Key Roles of Aliphatic Ligands over PbS Quantum Dots for Efficient Triplet Energy Transfer in a Hybrid TES-ADT/PbS...

Photon upconversion (PUC) via triplet-triplet annihilation (TTA), using inorganic quantum dots (QDs) as triplet sensitizers, is a promising strategy for harvesting near-infrared photons due to the negligible energy loss associated with intersystem crossing in QDs. A TTA PUC system comprising 5,11-bis(triethylsilylethynyl)-anthradithiophene (TES-ADT) and lead sulfide (PbS) QDs effectively takes advantage of such advantages. Notably, in the liquid system, TES-ADT functions as both the triplet acceptor and TTA material, eliminating requirement of the conventional transmitter ligands that create energy losses via triplet energy transfer (TET). This unique dual-function presumably stems from a dynamic attach/detach mechanism: TES-ADT molecules detach to be free-floating molecules after accepting triplet energy and subsequently proceed with TTA. The emerging dynamic attach/detach mechanism is of general interest for hybrid systems of organic molecules and inorganic QDs; however, its mechanism remains elusive. Herein, modulation of aliphatic ligands over the QDs in TES-ADT/PbS QDs systems reveals that the affinity of TES-ADT molecules to the ligands can be a key factor for achieving efficient net TET via the dynamic attach/detach mechanism. In the steady-state PUC measurement, among the employed ligands (carbon numbers of the ligands 4C-18C), middle length ligands (8C and 12C) exhibited relatively high PUC and TET efficiency of up to 0.083% and 29%, respectively. Pump-probe transient absorption (TA) measurements suggest that the long ligand (18C) leads to the stacking of TES-ADT within the ligand shell, which reduces its net TET efficiency. Meanwhile, the 4C ligand presumably resulted in a lower affinity of TES-ADT to the shorter ligand, hampering the first step of TET. Conversely, ligands of length comparable to that of the ADT backbone (8C and 12C) most likely led to sufficient affinity to TES-ADT, allowing TES-ADT to detach/attach the PbS surface efficiently. Consequently, the insights obtained in this work will be clues for the development of inorganic-organic hybrid systems exploiting triplet energies.