Unique solvent-dependent fluorescence of nitro-group-containing naphthalene derivatives with weak donor–strong acceptor system

We synthesized nitro-group-containing π-conjugated naphthalene derivatives with a weak donor–strong acceptor system and investigated their photophysical properties. The nitro group was introduced into naphthalene through the phenyl or phenylethynyl moiety at the C2 and C7 positions as the strong acceptor moiety, and a methoxy group was introduced into naphthalene directly at the position opposite to the nitro group, as a weak donor moiety. While 2-(4-nitrophenyl)naphthalene did not show fluorescence in various solvents, 2-methoxy-6-(4-nitrophenyl)naphthalene showed fluorescence in weakly polar solvents (ΦF = 0.11 in CH2Cl2), with a large Stokes shift (Δν = 12,000 cm−1). Additionally, 2-methoxy-6-(4-nitrophenyl)naphthalene did not show fluorescence in polar solvents (acetonitrile) and non-polar solvents (toluene). This unique solvent-dependent fluorescence is remarkable for environmental fluorescence sensor applications.