular charge transfer (TICT) model is proposed by photophysical behaviors observed with increasing solvent polarity. Red electroluminescence devices were fabricated by spin coating the blend of polymer host and fluorescent guest. Highly efficient energy transfer leads to a saturated red emission with emission peak at 623 nm, narrow bandwidth of 75 nm and an external quantum efficiency of 0.38%.

Catching a molecule at the air-water interface: Dynamic pore array for molecular recognition

As a soft and flexible porous structure, a pore array of a steroid cyclophane SC(OH), which consists of the rigid 1,6,20,25-tetraaza[6.1.6.1]paracyclophane ring connected to four steroid moieties (cholic acid) through flexible L-lysine spacers, was prepared at the air-water interface. As confirmed by surface pressure (π)-molecular area (A) isotherms, transition between open conformation and cavity conformation of the SC(OH) molecule was reversibly induced upon repeated compression and expansion of its monolayer at pH 11 where amino groups of the lysine residues are not fully deprotonated. Capture and release of an aqueous fluorescent guest (TNS) by SC(OH) was observed upon dynamic cavity formation through surface fluorescence spectroscopy. At pH 12, dynamic cavity formation of SC(OH) was sufficiently suppressed, and the capture and release of an aqueous TNS by the monolayer was not virtually observed. Lessened electrostatic repulsion between the SC(OH) molecules due to conversion of ammonium to free amine may prevent the cavity from reopening. The importance of dynamic nature of cavity formation on the guest binding was also proved by control experiments using SC(H), which cannot form cavity conformation at any surface pressures at both pH 11 and 12.

Functional properties of fluorescent poly(amidoamine) dendrimers in nematic liquid crystalline media

The effectiveness of poly(amidoamine) dendrimers from zero generation as a fluorescent guest for liquid crystal displays of the ‘guest–host’ type is discussed on the basis of their absorption and fluorescent properties. It has been shown that the dendrimers at concentration of 0.3 wt% do not destabilize the orientation of the liquid crystal matrix. The orientation order parameters S A and S F depends on the nature of the substituent at C-4 position of the 1,8-naphthalimide. The effect that poly(amidoamine) dendrimers have upon the phase transition temperature and the electro-optical properties of the LC/dendrimer mixtures has been also presented. All investigations reported have been carried out in surface stabilized display cells.

Crystalline structures of intercalate molecular complexes of syndiotactic polystyrene with two fluorescent guests: 1,3,5-Trimethyl-benzene and 1,4-dimethyl-naphthalene

The crystal structures of two molecular complex phases of syndiotactic polystyrene (s-PS) with 1,3,5-trimethyl-benzene (TMB) and 1,4-dimethyl-naphthalene (DMN) have been described. These structures present a monoclinic unit cell in which the s(2/1)2 polymer helices and guest molecules are packed according the space group P21/a and unit cell constants: a=17.3Å, b=15.4Å, c=7.8Å and γ=95.7° for s-PS/TMB and a=17.4Å, b=17.2Å, c=7.8Å and γ=116.4° for s-PS/DMN. Both structures can be described as intercalates, since they present ac layers of polymer helices alternated to layers of contiguous guest molecules and a guest/monomeric-unit molar ratio of 1/2, as recently observed only for s-PS/norbornadiene molecular complex. On the basis of a comparison between crystalline structures and X-ray diffraction data of several s-PS molecular complexes, a simple criterion to anticipate their clathrate or intercalate nature has been suggested.

