Interchain Energy Research Articles

Role of van der Waals, Electrostatic, and Hydrogen-Bond Interactions for the Relative Stability of Cellulose Iβ and II Crystals.

Naturally occurring cellulose Iβ with its characteristic parallel orientation of cellulose chains is less stable than cellulose II, in which neighboring pairs of chains are oriented antiparallel to each other. While the distinct hydrogen-bond patterns of these two cellulose crystal forms are well established, the energetic role of the hydrogen bonds for crystal stability, in comparison to the van der Waals (vdW) and overall electrostatic interactions in the crystals, is a matter of current debate. In this article, we investigate the relative stability of celluloses Iβ and II in energy minimizations with classical force fields. We find that the larger stability of cellulose II results from clearly stronger electrostatic interchain energies that are only partially compensated for by stronger vdW interchain energies in cellulose Iβ. In addition, we show that a multipole description of hydrogen bonds that includes the COH groups of donor and acceptor oxygen atoms leads to consistent interchain hydrogen-bond energies that account for roughly 70% and 75% of the interchain electrostatics in celluloses Iβ and II, respectively.

Highly fluorescent functionalized nano-poly(para-phenylene ethynylene) by polymer modification

Highly photostable conjugated polymer based nanoprobe, poly(para-phenylene ethynylene), had been developed. Tumor targeting ligands folate modification made those nanoprobes to be easily and selectively taken up by cells. By grafting extended polyester side chain acting as the molecular insultors, the interchain energy transfer by isolation of conjugated polymer backbones was able to minimize, which led to reduction of π-π stacking, resulting in the quantum yield increased accordingly from 0.14% up to 36%. This method of polymer modification led to highly fluorescent functionalized nano-poly(para-phenylene ethynylene).

Coupled Schrödinger equations as a model of interchain torsional excitation transport in the DNA model

In this report we consider two weakly coupled Schrödinger equations as a reduced model of interchain energy transport in the DNA double-helix torsional model. We employ a reduction of the Yakushevich-type model that considers the torsional dynamics of the DNA as effective chains of pendula. In previous works, only small amplitude excitations and stationary dynamics were investigated, whereas we focus on the nonstationary dynamics of the double helix. We consider the system to be a model of two weakly interacting DNA strands. Assuming that initially only one of the chains is excited in the form of a breather, we demonstrate the existence of an invariant that allows us to reduce the order of the problem and examine the system of the phase plane. The analysis demonstrates the utility of an analytical tool for predicting the periodic interchain excitation transitions of its localisation on one of the chains. The technique also takes into account the spreading of the excitations over time.

Influence of lactide vs glycolide composition of poly (lactic-co-glycolic acid) polymers on encapsulation of hydrophobic molecules: molecular dynamics and formulation studies

The work demonstrates the preparation of PLGA (PLGA 50:50, PLGA 75:25) nanoparticles, to encapsulate a hydrophobic molecule (coumarin-6), using the microreactor-based continuous process. The formulations were characterized using dynamic light scattering and transmission electron microscopy to determine their size, homogeneity, zeta potential, and surface morphology. The resulting nanoparticles were safe to the CHO cells (≈80% cell survival), at the concentration of ≤600 µg/mL and were successfully taken up by the cells, as demonstrated using confocal microscopy. Moreover, imaging flow cytometry confirmed that the nanoparticles were internalized in 73.96% of the cells. Furthermore, molecular dynamics simulation and docking studies were carried out to explore the effect of polymer chain length of PLGA and lactide vs glycolide (LA:GA) ratio on their compatibility with the coumarin-6 molecules and to study the coiling and flexibility of PLGA in the presence of coumarin-6 molecules. Flory–Huggins interaction parameter (χ) was calculated for polymer chains of varying lengths and LA:GA ratio, with respect to coumarin-6. χ parameter increased with increase in polymer chain length, which indicated superior interaction of coumarin-6 with the smaller chains. Amongst all the polymeric systems, PLGA55 exhibited the strongest interaction with coumarin-6, for all the chain lengths, possibly because of their homogeneous spatial arrangements and superior binding energy. PLGA27 showed better compatibility compared to PLGA72 and PGA, whereas PLA-based polymers exhibited the least compatibility. Analysis of the radius of gyration of the polymer chains in the polymer–coumarin-6 complexes, at the end of molecular dynamics run, exhibited that the polymer chains displayed varying coiling behavior and flexibility, depending upon the relative concentrations of the polymer and coumarin-6. Factors like intra-chain interactions, spatial arrangement, inter-chain binding energies, and polymer–coumarin-6 compatibility also influenced the coiling and flexibility of polymer chains.

