Fluorescent 4-(2,2,6,6-tetramethylpiperidin-4-ylamino)-1,8-naphthalimide pH chemosensor based on photoinduced electron transfer

Two isomeric 2,2,6,6-tetramethylpiperidinyl-4-amino-1,8-naphthalimides 5 and 7 were configured as “fluorophore–spacer–receptor” systems. Photophysical characteristics of the dyes were investigated in water/DMF (4:1, v/v) solution. Compound 5, which lacks the amino receptor at the 1,8-naphthalimide 4-amino moiety, did not show any changes in the emission properties as a function of pH. In contrast to compound 5, 4-(2,2,6,6-tetramethylpiperidin-4-ylamino)-1,8-naphthalimide 7 displayed “on–off” switching in its fluorescence as a function of pH. From the luminescent changes pKa value of 8.18 was determined. This indicates that the chemosensor 7 would be able to act as highly efficient “off–on” fluorescent switcher for pH.