A fluorescent "2 in 1" proton sensor and polarity probe based on core substituted naphthalene diimide.

A new core-substituted naphthalene diimide (cNDI) compound that is sensitive to both H(+) and solvent polarity has been synthesised and characterised. Optical absorption and emission change upon protonation while emission quantum yield and decay kinetics of the unprotonated form vary significantly with solvent polarity due to delayed fluorescence.