[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. The benzoxazole and pyridine substituents offer a particular advantage due to their persistence under acidic conditions. Thus, these compounds may be used as efficient pH-sensitive fluorescence switches. Nevertheless, the switching of benzoxazole 2c requires relatively strong acidic conditions. The anthrylpyridinium exhibits a red-shifted emission in chloroform; however, it is nonfluorescent in aqueous or alcoholic solution. Although the oxazoline is not persistent under permanent acidic conditions, this heterocycle may be useful as a substituent in fluorescence indicators since it may be used to detect acid concentrations of 10(-4)-10(-5) M, which are close to the biologically relevant range.

[reaction: see text] Experimental evidence is reported for the reversible formation of the singlet diazenyl diradical ((1)DZ), photolytically generated from the structurally elaborate DBH-type azoalkane. Reversiblity of the (1)DZ formation manifests itself through the decrease of the photodenitrogenation quantum yield over a ca. 40-fold viscosity variation (from 0.5 to 19.3 cP). This viscosity behavior is interpreted in terms of frictional effects on the competitive reaction modes of the diazenyl diradical.