Unravelling the origin of dual photoluminescence in Au2Cu6 clusters by triplet sensitization and photon upconversion

Ligand-protected, noble-metal clusters are promising as luminescent materials and photosensitizers because of their diverse chemical compositions and structures, as well as their easily tunable electronic and photophysical properties with single-atom precision. Many studies have shown that the photoluminescence (PL) properties of these clusters can be significantly enhanced by alloying the metal core or by increasing the rigidity of the ligand environment. However, the fundamental nature of their PL (whether fluorescent or phosphorescent) and the elementary process for enhancing their PL have not been fully clarified. In this study, we established that Au2Cu6(S-Adm)6(PPh3)2 (S-Adm = 1-adamantanethiolate) clusters exhibit not only fluorescence but also phosphorescence at room temperature. Further, we discovered that Au2Cu6 serves as a triplet sensitizer, and through the analysis of the photon-upconversion phenomenon, we determined the rate constants and quantum yields of all radiative and non-radiative processes involving the excited singlet and triplet states of the clusters. The temperature dependence of the PL and the theoretical calculations indicated that thermally activated intersystem crossing occurs through the spin–vibronic coupling mechanism mediated by higher excited states (S2 and/or T2) where direct spin–orbit coupling is possible.