Primarily, colorimetric properties of NPDM towards Al 3+ ions were investigated in aqueous DMSO solution (Fig. ). The characteristic short-wavelength absorption bands (329 nm, 357 nm), and the longer absorption bands (414 nm, 473 nm and 502 nm) in UV-Vis spectrum of NPDM (10 µM) were due to π → π* transitions and charge transfers (CT) from the electron donors ((D, phenolic hydroxyl) to the acceptors (A, cyanogen) groups respectively. Upon addition of Al 3+ ions (20 µM), significant variations of absorption pattern were marked. The typical absorption band at 414 nm was bathochromic-shifted to 436 nm, and the absorption maximum near 473 nm and 502 nm were enhanced along with a color change from yellowish brown to chrome yellow, which indicated that NPDM could coordinate with Al 3+ ions. The above observations illustrated that NPDM can undergo a chromogenic response to Al 3+ ions. In addition, the intrinsic optical properties of NPDM are very important. So, its emission spectrum was tested under the excitation of 380 nm in DMSO, and the emission maximum is observed around 547 nm (Fig. ). The dual emissions could be attributed to ESIPT behavior of molecular tweezers NPDM. Then, we tested the fluorogenic responses of NPDM in the presence of Al 3+ (100 µM) and other competitive metallic ions (i.e, Li + , Na + , Ag + , Zn 2+ , Mg 2+ , Ca 2+ , Co 2+ , Fe 3+ , Fe 2+ , Ni 2+ , Sn 2+ , Cd 2+ , Pb 2+ , Mn 2+ , VO 2+ , Cu 2+ , Hg 2+ , 200 µM) in DMSO/H2O (7/3, v/v, 10 mM, HEPES, pH 7.0). As shown in Fig. , NPDM alone has the faint fluorescence owing to C=N isomerizations and PET behaviors as well as the existence of strong electron withdrawing groups (-C≡N) in NPDM. By contrast, as Al 3+ was added to NPDM solution, a significant dual emission bands at 520 nm and 556 nm were observed, which may be the results of ESIPT processes, whereas there were no remarkable fluorescence variations of NPDM when other miscellaneous cations were introduced into NPDM solution respectively. The relative fluorescence intensity increased by 35 folds as compared with that of free NPDM (Fig. ). Further, the tolerance experiments were explored in the presence of competitive cations (Fig. ). It showed that only Cu 2+ obviously reduced the maximum emission intensities, however, this interference can be cleared up with the masking reagent dimethylglyoxime (DMG) for the distinction between Al 3+ and other metallic ions. These results implied that NPDM is highly selective to Al 3+ . Furthermore, the fluorescence titration experiments were employed for investigating the interaction properties of NPDM towards Al 3+ . As shown in Fig. , fluorescence intensities of NPDM solution at 520 nm and 556 nm showed the linear growth up to 100 µM Al 3+ and reached the equilibrium (200 µM) following the visual fluorescence change from colorless to strong green-yellow under a 365 nm UV lamp (Fig. ). The fluorescence enhancement of NPDM treated with Al 3+ could be atrributed to the CHEF and ICT processes. NPDM-Al 3+ (Φ = 0.3404) complex implied about 37 folds higher Φ value than that of NPDM solution (Φ = 0.0096) at 520 nm. It's noteworthy that the appearance of remarkable dual emission bands at 520 nm and 556 nm with the introduction of Al 3+ are contributed to coexistence of the forms (enol and keto) in phenolic Schiff base moieties and may be due to the excited tautomer of E* and K* forms arising because of ESIPT processes (Scheme 1). The emission bands at 520 nm and 556 belong to NPDM-Al 3+ enol form and NPDM-Al 3+ keto form respectively. Actually, if the chealation of NPDM with Al 3+ trigger deprotonations of the phenolic hydroxyl groups (-OH), the ESIPT processes would not happen and NPDM-Al 3+ complex only could emit a single emission band. However, the deprotonation behaviours could not be observed, and thus ESIPT is not blocked and the dual emissions arising, which was also further supported by HRMS, 1 H NMR, and DFT studies. By plotting the relative emission intensities (I/I0) at 520 nm versus the [Al 3+ ] (Fig. ), and according to the DL equation (3δ/slope), the LOD of NPDM for Al 3+ was found as 16.3 nM (0-40 μM, R 2 = 0.9975), which is sufficiently below the maximum allowable level of Al 3+ in potable water from the EPA (7.4 μM), revealing desirable sensitivity of NPDM for Al 3+ ions. The stoichiometric ratio of NPDM chelated with Al 3+ was obtained as 1:2 using Job's plot (Fig. ). Depending on fluorescence titration data, the association constant between the NPDM and Al 3+ was estimated to be 1.07 × 10 10 M -2 by the Benesi-Hildebrand plot (Fig. , ESI), which displayed the binding affinity of NPDM to Al 3+ was enough strong.