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

Abstract

[EN] 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 lambda(max) = 355 nm, tau(F) (MeOH) ca. 11.5 ns and tau(F) (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 mn and tau(F) = 1.1 ns. By contrast, the behaviour of solid (S)-NPX/(S)-Arg was similar to that of NPX/Na With; ax = 355 nm and tau(F) 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.