Host-guest interactions in the supramolecular incorporation of fullerenes into tailored holes on porphyrin-modified gold nanoparticles in molecular photovoltaics

Novel gold nanoparticles modified with a mixed self-assembled monolayer of porphyrin alkanethiol and short-chain alkanethiol were prepared (first step) to examine the size and shape effects of surface holes (host) on porphyrin-modified gold nanoparticles. The porphyrin-modified gold nanoparticles with a size of about 10 nm incorporated C60 molecules (guest) into the large, bucket-shaped holes, leading to the formation of a supramolecular complex of porphyrin-C60 composites (second step). Large composite clusters with a size of 200-400 nm were grown from the supramolecular complex of porphyrin-C60 composites in mixed solvents (third step) and deposited electrophoretically onto nanostructured SnO2 electrodes (fourth step). Differences in the porphyrin:C60 ratio were found to affect the structures and photoelectrochemical properties of the composite clusters in mixed solvents as well as on the SnO2 electrodes. The photoelectrochemical performance of a photoelectrochemical device consisting of SnO2 electrodes modified with the porphyrin-C60 composites was enhanced relative to a reference system with small, wedged-shaped surface holes on the gold nanoparticle. Time-resolved transient absorption spectroscopy with fluorescence lifetime measurements suggest the occurrence of ultrafast electron transfer from the porphyrin excited singlet states to C60 or the formation of a partial charge-transfer state in the composite clusters of supramolecular complexes formed between porphyrin and C60 leading to efficient photocurrent generation in the system. Elucidation of the relationship between host-guest interactions and photoelectrochemical function in the present system will provide valuable information on the design of molecular devices and machines including molecular photovoltaics.     

On Nonexistence of type II blowup for a supercritical nonlinear heat equation

In this paper we study blowup of radially symmetric solutions of the nonlinear heat equation u_t = Δu + |u|^p?1u either on ?^N or on a finite ball under the Dirichlet boundary conditions. We assume that the exponent p is supercritical in the Sobolev sense, that is, We prove that if p_s < p < p^*, then blowup is always of type I, where p^* is a certain (explicitly given) positive number. More precisely, the rate of blowup in the L^∞ norm is always the same as that for the corresponding ODE dv/dt = |v|^p?1v. Because it is known that “type II” blowup (or, equivalently, “fast blowup”) can occur if p > p^*, the above range of exponent p is optimal. We will also derive various fundamental estimates for blowup that hold for any p > p_s and regardless of type of blowup. Among other things we classify local profiles of type I and type II blowups in the rescaled coordinates. We then establish useful estimates for the so‐called incomplete blowup, which reveal that incomplete blowup solutions belong to nice function spaces even after the blowup time. © 2004 Wiley Periodicals, Inc.     

Direct Oxidation of Propylene with Molecular Oxygen Over Ti-Modified Sulfated Zirconia Catalyst, 2. In Situ Diffuse Reflectance FT-IR Study

During the reaction of propylene with O₂, in situ Diffuse Reflectance FT-IR measurements were performed over Ti-modified SZ and SZ catalysts. Without O₂, the main bands characteristic of (branched) hydrocarbons, formed by oligomerization leading to, finally, carbonaceous residue, appeared within the range of 3900-2750 cm^-1, which was affected by the bands of surface OH groups. Investigation of these IR bands showed the role of molecular oxygen not only to limit the formation of carbonaceous species on the catalyst surface, but also to form oxygenates and these findings were in good agreement with the results of catalytic reaction.     

Spontaneous molecular orientation of polyimides induced by thermal imidization (4): Casting- and melt-induced in-plane orientation

Driving forces of in-plane chain orientation of polyimides (PIs) and their precursors were discussed and the mechanisms were proposed. A polyimide precursor, poly(amic acid) (PAA) derived from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA) with p-phenylenediamine (PDA) showed a certain degree of in-plane orientation in its solution-casting process and clear molecular weight dependence. The results allowed us to propose the casting-induced in-plane orientation mechanism of the rigid PAA chains. The imidization-induced in-plane orientation mechanism was also discussed by investigating how residual solvent content influences the degree of in-plane orientation of resultant PI films. The results suggested that the magnitudes of the PI chain in-plane orientation are dominated by a combined effect of the initial PAA orientation, apparent stretching based on a great thickness decrease, and the molecular mobility during thermal imidization. In a system derived from s-BPDA with 2,2′-bis(trifluoromethyl)benzidine (TFMB), the effect of molecular mobility during thermal imidization was predominant when cured under usual thermal conditions owing to the presence of the trifluoromethyl groups contributing to weakened intermolecular interaction. In s-BPDA/TFMB and s-BPDA/m-TOL systems (m-TOL = m-tolidine), a melt-induced in-plane orientation phenomenon was observed at temperatures corresponding to their T_g’s when the extents of in-plane chain orientation (f values) were monitored as a function of temperature in the stepwise heating process. This behavior is very curious because there are no appreciable dimensional, morphological, and structural changes as some driving forces just above the T_g of s-BPDA/TFMB.     

J-aggregation of a sulfonated amphiphilic porphyrin at the air-water interface as a function of pH

π-A isotherms, ellipsometric measurements, Brewster angle microscopy (BAM) and reflection spectroscopy have been utilized to characterize the films of an amphiphilic porphyrin ((OD)(3)TPPS(3)) at the air-water interface as a function of pH. This porphyrin forms stable mono-molecular layers at such interfaces, and exhibits different J-aggregation as a function of pH. The J-aggregation of (OD)(3)TPPS(3) on neutral pH subphases is notable considering that the nitrogen atoms at the central macrocycle have a pK(a)≈4.9. The type of aggregates at neutral pH is like those detected at pH<4, because the central porphyrin ring is already protonated. However at basic pH the aggregation happens without protonation of the central ring but can be instead controlled by application of the surface pressure. At the air-water interface, (OD)(3)TPPS(3) shows two bands, a red component and a blue component, which have characteristics of non-degenerate linear oscillators being perpendicularly polarized between each other. The spectral behavior observed on subphases at different pHs is qualitatively interpreted by means of exciton coupling theory, assuming that the degenerate transitions attributed to the Soret band are split. Additionally, highly oriented molecular films of these J-aggregates were deposited onto transparent quartz slides.     

Synthesis of a phosphorus-containing hybrid porphyrin

A phosphorus-containing hybrid porphyrin was successfully prepared via the BF3-promoted dehydrative condensation between sigma4-phosphatripyrrane and 2,5-bis[hydroxy(phenyl)methyl]thiophene. The NMR and UV-vis absorption spectra, electrochemical measurements, and DFT calculations revealed that the sigma3-P,N2,S-hybrid porphyrin exhibits high aromaticity as an 18pi-electron system in terms of both geometric and magnetic criteria. [structure: see text]     

Efficient photocurrent generation by SnO2 electrode modified electrophoretically with composite clusters of porphyrin-modified silica microparticle and fullerene

A silica microparticle has been successfully employed as a nanoscaffold to self-organize porphyrin and C60 molecules on a nanostructured SnO2 electrode which exhibits efficient photocurrent generation.