Dissecting the Triplet-State Properties and Intersystem Crossing Mechanism of the Ligand-Protected Au₁₃ Superatom

Icosahedral Au-13 nanoclusters are among the most typical superatoms and are of great interest as promising building blocks for nanocluster-assembled materials. Herein, the key parameters involved in the intersystem crossing (ISC) process of [Au-13(dppe)(5)Cl-2](3+) (Au-13; dppe = 1,2-bis(diphenylphosphino)ethane) were characterized. Quenching experiments using aromatic compounds revealed that the T-1 energy of Au-13 is 1.63 eV. An integrative interpretation of our experimental results and the relevant literature uncovered important facts concerning the Au-13 superatom: the ISC quantum yield is unity due to the ultrafast ISC (similar to 10(12) s(-1)), the lowest absorption band includes contributions of direct singlet-triplet transitions, and there exists a large S-1-T-1 gap of 0.73 eV. To explain the efficient ISC, the El-Sayed rule was applied to the superatomic orbitals corresponding to the excited-state hole/electron distributions obtained from theoretical calculations. The strong spin-orbit coupling between the S-1 and T-2-T-4 states offers a reasonable explanation for the ultrafast ISC.