Ultrafast Atom Transfer Chemistry inside Nanocontainers!
10 July 2015
Photosynthesis is a process by which light energy is converted into chemical potential and stored in the chemical bonds in form of carbohydrates. The complex light triggered events that drive photosynthesis primary takes place in the confines of the membrane and inside large protein complexes. Confining synthetic organic photochemistry inside water-soluble molecular hosts underscores a biomimetic approach towards artificial photosynthesis. Organic transformations using light classically make use of diffusion based interactions or kinetics which occurs in nanoseconds to microseconds, thereby ensuring that some part of photon's energy is wasted via relaxation processes. If bond making or breaking can be performed within few picoseconds of optical excitation, then most of the photon energy can be utilized efficiently for chemical transformations. Previously ultrafast electron transfer and proton transfer reactions independently have been demonstrated by researchers in many artificial molecular systems however ultrafast atom transfer chemistry has been very rare.
At TIFR, we have now demonstrated for the first time that emergent host-guest charge transfer (CT) states can be directly used to trigger photoreactions inside molecular nanocages. Upon incarceration of a complementary electron donor host to the acceptor molecular cage, we generate an emergent host - guest CT optical band usually in the visible part of the electromagnetic spectrum. Optical excitation of the CT band can chemically couple the electron (e-) and proton (H+) transfer events such that an H-atom (H+ + e-) is formally abstracted.
As shown above in the schematic, we choose a well-known water soluble Pd6L4-nanocage for generating optical CT bands by incarcerating electron-rich donor amine (-NH functionality) molecules. We show that using the bulk solvent water interaction, one can control the timescale of hydrogen atom abstraction which occurs in ~900 fs timescale (fastest H-atom chemistry) from a trapped 4-hydroxy-diphenylamine molecule through a stepwise proton-coupled electron transfer (PCET) process. Our work illustrates that optical host?guest CT excitations can drive solvent-coupled ultrafast H-abstraction reactions inside nanocages, and if optimally tuned should provide a novel paradigm for visible-light mediated photocatalysis.
For more details see: Gera et al. J. Am. Chem. Soc., Volume 136, Issue 45, Pages: 15909 - 15912