Light-Induced Proton-Coupled Electron Transfer Inside a Nanocage

Abstract

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.

Photoinduced Charge Transfer State Probes the Dynamic Water Interaction with Metal–Organic Nanocages

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 subsequent to the dipolar photoexcitation using pump-probe spectroscopy. 4 The femtosecond pump pulse centered at 490 nm in combination with a broadband probe pulse (500-1300 nm)…”

“…The water reactivity within the timescale of the diffusive components was recently demonstrated by our group through the illustration of the fastest hydrogen abstraction reaction inside a nanocage. 4 The ultrafast water shell reorganization in few hundred femtoseconds formed the predominant reaction coordinate for the efficient proton transfer reaction. Therefore, we envision that water interactions with the guest in the primary coordination shell can be utilized on fast timescales that ensure high energy efficiency of the photocatalytic processes.…”

“…17 We recently demonstrated that host-guest CT excitations can be used to drive ultrafast hydrogen abstraction reactions using water as a reactive base with >80% efficiency. 4 In all the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 For photochemical transformations the precise advantage of using such reaction vessels is usually explained by the confinement effect of the molecular hosts. [18][19][20] Hunter, Ward and coworkers recently quantified the contribution arising from solvation upon guest incarceration inside a water soluble host in the ground state.…”

Selective Co‐Encapsulation Inside an M6L4 Cage

“…A self‐assembled metallocage, which has been reported previously (Figure ), was used as the host for the current studies. This metallocage has recently been demonstrated to facilitate exciplex formation with different guests and to incorporate metal complexes, whereupon a charge‐transfer band was observed . The water‐soluble octahedral nanosphere 1 (Figure ) can bind a large variety of guests, mostly on the basis of hydrophobic effects and interactions with the electron‐deficient sidewalls of its cavity.…”

Tunable Reduction of 2,4,6‐Tri(4‐pyridyl)‐1,3,5‐Triazine: From Radical Anion to Diradical Dianion to Radical Metal–Organic Framework

“…[2] As imilar cage-localized radical intermediate was also observed in al ight-induced protoncoupled electron transfer (PCET) process in 2014. [3] Moreover the metal-to-ligand charge transfer (MLCT) [4] and the light-induced ligand-to-ligand electron transfer (LLET) [5] of TPT have also been studied, further suggesting that TPT is redox active.T hus,i ti sn ecessary to investigate the redox behavior of TPT,e specially its direct reduction with alkali metals.…”