Synthesis and photophysical properties of fluorescence sensing ester- and amidoamine-functionalized 1,8-naphthalimides

Novel ester- and amidoamine-functionalized 1,8-naphthalimides have been synthesized based on a convergent approach. Their basic photophysical properties in both protic and aprotic organic solvents of different polarity have been determined and discussed. Depending on the solvent polarity the fluorescence maxima are in the spectral range 480–550 nm, the fluorescence quantum yields vary from 0.23 to 0.46, the fluorescence lifetimes are long, in order of 9 ns. Absorption and fluorescence characteristics of the dyes as a function of pH were investigated in water/ethanol (4:1, v/v) solution. The starting 1,8-naphthalimide, containing primary amino receptor at the 1,8-naphthalimide 4-amino moiety, did not show significant changes in the emission properties as a function of pH. On the contrary the ester- and amidoamine-functionalized 1,8-naphthalimides were found to display sensitive fluorescence signal amplification over a wide pH scale, which has been ascribed to a photoinduced electron transfer from the tertiary amine receptor to the fluorophore 4-amino donor. From the changes in the fluorescence intensity pKa values of 4.42 and 4.86 for ester- and amidoamine-functionalized 1,8-naphthalimides, respectively were determined, making the synthesized compounds of potential use as pH chemosensing materials.