Magnetic field dependence of delayed fluorescence of pyrene in solution

The magnetic field effect on the intensities of monomer (I_M) and excimer (I_D) delayed fluorescence of pyrene has been found to be identical. This result suggests a common spin-selective step in the processes giving rise to I_M and I_D, and is discussed in the light of previously proposed mechanisms. An unusual solvent effect is found on the I_M (I_D) versus field strength curves. This is attributed to photochemical doublet-radical formation. Doublet-radical formation affects the field dependence of I_M and I_D through spin selectivity in the triplet quenching reaction.     

Molecular dynamics of polystyrene model molecules: Solvent effects on intramolecular excimer formation in 2,4-diphenylpentanes

Intramolecular excimer formation in 2,4-diphenylpentanes has been examined in a homologous series of alkanes, in ethanol and in mixtures of ethanol, ethylene glycol and glycerol. The ratio of the emission intensities of dimer and monomer (I_D/I_M) is not affected in low viscosity solvents but, above 4 cP, viscosity effects are discernible and a relationship of the form I_D/I_M = Aη^?2 is obeyed. In methylene chloride, only the dl molecule exhibits a decrease of the efficiency of excimer sampling. The temperature dependence of I_D/I_M in isooctane and methylene chloride has been interpreted in terms of the activation energy of the excimer sampling.     

Luminescence studies in polymers—II : Temperature effect on polyvinylcarbazole fluorescence

Polyvinylcarbazole (PVCa) films and solutions emit normal and excimer fluorescence between 77 and 425 K. The absolute (I_M and I_D) and relative (I_M/I_D) intensities emitted strongly depend on temperature. The usual U-shaped curve is obtained for log I_M/I_D as a function of 1/T in the case of polymer films. In solution, two minima corresponding to two different excimers are observed. The formation and dissociation of PVCa excimers in films and solutions have been interpreted according to the usual kinetic scheme. The binding energies for the excimer in films and for the high temperature excimer in solutions are respectively 4 and 2·8 kcal mole⁻¹.     

The ratio of excimer to monomer emission in diesters of 1-pyrenoic acid and five glycols

The ratio of the intensities of excimer and monomer emission, I_D/I_M, has been measured for Py? COO? (CH₂)_m? OOC? Py (m = 2–6, Py = 1-pyrenyl) in solvents of different viscosity, η. These compounds are dimeric models for the corresponding polymers, selected to eliminate the influence of energy migration on the population of an excimer. The solvents are H(CH₂)_nOH, n = 1–7, and ethylene glycol. The values of I_D/I_M decrease as η increases, and the rate of this decrease is a function of m. Therefore the qualitative dependence of I_D/I_M on m is a function of η. In the limit as η → ∞, I_D/I_M is an odd–even function of m for m = 2–5, and the values at m = 5 and 6 are nearly identical. A rotational-isomeric state analysis of the conformations of the diesters can rationalize the appearance of the odd–even effect. This result is insensitive to assumptions about the extent of overlap of the two pyrene ring systems that is required for the production of emission in the excimer band. © 1993 John Wiley & Sons, Inc.     

Fluorescence of copolymers of 2-naphthyl methacrylate and methyl methacrylate

The fluorescence of a series of copolymers of 2-naphthyl methacrylate (2-NM) and methyl methacrylate (MMA) with various contents of 2-NM (obtained in chloroform, carbon tetrachloride and acetonitrile) was investigated. A linear dependence between the ratio of the excimer to monomer emission intensities (I_D/I_M) and the diad fraction (f_nn) of 2-NM monomer units was established. The relationship between I_D/I_M and f_nn · I_n (I_n = the mean sequence length of 2-NM units) fits a logarithmic curve. The results indicate that the excimer emission is determined mainly by the nearest neighbour naphthalene-containing monomer units in the copolymer chain. The copolymers obtained in acetonitrile have higher values of I_D/I_M than those obtained in chloroform and carbon tetrachloride. This difference is due to the higher content of mm-triads in copolymers from acetonitrile, confirmed by ¹H-NMR analysis of the samples of poly(methyl methacrylate) formed from copolymers of 2-NM and MMA.     

