Synthesis and energy-transfer properties of fluorescence sensing bichromophoric system based on Rhodamine 6G and 1,8-naphthalimide

This paper reports on the synthesis and investigation of a novel fluorescence sensing bichromophoric system 1 based on Rhodamine 6G and 1,8-naphthalimide, designed as a wavelength-shifting FRET chromophore. A similar donor–acceptor system 2, not containing photoinduced electron transfer (PET) receptor, was synthesized as a reference compound and investigated as well. The overlap between the emission of the donor (1,8-naphthalimide) and the absorbance of the acceptor (rhodamine) was more than 90%. Due to the efficient donor–acceptor energy transfer (96–97%) the acceptor fluorescence of the systems, excited in the maximal absorption of the donor chromophore, was enhanced more than 33 times with respect to the fluorescence intensity of the reference Rhodamine 6G. It was also found that the novel bichromophoric system 1 displayed sensitive acceptor fluorescence signaling over a wide pH scale, which has been ascribed to a PET nature of 4-(N-methylpiperazinyl)-1,8-naphthalimide donor in this system. The fluorescence enhancement of dyad 1 in water-DMF (4:1, v/v) between ca. pH 9 and 2 was remarkable (FE = 43.37). These changes, attributed to the protonation of the piperazine amine receptor, are of such magnitude that they can be considered as representing two different “off-on” states. This indicates the high potential of the novel wavelength-shifting chromophore 1 as efficient pH chemosensing material.