Complexation-induced Resonance Raman Effect as a Novel Platform for Monitoring Intracellular Fe2+

<jats:p> Intracellular Fe <jats:sup>2+</jats:sup> plays a crucial role in ferroptosis and other redox-regulated biological processes. However, the direct visualization of its dynamic behavior remains difficult. Herein, we report a resonance Raman-based strategy for imaging the intracellular Fe <jats:sup>2+</jats:sup> via the formation of Fe <jats:sup>2+</jats:sup> –phenanthroline complexes. Initial studies using phenanthroline and bipyridine revealed that Fe <jats:sup>2+</jats:sup> coordination selectively generates resonance Raman signals, but the signal intensity was insufficient for robust intracellular imaging. We developed a series of alkyne-functionalized and deuterated phenanthroline derivatives to improve sensitivity and spectral selectivity. Although resonance enhancement of the alkyne vibrational mode was limited, Fe <jats:sup>2+</jats:sup> coordination of 4,7-diethynyl-1,10-phenanthroline substantially increased visible absorption and aromatic ring-derived resonance Raman intensity. This was attributed to the extension of the conjugated system. This complexation-induced enhancement facilitated sensitive Fe <jats:sup>2+</jats:sup> visualization in living HeLa cells and enabled time-lapse monitoring of Fe <jats:sup>2+</jats:sup> accumulation on the minute timescale. Thus, ligand structural modification can tune the electronic absorption and resonance Raman response of Fe <jats:sup>2+</jats:sup> complexes in living cells. Complexation-induced resonance Raman enhancement provides a versatile platform for visualizing intracellular metal-ion dynamics while preserving the label-free molecular information from spontaneous Raman spectroscopy. </jats:p>