Effect of the directionality of the ester group on the formation of hairpins in polyesters containing naphthalene and a flexible spacer

Steady-state fluorescence measurements and molecular dynamics simulations have been used to study the intramolecular formation of excimers in five model compounds for polyesters containing naphthalene groups separated by flexible spacers. The model compounds are derived from 2-hydroxynaphthalene and HOOC (CH₂)_n COOH, n = 2–6. The ratio of the intensity of excimer and monomer emissions, I_D/I_M, is nearly independent of the viscosity of the medium, η, over the range covered in dilute solution. Although I_D/I_M is always very small, it shows an odd–even effect for the first four members of the series, with maxima when n is odd. Molecular dynamics simulations provide an explanation for the small values of I_D/I_M, their weak dependence on η, and the trend of I_D/I_M with n. The results for the present series of model compounds are compared with previous work, which reported larger values of I_D/I_M, and a stronger dependence of I_D/I_M on η, for bichromophoric compounds derived from 2-naphthoic acid and aliphatic glycols, where the direction of the ester groups is reversed. The origin of the difference in the behavior of I_D/I_M in the two series is identified. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1127–1133, 1997     

Emission studies of pyrene solutions

Emission spectra of pyrene in hexane have been obtained over a temperature range (from 130 to 260 K) that has not been explored before for concentrations ranging from 10^?4 mole/? to 2 × 10^?2 mole/?. A conventional and new approximation which does not depend on the experimental set-up response functions has been used for evaluating pyrene excimer association energy W_DM and other pyrene parameters. In both methods W_DM agrees quite satisfactorlly, at all concentrations used, with that reported in the literature and obtained by other techniques. However, both approximation fail to yield the right values of the rate parameters at concentrations ? 2 g/?. This set the upper limit of sample solubility to be at C = 2g/? for our range of temperature. Furthermore, the new approximation can probably be used at higher concentration (for higher range of temperature) and even may be used for other organic molecules. There was also no difference in the ratio of the excimer (I_D) to monomer (I_M) quantum yields when an intense laser beam was used as a source of excitation rather than a super-pressure Hg-lamp. This suggested that the concentrations we used might not be large enough. As a result, the laser beam would not create enough density of excited molecules to affect the ratio of I_D/I_M.     

Intramolecular formation of excimers in model compounds for polymers of 2,6-naphthalene dicarboxylic acid and five glycols in media of different viscosity

Steady-state fluorescence has been used to study the dependence of the intramolecular formation of excimers in media of differing viscosity, η, for model compounds of polyesters containing naphthalene groups. The model compounds are derived from 2-naphthalene carboxylic acid as rigid units and glycols, HO (SINGLEBOND) (CH₂)_m (SINGLEBOND) OH, m = 2(SINGLEBOND) 6, as flexible spacers. The ratio of the intensity of excimer and monomer emissions, I_D/I_M, exhibits two regimes: a regime at high η where I_D/I_M shows an odd(SINGLEBOND) even effect with m, with maxima at odd m, and a regime at low η where the odd-even effect is lost, and the maximum values are obtained for m = 3 and 6. Calculations performed for the equilibrium state using the rotational isomeric state model and Molecular Dynamics (MD) simulations allow the rationalization of the behavior of I_D/I_M with m in the media of high and low η, respectively. © 1996 John Wiley & Sons, Inc.     

Study of the diffusion of pyrene in silicone polymer coatings by steady state fluorescence technique: effects of pyrene concentration

The concentration dependence of the diffusion coefficient of pyrene in single component and two-component room temperature curing silicone polymer coatings is investigated by the steady state fluorescence technique by measuring the pyrene excimer fluorescence intensity. At pyrene concentrations lower than 10 mM, the intensity of excimer fluorescence is proportional to the concentration and at higher concentrations it deviates from this trend due to concentration quenching. Thermal aging studies show that this concentration quenching can be removed by thermal annealing and the excimer emission intensity approaches the value expected from the trend at lower concentrations. The diffusion coefficient of pyrene at low concentrations in silicone polymer coatings is obtained using the approximate solution of one-dimensional diffusion equation. A modified approach is employed to estimate the diffusion coefficient at higher pyrene concentrations. In this method, the excimer intensity and time scale are shifted, respectively to I_max the maximum value of excimer intensity attained during annealing and t_max, the time taken to reach this. The estimated diffusion coefficients at different pyrene concentrations show a negligible dependence on pyrene concentration in both types of polymers. These results are attributed to the high structural mobility of silicone polymer chains due to their molecular structure.     

