title: Origin of Water-Induced Fluorescence Turn-On from a Schiff Base Compound: AIE or H-Bonding Promoted ESIPT?
abstract: A nonfluorescent Schiff base compound (4) in an aprotic solvent (e.g., CH3CN) is found to give blue fluorescence turn-on (λem ≈ 475 nm) upon addition of H2O. By using a wide range of spectroscopic methods, including (1)H NMR and dynamic light scattering, the fluorescence response is shown to be not originating from the molecular aggregation-induced emission (AIE). Spectroscopic studies at low temperatures further reveal a dynamic response of 4 to temperature, showing that the excited state intramolecular proton transfer (ESIPT) can be ON or OFF through interaction with protic solvent. In the binary solvent (with composition CH3CN/H2O = 3:1), the Schiff base gives ESIPT emission (λem ≈ 524 nm) only at extremely low temperature (below -80 °C), which is turned off when being warmed to -60 °C, attributing to the increasing photoinduced electron transfer (PET) effect. When the temperature is further raised to -20 °C, ESIPT emission is reactivated to give blue emission (λem ≈ 475 nm) that is observed at room temperature. The observed dynamic fluorescence response reveals that ESIPT could be a predominant mechanism in the fluorescence turn-on of Schiff base compounds, although both AIE and ESIPT mechanisms could operate. The assumption is further verified by examining the response of Schiff base to Al(3+) cation.
authors.affiliation: Department of Chemistry, The University of Akron , Akron, Ohio 44325, United States.
authors.forename: Lucas
authors.initials: L
authors.lastname: McDonald
authors.affiliation: Department of Chemistry, The University of Akron , Akron, Ohio 44325, United States.
authors.forename: Junfeng
authors.initials: J
authors.lastname: Wang
authors.affiliation: Department of Chemistry, The University of Akron , Akron, Ohio 44325, United States.
authors.forename: Nick
authors.initials: N
authors.lastname: Alexander
authors.affiliation: Department of Polymer Science, The University of Akron , Akron, Ohio 44325, United States.
authors.forename: Hui
authors.initials: H
authors.lastname: Li
authors.affiliation: Department of Polymer Science, The University of Akron , Akron, Ohio 44325, United States.
authors.forename: Tianbo
authors.initials: T
authors.lastname: Liu
authors.affiliation: Department of Chemistry, The University of Akron , Akron, Ohio 44325, United States.
authors.forename: Yi
authors.initials: Y
authors.lastname: Pang
mesh_terms: D005453:Fluorescence; D006860:Hydrogen Bonding; D012545:Schiff Bases / Q000737:chemistry*; D014867:Water
chemical_list: D012545:Schiff Bases; D014867:Water