Sensing ability of fluorescence based chemosensor: Indole-7-carboxaldehyde (I7C) for environmentally hazardous metal ions (X n+=Zn2+, Pb2+, Co2+, Ni2+, Cu2+, Cd2+, Cr3+, and Fe3+) has been investigated. Based on excited state proton transfer (ESPT) phenomenon, the sensing capability of I7C is verified experimentally (UV–vis, FTIR, NMR) and theoretically (DFT, TD-DFT) in aqueous media. Chemical hardness (η), electrophilicity (ω) and electronic chemical potentials (μ) confirm the selective reactivity of I7C (N c and N t conformers) in the presence of metal ions by the formation of (1:1) metal ion: I7C complex (M X , M X ⁎ ). M X , M X ⁎ are found to display excellent sensing capability for Zn2+ and Pb2+ ions. “Turn-On” response for Zn2+ is observed through the appearance of a new enhanced fluorescence at ~430nm. By strong blue emission I7C establishes its strong candidature as “blue emitter”. “Turn-Off” response is observed for Pb2+ through the quenching of the existing fluorescence peak.