Urea-fused and π-extended single-benzene fluorophores with ultralarge Stokes shifts

Abstract

The excited-state tautomer equilibrium of the urea-fused single-benzene fluorophore was synthetically modulated to produce exceptionally large Stokes shifts (>12 400 cm⁻¹). The key N–H⋯N hydrogen bonding motif utilizes an endogenous proton for long-wavelength emission or an exogenous proton for acid–base chemistry, the balance of which is exploited for fluorescence switching in the solid state.