Asymmetrical Intramolecular Photochemistry
Symmetric multipolar molecules undergo two different symmetry-breaking mechanisms: solvent-driven - efficient in polar solvents and resulting in a concentration of the excitation triggering fast photochemical reactions, and chemical-driven in nonpolar solvents leading to inefficient photochemistry.
Excited-state symmetry breaking (ESSB) is an interesting phenomenon that we started to investigate with ultrafast IR in the Vauthey group in 2014. We found that upon photoexcitation of a large number of quadrupolar molecules, that are essentially two oppositely oriented dipolar charge-transfer arms, the initially prepared symmetric and delocalized excited state quickly asymmetrizes and can collapse onto a single arm. This behavior is universal when such molecules are placed in polar media, where femtosecond to picosecond wiggling of surrounding solvent molecules produces fluctuating electric fields that are at the origin of this effect. At the same time, in non-polar media, nothing interesting happens: the quadrupole pertains its symmetric delocalized character for the entire excited-state lifetime which can reach many nanoseconds. It seemed that solvent was always in charge of this process. And it does not depend on the exact multipolar nature of the molecule. Here we demonstrate that it works identically for quadrupoles and octupoles.
After we investigated all kinds of photophysical aspects of ESSB, the truly interesting chemical question emerged. How can symmetry breaking affect the ensuing intramolecular photochemistry? If excited state is so different in polar versus non-polar media, can we obtain different photoproducts by changing the solvent appropriately (Figure 1)? Moreover, since a symmetric excited state represents a coherent delocalized exciton, whereas the localized exciton is the one that underwent decoherence, it is a broad question of functional symmetry/asymmetry that boggles researchers’ minds today. With an array of state-of-the-art ultrafast multidimensional techniques, it is not a problem to demonstrate that coherence in the excited state exists and survives for sufficient time under appropriate circumstances, but can it be used to enhance chemical function? Can ESSB work as a switch between coherent and incoherent modalities?
Figure 1. Symmetrical coherent and asymmetrical incoherent photochemistry hypothesis.
Here, we provide an answer to these questions from the perspective of a class of multipolar charge-transfer molecules. It turns out that ESSB is an extremely efficient way of concentrating the harvested energy in a single specific place in the molecule where it may trigger fast chemical reactions (in our case, it was an isomerization of the alkyne into the allene). We show here with a toolbox of broadband femtosecond techniques that the reaction is fast, efficient and irreversible, and the final allene photoproduct persists for nanoseconds (Figure 2). It provides, in principle, enough time for a diffusional encounter with some reagent that can convert it into another product thus opening a possibility for domino reactions of the in situ generated allene.
Figure 2. Scheme of actually occurring processes deduced using a toolbox of broadband femtosecond spectroscopies.
However, in apolar solvents, the coherent delocalized exciton does not provide a new reaction pathway resulting in multiple isomerizations in a single molecule. Instead, symmetry still breaks spontaneously, now driven by structural fluctuations while the reaction is taking place in a single arm (Figure 2). However, it occurs slowly and inefficiently and, most importantly, it is reversible. The photoproduct is not well-stabilized and converts back to the initial alkyne state. Photochemistry does not take advantage of the available coherent delocalization. In fact, delocalization harms as it dilutes the excitation and the driving force is apparently reduced.
Our results demonstrate that symmetry breaking is a very efficient way to localize excitation in a single spot where photochemistry is needed. They provide a way of thinking why a number of efforts to produce symmetric molecules with multiple reaction sites (e.g., proton transfer) have not succeeded. Additionally, we found a new marker of symmetry breaking that was long-sought and missing, namely the electronic transient absorption signature of ESSB.
If you want to know more, please, check out our article “Solvent tuning of photochemistry upon excited-state symmetry breaking” in Nature Communications. Link to the article: https://www.nature.com/articles/s41467-020-15681-3
Written by Dr. Bogdan Dereka