Nanoscale physics

Transformation mechanisms of an excited molecule on the spiropyran-merocyanine path
P.O.Kondratenko1,  O. A. Kovalenko2, Yu.M. Lopatkin2, T.N.Sakun1, A.Marinchenko1
1  Department of Theoretical Physics, Aerospace Institute, National Aviation University, Kosmonavt Komarov Prosp., 1, Kiev 03680, Ukraine

2 Department of General and Theoretical Physics, Sumy State University, Rymsky-Korsakov St., 2,  Sumy  40007, Ukraine 
E-mail: olya.sumy.ua@gmail.com

In this work results of a quantum-chemical study of the transformation mechanisms between spiropyran and merocyanine molecules (Sp ↔ Mc) in the excited state are presented.
  
Scheme 1. General structures of spiropyran and merocyanine molecules.
The research was carried out using MOPAC program. Route of thermal Sp-Mc transformation was found and structures of molecules with different Cspiro-O distances were obtained. The energy structure of intermediate molecules was calculated for a fixed rotation of the phenyl group around the C(27) –C(28) axis using MNDO and AM1 methods. As a result, potential surfaces of excited states on the path of Sp-Mc photochemical transformation were obtained.
The structure of potential surfaces was analyzed, and it was shown that, in the case of merocyanine, it was formed due to the intersection of two surfaces. As a consequence, such a structure of potential surfaces assists to the rapid cis-trans isomerization of the Mc molecule. In the case of the Sp molecule, there is a significant limitation on the oscillatory processes in the area of Cspiro atom localization. However, excitation of the Sp molecule leads to the possibility of lengthening of the Cspiro-O bond, since the potential surface of the S1 state appeared to be dissociative on the Cspiro-O bond. The potential surface of the S0 state has a high barrier (~ 1 eV), caused by the need of sp3 → sp2 rehybridization of AO of the spiroatom. It was shown that the reverse process of photochemical Mc-Sp transformation occurs from the configuration of Mc-TTC. The excitation of the Mc-TTT configuration assists only to the isomerization to the Mc-TTC.