Full-visible color-tunable small molecular xenobiotics are at the forefront of bioimaging research. It received broad attention in multicolor organelles imaging through intracellular molecular interactions due to its non-invasive nature, simplicity, and high sensitivity. However, significant challenges are designing fluorophores with desired optical and biological properties due to their structural complexity. Here, we synthesized a unified interesting chemical toolbox, IndiFluors, which exhibited full-visible emission color (375-700 nm) by varying donor and acceptor strengths. Encouraged by the excellent optical properties of IndiFluors, we synthesized a unique small molecular morpholine conjugated probe, PM-Mor-OH as a ratiometric fluorescent probe at a single excitation wavelength (λ_ex. 488 nm). The lipophilic morpholine ligand has been extensively used to target lysosomes, but our probe unusually targets mitochondria. The morpholine ligand also plays a pivotal role in modulating the emission ratio (λ_green/_red) with internal pH variations (ΔpHi) via photo-induced electron transfer (PET). Furthermore, the time-dependent density functional theory (TD-DFT) explained the bathochromic emission shift in basic pH through intramolecular charge transfer (ICT). This probe was successfully applied to quantify the mitochondrial pH ratiometrically when treated with mitophagy-induced rapamycin, A53T α-synuclein misfolded protein, and starvation. In addition, it can monitor mCherry Parkin’s translocation from cytoplasm to damaged mitochondria. Thus, PM-Mor-OH is a suitable probe for measuring ΔpHi when lysosomes fuse with damaged mitochondria, known as mitophagy. The probe may be a potential chemical tool for diagnosing mitochondrial diseases.

Subrata Munan

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