[10]annulene: Electrocyclization mechanisms.

The mechanism of 6-π electrocyclization of all-cis, mono-trans, and double-trans [10]annulene to yield 4a,8a-dihydronaphthalene has been explored at various quantum-chemical methods. The mono-trans configuration cyclizes preferentially to trans-4a,8a-dihydronaphthalene, in agreement with the experimental results. The cyclization of the all-cis configuration requires firstly a bond-shifting to the naphthalene-like conformation of double-trans [10]annulene, which is the rate-limiting step, and finally its azulene-like conformation electrocyclizes quickly to cis-4a,8a-dihydronaphthalene. Its experimental rate coefficient is consistent with the computed one for the cyclization of the all-cis configuration, unlike the calculated one for the double-trans configuration. These results confirm the configurations assigned by Masamune et al. to the two isomers which they isolated.

Optomechanical Control of Quantum Yield in Trans-Cis Ultrafast Photoisomerization of a Retinal Chromophore Model.

The quantum yield of a photochemical reaction is one of the most fundamental quantities in photochemistry, as it measures the efficiency of the transduction of light energy into chemical energy. Nature has evolved photoreceptors in which the reactivity of a chromophore is enhanced by its molecular environment to achieve high quantum yields. The retinal chromophore sterically constrained inside rhodopsin proteins represents an outstanding example of such a control. In a more general framework, mechanical forces acting on a molecular system can strongly modify its reactivity. Herein, we show that the exertion of tensile forces on a simplified retinal chromophore model provokes a substantial and regular increase in the trans-to-cis photoisomerization quantum yield in a counterintuitive way, as these extension forces facilitate the formation of the more compressed cis photoisomer. A rationale for the mechanochemical effect on this photoisomerization mechanism is also proposed.

Mechanical Forces Alter Conical Intersections Topology.

Photoreactivity can be influenced by mechanical forces acting over a reacting chromophore. Nevertheless, the specific effect of the external forces in the photoreaction mechanism remains essentially unknown. Conical intersections are key structures in photochemistry, as they constitute the funnels connecting excited and ground states. These crossing points are well known to provide valuable information on molecular photoreactivity, including crucial aspects as potential photoproducts which may be predicted by just inspection of the branching plane vectors. Here, we outline a general framework for understanding the effect of mechanical forces on conical intersections and their implications on photoreactivity. Benzene S1/S0 conical intersection topology can be dramatically altered by applying less than 1 nN force, making the peaked pattern of the intersection become a sloped one, also provoking the transition state in the excited state to disappear. Both effects can be related to an increase in the photostability as the conical intersection becomes more accessible, and its topology in this case favors the recovery of the initial reactant. The results indicate that the presence of external forces acting over a chromophore have to be considered as a potential method for photochemical reactivity control.

Role of bifurcation in the bond shifting of cyclooctatetraene.

The present study of the cyclooctatetraene potential energy surface shows the presence of a bifurcation (valley ridge inflection point) in the intrinsic reaction coordinate path between the two transition states of D(8h) and D(4h) symmetries. This result is of capital importance for the correct understanding of the bond shifting and ring inversion processes in this compound.