Investigation of the substituent effect on the excited- state intramolecular proton transfer of 8-hydroxyquinoline

Abstract

Fluorescent sensors operating on the excited-state intramolecular proton transfer (ESIPT) mechanism have demonstrated excellent sensing performance owing to their attributes such as large Stokes shifts and excellent photostability. In the present study, we investigated the kinetics of the excited states of three 8-hydroxyquinoline (8HQ) derivatives(i.e.,2-amino-8-hydroxyquinoline(2A8HQ),2-carbaldehyde-8-hydroxyquinoline (2C8HQ), and 2-methyl-8-hydroxyquinoline (2Me8HQ)) in cyclohexane. The absorption and emission properties of the organic molecules were determined by UV–vis absorption and fluorescence spectroscopy. Additionally, the PBE0/TZVP method was used to explore the effect of substituents on 8HQ (i.e., –NH2, –CHO, and –CH3) on the photophysical properties and ESIPT behaviors of the 8HQ derivatives. Our findings revealed an enhancement in hydrogen bond interactions in the excited states of 2A8HQ and 2C8HQ, which served as a driving force behind the ESIPT process. Potential energy curve, hole–electron distribution, orbital contributions, infrared spectroscopy, and reduced density gradient analyses were conducted to better understand the ESIPT process. The results showed that the potential barriers were overcome by an excited-state proton transfer mechanism owing to differences between the substituent electron donor and electron acceptor. Moreover, this study lays the groundwork for the development of ESIPT-based 8HQ materials.

Keywords

• Hydrogen bond
• Time-dependent density functional theory
• Excited-state intramolecular proton transfer
• Substitution base effect