Chromoisomers and Polymorphs of 9-Phenylacridinium Hydrogen Sulfate

The debate about the meaning of the defunct term chromoisomerism is reviewed. The central role of red and green crystals of 9-phenylacridinium hydrogen sulfate (1) in fueling this dialogue is highlighted. Single-crystal structures of a red (1r) and a green (1g) form were determined by X-ray diffraction, 1r: monoclinic, space group P2₁/c, a = 9.266(1) Å, b = 8.582(1) Å, c = 20.690(3) Å, β = 98.03(1)°, Z = 4. 1g: triclinic, space group PĪ, a = 8.597(1) Å, b = 10.449(1) Å, c = 10.822(1) Å, α = 101.33(1)°, β = 108.73(1)°, γ = 114.65(1)°, Z = 2. Differential scanning calorimetry revealed that the polymorphs 1r and 1g underwent comparable endothermic transitions. The thermal transformations were monitored and characterized by thermal gravimetric infrared spectroscopy and X-ray powder diffraction. Taken together these data failed to reveal a thermal transformation between 1r and 1g. Diffuse reflectance spectra of powders, coupled with INDO/S calculations, did not identify the origin of the color difference. Optical processes in related dye polymorphs are discussed. The short-lived and physically ill-defined term chromoisomerism is illustrative of the introduction, refinement, and abandonment of a concept in organic chemistry.