W0188

Relative Stabilities of Acridine Polymorphs. R.D. Bailey,^[dagger] E.L. Rimmer,^§ T.W. Hanks,^§ and W.T. Pennington^[dagger], ^[dagger]Department of Chemistry, Clemson University, Clemson, SC 29634-1905, ^§Department of Chemistry, Furman University, Greenville, SC 29613

We have recently reported that the thermal decomposition of metastable tetrapyridylpyrazine·nI₂ complexes (n = 1, 2) provides a mechanism for interconversion of the monoclinic and tetragonal polymorphs of tetrapyridylpyrazine. To further investigate this phenomenon, we are systematically studying other polymorphic compounds capable of forming donor·acceptor complexes with iodine. Acridine forms a metastable complex with iodine ((acridine·I₂)₂·I₂, compound 1), and is known to crystallize in four different polymorphic forms (II - IV), as well as a hydrate (I). Thermal decomposition of 1 yields the form II polymorph of acridine, exclusively, as does dehydration of form I. Form IV can be converted into form III by heating to 70°C, and form III is converted into form II at 45°C. These results suggest that form II is the most thermodynamically stable form of acridine, however, the reported value for the density of form II (1.28 g/cm³) is somewhat lower than that of form III (1.30 g/cm³). We have found that acridine molecules pack as supramolecular columns in both forms II and III. The packing requirements and relative stabilities of these columns determine the thermal behavior of the compound and also the thermal decomposition product (through loss of I₂) of 1.