Photoexcited Singlet and Triplet States of a UV Absorber Ethylhexyl Methoxycrylene

The excited states of UV absorber, ethylhexyl methoxycrylene (EHMCR) have been studied through measurements of UV absorption, fluorescence, phosphorescence and electron paramagnetic resonance (EPR) spectra in ethanol. The energy levels of the lowest excited singlet (S₁) and triplet (T₁) states of EHMCR were determined. The energy levels of the S₁ and T₁ states of EHMCR are much lower than those of photolabile 4‐tert‐butyl‐4′‐methoxydibenzoylmethane. The energy levels of the S₁ and T₁ states of EHMCR are lower than those of octyl methoxycinnamate. The weak phosphorescence and EPR B_min signals were observed and the lifetime was estimated to be 93 ms. These facts suggest that the significant proportion of the S₁ molecules undergoes intersystem crossing to the T₁ state, and the deactivation process from the T₁ state is predominantly radiationless. The photostability of EHMCR arises from the ³ππ* character in the T₁ state. The zero‐field splitting (ZFS) parameter in the T₁ state is D** = 0.113 cm^?1.     

Electron transfer from axial ligand to S1- and S2-excited phosphorus tetraphenylporphyrin

Photoinduced processes of a series of phosphorus tetraphenylporphyrin (PTPP) derivatives ([PTPP-(NHC6H4X)2]+Cl-, X = OCH3, CH3, H, Cl, CF3, and CN) have been investigated by using femtosecond laser flash photolysis mainly. PTPP with OH as an axial ligand showed S2 fluorescence upon excitation of the Soret band. The S2 fluorescence lifetime was estimated to be 1.5 ps. On the other hand, both S2 and S1 fluorescence bands of PTPP-(NHC6H4X)2 were difficult to observe, indicating the existence of an additional deactivation process such as charge separation (CS). From MO calculation and cyclic voltammetry, PTPP and the axial ligand are expected to act as an acceptor and a donor, respectively, upon excitation of PTPP. CS via the S2 state was confirmed during the femtosecond laser flash photolysis by observing the transient absorption of radical anion of PTPP. Furthermore, CS via the S1 state of PTPP was also observed. The CS rate via the S1 state was faster than that from the S2 state. The free energy dependence of the electron-transfer rates was discussed on the basis of Marcus theory.     

Enhanced electro-optic response of a poled polymer in a reflective microcavity

We investigated the electro-optic (EO) response of a reflective planar microcavity with a poled polymer spacer using reflection ellipsometry. We designed and fabricated the microcavity to have an enhanced local field in the EO polymer spacer and evaluated the resulting enhancement in the voltage-induced phase difference between the s- and p-polarized components. The phase difference obtained was 56 times larger than that for the EO polymer alone. This enhancement is attributed to the polarization-dependent local field effect for oblique incidence.     

Photophysical and Photochemical Properties of Ru(bpy)3 2+ in Si-Ti Mixed Oxides Xerogels Prepared by the Sol-Gel Process

The luminescence spectra and lifetime of tris(2,2-bipyridine)ruthenium(II), Ru(bpy)₃ ²⁺, were studied in sol-gel reaction systems of tetramethoxysilane (TMOS) and titanium(IV) isopropoxide (TTIP) with HCl. Luminescence lifetime in the TMOS system increased as the sol-gel reaction proceeded, because diffusion-controlled luminescence quenching such as oxygen and collisional quenching with solvent molecules were suppressed in the rigid matrices. On the other hand, luminescence lifetime in the TTIP system decreased during the sol-gel reaction. The decrease in lifetime was ascribed to electron transfer from photoexcited Ru(bpy)₃ ²⁺ to the conduction band of the TiO₂ xerogels. Extended X-ray absorption fine structure (EXAFS) measurements were done to associate lifetime in the Si-Ti xerogels with the structures of Ti⁴⁺ sites in the xerogels.     

Diastereoselective Brook rearrangement-mediated [3+4] annulation: application to a formal synthesis of (+)-laurallene

The formal synthesis of (+)-laurallene, a halogenated eight-membered ring ether, was accomplished. The synthesis involves construction of a trans alpha,alpha'-disubstituted oxocene structure 16 through a Brook rearrangement-mediated [3+4] annulation using acryloylsilane 10 and 6-oxa-2-cycloheptenone 9 and its conversion into 2, which has been transformed into (+)-laurallene by Crimmins and co-workers.     

Liquid core waveguide spectrophotometry for the sensitive determination of nitrite in river water samples.

