Proton shuttle-assisted triplet energy transfer.

Electronic transition/motion coupled with proton transfer has a key role in natural and artificial energy conversion and storage materials. Previous examples include proton-coupled electron transfer and singlet energy transfer, but not triplet energy transfer. Here we report a mechanism termed proton shuttle-assisted triplet energy transfer. The system comprises ZnSe-based quantum dots surface anchored with phenol-pyridine dyadic acceptors. Ultrafast measurements and kinetic isotope effects establish that the photoexcitation of ZnSe leads to hole transfer from ZnSe to phenol, which is coupled with proton transfer from phenol to pyridine. A subsequent step of electron transfer from ZnSe to phenoxyl radical, coupled with back proton transfer from pyridinium, accomplishes a net process of spin-triplet migration from ZnSe to phenol-pyridine. Adding a strongly electron-withdrawing trifluoromethyl substituent on pyridine can switch the sequence of proton-coupled electron and hole transfer steps. Compared with a methylated analogue acceptor lacking the shuttle, the assistance of proton shuttle substantially increases the energy transfer rate and efficiency.