Ultrafast Charge‐Transfer Dynamics in Aggregated Gold Nanoparticle–Ru(bpy)₃²⁺ Assemblies Probed by Transient Absorption Spectroscopy

Gold nanoparticles (AuNPs) localize electromagnetic fields and generate energetic (“hot”) carriers under optical excitation, enabling photocatalysis, solar conversion, and molecular photochemistry. They are strong model systems for nanoscale light–matter studies, yet direct observation of interfacial charge transfer in plasmonic–molecular hybrids remains difficult, especially when molecular states couple to collective plasmon modes on femtosecond timescales.

We synthesized spherical AuNPs and induced controlled aggregation in the presence of tris(bipyridine)ruthenium(II) (Ru(bpy)₃²⁺, “Rubpy”), a photosensitizer with a well-defined MLCT excitation. Tuning ionic strength and surfactants yielded reproducible aggregates with optical access, allowing systematic variation of interparticle coupling and plasmon resonances.

Femtosecond transient absorption (TA) following MLCT excitation tracked the ultrafast response. Preliminary spectra show Rubpy excited-state decay, a broad plasmonic bleach, and long-lived absorption tails consistent with charge separation at the metal–molecule interface. Comparing dispersed and aggregated samples, aggregation markedly shifts spectral evolution and relaxation kinetics. Early kinetic fits indicate electron injection from photoexcited Rubpy into AuNP aggregates within a few picoseconds, followed by plasmon-mediated stabilization or redistribution of carriers across the nanoparticle network.

These results reveal interplay between molecular excited states and delocalized plasmonic fields. Aggregation appears to enhance both the rate and efficiency of electron transfer via stronger near-field coupling and increased electronic communication across particle junctions, creating plasmonically “active” environments that can channel photoexcited carriers into chemical or electronic pathways.

This framework advances mechanistic understanding of photoinduced charge transfer in molecular–plasmonic systems and informs design of plasmonic architectures for photoredox catalysis, optoelectronics, and hybrid solar platforms. Future work will probe two-photon and other nonlinear excitation under plasmonic confinement to map how structure, coupling, and excitation conditions govern energy flow in these hybrid nanosystems.