Dynamics of Photoinduced Energy Transfer in Fully and Partially Conjugated Polymers Bearing π-Extended Donor and Acceptor Monomers.

The photophysical properties of donor (D)-acceptor (A) polymers were studied by designing two types of polymers, (D-σ-A)n and (D-π-A)n, with non-conjugated alkyl (sp3) and π-conjugated (sp2) linkers using π-extended donor and acceptor monomers that exhibit planar A-D-A structures. The non-conjugated alkyl linker provides structural flexibility to the (D-σ-A)n polymers, while the π-conjugated linker retains the rigid structure of the (D-π-A)n polymers. Photoinduced energy transfer occurs from the large donor to acceptor units in both polymers. However, the photoinduced energy transfer dynamics are found to be dependent on the conformation of the polymers, where the difference is dictated by the types of linkers between the donor and acceptor units. In solution, intramolecular energy transfer is relatively favorable for the (D-σ-A)n polymers with flexible linkers that allow the donor and acceptor units to be proximally located in the polymers. On the other hand, intermolecular (or interchain) energy transfer is dominant in the two polymer films because the π-extended donor and acceptor units in polymers are closely packed. The structural flexibility of the linkers between the donor and acceptor repeating units in the polymers affects the efficiency of energy transfer between the donor and acceptor units and the overall photophysical properties of the polymers.

Light induced fluorescence enhancement in MEH-PPV thin films

Laser treatment of MEH-PPV films increased photoluminescence (PL) by up to 400% in nitrogen. Maximum enhancement was observed with irradiances between 20 and 70 μW/μm2, and a total fluence of ~700 μJ/μm2. (Below ~1 μW/μm2 only photobleaching was observed). PL continued to increase for 16 h after treatment ended suggesting that the morphology continued to evolve in the absence of light. After 1 day, emission remained unchanged suggesting the new morphology is stable at room temperature. We believe this increased PL efficiency in primarily the result of reduced efficiency of trap-assisted recombination due to decreased interchain energy transfer.

Role of Triplet-State Shelving in Organic Photovoltaics: Single-Chain Aggregates of Poly(3-hexylthiophene) versus Mesoscopic Multichain Aggregates.

Triplet excitons have been the focus of considerable attention with regards to the functioning of polymer solar cells because these species are long-lived and quench subsequently generated singlet excitons in their vicinity. The role of triplets in poly(3-hexylthiophene) (P3HT) has been investigated extensively with contrary conclusions regarding their importance. We probe the various roles triplets can play in P3HT by analyzing the photoluminescence (PL) from isolated single-chain aggregates and multichain mesoscopic aggregates. Solvent vapor annealing allows deterministic growth of P3HT aggregates consisting of ∼20 chains, which exhibit red-shifted and broadened PL compared to single-chain aggregates. The multichain aggregates exhibit a decrease of photon antibunching contrast compared to single-chain aggregates, implying rather weak interchain excitonic coupling and energy transfer. Nevertheless, the influence of triplet-quenching oxygen on PL and a photon correlation analysis of aggregate PL reveal that triplets are quenched by intermolecular interactions in the bulk state.

The role of chain conformation in energy transfer properties of single conjugated polymer molecule

