Integral equation formulation for buoyancy-driven convection problems

An integral equation formulation for buoyancy-driven convection problems is developed and illustrated. Buoyancy-driven convection in a bounded cylindrical geometry with a free surface is studied for a range of aspect ratios and Nusselt numbers. The critical Rayleigh number, the nature of the cellular motion, and the heat transfer enhancement are computed using linear theory. Green's functions are used to convert the linear problem into linear Fredholm integral equations. Theorems are proved which establish the properties of the eigenvalues and eigenfunctions of the linear integral operator which appears in these equations.     

Synthesis and characterization of donor-acceptor chromophores for unidirectional electron transfer

[reaction: see text] A series of electron-transfer chromophores containing a donor and acceptor linked by an alkyl spacer were synthesized, and their electronic spectra were investigated. By inclusion with amylose, the supramolecularly encapsulated chromophores exhibit enhanced fluorescence quenching with discrete distance dependence and acquire the ability to sustain self-assemblies of a densely packed supramolecular array on a SiOH/Si substrate.     

Synthesis of light-harvesting dendrimers focally anchored with crown ethers or terpyridine ligands

Crown ethers and terpyridine ligands have been successfully attached to the focal point of light harvesting phenylacetylene monodendrons through Pd-catalyzed coupling reactions. The structures of these functional monodendrons were characterized by 1H and 13C NMR spectroscopy, mass spectrometry and elemental analysis. Such binding-ligand anchored dendrons exhibit broad absorption, large molar extenction coefficients and high fluoresence quantum yields. Coordination of crown ethers with alkali ions results in a significant increase in absorption strength in the UV range, but little alteration in either intensity or position of fluorescence emission. Coordination of terpyridine ligands with Ru2+, however, does efficiently quench the fluorescence from the dendrons,albeit only the smallest dendron exhibits efficient binding.     

One-dimensional energy/electron transfer through a helical channel

We have synthesized A-D-A-type linear chain chromophores based on oligo(phenylene vinylene) as an electron donor (D) and several electron/energy acceptors (A), which are linked by an alkyl spacer with various lengths and are processed for a helical encapsulation with amylose. Photoinduced electron/energy transfers (eT/ET) of the chromophores are investigated in the presence and absence of the helical encapsulation with respect to D-A distance. Fluorescence intensity of the free chromophores is unusually small, not due to the advancement of eT/ET but most likely to self-quenching by aggregation and/or conformational flexibility in solution. By contrast, the helically encapsulated chromophores exhibit highly efficient eT/ET over a long D-A distance and a well-defined distance effect depending on the acceptor strength.     

Solid-state density functional theory investigation of the terahertz spectra of the structural isomers 1,2-dicyanobenzene and 1,3-dicyanobenzene

The high-resolution waveguide terahertz (THz) time-domain spectra (20-100 cm(-1)) of the two structural isomers 1,2-dicyanobenzene (1,2-DCB) and 1,3-dicyanobenzene (1,3-DCB) have been modeled and assigned using solid-state density functional theory. The THz spectra of these similar molecules are distinctly different in the low-frequency region with the differences being driven by modifications of the crystal packing arrangement between the isomers. Simulations utilizing the hybrid density functionals B3LYP and PBE0 were performed to determine the origins of the observed vibrational features. External lattice vibrations (hindered translations and rotations) are found to dominate these spectra, reinforcing the need for proper solid-state models in the analysis of the THz spectra of organic molecular solids. These calculations were able to account for all of the observed spectral features exhibited by both isomers, even in the case of 1,2-DCB, where the spectrum was found to be the result of two coexisting crystalline polymorphs.     

Synthesis and optical properties of unsymmetrical conjugated dendrimers focally anchored with perylenes in different geometries

Two sets of light-harvesting monodendrons with unsymmetrical conjugated phenylacetylene branches and perylene cores, one with pi-conjugation from the branches to the core and one without, were synthesized and their photophysical properties were studied by steady-state and time-resolved spectroscopic methods. These monodendrons show comparably high fluorescence quantum yields and efficient energy-transfer properties.