Chan Im

Aggregation induced enhanced emission of conjugated dendrimers with a large intrinsic two-photon absorption cross-section

Organic nonlinear optical materials combining high luminescence quantum yields and large two-photon absorption cross-sections are attractive for both fundamental research and practical applications, such as up-converted lasers and two-photon fluorescence microscopy. Herein, we reported a series of conjugated dendrimers (AnG0, AnG1, and AnG2) which showed weak emissions in dilute solution due to the twisted intramolecular charge transfer (TICT) and free intramolecular rotational motion. On the contrary, high luminescence quantum yields (up to 0.85) have been observed in the nanoaggregated and solid-state because of the restriction of intramolecular rotation (RIR) and the more planar conformation, which has been demonstrated by experimental and theoretical investigations. These dendrimers possess an obviously large intrinsic two-photon absorption cross-section (up to 5180 GM), which is enhanced with the increase of the generation number. Due to the high solid-state luminescence quantum yields, an intense two-photon excited fluorescence with a larger Stokes shift can be obtained from these dendrimers as nanoaggregates and thin films. Thus, these dendrimers successfully provide large two-photon action cross-sections in the nanoaggregated or solid-state and have the potential for nonlinear optical applications.

Enhanced photovoltaic properties of TiO2 film prepared by polycondensation in sol reaction

A TiO2 film of dye-sensitized solar cells was fabricated using a TiO2 sol prepared from titanium isopropoxide as a starting material. The effect of polycondensation time and temperature on physical and chemical properties of TiO2 film shows that the crystallinity, particle size, roughness, and surface area of TiO2 film can be successfully controlled by adjusting the polycondensation time and temperature in the synthesis process of the TiO2 sol. Adsorption properties (adsorption equilibrium, isosteric enthalpies of adsorption, and adsorption energy distributions) of N719 dye molecules on the prepared TiO2 film revealed that the short circuit current (Isc) and overall conversion efficiency (ηeff) are highly dependent on the crystallite size and the N719 dye adsorption properties. The TiO2 film has a pure anatase phase, high surface area including pore volume, and large anatase crystallite size and is capable of enhancing the photovoltaic performance. The energy conversion efficiency of the dye sensitized solar cells is highly dependent on the physical and chemical properties of TiO2 films.

Preparation of nanoporous TiO2 electrodes using different mesostructured silica templates and improvement of the photovoltaic properties of DSSCs

Nanoporous TiO2 (NP-TiO2) was synthesized using different mesostructured silica templates such as SBA-15, KIT-6, and MSU-H, which have highly ordered pore or well-connected inner-network structures. NP-TiO2 has a large surface area and a dye-adsorption capability 1.5 times greater than that of nanocrystalline TiO2 (NC-TiO2). We have investigated dye-sensitized solar cells (DSSCs) with bilayer TiO2 electrodes, which were prepared by applying a coating of NP-TiO2 onto an NC-TiO2 film. The photocurrent (Jsc) and fill factor (FF) for the DSSC with the bilayer TiO2 electrode were greatly improved. The solar energy conversion efficiency (ηeff) also increased over 15% compared with a DSSC with a monolayer NC-TiO2 electrode. For the DSSC with the bilayer TiO2 electrode, the charge recombination and dye-regeneration processes, which were studied by transient absorption spectroscopy, were slower and faster, respectively, than those of the DSSC with an NC-TiO2 electrode.

Photoinduced reduction of manganese(III) meso-tetrakis(1-methylpyridinium-4-yl)porphyrin at AT and GC base pairs.

The photoreduction of water-soluble cationic manganese(III) meso-tetrakis(1-methylpyridium-4-yl)porphyrin (Mn(III)(TMPyP)(4+)) bound to a synthetic polynucleotide, either poly[d(A-T)2] or poly[d(G-C)2], was examined by conventional absorption and circular dichroism (CD) spectroscopy, transient absorption, and transient Raman spectroscopy. Upon binding, Mn(III)(TMPyP)(4+) produced a positive CD signal for both polynucleotides, suggesting external binding. In the poly[d(A-T)2]-Mn(III)(TMPyP)(4+) adduct case, an interaction between the bound porphyrin was suggested. The transient absorption spectral features of Mn(III)(TMPyP)(4+) in the presence of poly[d(A-T)2] and poly[d(G-C)2] were similar to those of the photoreduced products, Mn(II)(TMPyP)(4+), whereas Mn(III)(TMPyP)(4+) in the absence of polynucleotides retained its oxidation state. This indicated that both poly[d(A-T)2] and poly[d(G-C)2] act as electron donors, resulting in photo-oxidized G and A bases. The transient Raman bands (ν2 and ν4) that were assigned to porphyrin macrocycles exhibited a large downshift of ~25 cm(-1), indicating the photoreduction of Mn(III) to Mn(II) porphyrins when bound to both polynucleotides. The transient Raman bands for pyridine were enhanced significantly, suggesting that the rotation of peripheral groups for binding with polynucleotides is the major change in the geometry expected in the photoreduction process. These photoinduced changes do not appear to be affected by the binding mode of porphyrin.

