Towards Chemical Control of Photoisomerization: Substituent Effects on the Excited-state Dynamics of Conjugated Polyenes

The photochemical isomerization of alkenes is an important aspect of many light-sensitive processes in chemistry and biology. It involves the internal conversion from an excited electronic state of a molecule to its ground state via geometries which twist about a carbon-carbon bond. In nature, the electronic environment around the molecule influences photoisomerization to occur at specific locations in conjugated polyenes, i.e. molecules with multiple adjacent carbon-carbon double bonds. Without the environmental influences, polyenes may undergo isomerization at many competing sites. A potential alternative approach to influencing photoisomerization is the use of functional group substituents. A substituent may lead to significant changes in the electronic excitation energies and the nuclear dynamics at or near the substituted site. For alkenes, substituents can be used to influence the electron density by favouring or inhibiting charge at specific positions in a molecule. The mechanism of photoisomerization is exemplified by the smallest alkene, ethylene. After photoexcitation to the lowest absorption band, carbon-carbon bond torsion is followed by a pyramidalization at one carbon atom to reach a conical intersection (i.e. a point of degeneracy between two electronic states) with a concurrent "sudden polarization" across the carbon-carbon bond, such that there is a lone pair at the pyramidalized site. Conjugated polyenes have excited-state decay pathways similar to that of ethylene. They also have "kinked-diene" pathways, which involve conical intersection regions with no significant polarization. These pathways are barrierless and lead to competing relaxation processes during a photochemical reaction. Being able to predict isomerization sites and design photochemical reactions is desirable for many applications in light-driven chemical properties and chemical synthesis. We show that the principles of chemical substitution to favour or inhibit electron density (and thus pyramidalization) at specific sites can be applied to excited-state processes of cyano-substituted ethylenes through ab initio molecular dynamics simulations, verified experimentally by ultrafast spectroscopy techniques. The effect of these electronic substituents is consistent for a range of substituted ethyenes, and the form of the excited-state potential can be qualitatively predicted by modification of a biradical model of ethylene. The ability to significantly alter the branching ratios of photochemical reactions is the most unambiguous demonstration of substituent effects in excited-state processes. We simulate the excited-state dynamics of 1,3-butadiene and its cyano-substituted analogs to show how the conical intersection energies are predictably lowered and raised for pyramidalization at the subsituted and adjacent sites, respectively. Dynamics simulations reveal a more important change in the potential energies: the gradients of the exicted-state surface direct nuclear rearrangement towards the favoured ethylenic conical intersection region, despite the fact that the kinked-diene conical intersections are lower in energy. We use methyl-substituted butadienes to show how this change in gradients compares to the change in the masses and thus the frequencies of motion relevant to dynamics. Dynamical simulation of mass-weighted butadienes reveal that, despite having a minor effect on potential energies, the difference in excited-state gradients caused by the electronic effect of methyl groups plays an important role in the decay pathway. Analysis of the rearrangement of charge caused by substitution and by nuclear dynamics towards a conical intersection require methods which show differences in electron density of electronic excited states. We show how partial atomic charge methods, particularly real-space methods, can be used to characterize the complex evolution of excited-state electronic character. As an example, we show that the polarization effect is consistent for alkenes ranging from ethylene to 1,3,5-hexatriene with and without cyano and amino substituents. These results give a consistent picture of how substituents may be employed to achieve site-specificity in photochemical isomerization, and to tailor photochemical properties of conjugated polyenes with potential applications in light-harvesting, molecular motors and chemical synthesis.

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