Synthesis, Structural and Photophysical Properties of Pentacene Alkanethiolate Monolayer-Protected Gold Nanoclusters and Nanorods: Supramolecular Intercalation and Photoinduced Electron Transfer with C₆₀

6,13-Bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene: TP) alkanethiolate monolayer-protected gold nanoclusters (TP-Cn-X-MPCs: X stands for small (S) and large (L) nanocluter sizes) and nanorods (TP-Cn-MPRs) with different alkyl chain lengths (n = 7, 11) were synthesized to examine the structural and photophysical properties as well as intercalation trends with C₆₀. The syntheses of TP-Cn-X-MPCs and TP-Cn-MPRs were successfully performed using two different precursors: TP disulfides and TP alkanethiols. The detailed structural properties were confirmed by ¹H NMR, elemental analyses, and transmission electron micrograph (TEM). In the spectroscopic absorption and fluorescence excitation measurements, spectral shapes of TP units on the gold surface were clearly observed, whereas fluorescence intensities of TP units were strongly quenched as compared to the corresponding reference monomer (TP-Ref). Then, fluorescence quenching titration experiments to determine the association constants (K_app) between C₆₀ and TP assemblies (TP-Cn-X-MPCs and TP-Cn-MPRs) were performed by adding C₆₀ in toluene. The K_app values were largely dependent on the sizes of nanoclusters and alkyl chain lengths in TP-Cn-X-MPC. For example, the K_app value of TP-C7-S-MPC (73 800 M^-1) was much larger than those of TP-C11-S-MPC (37 800 M^-1) and TP-C7-L-MPC (5350 M^-1). This trend is in sharp contrast with the similar K_app values (∼66 000 M^-1) in TP-Cn-MPR (n = 7, 11). These results suggest that the intercalation behaviors are dependent on the surface structures (nanocluster vs nanorod). Such fluorescence quenching processes by photoinduced electron transfer (PET) in the complex between TP-C7-S-MPC and C₆₀ were directly observed by femtosecond transient absorption measurements, monitoring the TP radical cation and C₆₀ radical anion.