Tailoring Transition Intensity of Ruthenium(II) Complexes with π-Elongated Phenyl Ligands: Experimental and Computational Insights
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A series of novel ruthenium(II) complexes bearing π -elongated phenyl ligands have been synthesized via a 3-step reaction involving Ru(phen) 2 Cl 2 and functionalised phenanthroline ligands (phen- p -RCA; phen = phenanthroline, R = H, OCH 3 , or NO 2 ; CA = cinnamic acid/chloride) to evaluate the substituent effects on metal-to-ligand charge transfer (MLCT) absorption. The complexes were characterised by CNHS elemental analysis and IR, NMR, and UV-Vis spectroscopies. Complementary DFT/TDDFT calculations were employed to probe the electronic factors governing transition intensity. The molar extinction coefficient ( ε ) of the MLCT band increased in the order NO 2 < H < OCH 3 , with the methoxy OCH 3 group enhancing transition dipole moment through broader transition density distribution, while NO 2 group displayed the opposite trend. Computational investigation on π -elongation revealed that increasing ligand conjugation enhanced the existing MLCT bands and introduced new low-energy intra-ligand charge transfer (ILCT) transitions. Transition density and frontier molecular orbital analyses revealed reduced HOMO–LUMO gaps, increased ligand participation, and higher ε values, particularly in the ILCT region, largely independent of substituent type. Additionally, substituents show minimal effects on MLCT transitions, whereas NO 2 -substituted complexes exhibited stronger ILCT bands due to carbon–nitrogen orbital hybridization. Structural modifications also influenced photophysical behaviour: removal of amide functional groups decreased dihedral angles, improved planarity, and enhanced π -conjugation, leading to increased ε . Incorporation of nitrogen-containing moieties also boosted ε via lone-pair electron donation. Overall, these findings highlight the critical roles of π -elongation, substituent effects, and specific functional groups in tailoring the absorption features of Ru(II) complexes, providing valuable guidelines for optimizing photophysical performance.
