Bidirectional Intramolecular Singlet and Triplet Energy Transfer in Tetracene-Ultrasmall Gold Nanocluster Dyads: An Evaluation of the Triplet Behavior of Gold Nanoclusters

A series of tetracene (Tc)-alkanethiol-functionalized gold nanoclusters (Aum: m = 25, 38) dyads with varying alkyl chain lengths (n = 5, 11) (denoted as Tc-Cn-Aum) were synthesized to investigate the excited-state dynamics through the triplet excited state of these gold nanoclusters. Transient absorption measurements revealed bidirectional intramolecular energy transfer processes, including the initial singlet-singlet energy transfer (S-SEnT) from Tc to Au25, followed by the subsequent triplet-triplet energy transfer (T-TEnT) from Au25 to Tc in Tc-Cn-Au25 (n = 5, 11), with nearly similar rate constants regardless of the alkyl chain length (n). In the case of Tc-C5-Au38, intramolecular S-SEnT from Tc to Au38 was similarly observed; however, T-TEnT from Au38 to Tc was not detected. The differences in excited-state dynamics between Tc-C5-Au25 and Tc-C5-Au38 can be attributed to variations in the triplet energies of Aum. These results clearly demonstrate the triplet character of Au25 and Au38.