Fluorescence study on a thermotropic liquid-crystalline polyester at high temperatures

A thermotropic liquid-crystalline (LC) polyester, poly[(ethylene terephthalate)-co-(p-oxybenzoate)] (PET40/OBA60) (OBA content: 60 mol %), is investigated by fluorescence technique using two model compounds: dimethyl terephthalate (DMT) and methyl methoxybenzoate (MMB) and is demonstrated to form an intermolecular ground-state complex between the terephthalate and OBA moieties. The change in fluorescence of PET40/OBA60 film is studied from 25°C to 450°C. The peak wavelength change for fluorescence of the intermolecular ground-state complex from 394 to 430 nm was observed in the temperature range between T_g and the LC transition temperature (115～ 250°C). This is attributed to the electronic distribution change between terephthalate and OBA moieties in the excited state, which play roles of acceptor and donor, respectively. The increase in the fluorescence intensity from the temperature near the annealing temperature to the temperature near the isotropic temperature (287～370°C) is suggested to be the increase in LC configuration and the formation of a more stable excited state due to the electronic distribution change between terephthalate and OBA moieties. The lifetime of PET40/OBA60 film quenched from LC temperature (300°C) to room temperature is in agreement with that of the nonannealed one, which is due to the fact that the deactivation process of the sample quenched from LC temperature is in accord with that of the nonannealed one. © 1995 John Wiley & Sons, Inc.     

Fluorescence study on dynamics of liquid-crystalline polymers

Fluorescence measurements of various main-chain liquid-crystalline (LC) polyesters, poly[oxybiphenyl-4, 4'-diyloxy(2, 5-dihexadecyloxy)-1, 4-pyromellitoyl] (B-C16), poly[(ethylene terephthalate)-co-(p-oxybenzoate)] (PET40/OBA60), poly(oxybiphenyl-4, 4'-diyloxy-1, 10-decanedioyl) (PB-10), low-molecular-weight LC bis(p-hexyloxyphenyl) terephthalate (PP6), and a lyotropic LCP, poly(sulfo-1, 4-phynelene 3'-nitroterephthalate) (PSPNT), were carried out to investigate their dynamics and intermolecular interaction changes.     

Charge transfer interactions in liquid crystalline polymers probed by fluorescence spectroscopy

Rigid-rod aromatic LC polyester with long alkyl side chains and two thermotropic LC polyesters (PET40/OBA60 and PB-10) were studied by fluorescence spectroscopy to investigate their charge transfer interactions corresponding to LC configuration and changes during phase transition.     

Observation of the diffusion-free intermolecular excimer of 9-methyladenine in aqueous solution by picosecond time-resolved spectroscopy

Diffusion-free intermolecular excimer formation in liquid solution is observed for an aqueous solution of 9-methyladenine using the synchroscan streak camera technique. The excimer fluorescence (330 ps lifetime) appears at 380 nm, at longer wavelength than the monomer fluorescence (5 ps lifetime). The excimer is formed via a weakly coupled stacking dimer composed of ground-state monomers.     

Thiazole Orange Dimers in DNA: Fluorescent Base Substitutions with Hybridization Readout

By using (S)‐2‐amino‐1,3‐propanediol as a linker, thiazole orange (TO) was incorporated in a dimeric form into DNA. The green fluorescence (λ=530 nm) of the intrastrand TO dimer is quenched, whereas the interstrand TO dimer shows a characteristic redshifted orange emission (λ=585 nm). Steady‐state optical spectroscopic methods reveal that the TO dimer fluorescence is independent of the sequential base contexts. Time‐resolved pump–probe measurements and excitation spectra reveal the coexistence of conformations, including mainly stacked TO dimers and partially unstacked ones, which yield exciton and excimer contributions to the fluorescence, respectively. The helicity of the DNA framework distorts the excitonic coupling. In particular, the interstrand TO dimer could be regarded as an excitonically interacting base pair with fluorescence readout for DNA hybridization. Finally, the use of this fluorescent readout was representatively demonstrated in molecular beacons.     

Spectroscopic studies of the conformational properties of naphthoyl chitosan in dilute solutions

