Metal quinolinolate-fullerene(s) donor-acceptor complexes: evidence for organic LED molecules acting as electron donors in photoinduced electron-transfer reactions

Tris(quinolinolate)aluminum(III) (AlQ3) is the most widely used molecule in organic light-emitting devices. There exists a strong demand for understanding the photochemical and photophysical events originating from this class of molecules. This paper provides the first report on the electron donor ability of MQ(n) (M = Al or Zn for n = 3 or 2) complexes covalently connected to a well-known electron acceptor, fullerene. To accomplish this, fullerene was functionalized with 8-hydroxyquinoline at different ligand positions and their corresponding zinc(II) and aluminum(III) complexes were formed in situ. The weakly fluorescent metal quinolinolate-fullerene complexes formed a new class of donor-acceptor conjugates. The stoichiometry and structure of the newly formed metal quinolinolate-fullerene complexes were established from various spectroscopic methods including matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and computational density functional theory studies. Electrochemical studies involving free-energy calculations suggested the possibility of photoinduced electron transfer from excited metal-quinolinolate complex to the appended fullerene entity. Femtosecond transient absorption studies confirmed such a claim and analysis of the kinetic data allowed us to establish the different photophysical events in sufficient detail. The novel features of this class of donor-acceptor conjugates include faster charge recombination compared to charge separation and decay of the charge-separated state to populate the low-lying fullerene triplet state in competition with direct charge recombination to the ground state.