Supramolecular Structures of Zinc (II) (8-Quinolinolato) Chelates

Abstract

We investigate the oligomeric purity and stability of zinc (8-quinolinolato) (Znq2) and its methylated derivatives (nMeq2Zn, n = 2, 4, 5) through a combination of theoretical modeling of oligomerization energetics leading to supramolecular structures and experimental size-exclusion chromatography studies. Gas- and solution-phase (CHCl3) formation energies for dimeric, trimeric, and tetrameric species are reported. Favorable gas-phase thermodynamics were calculated and found to favor tetrameric structures for all Znq2 chelates (∼−50 kcal/mol for monomer dimerization to ∼−35 kcal/mol for dimer dimerization), with the exception of 2Meq2Zn, which gave lower formation energies by 30−45% due to steric hindrance. Solvation model computations indicate that these energies are reduced by ∼10−25% with the introduction of a dielectric medium. Computed structural parameters for the basic Zn−O core structure formed via bridging of phenolato oxygens do not change significantly as oligomer growth progresses. Size-exclusion chromatography experiments of crystalline and amorphous films (vapor deposited) dissolved in CHCl3 or CHCl3/DMSO mixtures showed that the dominant species for Znq2, 4Meq2Zn, and 5Meq2Zn is tetrameric, but partial disassociation to monomers can occur in the presence of nucleophilic solvent. The sterically hindered 2Meq2Zn was monomeric in all solvent systems. Implications for organic light-emitting devices using these materials are discussed.