Naphthalimide‐Containing Isomeric Urea Derivatives: Mechanoluminescence and Fluoride Recognition

Isomeric ortho , meta and para naphthalimide–urea derivatives ND‐2U , ND‐3U and ND‐4U were synthesized and characterized. Upon gentle mechanical stimulation, the weakly emissive ortho isomeric sample ND‐2U was transformed into a yellow‐green emissive material ( λ em =500 nm), with Δ λ ≈125 nm. The mechanoluminescent emission of this isomer could be “switched off” by exposing the sample to solvent vapour. Moreover, ND‐2U could respond to fluoride anions in solution with a dramatic enhancement of fluorescence at λ =446 nm. The sigmoidal nature of the binding isotherm suggested cooperative binding of fluoride to the ortho  isomer. 1 H NMR spectroscopic studies provided vital insights into the nature of ND‐2U /F − complexation, highlighting the role of the proximal 1,8‐naphthalimide motif.