Cholesteric Polymer Guest−Host Mixture with Circularly Polarized Fluorescence: Two Ways for Phototuning of Polarization and Its Intensity

A photosensitive fluorescent cholesteric guest-host mixture consisting of a nematic polyacrylate, a chiral, photochromic dopant sensitive to UV light, and a fluorescent dopant was prepared. The nematic polyacrylate contains 4-phenyl-4'-methoxybenzoate nematogenic side groups and photochromic 4-cyanoazobenzene side groups. The chiral-photochromic dopant formed by isosorbide and cinnamic acid is capable of E-Z photoisomerization and [2 + 2] photo-cycloaddition under light irradiation. The planarly oriented films possess a selective light reflection in the visible spectral region coinciding with the emission peak of the fluorescent dopant. The fluorescence emitted by the planarly oriented films of the mixture is strongly circularly polarized and characterized by a large value of the dis-symmetry factor. At temperatures below glass transition (T(g)) the polarized light action of an Ar(+) laser (488 nm) leads to the photo-orientation of the azobenzene fragments resulting in a strong and reversible disruption of the selective reflection and a decrease of the dis-symmetry factor of fluorescence. UV irradiation leads to E-Z isomerization and/or [2 + 2] cycloaddition of the chiral-photochromic dopant, causing an irreversible shift of the maximum of the dis-symmetry factor to a long-wavelength spectral region under subsequent annealing at temperatures higher than T(g). Such multifunctional glass-forming guest-host mixtures combining photosensitive and fluorescent properties with the unique optical properties of cholesteric liquid crystals can be considered as promising material for optical data processing technologies and photonic applications.

Distributed feedback micro-laser array: helixed liquid crystals embedded in holographically sculptured polymeric microcavities

We report a detailed physical characterization of a novel array of organic distributed feedback microcavity lasers possessing a high ratio between the quality factor Q of the resonant cavity and its volume V. The optical microcavity was obtained by confining self-organized mesophases doped with fluorescent guest molecules into holographically patterned polymeric microchannels. The liquid crystal microchannels act as mirror-less cavity lasers, where the emitted laser light propagates along the liquid crystal helical axis behaving as Bragg resonator. This miniaturization process allows us to obtain a micro-laser array possessing an ultralow lasing threshold (25nJ/pulse) while having directional control on the lasing emission, a fine wavelength tunability and the control over the emission intensity.

Two-dimensional molecular patterns and their dynamic functions: Molecular recognition of aqueous guest by mixed monolayer of alkyl cyclophane and amphiphilic guanidinium

Molecular tiling has been proposed as a novel technique for controlled molecular arrangements in two-dimensional plane, and their capability of molecular recognition of aqueous guest has been investigated. The surface pressure–molecular area ( π– A) isotherms of alkyl cyclophane derivatives revealed the importance of flexible spacer between the cyclophane ring and alkyl chains for better molecular packing. Molecular recognition of an aqueous fluorescent guest (TNS) by the alkyl cyclophane with eight alkyl chains with l-lysine spacer was next investigated through π– A isotherm measurement and surface fluorescence spectroscopy. A single-component monolayer of the alkyl cyclophane did not show any recognition capability of aqueous TNS. In contrast, an apparent increase in fluorescence intensity was observed when the mixed monolayers of the amphiphilic guanidinium and alkyl cyclophane were compressed on aqueous TNS. TNS would be bound through strong electrostatic interaction with the guanidinium and the formed complex was included by the alkyl cyclophane. Efficient binding site for aqueous TNS seems to be formed through self-assembling of the two host components at the air–water interface.

Fluorescent Guest Molecules Report Ordered Inner Phase of Host Capsules in Solution

Despite recent strides in the synthesis of elaborate nanometer-scale molecular hosts, the internal structure of these self-assembled cages remains ill characterized. We used fluorescent probe molecules, pyrene butyric acid (PBA), as guests in C-hexylpyrogallol[4]arene capsules to relay information about the chemical environment on the interior of the assemblies. Spectroscopic and single-crystal x-ray diffraction studies show that, in both solution and the solid state, the host can encapsulate two PBA guests and keep them well separated through specific interactions with the capsule walls.

