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.

We study the molecular-scale structure of the surface of Reline, a DES made from urea and choline chloride, using heterodyne-detected vibrational sum frequency generation (HD-VSFG). Reline absorbs water when exposed to the ambient atmosphere, and following structure-specific changes at the Reline/air interface is crucial and difficult. For Reline (dry, 0 wt %, w/w, water) we observe vibrational signatures of both urea and choline ions at the surface. Upon increase of the water content, there is a gradual depletion of urea from the surface, an enhanced alignment, and an enrichment of the surface with choline cations, indicating surface speciation of ChCl. Above 40% w/w water content, choline cations abruptly deplete from the surface, as evidenced by the decrease of the vibrational signal of the −CH 2 – groups of choline and the rapid rise of a water signal. Above 60% w/w water content, the surface spectrum of aqueous Reline becomes indistinguishable from that of neat water.

The effects of ligand structural variation on the ultrafast dynamics of a series of copper coordination complexes were investigated using polarization-dependent mid-IR pump− probe spectroscopy and two-dimensional infrared (2DIR) spectroscopy. The series consists of three copper complexes [( R 3 P 3 tren)Cu I I N 3 ]BAr 4 F ( 1 P R 3 , R 3 P 3 tren = tris[2-( p h o s p h i n i m i n a t o ) e t h y l ] a m i n e , B A r 4 F = tetrakis-(pentafluorophenyl)borate) where the number of methyl and phenyl groups in the PR 3 ligand are systematically varied across the series (PR 3 = PMe 3 , PMe 2 Ph, PMePh 2 ). The asymmetric stretching mode of azide in the 1 PR3 series is used as a vibrational probe of the small-molecule binding site. The results of the pump−probe measurements indicate that the vibrational energy of azide dissipates through intramolecular pathways and that the bulkier phenyl groups lead to an increase in the spatial restriction of the diffusive reorientation of bound azide. From 2DIR experiments, we characterize the spectral diffusion of the azide group and find that an increase in the number of phenyl groups maps to a broader inhomogeneous frequency distribution (Δ 2 ). This indicates that an increase in the steric bulk of the secondary coordination sphere acts to create more distinct configurations in the local environment that are accessible to the azide group. This work demonstrates how ligand structural variation affects the ultrafast dynamics of a small molecular group bound to the metal center, which could provide insight into the structure−function relationship of the copper coordination complexes and transition-metal coordination complexes in general.

Photoexciting charge transfer (CT) transitions arising from host-guest interactions in a confined environment can efficiently yield kinetically trapped radicals. In order to predispose these photogenerated radicals for diffusion limited reactions...