Multiple fluorescences. 6. The case of 1,8-naphthosultam

The emission from 1,8-naphthosultam (2H-naphth[1,8-cd]isothiazole 1,1-dioxide) (1a) is composed of two bands, one for the acid (λ_max = 480 nm) and one for the corresponding anion (2, λ_max = 600 nm). The fluorescence of both 1a and 2, as well as that of the N-methyl- and N-ethyl-1,8-naphthosultams (1b and 1c), has a modest sensitivity to solvent polarity. The ground-state pK_a for 1a is 6.2, with pK_a* of -3.0. The proton transfer rate for 1a to water is >5 × 10¹⁰ s^-1, from the rise time of anion fluorescence by picosecond pulse techniques. The fluorescence lifetimes for 1a and 2 are 16 (φ_F = 0.37, dioxane) and 1.4 (φ_F = 0.005, H₂O, pH 10) ns, respectively.