Influence of a polyelectrolyte based-fluorene interfacial layer on the performance of a polymer solar cell

A new quaternized ammonium polyfluorene polyelectrolyte poly[3,3′-(2-(3-hexyl-5-(7-(4-hexyl-5-methylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)-7-methyl-9H-fluorene-9,9-diyl)bis(N,N-dimethylpropan-1-amine)]dibromide (PFBTBr) is applied as the cathode interfacial layer of a polymer solar cell based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PC61BM). Electrostatic force microscopy (EFM) measurements of PFBTBr layers demonstrate the formation of the interfacial dipole between the active layer and the cathode by inserting a PFBTBr interfacial layer. Atomic force microscopy (AFM) measurements of PFBTBr layers with varied concentrations show that the morphology of the PFBTBr layer plays a direct, important role in the contact quality between the active layer and the PFBTBr interfacial layer, which can strongly affect the performance of devices. X-ray photoelectron spectroscopy measurements (XPS) indicate that PFBTBr may serve as a protective agent for the active layer against Al-induced degradation, since it prevents hot aluminum atoms from diffusing into the active layer. The power conversion efficiency (PCE) of the PSCs with the PFBTBr layer reaches 3.9% under the illumination of AM 1.5G, 100 mW cm−2, which is 1.6 times higher in comparison with that (2.4%) of the device without the PFBTBr layer. The significant increase in efficiency and easy utilization indicate that this interfacial material has promising and practical application prospects.

Synthesis and characterization of N-acyl-tetra-O-acyl glucosamine derivatives

Novel 1,3,4,6-tetra-O-acyl-N-acyl-D-glucosamine derivatives were synthesized from glucosamine hydrochloride (GlcN·HCl) by the acylation with pyridine as a catalyst. A derivative of tetra-O-acetyl glucosamine contained ketoprofen, a non-steroidal anti-inflammatory drug (NSAID) with analgesic and antipyretic effects, was first synthesized. In analysis of the NMR spectra, the ratio of α:β-anomer showed that penta-acyl-D-glucosamine derivatives and N-acetylated glucosamines containing O-acyl groups have been only the α-anomer. Meanwhile, both the intermediates and the glucoconjugate compound of ketoprofen have only the β-anomer.

Synthesis of corn rootworm pheromones from commercial diols

A mixture of stereoisomers of the corn rootworm pheromones was synthesised via the Grignard coupling of protected bromohydrins with alkylcuprate as a key step. The synthesis of 8-methyldec-2-yl propanoate (I), the northern corn rootworm Diabrotica longicornis Say pheromone, was achieved from pentane-1,5-diol in four steps with an overall yield of 35.1 % and 10-methyltridecan-2-one (II), the southern corn rootworm Diabrotica undecimpunctata howardi Barber pheromone, was synthesised from octane-1,8-diol as commercially available starting material in five steps with an overall yield of 28.7 %.

Synthesis of Novel Ruthenium Dyes with Thiophene or Thienothiophene Substituted Terpyridyl Ligands and Their Characterization

We synthesized new materials for dye-sensitized solar cell (DSSC) which is incorporating highly conjugated π-electron-donating groups, such as thiophene or thieno[3,2-b]thiophene. The synthesized dyes showed similar molar extinction coefficient compared to that of N749. All compounds are analyzed by 1H- and 13C-NMR, and FT-IR spectroscopic measurements. The physical properties of dyes are characterized by UV-vis absorption spectroscopy, cyclic voltamogram and DFT calculation. The power conversion efficiencies (ηeff) of two synthesized dyes were 4.0 and 3.4%, respectively, under AM1.5G light source. The reference cell (N749) was evaluated the ηeff of 6.0% at the same conditions.

Improvement in power conversion efficiency by blending of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) into poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester active layer

The improvement in power conversion efficiency (PCE) by the addition of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) into regioregular poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM)-based bulk heterojunction (BHJ) organic photovoltaic (OPV) devices was investigated. The addition of PEDOT:PSS into the P3HT:PCBM active layer was found to promote a significant enhancement of the fill factor, which ultimately increased the PCEs of the corresponding OPV devices, although the exciton generation rates of the BHJ layers were virtually the same, as observed by UV-Vis absorption spectra. Therefore, we conclude that the reduction of internal series resistance (RS) was the most crucial reason for the PCE improvement by the addition of PEDOT:PSS.

Charge carrier photogeneration and hole transport properties of blends of a π-conjugated polymer and an organic-inorganic hybrid material

This study examined the charge carrier photogeneration and hole transport properties of blends of poly (9-vinylcarbazole) (PVK), a π-conjugated polymer, with different weight proportions (0~29.4 wt%) of (PEA)-VOPO4·H2O (PEA: phenethylammonium cation), a novel organic-inorganic hybrid material, using IR, UV-Vis, and energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), steady state photocurrent (SSPC) measurement, and atomic force microscopy (AFM). The SSPC measurements showed that the photocurrent of PVK was reduced by approximately three orders of magnitude by the incorporation of a small amount (~12.5 wt%) of (PEA) VOPO4·H2O, suggesting that hole transport occurred through the PVK carbazole groups, whereas a reverse trend was observed at high proportions (>12.5 wt%) of (PEA)VOPO4·H2O, suggesting that transport occurred via (PEA)VOPO4·H2O molecules. The transition to a trap-controlled hopping mechanism was explained by the difference in ionization potential and electron affinity of the two compounds as well as the formation of charge percolation threshold pathways.