Abstract
Novel bi-chromophoric naphthalimide derivatives containing benzo-15-crown-5 and N-phenyl-aza-15-crown-5 receptor moieties BNI2 and BNI3 were designed and prepared. Significant Förster resonance energy transfer (FRET) from donor (D) amido-naphthalimide to acceptor (A) amino-naphthalimide chromophores as well as photoinduced electron transfer (PET) between the N-aryl receptor and amido-naphthalimide fragment was revealed by the steady-state and time-resolved UV/Vis absorption and fluorescence spectroscopy. Upon the addition of alkaline-earth metal perchlorates to an acetonitrile solution of ligands, FRET mediated fluorescence enhancement was observed, which was a result of inhibition of the PET competitive deactivation pathway. The studied compounds provide an opportunity to register a two-channel fluorescence response upon selective excitation of either of the photoactive units and, thus, might be of interest as ratiometric probes.
Highlights
Forster resonance energy transfer is a unique process making possible the generation of fluorescence signals sensitive to molecular conformation, association and separation in the 1–10 nm range.[1,2] This mechanism has been widely used in medicinal diagnostics, optical imaging and molecular biology as a spectroscopic ruler to study the structure of proteins and nucleic acids
Crown ether groups were incorporated in the N-aryl fragment of a more electron deficient amido-naphthalimide chromophore, because in this case a strong photoinduced electron transfer (PET) interaction is expected for both benzo-15crown-5 and N-phenylaza-15-crown-5 ether receptors.[41,43]
The compounds were designed as ratiometric cation Forster resonance energy transfer (FRET) chemosensors comprising the PET switching amido-naphthalimide fluorophore linked with the amino-naphthalimide fragment
Summary
Forster resonance energy transfer is a unique process making possible the generation of fluorescence signals sensitive to molecular conformation, association and separation in the 1–10 nm range.[1,2] This mechanism has been widely used in medicinal diagnostics, optical imaging and molecular biology as a spectroscopic ruler to study the structure of proteins and nucleic acids. We have recently developed mono-chromophoric aminoand amido-naphthalimide derivatives MNI4–6 (Scheme 1) bearing crown ether groups as fluorescent sensors exploiting the photoinduced electron transfer process.[41,42,43] These compounds displayed pronounced enhancement of emission intensity by coordination with metal cations, which was a result of inhibition of PET between the crown ether receptor conjugated with the N-phenyl ring and the fluorophore. We report the design, synthesis and investigation of cation-dependent behavior of FRET-based ratiometric sensors BNI2–3 by integrating amido-naphthalimide probes MNI5 and MNI7 as FRET donors and amino-naphthalimide MNI1 as an FRET acceptor In this case, the strategy for detection of metal ions is based on modulating the FRET process, and emission intensity of the acceptor aminonaphthalimide fragment, by means of incorporation of a competitive PET deactivation pathway. Naphthalimides MNI1,44 MNI2,44 MNI3,44 MNI544 and MNI743 have been synthesized earlier and were included in photophysical studies as reference compounds
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