To strengthen the interaction mechanism, the optimized geometries and the HOMO/LUMO orbital energies of NPDM, and NPDM-Al 3+ (enol-keto forms) were again computed using (TD)DFT calculations through Gaussian 09 (Fig. , Fig. ). The energy gaps of NPDM, and NPDM-Al 3+ in the S0 states were examined as 3.0047 eV, and 2.7524 eV respectively. The data show that the binding of Al 3+ to NPDM narrowers the energy gap and stabilizes the system, also confirming the red-shift of NPDM-Al 3+ in the absorption maxima, which coincides well with the aforementioned absorption spectra analysis. Additionally, the energy gaps of NPDM-Al 3+ complex (E*) and NPDM-Al 3+ complex (K*) were determined to be 2.1598 eV and 2.0074 eV, respectively. These calculations demonstrated that the energy gaps in the NPDM-Al 3+ complexes (E*-K* forms) were quite lower than that of NPDM, verifying the enhancement of ICT effect. Furthermore, the energy gap of NPDM-Al 3+ complex (E*) was larger than that of NPDM-Al 3+ complex (K*), which could be responsible for dual emissions of NPDM-Al 3+ because of ESIPT process.