Browsing by Author "Deneva V."

Now showing 1 - 2 of 2
  • No Thumbnail Available
    Item
    Chercher de l'eau: The switching mechanism of the rotary switch ethyl-2-(2-(quinolin-8-yl)hydrazono)-2-(pyridin-2-yl)acetate
    (2020-05-01) Deneva V.; Vassilev N.; Hristova S.; Yordanov D.; Hayashi Y.; Kawauchi S.; Fennel F.; Völzer T.; Lochbrunner S.; Antonov L.
    The E/Z switching mechanism of the rotary switch ethyl-2-(2-(quinolin-8-yl)hydrazono)-2-(pyridin-2-yl)acetate was studied by NMR, UV–Vis, and ultrafast spectroscopy and modeled by advanced quantum-chemical calculations. Three possible mechanisms were considered theoretically: out-of-plane rotation, in-plane inversion and proton transfer. Neither of them correctly describes the experimental data by using implicit solvation. Taking into account the existence of water in the used solvents, which influence the measured rate constants, an explicit solvation was attempted in the quantum-chemical calculations. According to a simplified model, the water molecules form a wire, being able to transfer the proton from Z to E form of the switch. This leads to substantially lower transition states and corresponds to the experimentally observed rate constants. This information shines new light on the mechanism of isomerization in the rotary switches and on the understanding of the mechanism of hydrazone tautomerism as a whole.
  • No Thumbnail Available
    Item
    Indirect solvent assisted tautomerism in 4-substituted phthalimide 2-hydroxy-Schiff bases
    (2020-08-15) Yordanov D.; Deneva V.; Georgiev A.; Crochet A.; Fromm K.M.; Antonov L.
    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 (KT 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.