Indirect solvent assisted tautomerism in 4-substituted phthalimide 2-hydroxy-Schiff bases

Dancho Yordanov; Vera Deneva; Anton Georgiev; Aurelien Crochet; Katharina M Fromm; Liudmil Antonov

doi:10.1016/j.saa.2020.118416

Indirect solvent assisted tautomerism in 4-substituted phthalimide 2-hydroxy-Schiff bases

Spectrochim Acta A Mol Biomol Spectrosc. 2020 Aug 15:237:118416. doi: 10.1016/j.saa.2020.118416. Epub 2020 Apr 27.

Authors

Dancho Yordanov¹, Vera Deneva¹, Anton Georgiev², Aurelien Crochet³, Katharina M Fromm³, Liudmil Antonov⁴

Affiliations

¹ Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria.
² Department of Organic Chemistry, University of Chemical Technology and Metallurgy, 1756 Sofia, Bulgaria; Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria. Electronic address: antonchem@abv.bg.
³ Department of Chemistry and Fribourg Center for Nanomaterials FriMat, University of Fribourg, CH-1700 Fribourg, Switzerland.
⁴ Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria; Institute of Electronics, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria. Electronic address: lantonov@orgchm.bas.bg.

PMID: 32388169
DOI: 10.1016/j.saa.2020.118416

Abstract

The paper presents the synthesis and characterization of two 4-substituted phthalimide 2-hydroxy-Schiff bases containing salicylic (4) and 2-hydroxy-1-naphthyl (5) moieties. The structural differences of 2-hydroxyaryl substituents, resulting in different enol/keto tautomeric behaviour, depending on the solvent environment were studied by absorption UV-Vis spectroscopy. Compound 5 is characterized by a solvent-dependent tautomeric equilibrium (K_T in toluene = 0.12, acetonitrile = 0.22 and MeOH = 0.63) while no tautomerism is observed in 4. Ground state theoretical DFT calculations by using continuum solvation in MeOH indicate an energy barrier between enol/keto tautomer 5.6 kcal mol^-1 of 4 and 0.63 kcal mol^-1 of 5, which confirms the experimentally observed impossibility of the tautomeric equilibrium in the former. The experimentally observed specific solvent effect in methanol is modeled via explicit solvation. The excited state intramolecular proton transfer (ESIPT) was investigated by steady state fluorescence spectroscopy. Both compounds show a high rate of photoconversion to keto tautomers hence keto emissions with large Stokes shifts in five alcohols (MeOH, EtOH, 1-propanol, 1-butanol, and 1-pentanol) and various aprotic solvents (toluene, dichlormethane, acetone, AcCN). According to the excited state TDDFT calculations using implicit solvation in MeOH, it was found that enol tautomers of 4 and 5 are higher in energy compared to the keto ones, which explains the origin of the experimentally observed keto form emission.

Keywords: 2-hydroxy-Schiff bases; Crystallography; DFT; ESIPT; Enol/keto tautomerism; Phototautomerization; Phthalimides; UV–Vis spectroscopy.