Ultrafast Intra- and Intermolecular Energy Transfer in Photo-Cyclic Ring-Closure Reactions
A second research aim is to elucidate energy-transfer processes that underlie photo-cyclic ring closure reactions of ortho-substituted arenes (e.g. ortho-terphenyl) and related stilbenoids in solution. A primary objective of this work is to study these as model systems for developing a molecular-level understanding of non-adiabatic processes and how they are influenced by chemical environment (e.g., molecular structure and interactions with the local solvent environment). Non-adiabatic transitions between electronic states are fundamental intramolecular energy-transfer pathways; these processes and, more specifically, fast crossings along conical intersections, hold promise for efficient photochemical synthesis and energy-conversion schemes. In current work we are developing a detailed picture of non-adiabatic dynamics in ortho-terphenyl and 1,2-diphenylcyclohexene. By comparing their excited-state dynamics as measured via ultrafast transient absorption spectroscopy we have been able to assess how structural constraints influence the course of dynamics on excited-state potential-energy surfaces: Specifically, approach to ring closure is inhibited sterically by torsional rotation of phenyl rings, such that the ring-closure rate increases appreciably by relaxing structural constraints (manuscript in preparation).
In addition to serving as model systems for studying non-adiabatic dynamics, photo-cyclic ring-closure creates carbon-carbon bonds and is the first step in Mallory synthesis, which can be used to make two-dimensional, graphene-line poly-aromatic hydrocarbons with interesting and useful electronic properties. A secondary objective of our work is to establish what (and how) structural constraints influence bond formation in these systems. We aim to address how molecular complexity and size fundamentally influence intra- and intermolecular energy-transfer pathways necessary to fuse together extended networks of arenes via multiple ring-closures, particularly in cases where the sequence in which ring-closure reactions proceeds will matter. By interrogating the molecular underpinnings of these energy-transfer pathways in detail and determining the conditions under which these steps can be utilized, avoided, or even controlled, we are not only adding to the understanding of fundamental photophysical processes, but also fleshing out dynamical considerations that should be heeded when using these processes in synthetic schemes for making useful materials.