Intra- and InterMolecular Processes

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 Institute of Organic Chemistry


First tautomeric compounds have been reported more than 120 years ago. But even now, the study of the tautomerism and proton transfer mechanism remains a challenge. Tautomeric processes are in the basis of the information transfer/storage in the living organisms and tautomeric systems are used as photoprotectors, molecular switches, laser dyes, optical sensors, etc.
We have concentrated our efforts to investigate equilibrium tautomeric systems, i.e. the tautomers co-exist in solution. They cannot be experimentally separated and analyzed conventionally. Due to the methods for quantitative analysis of undefined mixtures we have developed many tautomeric systems have been investigated quantitatively and structure - tautomeric properties relations have been found.
Three are the main classes of tautomeric systems we have studied:
Keto-enol tautomerism in azodyes and Schiff bases: Important information about the effects of the substituents, strength of the intramolecular hydrogen bonding, solvents, temperature and acidity have been found by using conventional UV-Vis spectroscopy and advanced chemometrics, steady-state and time-dependent fluorescence, quantum chemistry and linear solvation energies relationships. As a result, using the accumulated knowledge, we have developed a tautomeric molecular switch, which allows directed shift in the position of the tautomeric equilibrium in 1-phenylazo-4-naphthol under addition of acid or metal ions.
Below the controlled shift in the equilibrium is shown. In the non-modified dye (left) the enol form is more stable with 1.4 kcal/mol. Always in solution a tautomeric mixture is observed. The addition of N-containing unit connected to the chromophore system by a spacer (middle) leads to full shift of the equilibrium towards the enol being stabilized through H-bonding with the nitrogen (the energy difference becomes 4 kcal/mol). Upon protonation (right), the protonated N-atom stabilizes the keto form (2 kcal/mol) as a result of new hydrogen bonding with the keto carbonyl oxygen.
Ammonium-azonium tautomerism in protonated amino azodyes: Depending on the structure and substituents a clear color change is observed under protonation, allowing these dyes to be used as pH-indicators.
Tautomerism, stability and aggregation of heterocyclic compounds: Here we have studied the tautomeric properties and stability of thiopyridines and pyrimidines and the concentration effects on the tautomerism in 2-aminobenzimidazole.

Basic papers:

  • Exploiting the tautomerism for switching/signaling purposes. L.Antonov, V.Deneva, S.Simeonov, V.Kurteva, D.Nedeltcheva & J.Wirz; Angewandte Chemie International Edition, 48, 7875 (2009)
  • Tautomerism in 1-phenylazo-4-naphthols: experimental results vs quantum-chemical predictions. L.Antonov, V.Kurteva, A.Crochet, L.Mirolo, K.Fromm & S.Angelova; Dyes & Pigments, 92, 714 (2012)
  • Chemometric models for quantitative analysis of tautomeric Schiff bases and azodyes. D.Nedeltcheva, L.Antonov, A.Lycka, B.Damyanova & S.Popov; Current Organic Chemistry, 13, 217 (2009)
  • Tautomerism in hydroxynaphthaldehyde anils and azo analogues: a combined experimental and computational study. W.M.F.Fabian, L.Antonov, D.Nedeltcheva, F.S.Kamounah & P.J.Taylor; Journal of Physical Chemistry, 108A, 7603 (2004)
  • Gradual change of one- and two-photon absorption properties in solution – protonation of 4-N,N-dymethylamino-4’-aminoazobenzene. L.Antonov, K.Kamada, D.Nedeltcheva, K.Ohta & F.S.Kamounah; Journal of Photochemistry and Photobiology, 181A, 274 (2006)
  • Aggregation of 2-aminobenzimidazole – a combined experimental and theoretical investigation. S.Angelova, M.Spassova, V.Deneva, M.Rogojerov & L.Antonov; ChemPhysChem, 12, 1747 (2011)


How a controlled shift in the position of the tautomeric equilibrium can be achieved. The story.....

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AG Antonov

Institute of Organic Chemistry with Center of Phytochemistry

Acad. G.Bonchev street, bl. 9

Sofia 1113, Bulgaria

fax: 359 2 8700225