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Direct single-molecule dynamic detection of chemical reactions.
Single-molecule detection can reveal time trajectories and reaction pathways of individual intermediates/transition states in chemical reactions and biological processes, which is of fundamental importance to elucidate their intrinsic mechanisms. We present a reliable, label-free single-molecule approach that allows us to directly explore the dynamic process of basic chemical reactions at the single-event level by using stable graphene-molecule single-molecule junctions. These junctions are constructed by covalently connecting a single molecule with a 9-fluorenone center to nanogapped graphene electrodes. For the first time, real-time single-molecule electrical measurements unambiguously show reproducible large-amplitude two-level fluctuations that are highly dependent on solvent environments in a nucleophilic addition reaction of hydroxylamine to a carbonyl group. Both theoretical simulations and ensemble experiments prove that this observation originates from the reversible transition between the reactant and a new intermediate state within a time scale of a few microseconds. These investigations open up a new route that is able to be immediately applied to probe fast single-molecule physics or biophysics with high time resolution, making an important contribution to broad fields beyond reaction chemistry
Label-Free Dynamic Detection of Single-Molecule Nucleophilic-Substitution Reactions
The
mechanisms of chemical reactions, including the transformation
pathways of the electronic and geometric structures of molecules,
are crucial for comprehending the essence and developing new chemistry.
However, it is extremely difficult to realize at the single-molecule
level. Here, we report a single-molecule approach capable of electrically
probing stochastic fluctuations under equilibrium conditions and elucidating
time trajectories of single species in non-equilibrated systems. Through
molecular engineering, a single molecular wire containing a functional
center of 9-phenyl-9-fluorenol was covalently wired into nanogapped
graphene electrodes to form stable single-molecule junctions. Both
experimental and theoretical studies consistently demonstrate and
interpret the direct measurement of the formation dynamics of individual
carbocation intermediates with a strong solvent dependence in a nucleophilic-substitution
reaction. We also show the kinetic process of competitive transitions
between acetate and bromide species, which is inevitable through a
carbocation intermediate, confirming the classical mechanism. This
unique method creates plenty of opportunities for carrying out single-molecule
dynamics or biophysics investigations in broad fields beyond reaction
chemistry through molecular design and engineering