Molecular structure and conformational dynamics of 3,12-dichloro-5,14-diphenyl-7,9a,16,18a-tetrazadibenzo[a,l]pentacene-9,18-dione

The X-ray study of 3,12-dichloro-5,14-diphenyl-7,9a,16,18a-tetrazadibenzo(a,l)pentacene-9,18-dione (1), a model compound for a novel class of thermostable polyheteroarylenes, polyquinoquinazolones, has been carried out. The nonsymmetric flattened structure of the molecule observed is a result of intermolecular interactions. It was established using quantum-chemical calculations by the semiempirical AM1 method that the annelation of the 1,6-dihydropyrimidinone ring by aromatic cycles results in increasing the conformational flexibility of the dihydrocycle due to weakening the conjugation between the carbonyl group and the remaining π-system of the molecule. It was shown by X-ray study and quantum-chemical calculations that protonation of 1 results in a substantial change in the molecular structure due to the large contribution of the 1,4-dihydro tautomeric form to the structure of the 1,6-dihydropyrimidinone ring. A tendency for the conformational flexibility of the heterocycle to increase upon protonation was revealed.