Density functional theory investigation of photoelectric conversion in graphene quantum dot/Ir(III) complex nanocomposites: the influence of π-conjugation in cyclometalating ligands

Abstract

Using density functional theory (DFT), this study investigates the photoelectric performance of nanocomposites formed by coupling graphene quantum dots (GQDs) with Ir(III) complexes. The goal is to evaluate the influence of different π-conjugation levels in cyclometalating ligands and determine the most efficient ligand for energy conversion in the nanocomposite. The analysis covers seven distinct Ir(III) complexes, each featuring a common bpy ligand but differing diimine ligands. These complexes are linked to GQDs through amide connections. The study comprehensively examines electronic structure, absorption spectra, charge transfer, and chemical reactivity. Our results show that increased ligand π-conjugation causes a redshift in the absorption spectrum due to smaller highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps, ultimately enhancing light harvesting. This effect becomes more pronounced when GQDs are incorporated. For less-conjugated ligands, attaching GQDs enhances metal-to-ligand charge transfer, facilitating electron injection into TiO₂. Moreover, higher conjugation and GQD coupling reduce chemical hardness while increasing chemical potential and electrophilicity, thus improving electron acceptance. Furthermore, strategic structural variations modify free energy changes for electron injection and ground-state regeneration. Notable is the inclusion of perylene and pyrene moieties in the ligand, which accelerates injection and extends recombination lifetimes, while GQD incorporation accelerates injection across all ligands. Additionally, photocurrent generation primarily influences energy conversion efficiency. Finally, adding GQDs enhances the first-order hyperpolarizability, further boosting light harvesting. This study demonstrates the potential of tuning ligand π-conjugation and GQD interfaces to optimize optoelectronic properties and charge transfer dynamics, thereby enhancing solar energy conversion in GQD/Ir(III) complex systems.

Keywords: Cyclometalating ligands; Density functional theory; Graphene quantum dot; Ir(III) complex; Nonlinear optics.