Well-packed chains and aggregates in the emission mechanism of conjugated polymers

We synthesized dialkoxy-substituted poly[phenylene vinylene]s (dROPPV-1/1, 0.2/1, and 0/1) consisting of two repeating units with different side-chain lengths (methoxy and 3,7-dimethyloctyloxy). These polymers can serve as a model system to clarify roles of aggregates (the sites with ground-state interchain interactions) and the independent chain segments in the well-packed chains (the chain segments that are compactly packed without interaction) in the emission mechanism of conjugated polymers. Due to the packing of polymer chains, films of all of these polymers are accessible to interchain excitations, after which excitons can re-form to result in delayed luminescence. Besides, some chains form aggregates so that the delayed luminescence is no more the ordinary single-chain emission but red-shifted and less structured. Not only the re-formation of these indirect excitons but also the aggregation of chains are facilitated in the polymers with short methoxy side groups, revealing that both packing and aggregation of chain segments require a short spacing between polymer chains. However, the incorporation of other side chains such as the 3,7-dimethyloctyloxy group to dROPPVs is necessary for the formation of aggregates because these long branched side chains can reduce the intrachain order imposed by the short methoxy groups, which accounts for the absence of aggregate emission in the well-studied poly[2,5-dimethoxy-1,4-phenylene vinylene]. This study reveals that the well-packed chains do not necessarily form aggregates. We also show that the photophysical properties and the film morphology of conjugated polymers can be deliberately controlled by fine-tuning of the copolymer compositions, without altering the optical properties of single polymer chains (e.g., as in dilute solutions).     

Multiresolution quantum chemistry in multiwavelet bases: Hartree-Fock exchange

In a previous study we reported an efficient, accurate multiresolution solver for the Kohn-Sham self-consisitent field (KS-SCF) method for general polyatomic molecules. This study presents an efficient numerical algorithm to evalute Hartree-Fock (HF) exchange in the multiresolution SCF method to solve the HF equations. The algorithm employs fast integral convolution with the Poission kernel in the nonstandard form, screening the sparse multiwavelet representation to compute results of the integral operator only where required by the nonlocal exchange operator. Localized molecular obitals are used to attain near linear scaling. Results for atoms and molecules demonstrate reliable precision and speed. Calculations for small water clusters demonstrate a total cost to compute the HF exchange potential for all n(occ) occpuied MOs scaling as O(n(occ) (1.5)).     

Efficient light harvesting and energy transfer in organic-inorganic hybrid multichromophoric materials

Thin films of silica hybrid materials consisting of two to three covalently bound organic chromophores at different ratios were conveniently synthesized and fabricated. The photophysical properties of these materials have been studied. The fluorescence spectra reveal complete fluorescence resonance energy transfer (FRET) from donor to acceptor, and the light-harvesting ability of these hybrid materials increases with increasing the molar fraction of donor chromophore. In a three-chromophore system, the energy is transferred from 300 to 530 nm successfully. Time-resolved fluorescence experiments are employed to elucidate the average rates and efficiencies (84-97%) of energy transfer in these organic/inorganic hybrid systems. The hybrid materials have been shown to provide antenna effect to facilitate energy transfer and light harvesting.     

Coherently Aligned Porphyrin‐Appended Polynorbornenes

You can append on me! Porphyrin‐appended polynorbornenes derived from 5,6‐endo‐fused N‐arylpyrrolidenonorbornenes have been shown to have coherently aligned pendant groups that exhibit exciton coupling and fluorescence quenching in the absorption and emission profiles (see figure).

     

Multiresolution quantum chemistry in multiwavelet bases: Analytic derivatives for Hartree-Fock and density functional theory

An efficient and accurate analytic gradient method is presented for Hartree-Fock and density functional calculations using multiresolution analysis in multiwavelet bases. The derivative is efficiently computed as an inner product between compressed forms of the density and the differentiated nuclear potential through the Hellmann-Feynman theorem. A smoothed nuclear potential is directly differentiated, and the smoothing parameter required for a given accuracy is empirically determined from calculations on six homonuclear diatomic molecules. The derivatives of N2 molecule are shown using multiresolution calculation for various accuracies with comparison to correlation consistent Gaussian-type basis sets. The optimized geometries of several molecules are presented using Hartree-Fock and density functional theory. A highly precise Hartree-Fock optimization for the H2O molecule produced six digits for the geometric parameters.     

Voltammetric Study of Ferrocene/Ferricinium Ion Redox Reactions in Acetonitrile

Cyclic voltammetric and differential pulse voltammetric characteristics of the oxidation reaction of ferrocene in acetonitrile solvent have been evaluated using a glassy carbon indicator electrode. The redox couple ferrocene/ferricinium ion approaches thermodynamic reversibility in acetonitrile medium in only a low concentration range.     

Soluble electroluminescent poly(phenylene vinylene)s with balanced electron- and hole injections.

We report a new route for the design of efficient soluble electroluminescent PPV-based copolymers bearing electron-deficient oxadiazole (OXD) moieties on side chains. The introduction of OXD through a long alkylene spacer with PPV backbone provides a molecular dispersion of OXD in the film; both the side chain OXD and the main chain PPV do retain their own electron-transport and emissive properties, respectively. The use of phenylene vinylene derivatives with asymmetric and branched substituents and a long spacer provides solubility for ease of device fabrication as well as amorphous structure to allow a well-mixing of OXD groups with the main chains. By properly adjusting the OXD content through copolymerization, we can tailor the chemical structure of electroluminescent material to give a balance of hole- and electron injections for various metal cathodes, such that the quantum efficiency is significantly improved and the turn-on voltage is reduced for the devices with aluminum and calcium. For the device with calcium fabricated in open air, a maximum brightness of 15000 cd/m(2) at 15 V/100 nm and a maximum luminance efficiency of 2.27 cd/A can be obtained, respectively, about 30 times brighter and 9.4 times more efficient than those with the corresponding homopolymer, poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV). The use of physical blends to simulate the copolymers provides no significant improvement, since phase-separation structures appear, causing an inefficient utilization of OXD and sometimes voltage-dependent emission spectra. The present route permits a fabrication of single layer PLED with high brightness, high efficiency, and low turn-on voltage.     

Application of pressurized liquid extraction followed by gas chromatography-mass spectrometry to determine 4-nonylphenols in sediments

A time- and solvent-saving method, pressurized liquid extraction (PLE), to extract 4-nonylphenol (4-NP) in sediment was developed. The effects of various operational parameters (i.e., temperature, pressure, etc.) for the quantitative extraction of 4-NP by PLE were investigated. The analytes were then identified and quantitated by a large-volume injection GC-MS technique. The 4-NP can be completely extracted by methanol at 100 degrees C and 100 atm combined with 15 min static and then 10 min dynamic extraction steps (1 atm = 101,325 Pa). Recovery of 4-NP in spiked blank kaolin samples was 98% with 5% RSD. The degrees of recovery of 4-NP in the spiked sediment samples from a reservoir and a polluted river were 111% with 4% RSD and 106% with 5% RSD, respectively. The perfect applicability of PLE for 4-NP was determined after testing it with spiked and aged samples. The extraction efficiency of the PLE was compared with conventional Soxhlet and bath ultrasonication extraction methods using the spiked sediment samples.