94 research outputs found
Kinetics of photoinduced ordering in azo-dye films: two-state and diffusion models
We study the kinetics of photoinduced ordering in the azo-dye SD1
photoaligning layers and present the results of modeling performed using two
different phenomenological approaches. A phenomenological two state model is
deduced from the master equation for an ensemble of two-level molecular
systems. Using an alternative approach, we formulate the two-dimensional (2D)
diffusion model as the free energy Fokker-Planck equation simplified for the
limiting regime of purely in-plane reorientation. The models are employed to
interpret the irradiation time dependence of the absorption order parameters
extracted from the available experimental data by using the exact solution to
the light transmission problem for a biaxially anisotropic absorbing layer. The
transient photoinduced structures are found to be biaxially anisotropic whereas
the photosteady and the initial states are uniaxial.Comment: revtex4, 34 pages, 9 figure
Photoinduced reordering in thin azo-dye films and light-induced reorientation dynamics of nematic liquid-crystal easy axis
We theoretically study the kinetics of photoinduced reordering triggered by
linearly polarized (LP) reorienting light in thin azo-dye films that were
initially illuminated with LP ultraviolet (UV) pumping beam. The process of
reordering is treated as a rotational diffusion of molecules in the light
intensity-dependent mean-field potential. The two dimensional diffusion model
which is based on the free energy rotational Fokker-Planck equation and
describes the regime of in-plane reorientation is generalized to analyze the
dynamics of the azo-dye order parameter tensor at varying polarization azimuth
of the reorienting light. It is found that, in the photosteady state, the
intensity of LP reorienting light determines the scalar order parameter (the
largest eigenvalue of the order parameter tensor), whereas the steady state
orientation of the corresponding eigenvector (the in-plane principal axis)
depends solely on the polarization azimuth. We show that, under certain
conditions, reorientation takes place only if the reorienting light intensity
exceeds its critical value. Such threshold behavior is predicted to occur in
the bistability region provided that the initial principal axis lies in the
polarization plane of reorienting light. The model is used to interpret the
experimental data on the light-induced azimuthal gliding of liquid-crystal easy
axis on photoaligned azo-dye substrates.Comment: 27 pages, 11 fugure
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Photodimerization of acenaphthylene within a nanocapsule: excited state lifetime dependent dimer selectivity
Direct excitation of acenaphthylene molecules included in a syn fashion within the octa acid nanocapsule dimerizes quantitatively to a syn dimer, and upon triplet sensitization, yields both syn and anti dimers probably by reacting within and outside the capsule
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A latent photoreaction enhanced upon cyclodextrin encapsulation: photochemistry of α-alkyl dibenzyl ketones in water
α-Alkyl dibenzyl ketones that are capable of undergoing both Norrish Type I and Type II reactions generally yield products of Norrish Type I reaction in organic solvents. These water insoluble ketones form water-soluble host-guest complexes with β and γ-cyclodextrins. Irradiation of the above β-cyclodextrin complexes in water leads to products of Norrish Type II reaction. This change in behavior between the guest alone in organic solvent and as host-guest complexes in water is consistent with the influence of other ordered media such as micelles and reactants such as benzoin alkyl ethers and alkyl deoxy benzoins
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Alkali ion exchanged nafion as a confining medium for photochemical reactions
Methanol-swollen Nafion beads were used as microreactors to control the photochemical reaction pathways. Product selectivity in three unimolecular reactions, namely, the photo-Fries rearrangement of naphthyl esters, Norrish Type I reaction of 1-phenyl-3-p-tolyl-propan-2-one and Norrish Type I and Type II reactions of benzoin alkyl ethers were examined. The influence of cations over the photodimerization of acenaphthylene and cross-photodimerization between acenaphthylene and N-benzyl maleimide included within Nafion were also examined. The photochemical behaviors of the above substrates were significantly altered within Nafion compared with their solution photochemistry. Of particular interest, the product distributions were found to depend on the counter cations of Nafion
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Controlling photochemical geometric isomerization of a stilbene and dimerization of a styrene using a confined reaction cavity in water
Utility of a water-soluble deep cavity cavitand, octa acid, as a reaction medium is illustrated by carrying out photochemical reactions of a stilbene and a styrene included within the octa acid in water. Geometric isomerization of trans-4,4'-dimethyl stilbene is restricted while dimerization of 4-methyl styrene is facilitated within the octa acid cavity. The excited-state chemistry of both systems is different in this medium from that in organic solvents. The change in chemistry is attributed to the supramolecular effects provided by the host cavity
Photoproduct selectivity in reactions involving singlet and triplet excited states within bile salt micelles
Generally, photochemical reactions tend to give more than one product. For such reactions to be useful one should be able to control them to yield a single product. Of the many approaches used in this context, the use of reaction media with features different from those of isotropic solutions has been very effective. We provide results of our studies on four reactions within bile salt micelles (cholic acid and deoxycholic acid). These four reactions involve homolytic cleavage of a C-C or C-O bond to yield either a singlet or triplet radical pair. The bile salt micelles control the rotational and translational mobilities of the radical pair, resulting in photoproduct selectivity. The dynamic nature of the bile salt micelles results in differential effects on the singlet and triplet radical pairs
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Alkali metal ion controlled product selectivity during photorearrangements of 1-naphthyl phenyl acylates and dibenzyl ketones within zeolites
Photochemical behaviors of 1-naphthyl phenyl acylates and dibenzyl ketones included in zeolites have been compared. 1-Naphthyl phenyl acylates while in solution produce eight photoproducts; within NaY it gives a single product. The selectivity is attributed to the restriction brought on the mobility of the primary radical pair by the alkali metal ions present in zeolites. Photochemistry of dibenzyl ketones within NaY reveals that the intersystem crossing in caged radical pairs could be influenced by the heavy alkali metal ions. Structures of complexes among Li+ ion and the guest 1-naphthyl phenyl acetates and dibenzyl ketone computed at the B3LYP level have been useful to understand the origin of the observed product selectivity within zeolites.Key words: photo-Fries reaction, zeolites, cation?p interaction, spin-orbit coupling, heavy atom effect
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A Latent Photoreaction Predominates Within Water-Soluble Calixarenes: Photochemistry of Benzoin Alkyl Ethers
Manipulating Photochemical Reactions
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