Microscopic investigation of the optoelectronic and electronic structure characteristics of OLED derivatives with CH/N substitutions on pyridyl and phenoxide rings.

CONTEXT: Organic light-emitting materials based on tris(8-hydroxyquinoline) aluminum (Alq₃) and gallium (Gaq₃) remain benchmark emitters in OLED research due to stable emission behaviors. The present work is concerned with the molecular and optical characteristics of these compounds by introducing CH/N substitutions at different positions. A series of derivatives was constructed to evaluate the impact of structural modifications on absorption, emission, and Stokes shift. The results reveal that substitution position apparently affects electronic transitions and spectral behavior. Modifications on the pyridyl side tend to induce red shifts, whereas those on the phenoxide side result in blue shifts. Molecular orbital analyses further indicate that parent complexes exhibit multi-orbital excitations, whereas the 5n₂p derivatives show localized HOMO to LUMO + 1 transitions, leading to higher excitation energies and narrower Stokes shifts. METHODS: Molecular structures were constructed using GaussView and subsequently optimized by density functional theory (DFT) with the B3LYP functional and the 6-31G(d) basis set. The first singlet excited-state (S₁) geometries were optimized using the CIS method with the 3-21G* basis set. Then, based on the optimized ground- and excited-state structures, the absorption and emission spectra were obtained at the B3LYP/3-21G* level from time-dependent DFT (TD-DFT). All calculations were carried out using the Gaussian software package. The computed results have a good agreement with the experimental UV-vis and photoluminescence trends, providing confidence in the modeling approach.