Fluorescence Phenomena in Nerve-Labeling Styryl-Type Dyes.

Several classes of diversely substituted styryl type dyes have been synthesized with the goal of extending their expected fluorescent properties as much towards red as possible given the constraint that they maintain drug-like properties and retain high affinity binding to their biological target. We report on the synthesis, optical properties of a series of styryl dyes ( ), and the anomalous photophysical behavior of several of these Donor-Acceptor pairs separated by long conjugated π-systems ( ). We further describe an unusual dual emission behavior with two distinct ground state conformers which could be individually excited to locally excited (LE) and twisted intramolecular charge transfer (TICT) excited state in push-pull dye systems ( , and ).

Regioselective photodimerization of pyridyl-butadienes within cucurbit[8]uril cavities.

The cucurbit[8]uril (CB8) templation strategy that is known to yield stereoselective photodimers of organic olefins has been extended to substituted butadienes. By virtue of its strong binding interactions with guests the rigid cavity of CB8 is capable of preorienting the diene guests to result in greater yields of stereoselective photodimers upon irradiation. The symmetry of the butadiene monomers influences the relative arrangement of the monomers in complexes leading to the observed product selectivity.

Pyrophthalones as blue wavelength absorbers in thermoplastic media.

We have explored the utility of pyrophthalones as violet-blue light filtering dyes in polymer matrices for wavelengths below 450 nm. Further, we have investigated the photodegradation of these molecules in thermoplastic media and the mechanisms behind their degradation. Finally, a range of additives have been explored to improve the photostability of these molecules to achieve the desired performance.

Stable radicals during photodecarbonylations of trityl-alkyl ketones enable solid state reactions through primary and secondary radical centers.

The solid state photoexcitation of several triphenylmethyl-alkyl ketones resulted in the loss of CO and the exclusive formation of radical-radical combination products. Differences in reactivity suggest a stepwise mechanism with the unprecedented formation of primary and secondary radicals in some of the radical pair intermediates in the solid state.

The synthesis and stereospecific solid-state photodecarbonylation of hexasubstituted meso- and d,l-ketones.

Tertiary carbanions were trapped with half an equivalent of diphosgene to give meso- and d,l-hexasubstituted ketones in moderate yields and modest diastereoselectivities. The ketones were also synthesized by a step-wise synthesis in which the carbonyl group was first installed as an acid before activation and the second nucleophilic attack. This second method gave lower yields but similar diastereoselectivites.

Radical pairs with rotational fluidity in the photochemical reaction of acetophenone and cyclohexane in the zeolite NAY: a (13)C CPMAS NMR and product analysis study.

The photochemical reaction of acetophenone and cyclohexane in the zeolite NaY occurs by combination of the geminate radical pairs to give products that reveal a significant amount of rotational fluidity, which was also documented by intermolecular nuclear dipolar interaction measurements using cross polarization (13)C NMR (CPMAS) experiments.

Pump-probe spectroscopy and circular dichroism of nanocrystalline benzophenone--towards absolute kinetic measurements in solid state photochemical reactions.

Nanocrystals suspended in water can be used to record steady state and pump-probe absorption spectra, which should be useful for the study of excited states and reactive intermediates in the solid state.

Diastereoselective synthesis and spin-dependent photodecarbonylation of di(3-phenyl-2-pyrrolidinon-3-yl)ketones: synthesis of nonadjacent and adjacent stereogenic quaternary centers.

A diastereoselective procedure to obtain N-para-methoxybenzyl bis-alpha,alpha'-3-(3-phenyl-2-pyrrolidinone)yl substituted ketones with non-adjacent quaternary stereocenters, DL-2 and meso-3 was followed by a photoinduced, spin-dependent, and diastereoselective decarbonylation to give compounds DL-4 and meso-5, with adjacent all-carbon quaternary stereogenic centers.

Synthesis of a triply-bridged molecular gyroscope by a directed meridional cyclization strategy.

A crystalline triply bridged molecular gyroscope has been prepared and analyzed by single-crystal X-ray diffraction. A convergent synthetic strategy was developed to control the direction of the three bridges, from the preferred two zonal and one meridional arrangement of a one-step cylclization process to the directed three meridional bridges achieved by a north-south desymmetrization.

Preorientation of olefins toward a single photodimer: cucurbituril-mediated photodimerization of protonated azastilbenes in water.

With the view to establishing the generality of cucurbit[8]uril as a template, the photodimerization of hydrochloride salts of eight azastilbenes has been investigated in an aqueous medium. Whereas in solution upon excitation all of these olefins yield products of geometric isomerization, cyclization, and hydration, in the presence of cucurbit[8]uril the predominant product is that of dimerization. Such a change in product distribution is attributed to the localization of the olefins by the host cucurbit[8]uril.

