HARMONIA Results

  

Obtaining and then observing and describing the properties of rare tautomeric forms turned out to be possible. This goal was achieved in the following ways:

 

(i) using light, so that tautomerization took place in an excited state;

  

(ii) placing the molecule on the surface of the metal, which significantly stabilized the forms which in an isolated, non-interacting molecule have much higher energy;

   

(iii) synthesizing compounds in which a properly placed substituent selectively lowered the energy of one of the rare tautomers.

 

The research carried out during the project, especially the measurements of single molecules, allowed to conclude that the dominant mechanism of tautomerism is tunnelling. While this conclusion is obvious in the case of very low temperatures, the demonstration of a significant contribution from room temperature tunneling may be considered an important achievement. This is related to the demonstration that the rate of a chemical reaction (double or single hydrogen transfer) can be very strongly slowed down by a slow-relaxing environment (polymer as opposed to solution).

  

An important result of a general nature was proving, for a large series of porphycenes, the correlation between the strength intramolecular hydrogen bonding and fluorescence quantum yield. It is worth noting that this relationship it is valid over a very large range: four orders of magnitude of quantum efficiencies.

 

The above correlation is related to the demonstration that the quantum yield of weakly emitting porphycenes is very strongly dependent on the viscosity of the environment and can be greatly enhanced by increasing it. At the same time, fluorescence quantum yields  do not depend on the polarity of the solvent. Therefore, porphycenes seem to be great candidates for their use as viscosity probes.

  

A result with application potential is the detection - for both porphycene and hemiporphycene - of tautomerization controlled by the relaxation dynamics of the polymer environment. This finding makes it possible to use both of these porphyrin isomers to track, in a wide time range, the dynamics of polymer relaxation.

  

An observation giving a chance for application is the demonstration of the possibility of conversion between various tautomeric forms by using an external factor, e.g.,  light  or a stream of electrons. Such behaviour suggests a switch design, and in cases where the two tautomeric forms do not interconvert without the outer one stimulation, including molecular memories.

  

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