A computational investigation on the counterion-aided thermal and photo isomerizations of potassium ion-coordinated butterfly crown ether

Abstract

This computational study has captured thermal and photo cis → trans isomerization mechanisms of potassium ion-coordinated azobis(benzo-15-crown-5) aided by counter anions. Earlier, experimental studies on this crown ether (here mentioned as AZB15C5) have reported its alkali metal ion transport feature across the organic liquid membrane phase (primarily composed of o-dichlorobenzene), which involves this isomerization. In these studies, the formation of the trans isomer from cis was mentioned as the prerequisite for the metal ion release from the mentioned organic phase to the aqueous phase. Our study reveals that in the absence of any counter anion, the metal ion complex (AZB15C5-K+) in a sandwich form is the more stable cis-form than the open-cis form (a more open structure). However, the approach of a counter anion such as picrate (AZB15C5-K+-pic, where pic = picrate) makes these two forms equally stable, and the open-cis form is found to be responsible for the cis → trans isomerization. Consequently, thermal and photo isomerization studies have been performed with this particular isomeric form. The calculated rate constants of the thermal isomerization in the presence of picrate and other counter anions are in line with the reported experimental observations. Under S0 → S1 photo-excitation (in the visible region), the open-cis form undergoes photo-isomerization through a low-lying S0/S1 conical intersection (CI), leading to the trans isomer formation. This torsional CI geometry (AZB15C5-K+-pic-CIS1/S0) is located at 5 kcal/mol below the vertically excited S1 state, and it is characterized by a central dihedral angle (<CNNC) value close to 90°. The cis → trans photo-isomerization path has been tracked through the geodesic interpolation studies.