Study of the relationship between conformation and photophysics of individual -conjugated polymer chain is one of the most important problems in polymer nanoscience and nanotechnology, which will facilitate the application of conjugated polymer in a range of electronic devices such as organic field-effect transistors, light-emitting diodes, and solar cells. Single-molecule spectroscopy has emerged as a powerful tool to unravel structure and dynamic heterogeneities that are hidden in ensemble average. Identification of the emitting segments through fluorescence of single conjugated polymer molecules and their dependence on the conformation can help reveal the mechanism and the extent of energy transfer process in a single polymer chain. In this paper, the photophysical properties of individual poly[2, 7-(9, 9-dioctylfluorene)-alt-4, 7-bis(thiophen-2-yl) benzo-2, 1, 3-thiadiazole] (PFO-DBT) conjugated polymer molecules are measured based on the defocused wide-field microscopy of single molecules. The single PFO-DBT molecules are prepared on cleaned glass coverslips by spin-coating solution of poly[methyl methacrylate] (PMMA) containing 110-9 mol/L PFO-DBT molecules in chloroform and toluene, respectively. Defocused imaging of single conjugated polymer molecule is performed based on a wide-field fluorescence microscope system. The change of defocused patterns of individual polymer chain maps the angular distribution of emitted chromophore and thus the emitting dipole orientation. Fluorescence trajectory and corresponding emission dipole moments of single conjugated polymer molecules are analyzed to identify the emitting conjugated segments. It is found that single PFO-DBT conjugated polymer molecules prepared by chloroform solvent show extended conformation. The intrachain energy transfer is dominant in the single conjugated polymer molecules that take extended conformation, which leads to photophysical properties of multiple chromophores. In contrast, single PFO-DBT conjugated polymer molecules prepared by toluene solvent hold folded conformation, which exhibit emission from single chromophore due to efficient interchain energy transfer. The emitting chromophore is not constant in a single PFO-DBT conjugated polymer molecule with folded conformation. About 35% of the single conjugated molecules prepared with toluene show only one constant emitting chromophore before photobleaching. However, about 65% of single conjugated polymer molecules prepared with toluene show two or more sequencely emitting chromophores. It can be concluded that the energy transfer properties of single PFO-DBT conjugated polymer molecule is greatly dependent on the conformation, which can be reflected in its photophysical properties. The study on the influence of single conjugated polymer conformation on energy transfer efficiency can provide the reference for the preparation and performance of optoelectronic devices and molecular devices based on conjugated polymer.

Excitonic energy migration in conjugated polymers: the critical role of interchain morphology.

Excitonic energy migration was studied using single molecule spectroscopy of individual conjugated polymer (CP) chains and aggregates. To probe the effect of interchain morphology on energy migration in CP, tailored interchain morphologies were achieved using solvent vapor annealing to construct polymer aggregates, which were then studied with single aggregate spectroscopy. We report that highly ordered interchain packing in regioregular poly(3-hexylthiophene) (rr-P3HT) enables long-range interchain energy migration, while disordered packing in regiorandom poly(3-hexylthiophene) (rra-P3HT), even in aggregates of just a few chains, can dramatically impede the interchain mechanism. In contrast to rr-P3HT, interchain energy migration in poly(3-(2'-methoxy-5'-octylphenyl)thiophene) (POMeOPT), a polythiophene derivative with bulky side chains, can be completely inhibited. We use simulated structures to show that the reduction in interchain coupling is not due simply to increased packing distance between backbones of different chains, but reflects inhibition of stacking due to side-chain-induced twisting of the contours of individual chains. A competition from intrachain coupling has also been demonstrated by comparing POMeOPT aggregates with different polymer chain sizes.

Conjugated Polyelectrolyte (CPE) Poly{3-[6-(N-methylimidazolium)hexyl]-2,5-thiophene} Complexed with DNA: Relation between Colloidal Level Solution Structure and Chromic Effects

We report on the solution structure and colorimetric changes for aqueous cationic conjugated polyelectrolyte, poly[3-[6-(N-methylimidazolium)hexyl]-2,5-thiophene] bromide (P3ImiHT), mixed with double stranded (anionic) DNA–P3ImiHT(DNA)x, where x is the molar ratio of DNA base pair to polymer repeat unit. Small-angle X-ray scattering (SAXS) indicates that P3ImiHT forms loose arrays of rodlike chains involving electrostatic repulsion. SAXS and transmission electron microscopy confirm that the DNA addition reduces this repulsion and leads to networks of ∼10 nm sized aggregates (x < 0.5 or 1.3 < x ≤ 2.4); or compact submicrometer sized particles with rough surface and ∼2 nm thick rodlike chains (0.5 ≤ x ≤ 1.3). The absorption spectra red-shift from 440 nm (x < 0.5) to 510 nm (x ≥ 0.5) on increasing x, indicating an increased chain planarity and conjugation length concomitant with the rodlike segments. The photoluminescence maxima blue-shift from 680 nm (x = 0) to 661 nm (x = 0.5) and 628 nm (x = 2.4), indicating a decrease in interchain interactions. 16-fold dilution does not change the solution structure but reduces interchain energy transfer.

