Sequence distribution and intramolecular excimer formation in styrene-acrylonitrile copolymers

Styrene-acrylonitrile copolymers, like many other copolymers containing styrene, exhibit both normal and excimer fluorescence. We have shown that the ratio of the excimer to monomer fluorescence intensities in random styrene-acrylonitrile copolymers is linearly dependent upon the concentration of styrene-styrene bonds in the copolymer. This observation is consistent with a photophysical model which allows the energy absorbed by styrene units to migrate freely along the copolymer chain. Some of the energy is emitted in the form of normal fluorescence; some of the energy, trapped by neighbouring styrene-styrene pairs suitably oriented to allow excimer formation, is emitted as excimer fluorescence. The fluorescence characteristics of acrylonitrile-styrene copolymers are contrasted with those of methyl methacrylate-styrene copolymers, in which the methylmethacrylate sequences are believed to present partial barriers to energy migration along the copolymer chains.     

Steric effects on excimer formation in isotactic and atactic polystyrene and poly(ar-methylstyrene)s in solution

Effects of tacticity and steric hindrance on excimer formation were investigated in isotactic and atactic polystyrene, poly(o-methylstyrene), poly(m-methylstyrene), and poly(p-methylstyrene) in the presence and absence of a quencher (CCl₄). The calculated rate constants for excimer formation in the isotactic polymers except for poly(o-methylstyrene) were almost the same and larger than those in the corresponding atactic polymers. These results indicate that excimer formation was due to not only rotational sampling but also energy migration to trapping sites. It was found that steric hindrance on excimer formation was intimately related to the excition diffusion length in the polymer chain.     

Double-exponential decay in intramolecular excimer formation: 1,3-di(2-pyrenyl)propane

The fluorescence response functions of 1,3-di(2-pyrenyl)propane in methylcyclohexane can be fitted with two exponentials having the same decay parameters for the excimer and the monomer. This is in contrast to what had been observed before with 1,3-di(1-pyrenyl)propane where three exponentials are required. It is concluded that the different conformers of the propane chain act as a single kinetically uniform group in forming excimers. This signifies that the kinetic scheme applicable to intermolecular excimer formation (the so-called Birks scheme) can also be utilized in the intramolecular case, the more complex kinetics being exceptional.     

Molecular simulation of excimer fluorescence in polystyrene and poly(vinylcarbazole)

In fluorescence emission spectra of poly(vinylcarbazole) (PVK), two types of excimers are observed, the fully and the partially overlapped excimers, namely, excimers and exciplexes. In this work, we investigated the structural changes induced by the transition between electronic levels S(0) and S(1). Furthermore, the widely used assumption of similar potential energy surfaces in the S(0) and S(1) states and its use in molecular dynamics simulations are thoroughly examined for PVK and polystyrene (PS). The ground-state and excited-state intermolecular potentials between phenyl or carbazyl substituents in PS or PVK, respectively, are computed from high-level ab initio calculations and fit to analytic potentials. Finally, molecular dynamics simulations are performed at room temperature for PS and for isotactic and syndiotactic PVK. This treatment enabled the decoupling of excimer and exciplex contributions from the simulated spectra.     

Molecular dynamics of polystyrene solutions in microwave fields

Equilibrium and nonequilibrium molecular dynamics simulation techniques were used to assess the influence of an applied microwave field on the dynamics of methylamine-methanol and methylamine-dimethylformamide (DMF) solutions bound within atactic polystyrene over a range of polymer densities from 35 to 96 wt % polymer. Atomistically detailed systems were studied, ranging from 3000 to 10 644 particles, using previously established potential models. Structural and dynamical properties were determined in the canonical (NVT) ensemble at 298 K. The simulated DMF self-diffusion coefficients in polystyrene solutions were compared with the zero-field experimental results established with pulsed-gradient spin-echo NMR spectrometry. A simulated external microwave field, with a rms electric field intensity of 0.1 VA, was applied to these systems and the simulated dynamical results over field frequencies up to 10(4) GHz were compared with the zero-field values. Simulated evidence of athermal effects on the diffusive characteristics of these mixtures is reported.     

