Structure and three-dimensional crystal packing preferences for mer-tris(8-quinolinolato)indium(III) vapor-phase-grown crystals

Evaluation of indium(III) tris(8-quinolinolato) (Inq(3)) chelate crystals grown slowly from the vapor phase under vacuum (similar to10(-6) Torr) was compared to the aluminum analogue, Alq(3). In both cases, a previously unidentified close pi-pi interaction is identified, which likely impacts charge conduction. In the case of Inq3, we observe a mixture of both meridianal (mer) and facial (fac) crystals. The larger pseudohexagon mer-Inq(3) crystals were analyzed by single-crystal X-ray diffraction and showed average In-N and In-O bond lengths significantly longer (0.22 and 0.26 Angstrom, respectively) than Alq(3) and bond angles about the In3+ ion more distorted from an ideal octahedral geometry. The crystalline packing of mer-Inq(3) is found to closely resemble the beta-Alq(3) phase, where both metal chelates are characterized by pairing of symmetry-related mer-optical isomers in three-dimensional pi-pi stacking interactions (similar to3.5 Angstrom or less). The smaller needlelike crystals were determined to be pure fac-Inq(3) by powder X-ray diffraction studies. An equivalent is not observed when Alq(3) is processed in an identical manner.