Photoisomerization and thermal reversion of 5-arylmethylene-2-thioxoimidazolidin-4-one

JULIETA PEPINO; MAXI A. BURGOS PACI; WALTER PELAEZ; GUSTAVO A ARGÜELLO

Santiago de Chile

Congreso; 10th Congress of the World Association of Theoretical and Computational Chemists (WATOC 2014); 2014

Photoinduced isomerization Z/E (cis/trans) reactions are elementary processes of fundamental importance in biological photoreceptors. These kind of reactions are the mechanism of choice to trigger large amplitude motion in molecular photo-switches and used to photo-induce peptide folding, control ion complexation or gate the transport properties of ion channels. [1] Thus, looking for new molecules with the possibility of photoisomerization and understanding the details of the reaction mechanism has become of great importance. To the best of our knowledge, there is no study on the Z/E isomerization mechanism of 5-arylmethylene-2-thioxoimidazolidin-4-one (1). The pharmaceutical activities such as antimycobacterials, immunomodulators, anticonvulsivants and antifungal [2] presented by the 5-arylmethylene-2-thioxoimidazolidin-4-one nucleus have motivated the development of new methods for studying their synthesis and reactivity. It is known that they can exist in their Z and E forms, where Z is the thermodinamically favored, but there are no studies devoted to explain that stability and the mechanism of conversion. In this work, we studied the photochemical isomerization Z→E at 355 nm and the thermal reversion (TR) of 1 (figure 1) supporting our experimental results with ab initio calculations at the CASSCF level. The reaction coordinate was explored in the ground and excited states (S0, S1, S2 and T1). The results lead us to propose a first excitation to S2 state of π← π* character, followed by a conical intersection to the S1 (n←π* character) where the izomerization through a new conical intersection S0/S1 occurs. For thermal reversion three different mechanisms were suggested. From the energy analysis of the different channels it is concluded that TR involves the formation of a birradical intermediate that connects S0 and T1 states by an intersystem crossing (IRC).