Polymorphs and a pseudo-polymorphs based on a luminescent boron-containing compound: structural diversity arising from conformational isomers and noncovalent interactions

Abstract

The synthesis of an organoboron compound 2,6-bis(5-methyl-2-hydroxylphenyl)pyridyl boron (p-methoxyl)benzene (MDPPBPM(p)) is reported. The crystallizations of MDPPBPM(p) gave two crystalline polymorphs A and B (A from THF/diethyl ether system and B from pyridine/diethyl ether system, respectively) and a chloroform solvate C (pseudo-polymorph from chloroform/diethyl ether system). It was demonstrated that in crystalline polymorphs A and B, MDPPBPM(p) molecules adopt two different conformations, a cis-conformer for A and a trans-conformer for B, respectively. The pseudo-polymorph C also exhibits a trans-conformer. The MDPPBPM(p) molecules in the three phases vary remarkably in the rotational position of methoxy groups. Apart from the different conformational characteristic and structural motifs, non-covalent intermolecular interactions such as C–H⋯O hydrogen bonds and π⋯π stacking interactions, physical properties such as shape, calculated density and thermal stability, cause significant differences between the three crystals. All the polymorphs and the pseudo-polymorph exhibit solid-state photoluminescence due to the intraligand π → π* electron transition.