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.

Excited state conformational dynamics in carotenoids: Dark intermediates and excitation energy transfer

“…The analysis by Olivucci, Robb, Schlegel, and coworkers [27,[36][37][38][39][40] of the potential energy surfaces of models for retinal protonated Schiff bases (PSBs) and the structurally related cyanines emphasizes the crucial role of the barrier between planar and twisted conformations on the excited state dynamics. The gradients of the potential energy surface near the Franck-Condon geometry are effectively controlled by the conjugation length [27].…”

“…Torsional gradients encountered near the planar minimum result in an intramolecular redistribution of the Franck-Condon structure's momentum towards twisted conformations, which promotes crossing of the activation barrier and launching of trajectories leading to the conical intersection [37]. The fluorescence of cyanines and PSBs is largely quenched by crossing to the ground state, either with isomerization or by returning to the original ground state minimum [27,[38][39][40].…”

Computational and Spectroscopic Characterization of the Photocycle of an Artificial Rhodopsin

“…At this point, it is important to recall information on two important CIs related to the photoisomerization of conjugated polyene systems. − The first is a crossing between two states having 1A g and 1B u electronic structures (1B u /1A g crossing featuring a single 90° twisted CC bond) and is found in cationic conjugated systems such as rPSB. The second, a crossing between 2A g and 1A g like states (2A g /1A g crossing, also called a (CH) 3 -kinked CI) is found in nonpolar systems …”

Constraint Trajectory Surface-Hopping Molecular Dynamics Simulation of the Photoisomerization of Stilbene

“…The vibrational normal-mode analysis with specified PES involving the steep route indicated that the vibrational mode with frequency 240 cm −1 also strengthens the OBF mechanism [54]. Fuß and coworkers [43,55,56] suggested that the HT and the aborted HT mechanisms were deduced from the systematic features of the cis-and trans-photoisomerizations of nonpolar conjugated molecules and these were investigated by the experimental measurements as well [58][59][60].…”

“…The isomerization of cis-and trans-Stilbene has been extensively studied for more than forty years and the quantum yields of the photoisomerization were measured experimentally [39,40]. Three mechanisms were characterized for the isomerization; they are the conventional one-bond flip (OBF) [41], Hula-Twist (HT) [42], and the aborted HT mechanisms [43]. The OBF mechanism involves a 180 ∘ rotation around the central ethylenic C=C bond.…”