Multiple Fluorescences. 4. The Protonated Form of N-Alkyl-2-N-arylamino-6-naphthalenesulfonates

The unusual fluorescence maximum (λmax 405 nm; λmax expected 468 nm) which can be observed from 2-N-alkyl-2-TV-phenylaminonaphthalene-6-sulfonates (1) in glycerol at 25 °C is now identified as arising by way of a protonation reaction of a vibrationally excited state of 1. The structure of the protonated product is shown by NMR to be the N-alkyl betaine (N-methyl in the case of 3M) of 6-sulfonato-3β-tetralenone N-phenylimine (3). The high quantum yield of photoproduct (3), φh = 0.05, in glycerol at 25 °C, the stability of the product and its strong fluorescence (φ = 0.3) are responsible for the confusion about the fluorescence spectrum of 1M in glycerol, in which the fluorescence of 3 can alter or cover the spectrum of 1 in the course of a usual measurement. With low exciting light intensities, the fluorescence spectrum of 1 in glycerol can be observed (λmax 470 nm). The emissions are also readily distinguished by different excitation spectra. The protonated product can also be produced in 1,2-ethanediol and 1,2-propanediol via the quenching of charge-transfer states. Lack of wavelength dependence, temperature effect, or isotope effect on the quantum yield of the protonation reaction point to a fast proton transfer (estimated k, 1.1 X 108 s-1). A complete scheme for the excited state behavior of 2,6-arylaminonaphthalenesulfonate derivatives is presented.