Adventures with Fluorescent Compounds

Abstract

4-Amino-1,8-naphthalimide derivatives1 are highly fluorescent compounds whose relatively simple synthesis and high quantum yields have led to their widespread use, for example, as fluorescent sensors for metal ions and pH, and as elements in the design of molecular logic gates. The emission properties of these systems are strongly influenced by the solvent and by the nature and degree of substitution on the nitrogen5 since the ability of the compounds to form twisted charge transfer states may be a major determinant of the excited state properties. The emission properties of the naphthalimide nucleus is also sensitive to environ¬ment even when conformationally locked, as shown by the fluorescence emission of Tröger's bases based on this fluorophore. The bromination of saturated 4-alkylamino-N-alkyl-1,8-naph-thalimides proceeds well in halogenated solvents, but in typ¬ical experiments several hours are required for the complete dis¬charge of the color of the halogen. The first product formed with bromine is the 3-bromo compound; using excess bromine and long reaction times leads to the 4-amino compound rather than a second ring bromination. In the process of developing site-selective probes for use in flu¬orescence microscopy of live cells, we needed a dibromide of gen¬eral structure 4, which we expected should be easily obtained by addition of bromine to an N-allyl-4-alkylamino-1,8-naphthalimide. This addition reaction did not, however, proceed as expected.