Solvent effects on hydrogen bond formation

Enthalpies of solution have been used to calculate transfer enthalpies for phenol, pyridine, and DMSO between the solvent cyclohexane and the solvents CCl₄, benzene, and CHCl₃. By use of model compounds, enthalpies due to interactions with phenol, pyridine, and DMSO have been determined. These enthalpies are used to calculate the effect of solvation relative to cyclohexane on hydrogen bonded complexes in CCl₄ and benzene solvents. Correlations with enthalpies due to interactions and frequency shifts for the hydroxyl stretch in these solvents have also been made.     

Distributional effects on excimer formation in micellar surfactant solutions

An analysis is presented of excimer formation of molecules solubilized in micellar surfactant solutions. This treatment takes account of both the statistical distribution of solubilizate molecules among the micelles and the kinetic factors affecting excimer formation. By using this analysis to interpret the results of steady-state fluorescence-intensity measurements, where the excimer and monomer emissions of pyrene solubilized in a number of surfactants are measured, it is possible to gain information about micellar size and about the mobility and disposition of pyrene molecules within the micelle.     

Aromatic molecules in restricted geometries: pyrene excimer formation in an anchored bilayer

The galleries of an anionic clay, Mg-Al Layered Double Hydroxide (Mg-Al LDH) have been functionalized by intercalating the anionic surfactant do-decyl sulfate. Within the galleries, the alkyl chains of the surfactant adopt a bilayer structure with the sulfate headgroup anchored to the inorganic sheet. Pyrene molecules have been solubilized in the anchored bilayer by partitioning from polar solvents. The presence of pyrene molecules induces conformational disorder in the alkyl chains of the bilayer and more importantly inhibits the rotational disordering motion of the sulfate headgroup. Pyrene fluorescence indicates formation of excimers whose intensity increases with concentration of solubilized pyrene indicating that they are mobile. Pyrene solubilized in the anchored bilayer exhibits unusual phenomena; on evacuation the excimer band disappear but reappears on releasing vacuum. It is shown that this behavior arises due to the loss of water of hydration of the headgroup on evacuation and as a consequence the pyrene moves into the less polar interior of the bilayer where it is immobile and can no longer diffuse and form excimers. The motion of pyrene into the interior of the bilayer creates free space near the surfactant chain termini, which manifests in the disappearance of the methyl-rocking mode of the ordered (-tt) end-chain conformer in the Raman spectra.     

Molecular dynamics of a model of one ion in 225 polar molecules

Calculations are reported which explore the effect of an ion on the arrangement and motion of the surrounding solvent molecules in a highly polar medium. It is found that the ion and its nearest neighbours are strongly associated and this also applies - to a lesser extent - to groups of solvent molecules.     

Solvent dynamics in a model system

We report herein a study of the solvent reorganization process in an electron transfer reaction. The calculations are based on a model consisting of 26 or 62 solvent particles. Molecular dynamics simulations are performed to calculate the electric field fluctuations during the orientational and translational motion of the solvent molecules. The changes in the electric fields at various points near the reacting sites in the system are evaluated as a function of time. From these electric fields, electric field time correlation functions are calculated. The main conclusion in this work is that it requires nearly 3 ps for the model solvent to reorient during the charge transfer. These results suggest ways of incorporating solvent dynamics based on molecular models into theoretical studies of electron transfer rates in condensed media.     

