Exploring the isomerization paths of push–pull hexatrienes

This computational study is an attempt to reveal the mechanism of the isomerization processes happening in donor (D)-acceptor (A) hexatriene systems. The photo-excitation of all-trans isomers of these conjugated systems with terminal donor (amine, methoxy) and acceptor (cyano) groups populates the first (S-1) and second (S-2) singlet excited states which correspond to transitions with reasonably high oscillator strength values. The S-1 state of the amine (D), cyano (A)-substituted system forms a twisted and couple of slightly off-planar minima connected by low-energy transition states with configurations equally dominated by HOMO -> LUMO and HOMO2 -> LUMO2 excitations. Two important low-lying S-0/S-1 conical intersections have been identified in this system at 7-8 kcal mol(-1) and 8-11 kcal mol(-1) above the twisted excited state minima. The first one has been identified as the source of a cis-trans-trans isomer while the latter one may be responsible for the trans-cis-trans isomer. In comparison, the presence of a weaker donor group (methoxy) produces a more stable cis-trans-trans isomer from a lower energy S-0/S-1 conical intersection, situated around 20-23 kcal mol(-1) below the vertically excited geometry. Both the isomers have an alternate thermal route of formation from the all-trans isomer through ground state transition states with activation energy values close to 50 kcal mol(-1).