Ultrafast relaxation dynamics of Au₃₈(SC₆H₁₃)₂₄ monolayer-protected clusters resolved by two-dimensional electronic spectroscopy

Electronic relaxation dynamics of neutral Au₃₈(SC₆H₁₃)₂₄ monolayer-protected clusters (MPCs), following excitation of the mixed 15 875 cm⁻¹ charge transfer resonance, were studied using femtosecond transient absorption (fsTA) and two-dimensional electronic spectroscopy (2DES). The excited carriers relax by three different mechanisms, including an ∼100 fs HOMO_−12/−13 to HOMO_−4/−6 hole transfer, picosecond HOMO_−4/−6 to HOMO hole transfer, and subsequent electron–hole recombination that persisted beyond the hundreds of picoseconds measurement range. The fsTA data revealed two transient bleach components at 15 820 and 15 625 cm⁻¹, where the lower frequency component exhibited a delayed first-order buildup of 80 ± 25 fs that matched the decay of the high-energy bleach component (110 ± 45 fs). These results suggested that the excited charge carriers internally relax within the exited-state manifold in ≈100 fs. 2DES resolved multiple electronic fine-structure transient peaks that spanned excitation frequencies ranging from 15 500 to 16 100 cm⁻¹. State-to-state dynamics were understood by the analysis of time-dependent 2DES transient signal amplitudes at numerous excitation-detection frequency combinations. An off-diagonal cross peak at 15 825–15 620 cm⁻¹ excitation-detection signified the HOMO_−12/−13 to HOMO_−4/−6 hole transfer process. The lowest-frequency (15 620 cm⁻¹) 2DES diagonal fine-structure peak exhibited instantaneous amplitude but intensified following a 75 ± 10 fs buildup when compared to diagonal peaks at higher frequencies. This observation indicated that the charge transfer resonance in Au₃₈(SC₆H₁₃)₂₄ MPCs is comprised of several electronic transitions of unique spectral weights, which may result from different orbital contributions associated with specific cluster domains. The use of 2DES in combination with structurally precise MPCs can provide a platform for understanding structure-dependent electronic dynamics in metal nanoclusters and technologically important metal–chalcogenide interfaces.