Structure of the Conical Intersections Driving the cis–trans Photoisomerization of Conjugated Molecules¶

Abstract

High-level ab initio calculations show that the singlet photochemical cis-trans isomerization of organic molecules under isolated conditions can occur according to two distinct mechanisms. These mechanisms are characterized by the different structures of the conical intersection funnels controlling photoproduct formation. In nonpolar (e.g. hydrocarbon) polyenes the lowest-lying funnel corresponds to a (CH)3 kink with both double and adjacent single bonds twisted, which may initiate hula-twist (HT) isomerization. On the other hand, in polar conjugated systems such as protonated Schiff bases (PSB) the funnel shows a structure with just one twisted double bond. The ground-state relaxation paths departing from the funnels indicate that the HT motion may take place in nonpolar conjugated systems but also that the single-bond twist may be turned back, whereas in free conjugated polar molecules such as PSB a one-bond flip mechanism dominates from the beginning. The available experimental evidence either supports these predictions or is at least consistent with them.

Assessment of approximate computational methods for conical intersections and branching plane vectors in organic molecules

“…It has a transoid MECI with a -(CH 3 )-kink of the carbon backbone. 4,122 Electronically, this transoid MECI (see Figures 3 and 9) contains a kinked allylic fragment coupled to a terminal methylene radical. It was originally believed to provide the major non-adiabatic decay channel, 4, 122 but this conjecture has recently been challenged.…”

Direct evidence for hula twist and single-bond rotation photoproducts

“…In line with this interpretation is the observed formation of all three possible photoproducts (although in varying efficiencies) after irradiation of A , B , C , and D , which is indicative for a kinked conical intersection 60 , 61 , 63 , 64 . As solvent polarity increase favors the HT product in our case, a polar ionic conical intersection leading to sole DBI (usually observed in permanently charged systems 62 , 65 ) is likely to be absent in our system. Here the found solvent dependence could be explained by the involvement of an ionic charge-transfer state perturbing the photochemically active state and its decay to the ground state 65 , 66 .…”

“…To speculate, the observed strong temperature dependence of the photoreactions of B could be interpreted in terms of a hot-ground state mechanism 59 . On the other hand, a diabatic process where the product distribution is determined earlier by a conical intersection 60 – 62 is also possible. In line with this interpretation is the observed formation of all three possible photoproducts (although in varying efficiencies) after irradiation of A , B , C , and D , which is indicative for a kinked conical intersection 60 , 61 , 63 , 64 .…”

Photoisomerization by Hula Twist: 2,2′‐Dimethylstilbene and a Ring‐Fused Analogue

“…These results reinforce the suggestion that all photoisomerizations of polyenes in rigid media (pre‐vitamin D in organic glass,5 1,3‐butadiene in argon matrix,11 1,3,5,7‐octatetraene in an n ‐octane matrix,12 and selected cases of diarylethylenes in organic glasses13) proceeded by way of HT 1. However, at the same time, we recognize that the current results do not necessarily rule out the possibility that the isomerization at room temperature could also proceed by way of HT 14. The fluid medium and the higher temperature would complicate detection of the unstable conformer.…”