Global and target analysis of time-resolved fluorescence spectra of Di-9H-fluoren-9-yldimethylsilane: dynamics and energetics for intramolecular excimer formation

Dynamics and energetics for intramolecular excimer formation of a diarylsilane, di-9H-fluoren-9-yldimethylsilane (DFYDMS) have been investigated by means of ps time-resolved fluorescence spectroscopy and ab initio calculation. Multiple fluorescence decay curves were globally deconvolved to generate time-resolved fluorescence spectra and decay-associated spectra (DAS), from which species-associated spectra (SAS) were obtained. It is shown in the global analysis that there are at least three excited states: Two states are the locally excited (LE) states (lambda(max) approximately 320 nm) having lifetimes of 0.70 +/- 0.04 and 1.75 +/- 0.02 ns, and another is the excimer state (lambda(max) approximately 400 nm) having a lifetime of 7.34 +/- 0.02 ns. The species which decays with 0.70 ns evolves into a species with a red-shifted spectrum, which in turn decays in 7.34 ns. The experimental and ab initio results indicate that the rise time of 0.70 ns corresponds to the conversion of the initial S(1) LE state having a near sandwich geometry to the S(1) excimer state adopting a true sandwich geometry.     

Molecular association of singlet excited state methyl 2-naphthoate; effects of excimer formation on photocycloaddition

Research on Chemical Intermediates - We have established that singlet excited methyl 2-naphthoate (*2MN) forms an emissive excimer reversibly in CH3CN at concentrations above 10-2 M. The rate...     

Violet-to-blue tunable emission of aryl-substituted dispirofluorene-indenofluorene isomers by conformationally-controllable intramolecular excimer formation

Two series of DiSpiroFluorene-IndenoFluorene (DSF-IF) positional isomers, namely dispiro[2,7-diarylfluorene-9',6,9',12-indeno[1,2-b]fluorenes], (1,2-b)-DSF-IFs 1 and dispiro[2,7-diarylfluorene-9',6,9',12-indeno[2,1-a]fluorenes], (2,1-a)-DSF-IFs 2 have been synthesized. These violet-to-blue fluorescent emitters possess a 3π-2spiro architecture, which combines via two spiro links two different indenofluorene cores, that is, (1,2-b)-IF or (2,1-a)-IF and 2,7-substituted-diaryl-fluorene units. Due to their different geometric profiles, the two families of positional isomers present drastically different properties. The marked difference observed between the properties of (1,2-b)-DSF-IF (1) and (2,1-a)-DSF-IF (2) is discussed in terms of intramolecular π-π interactions occurring in (2,1-a)-DSF-IF (2) leading to conformationally-controllable intramolecular excimer formation. Indeed, the original geometry of the (2,1-a)-DSF-IF (2) family, with face-to-face "aryl-fluorene-aryl" moieties, leads to remarkable excimer emission through intramolecular π-π interactions in the excited state. Furthermore, the emission wavelengths can be gradually modulated by the control of the steric hindrance between the adjacent substituted phenyl rings. Thus, through a comparative and detailed study of the (1)H NMR, electrochemical and photophysical properties of DSF-IFs 1 and 2, we have evidenced the intramolecular π-π interactions occurring between the two "aryl-fluorene-aryl" moieties in the ground state and in the excited state. These properties have been finally correlated to the spectacular conformational change modeled by density functional theory (DFT) calculations. Indeed, the two "aryl-fluorene-aryl" moieties switch from a staggered conformation in the ground state to an eclipsed conformation in the first excited state.     

Solvent effects on jet evolution during electrospinning of semi-dilute polystyrene solutions

Linear polystyrene with a weight average molecular weight of 393,400 g/mol was used with various solvents including tetrahydrofuran (THF), chloroform, carbon disulfide (CS₂), 1-methyl-2-pyrrolidinone (NMP), and N,N-dimethylformamide (DMF) to produce solutions, corresponding to a Berry number of about 9. The jet breakdown behavior of each of these solutions was studied with a high speed camera (2000 frames/s). The structure of the electrospun polymer was examined with a scanning electron microscope. The results indicate that jet breakdown with THF and chloroform entailed significant extensional flow, followed by the onset of instabilities, leading to the formation of numerous secondary jets under steady-state conditions. By comparison, the solution jets with DMF and NMP exhibit extensive whipping and splaying to produce a cloud of jets. In this case, few secondary jets were observed under steady-state conditions. A highly refined structure was observed in the electrospun polymer for NMP and DMF, in accordance with the extensive instabilities observed during jet breakdown. Limited jet instability observed with CS₂ solution suggests the significant effect of solvent evaporation. Typical primary jet velocities were measured to be on the order of 2-5 m/s.     

