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Ligand shell size effects on one- and two-photon excitation fluorescence of zwitterion functionalized gold nanoclusters

Abstract : Gold nanoclusters (Au NCs) are an emerging class of luminescent nanomaterials but still suffer from moderate photoluminescence quantum yields. Recent efforts have focused on tailoring their emission properties. Introducing zwitterionic ligands to cap the metallic kernel is an efficient approach to enhance their one-photon excitation fluorescence intensity. In this work, we extend this concept to the nonlinear optical regime, i.e. two-photon excitation fluorescence. For a proper comparison between theory and experiment, both ligand and solvent effects should be considered. The effects of explicit ligands and of aqueous solvent on the optical properties of zwitterion functionalized gold nanoclusters have been studied by performing quantum mechanics/molecular mechanics (QM/MM) simulations. A. Introduction In the past decade, gold nanoclusters (Au NCs) 1 have emerged as fascinating luminescent nanomaterials. 2, 3 However, the photoluminescence quantum yield (QY) of most nanoclusters is still relatively low (generally of a few percent). Recent efforts focus on tailoring the emission properties, 4 including photoluminescence (PL) intensity and emission wavelength, the two basic characteristics of fluorescence. Some strategies such as surface ligand engineering, 5-7 aggregation-induced fluorescence, 8-10 silver doping, 11-16 and ligand-shell rigidifying 17, 18 have been achieved. In particular, the surface shell rigidification permits to decrease the energy loss due to intramolecular rotations and vibrations, thus increasing the PL intensity of nanoclusters. Introducing bidentate thiol zwitterionic ligands to cap the metallic kernel is another efficient approach to rigidify the surface structure of metal nanocluster. 19 Several groups have prepared bright fluorescent nanoclusters using this strategy. 20-23 The PL intensity and emission wavelength of these nanoclusters can be easily tailored via controlling the functional groups of the zwitterionic ligands. 24 Indeed, due, to the zwiterionic form of the tails of ligands, strong intermolecular electrostatic interactions occur allowing for the formation of different ligand shells on the surface of the gold clusters. 24, 25 Although it was found that the surface of Au NCs plays a major role in fluorescence generation, several fundamental issues are still not well understood. In particular, the extent to which the ligand shell and the metal core are coupled and how this coupling modulates the emission properties remain to be clarified. Therefore, theoretical explorations of the structural and electronic properties of various nanoclusters in the ground and excited states have been reported to address the origin of the photoluminescence enhancement. 26, 27 The Aikens group investigated the geometric and electronic structural changes of gold nanoclusters upon photoexcitation using time-dependent density functional theory (TD-DFT) method. 28-30 Moreover it was also shown that the inclusion of full ligands in calculations was needed to accurately describe the experimental optical spectra. 27, 28, 31 The combination of DFT with molecular mechanics within QM/MM approach appears to be appropriate and was recently used for structural elucidation of the clusters with explicit ligands. 32 Akola and coworkers 33, 34 used QM/MM simulations to study the influence of aqueous solvent and explicit ligands (e.g. glutathione ligands) on the structural and electronic properties of thiolate-protected Au25(SR)18-clusters. In this article, we show that the introduction of bidentate thiol zwitterionic ligands to cap the metallic kernel is another efficient approach to enhance nonlinear response in particular two-photon excitation fluorescence. In order to address better the ingredients responsible for their enhanced photo emission, we employ a QM/MM approach with explicit ligands. This approach permits us to address the linear and nonlinear optical regime of zwitterion functionalized gold nanoclusters (AuZw NCs) evaluating effects of ligand shell size. Optical properties including one-and two-photon absorption spectra obtained at the QM/MM level of theory are in qualitative agreement with optical measurements. Ligand shell rigidification should prevent excited state geometry relaxation through intramolecular motions, as well as through the associated non-radiative energy decay pathways. For this purpose we
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Martina Perić, Zeljka Sanader, Isabelle Russier-Antoine, Hussein Fakhouri, Franck Bertorelle, et al.. Ligand shell size effects on one- and two-photon excitation fluorescence of zwitterion functionalized gold nanoclusters. Physical Chemistry Chemical Physics, Royal Society of Chemistry, 2019, 21 (43), pp.23916-23921. ⟨10.1039/c9cp05262c⟩. ⟨hal-02355427⟩

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