Asymmetric induction during photocyclization of chiral and achiral alpha-oxoamides within achiral zeolites.

The photochemistry of 31 alpha-oxoamides capable of undergoing gamma-hydrogen transfer has been examined within zeolites. These molecules, upon excitation, yield two products--a beta-lactam and oxazolidinone--in solution, both resulting from gamma-hydrogen transfer. While in benzene the major product is oxazolidinone, within an MY zeolite, the main product is a beta-lactam.

Templating photodimerization of stilbazoles with water-soluble calixarenes.

Water soluble six and eight membered calixarenes template the dimerization of trans-stilbazoles. In the absence of calixarenes at the concentrations employed stilbazoles mainly isomerize to the corresponding cis isomers. Calixarenes help to localize the olefins and orient them in a specific geometry to yield anti-head-tail dimers.

Regioselective photodimerization of cinnamic acids in water: Templation with cucurbiturils.

Cinnamic acids upon irradiation in solution undergo geometric isomerization while dimerizing to different dimers in the crystalline state. Controlling the nature of the dimer formed upon irradiation remains a challenging task. We have aligned a variety of cinnamic acid molecules in a head-head fashion employing cucurbit[8]uril, a weakly water soluble host as a template.

Asymmetric induction during electron transfer mediated photoreduction of carbonyl compounds: role of zeolites.

Photochemistry of 17 aryl alkyl ketones included within cation exchanged zeolites has been examined. In solution five of the 17 ketones undergo intramolecular hydrogen abstraction reaction even in the presence of a chiral amine and the rest are photoreduced to the corresponding alcohol. Within zeolites all 17 ketones yielded in presence of a chiral amine, the corresponding alcohol as the major product.

Volume-demanding cis-trans isomerization of 1,2-diaryl olefins in the solid state.

Volume-demanding cis-trans photoisomerization of the aromatic substituted alkenes 1-3 in the solid state at room temperature and at 50 degrees C is presented. Alkene 3 did not undergo the cis-trans isomerization in the solid state either at room temperature or at 50 degrees C. The importance of the presence of void space near the reaction center to facilitate the large volume change during cis-trans photoisomerization is discussed.

Template directed photodimerization of trans-1,2-bis(n-pyridyl)ethylenes and stilbazoles in water.

Template induced photodimerization of trans-1,2-bis(n-pyridyl)ethylene dihydrochlorides and trans-n-stilbazole hydrochlorides within cucurbit[8]uril in aqueous media leads to high yields of the syn dimer.

Large molecular motions are tolerated in crystals of diamine double salt of trans-chlorocinnamic acids with trans-1,2-diaminocyclohexane.

Contrary to the general assumption that photoreactions in crystals may not proceed with large molecular motions, a pedal-like motion prompted by electronic excitation is believed to be involved during the beta-dimer formation from the crystals of the diamine double salt of trans-2,4-dichlorocinnamic acid and trans-1,2-diaminocyclohexane.

Asymmetric induction during Yang cyclization of alpha-oxoamides: the power of a covalently linked chiral auxiliary is enhanced in the crystalline state.

Gamma-hydrogen abstraction has been revealed to be the primary photoprocess in the crystalline state of alpha-oxoamides through photochemical and X-ray structural studies. The outstanding ability of a covalent chiral auxiliary in generating asymmetric induction in the photoproduct beta-lactam has been established with 10 examples. We have shown that the crystal lattice preorganizes the reactant molecules toward a single diastereomer of the beta-lactam and prevents large motions of the 1,4-diradical intermediate that would result in the loss of stereochemical memory.

Templating photodimerization of trans-cinnamic acids with cucurbit[8]uril and gamma-cyclodextrin.

Cucurbit[8]uril and gamma-cyclodextrin are able to align two olefin molecules in a head-head fashion within their large cavities. Excitation of such templated olefins results in syn head-head cyclobutanes in nearly quantitative yields. The methodology revealed here works with trans-cinnamic acids that do not dimerize either in solution or in the solid state and with the ones that yield only anti head-tail dimer in the solid state.

Asymmetric photoreactions within zeolites: role of confinement and alkali metal ions.

In this Account strategies using zeolites as media to achieve chiral induction are presented. Diastereomeric excesses as high as 90% and enantiomeric excesses up to 78% have been obtained with selected systems within zeolites. The same systems show no asymmetric induction in solution.

Enhanced enantio- and diastereoselectivity via confinement and cation binding: yang photocyclization of 2-benzoyladamantane derivatives within zeolites.

Irradiation of 2-benzoyladamantane derivatives in zeolites yields the endo-cyclobutanols as the only photoproduct via a gamma-hydrogen abstraction process. The cyclobutanols readily undergo retro-aldol reaction to give delta-ketoesters. The enantiomeric excess (ee) in the endo-cyclobutanols is measured by monitoring the ee in the ketoesters.