Not π stacking but CH⋅⋅⋅π‐ring center interactions lead to the fluorescence emission observed for the first time from a heteromolecular excimer complex. In single crystals a toluene molecule is located perpendicular to a Ph2P(S)‐disubstituted anthracene chromophore. This toluene complex fluoresces (see picture) but the benzene complex does not.

[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] The fluorescence properties of selected derivatives of 6-methoxyanthracene-2-carboxylic acid were studied. The corresponding ester and oxazoline derivatives exhibit a moderate solvatochromism, which is caused by the donor-acceptor interplay. The fluorescence band shift and intensity of oxazoline 1f may be reversibly modulated by protonation or complexation with silver ions; thus, the oxazoline heterocycle serves as a useful substituent with switchable acceptor strength.

Keywords: Cycloadditions / Heterocycles / π interactions / Photochemistry / Solid-state reactionsThe photodimerization of benzo-annellated acridizinium derivatives, namely naphtho[1,2-b]quinolizinium and naphtho[2,1-b]quinolizinium bromide, has been investigated in solution and in the solid state. Irradiation of the naphthoquinolizinium salts in solution gave all possible regioisomers in an unselective [4+4] photocycloaddition reaction. In contrast, the irradiation of crystalline samples resulted in regioselective dimerization to give the anti-head-to-tail photodimers.