Axial coordination of biologically relevant ligands offers an effective strategy for tuning the photophysical and redox behavior of magnesium porphyrins, widely used as models for chlorophyll a . Herein, we report the first example of a melatonin-coordinated magnesium porphyrin, MgT(4-Cl)PP(MT), 1 T(4-Cl)PP = 5,10,15,20-tetrakis(4-chlorophenyl)porphyrin, MT = melatonin, in which melatonin binds axially to the Mg(II) center through its carbonyl oxygen atom with the Mg–O MT distance of 2.008 Å, resulting in a penta-coordinated square-pyramidal geometry. Compound 1 was characterized by UV–visible spectroscopy, NMR, MALDI-TOF, FTIR, and single-crystal XRD. Moreover, the redox properties were studied using cyclic voltammetry. Photophysical studies reveal that melatonin coordination significantly alters the excited-state dynamics of the porphyrin, resulting in reduced fluorescence lifetime and suppressed singlet oxygen generation, which correlate with enhanced resistance toward photoinduced degradation. The melatonin-bound complex also exhibits improved antioxidant activity relative to the parent magnesium porphyrin. Notably, freshly isolated spinach chlorophyll a exhibits similar resistance to oxidative stress and photodegradation in the presence of melatonin. Complementary density functional theory calculations and electrostatic analyses provide insight into ligand-induced modulation of the porphyrin π-electronic framework. Overall, these results demonstrate that axial coordination of melatonin offers an effective strategy for tuning the photochemical stability and reactivity of magnesium porphyrins.
Choudhury et al. (Fri,) studied this question.