Primary nucleation of polyethylene: Embryogenesis from a semidilute solution

Using “realistic” molecular dynamics simulation extended up to 100 ns, we have investigated the evolution of cluster size, intrachain vs. interchain potential energies and pair correlations of polyethylene (PE) in a semidilute (ca. 28 wt%) 1,2,4-trichlorobenzene solution at 300 K. Results indicate that the embryonic development begins with the aggregation of trans-rich sequences of characteristic length lo ≈ 2 nm, forming clusters of short stems. This is immediately followed by reorganization/thickening via intracluster axial translation and reeling-in of segments from the surrounding matrix in dynamic competition with neighboring embryos. Up to this stage, the embryonic clusters are loosely packed, retaining largely the conformer populations in the solution state but with gauche conformers enriched in the loose fold loops. After reaching a critical size with l∗ ≈ 4 nm, the intracluster order starts to significantly improve via a “solidification” process with sigmoidal decreases of valence and nonbonding energies, while axial diffusion dramatically slows down and intracluster torsions become fully adjusted to trans conformation by annihilation of gauche conformers. In these “solidified” embryos, although molecular packing remains deviated from the orthorhombic structure (as reflected in significant differences in pair correlations) while reminiscent of the mesomorphic “rotator” or hexagonal phase, the decrease in potential energy is already significant (corresponding to about half of the heat of crystallization) as the intrachain valence contribution is fully realized.

Nonstationary regimes of homogeneous Hamiltonian systems in the state of sonic vacuum

In the present paper we study the mechanism that leads to the formation of regular patterns of energy localization and complete recurrent energy transport in the homogeneous systems of anharmonic oscillators and oscillatory chains subjected to a state of sonic vacuum. The basic model under investigation comprises a system of purely anharmonic oscillators as well as oscillatory chains given to a localized excitation where the initial energy is imported to one of the oscillators or oscillatory chains. The results of numerical simulations reveal the existence of a strong classical beating phenomenon, characterized by complete, recurrent, resonant energy exchanges between the oscillators and oscillatory chain and this in the state of sonic vacuum where no regular resonant frequencies can be defined. In this study we show that formation of the recurrent energy exchanges in this highly degenerate model is strictly stipulated by the system parameters. Thus, for instance, choosing the parameter of coupling below a certain threshold leads to significant energy localization on one of the oscillators or oscillatory chains. However, increasing the strength of coupling above the threshold leads to the formation of a strong beating response. The analytical study pursued in this paper predicts the origin of formation of a strong beating phenomenon and provides the necessary conditions on the system parameter for its excitation. Moreover, careful analysis of the beating phenomenon reveals the qualitatively different global bifurcation undergone by this type of highly nonstationary regime. The theoretical study is further extended to the system of coupled purely anharmonic lattices. Thus we show analytically and numerically that excitation of some particular solutions (e.g., spatially periodic standing waves and standing breathers) on one of the lattices results in the formation of similar patterns of energy (wave) localization as well as the regime of complete recurrent interchain energy transport. In particular we demonstrate that the formation of these regimes is solely affected by a particular choice of system parameters. The results of the analytical study are found to be in very good agreement with those of numerical simulations.

Excited-State Dynamics of Water-Soluble Polythiophene Derivatives: Temperature and Side-Chain Length Effects

We report synthesis and detailed spectroscopic study of three water-soluble polythiophene derivatives with distinct homologous oligo(ethylene oxide) side-chain lengths and lower critical solution temperatures (LCSTs). The linear absorption spectra exhibit reversible shifts and broadening with the variation of their aqueous solution temperature, whereas the corresponding steady-state fluorescence emission spectra were found to show negligible shifts and only minor changes in their line shape. Measurements of picosecond time-resolved fluorescence at chosen emission wavelengths reveal a strong dependence of the isotropic decays on both side-chain length and aqueous solution temperature. With lengthening of the side chain, the isotropic decays become not only remarkably slow but also increasingly complex. Except for the polymer with the shortest side chain, significant acceleration of the isotropic decays was found when the solution temperature was raised to the corresponding LCSTs and beyond, which further causes formation of large aggregates as evident by the physical appearance change from clear solutions to turbid suspensions. Direct evidence for a temperature-induced change of polymer chain conformation was obtained through measurements of time-resolved fluorescence anisotropies, which are characterized by a substantial increase of the initial values from ~0.2 to 0.4 and the appearance of a pronounced fast decay component with an estimated lifetime of 36 ps. The high initial anisotropy of ~0.4 observed for the two polymers with longer side-chains above their LCSTs suggests that the polymer chains are highly ordered in the aggregates. The observed effects of side-chain length and solution temperature are discussed by considering the conformational relaxation of the polymer backbones and occurrence of interchain energy transfer.

