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.

Isabelle Russier-Antoine,Vlasta Bonačić-Koutecký,Nathalie Calin,Marjan Krstić,Franck Bertorelle,Pierre-François Brevet,Philippe Dugourd,Rodolphe Antoine,Clothilde Comby-Zerbino,Željka Sanader

doi:10.1039/c6nr07989j

Abstract

We report a combined experimental and theoretical study of the two-photon absorption and excited emission properties of monodisperse ligand stabilized Ag11, Ag15 and Ag31 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.

Highlights

IntroductionNanoscale photon, is gaining a widespread popularity within the same community.[4] Here, the advantages lie in the increased penetration depth in tissues and in the reduced background fluorescence of the images collected
Two-photon absorption (TPA) defined as the electronic excitation of a molecular system induced by the simultaneous absorption of a pair of photons was first proposed theoretically PaperNanoscale photon, is gaining a widespread popularity within the same community.[4]
Three of the smallest silver nanoclusters, previously denoted as bands 1, 2, and 6 in the synthesized mixtures obtained by Bigioni and collaborators[27] were produced in a “size-focusing” way using a cyclic reduction under oxidative conditions.[28,29]

Summary

Introduction

Nanoscale photon, is gaining a widespread popularity within the same community.[4] Here, the advantages lie in the increased penetration depth in tissues and in the reduced background fluorescence of the images collected Within this context, considerable efforts have been made to engineer molecular systems or nanoparticles with the largest TPA and TPEF cross-sections. In the case of SHG for instance, the standard class of molecular systems is known as asymmetric push–pull chromophores whereas for the TPEF process it is rather customary to have centrosymmetric systems.[5,6,7] Push–pull chromophore systems consist of donor and acceptor end-groups connected by a charge conjugation path The latter introduces the possibility of a large charge transfer along the conjugated molecular axis upon excitation.

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Results

Conclusion