Dynamic viscosity versus probe-reported microviscosity of aqueous mixtures of poly(ethylene glycol)

Correlation between the dynamic viscosity (η) and the microviscosity of a hybrid green medium constituted of water and poly(ethylene glycol) (PEG) of average molar mass (200, 400, and 600) g · mol^?1, respectively, is explored over the temperatures range (10 to 90) °C across the complete composition regime. The microviscosity is obtained using a fluorescence probe 1,3-bis-(1-pyrenyl)propane (BPP), which is manifested through the ratio of the monomer-to-intramolecular excimer intensities (I_M/I_E). Aqueous PEG mixtures are observed to behave similar to Newtonian fluids as the temperature dependence of dynamic viscosity follows Arrhenius-type behavior. Surprisingly, a simple and convenient linear dependence of ln η with wt% PEG of the mixture is established. The BPP I_M/I_E is observed, in general, to increase with the bulk dynamic viscosity of the mixture having >10 wt% PEG suggesting a good correlation between the bulk dynamic viscosity and BPP-reported microviscosity when the viscosity of the aqueous PEG mixture is relatively high.     

Rationalization of the ratio of excimer-to-monomer fluorescence emission intensity in bichromophoric diesters of 1-pyrenoic acid and mono-, di-, tri-, and tetraethylene glycols

The ratio of excimer to monomer emission intensities, denoted by I_D/I_M, was measured for Py–COO(CH₂CH₂O)_mCO ? Py, where Py denotes the 1-pyrenyl group and m = 1–4, in solvents of different viscosity, η. Three different systems were used to change the viscosity of the medium: (a) Mixtures of methanol and ethylene glycol at 25°C, (b) linear aliphatic alcohols, H(CH₂)_nOH, where n =1–6, also at 25°C, and (c) ethylene glycol over the range 6.6–35°C. The ratio I_D/I_M decreases sharply as η increases, and the rate of the decrease in I_D/I_M is a function of m. Quantitatively, the dependence of I_D/I_M on η at high viscosity, i.e., the slope [d(I_D/I_M)/d(1/η)]_∞, is larger in the present work than in another series of 1-pyrenyl diesters in which the flexible spacer is an oligomer of polyethylene, instead of an oligomer of polyoxyethylene. In the limit where η → ∞, the ratio I_D/I_M assumes its largest value in the bichromophoric compound with m = 2. However, as η decreases the compound with m = 3 becomes the one with the largest I_D/I_M. A complete rotational isomeric state analysis (for the compounds with m = 1–3) and a Monte Carlo simulation (for the compound m = 4) of the conformations of the diesters can rationalize the behavior of I_D/I_M in the high viscosity limit. ©1995 John Wiley & Sons, Inc.     

Structure and hydrodynamics of polyamic acid chains in solutions. Effect of “pin-joint” oxygen atom

The hydrodynamic characteristics and weight-average molecular weights of some samples of poly-(4,4′-oxydiphenylene) pyromellitamic acid (I) and poly (4,4′-phenylene) pyromellitamic acid (II) in DMF have been obtained by sedimentation, viscometry and light scattering. For (I) the dependence of S₀ on M was obtained and it was found that the hydrodynamic parameters $\text{F}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{P}{}^{\mathrm{?1}}$ depend on M up to M = 10⁵. The experimental results were treated by using two models, viz. a Yamakawa-Fujii persistent chain and a rotational isomeric chain with free rotation about valence bonds. It was shown that the hydrodynamic behaviour of macromolecules of (I) with a pin-joint oxygen atom in the main chain is satisfactorily described by the rotational isomeric chain model with free rotation about valence bonds. The dependence of [η] on M was obtained and the constants K and a in an equation of the Mark-Kuhn-Houwink type were determined for (II). The flexibility of the chain was estimated and the Kuhn segment was found to be 200 Å using the Yamakawa-Fujii theory. The flexibilities of the chains of (I) and (II) are compared; it is shown that the rigidity of the chain of (II) without a pin-joint oxygen atom in the monomer unit is much higher than that of the chain of (I). The dependence of S₀ on M obtained for (II) suggests that its macromolecules show a more pronounced tendency for formation of aggregates than those of (I).     

Fluorescence observations on complex formation between linear and hyperbranched polyelectrolytes in dilute aqueous solutions