A novel approach to electrospinning of pristine and aligned MEH-PPV using binary solvents

Pristine and aligned electrospun fibers of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) have been fabricated by electrospinning. The key to success was using a binary solvent system, in which a poor solvent was introduced to increase interchain interactions of MEH-PPV. At the same time, the poor solvent should have a low surface tension and high conductivity. The emission spectra of pristine MEH-PPV in electrospun fibers, spin coating film and dilute solution were acquired and analyzed. The emission anisotropy of aligned electrospun fibers was investigated by polarized emission measurement. For pristine MEH-PPV fibers, the emission anisotropy is 0.47 at emission peak, indicating alignment of MEH-PPV chains along the electrospun fiber axis. For MEH-PPV/poly (e-caprolactone) (PCL) electrospun fibers, the interchain energy transfer in MEH-PPV was significantly depressed. As a result, MEH-PPV/PCL electrospun fibers exhibited a strong blue shift of emission maximum and increased emission anisotropy of 0.66 compared with pristine MEH-PPV electrospun fibers. The ease of electrospinning of pristine MEH-PPV provides new opportunities for its application in unique optoelectronic devices.

A Biomimetic Nose by Microcrystals and Oriented Films of Luminescent Porous Metal–Organic Frameworks

Guest-free and porous [In(OH)(bdc)]n (bdc =1,4-benzenedicarboxylate) microcrystals were produced by heating indium(III) nitrate hydrate (In(NO3)3·xH2O) and terephthalic acid (H2bdc) in N,N-dimethylformamide (DMF) with the addition of ethyl acetate as a crystal habit modifier at 100 °C for 10 h and then vacuum drying them in an oven at 250 °C for 48 h to remove lattice DMF. These microcrystals were monodispersed and hexagonal rod-like in shape with a length of 30 μm, an aspect ratio of 7.5, a BET surface area of 1148.74 m2/g, and the total pore volume of 0.57 cm3/g. The oriented films of those hexagonal rods were fabricated first through the self-assembly of as-synthesized [In(OH)(bdc)·2DMF]n microcrystals at the water/n-heptane interface and then followed by the removal of DMF upon vacuum drying in an oven at 250 °C for 48 h. The stereochemical sensing capability of both guest-free and porous [In(OH)(bdc)]n microcrystals and oriented films toward odorants emanated from the analytes such as cumin, cinnamon, vanillin, p-xylene, m-xylene, o-xylene, water, and ethanol, was successfully transduced to the first set of photoluminescence (PL) emission responses. The original emission of the guest-free, porous [In(OH)(bdc)]n framework might be attributed to the ligand-to-metal charge transfer (LMCT). The inclusions of guest odorant molecules in the guess-free, porous [In(OH)(bdc)]n microcrystals could have quenched the excitons through: (1) delocalization over the conjugated polymer backbone, (2) interchain energy migration in the solid state, and (3) a highly organized molecular stacking structure, attributed to the pore confinement of the analyte inside the molecular-sized cavities of [In(OH)(bdc)]n framework which facilitated strong interactions between the analyte (or the odorant) and the host framework. Therefore, specific guest–host stereochemical interactions would dictate the characteristic quenching response of a given analyte. Red shifts were observed for cumin-, cinnamon-, vanillin-, p-xylene-, m-xylene-, o-xylene-, water-, and ethanol-adsorbed samples of [In(OH)(bdc)]n microcrystals, with emission λmax at 390, 425, 343, 428, 422, 400, 390, and 389 nm respectively, upon excitation at 270 nm in solid state. A second set of PL emission responses of the same analytes produced from the analyte-adsorbed Zn4O(bdc)3 (MOF-5) microcrystals for a demonstration purpose was also determined and coplotted with the first set of PL emission values to construct a 2D map of PL emission responses for our MOF-based “biomimetic nose”. The biomimetic nose could distinguish the odors of the analytes based on a pattern recognition method (i.e., principal component analysis) because on the 2D map of PL emission responses, ethanol, m-xylene, o-xylene, vanillin, cumin, p-xylene, cinnamon, and water, were represented by a line, a point, a point, a point, a line, a point, a rectangle and a point respectively.

