[en] This work was focused on the study of organic photochromic compounds, dithienylethenes (DTEs), using computational chemistry methods. The density functional theory method (DFT) with 6-31G(d), as basis set, was used to study the ground state, and the conductor-like polarizable continuum model (CPCM) -implicit solvation model- was used to simulate solvents, mainly chloroform. Excitation and emission processes were studied using time-dependent density functional theory (TDDFT). All calculations were carried out with the suite of programs Gaussian09. Those quantum chemistry calculations allowed the study of geometries, kinetic, thermodynamic and photophysical parameters belonging to the closed and open forms (ground and excited states) of DTEs: [3-[thiophen-3-yl]-4-(thiophen-3-yl)-1H pyrrole- 2,5-dione (1), [2-methylthiophen-3-yl]-4-(2-methylthiophen -3-yl)-1H pyrrole-2,5-dione (2) and 1-Benzyl-3-[2-methyl-5-(naphthalen-1-yl)thiophen-3-yl]-4-(2-methyl-5- phenylthiophen -3-yl)-1H pyrrole-2,5-dione (3)], and to their interconversion equilibria (closed ⥂ open). The obtained results suggest the equilibrium is displaced towards the open form, the lower the electric permittivity of the solvent the higher the equilibrium constant. UV-Vis spectral results reasonably match the empirical observations. The presence of substituents increases the equilibrium constant (closed ⥂ open) several orders of magnitude. Although it was not possible the calculation of the thermal transition state between the closed and the open form, a crude estimation of the activation energy suggests the photochemical nature of the ring opening/closure reaction of the studied dithienylethenes.

[gl] Este traballo centrouse no estudo de compostos orgánicos fotocrómicos, ditieniletenos (DTE), usando métodos de química computacional. O método da teoría de densidade funcional (DFT), con 6-31G(d) como funcións base, foi usado para estudar o estado fundamental, e o modelo de condutor continuo polarizable (CPCM) –modelo de solvatación implícito- foi empregado para simular o disolvente, principalmente cloroformo. Os procesos de excitación e emisión foron estudados mediante a teoría de densidade funcional dependente do tempo (TD-DFT). Todos os cálculos foron levados a cabo usando o programa Gaussian09. Os cálculos de química cuántica permitiron estudar a xeometría, a cinética, a termodinámica e determinar os parámetros fotofísicos pertencentes ás formas aberta e pechada (estados fundamental e excitados) dos DTEs: [3-tiofen-3-il]-4-(tiofen-3-il)-1H pirrol-2,5-diona (1), [2-metiltiofen-3-il]-4-(2 metiltiofen-3-il)-1H pirrol-2,5-diona (2) e 1-bencil-3-[2-metil-5-(naftalen-1-il)tiofen-3-il]-4-(2-metil-5-feniltiofen-3-il)-1H pirrol- 2,5-diona (3), e para o seu equilibrio de interconversión (pechada ⥂ aberta). Os resultados obtido suxire que o equilibrio desprazase cara a forma aberta, e que canto menor é a permitividade eléctrica do disolvente maior é a constante de equilibrio pechada ⥂ aberta. Os resultados dos espectros UV-VIS coinciden razoablemente coas observacións empíricas. A presenza de substituíntes aumenta a constante de equilibrio (pechada ⥂ aberta) en varios ordes de magnitude. Aínda que non foi posible calcular o estado de transición térmico entre a forma pechada e aberta, unha crúa estimación da enerxía de activación confirma a natureza fotoquímica da reacción de apertura/peche nos compostos ditieniletenos estudados.