Red-to-blue photon upconversion based on a triplet energy transfer process not retarded but enabled by shell-coated quantum dots

Charge and/or energy transfer from photoexcited quantum dots (QDs) is often suppressed by a wide-bandgap shell. Here, we report an interesting, counter-intuitive observation that interfacial triplet energy transfer from QDs is not retarded but rather enabled by an insulating shell. Specifically, photoluminescence of red-emitting CdSe QDs could not be quenched by surface-anchored Rhodamine B molecules; in contrast, after ZnS shell coating, their emission was effectively quenched. Time-resolved spectroscopy reveals that the shell eliminates ultrafast hole trapping in the QDs and hence opens up the triplet exciton transfer pathway. The triplet energy of Rhodamine B can be reversely transferred back to QDs by thermal activation, or it can be passed to triplet acceptors in the solution. Capitalizing on the latter, we demonstrate red-to-blue photon upconversion based on QD-sensitized triplet-triplet annihilation with an efficiency of 2.8% and an anti-Stokes shift of 1.13 eV.