Quantum-Chemical Investigation of Molecular Structure and Thermodynamic Properties of Spiropyran Molecules
Authors: Govorov Dmitrii N. , Kudin Lev S. , Dunaev Anatoliy M. 
Published: February 2020

Spiropyrans are heterocyclic organic compounds known for their photochromic properties which are of perspective use for technology. They can be potentially used as the elements of molecular machines, optical data recording, modulated light filters and numerous hybrid materials. The key characteristic of such compounds is an ability to enter a photochromic reaction, taking place upon irradiation with UV light (see Figures 1 and 2). To develop more compounds which can possibly be of practical use the most important problem is to investigate their chemical properties both theoretically and experimentally.
To determine the optimal geometric structures for both spiro- and mesocyanine forms of 1,3,3,6’-tetramethyl-8’-formylsporoindoline-2,2’-[2H]-chromene-3 and 3,8’-dimethyl-6’formylspiro-(-4-oxo-3,4-dihydro-2H-1,3)-benzoxasine-2,2’-[2H]-chromene-7 (objects 1 and 2 respectively) using the DFT approach, calculations of quantum chemistry were used. All the computations were carried out in GAMESS software. In this research we used B3LYP, CAM-B3LYP, wB97, M11 and TPSSh functionals in conjunction with 6-311++G(d,p) basis set. In our studies we were able to compare experimental gas-phase IR spectra with the quantum-chemically computed one. This allowed us to consider the TPSSh functional as the best model for these compounds due to the most accurate geometrical structures and thermodynamic functions obtained with its use. In addition, it was possible to compute the photoinduced products of the isomerization reaction (see Figure) of our objects and their UV/Vis spectra which were also compared with the experimental spectra of thin films. Comparison allowed us to consider high accuracy of computed data.
However, an attempt to find the optimal molecular structure for the cis-isomer of the second object was unsuccessful. Except for the B3LYP functional, none of the listed above could provide us with the reliable structure and, therefore, other properties. For this reason, it was only possible to compute the reliable reaction enthalpies for the one precursor molecule.
The reaction enthalpies listed in the Table 1 are considered to be optimal for using such a compound as a photosensitive switch element because it is bigger than 10 kJ/mole but no more than 100 kJ/mole. From this it follows, that spontaneous switching probability is acceptably small, although at the same time such an amount of energy is practically feasible.

Reaction

∆r

H(298.15 K), kJ/mol

1
56.39

2
-11.78

3
-68.17

References:

S. Lukyanov and M. B. Lukyanova, Chemistry of Heterocyclic Compounds, Vol. 41, No. 3, 2005.

"General Atomic and Molecular Electronic Structure System" M. W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert, M. S. Gordon, J. H. Jensen, S. Koseki, N. Matsunaga, K. A. Nguyen, S. Su, T. L. Windus, M. Dupuis, J. A. Montgomery Comput. Chem., 14, 1347-1363(1993).

"Advances in electronic structure theory: GAMESS a decade later" M. S. Gordon, M. W. Schmidt pp. 1167-1189, in "Theory and Applications of Computational Chemistry: the first forty years" C. E. Dykstra, G. Frenking, K. S. Kim, G. E. Scuseria (editors), Elsevier, Amsterdam, 2005.

Figure 1