Synthesis, photophysical, and DNA binding studies of fluorescent Tröger's base derived 4-amino-1,8-naphthalimide supramolecular clefts.

The synthesis and characterization of three bis-1,8-naphthalimide-containing Tröger's bases 1-3, formed from the corresponding 4-amino-1,8-naphthalimide precursors 7-9 in a single step, is described. The photophysical investigation of 1-3 and 7-9 was carried out in various organic solvents as well as in water and as a function of pH using UV/vis and fluorescence spectroscopy. As for their 4-amino-1,8-naphthalimide precursors 7-9, both the ground-state and excited-state characteristics of 1-3 were dependent on the polarity and the hydrogen-bonding ability of the solvent medium. The DNA-binding affinities of 1-3 were also studied in aqueous solution at pH 7.4, in the presence of calf-thymus DNA (ct-DNA), using various UV/vis absorption and fluorescence spectroscopic methods. These molecules exhibited significant DNA-binding ability, where large binding values K(b) in the range of 10(6) M(-1) were determined. Such strong binding to ct-DNA was maintained even in competitive media (50 and 160 mM NaCl) and was also found to be irreversible regardless of the concentration of the ionic strength. Thermal denaturation experiments also demonstrated that the interaction of 1-3 with ct-DNA gave rise to significant stabilization in the double-helical structure of DNA. The binding affinity of 1-3 for ct-DNA was also compared to that of their 4-amino-1,8-naphthalimide precursors 7-9, determined by fitting of data using "intrinsic" methods and ethidium bromide displacement assays. The latter method gives outstanding binding constants for 1-3 in the range of 10(7) M(-1).