Abstract
Photochemical reactions in solution often proceed via competing reaction pathways comprising intermediates that capture a solvent molecule. A disclosure of the underlying reaction mechanisms is challenging due to the rapid nature of these processes and the intricate identification of how many solvent molecules are involved. Here combining broadband femtosecond transient absorption and quantum mechanics/molecular mechanics simulations, we show for one of the most reactive species, diphenylcarbene, that the decision-maker is not the nearest solvent molecule but its neighbour. The hydrogen bonding dynamics determine which reaction channels are accessible in binary solvent mixtures at room temperature. In-depth analysis of the amount of nascent intermediates corroborates the importance of a hydrogen-bonded complex with a protic solvent molecule, in striking analogy to complexes found at cryogenic temperatures. Our results show that adjacent solvent molecules take the role of key abettors rather than bystanders for the fate of the reactive intermediate.
Highlights
Photochemical reactions in solution often proceed via competing reaction pathways comprising intermediates that capture a solvent molecule
Studies by Eisenthal and coworkers[2,3,4,5,6,7,8,9,10,11] have pioneered many aspects of Ph2C reactivity in solution, for example, effects of solvent polarity and selected cosolvents, but without the direct spectroscopic observation of the singlet 1Ph2C. This was possible in ultrafast studies, performed initially by the Chergui group[14] and further extended by Kohler and coworkers[15] in several pure solvents. The latter revealed that ultraviolet excitation of the diazo-compound precursor diphenyldiazomethane (Ph2CN2) leads to formation of 1Ph2C on a subpicosecond time scale, which can further react via intersystem crossing (ISC) to the triplet 3Ph2C in a few hundreds of picoseconds in aprotic solvents
Our results emphasize that ultrafast photodynamics in solvent mixtures are more than just a linear combination of the behaviour in neat solvents, and that the amount of molecules of a certain solvent in the vicinity of the solute may be of critical importance for a desired reaction outcome
Summary
Photochemical reactions in solution often proceed via competing reaction pathways comprising intermediates that capture a solvent molecule. Studies by Eisenthal and coworkers[2,3,4,5,6,7,8,9,10,11] have pioneered many aspects of Ph2C reactivity in solution, for example, effects of solvent polarity and selected cosolvents, but without the direct spectroscopic observation of the singlet 1Ph2C. This was possible in ultrafast studies, performed initially by the Chergui group[14] and further extended by Kohler and coworkers[15] in several pure solvents. Our results emphasize that ultrafast photodynamics in solvent mixtures are more than just a linear combination of the behaviour in neat solvents, and that the amount of molecules of a certain solvent in the vicinity of the solute may be of critical importance for a desired reaction outcome
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