The complexation between poly(ethylene imine) (PEI) and poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) or AMPS copolymers was investigated with the relative excimer emission intensity I_E/I_M of a cationic probe 1-pyrenemethylamine hydrochloride (PyMeA · HCl), fluorescence nonradiative energy transfer (NRET) I_Py/I_Np of naphthalene to pyrene labels, the fluorescence anisotropy r and I_E/I_M of pyrene labels. PEI was a hyperbranched weak polycation in acid solution, which formed complex with anionic polyelectrolytes due to the electrostatic attraction. The I_E/I_M of PyMeA · HCl probe decreased to zero, the intra-, intermolecular NRET I_Py/I_Np and I_E/I_M of pyrene labels on the AMPS copolymers reached their maxima when χ was increased from 0 to 2.4, which was defined as the mole ratio of the amino group in PEI to the AMPS group in the polyanion. These facts indicated the formation of nonstoichiometric complex of the oppositely charged polyelectrolytes when χ = 2.4 at the concentration much lower than their overlap concentrations. The intermolecular aggregate appeared as indicated by an increase in the intermolecular I_Py/I_Np and r with χ up to 2.4 due to neutralizing and hydrophobizing the polyelectrolytes and the bridging effect of the PEI chain bound on different polyanion chains. At high pH, PEI became a neutral polymer and did not bind with the AMPS anion to form the complex as illustrated by the constant value of r for the pyrene labels attached to the AMPS polyanion as that without addition of PEI. The amino group in PEI quenched pyrene and naphthalene emission, resulting a decrease in both I_Np and I_Py.     

Interaction of cetyltrimethylammonium bromide and poly(2-(acrylamido)-2-methylpropanesulfonic acid) in aqueous solutions determined by excimer fluorescence

 The binding interaction of the cationic surfactant cetyltrimethylammonium bromide (CTAB) and anionic poly(2-(acrylamido)-2-methylpropanesulfonic acid) (PAMPS) in dilute aqueous solutions was studied using the excimer fluorescent emission of the cationic probe 1-pyrenemethylamine hydrochloride (PyMeA·HCl). In the absence of CTAB, the saturation binding of PyMeAH⁺ on PAMPS is about 2.4 AMPS repeat units for one probe cation as determined by the relative emission intensity, I _E/I _M, of the excimer to monomer. With increasing CTAB concentration, I _E/I _M firstly increases, reaches a maximum, then decreases to zero. The I _E/I _M maximum indicates a critical aggregation concentration (cac) of 10⁻⁵ mol/l for CTAB in PAMPS solutions. The CTAB concentration at which I _E/I _M is zero is exactly equal to the PAMPS concentration, indicating that the probe cation is thoroughly excluded from the binding site of PAMPS by the CTAB cation and the equivalent stoichiometric aggregation is formed between CTAB and PAMPS. The blueshift of the excimer emission and the excitation spectra shows that the decrease of I _E/I _M with increasing CTAB concentration above the cac is caused mainly by the decrease of the static excimer. Received: 26 July 2000 Accepted: 23 November 2000     

Temperature effects on the relaxation parameters of the 1-methylnaphthalene excimer

The excimer lifetime τ_D and the excimer fluorescence efficiency for 1-methylnaphthalene in ethanol have been determined between −30 and 60°C. Expressing the rate constant for excimer deactivation. k_D, in terms of radiative k_FD) and nonradiative (k_ID) processes as k_D = k_FD + k_ID it is found that k_FD is independent of tempeature and k_ID = 5 × 10⁶ + 3.2 × 10¹¹ exp (-ΔE/RT) sec⁻¹, where ΔE = 6.7 kcal mole⁻¹. The behaviour of k_FD and k_ID with temperature and the appearance of isoemissive points in a limited region of temperature are discussed.     

Isotopic effect in the radiolysis of water. Diffusion-kinetic modelling up to 300°C

Diffusion-kinetic calculations [1-3] have been analysed to determine the isotopic effect in the radiolysis of water with ionising radiation of linear energy transfer characteristics (LET) from 0.2 to 60 eV/nm and at temperatures up to 300°C. This analysis shows that, for low LET radiation, the spur decay of e^- _aq is slower in D₂O and results in a higher yield of e^- _aq, g(e^- _aq), at 10^-7 -10^-6s after the ionisation event. In low LET radiolysis, g(OD) ≈ g(OH) over the whole range of temperature but in high LET radiolysis g(OD) is clearly lower than g(OH). The isotopic effect on the yields of the radical products is enhanced by increasing LET but diminished by increasing temperature. The yields of the molecular products show the opposite isotopic effect to their radical precursors, namely g(D₂) is 10-20% lower than g(H₂) and g(D₂O₂) > g(H₂O₂). A particularly significant difference between g(D₂O₂) and g(H₂O₂) has been found at LET = 20 eV/nm. The isotopic dependence of the g-values estimated for fast neutron radiolysis is also presented.     

