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1,8-Naphthalimide derivatives have been widely used in the field of fluorescent probe and attracted immense interest.In this work,the fluorescence quenching and enhancing mechanisms of a 1,8-naphthalimide-based fluorescent pH probe molecule,namely N-(2-(dimethylamino)ethyl)-4-(4-piperidinylphenyloxy)-1,8-naphthalimide(Napa-pp),are studied via density functional theory(DFT)and time-dependent density functional theory(TDDFT)methods.The results show that for the ground state of neutral probe molecules,HOMO-2 and LUMO are located on the fluorophore,while the HOMO is located on the whole piperidine phenyl unit and HOMO-1 is located on the alkylamine unit.It will be possible to produce photoinduced electron transfer(PET)and intramolecular charge transfer(ICT)effect and cause fluorescence quenching.Upon the addition of H+,the alkylamine N atom is protonated in the form of NH+,the HOMO energy of alkylamine unit becomes lower than that of the excited fluorophore moiety and the LUMO energy of alkylamine unit becomes higher than that of the excited fluorophore moiety,leading to a PET quenching process and the fluorescence is intensified.When the piperidine N atom is protonated in the form of bis-protonated Napapp,both the HOMO and LUMO are located almost completely on the naphthalimide moiety,so there is no PET and ICT,leading to an intensive intrinsic fluorescence from the naphthalimide moiety of this compound.The calculated results have showed a good agreement with experimental data.