Designing highly efficient second-order v
(2) and third-order v
(3) NLO chromophores is largely a matter of finely combining a high density of delocalized
electrons in a symmetrical or unsymmetrical environment. Gold and silver NCs
constitute therefore good candidates (Fig. 5.1; right), although the reported v
(2)
values are still weaker than those of push–pull dyes [8] and are not currently
competitive as contrast agents for MPE microscopy. Our theoretical and experimental joint investigation on ligand-protected silver and gold clusters [9–17] has
shown that the structure of the metal atom core, its charge, and symmetry, dramatically influencing the NLO cross sections and the core stabilizing ligands, play a
major role in NLO efficiencies. This new class of NLO materials is coined as
“ligand-core” NLO-phores [18, 19].
This feature article overviews mainly experimental methodologies used for
determination and analysis of two-photon absorption (TPA) and two-photon excited
(TPEF) emission properties of atomically precise clusters of gold and silver. Using
results obtained on atomically precise clusters of silver, I will show that both
experimental and theoretical data can be used to get an extensive comprehension of
the physics underlying the two-photon absorption process and its amplitude, as well
as to suggest an exploratory root for novel molecular engineering for further
enhancement of TPA [20]. Also, I will describe some strategies that have been
suggested recently to enhance TPEF properties. This includes the possibility to
increase the rigidity of the protective shell [21] as well as metal core-doping [22], a
promising strategy as it distorts the metallic atom core.
The paper is organized as follows. In Sect. 5.2, I define the atomically precise
clusters of gold and silver, from their synthesis and characterization toward their
(linear) optical properties. In Sect. 5.3, I summarize the main theoretical and
computational models used for calculation of the nonlinear responses and describe
the leading factors responsible for enhanced NLO efficiencies. Section 5.4 deals
with the principal experimental techniques and challenges for measurement of the
TPA/TPEF cross section. A variety of representative “ligand-core” NLO-phores are
investigated in Sect. 5.5, allowing for rationalization of different structural effects
on the TPA/TPEF cross section. Finally, the main conclusions as well as some
future perspectives are drawn in Sect. 5.6.
5.2 Atomically Precise Clusters of Gold and Silver:
Synthesis, Characterization, and Optical Properties
5.2.1 Atomically Precise Clusters of Gold and Silver
For small noble metal clusters—in the size range where each atom counts—
Mie-Drude-like model [23] that predicts the optical response of free-electron metals
in the bulk state is no longer appropriate to discuss absorption spectra in details.
5 Ligand-Core NLO-Phores
141
(2) and third-order v
(3) NLO chromophores is largely a matter of finely combining a high density of delocalized
electrons in a symmetrical or unsymmetrical environment. Gold and silver NCs
constitute therefore good candidates (Fig. 5.1; right), although the reported v
(2)
values are still weaker than those of push–pull dyes [8] and are not currently
competitive as contrast agents for MPE microscopy. Our theoretical and experimental joint investigation on ligand-protected silver and gold clusters [9–17] has
shown that the structure of the metal atom core, its charge, and symmetry, dramatically influencing the NLO cross sections and the core stabilizing ligands, play a
major role in NLO efficiencies. This new class of NLO materials is coined as
“ligand-core” NLO-phores [18, 19].
This feature article overviews mainly experimental methodologies used for
determination and analysis of two-photon absorption (TPA) and two-photon excited
(TPEF) emission properties of atomically precise clusters of gold and silver. Using
results obtained on atomically precise clusters of silver, I will show that both
experimental and theoretical data can be used to get an extensive comprehension of
the physics underlying the two-photon absorption process and its amplitude, as well
as to suggest an exploratory root for novel molecular engineering for further
enhancement of TPA [20]. Also, I will describe some strategies that have been
suggested recently to enhance TPEF properties. This includes the possibility to
increase the rigidity of the protective shell [21] as well as metal core-doping [22], a
promising strategy as it distorts the metallic atom core.
The paper is organized as follows. In Sect. 5.2, I define the atomically precise
clusters of gold and silver, from their synthesis and characterization toward their
(linear) optical properties. In Sect. 5.3, I summarize the main theoretical and
computational models used for calculation of the nonlinear responses and describe
the leading factors responsible for enhanced NLO efficiencies. Section 5.4 deals
with the principal experimental techniques and challenges for measurement of the
TPA/TPEF cross section. A variety of representative “ligand-core” NLO-phores are
investigated in Sect. 5.5, allowing for rationalization of different structural effects
on the TPA/TPEF cross section. Finally, the main conclusions as well as some
future perspectives are drawn in Sect. 5.6.
5.2 Atomically Precise Clusters of Gold and Silver:
Synthesis, Characterization, and Optical Properties
5.2.1 Atomically Precise Clusters of Gold and Silver
For small noble metal clusters—in the size range where each atom counts—
Mie-Drude-like model [23] that predicts the optical response of free-electron metals
in the bulk state is no longer appropriate to discuss absorption spectra in details.
5 Ligand-Core NLO-Phores
141