Intraligand Charge Transfer in Zn(II) Complexes for Singlet Oxygen Photocatalysis: A Sustainable Alternative to Precious Metal Systems

Triplet photosensitizers are essential in photocatalytic hydrogen production and photoredox organic transformations, relying on efficient light absorption, intersystem crossing, and subsequent electron or energy transfer processes. Recent developments have focused on transition metal complexes with favorable electronic configurations, such as Cu(I) complexes, which feature extended excited-state lifetimes and reduced nonradiative decay. In contrast, Zn(II) complexes, despite being isoelectronic with Cu(I), are generally dismissed as photosensitizers because they predominantly exhibit ligand-centered emission, and MLCT states, when observed, are short-lived and poorly suited for photocatalysis. Nonetheless, their tunable coordination chemistry, low toxicity, and catalytic potential make them promising candidates. In this work, we demonstrate that Zn(II) complexes can be deliberately engineered to act as efficient triplet photosensitizers by exploiting intraligand charge transfer (ILCT) rather than MLCT. We report a comparative study of Zn(II) complexes supported by redox-noninnocent tridentate pincer ligands bearing different aryl substituents, revealing pronounced differences in triplet-state behavior. Structural analysis shows that coplanarity between the aryl-azo and phenanthroline moieties is essential for efficient ILCT and long-lived triplet emission. To elucidate the role of the metal center, a structurally analogous Cd(II) complex was examined, which exhibits photophysical behavior nearly identical to its Zn(II) counterpart, confirming that the metal primarily enforces ligand geometry rather than participating electronically in the excited state. Both Zn(II) and Cd(II) complexes function as effective photosensitizers for singlet-oxygen-mediated oxidation, achieving photocatalytic efficiencies of approximately 50%. Although this efficiency is lower than that of certain Ir-based systems, this work establishes a design strategy in which unfavorable metal-centered electronic interactions are intentionally bypassed, and the metal serves chiefly as a structural anchor. These findings position Zn(II) complexes as sustainable, earth-abundant photosensitizers and provide a new framework for designing triplet photosensitizers based on closed-shell metal ions.