A naphthoyl chitosan derivative was prepared, and its conformations in dilute solutions were characterized with spectroscopic methods, including circular dichroism (CD) spectroscopy and fluorescence emission spectroscopy. The CD spectrum of this polymer showed a negative band at about 295 nm in dimethyl sulfoxide (DMSO), indicating that the polymer adopted a helical secondary structure. A helix reversion occurred at concentrations greater than 1 mg/mL. The intensity of the CD signal decreased with the addition of water to the solution, and this suggested a change from a helical conformation to a looser one as a result of the collapse of intramolecular hydrogen bonds. In the fluorescence emission experiments, two kinds of excimer emission bands were detected at 375 and 425 nm, and they were assigned to a partially overlapped dimer with a twisted geometry and a fully overlapped dimer with a sandwichlike geometry, respectively. Adding water to a solution of naphthoyl chitosan in DMSO resulted in a gradual reduction of the emission intensity at 375 nm, and this implied that the twisted arrangement of the chromophore was destroyed by the presence of water. The relative intensity (i.e., the ratio of the intensity of the excimer emission at 425 nm to that of the excimer emission at 375 nm) depended on the solvent (DMSO, N,N‐dimethylformamide, N,N‐dimethylacetamide, and 1‐methyl‐2‐pyrrolidinone), and this indicated that the conformation of naphthoyl chitosan was solvent‐dependent. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2747–2758, 2004     

Normal and second excimers in macromolecules—I: Poly(1-methoxy-4-vinylnaphthalene) in fluid solution

The emission properties of poly(1-methoxy-4-vinylnaphthalene) (PMVN) in 2-methyltetrahydrofuran solution have been investigated over the range 77–350 K. It exhibits, in addition to the monomer fluorescence (348 and 360 nm), two structureless emissions derived from two different types of excimer, viz. the normal excimer (420 nm) and the second excimer (380 nm), the latter having a partially overlapping structure of aromatic rings. The intensity of second excimer emission of PMVN increases with decreasing temperature, while the normal excimer emission shows a maximum at 215 K. Kinetic analysis of transient decay curves for the fluorescence of PMVN gave results consistent with the previously reported kinetic scheme for the dimer model, 1,3-bis(4-methoxy-1-naphthyl)propane, showing that the second excimer is not formed from or converted to the normal excimer and that two types of excimer are formed independently from the excited monomer.     

Solvent effect on thermal degradation of plasticized para-substituted polystyrenes

The thermal effect on stability of a series of para-substituted polystyrenes with methyl, methoxy and α-methyl substituents in various solvents was studied in the temperature range of 298-363 K. They gave a monomer fluorescence as a minor part and excimer fluorescence as a major part. Thermal heating of para-substituted polystyrenes shows a decrease in both monomer and excimer fluorescences in all used solvents. Thermal heating causes a small fluorescence quenching effect at lower temperatures in solution but becomes very dominant at higher temperatures. Added terephthalate and phthalate plasticizers to these para-substituted polystyrenes caused a quenching of both monomer and excimer fluorescences without the formation of exciplex emission. The thermal quenching processes of the plasticized polymers were accompanied by a change in the structure of the fluorescence spectra at high heating temperatures. This may indicate that thermodestruction of these polymers starts from a random chain scission. The change in solvent polarity has considerable effect on fluorescence quenching but it has a minor effect on the thermal degradation of these polymers. The binding energies for excimer formation were calculated in the used solvents.     

EXCIMER FORMATION OF N-(I-PYRENESULFONYL)-DIPALMITOYL-L-α- PHOSPHATIDYLETHANOLAMINE AT THE LIPID-WATER INTERFACE OF FAT CELL PLASMA MEMBRANES

Abstract— The mechanism of excimer formation of N-(l-pyrenesulfonyl)-dipalmitoyl-L-α-phosphatidylethanolamine incorporated into fat cell plasma membranes was studied by means of steady-state and pulse fluorometry. It was found that the pyrenesulfonyl group at the lipid-water interface of the membrane formed an appreciable amount of ground-state dimers and that the excimer of the pyrenesulfonyl group was formed through two different processes: (i) the collisional interaction of an excited monomer with a ground-state monomer, and (ii) the direct excitation of a ground-state dimer (approximate ratio of the two processes, 1:1). Analysis of fluorescence decay curves revealed that the detailed mechanism of excimer formation by process (i) is essentially similar to the mechanism established for the reaction in organic solvents but not to that in a highly viscous medium such as the hydrocarbon core of the membrane.     

A Cu<Superscript>2+</Superscript>ion-selective fluoroionophore with dual off/on switches

A new malonamide fluoroionophore possessing two pyrene moieties was synthesized. This bispyrene exhibited the fluorescence of the pyrene monomer (λ_em = 395 nm) and intramolecular excimer (λ_em = 467 nm) emissions. The designed derivative showed the excellent ion sensing ability to Cu²⁺. The "on-off-off" and "off-on-off" fluorescence responses were demonstrated by the addition of the variable Cu²⁺ concentration. The utilization of the dual off/on responses could apply to the estimation of the rough Cu²⁺ concentration.     

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.     