Preparation,Crystallographic, Spectroscopic and Magnetic Characterization of Low‐Valency Nitridometalates Li2[(Li1‐xMx)N] with M = Cu,Ni

Thermogravimetric and difference thermal analyses show that the reactions of lithium nitride with the transition metals Cu and Ni under molecular nitrogen to form phases Li₂[(Li_1‐xM^I_x)N] take place above 673 K. The maximum weight gains are reached at 926 K and 968 K for M = Cu and Ni, respectively. At higher temperatures, the ternary phases Li₂[(Li_1‐xM^I_x)N] decompose, limiting the substitutional level x. In the temperature range of 773 K — 873 K, the successful synthesis of Li₂[(Li_1‐xNi^I_x)N] (0 < x ≤ 0.85(1)) single phase products is demonstrated. Maximum substitution obtained for the Cu phases is x_max= 0.43(1). The dependence of the lattice parameters of the hexagonal unit cell on x is almost linear. The magnetic moment of M strongly depends on x. At low x the magnetic moments in phases with M = Ni are presumably enhanced by orbital effects. A decrease of μ_eff with x to μ_eff(x = 1) → 0 is explained by delocalization of the magnetic moments and by the gradual formation of a metal for the hypothetical compound Li₂[NiN] (x = 1). XAS spectroscopy at the transition metal K‐edges shows that Cu and Ni principally correspond to d¹⁰‐ and d⁹‐configurations, respectively.     

TEMPERATURE DEPENDENCE OF THE FLUORESCENCE SPECTRA OF LADDERLIKE POLYPHENYLSILSESQUIOXANE AND LADDERLIKE 1,4-PHENYLENE-BRIDGED POLYVINYLSILOXANE

Fluorescence spectra of ladderlike polyphenylsilsesquioxane (LPPS) and ladderlike 1,4-phenylene-bridgedpolyvinylsiloxane (LPPVS) have been measured as a function of temperature (4-55℃), in dilute tetrahydrofuran solution.The excimer (I_E) to monomer (I_M) intensity ratio (I_E/I_M) of LPPS dilute solution shows a double linear Arrhenius plot with abreak point ascribable to a transition temperature T_t This behavior has not been found for single chain polyorganosiloxanes.When InI_E of LPPS was plotted against I/T it also gave a double linear plot with one break point, which was found in singlechain polyorganosiloxanes. The different behaviors between LPPS and single chain polyorganosiloxanes may be mainlyattributed to the relatively rigid double-chain macromolecular backbone of LPPS. However, the In(I_E/I_M) of dilute LPPVSsolution versus I/T shows a simple linear variation with a positive slope which confirms our proposition. The differencebetween the fluorescence results of LPPS and those of LPPVS may possibly derive from their structure differences andcooperative motion in backbone chain bonds.     

The Fe2+ chelate of hydrazinecarboxylic acid studied by the Mössbauer effect

The chelate compounds K[Fe(hyc)₃] and N₂H₅[Fe(hyc)₃]·H₂O (hyc = N₂H₃COO) were studied by the Mössbauer effect of ⁵⁷Fe at various temperatures. At room temperature the quadrupole splitting parameter is 2.77 mm/sec for K[Fe(hyc)₃] and 2.35 mm/sec for N₂H₅[Fe(hyc)₃]·H₂O, and the center shift is 1.08 mm/sec for both compounds. The temperature dependences of the quadrupole parameters yielded the crystal field splittings of the ⁵T_2g levels of the Fe²⁺ ions which indicate large trigonal distortion of the Fe(hyc)₃ anion. Using a molecular crystal-like treatment of the ferrous ion vibrations the temperature dependence of the recoilless fraction gave an effective Debye temperature Θ_D = 71°K for K[Fe(hyc)₃] and Θ_D = 90°K for N₂H₅[Fe(hyc)₃]·H₂O. No evidence for magnetic ordering was found down to 4.5°K in either compound.     

Physicochemical constants as a factor determining the need for the derivatization of organic substances in analysis by gas chromatography

A criterion was proposed to estimate the necessity of the derivatization of organic substances for their determination on conventional nonpolar phases, based on such characteristic of analytes as molecular weight (M _r), normal boiling point (T _bp), and molar refraction (MR _D). All these constants can be presented as indices relative to nonpolar n-alkanes (similarly to chromatographic retention indices), I(M), I(T), and I(MR _D), which can be compared to each other as differences Δ_{T − M} = I(T) − I(M) and Δ_{T − M} R _D = I(T) − I(MR _D). Substances do not require derivatization if Δ_{T − M} < 400 and Δ_{T − M} R _D < 600, while at Δ_{T − M} > 600 and Δ_{T − MRD} > 800, derivatization is necessary.