Porphyrin-Acetylide Grafted Gold Clusters: Synthesis, Photophysical Studies, and Explosive Sensing

A porphyrin acetylide (P)-stabilized gold cluster (Au:P) with an average cluster diameter of 1.5 ± 0.2 nm is synthesized using a poly(vinylpyrrolidone) (PVP)-supported gold cluster (Au:PVP) precursor following the optimized biphasic ligand-exchange method. Other reference gold clusters are also synthesized by using phenylacetylene (PA), 2-phenylethanethiol (PET), and 9-ethynylphenanthrene (EP), following the same synthetic strategy. Detailed photophysical studies revealed an efficient through-space partial charge/electron transfer (CT/ET) process from porphyrin to the gold cluster in the case of Au:P, causing substantial luminescence quenching as evidenced by the shortening of excited-state lifetime values. The process of ligand-to-metal partial CT/ET is higher in the case of the porphyrin acetylide-appended gold cluster {Aun(−C≡C–R1)m, where R1 = P–H} compared to that of the thiol-ligated gold cluster {Aun(−S–R2)m, where R2 = PET–H}, which might be due to the better electronic communication through the rigid gold–carbon bonds present in the Au:P cluster. In addition, compared to PA, PET, and EP ancillary ligands used in the present study, porphyrin is a more potent electron donor to trigger an efficient CT/ET process. Among this series of gold clusters, Au:P shows better efficacy in detecting nitro-organic explosives (NOEs) with enhanced porphyrin-based emission under ambient conditions. It is speculated that this turn-on luminescence response in the presence of NOEs is associated with the inhibition of the through-space CT/ET process operating within the NOE-intercalated porphyrin–gold cluster.