Four new betaine dyes of the Reichardt's type featuring two tolyl substituents in different attachment (2)(3)(4) or being an analogous 1-naphthyl derivative (5) have been synthesized and described with reference to their negative solvatochromism. The dinaphthyl derivative 5 and also a corresponding intermediate nitrophenol 9d were shown with variable-temperature NMR spectroscopy to form conformational atropisomers in solution. Crystal structures of seven alcoholic solvent complexes including the unsubstituted parent compound (1) and different alcohols [1ÁEtOH (1 : 1), 1Ái-PrOH (1 : 1), 2ÁMeOH (1 : 2), 3Ái-PrOH (1 : 1), 3Án-BuOH (1 : 1), 4ÁEtOH (1 : 2) and 5ÁMeOH (1 : 3)] have been studied using X-ray diffraction methods. The betaine dyes exist as genuine zwitterion ground state structures in the solid state with twisted conformation between the phenoxide and pyridinium rings. With the exception of 3a [3Ái-PrOH (1 : 1)], the alcohol guests in the complexes are hydrogen bonded to the oxygen atom of the phenoxide units, whereas in the i-PrOH complex of 3, the alcohol forms trimeric clusters without specific interaction with the betaine framework. From the packing modes in the crystals, the solvated betaines may be classified either as co-ordination type inclusion compounds with pocket-like host arrangements [1ÁEtOH (1 : 1), 1Ái-PrOH (1 : 1), 2ÁMeOH (1 : 2), 3Án-BuOH (1 : 1), 4ÁEtOH (1 : 2) and 5ÁMeOH (1 : 3)], or a true clathrate with channel-type topology [3Ái-PrOH (1 : 1)].

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