Site-Selective Isomerization of Cyano-Substituted Butadienes: Chemical Control of Nonadiabatic Dynamics

The photochemistry of organic chromophores generally involves the co-evolution of the electronic and nuclear degrees of freedom. To obtain a specific and predetermined photochemical reaction outcome, chemical substitution can be used to selectively alter the underlying electronic potential energy surfaces to favor a particular reaction pathway. We show using ab initio simulation that the substitution of s- trans-1,3-butadiene with a cyano group can effectively "direct" a molecular wavepacket to particular regions of the seam of conical intersection and either favor or inhibit the photoinitiated cis-trans isomerization. The substituent is able to effect this control due to the formation of transient charge-separated electronic structures that arise during the nonadiabatic dynamical process. The atomic site at which this charge develops can be selectively stabilized (or destabilized) depending on the location of the cyano substituent and gives rise to a single dominant decay pathway. This work aims to demonstrate how the application of known electron density effects to ultrafast dynamics may be used to obtain desired photochemical reactions and properties.