Prototropic behavior of naphthalene derived probes in deep eutectic solvents

• Published in: Journal of photochemistry and photobiology. A, Chemistry. Chemistry., 2022-05, Vol.427, p.113798, Article 113798
• Main Authors: Khokhar, Vaishali, Deepika, Pandey, Siddharth
• Format: Article
• Language: English
• Subjects: Deep Eutectic Solvents; Emission Spectroscopy; Lifetime; Proton Transfer; Uv–vis absorbance
• ISSN: 1010-6030; 1873-2666
• DOI: 10.1016/j.jphotochem.2022.113798

[Display omitted] •Naphthalene derived probes are investigated within 5 deep eutectic solvents (DESs).•Role of different Hydrogen bond donors (HBDs) of DESs on prototropism is assessed.•Neutral form of 2-naphthol and 2-naphtylamine are supported in all the DESs.•Urea-based DES supports anionic and neutral form of 1-naphthol in excited-state.•Cationic form of 1,8-bis(dimethylamino)naphthalene is present within all the DESs. 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.