Triplet properties and intersystem crossing mechanism of PtAg28 nanocluster sensitizers achieving low threshold and efficient photon upconversion.

Ligand-protected metal nanoclusters have emerged as a promising platform for providing sensitizers for triplet-triplet annihilation upconversion (TTA-UC). Herein, we report [PtAg28(BDT)] (PtAg28; BDT = 1,3-benzenedithiolate) as a sensitizer enabling TTA-UC at low excitation intensities. PtAg28 exhibits a long-lived triplet state (approximately 7 μs) generated with a 100% intersystem crossing (ISC) quantum yield. The mechanism driving this efficient ISC was unveiled with the aid of theoretical calculations. Specifically, the S-T ISC reveals a small spin-orbit coupling (SOC) matrix element, attributed to their similar electron configuration. In contrast, the T state, which is energetically close to S, features a hole distribution derived from the P superatomic orbital of the icosahedral Pt@Ag core. This distribution enables direct SOC based on the orbital angular momentum change from the S state with a P-derived hole distribution. Consequently, the efficient ISC was rationalized by the S → T → T pathway. The T state possesses a metal core-to-surface metal charge transfer character, facilitating triplet energy transfer and conferring superior sensitization ability. Leveraging these characteristics, the combination of PtAg28 sensitizer with a 9,10-diphenylanthracene annihilator/emitter attained an extremely low UC threshold of 0.81 mW cm at 532 nm excitation, along with efficient green-to-blue TTA-UC with an internal quantum yield () of 12.2% (50% maximum). This results in a pseudo-first-order TTA process with strong UC emission under 1-sun conditions.