The sole existence of a t-bone-shaped naphthalimide derivative [2-(2-aminoethyl)-1H-benzo[de]isoquinoline-1,3(2H)dione] (NAP), which gives rise to a photoinduced electron transfer (PET) mechanism, has been established using a combination of experimental and theoretical studies. In parallel an in vitro-in cell PET mechanism has also been shown. To understand the photophysics of NAP, solvent studies have been carried out in different solvents. In addition, theoretical calculations have been conducted to explain the spectroscopic properties through optimized structures. A "turn off" PET mechanism has also been observed in the presence of specific metal ions, namely, Cr , Fe and Hg among a series of metal ions. Theoretical studies reveal that NAP-Cr , NAP-Fe and NAP-Hg have their HOMO energy states lying in between a HOMO-LUMO energy state of the t-bone-type NAP molecule. On the contrary, the HOMO state of the other metal ion-NAP conjugate (NAP-M ) does not lie in between the HOMO-LUMO energy gap of the t-bone-type NAP molecule. Coupled with in vitro studies, in cell investigations reveal an enhancement of fluorescence intensity of NAP upon cytosolic metal sensing. Furthermore, a very high cell viability of NAP treated cells as tested by MTT assay and a fast permeation of the said compound as revealed by flow cytometry suggest NAP to be a potential candidate in metal sensing and bioimaging applications.

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Since zinc ions (Zn(2+)) are involved in numerous biological phenomena and go through subsequent interactions with zinc-binding proteins, we have attempted a sensitive fluorescence based detection of this second most abundant metal ion using an engineered and synthesized Schiff-base ligand, namely 2,4-bis((Z)-2-(1-(pyridin-2-yl)ethylidene)hydrazinyl)pyrimidine (PyHP). The ligand exhibits a zinc-induced fluorescence response when investigated in a MeOH-buffer (10 mM HEPES, pH = 7) (4 : 1) solvent mixture. The presence of zinc ions (λ(ex) = 410 nm, quantum yield, ϕ = 0.20) causes approximately 45 fold fluorescence enhancement at 489 nm. Formation of the metal-ligand complex was ascertained by (1)H NMR and mass spectra analysis. 1 : 1 binding affinity was ascertained according to Job's plot. Apart from this, theoretical interpretation of the experimental outcome was also obtained by applying density functional theory (DFT) to the PyHP-Zn(2+) complex formation. The practical applicability of the ligand has been tested in bacterial cells as well as in mammalian cell imaging and also by measuring and comparing the amount of Zn(2+) in some real samples such as liquid milk, tomato juice, banana stem juice and commercial fruit juice.

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