Intra- and InterMolecular Processes

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 Institute of Organic Chemistry


Two are the fields we work in:
Nonbonding interactions: We are interesting how the formation of aggregates influences the optical and tautomeric properties of dyes. The simplest case - monomer-dimer equilibrium - is usually considered in the frame of Kasha's (exciton) theory. According to it two kinds of dimers are formed: sandwich (H-type, a/c in the picture below) and head-to-tail (J-type, b/d in the picture below). In the practice the usual problem is to find the spectrum of the dimer and to calculate the structural parameters (distance between the monomeric units R and angle between the monomer transition moments a). This can be solved by number of methods, one of them is our Fishing-Net-Algorithm. More complicated is to study how the dimer formation can influence the relative stability of the tautomers in a tautomeric mixture, which we do in the case of azodyes, Schiff bases and heterocyclic compounds.
Design of macrocyclic ligands: For couple of years we have tried to develop various aza-15-crown-5 based ligands. In most of the cases the azacrown was directly conjugated to the chromophore system, which leads to reduced electron density at the nitrogen atom (and as a result - low stability constants of the complexes obtained) and a blue shift of the absorption maximum under complexation (and as a result - limited applicability). Then we decided to exploit the tautomerism as signal converter using the fact that the tautomeric forms have different colors. The idea is shown below. The off-state is achieved through engagement of the tautomeric hydrogen using the hydrogen bonding ability of an antenna, which is connected to the tautomeric unit by a spacer. The engagement of this antenna by external stimuli (like addition of acid or metal ions) causes change in the tautomeric state, switching to the on-state. When the action of the stimuli is terminated the tautomeric equilibrium returns back to the off-state.
With this idea in mind we have developed a ligand, where an aza-15-crown-5 moiety is used as antenna and 1-phenylazo-4-naphthol as signaling unit. This compound gives red shift under complexation and the stability constants (alkali and alkaline-earth metal ions) are fairly high. This is not surprising, because the metal ion interacts better with the carbonyl oxygen, shifting the equilibrium towards the keto tautomer. On the other side, the keto carbonyl group plays a role of an active side-arm, supporting the process of complex formation.

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)
  • Tautocrowns: a concept for a sensing molecule with an active side-arm. L.Antonov, V.Kurteva, S.Simeonov, V.Deneva, A.Crochet & K.M.Fromm; Tetrahedron, 66, 4292 (2010)
  • Gas phase study of molecular switches based on tautomeric proton transfer. D.Nedeltcheva, V.Kurteva & L.Antonov; European Journal of Mass Spectrometry, 17, 47 (2011)
  • Complexation properties of Schiff bases containing N-phenylaza-15-crown-5 moiety. L.Antonov, M.Vladimirova, E.Stanoeva, W.M.F.Fabian, L.Ballester & M.Mitewa; Journal of Inclusion Phenomena, 40, 23 (2001)
  • UV-Vis spectroscopic and chemometric study on the aggregation of ionic dyes in water. L.Antonov, G.Gergov, V.Petrov, M.Kubista & J.Nygren; Talanta, 49, 99 (1999)
  • 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