Hemali Rathnayake

Luminescence of fluorenes 2,7-conjugatively extended with pyrenylvinylene and pyrenylvinylene-phenylenevinylene

2,7-Bis(1′-pyrenylvinylene)-9,9-diethylfluorene (1) and 2,7-bis(1′-pyrenylvinylene-4″-phenylenevinylene)-9,9-diethylfluorene (2) were synthesized and their static luminescence behavior assessed. They show solution photoluminescence (PL) maxima in chloroform at 475 nm and 467 nm, with quantum efficiencies of 54% and 52%, respectively. Double-layer LED devices with configuration ITO/PEDOT-PSS/(1 or 2)/Ca–Al emitted blue-green light with turn-on voltages of 2.5 V and emission maxima at 500 nm (2.48 eV); their luminance efficiencies were 0.36 and 0.30 cd A−1, respectively. Blending of 1 at 20% (w/w) in PVK improved the luminance efficiency to 1.81 cd A−1 for the same device configuration, with only a small increase in turn-on voltage to 3 V; the emission maximum was 497 nm (blue-green emission), the maximum luminance 7600 cd m−2 with CIE color coefficients of (0.12, 0.45). For a 10% (w/w) blend of 2 in PVK using the same device configuration, the luminance efficiency was 1.47 cd A−1, turn-on voltage 3 V, maximum luminance 2600 cd m−2 with CIE coefficients of (0.13, 0.45). Simple π-MO calculations show that structural extension of the nominal conjugation length in 2 does not significantly decrease the effective band gap relative to 1, consistent with the observed lack of red shift in 2.

A versatile method to prepare size- and shape-controlled copper nanocubes using an aqueous phase green synthesis

We demonstrate a versatile, simple, and environmentally friendly method for preparing copper nanocubes with controlled morphology in aqueous solution at room temperature. Copper(II)chloride is used as a precursor and the reduction is performed under an argon atmosphere with milder and non-toxic sodium borohydride in de-ionized water. The molar ratios of the precursor and the reducing agent, and sodium borohydride addition time play a key role in the preparation of copper nanocubes with an average edge length in the range of 100 ± 35 nm. With the addition of 20 w% poly(vinylpyrrolidone) prior to the addition of the reducing agent, well-dispersed PVP-capped copper nanocubes were also prepared in considerably good yield. The UV-visible absorption traces and transmission electron microscopy analysis were used to monitor the formation of copper nanocubes. The powder X-ray diffraction (XRD) and selected area electron diffraction (SAED) reveals the packing of copper nanocrystals to cubic 3D structures.

Covalent synthesis of perylenediimide-bridged silsesquioxane nanoribbons and their electronic properties

Perylenediimide-functionalized silsesquioxane nanostructures were prepared from base-catalyzed polymerization of their respective monoalkoxysilane precursor. The shapes of nanostructures varied from nanoribbons to nanochains to nanorods upon changing the base concentration. Transmission electron microscopy confirms the twisted nature of nanoribbons with lengths up to 15 μm, whereas the dimensions of nanorods were in the range of 9 μm in length and 200 nm in width. The photovoltaic performance of the nanoribbons and nanorods were evaluated and compared for bulk heterojunction solar cells and it was discovered that morphology plays an important role in the PV performance.

Fused arene-functionalized polyhedral oligomeric silsesquioxanes as thermoelectric materials

Two derivatives of fused-arene functionalized polyhedral oligomeric silsesquioxanes (POSS-ANT and POSS-PDI-POSS) were synthesized by the reaction of aminopropylisobutyl POSS with either 9-anthracenecarboxylic acid or perylene-3,4,9,10-tetracarboxylic dianhydride. Proton NMR and FT-IR spectroscopies confirmed their purity and structures. The UV-visible and emission spectra of POSS-ANT and POSS-PDI-POSS in solution follow similar spectral patterns as their building blocks. These spectral signatures suggest that there is no interruption on the optical properties from POSS cages. Electrical characterization conducted by casting thin films either by spin coating or drop casting on glass substrates showed subohmic semiconductor behavior with electrical conductivity of 110.5 × 10−3 S cm−1 for POSS-ANT and 11.76 × 10−3 S cm−1 for POSS-PDI-POSS at room temperature. The temperature dependence IV curves obtained for the test devices prepared from drop casting of PDI-POSS-PDI exhibit a noticeable improvement in electrical conductivity with the highest conductivity of 115.3 × 10−3 S cm−1 where as POSS-ANT shows no improvement in conductivity above the room temperature. From the temperature dependent surface morphology analysis and DSC traces, it is revealed that the microstructural morphology of the thin films determines almost all the electrical properties of both compounds. Thermoelectric measurement of POSS-ANT showed a maximum power factor of 2.8 μW K−2 m−1 and Seebeck coefficient of 160.85 μV K−1 at room temperature where as POSS-PDI-POSS showed an improved thermoelectric performance above room temperature with a Seebeck coefficient of 140.30 μV K−1 and a power factor of 1.7 μW K−2 m−1.

