Prototropic behavior of naphthalene derived probes in deep eutectic solvents

Abstract

Deep eutectic solvents (DESs) have garnered widespread acceptance as novel and biodegradable substitutes to the volatile organic solvents and ionic liquids in both biological and chemical sciences. The prototropic equilibria of fluorescent probes is known to be largely controlled by its surrounding milieu. The prototropic behavior of four naphthalene derivatives: 1-naphthol, 2-naphthol, 1-naphthylamine, and 1,8-bis(dimethylamino)naphthalene (the proton sponge) is investigated within five different DESs constituted of choline chloride (ChCl) as a common H-bond acceptor (HBA) with five different H-bond donors (HBDs). UV–vis molecular absorption and steady-state fluorescence spectroscopies along with time-resolved fluorescence measurements are used to investigate the ground-state and excited-state prototropic behavior of these four probes dissolved in the five DESs. For 2-naphthol, neutral form is the only absorbing and emitting form observed in all the investigated DESs. In the ground-state, 1-naphthol is predominantly present in its neutral form within all the investigated DESs. However, both neutral and anionic forms of 1-naphthol are supported in the excited-state of ChCl:Urea DES whereas neutral form remains the preferred form in other DESs. In all the investigated DESs, the neutral form of 1-naphthylamine is the only prominent form in both ground- and excited-state. The cationic form of 1,8-bis(dimethylamino)naphthalene remains the dominant prototropic form in ground- as well as excited-state in all the investigated DESs upholding the high photo-basicity of this probe in DESs as well.