Abstract

We report the design, synthesis, and characterization of YBL-1, a coumarin-triphenylphosphonium (TPP+) fluorescent probe engineered for mitochondrial targeting and iron sensing. YBL-1 was obtained through a threestep sequence culminating in quaternization with (bromomethyl)triphenylphosphonium bromide. Steady-state photophysics reveal minimal solvatochromism (lambda abs 348-355 nm; lambda em 401-408 nm) and allowed transitions (epsilon max approximate to 2.2-2.6 & times; 104 M-1 cm-1). The probe remains bright in water, with high quantum yields (Phi F approximate to 0.76). Screening against common metal cations indicates a pronounced and selective turn-off response to Fe3+. Job's analysis supports 1:1 complexation, and Benesi-Hildebrand treatment (1:1 model) indicates a modest association constant on the order of 102 M-1 in water. Complementary quantum-chemical calculations support a predominantly it -> it* locally excited S1 state with only weak charge-transfer character, consistent with the small solvatochromism. Electrostatic potential maps and population analyses highlight the carbonyl/urea/alkoxy region as the primary hard-donor site, rationalizing Fe3+ binding; simple adduct models in water predict modest stabilization in line with the measured Ka and a rigidified complex consistent with static quenching. In SH-SY5Y cells co-stained with MitoTracker Red, YBL-1 colocalizes strongly with mitochondria (Pearson r approximate to 0.95), consistent with TPP+-driven membrane-potential-mediated accumulation. Complementary computations (DFT/ MEP) indicate robust electrostatic features and preferential stabilization in polar media, with the lowest relative energy in water. Together, these results establish YBL-1 as a water-bright, mitochondria-addressable fluorescent probe exhibiting selective Fe3+-responsive quenching suitable for live-cell imaging.