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.     

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.     

Magnetic field dependence of monomer and excimer delayed fluorescence of aromatics in fluid solution

We have extended our study of the magnetic field dependence of monomer (M_M) and excimer (I_D) delayed fluorescence of aromatics in fluid solution. I_M and I_D show a monotonic decrease with increasing field strength at room temperature in agreement with theoretical predictions. At lower temperatures both I_M and I_D show an initial increase followed by a monotonic decrease. This marked temperature dependence can be attributed to a “freezing-out” of the effect of molecular motion on the field effect. At even lower temperatures the field effect curves of I_M and I_D start to diverge. Two models have been proposed to explain the mechanism of excimer formation in triplet—triplet annihilation. Qualitatively at least the divergence can be rationalized in terms of both models.     

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.     

Characteristics of excimer formation in langmuir-blodgett assemblies of 1-palmitoyl-2-pyrenedecanoylphosphatidylcholine and dipalmitoylphosphatidylcholine

Langmuir-Blodgett film alloys of PPDPC (1-palmitoyl-2-[10-(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine) and DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine) were assembled in varying molar proportions on quartz glass slides. The transferred layers were then characterized according to their pyrene excimer to monomer fluorescence intensity ratio i_E/i_M and fluorescence quantum yields as a function of film composition. The observed deviation from non-ideal mixing is considered to be due to the formation of regular distribution patterns of PPDPC in a DPPC lattice. The observed critical mole fractions of PPDPC evident as steps in I_E/I_M versus X_PPDPC plots can be accounted for by a model involving a trigonal distribution of pyrenedecanoyl chains in the phospholipid acyl chain lattice. The implications of the distribution of PPDPC as superlattices to the excimer formation mechanism are discussed.     

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.     

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.     

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.     

Preparation of substituted ionic carbohydrate polymers and their interactions with ionic surfactants

The formation of micelles of hexadecyltrimethylammonium chloride (CTAC) and sodium dodecylsulfate (SDS) in aqueous solutions containing charged polysaccharides was studied by steady-state and time-resolved fluorescence measurements using pyrene as a photophysical probe. Micropolarity studies using the I₁/I₃ ratio of the vibronic emission bands of pyrene and the behaviour of the I_E/I_M ratio between the excimer and monomer emissions show the formation of hydrophobic domains. The interactions between the polyelectrolytes and surfactants of opposite charge lead to the formation of induced pre-micelles at surfactant concentrations lower than the critical micellar concentration (cmc) of the surfactants. At similar concentrations, the I_E/I_M ratio shows a peak. This aggregation process is assumed to be due to electrostatic attractions. At higher surfactant concentrations, near the critical micellar concentration, micelles with the same properties as those found in pure aqueous solution are formed. On the other hand, systems containing polyelectrolytes and surfactants of the same charge do not show this behaviour at low concentrations. The presence of long alkyl chains bound to the polyelectrolytes also induces the formation of free micelles at concentrations somewhat below the aqueous cmc.     

Fluoride-induced intermolecular excimer formation of bispyrenyl thioureas linked by polyethylene glycol chains

New bispyrenyl thioureas linked by polyethylene glycol (PEG) chains, L1-L3, and methoxy benzene pyrene thiourea, L4, were synthesized. Upon binding with F⁻ in CHCl₃, L1-L3 exhibited strong excimer emission bands (I_E) and weak monomer emission bands (I_M), while L4 displayed the same intensity of both bands. However, little or no change was observed in fluorescence spectra of L1 upon adding OH⁻, AcO⁻, BzO⁻, H₂PO₄⁻, Cl⁻, Br⁻, and I⁻. Therefore, only F⁻ induced the pyrene excimer formation. Job’s plots showed 1/1 or 2/2 complexation of L1 with F⁻. Ratios of I_E/I_M of L1·F⁻ complex were dependent on the concentration of L1, implying that the dimerization of L1 proceeded via the intermolecular excimer formation. Among L1-L4, L1 possessed the highest binding constant and sensitivity toward F⁻ implying the importance of the linking PEG chain. L1 was demonstrated to be an excellent probe for F⁻ in CHCl₃ with the detection limit as low as 46.2 μg/L.     

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     

The effect of γ‐cyclodextrin addition in the self‐assembly behavior of pyrene labeled poly(acrylic) acid with different chain sizes

The interaction between poly(acrylic acid) polymers (PAA) of low‐ (2000 g/mol) and high‐ (450,000 g/mol) molecular weight (M_w) hydrophobically modified with pyrene (PAAMePy) and β‐ and γ‐cyclodextrins (β‐CD, γ‐CD) was investigated with fluorescent techniques. The interaction with β‐CD promotes little variation in the spectral and photophysical behavior of the polymer, whereas significant changes are observed upon addition of γ‐CD. The degree of inclusion (between the pyrene groups of the polymer and the cyclodextrins) is followed through the observation of the changes in the absorption, excitation (collected in the monomer and excimer emission regions) and emission (I_E/I_M ratio) spectra and from time‐resolved data. Within the studied range of γ‐CD concentration, the fluorescence decays of the long chain (high M_w) PAAMePy polymers were found tri‐exponential in the monomer and excimer emission regions in agreement with previous studies. In the case of the low M_w PAAMePy polymers, tri‐exponential decays were observed at the monomer and excimer emission wavelengths. However, when a γ‐CD concentration of 0.01 and 0.03 M is reached for, respectively, the low‐ and high‐labeled pyrene short chain (low M_w) polymers, the fluorescence decays in the excimer region become biexponential (two excimers) with no rising component, thus showing that all pyrene groups are encapsulated (and preassociated) into the γ‐CD cavity. In the case of the high M_w polymers, the addition of γ‐CD has been found to change the level of polymer interaction from pure intramolecular (water in the absence of cyclodextrin) to a coexistence of intra‐ with intermolecular interactions. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1402–1415, 2008     

