Triggering proton-coupled electron-transfer (PCET) reactions with light in a nanoconfined host environment would bring about temporal control on the reactive pathways via kinetic stabilization of intermediates. Using a water-soluble octahedral Pd6L4 molecular cage as a host, we show that optical pumping of host-guest charge transfer (CT) states lead to generation of kinetically stable phenoxyl radical of the incarcerated 4-hydroxy-diphenylamine (1-OH). Femtosecond broadband transient absorption studies reveal that CT excitation initiates the proton movement from the 1-OH radical cation to a solvent water molecule in ~890 fs, faster than the time scale for bulk solvation. Our work illustrates that optical host-guest CT excitations can drive solvent-coupled ultrafast PCET reactions inside nanocages and if optimally tuned should provide a novel paradigm for visible-light photocatalysis.
We study the properties of the interface
of water and the surfactant
hexaethylene glycol monododecyl ether (C12E6) with a combination of
heterodyne-detected vibrational sum frequency generation (HD-VSFG),
Kelvin-probe measurements, and molecular dynamics (MD) simulations.
We observe that the addition of the hydrogen-bonding surfactant C12E6,
close to the critical micelle concentration (CMC), induces a drastic
enhancement in the hydrogen bond strength of the water molecules close
to the interface, as well as a flip in their net orientation. The
mutual orientation of the water and C12E6 molecules leads to the emergence
of a broad (∼3 nm) interface with a large electric field of
∼1 V/nm, as evidenced by the Kelvin-probe measurements and
MD simulations. Our findings may open the door for the design of novel
electric-field-tuned catalytic and light-harvesting systems anchored
at the water–surfactant–air interface.
Kinetic stabilization of reactive photo-intermediates inside nanocages is conjectured to arise through a subtle interplay of cavity-induced hydrophobicity, cage electrostatics and solvent-mediated interactions. Herein we enunciate the dynamic role of the water shell around an electron deficient Pd 6 L 4 12+ nanohost by optically triggering the emergent host-guest charge transfer (CT) band arising from incarcerated guest 9-anthracenealdehyde (9-AA). Using time-resolved spectroscopy complemented with resonance Raman, we determine two temporal regimes of aqueous solvation i.e. ~120 fs and ~4 ps around the host-guest complex.Solvent-isotope dependent lifetime changes of the emissive twisted intramolecular charge transfer state for "caged" 9-AA unravels a slower ~1.65 ns water interaction through the exposed -CHO group. Our work provides the first comprehensive physical glimpse of all excited state water-mediated interactions, and thus motivates rational design of new reactions that would use the aqueous shell for supramolecular photochemistry.
Interfacial vibrational
footprints of the binary mixture of sodium
dodecyl sulfate (SDS) and hexaethylene glycol monododecyl ether (C12E6) were probed using heterodyne detected vibrational
sum frequency generation (HDVSFG). Our results show that in the presence
of C12E6 at CMC (70 μM) the effect of
SDS on the orientation of interfacial water molecules is enhanced
10 times compared to just pure surfactants. The experimental results
contest the traditional Langmuir adsorption model predictions. This
is also evidenced by our molecular dynamics simulations that show
a remarkable restructuring and enhanced orientation of the interfacial
water molecules upon DS– adsorption to the C12E6 surface. The simulations show that the adsorption
free energy of DS– ions to a water surface covered
with C12E6 is an enthalpy-driven process and
more attractive by ∼10 k
B
T compared to the adsorption energy of DS– to the surface of pure water.
Self-assembled coordination cages form host-guest complexes through weak non-covalent interactions. Knowledge of how these weak interactions affect the structure, reactivity, and dynamics of guest molecules is important to further the design principles of current systems and optimize their specific functions. In this manuscript, we apply ultrafast mid-IR polarization dependent pump-probe spectroscopy to probe the effects of two Pd 6 L 4 self-assembled nanocages on the properties and dynamics of fluxional group VIII metal carbonyl guest molecules. We find that the interactions between the Pd 6 L 4 nanocages and guest molecules act to alter the ultrafast dynamics of the guests; restricting rotational diffusional motion and decreasing the vibrational lifetime.
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