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Phys Chem Chem Phys ; 22(9): 5272-5285, 2020 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-32095793


We investigate the excited electron dynamics in [Au25(SR)18]-1 (R = CH3, C2H5, C3H7, MPA, PET) [MPA = mercaptopropanoic acid, PET = phenylethylthiol] nanoparticles to understand how different ligands affect the excited state dynamics in this system. The population dynamics of the core and higher excited states lying in the energy range 0.00-2.20 eV are studied using a surface hopping method with decoherence correction in a real-time DFT approach. All of the ligated clusters follow a similar trend in decay for the core states (S1-S6). The observed time constants are on the picosecond time scale (2-19 ps), which agrees with the experimental time scale, and this study confirms that the time constants observed experimentally could originate from core-to-core transitions and not from core-to-semiring transitions. In the presence of higher excited states, R = H, CH3, C2H5, C3H7, and PET demonstrate similar relaxations trends whereas R = MPA shows slightly different relaxation of the core states due to a smaller gap between the LUMO+1 and LUMO+2 gap in its electronic structure. The S1 (HOMO → LUMO) state gives the slowest decay in all ligated clusters, while S7 has a relatively long decay. Furthermore, separate electron and hole relaxations were performed on the [Au25(SCH3)18]-1 nanocluster to understand how independent electron and hole relaxations contribute to the overall relaxation dynamics.

J Chem Phys ; 151(9): 094702, 2019 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-31492077


Experimental findings of Au18(GSH)14 as a photosensitizer with the highest potential compared to other glutathione-protected clusters demand understanding the photophysics and relaxation dynamics of the Au18(SR)14 cluster. To this end, we perform ab initio real-time nonadiabatic molecular dynamics simulations on Au18(SH)14 to investigate its relaxation dynamics compared to the well-studied [Au25(SR)18]-1 relaxation dynamics. In this work, the excitations covering up to ∼2.6 eV in the optical absorption spectrum are analyzed to understand the electronic relaxation process of the Au18(SH)14 cluster. The ground state growth times of Au18(SH)14 are several orders of magnitude shorter than the growth times observed for the [Au25(SH)18]-1 nanocluster. The S1 (HOMO-LUMO) state gives the slowest decay time (∼11 ps) among all the states (S1-S30) considered similar to [Au25(SH)18]-1. However, the S1 state in Au18(SH)14 is a semiring-to-core charge transfer state, whereas S1 in the [Au25(SH)18]-1 cluster is a core-to-core transition. The remaining higher excited states have very short decay time constants less than 1.4 ps except for S2 which has the second slowest decay of 6.4 ps. The hole relaxations are faster than the electron relaxations in Au18(SH)14 due to the closely packed HOMOs in the electronic structure. Radiative relaxations are also examined using the time-dependent density functional theory method, and the excited state emission energy and lifetime are found to be in good agreement with experiment.

Dalton Trans ; 48(11): 3635-3640, 2019 Mar 12.
Artigo em Inglês | MEDLINE | ID: mdl-30747941


A diphosphine-protected 18-gold-atom nanocluster was isolated via a facile reduction of an AuI precursor by NaBH4. Its composition was identified as {[Au18(dppm)6Cl4]·C6H6·3Cl·PF6} (SD/Au18, SD = SunDi; dppm = bis-(diphenylphosphino)methane) by X-ray single crystal structural analysis. This nanocluster possesses a prolate shape and is built from an Au10 kernel (bi-octahedral Au6 units sharing one edge) fused with two Au7 caps via sharing six gold atoms. The identity of the Au18 cluster is further demonstrated by ESI-MS. The number of valence electrons of [Au18(dppm)6Cl4]4+ is 10 (n* = 18-4-4), which does not match with the known magic numbers according to the spherical jellium model, and elongated models must be considered. The special stability of the Au18 cluster likely arises from geometrical factors in the metallic core. Two charge states are reported for this system. This work not only presents the structure elucidation of a diphosphine-protected Au18 nanocluster, but also provides an important insight into the growth pattern of gold nanoclusters and the charge states they can achieve.

Chem Sci ; 9(5): 1251-1258, 2018 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-29675171


Due to distinctive quantum confinement effects, ultrasmall gold nanoparticles usually exhibit interesting electronic structure and molecular-like properties. However, the lack of atomically-precise structural information makes the understanding of them almost impossible, such as understanding the relationships between their compositions and unique properties. Herein, by reducing a diphosphine AuI precursor (Au2(dppm)2Cl2; dppm = Ph2PCH2PPh2) with or without a S2- releasing reagent, we enriched our knowledge of the members in the families of Au13 and Au8 by the structural determinations of two new dppm-protected gold nanoclusters, [Au13(dppm)6]5+ (SD/Au1) and [Au8(dppm)4S2]2+ (SD/Au2), respectively. Within SD/Au1, the Au13 kernel significantly deviates from the ideal Ih icosahedron by the elongation of three surface Au-Au bonds to ∼3.5 Å, giving it C3 symmetry, whereas SD/Au2 has a novel heart-shaped C2 symmetric Au8S2 core (central Au4 tetrahedron + two Au2S units) protected by four µ2-dppm ligands in the outer shell. Of note, SD/Au1 represents a rare Au13 nanocluster with an opened icosahedral geometry, and SD/Au2 shows a new edge-shared "core + 4exo" structure type that has never been observed before. The electronic structures and optical absorption spectra of these systems are correlated with time-dependent density functional theory (TDDFT) calculations. Based on the spherical jellium model, the stability of the Au13 and Au8 nanoclusters can be ascribed to 8- and 2-electron superatoms with 1S21P6 and 1S2 configurations, respectively. Interestingly, the cluster SD/Au2 exhibits bright yellow luminescence with an emission maximum at 591 nm that slightly hypsochromically shifts to 581 nm upon cooling to 93 K. Our findings not only enrich the family of diphosphine-protected ultrasmall gold nanoclusters, but also demonstrate the rich variations of gold kernels during the transformation from a simple AuI precursor to Au nanoclusters.