Plasmonic Field Effects on the Energy Transfer between Poly(p-phenyleneethynylene) Fluorescent Polymer and Au Nanocages

Colloidal gold nanocages (AuNCs) with a wall length of 60 nm and wall thickness of 6 nm were prepared by the galvanic replacement method. The AuNCs were assembled into monolayers with different percents of surface coverage, using the Langmuir–Blodgett technique. The same technique was used to coat the assembled monolayers of AuNCs with another monolayer of poly(p-phenyleneethynylene) (PPE) fluorescent polymer. The surface plasmon resonance (SPR) spectrum of the AuNCs red-shifts as the percent of AuNCs coverage increases due to growth of the interparticle surface plasmon resonance field coupling as interparticle distance decreases. While after coating the AuNCs array with PPE, the SPR of all the AuNCs monolayers arrays were red-shifted to the same position (693 nm). This unsystematic red-shift behavior is based on the presence of two plasmon fields (inside and outside of AuNCs) and is described by discrete dipole approximation (DDA) simulation. The polymer fluorescence intensity is found to decrease when the polymer is deposited over the AuNCs. This observation is due to the energy transfer between the excited PPE and the AuNCs as well interchain energy transfer. Upon turning on the plasmonic field (exciting the surface plasmon resonance band), it is found that the fluorescence intensity of the polymer increased to a maximum value at AuNCs percent coverage of 10% and then decreased as the interparticle separation distance decreased. DDA calculations showed that this is consistent with the dependence of the SPR field strength on the interparticle separation. Physically, this is a result of the interplay between the changes in the surface plasmon fields within the cavity of each nanoparticle and their external surface fields as a pair of AuNCs approach one another.

Effect of Aggregation on the Photophysical Properties of Three Fluorene–Phenylene-Based Cationic Conjugated Polyelectrolytes

The effect of aggregation on the photophysical properties of three cationic poly{9,9-bis[N,N-(trimethylammonium)hexyl] fluorene-co-l,4-phenylene} polymers with average chain lengths of ∼6, 12, and 100 repeat units (PFP-NR3(6(I),12(Br),100(Br))) has been studied by steady-state and time-resolved fluorescence techniques. Conjugated polyelectrolytes are known to aggregate in solution and for these PFP-NR3 polymers this causes a decrease in the fluorescence quantum yield. The use of acetonitrile as a cosolvent leads to the breakup of aggregates of PFP-NR3 in water; for PFP-NR3(6(I)), this results in an ∼10-fold increase in fluorescence quantum yield, a ca. 2-fold increase in the molar extinction coefficient at 380 nm, and an increase in the emission lifetime, as compared with polymer behavior in water. Fluorescence anisotropy also decreases with increasing aggregation, and this is attributed to increased fluorescence depolarization by interchain energy transfer in aggregate PFP-NR3 clusters. Förster resonance energy transfer along the polymer chain is expected to be very fast, with a calculated FRET rate constant of 7.3 × 10(12) s(-1) and a Förster distance of 2.83 nm (cf. the polymer repeat unit separation of 0.840 nm) for PFP-NR3(100(Br)). The complex polymer excited-state decay kinetics in aggregated PFP-NR3 systems have been successfully modeled in terms of intrachain energy transfer via migration and trapping at interchain aggregate trap sites, with model parameters in good agreement with data from picosecond time-resolved studies and the calculated theoretical Förster energy-transfer rates.

Photodynamics of a PV Trimer in High‐Viscosity Solvents and in PMMA Films: A New Insight into Energy Transfer versus Conformational Relaxation in Conjugated Polymers

