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

Abstract

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.

Symmetry-Dependent Dynamics in Au38(SC6H13)24 Revealed by Polarization-Dependent Two-Dimensional Electronic Spectroscopy

“…Experimental details of the femtosecond 2DES setup have been described previously. ,, A brief description is included here. Fundamental seed pulses at 1040 nm from a solid-state ytterbium amplifier (Spirit; Spectra-Physics) were modulated to a 100 kHz repetition rate and used to seed a dual-stage noncollinear optical parametric amplifier (NOPA).…”

“…31,32 Recently, our group used 2DES to study ultrafast carrier relaxation through the manifold of charge transfer resonances in neutral Au 38 (SC 6 H 13 ) 24 . 6 It was determined that (i) excitation is dominated by the higher-lying LUMO ← HOMO − 8 transition; (ii) charge carriers relax through internal conversion into a manifold (LUMO ← HOMO − n) of mixed, inorganic semiring and metal kernel states within 150 fs; and (iii) carriers relax through a picosecond charge transfer process prior to radiative recombination in the kernel-localized LUMO ← HOMO manifold. However, the high density of electronic states limited the identification of specific states that govern relaxation.…”

“…For example, gold nanospheres exhibit size-dependent collective electron–phonon coupling that increases from a few hundred femtoseconds to a few picoseconds for particle diameters between 2 and 8 nm . For metal nanoclusters in the 1–2 nm range, a few reports have demonstrated evidence of electronic-state-specific carrier relaxation mechanisms. − In particular, ultrafast two-dimensional electronic spectroscopy (2DES) has been used to resolve carrier dynamics in the superatomic P and D states in Au 25 (SC 8 H 9 ) 18 monolayer-protected clusters (MPCs) . In addition to electronic structure, geometric structure determines picosecond metal-to-ligand charge transfer processes for metal nanoclusters of the sub-2 nm size .…”

“…Two-dimensional electronic spectroscopy (2DES) has been used to obtain insights on state-specific relaxation dynamics in several biological, , molecular, , and nanomaterial systems. − In time-resolved 2DES, Fourier transformation of the transient signal collected as a function of the time delay between two phase-locked excitation pulses provides an excitation-energy axis that is used to map the initial electronic excitation of the system to its detected time-dependent third-order response, yielding a frequency–frequency correlation that reports on coupling of excited states and thermalization processes via line shape and amplitude modulations. , Recently, our group used 2DES to study ultrafast carrier relaxation through the manifold of charge transfer resonances in neutral Au 38 (SC 6 H 13 ) 24 . It was determined that (i) excitation is dominated by the higher-lying LUMO ← HOMO – 8 transition; (ii) charge carriers relax through internal conversion into a manifold (LUMO ← HOMO – n ) of mixed, inorganic semiring and metal kernel states within 150 fs; and (iii) carriers relax through a picosecond charge transfer process prior to radiative recombination in the kernel-localized LUMO ← HOMO manifold.…”