A flow injection analysis system was built with a liquid core waveguide spectrophotometric detector using an 80 cm Teflon AF-1600 capillary tube (2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole/tetrafluoroethylene). The system was applied to determine nitrite ion in river water samples. The lower limit of detection for nitrite was 2.1 nmol dm(-3) (0.1 ng dm(-3) as NO(2)(-)) and the relative standard deviation of measurements was typically 0.56% (n = 5) at 0.21 micromol dm(-3).     

Pyrolytic and non-pyrolytic boron nitride platforms in electrothermal AAS for the determination of cadmium in sea and estuarine water without chemical modification.

The analytical performance of pyrolytic and non-pyrolytic boron nitride (PBN and NBN) platforms, attached to a commercially available graphite tube furnace, in electrothermal atomic absorption spectrometry (ETAAS) for Cd was studied. Although the tolerable pyrolysis temperature was 300 degrees C with the conventional pyrolytic graphite platform, it increased to 600 and 950 degrees C with the PBN and NBN platforms, respectively. The lifetime of the ceramic platform was 500 firings. The NBN platform provided an enhanced sensitivity with a better reproducibility than others. Using the NBN platform allowed the LOD, based on the variability of the blank (3sigma), to be 0.1 microg l(-1) within a seawater matrix (20,000 mg NaCl l(-1)) and a constant sensitivity in the range 0-30,000 mg NaCl l(-1). Good recovery in the range of 90-105% was observed for Cd (2.0 microg l(-1)) spiked into sea, estuarine and river water samples using the recommended procedure. This work proposes that using the NBN platform allows the direct monitoring and control of contaminated water for Cd by ETAAS without any chemical modification.     

Optical anisotropy in solution-cast film of cellulose triacetate

The out-of-plane birefringence and its wavelength dispersion are studied employing solution-cast films of cellulose triacetate (CTA). In solution-cast process, CTA molecules are induced to align in the film plane. Although refractive index is the lowest in the oriented direction for the CTA films stretched more than 110 %, refractive index is found to be the lowest in the normal direction for the unstretched cast film. Attenuated total reflection measurements reveal that in-plane alignment of the acetyl group which provides strong polarizability anisotropy is responsible for the phenomenon. Furthermore, the out-of-plane birefringence is found to increase with increasing wavelength, i.e. extraordinary wavelength dispersion, whereas a stretched CTA film shows ordinary wavelength dispersion. The level of the out-of-plane birefringence in cast films depends on the preparation conditions, which is predictable considering the evaporation rate. Moreover, it is demonstrated for the first time that the out-of-plane birefringence and its wavelength dispersion can be modified by addition of a certain plasticizer such as tricresyl phosphate (TCP). During the evaporation, TCP molecules orient in the film plane accompanying the orientation of CTA chains by intermolecular orientation correlation, called nematic interaction. This technique will widen the scope of material design of retardation films because there are numerous liquid compounds having strong polarizability anisotropy.     

Synthesis of Phenanthrene Derivatives by Intramolecular Cyclization Utilizing the [1,2]‐Phospha‐Brook Rearrangement Catalyzed by a Brønsted Base

The synthesis of functionalized phenanthrene derivatives was achieved by intramolecular cyclization utilizing the [1,2]‐phospha‐Brook rearrangement under Brønsted base catalysis. Treatment of biaryl compounds having an α‐ketoester moiety and an alkyne moiety at the 2 and 2′ positions, respectively, with diisopropyl phosphite in the presence of a catalytic amount of phosphazene base P2‐tBu provides 9,10‐disubstituted phenanthrene derivatives in high yields. This reaction involves the generation of an ester enolate through an umpolung process, that is, addition of diisopropyl phosphite to a keto moiety followed by the [1,2]‐phospha‐Brook rearrangement, the intramolecular addition to an alkyne, and the [3,3] rearrangement of the allylic phosphate moiety in a consecutive fashion.     

Elastic modulus and atomic displacements of skeleton and side groups in stretching of a polymer chain

We propose a new method of calculating elastic modulus and atomic displacements of a stretched polymer chain. We use a stretched Cartesian coordinate system and a stretched B matrix defined by the Cartesian displacement components of the bonds to express the potential energy matrix F_x for stretching. The method is used to calculate elastic moduli of planar-zigzag-chain vinyl polymers and helical-chain isotactic polypropylene and the atomic displacements of their skeleton and side groups. The skeletal displacements of planar-zigzag-chain polymers are about ten times larger than those of the side groups and reverse displacements of the two adjacent skeletal units occur in the direction perpendicular to the stretching direction along the fiber axis. In the case of isotactic polypropylene, the methyl group has more effect than the torsional change around the C? C bond on the skeletal deformation. © 1993 John Wiley & Sons, Inc.