Geometries of ground and first excited states of mer-tris(8-hydroxyquinolinato)metal (Mq3, M=Al-III, Ga-III) are optimized at B3LYP/6-31G(d) and CIS/6-31G level, respectively. In order to investigate the difference for individual ligands in mer-Mq3, the energy partitioning analysis has been carried out at the BP86 level using TZ2P basis functions for the bonding interactions between each fragment Mq2 and single ligand q. HOMO and LUMO distribution fashion can be traced back to the lowest electrostatic attractive and highest orbital interaction energy between fragments A-quinolate ligand and Mq2 and B-ligand and Mq2, respectively. (C) 2004 Elsevier B.V. All rights reserved.