Preparation of n-type semiconducting polymer nanoarrays by covalent synthesis followed by crystallization

In situ covalent synthesis followed by solution crystallization of the n-type semiconducting polymer derived from perylenediimide-bridged silsesquioxanes (PDIB-SSQ) yielded 1D-nanoarrays of 40–100 nm widths and up to 80 μm lengths. Morphologies and dimensions of nanostructures resulting from different base concentrations were characterized by SEM, fluorescence optical microscopy, SAXS, elemental analysis, MALDI-TOF-MS, and absorption and fluorescence spectroscopies. As revealed by SAXS, the nanostructures are composed of crystalline unit cells with cell parameters of d[001] = 24.8 A and d[100] = 10.3 with multiple π–π stacking distances ranging from 4.71 to 2.57 A. The ordering in polymer nanoarrays is favoured by the π–π interactions between the cofacially arranged PDI cores, resulting closely packed polymer arrays with the d-spacing ranging from 3.67 A to 3.24 A. The spectroscopic traces of nanoarrays in solution resembled that of the monomer except the slightly red shifted features of the emission spectrum associated with π–π stacking of polymer chains in aggregated form. Thin film emission spectra followed the similar spectral pattern with a noticeable shoulder corresponds to cofacial π–π interactions. The excited state lifetimes of aggregated polymer nanoarrays in both solution and solid phase were nearly identical. The electrical characterization of thin films made from polymer nanoarrays shows typical semiconducting behaviour with the electrical conductivity of 0.48 × 10−3 S cm−1. The covalent synthesis followed by solution-based crystallization of PDIB-SSQ reported herein provides a new synthesis path to make ordered-crystalline semiconducting polymer nanoarrays and ultimately a benefit for better understanding of their role in organic electronics.

A Novel n-Type Organosilane–Metal Ion Hybrid of Rhodamine B and Copper Cation for Low-Temperature Thermoelectric Materials

An n-type organosilane-metal ion hybrid of Rhodamine B-silane and copper cation (Cu-RBS) was investigated as a low-temperature thermoelectric material. Computational analysis revealed the most likely localized binding site of Cu2+ was to the Rhodamine B core and provided predictions of molecular orbitals and electrostatic potentials upon complexation. The concentration-dependent optical absorption and emission spectra confirmed the effective metal-ligand charge transfer from Cu2+ to the xanthene core of RBS, indicating the potential for improved electrical properties for the complex relative to RBS. The electrical conductivity and Seebeck thermoelectric (TE) behavior were evaluated and compared with its precursor complex of Rhodamine B and copper cation. While a moderately high electrical conductivity of 4.38 S m-1 was obtained for the Cu-RBS complex, the relatively low Seebeck coefficient of -26.2 μV/K resulted in a low TE power factor. However, compared to other organic doped materials, these results were promising toward developing n-type thermoelectric materials with no doping agents. Both phase segregation and thin film heterogeneity remain to be optimized; thus, the balance between Cu2+ domains and RBS domain phases will likely yield higher Seebeck coefficients and improved power factors.

Single Molecule Studies of a 2,7-Bis-(Phenylethenyl)fluorenone: Implications for Green-Emission Bands in Fluorene-Based OLEDs

The color purity of blue polyfluorene fluorescence emission is often adversely contaminated by green emission bands. The origin of this green emission has been attributed to various factors including on-chain oxidation of fluorene leading to monomeric fluorenone emission and inter/intra-chain interaction between segments of fluorenone units leading to excimer formation on the polyfluorene backbone. We report here emission properties at the bulk and single molecule level of a molecular fluorene derivative and compare them to those of its oxidative fluorenone derivative. Whereas the bulk fluorescence emission of 2,7-bis(3,4,5-trimethoxyphenylethenyl)-9,9-diethyl- 9H -fluorene (OFPV) exhibits a blue to blue-green emission at about 480 nm, 2,7-bis(3,4,5-trimethoxyphenylethenyl)-9,9-diethyl- 9H -fluorenone (OFOPV) shows red luminescence with a peak centered at about 630 nm. However, the peak position for OFOPV shifts to higher energies (∼540 nm) upon dilution or dispersion in polymer matrices like PMMA or Zeonex. Single molecule measurements of OFOPV show fluorescence spectra dominated by peaks around 540 nm, with a small minority at longer wavelengths that are attributed to emission from dimers or higher aggregates. This distribution indicates that monomeric emission of OFOPV is green, consistent with green emission bands seen in polyfluorenes.