Photophysical properties of a water-soluble cluster Au102(pMBA)44 (pMBA = para-mercaptobenzoic acid) are studied by ultrafast time-resolved mid-IR spectroscopy and density functional theory calculations in order to distinguish between molecular and metallic behavior. In the mid-IR transient absorption studies, visible or near-infrared light is used to electronically excite the sample, and the subsequent relaxation is monitored by studying the transient absorption of a vibrational mode in the ligands. Based on these studies, a complete picture of energy relaxation dynamics is obtained: (1) 0.5-1.5 ps electronic relaxation, (2) 6.8 ps vibrational cooling, (3) intersystem crossing from the lowest triplet state to the ground state with a time constant 84 ps, and (4) internal conversion to the ground state with a time constant of ∼3.5 ns. A remarkable finding based on this work is that a large cluster containing 102 metal atoms behaves like a small molecule in a striking contrast to a previously studied slightly larger Au144(SC2H4Ph)60 cluster, which shows relaxation typical for metallic particles. These results therefore establish that the transition between molecular and metallic behavior occurs between Au102 and Au144 species.

Injection of an electron from the excited dye molecule to the semiconductor is the initial charge separation step in dye-sensitized solar cells (DSC's). Though the dynamics of the forward injection process has been widely studied, the results reported so far are controversial, especially for complete DSC's. In this work, the electron injection in titanium dioxide (TiO 2 ) films sensitized with ruthenium bipyridyl dyes N3 and N719 was studied both in neat solvent and in a typical iodide/triiodide (I − /I 3 − ) DSC electrolyte. Transient absorption (TA) spectroscopy was used to monitor both the formation of the oxidized dye and the arrival of injected electrons to the conduction band of TiO 2 . Emission lifetime of the dye-sensitized films was recorded with time-correlated single photon counting to reveal nanosecond time scales of injection. It was found that the injection dynamics of the N3 and N719 dyes are similar. In solvent the injection from both dyes occurs in the femto-to picosecond time scale while in the I − /I 3 − electrolyte, it slows down significantly, extending to the nanosecond time domain. The presence of the electrolyte was found to increase the excited state lifetime of the dyes, implying that injection efficiency remains high despite the slower kinetics of injection compared to neat solvent. A remarkable new finding was that the prominent absorption signal of the oxidized dye observed in neat solvent vanished almost completely in the presence of the electrolyte, while the arrival of electrons to the conduction band of TiO 2 was practically unaltered, only slowed down. The observed disappearance of the oxidized dye population in the I − /I 3 − electrolyte is most likely related to the reduction of the oxidized dye by iodide I − , which is the first step of the dye regeneration process. To the best of our knowledge, this is the first time initial dye regeneration has been shown to occur in a few picoseconds after injection.

Energy relaxation dynamics of a gold nanocluster with atomically precise composition, Au144(SC2H4Ph)60, is studied by transient mid-IR spectroscopy. The experiment is designed to simultaneously probe electronic and vibrational dynamics by using excitation at 652 nm to prepare an electronic state localized on the gold core (as shown by high level DFT calculations) and by probing a stretching vibration localized on phenyl ring of the ligand molecules. We found that electronic relaxation proceeds with a time constant of 1.5 ps simultaneously heating the phonon bath of the cluster. The heat is further dissipated to solvent with a time constant of 29 ps. The electronic relaxation time increases with increasing pump power. Absence of long-lived electronic states and power dependence of relaxation time indicate metallic behavior. The metal–ligand interface modes are strongly anharmonically coupled to the probed mode which provides connection between the cluster core temperature and the vibrational shift of the ligand molecules. The obtained results are relevant for understanding energy relaxation dynamics of nanoclusters and together with the measured absolute molar absorption coefficient of the cluster allow designing experiments for controlled heating of the cluster by continuous wave irradiation.

Ligand-stabilized, atomically precise gold nanoclusters with a metal core of a uniform size of just 1-3 nm constitute an interesting class of nanomaterials with versatile possibilities for applications due to their size-dependent properties and modifiable ligand layers. The key to extending the usability of the clusters in applications is to understand the chemical bonding in the ligand layer as a function of cluster size and ligand structure. Previously, it has been shown that monodispersed gold nanoclusters, stabilized by meta-mercaptobenzoic acid (m-MBA or 3-MBA) ligands and with sizes of 68-144 gold atoms, show ambient stability. Here we show that a combination of nuclear magnetic resonance spectroscopy, UV-vis absorption, infrared spectroscopy, molecular dynamics simulations, and density functional theory calculations reveals a distinct chemistry in the ligand layer, absent in other known thiol-stabilized gold nanoclusters. Our results imply a low-symmetry C ligand layer of 3-MBA around the gold core of Au and Au and suggest that 3-MBA protects the metal core not only by the covalent S-Au bond formation but also via weak π-Au and O═C-OH···Au interactions. The π-Au and -OH···Au interactions have a strength of the order of a hydrogen bond and thus are dynamic in water at ambient temperature. The -OH···Au interaction was identified by a distinct carbonyl stretch frequency that is distinct for 3-MBA-protected gold clusters, but is missing in the previously studied Au(p-MBA) cluster. These thiol-gold interactions can be used to explain a remarkably low ligand density on the surface of the metal core of these clusters. Our results lay a foundation to understand functionalization of atomically precise ligand-stabilized gold nanoclusters via a route where weak ligand-metal interfacial interactions are sacrificed for covalent bonding.

We have determined vibrational signatures and optical gap of the Au144(PET)60 (PET: phenylethylthiol, SC2H4Ph) nanocluster solvated in deuterated dichloromethane (DCM-D2, CD2Cl2) and in a single crystal. For crystals, solid-state (13)C NMR and X-ray diffraction were also measured. A revised value of 2200 cm(-1) (0.27 eV) was obtained for the optical gap in both phases. The vibrational spectra of solvated AU144(PET)60 closely resembles that of neat PET, while the crystalline-state spectrum exhibits significant inhomogeneous spectral broadening, frequency shifts, intensity transfer between vibrational modes, and an increase in the overtone and combination transition intensities. Spectral broadening was also observed in the (13)C NMR spectrum. Changes in the intensity are explained due to vibrational coupling of the normal modes induced by the crystal packing, and the vibrational broadening is caused by ligand-environment inhomogeneity in the crystal. This indicates a pseudocrystalline state where the cluster cores are arranged in periodic fashion, while the ligand-layer molecules between the cores form amorphous structures.