Coupling of a Quinoline Derivative and Rare-Earth-Doped TiO₂ Thin Films: Prospective Application to Hybrid...

Hybrid organic-inorganic heterostructures offer a promising design toward low-cost light-emitting devices by combining the high luminescence efficiency of organic semiconductors with the stability and charge transport capability of metal oxides. In this work, TiO₂ thin films doped with trivalent lanthanide ions (Er³⁺ and Yb³⁺, 2 and 4 at % concentrated) were prepared by the sol-gel dip-coating method and coupled to a quinoline-based donor-π-acceptor small molecule, 4-(6-(diethylamino)-4-phenylquinolin-2-yl) benzoic acid. X-ray diffraction confirmed preservation of the anatase phase after doping, while optical absorption revealed defect-related features and characteristic intra4f transitions of the lanthanides. Fourier-transform infrared spectroscopy demonstrated deprotonation of the carboxylic group and the formation of interfacial coordination bonds, with Er³⁺-doped TiO₂ exhibiting a well-defined bridging bidentate configuration. Electroluminescence measurements showed diode-like behavior for all doped systems. The Er³⁺-doped TiO₂ heterostructure exhibited a lower turn-on voltage (≈5 V), whereas the Yb³⁺-doped TiO₂ system showed enhanced emission at a higher applied bias. A blue shift of the photoluminescence peak from 500 to 483 nm upon heterostructure formation indicates modification of the electronic environment of the excited state due to interfacial coupling. Photoinduced electrical measurements in the quinoline derivative (QD) layer show a slow recombination process consistent with local lattice relaxation and Variable Range Hopping (VRH) mechanism, with a characteristic recombination time of 59.2 s. These results demonstrate that rare-earth doping modulates interfacial coordination strength and charge injection dynamics, directly impacting the electroluminescent performance of TiO₂/QD heterostructures.