Regioselective Ligand Substitution of Thiolate-Protected Gold Nanoclusters with Porphyrins

Abstract

Hybridization of photofunctional organic molecules like porphyrins with inorganic nanomaterials provides interesting photophysical properties like photo-induced electron transfer, energy transfer, or photoemission from charge transfer states. For fine tuning of these photo-functionalities, precise control over organic-inorganic interfaces, including spatial distances and orientations of organic molecules against inorganic nanomaterial, should be crucial. However, control and evaluation of the interfaces are quite difficult, mainly due to heterogeneous and unclear environments on the surface of inorganic nanomaterials like metal nanoparticles and metal nanosheet, which basically have size and/or composition distributions.Among the inorganic nanomaterials, atomically-precise metal nanoclusters (MNCs) are one of the homogeneous materials with uniform composition and distinct surface structure at an atomic level. Thus, MNCs can be an appropriate model to evaluate the effects of organic-inorganic interfaces on photophysical properties of the hybridized materials. Nevertheless, even in one of the most common and simplest MNCs like thiolate-protected Au25 cluster ([Au25(SR)18]–, SR = thiolate), two kinds of inequivalent ligands (S1, S2 in Figure 1) are present on surface. Thus, control over the regioselectivity should be required to archive precise substitution of photo-functional molecules, which has yet to be reported. In this work, we have controlled regioselectivity in the modification of [Au25(SC2Ph)18]– (SC2Ph = phenylethanethiolate) with porphyrinthiol ligands (Por) through ligand-exchange reactions. An advantage of Por as a photofunctional ligand for Au25 cluster is more bulkiness than that of original ligands (SC2Ph), which aids the isolation of mono-porphyrin-coordinated Au25 cluster ([Au25(SC2Ph)17(Por)1]–, Au25-Por1 ). Moreover, synthetic flexibility of porphyrins enabled us to control the regioselectivity in Au25-Por1 systematically.We sythesized a series of Por with a thiol moiety at different positions to evaluate the steric effect on the regioselectivity in ligand-exchange reactions (Figure 1). Porphyrin thiol with different substituents at meso-phenyl groups were also synthesized to evaluate the electronic effect. Substitution of porphyrins onto Au25 clusters was conducted by ligand-exchange reactions of [Au25(SC2Ph)18]– with 1 equivalent of Por in tetrahydrofuran. After the careful purification of the reaction mixture through size-exclusion column chromatography, we have successfully isolated Au25-Por1 in ca. 20% yield. The characterizations of Au25-Por1 were performed by MALDI-TOF-MS and 1H NMR spectroscopy, which confirmed the high purities of a series of Au25-Por1 .The regioselectivity in Au25-Por1 was evaluated through 1H NMR measurements. When using para-substituted porphyrin (PorSH) as an incoming ligand, 1H NMR spectrum of Au25-SPor1 in acetone-d 6 indicated the presence two kinds of regioisomers with the ratio of 2.5:1. Interestingly, the regioselectivity increased up to 80% (4:1) in Au25 cluster with more steric meta-substituted porphyrin (Au25-S m Por1 ), whereas the ratio reached to almost 1:1 in the case of Au25 cluster with less steric phenyl-spacer introduced porphyrin (Au25-SPhPor1 ). Because S2 position in the Au25 clusters was less steric than S1 position, we concluded that more steric ligands like m PorSH tend to be introduced at S2 position favorably. On the other hand, we hardly observed the electronic effect of Por on the regioselectivity in Au25-Por1 , indicating the importance of steric factor of an incoming ligand for precise control over the exchanged position (Figure 1). This is the first report to control over ligand-exchange position of MNCs, which could be achieved through ligand design.Finally, we briefly evaluated the photophysical properties of Au25-Por1 in toluene solution. When the Soret band of coordinated-porphyrins was photoexcited, the fluorescence quenching was clearly observed in each Au25-Por1 . Judging from the energy diagram of Au25-Por1 , electron transfer from Au25 to singlet excited state of porphyrin ligand would occur to form charge separated state. It should be noted that the quenching efficiency depended on not only the distance between electron donor and acceptor, but also the position of the coordinated chromophore. These results suggest the importance of precise modification of organic-inorganic interfaces to tune photodynamics of chromophore-nanocluster systems.In conclusion, we have successfully controlled the regioselectivity in the ligand-exchange reactions of thiolate-protected gold nanoclusters with porphyrinthiol derivatives through the steric effect. The essential roles of the ligand-introduced position on altering photodynamics of porphyrin-coordinated MNCs were also clarified, which provides an interesting insight into fine-tuning of photofunctional materials.Figure 1.Reaction scheme of ligand-exchange reaction of [Au25(SC2Ph)18]– with Por. The R groups of SR ligands were omitted for clarity. Figure 1