Studies of the solvatochromic emission properties of N-aroylurea derivatives I: Influence of the substitution pattern.

The influence of the substituent of the N-aroylurea functionality on the solvatochromic properties of this class of compounds was investigated with eight examples. The absorption spectra of these compounds exhibit the characteristic spectroscopic properties of the corresponding arene fragment and are only slightly dependent on the solvent. In contrast, all investigated aroylurea derivatives exhibit a strong solvatochromism with a good linear correlation between the emission energy and the acceptor numbers (AN) of the solvents; that is, the emission maximum shifts bathochromically (Δλ = 50-93 nm) with increasing AN.

Dual fluorescence of 2-methoxyanthracene derivatives.

The time-resolved emission properties of selected anthracene derivatives, namely anthracene (1a), 2-methylanthracene (1b), 2-chloroanthracene (1c), 2-methoxyanthracene (1d), 2-methoxy-6-methylanthracene (1e), 2-(N,N'-dicyclohexylureidocarbonyl)-6-methoxyanthrace ne (1f), 2-(6-methoxyanthr-2-yl)-4,4-dimethyl-2-oxazoline (1g), 2-(6-methoxyanthr-2-yl)-pyridine (1h), and N-cyclohexylanthracene-2-carboxamide (1i) were investigated. In contrast to anthracene (1a), 1b, and 1c, the 2-methoxy-substituted anthracene derivatives 1d-1h exhibit two emission lifetimes. The determination of the lifetimes at different emission wavelengths and additional time-resolved emission spectroscopy (TRES) reveal that the dual emission originates from two different, interconvertible emissive species, with the s-cis and s-trans conformation relative to the exocyclic C2-O bond.

N-Acylureido functionality as acceptor substituent in solvatochromic fluorescence probes: detection of carboxylic acids, alcohols, and fluoride ions.

N,N'-Dicyclohexyl-N-(6'-methoxyanthryl-2-carbonyl)urea (1a) exhibits a strong solvatochromism with respect to its fluorescence properties. The emission maxima of 1a correlate well with the anion-stabilizing properties of the solvent. This feature allows the detection of analytes with high acceptor numbers, such as alcohols and carboxylic acids, and the detection of analytes such as fluoride ions which form strong hydrogen bonds with the amido hydrogen atom.

Anthryl-substituted heterocycles as acid-sensitive fluorescence probes.

[reaction: see text] Four pH-sensitive fluorescence probes are presented which consist of an anthracene fluorophore and a pi-conjugated oxazoline, benzoxazole, or pyridine substituent. The protonation of the heterocycles increases their acceptor properties and results in significant red-shifts of the absorption and emission maxima of the anthracene chromophore. The comparison between 2-[2'-(6'-methoxyanthryl)]-4,4-dimethyl-2-oxazoline and 2-[2'-(anthryl)]-4,4-dimethyl-2-oxazoline reveals that the donor-acceptor substitution pattern of the fluorophore is not required to achieve a red shift upon protonation.

Design of a chiral mesoporous silica and its application as a host for stereoselective di-pi-methane rearrangements.

[reaction: see text] A chiral periodic mesoporous organosilica (ChiMO) was prepared, in which a 1,2-bis-(ureido)cyclohexyl linker (38 wt %) is introduced into the walls of the hybrid organic-inorganic material. This silica was used as a host for 11-formyl-12-methyldibenzobarrelene (2), and the stereoselectivity of the di-pi-methane rearrangement of 2 within this host-guest complex was studied. At low conversions, the only product was the corresponding dibenzosemibullvalene.

Medium-dependent type selectivity in photoreactions of a crown ether-annelated dibenzobarrelene derivative.

[reaction: see text] A concept for modifying the photochemical properties of a dibenzobarrelene derivative by the combination of supramolecular assembly and solid-state photochemistry is presented. The irradiation of dibenzobarrelene 1b and alkali metal complexes thereof (1:1) in acetonitrile or benzene solution leads preferentially to the dibenzocyclooctatetraene 4b as the photoproduct. In contrast, the dibenzosemibullvalene 5b is formed as the main photoproduct in the solid state because of a strong cation effect.