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, λ max = 535 nm, Φ f = 38%) and 2 (greenish-yellow, λ max = 525 nm, Φ f = 21%). Surprisingly, nanofabrication of 1 and 2 produced metastable white light emitting nanoparticles (Φ f = 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.