Molecular dynamics simulations of the effects of carbon dioxide on the interfacial bonding of polystyrene thin films

Fabrication of nanoscale polymer‐based devices, especially in biomedical applications, is a challenging process due to requirements of precise dimensions. Methods that involve elevated temperature or chemical adhesives are not practicable due to the fragility of the device components and associated deformation. To effectively fabricate devices for lab‐on‐a‐chip or drug delivery applications, a process is required that permits bonding at low temperatures. The use of carbon dioxide (CO₂) to assist the bonding process shows promise in reaching this goal. It is now well established that CO₂ can be used to depress the glass transition temperature (T_g) of a polymer, allowing bonding to occur at lower temperatures. Furthermore, it has been shown that CO₂ can preferentially soften a polymer surface, which should allow for effective bonding at temperatures even below the bulk T_g. However, the impact of this effect on bonding has not been quantified, and the optimal bonding temperature and CO₂ pressure conditions are unknown. In this study, a molecular dynamics model is used to examine the atomic scale behavior of polystyrene in an effort to develop understanding of the physical mechanisms of bonding and to quantify how the process is impacted by CO₂. The final result is the identification of a range of CO₂ pressure conditions which produce the strongest bonding between PS thin films at room temperature. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Molecular structure effects in intramolecular electronic energy transfer

A study of intramolecular energy transfer (intra-ET) in a series of bichromophoric molecules composed of cyclic α-diketones and para substituted benzene ring is reported. Preliminary results show that the transfer efficiency is strongly structure dependent suggesting that Dexter-type exchange interaction is responsible for intra-ET between close chromophores in a bichromophoric molecule.     

The kinetic parameters for intramolecular excimer formation from the meso and racemic diastereoisomers of 2,4-DI(2-pyrenyl) pentane: evidence for rapid conformer interconversion

Absorption and fluorescence spectra of the meso and racemic diastereoisomers of 2,4-di(2-pyrenyl)pentane (2DPP) which form intramolecular excimers in methylcyclohexane at 25° C are presented. From time-correlated single-photon counting measurements of the double-exponential monomer and excimer fluorescence decays, the rate constants for excimer formation and dissociation and the excimer lifetime have been determined. These data are compared with similar data obtained for the related compounds 1,3-di(2-pyrenyl) propane and l,3-di(2-pyrenyl) butane. It is concluded that the singlet excited state monomer conformers of 2DPP interconvert on a subnanosecond time scale.     

三发色基团体系分子内激基复合物形成的环境效应

芳香烃的荧光可以被电子给体如叔胺化合物淬灭,这一现象早为人们所熟知.随着芳烃分子荧光的淬灭出现一个向红移无结构的新荧光峰,它被认为是激发态芳香烃分子A与电子给体D相互作用形成的激基复合物(exciplex)DA所发射的荧光.将两个、三个或更多个发色基团用非共轭链连接起来,受光激发后则可以形成分子内激基复合物.通过这些体系我们可以研究电子给体和电子受体相互作用的机制,研究在复杂的氧化、还原体系(如光合作用)     

Intramolecular excimer formation with non-planar molecules: 1,3-dibiphenylpropane. Kinetic and thermodynamic results

Intramolecular excimer formation with dibiphenylpropane in several alkane solvents is reported. By a combination of fluorescence decay time and quantum yield measurements the rate constants describing the excimer formation and deactivation and the values for ΔH and ΔS of the excimer formation equilibrium could be determined. These data are compared with results obtained with di(2-naphthyl) propane and di(1-pyrenyl) propane. It is argued that the difficulty to observe excimer emission with biphenyl under photoexcitation results from a very large value of ?ΔS. A comparison between the excimer fluorescence of dibiphenylpropane and [2.2] biphenylophane points to an unusually weak transannular interaction in the phane.