Ligand-core NLO-phores: a combined experimental and theoretical approach to the two-photon absorption and two-photon excited emission properties of small-ligated silver nanoclusters

Abstract

We report a combined experimental and theoretical study of the two-photon absorption and excited emission properties of monodisperse ligand stabilized Ag, Ag and Ag nanoclusters in aqueous solutions. The nanoclusters were synthesized using a cyclic reduction under oxidative conditions and separated by vertical gel electrophoresis. The two-photon absorption cross-sections of these protected noble metal nanoclusters measured within the biologically attractive 750-900 nm window are several orders of magnitude larger than that reported for commercially available standard organic dyes. The two-photon excited fluorescence spectra are also presented for excitation wavelengths within the same excitation spectral window. They exhibit size-tunability. Because the fundamental photophysical mechanisms underlying these multiphoton processes in ligand protected clusters with only a few metal atoms are not fully understood yet, a theoretical model is proposed to identify the key driving elements. Elements that regulate the dipole moments and the nonlinear optical properties are the nanocluster size, its structure and the charge distribution on both the metal core and the bound ligands. We coined this new class of NLO materials as "Ligand-Core" NLO-phores.

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) …”

“…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 . However, the main conclusion so far is that liganded silver and gold quantum clusters are excellent two‐photon absorbers but are rather poor two‐photon excited emitters .…”

“…[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. [10] For example, the two-photon absorption cross-section at 780 nm for Au 15 is % 65700 GM, whereas its TPEF cross-section at 475 nm is only % 0.022 GM.…”

“…[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.…”

Atomically precise clusters of gold and silver: A new class of nonlinear optical nanomaterials

“…Our investigation on ligand protected silver and gold clusters [8][9][10][11][12][13][14][15] has shown that the structure of the metal atom core, its charge and symmetry, dramatically influence the NLO cross-sections and the core stabilizing ligands play a key role in NLO efficiencies. We coined this new class of NLO materials : "Ligand-Core" NLO-phores 16,17 .…”