Theory of emission state of tris(8-quinolinolato)aluminum and its related compounds

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Abstract
Fluorescence of fac-AlQ3 (Q=8-quinolinolato), mer-AlQ3, mer-Al(mQ)3 (mQ=4-methyl-8-quinolinolato), and BeQ2 were investigated with electronic structure calculations. The molecular structure of the first singlet excited state (the emission state) was optimized with the ab initio “configuration interaction with single excitations” (CIS) method. Ab initio CIS and semiempirical “Zerner’s intermediate neglect of differential overlap” (ZINDO) methods were used to calculate the emission energies (ΔE) and also the corresponding absorption energies. Although the ab initio CIS method overestimated the experimental value of ΔE by 1.09–1.16 eV, the ZINDO method reproduced it to a reasonable accuracy (within 0.26 eV). The optimized excited-state structure has an interesting feature in that one of the equivalent ligands distorts appreciably, while the thers keep their ground-state structures. As a result the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO) are localized on the distorted ligand. Since the excited state is characterized as the HOMO–LUMO transition, the emission from AlQ3 (and its analogues) directly reflects that of the ligand. In order to analyze this intrigung excited-state structure, exciton transfer theory was applied, where the exciton coupling between ligands and the structural relaxation of the ligand upon excitation were taken into account. By examining these two factors for BeQ2, it is shown that the exciton localization results from weak exciton coupling and/or large structural relaxation energy.