2‐Aryl‐5,8‐di‐tert‐butyl‐1H‐benz[de]isoquinolinium tetrafluoroborates were synthesized and crystalized to form yellow to orange crystals. Absorption and fluorescence maxima (λABS and λFL‐SL) of the compounds in CH2Cl2 depend on the electronic and steric properties of the aryl ring substituents and closely match those arising from theoretical calculations. λABS and λFL‐SL values of the compound possessing bulky 2,6‐diisopropylphenyl substituents are the shortest among the substances because of sterically promoted retardation of conjugation between the isoquinolinium and aryl ring planes. In contrast, solid‐state fluorescence maxima (λFL‐SD) of the compounds are not closely correlated with the electronic properties of the aryl substituents. Furthermore, one crystalline form of both the 4‐trifluoromethylphenyl and 4‐nitrophenyl derivatives exhibit mechanochromic and crystallochromic behavior reflected in absorption and fluorescence color changes. X‐ray diffraction and crystallographic analysis reveal that the solid state photophysical properties of these substances are related to different molecular stacking interactions occurring in different crystalline forms.