Aggregation Induced Emission of Excited-State Intramolecular Proton Transfer Compounds: Nanofabrication Mediated White Light Emitting Nanoparticles

Abstract

Excited-state intramolecular proton transfer (ESIPT) compounds, 2-(2-((2-hydroxybenzylidine)amino)phenoxy)benzonitrile (1) and 2-(4-((2-hydroxy-4-methoxybenzylidine)amino)phenoxy)benzonitrile (2) exhibited aggregation induced enhanced emission (AIEE) in the solid state and nanofabrication mediated white light emission. The strong intermolecular interactions (H-bonding, C–H···π and π··· π) restrict the free rotation and rigidify the fluorophores, 1 and 2, in the crystal lattice that lead to enhanced fluorescence in the solid state of 1 (yellow, λmax = 535 nm, Φf = 38%) and 2 (greenish-yellow, λmax = 525 nm, Φf = 21%). Surprisingly, nanofabrication of 1 and 2 produced metastable white light emitting nanoparticles (Φf = 24% (1), 19% (2)) that slowly converted into stable yellow (1) or greenish-yellow (2) fluorescent nanoparticles with increasing time. The time dependent fluorescence, morphological, and metal ion interacting studies indicate the formation of soft nanoparticles in which both blue fluorescent cyanophenoxy and yellow/greenish-yellow fluorescent salicylidene group behave independently. 1 and 2 nanoparticles undergo morphological change from spherical to plates with increasing time. Thus, simple ESIPT molecules showed solid-state fluorescent and nanofabrication induced tunable fluorescence, particularly white fluorescent metastable nanoparticles.