Optical pH Sensor Based on Ruthenium(II) Complexes with Electron-withdrawing and Electron-donating Substituents: Experimental and Computational Studies

Yeoh, Zi Ying (2024) Optical pH Sensor Based on Ruthenium(II) Complexes with Electron-withdrawing and Electron-donating Substituents: Experimental and Computational Studies. Masters thesis, Tunku Abdul Rahman University of Management and Technology.

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Abstract

A series of novel ruthenium(II) polypyridine complexes bearing an extended π-conjugated ligand with the formula, [Ru(phen)2(phen-p-RCA)](PF6)2 [phen = phenanthroline, CA = cinnamide, R = H (1), CH3 (2), OCH3 (3), Cl (4), and NO2 (5)] were successfully synthesized and structurally characterized using fourier transform infrared spectrometer (FTIR), CHNS elemental analyzer, cyclic voltammetry (CV), nuclear magnetic resonance (NMR) spectrometer, and ultraviolet-visible (UV-Vis) spectrophotometer. In this work, we investigated the influence of pH on the lowest-energy absorption band of these complexes with different substituent groups, both experimentally and theoretically. Computational studies employing density functional theory (DFT) and time-dependent DFT (TDDFT) were conducted to elucidate the underlying mechanisms governing the spectral changes induced by varying pH. Complexes 1-5 exhibited a prominent absorption band at approximately 450 nm, which showed enhancement in molar absorptivity (ε) when pH increased from pH 3 to pH 11. Interestingly, complexes 3 (1.467) and 5 (1.389), which featured OCH3 (a strong electron-donating group) and NO2 (a strong electron-withdrawing group) respectively, showed higher on-off ratios and greater pH-sensitivity than the unsubstituted complex 1 (1.324). This finding suggested that the incorporation of strong electronic substituents into the ligand framework could enhance the sensitivity of the complexes to pH based on their molar absorptivity. Besides, Mulliken Population Analysis was performed to clarify the origins of the lowest-energy band, revealing that the deprotonated ligands are more negatively charged (-0.714 to -0.720 A.U.) than the neutral (0.229 to 0.234 A.U.) and protonated (1.205 to 1.209 A.U.) ligands. Consequently, the ligands states (-6.025 to -5.890 eV) of deprotonated complexes were destabilized above the Ru-derived states (-6.120 to -6.102 eV), leading to dominant intra-ligand charge transfer (ILCT) observed at high pH, while metal-to-ligand charge transfer (MLCT) was primarily detected at both neutral and low pH. Moreover, the high ε (18,034 to 29,691 M-1cm-1) observed under basic conditions was attributed to a greater extent of transition density during excitations induced by the smaller dihedral angles of the conjugated ligand in deprotonated complexes. In short, our findings successfully provided valuable insights into the strategic development of potential complexes for absorption-based pH monitoring applications.