Unexplored Isomerization Pathways of Azobis(benzo-15-crown-5): Computational Studies on a Butterfly Crown Ether

Abstract

Computational studies on trans → cis and cis → trans isomerizations of photoresponsive azobis(benzo-15-crown-5) have been reported in this work. The photoexcited ππ* state (S 2 ) of the trans isomer relaxes through the planar S 2 minimum and the planar S 2 /S 1 conical intersection (both situated around 9 kcal/mol below the vertically excited S 2 state) arising along the N�N stretching coordinate. The nπ* state (S 1 ) of this isomer has both planar and rotated (clockwise and anticlockwise) minima, which may lead to a torsional conical intersection (S 0 /S 1 ) geometry having a <CNNC dihedral angle value close to 90°. This rotational isomerization path is found to have an energy barrier. It has been noticed that an N−N-out-of-plane motion coupled with the torsion can bring down the barrier and may facilitate the isomerization process. On the other hand, the vertically excited S 1 state of the cis-isomer undergoes a barrierless path to reach a torsional conical intersection (S 0 /S 1 ) geometry (<CNNC = 88°), responsible for the trans-isomer formation. The cis-S 2 /S 1 CI (conical intersection) torsional geometry (<CNNC = 45°) is situated around 16 kcal/mol above the vertically excited ππ* state of the cis-isomer and not reachable on S 2 photoexcitation. The two optimized torsional S 2 minima of this system are close to the S 1 surface and seem to be involved in the S 2 −S 1 internal conversion. A less efficient concerted inversion photoisomerization path through a linear geometry has also been identified. The thermal cis → trans isomerization has been found to undergo an inversion motion, which forms a less stable trans isomer. Possible differences between the isomerization channels of this azobis(benzo-15-crown-5) and the unsubstituted azobenzene system have also been highlighted in the work.