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<p>A series of novel ruthenium(II) complexes bearing <i>π</i>-elongated phenyl ligands have been synthesized via a 3-step reaction involving Ru(phen)<sub>2</sub>Cl<sub>2</sub> and functionalised phenanthroline ligands (phen-<i>p</i>-RCA; phen = phenanthroline, R = H, OCH<sub>3</sub>, or NO<sub>2</sub>; 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 (<i>ε</i>) of the MLCT band increased in the order NO<sub>2</sub> &lt; H &lt; OCH<sub>3</sub>, with the methoxy OCH<sub>3</sub> group enhancing transition dipole moment through broader transition density distribution, while NO<sub>2</sub> group displayed the opposite trend. Computational investigation on <i>π</i>-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 <i>ε</i> values, particularly in the ILCT region, largely independent of substituent type. Additionally, substituents show minimal effects on MLCT transitions, whereas NO<sub>2</sub>-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 <i>π</i>-conjugation, leading to increased <i>ε</i>. Incorporation of nitrogen-containing moieties also boosted <i>ε</i> via lone-pair electron donation. Overall, these findings highlight the critical roles of <i>π</i>-elongation, substituent effects, and specific functional groups in tailoring the absorption features of Ru(II) complexes, providing valuable guidelines for optimizing photophysical performance.</p>

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Tailoring transition intensity of ruthenium(II) complexes with π-elongated phenyl ligands: experimental and computational insights

  • 期刊论文

  • 发表日期：

    2026年1月12日

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  • Zi Ying Yeoh,
  • Yoshitada Morikawa,
  • Mohammad B. Kassim,
  • Siow-Ping Tan,
  • Mohd Azlan Nafiah,
  • Siew San Tan

摘要

A series of novel ruthenium(II) complexes bearing π-elongated phenyl ligands have been synthesized via a 3-step reaction involving Ru(phen)2Cl2 and functionalised phenanthroline ligands (phen-p-RCA; phen = phenanthroline, R = H, OCH3, or NO2; 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 NO2 < H < OCH3, with the methoxy OCH3 group enhancing transition dipole moment through broader transition density distribution, while NO2 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 NO2-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.