Resumo:
Spiropyrans constitutes one of the most well-known and well-studied class of
chromophores nowadays. This occurs since this class of compound undergoes a reversible
isomerization which alters several of its properties, colors mainly, when submitted to a series of
different stimuli, such as light, temperature, solvent, among others. Such versatility allowed
spiropyrans to be utilized in several fields: optical storage, molecular switches, sensors.
Herein, Density Functional Theory (DFT) level calculations of the thermal isomerization
reaction SP→MC, where SP is the substituted 3-(3',3'-dimethyl-6-nitrospiro[chromene-2,2'-
indole]-1'-yl) propanoic acid spyropiran in the closed-form and MC (merocyanine) is the
corresponding open form are presented. Calculations were carried out using the CAM-B3LYP
functional with the 6-31G(d,p) basis set. For the most stable conformers of the MC, an Atoms in
Molecules (AIM) analysis was carried out to evaluate non-covalent intramolecular interactions.
The influence of the carboxyl acid substituent group was evaluated through structural scan
calculations. The solvents were treated implicitly using the Intrinsic Equation Formalism
Polarizable Continuum Model (IEFPCM) method and by applying an electrostatic field upon the
structure.
The adopted methodology unveiled structural and thermodynamic parameters of the SP
form and the several MC conformers both in the gas phase and in solution with the usage of a
series of different solvents, their influence was computed within the analysis of properties such as
bond length alternance (BLA), Gibbs Free Energy and dipole moment as a function of the dielectric
constant of the solvents. A reactional mechanism proposal was made for the thermal pathway in
the SP → MC ring-opening conversion. The NCI method allowed the elucidation of the relative
stability of the different MC conformers in terms of intramolecular interactions and the results
were in great agreement with experimental and theoretical data.
Some of the obtained results provided a greater understanding of the conversion between
the SP and MC isomers and extended the discussions upon the behavior of these structures in
solution and the impact of the solvent effect in several of their properties. The applied electrostatic
field upon the MC molecule unveiled a solvatochromic reversal in the UV-Vis spectrum and others
electronic effects and structural changes.
