Irradiation 9-donor-substituted acridizinium salts in the solid state gave exclusively the anti-head-to-tail dimers. The regioselectivity of the photodimerization of the bromo-substituted derivative was independent of the morphology of the solid sample, and this selectivity is proposed to be the result of efficient directional a stacking in the solid state. X-ray structure analysis and solid-state emission spectroscopy are used to characterize the solid samples.