Solvent-Assisted Prototopic Switching of Norharmane Along Hydrogen-Bonded Network: Assessing the Precise Length of Network

Abstract

In this article, the proton transfer dynamics along a stable norharmane•(H₂O)_n (n = 2–4) hydrogen-bonded cluster on conversion from the neutral to cationic form of norharmane (NHM) in water medium was demonstrated experimentally and theoretically. The distinct absorption and emission bands of different prototropic forms of NHM are well-known in the literature. Initially, the conversion from neutral to cationic form of NHM on moving from a polar aprotic (acetonitrile) to a polar protic (water) solvent was ensured by steady-state absorption and fluorescence studies. The analysis of IR spectra and steady-state anisotropy data of NHM confirmed the possibility of the formation of a hydrogen-bonded network in the presence of water. The length of the network was explored and assumed by extensive Density Functional Theory (DFT) calculations. Then, by time-dependent density functional theory (TD-DFT), the excited state proton transfer (ESPT) pathway was established interrogating the NHM-water cluster with different numbers of water molecules. The theoretical analysis assured that the NHM•(H₂O)₂ cluster was incapable of maintaining the stable hydrogen bonding wire in the course of the ESPT mechanism. Rather, NHM•(H₂O)₃ and NHM•(H₂O)₄ clusters were simultaneously involved in operating the ESPT mechanism. The NHM•(H₂O)₄ cluster was more feasible to carry out the proton transfer than the NHM•(H₂O)₃ cluster. To the best of our knowledge, this was possibly the first theoretical evidence behind the conversion from neutral to cationic form of NHM via the formation of a hydrogen-bonded network.