FLUORESCENCE TECHNIQUE TO STUDY FREE-RADICAL POLYMERIZATION OF 2-VINYLNAPHTHALENE

A novel technique based on steady-state fluorescence measurements was introduced to study free-radical polymerization of 2-vinylnaphthalene (2VN). Both spectroscopic and kinetic studies of polymerization of 2VN in toluene solution have been carried out over the temperature range between 50 and 70°C. The effective radius of interaction of 2VN required for excimer formation was found to be around 3.5 nm. The product of reaction rate constant kp and steady-state radical concentration [M?], k_p[M?] was calculated for 2-vinylnaphthalene (2VN) polymerization carried out at the temperatures 50, 60, 65, and 70°C. A sudden change in the value of k_p[M?] was observed above 60°C, indicating that polymerization proceeds effectively just at and above this temperature.     

EXCITED STATES OF DNA AND ITS COMPONENTS AT ROOM TEMPERATURE—III. SPECTRA, POLARISATION AND QUANTUM YIELDS OF EMISSIONS FROM ApA AND poly rA*

Abstract— Corrected normalised emission spectra from 300 to 480 nm and their polarisation are reported for neutral and poly rA at room temperature. Yields are 4 times (ApA) and 6 times (poly rA) greater than the monomer. Comparison of emission spectra and polarisation spectra demonstrates heterogeneity of emission and, following attempts at spectral synthesis, comparison with experimental reference spectra allows the observed total emission spectra to be resolved into four components assigned as I monomer-like emission, II excimer fluorescence, HI monomer phosphorescence, IV excimer phosphorescence; the polarisation of the excimer fluorescences of ApA and poly rA are deduced. The observations can be understood quantitatively using a simple stacking model with excimer emissions originating in the stacked components. Quantitative differences between ApA and poly rA are due to their different hypochromism and extent of stacking. Differences of polarisation are attributed to different stacking geometries, and the directional properties of the transition moment suggest that the excimer fluorescence is largely charge-resonance in nature.     

Controllable Electronic Structures and Photoinduced Processes of Bay‐Linked Perylenediimide Dimers and a Ferrocene‐Linked Triad

A series of perylene‐3,4,9,10‐bis(dicarboximide) (PDI) dimers linked through the bay regions was systematically synthesized to examine the electronic structures and photophysical properties in dependence on the distance and orientation between the two PDI units. The spectroscopic and electrochemical measurements suggested that the coupling value of a directly linked PDI dimer (PDI)₂ is much larger than those of para‐ and meta‐phenylene‐bridged PDI dimers p‐(PDI)₂ and m‐(PDI)₂. The width of Davydov splitting was quantitatively evaluated to compare the coupling values between the two PDI units in these dimers by absorption spectroscopy in frozen 2‐methyl‐THF. Excimer formation of PDI dimers induced the strong fluorescence quenching and large red‐shifts. Femtosecond transient absorption revealed a broad absorption derived from an excimer in the range from about 600 nm to the near‐IR region. The rate constants of formation and decay of the excimer are strongly dependent on the coupling values. Time‐resolved measurements on ferrocene‐linked p‐(PDI)₂ revealed a competition between the photoinduced processes of electron transfer and excimer formation in PhCN, which is in sharp contrast with the sole electron‐transfer process in toluene.     

Transesterification kinetics of poly(ethylene terephthalate) and poly(ethylene 2,6‐naphthalate) blends with the addition of 2,2′‐bis(1,3‐oxazoline)

The kinetics of the transesterification reaction between poly(ethylene terephthalate) (PET) and poly(ethylene 2,6‐naphthalate) (PEN) with and without the addition of a chain extender were studied with ¹H NMR. Different kinetic approaches were considered, and a second‐order, reversible reaction was accepted for the PET/PEN reactive blend system. The addition of 2,2′‐bis(1,3‐oxazoline) (BOZ) promoted the transesterification reaction between PET and PEN in the molten state. The activation energy of the transesterification reaction for the PET/PEN reactive blend with BOZ (94.0 kJ/mol) was lower than that without BOZ (168.9KJ/mol). The rate constant k took an almost constant value for blend samples with different compositions mixed at 275 °C. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2607–2614, 2001     

