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 (Au_m: m = 25, 38) dyads with varying alkyl chain lengths (n = 5, 11) (denoted as Tc-C_n-Au_m) 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-C_n-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 Au_m. These results clearly demonstrate the triplet character of Au₂₅ and Au₃₈.