Norharmane prototropism in choline chloride-based deep eutectic solvents

Abstract

Deep eutectic solvents (DESs) are gaining widespread attention as novel and alternate solvents in chemical and biological sciences. Prototropic equilibria are known to be governed by the solubilizing milieu, and thus, it is imperative to study them within DESs. Prototropic behavior of β-carboline commonly named as norharmane (9H-pyrido[3,4-b]indole) is investigated in eight different DESs constituted of the salt choline chloride with eight different H-bond donors (HBDs) to ascertain their role in controlling prototropism within DESs. UV–vis molecular absorbance is used to obtain information on ground-state prototropism whereas steady-state and time-resolved fluorescence are used to investigate prototropic forms in the excited-state. In the ground-state, DESs with HBDs glycerol and ethylene glycol support both neutral and cationic forms of norharmane, however, within DESs constituted of HBDs urea, 1,4-butane-diol, and acetamide, respectively, only the neutral form exists. Within the remaining three DESs with HBDs tetraethylene glycol, 3-phenylpropionic acid, and malonic acid, respectively, only cationic form is supported. As the cationic form is preferred more over the neutral form of norharmane in the excited-state, DESs with HBDs tetraethylene glycol, 3-phenylpropionic acid, and malonic acid, respectively, support only cationic form in the excited-state. DES with glycerol also supports only the cationic form in the excited-state, DES with ethylene glycol, acetamide, and 1,4-butane-diol support both cationic and neutral forms. The DES with urea as the HBD is the only DES that supports only the neutral form in the exited-state. Excited-state intensity decay data confirms the presence of different prototropic forms of norharmane within investigated DESs with distinct decay times for the neutral and cationic forms. Correlation between the common empirical solvent polarity parameter, ETN, along with Kamlet-Taft parameters, H-bond donating acidity (α) and H-bond accepting basicity (β), of the DESs and the relative presence of the norharmane prototropic forms is established.