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

Ligand-protected noble-metal clusters are promising candidates for photosensitizers and luminescent materials owing to their diverse structures and chemical compositions as well as easily tunable photophysical and electronic properties. Realizing these applications requires a profound understanding of the fundamental photophysics of these clusters, including their photoluminescence and photoabsorption properties. Although photoluminescence has been frequently reported in ligand-protected clusters like silver and gold, its origin remains unclear. This has been mainly attributed to the presence of numerous emission sites and the contribution of electronic states with various spin multiplicities to these sites.

Establishing the quantum yields and rate constants of both radiative and non-radiative processes associated with excited state of clusters is imperative for accurate understanding of the complex characteristics of the excite-state processes. A plethora of studies have shown that increasing the ligand environment rigidity or alloying the metal core can significantly enhance the photoluminescence properties of these clusters. This has been observed in Au₂Cu­₆-based clusters that also display great flexibility in controlling their photophysical and electronic properties. Nevertheless, the underlying nature of the PL properties of Au₂Cu­₆-based clusters and the elementary process of improving the PL have not been fully explored.

On this account, Mr. Daichi Arima, Dr. Yoshiki Niihori and Professor Masaaki Mitsui from Rikkyo University investigated the origin of dual PL in Au₂Cu­₆-based clusters (specifically Au₂Cu₆(S-Adm)₆(PPh₃)₂ (S-Adm = 1-adamantanethiolate) clusters) using photon upconversion and triplet sensitization techniques. Phosphorescence and fluorescence were the two fundamental photoluminescence properties studied. The quantum yields and rate constants of all the radiative and non-radiative processes involved in the excitation of the clusters, as well as the various factors affecting the PL of the clusters, were examined and discussed. Their research work is currently published in the Journal of Materials Chemistry C.

The authors observed that Au₂Cu­₆ clusters exhibited both phosphorescence and fluorescence properties at room temperature. The clusters also serve as molecular triplet sensitizers and will add to the already developed triplet sensitizers like semiconductor nanocrystals and heavy-metal complexes. A detailed analysis of the tripled fusion upconversion phenomenon involved in the Au₂Cu₆ sensitizer-perylene emitter pair led to accurate determination of the quantum yields and rate constants of all the radiative and non-radiative processes associated with the excited single and triplet states of the clusters.

The obtained theoretical computations and temperature dependence of the photoluminescence showed that the thermally activated intersystem crossing in the Au₂Cu₆ clusters occurred via a spin-vibronic coupling mechanism. This mechanism was influenced by the higher excitation states where the coupling was possible, as shown by the time-dependent and density functional theory calculations. It was worth noting that the metal clusters comprising heavy metal elements such as silver and gold atoms possess remarkably larger spin–orbit coupling constants compared with those of organic molecules, which often results in stronger S-T mixing.

In summary, the phosphorescence and fluorescence properties of Au₂Cu­₆-based clusters at room temperature were successfully demonstrated. The results also established the high probability that the excited triplet states might be involved in different excited-state relaxation processes like the photoluminescence phenomena. To this end, the presented metal cluster-induced triplet sensitization approach would be useful in establishing the origin of photoluminescence in clusters. In a statement to Advances in Engineering, Professor Masaaki Mitsui, the lead and corresponding author said their research work will deepen our understanding of the photophysical properties of metal clusters and further expand their applications as optical and photoenergy conversion nanomaterials.