Energy Structure and Stability of Merocyanine Isomers as Memory Elements or Switches

Creation of a stable and reliable molecular device is an important task of nanotechnology. Photochromic molecules are the most promising candidates for such a purpose. One of the most prospective classes of photochromic compounds is spiropyran and its open form merocyanine. To use the spiropyran-mericyanine system it is essential to know properties of these molecules. The transformation between spiropyran and merocyanine molecules can occur in different ways. Therefore, it is important to have a complete picture of the merocyanine structures and transition routes between them. Thus, the investigation of geometric structure of the merocyanine molecule was carried out. It shows that the merocyanine molecule can be in one of the eight possible isomers taking into account the rotation around the three central bonds C(25)-C(26), C(26)-C(27), C(27)-C(28). TTT, TTC, CTT and CTC isomers have a flat geometric structure and similar absorption spectra (from 490 nm to 510 nm), thus they cannot be separated in experimental studies. All these isomers exhibit dark stability, since the energy barrier for the transition between them is more than 1 eV. The other four isomers TCT, TCC, CCT and CCC have a spiral structure, at which each of these isomers can have right-handed or left-handed structure. Transformations between neighboring isomers have been studied. The results have shown that spiral structure isomers are thermally unstable, and they turn into one of the flat isomers rapidly. That is why their contribution to the absorption spectrum of the merocyanine molecule is absent. At the same time, all flat structures are characterized by identical conductivity properties along the π-electron system and the same length of the long axis of the molecule. The obtained results are important in terms of using the spiropyran-merocyanine system as a memory element or switch in future computer systems.