In addition, the TD-DFT calculations were also performed to describe the specific ESIPT luminescence mechanism observed above (Fig. , Table ESI). The computational vertical excitation energy in the S0(E) → S1(E*) process is 2.8764 eV (431 nm), approximate to the absorption peak value of NPDM-Al 3+ in solution (436 nm). The vertical emission energy in the S1(K*) → S0(K) was calculated as 2.4311 eV (510 nm), approximating the emission peak value of NPDM-Al 3+ in keto form (556 nm). There is not a local minimum for S0(K), hence NPDM-Al 3+ returns to S0(E) without energy barrier (Fig. ). With the aid of the theoretical calculations, the photophysical process of NPDM-Al 3+ has been elucidated (Fig. ). In the electronic S0 state, NPDM-Al 3+ exists solely as a S0(E) form like other quintessential ESIPT molecules for the steady intramolecular H-bonds. Upon photoexitation, S1(E*) was transformed into tautomer S1(K*) through an ultrafast ESIPT process, which caused the transient change of the electronic characteristics and simultaneously the formation of a new S1 state PES. After decaying radiatively to S0(K), followed by reverse proton transfer process happened to the original S0(E) form, eventually finishing the ESIPT photocycle.