Unusual large Stokes shift and solvatochromic fluorophore: Synthesis, spectra, and solvent effect of 6-substituted 2,3-naphthalimide

In this article, four new series of 6-substituted 2,3-naphthalimides (1a-d, 2a-d, 3a-d, 4a-d and 4 e-h) have been designed and synthesized through the formation of key cyclic anhydride intermediate, which was the precursor of the well known environment-sensitive fluorophore [6-N,N-dimethylaminonaphthalimides (6-DMN)] and other 6-substituted 2,3-naphthalimide series (2, 3, 4). Based on 6-amino-2,3-naphthalimide (6-ANP) compound, a new type of fluorophore was found to exhibit moderate to unusual large Stokes shift (297-303 nm). 6-ANP derivatives display relatively low fluorescence quantum yields in high polar protic solvents such as water (Φ_F ∼ 0.004, 571-576 nm) and a significant unusual red shift due to (1) hydrogen bonding interaction of the excited state of the molecule with the solvents, which presumably enhance the intersystem crossing process in the system to quench fluorescence, (2) this large Stokes shift was assumed to be the consequence of a substantial change of the geometric structure from the ground state (S₀) to the first excited state (S₁). Compared with the other compounds studied, the fluorescence of the nitro- and halo-derivatives was rather weak, probably due to the efficient intersystem crossing leading to a non-reactive triplet state.