Exciton Mobility and Trapping in a MALDI Matrix

Energy transfer (ET) from excited matrix to fluorescent traps is used to probe the mobility of excitations in the matrix-assisted laser desorption/ionization (MALDI) matrix material 2,5-dihydroxybenzoic acid. The dependence of host and guest fluorescence on excitation density (laser intensity) and trap concentration gives clear evidence for long-range energy transport in this matrix. This conclusion is further supported by time-resolved emission data showing a 2 ns delay between matrix and trap emission. Rate equation and random walker models give good agreement with the data, allowing determination of hopping, collision, and trapping parameters. Long-range energy transfer contributes to the pooling reactions which can lead to primary ions in MALDI. The results validate the pooling aspect of the prior quantitative MALDI ionization model (J. Mass Spectrom. 2002, 37, 867-877). It is shown that exciton trapping can decrease MALDI ion yield, even at low trap concentration.

Orientation and Microenvironment of Naphthalene Guest in the Host Nanoporous Phase of Syndiotactic Polystyrene

Syndiotactic polystyrene (s-PS) films including a fluorescent guest (naphthalene, NP) in their nanoporous δ crystalline phase have been prepared and characterized by X-ray diffraction, infrared linear dichroism, molecular modeling, and fluorescence depolarization techniques. A nearly perpendicular orientation of the fused rings of NP guest molecules with respect to the chain axis of the δ crystalline structure has been established by molecular mechanics and by evaluation of directions of transition moment vectors of infrared vibrational modes. The fluorescence depolarization is more efficient for NP guest molecules into the host phase than for NP molecules simply absorbed in the amorphous phase. This could be due to a more efficient resonance energy transfer between guest molecules, possibly associated with their ordered positioning and orientation into the host nanoporous polymeric crystallites.

A Cucurbit[6]uril Analogue: Host Properties Monitored by Fluorescence Spectroscopy

This paper describes the host properties of a new cucurbit[6]uril analogue, studied by fluorescence and 1H NMR spectroscopy. This host has an elongated cavity with oval-shaped portals. It is intrinsically fluorescent, and more importantly, this fluorescence is sensitive to guest encapsulation, allowing for the study of the inclusion of nonfluorescent guests by fluorescence spectroscopy. In the case of benzene as guest, significant enhancement of the cucurbit[6]uril analogue host fluorescence was observed upon addition of benzene; this allowed for the determination of the binding constant for 1:1 host-guest complexation, yielding a value of K = 6900 +/- 1100 M(-1). This complexation was also studied by 1H NMR, yielding a similar value of K = 8980 +/- 500 M(-1). The binding of a much larger guest, the dye Nile Red, was also studied, but in this case using guest fluorescence. Significant suppression of the Nile Red fluorescence was observed upon 1:1 complexation with the cucurbit[6]uril analogue, with an extremely large binding constant of 8.2 +/- 0.5 x 10(6) M(-1), indicating a very strong host-guest interaction and an excellent size and shape match. In both cases, binding was much stronger than in the case of the same guests with cucurbit[6]uril itself, and in the case of Nile Red, binding was also much stronger than with modified beta- or gamma-cyclodextrins. This is partly a result of the partial aromatic nature of the host walls, which allow for pi-pi interactions not possible in cucurbiturils or cyclodextrins. The ability to study its inclusion complexes using either host or guest fluorescence, and the very high binding constants observed, illustrates the versatility and potential usefulness of this new host compound.

Probing the Photonic Properties of a Cholesteric Elastomer under Biaxial Stress

We have investigated the photonic properties of a free-standing cholesteric elastomer film under biaxial mechanical load. The sample, which was obtained by UV cross-linking of a cholesteric side- chain polymer, shows substantially improved optical properties compared to cholesteric elastomers obtained by the anisotropic deswelling method. The selective reflection of circularly polarized light and the modified spontaneous emission of dispersed fluorescent guest molecules show that the cholesteric order is well preserved in the cross-linked film. Application of biaxial stress leads to a shift of the photonic stop band to shorter wavelengths due to a contraction of the cholesteric helix; the change of the cholesteric pitch is affine to the film thickness. Pulsed excitation of the fluorescent guest molecules gives rise to photonic band edge laser emission, which is mechanically tunable in a wavelength range of almost 100 nm. The lasing performance is found to be comparable to that of conventional cholesteric systems.