A Study on Cation-Dependent Resonance Energy Transfer in Crown-Containing Bischromophoric System Based on 4-Methoxy-and 4-Styryl-1,8-naphthalimide

Azadithia-15-crown-5 ether based bis-chromophoric chemosensor BNI bearing 4-alkoxy-and 4-styryl-1,8-naphthalimide fragments was obtained using copper(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition click reac-tion. Excitation ofBNI with 360 nm light results in resonance energy transfer (RET) in the system and formation of a fluorescence band at 617 nm which corresponds to the emission of 4-styrylnaphthalimide chromophore. Analysis of the steady-state UV-Vis absorption and fluorescence spectra showed that the efficiency of energy transfer (Phi RET ) in the free ligand BNI was as high as 65%. The estimated Phi RET value was found to be considerably lower compared to that obtained by the theoretical calculations using the inductive -resonance Forster model (99.98%). This difference can be explained by the occurrence of photoinduced electron transfer (PET) process from the receptor group to theexcited 4-alkoxynaphthalimide residue competing with energy transfer. The possibility of PET was confirmed by the quantum chemical calculations using PM6 method. Binding of silver cations by BNI in an aqueous solution at pH 7.4 (HEPES buffer) caused fluorescence enhancement at 620 nm which was related to the inhibition of PET upon com-plexation and increase in RET efficiency as a result. Coordination of Ag+ with the crown ether moiety was demon-strated by the 1H NMR spectroscopy.