Non-Stokes-Einstein relaxation: The rate constant of conformational relaxation of a phenylenevinylene trimer (see picture) in different solvents is proportional to eta(-alpha), with alpha values decreasing from close to unity (low viscosity) to zero at sufficiently high solvent viscosity. This behaviour is attributed to the flexible methylbutyl side chains of the trimer, which partially screen the solvent friction. The p-phenylenevinylene (PV) trimer (MBOPV3) was used to probe the effect of conformational relaxation on fluorescence decays of PV-based polymers in the high solvent viscosity regime, from n-hexadecane (3.45 cP at 293 K) to liquid paraffin (123 cP at 293 K), and in dilute poly(methyl methacrylate) (PMMA) solid films. The effect of intermolecular energy transfer and radiative transport on the fluorescence decays was also analysed by increasing the concentration of MBOPV3 in the PMMA films up to a pure MBOPV3 film. The rate constant of conformational relaxation was found to decrease with increasing solvent viscosity up to about 23 cP, but then becomes viscosity independent. The non-Stokes-Einstein behaviour of k(CR) versus eta over the whole viscosity range apparently results from the presence of the flexible methylbutyl side chains of MBOPV3, which partially screens the solvent friction. Conformational relaxation is not observed in very dilute PMMA solid films, where the fluorescence decay becomes single exponential. The decays become multi-exponential again with an increase of MBOPV3 concentration in the PMMA films, but in this case it is due to intermolecular (interchain) energy transfer from less planar to more planar conformations of MBOPV3. In the pure MBOPV3 film, interchain energy transfer (radiative and non-radiative) is the major process responsible for the observed (tetra-exponential) decays.

Single molecule spectroscopy of red- and green-emitting fluorene-based copolymers

Single molecule fluorescence spectroscopy is used to study the optical properties of two polymers: a fluorene-based statistical copolymer that contains a low fraction (10%) of a red-emitting thiophene group, and the green-emitting polymer poly(9,9-dioctylfluorene-alt-benzothiadiazole). These polymers were studied when isolated at a low concentration in a polymer matrix (either polynorbornene or polystyrene). For the red-emitting polymer, we compare the relative emission intensity from the green-emitting benzothiadiazole groups with the red-emitting thiophene. We find that red emission from the thiophenes is significantly suppressed in the single molecule regime, suggesting that interchain energy transfer dominates intrachain processes in such polyfluorene copolymers. We then use fluorescence spectroscopy and polarization anisotropy measurements to show that the conformations of both polymers are dependent on their host matrix, adopting a more collapsed, globular conformation in polystyrene and a more extended chain conformation in polynorbornene.

Ultrasound‐assisted bulk synthesis of Cds‐PVK nanocomposites via RAFT polymerization

With the development of advanced synthetic, spectroscopic, and device fabrication techniques in nanoscience, the performance of most of the optoelectronic and photonic devices, for example, nanocrystalline photovoltaic cells, photo-electrochromic devices, LEDs, tunable lasers, and optical limiters, may be significantly improved by incorporating nanotechnology during the materials synthesis and device fabrication. As one of the most investigated promising inorganic semiconductive materials, CdS nanoparticles exhibit outstanding properties including high carrier generation efficiency and mobility, broad absorption spectra, discreet energy bands, and narrow emission profiles as well. The practical application of these nanoparticles would inevitably involve incorporating the CdS unites into some form of a single layered or, multilayered solid-state entity. Embedding them as inclusions in a polymer host to form a composite material would allow traditional methods such as spin casting to be employed to produce suitable films for these solidstate applications. For the polymer/CdS nanoblends, a partial incompatibility of CdS nanoparticles and polymer matrices usually leads to difficulties in achieving homogeneous dispersions and ultimately phase separation at high loadings. Distortion of the conjugation and coplanarity of the polymer backbones and an increase of the interchain distance in these systems would hinder both intraand interchain charge transfer or energy transfer processes, leading to a lower device performance. A convenient route to restrain phase separation is linking soluble polymer or oligomer and prefunctionalized CdS nanoparticles via a covalent bond. Our current interest is designing and synthesizing CdS-PVK nanocomposites in which two components were directly connected with a covalent bond. Poly(N-vinylcarbozole) (PVK) is a wellknown good hole transporting material widely used for the fabrication of some optoelectronic devices, for example, photoconductors, PLED, and organic solar cells. The fabrication of nanocomposites by covalent inclusion of inorganic nanoparticles, for example, CdS, in an organic polymer matrix is highly topical and may find applications in the electronics, optics, and energy sectors. To date, only a few of literatures concern the synthesis of CdS nanoparticles chemically modified PVK functional materials. Incorporation of CdS nanoparticles into the PVK and its derivatives matrices could be expected to display improved or enhanced optoelectronic and optical properties. In this contribution, we developed a new viable and versatile synthetic method to prepare CdS-PVK Correspondence to: Y. Chen (E-mail: chentangyu@yahoo. com) or H. Zhan (E-mail: hbzhan@fzu.edu.cn)