Aquation of chloropentaamminecobalt(III) Perchlorate in chloride-ion containing water–nonaqueous solvent mixtures

The equilibrium quotients for the formation of Co(NH₃)₅Cl²⁺ from Co(NH₃)₅OH₂³⁺ and Cl^? were 3.74±0.25 M^?1 and 6.07±0.54 M^?1 at 45.0°C in 10:1 mole ratio water: dimethyl sulfoxide and in 25 w/w % aqueous ethanol, respectively, and those forthe formation of the ion pair Co(NH₃)₅OH₂³⁺ . Cl^? were 1.21±0.20 M^?1 and 1.58±0.17 M^?1, respectively, in the same solvents. The aquation and anation rateconstants were determined at 45.0°C for these two solvents over the range of chloride-ion concentrations 0.0 ≤ [Cl^?] ≤ 0.9 M. The aquation rate constant was essentially independent of chloride-ion concentration in each solvent over this range. The inverse of the pseudo-first-order anation rate constant was linearly dependent on the inverse of the chloride-ion concentration in each solvent. The least squares relationships between (1/k_an) and (1/[Cl^?]) gave intercepts and ratios of intercept to slope which were analyzed interms of I_d and D mechanisms. It was concluded that the data were not satisfied by a D mechanism, but that they were consistent with an I_d mechanism.     

Polymerization of acrylamide with persulfate–thiomalic acid initiator

The redox system of potassium persulfate–thiomalic acid (I₁–I₂) was used to initiate the polymerization of acrylamide (M) in aqueous medium. For 20–30% conversion the rate equation is where R_p is the rate of polymerization. Activation energy is 8.34 kcal deg^?1 mole^?1 in the investigated range of temperature 25–45°C. M_n is directly proportional to [M] and inversely to [I₁]. The range of concentrations for which these observations hold at 35°C and pH 4.2 are [I₁] = (1.0–3.0) × 10^?3, [I₂] = (3.0–7.5) × 10^?3, and [M] = 5.0 × 10^?2–3.0 × 10^?1 mole/liter.     

Binding on strong polyelectrolytes of mixed ionic and nonionic surfactants below their critical micelle concentration observed by fluorescence

The binding of mixed surfactants of cationic cetyltrimethylammonium bromide (CTAB) and nonionic octaethylene glycol monododecyl ether (C ₁₂E ₈) on anionic polyelectrolyte poly[2-acrylamido-2-methylpropanesulfonic acid (PAMPS)] and fluorophore-labeled copolymers containing about 40 mol% of AMPS was investigated at different mole fractions, Y , of CTAB in the surfactant mixture. The excimer emission of the cationic probe 1-pyrenemethylamine hydrochloride (PyMeA·HCl), nonradiative energy transfer (NRET) between pyrene and naphthalene labels and I ₁/ I ₃ of the pyrene label were determined by varying the total surfactant concentration, c _Surf. The I _E/ I _M value of PyMeA·HCl firstly increases and then decreases to 0 with c _Surf, showing a maximum on every curve. The critical aggregation concentration of the mixed surfactants determined from the I _E/ I _M maximum decreased from 5×10 ^-5 to 1×10 ^-5 mol/l as Y increased from 0.1 to 0.50, and then leveled off as Y increased up to unity. And at least 5×10 ^-6 mol/l CTAB was required for the mixed surfactants to bind on the PAMPS cooperatively. Equimolar binding of CTAB on AMPS was formed at I _E/ I _M=0 when Y =0.25, while at Y =0.1 some CTAB molecules in the mixed micelle were directed to the water phase without binding with AMPS. Both the intramolecular and the intermolecular NRET increased and then decreased with c _Surf, having a maximum on each curve corresponding to the equimolar binding of CTAB and AMPS so long as Y >0, indicating the coiling of the chain and interchain aggregation upon bound surfactants. The I _Py/ I _Np value at the maximum decreased with decreasing Y because more nonionic surfactant C ₁₂E ₈ participated into the polyelectrolyte-mixed surfactant complexes together with bound CTAB.     

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     

Investigations of the effect of viscosity of resin on the diffusion of pyrene in silicone polymer coatings using steady state fluorescence technique

The diffusion phenomenon of pyrene in silicone coatings prepared from various commercial silicone resins with different viscosities was investigated using steady state excimer fluorescence technique. The amount of pyrene lost from the coatings by diffusion at different temperatures ranging from 25 to 70 °C was estimated from the excimer emission intensity. The diffusion coefficients (D) and activation energy of diffusion of pyrene in silicone coatings were determined. It was found that the kinetic parameters of pyrene were comparable in all the four silicone coatings and independent of the viscosity of the resin. The D values were lower than those expected for pyrene in PDMS resins in the corresponding viscosity range. This may be attributed to restricted mobility of pyrene due to cross-linking of polymer chains in the cured silicone coatings.     

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.