Origin of green photoluminescence in four-ring bent-core molecules with ESIPT, selective sensing of zinc ions by turn-on emission and their liquid crystal properties

Abstract

Fluorescent four-ring symmetrical/unsymmetrical molecules containing alkyl chains of a varied number of –CH₂– groups with a bent-core have been synthesized to explore their liquid crystalline (LC) and photophysical properties. Some of these molecules depending upon their alkyl chain length were found to exhibit B1 and B7 liquid crystalline phases and are characterized by various analytical techniques such as FT-IR, ¹H NMR, mass, POM, DSC, single crystal XRD, etc. Crystal structure determination reveals the hydrogen bonded enol form of these molecules with non-planar bent-molecular geometry. Intramolecular hydrogen bonding was found to play an important role in the stabilization of these molecules and in the origin of their green photoluminescence (GPL). The photo-physical experimental results through various control experiments clearly demonstrate that the origin of the large Stokes shifted GPL of these molecules can be attributed to the excited state intramolecular proton transfer (ESIPT) process. The formation of various types of anionic species and their stability were explored through steady-state and time-resolved fluorescence measurements. These compounds are found to be good turn-on PL probes in the selective detection of zinc ions at the micromolar level. Upon binding of zinc ions with the bent-core molecule, the structural changes have been investigated through NMR spectroscopy.