Análise teórica da influência do solvente em propriedades estruturais, termodinâmicas e eletrônicas de merocianinas substituídas e seus isômeros

Abstract

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.