Sensitive and Selective PET-Based π-expanded Phenanthrimidazole Luminophore for Zn2+ Ion

The novel photoinduced electron transfer (PET) chemosensor, 1-(1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)naphthalen-2-ol [MPPN] and its zinc complex were synthesised and characterized by electronic spectral and Frontier molecular orbital energy analysis. MPPN becomes efficient fluorescent chemosensor upon binding with metal ions and shows a strong preference toward Zn2+ ion. Density Functional theory (DFT) calculations reveal that luminescence of free MPPN originates from its orbital structure in which two π-orbitals (HOMO and HOMO-1) of the imidazole ring are situated between two π-orbitals (HOMO-2 and LUMO) of the naphthyl fragment. Therefore the absorption and emission processes occur between the two π- orbitals (HOMO-2 and LUMO). The two higher energy imidazole orbitals (HOMO and HOMO-1 ) serve as quenchers for the excited state of the molecule through nonradiative processes. Upon binding with Zn2+ ion, MPPN becomes a highly luminescent with λemi − 421 nm. The significant enhancement of luminescence upon binding with Zn2+ ion is attributed to the stabilization of HOMO-2 and HOMO-1 π-orbitals of imidazole ring upon their engagement in new bonds with Zn2+ ion. The affinity of MPPN to zinc ion is found to be very high [K = 6 × 106 M−1] when compared with other metals ions. The nonlinear absorption coefficient γ for MPPN is 1.9 × 10−12 m/W and 3.9 × 10−11 m/W for MPPN-Zn complex.