Two-photon absorption of ligand-protected Ag₁₅ nanoclusters. Towards a new class of nonlinear optics nanomaterials

Abstract

We report theoretical and experimental results on two-photon absorption (TPA) cross section of thiolated small silver cluster Ag15L11 exhibiting extraordinary large TPA in red. Our findings provide the responsible mechanism and allow proposing new classes of nanoclusters with large TPAs which are promising for biological and medical applications.

Importance of double-resonance effects in two-photon absorption properties of Au25(SR)18−

“…Previous simulations have found very large TPA cross-sections for small monolayer protected gold and silver clusters due to double-resonance effects. 24,51,52 A TPA cross-section of 620 000 GM at a photon energy of 1.58 eV was reported for the Au 25 (SH) 18 – cluster using B3LYP and a SDD-DZ basis set, with similar values found for the other functionals tested. 24 The large TPA cross-section is in good agreement with the experimental observation and was attributed to resonance enhancement from the lowest excited state.…”

Bulky Counterions: Enhancing the Two‐Photon Excited Fluorescence of Gold Nanoclusters

“…Such a difference between emission resulting from absorption of one or two photons was already reported on DNA‐protected and thiolate‐protected silver clusters . This behavior might be accounted for the different nature of the metal‐to‐metal excitations within the gold cluster core (blue emission) and the ligand‐metal and ligand‐metal‐metal charge transfer excitations (red emission) …”

“…Thus, these ligated nanoclusters may provide a valuable route within the nonlinear optical regime. We have described in a previous experimental and theoretical investigation how to improve two‐photon absorption (TPA) properties in such quantum nanoclusters, through the concept of “ligand‐core” NLO‐phores . The structure of the metallic core, its charge and symmetry (influencing the NLO cross sections), and the ligands stabilizing the core play major roles in TPA efficiencies .…”

“…We have described in ap revious experimental and theoretical investigation how to improve two-photon absorption (TPA) properties in such quantum nanoclusters, through the concept of "ligand-core" NLOphores. [8] The structure of the metallic core, its chargea nd symmetry (influencing the NLO cross sections), and the ligands stabilizing the core play major roles in TPAe fficiencies. [9] However,t he main conclusion so far is that liganded silver and gold quantum clustersa re excellent two-photon absorbers but are rather poor two-photon excited emitters.…”

“…[19] This behavior might be accounted for the different nature of the metal-to-metale xcitations within the gold cluster core (blue emission) and the ligand-metal and ligand-metal-metal charget ransfer excitations (red emission). [8,9,19] We also explored the solvent dependence of TPEF.T he TPEF intensity of both paired bulky cations,A u 15 and Au 18 NCs, significantly increases with decreasing dielectric constant of the solvent( methanol:d ielectric constant e = 32.7 versus DCM:d ielectric constant e = 8.9, see Figure 2). Indeed, more polar is the medium, weaker is the effect of ion pairing.…”