title: TDDFT study of the polarity controlled ion-pair separation in an excited-state proton transfer reaction.
abstract: 6-Hydroxyquinoline (6HQ) is an ideal photoacid system for exploring excited-state proton transfer (ESPT) reactions. We have previously (Mahata et al. (2002)) shown that the ESPT reaction between 6HQ and trimethylamine (TMA) leads to an "unusual" emission in the 440-450 nm range, containing two decay components (∼5 ns and ∼12 ns). The observed results suggest the presence of a contact ion-pair and a solvent separated ion-pair. In this work, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) have been employed to study the nature of the contact ion-pair formed between 6HQ and TMA and to determine why the decay component ∼12 ns is absent in a non-polar solvent. Calculations of the hydrogen-bonded complexes formed between 6HQ and TMA and its ESPT reaction product, namely 6HQ-TMA and 6HQ-TMA-PT, respectively, have been carried out, both in the electronic ground and excited states. Moreover, by using the CPCM model, different dielectric constants have been introduced into the calculations. On increasing the dielectric constant, the hydrogen bond in 6HQ-TMA-PT becomes weaker and the hydrogen bond length becomes larger; this effectively facilitates the proton transfer reaction and formation of separated ion-pair. Thus, the separation and diffusion of the contact ion-pair can be controlled by changing the polarity of the surroundings.
authors.affiliation: Department of Physics, College of Mathematics and Physics, Bohai University, Jinzhou 121013, China. Electronic address: yhliu@dicp.ac.cn.
authors.forename: Yu-Hui
authors.initials: YH
authors.lastname: Liu
authors.affiliation: Department of Applied Physics, Delhi Technological University, Bawana Road, Delhi 110042, India.
authors.forename: Mohan Singh
authors.initials: MS
authors.lastname: Mehata
authors.affiliation: Department of Physics, College of Mathematics and Physics, Bohai University, Jinzhou 121013, China.
authors.forename: Sheng-Cheng
authors.initials: SC
authors.lastname: Lan
mesh_terms: D006912:Hydroxyquinolines / Q000737:chemistry*; D008744:Methylamines / Q000737:chemistry*; D008956:Models, Chemical; D011522:Protons
chemical_list: D006912:Hydroxyquinolines; D008744:Methylamines; D011522:Protons; C071287:6-hydroxyquinoline; C023336:trimethylamine
keywords: Excited state; Excited-state proton transfer; Hydrogen bond; Ion-pair