Polymorphism in Sn(IV)-Tetrapyridyl Porphyrins with a Halogenated Axial Ligand: Structural, Photophysical, and Morphological Study

In this study, we report the first example of polymorphic Sn(IV)-tetrapyridyl porphyrins axially armed with halogen (Cl, Br)-substituted carboxyphenyl ligands (structural formula [Sn(tPyP)2+(A−)2], where A is the axial ligand = 3,5-dichloro/dibromo benzoic acid). The two polymorphs of chloro-substituted Sn(IV)-tetrapyridyl porphyrin (1α and 1β) display distinct photophysical and morphological properties in the solid state. X-ray diffraction study reveals that these polymorphs 1α and 1β greatly differ in supramolecular architecture and noncovalent interactions, which is responsible for their distinct solid-state properties. Molecules of the two polymorphs adopt different conformations of the axial carboxyphenyl ligand. Crystal packing of these polymorphs are dominated by intermolecular C–H···Npyridine, C–H···O, C–H···π, and Cl···Npyridin interactions. Theoretical study showed that the energy difference between the two polymorphs is ∼8 kcal/mol. In addition to these polymorphs, a reference compound 2 with an axial bromo-substituted carboxyphenyl ligand is also synthesized and structurally characterized.