Ionic and excited species in irradiated poly(dimethylsiloxane) doped with pyrene

The influence of temperature (77–230 K) on the fate of pyrene (Py) radical ions and Py excited states in irradiated poly(dimethylsiloxane) (PDMS) doped with Py is described. At 77 K, the Py radical ions seem to be stable, whereas the Py excited states [fluorescence (λ = 395 nm) and phosphorescence (λ = 575–650 nm)] are generated via tunneling charge transfer. In the range of the glass‐transition temperature (T_g = 152–153 K), the Py radical ions start to decay, taking part in a recombination process and leading to the Py monomer and Py excimer fluorescence (λ = 475 nm). The wavelength‐selected radiothermoluminescence (WS RTL) observed at approximately 395, 475, and 600 nm has helped us to identify the T_g range (152–153 K). The absorption maximum at approximately 404 nm, found in the temperature range under consideration, is thought to represent PyH^?, cyclohexadienyl‐type radicals produced as a result of the reaction of Py^?? with protonated PDMS macromolecules. With the initial‐rise method of evaluating the activation energy (E_a) with the WS RTL peaks observed in the T_g range, E_a values of 123–151 kJ mol^?1 have been found. Such high E_a values can be explained by the contribution of energy connected to the molecular relaxation of the matrix in the T_g range. The well‐known Williams–Landel–Ferry equation, with universal constants C₁ = 17.4 and C₂ = 12.7, has been successfully applied to the interpretation of old pulse‐radiolysis/viscosity data found for crosslinked PDMS doped with Py. The mechanisms involved in these phenomena are discussed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6125–6133, 2004     

Water sorption/desorption kinetics in poly(ethylene naphthalene-2,6-dicarboxylate) and poly(ethylene terephthalate)

Water sorption/desorption experiments were carried out on films (～ 220 μm thick) of amorphous poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) stored in ambient conditions for different periods of time (0.5-4 years) and of poly(ethylene terephthalate) (PET) with different degrees of crystalinity levels (0-29%) by means of FTIR spectroscopy. Water sorption/desorption kinetics follows Fick's law for all samples investigated. Water sorption isotherms, obtained from gravimetric methods, indicate a larger sorption capacity in the case of PEN materials. The apparent diffusion coefficients (D) are larger in the case of PET samples. The observed D values decrease with storage time (physical aging) of PEN samples and with the crystallinity of PET samples. © 1995 John Wiley & Sons, Inc.     

Studies of miscibility,transesterification and crystallization in blends of poly(ethylene terephthalate) and Poly(ethylene-2,6-naphthalene dicarboxylate)

Blends of poly(ethylene terephthalate) (PET) and poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) were obtained by coprecipitation from solution followed by melt-pressing for different timest _mand quenching in iced water. When the melt-pressing time was 0.2 and 0.5 min, two glass transition temperaturesT _gwere observed by means of dynamic mechanical analysis (DMA), indicating that there are two phases present, a PEN-rich phase and a PET-rich phase. The differential scanning calorimetry (DSC) curves show two crystallization peaks and two melting peaks which, according to wide-angle x-ray scattering (WAXS) measurements, can be attributed to PET and PEN, respectively. In the case oft _m=2 min or longer, a single value ofT _gand thus a single phase is found to exist. Fort _m=10 min and 45 min no crystallization and melting at all is observed during heating with 10°C/min, indicating that a copolyester of PET and PEN has been formed by transesterfication during melt-pressing.Time-resolved WAXS measurements during isothermal crystallization show that, in the blend, the half-time of crystallization of PET is different from that of PEN, and not the same as that which is found in the pure polymer.Dedicated with best wishes to Prof. Dr. E.W. Fischer on the occasion of his 65th birthday     

A fluoride-selective PCT chemosensor based on formation of a static pyrene excimer

[reaction: see text] Calixarene-based fluorescent chemosensor 1 with two fluorogenic pyrene units conjugated to amide groups as guest recognition sites is synthesized. Complexation of F(-) by 1 causes a red shift of its absorption band to 400 nm (Deltalambda = 54 nm) and a blue shift of the excimer emission to 470 nm (Deltalambda = 12 nm) together with enhanced fluorescence intensity. The blue-shifted excimer emission is attributed to a pyrene dimer formed in the ground state, a so-called static excimer.     

Thermal properties of random copolyesters of PET and 3- and 3,5-bromo-substituted p-hydroxybenzoic acids. II

Melt-polycondensation of poly(ethylene terephthalate) (PET) and 3-bromo-p-acetoxybenzoic acid or 3,5-dibromo-p-acetoxybenzoic acid in different mole ratios yielded random copolyesters. The copolyesters have higher T_gs than PET because of an increase in mol % of the substituted p-oxy-benzoate units and follow Wood's equation for copolymer T_gs. Using this equation, we calculated the T_gs of the homopolymers of 3-bromo-p-oxybenzoate and 3,5-dibromo-p-oxybenzoate, which are not available experimentally, to be 113 and 123°C, respectively. Up to certain percentages of the comonomer composition the copolyesters exhibited cold crystallization and melt transitions which we attribute to the crystallizable segments of PET. The variation in melting temperatures in the composition of the copolymer was explained by Flory's theory. The differences in the melting behavior of the polymer, annealed at various crystallization temperatures for a constant time interval, throws light on the morphological changes that took place in it. Using the Hoffman and Weeks method, we determined the extrapolated equilibrium melting temperatures of these copolyester which were used to calculate the enthalpy of melting for the crystallizable units.