Stereodifferentiation in the fluorescence of naproxen–arginine salts in the solid state

Three stereoisomeric salts of naproxen (NPX) with arginine (Arg), namely (S)-NPX/(S)-Arg, (R)-NPX/(S)-Arg and (S)-NPX/(R)-Arg, have been prepared, and their fluorescence spectra recorded in solution and in the solid state. While the emission properties in solution did not show significant differences with λmax = 355 nm, τF (MeOH) ca. 11.5 ns and τF (H2O) ca. 9 ns (as NPX/Na), the (R)-NPX/(S)-Arg and (S)-NPX/(R)-Arg solid salts displayed red-shifted fluorescence spectra with maxima at 375 nm and τF = 1.1 ns. By contrast, the behaviour of solid (S)-NPX/(S)-Arg was similar to that of NPX/Na with λmax = 355 nm and τF ca. 5.5 ns. These results are explained based on the X-ray crystal structures and attributed to formation of NPX excimers emitting at longer wavelengths. Accordingly, such excimer emission was also observed in the fluorescence spectrum of a model NPX dyad in solution.

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