The recent emergence of advanced technological applications for colloidal gold suspensions and related particle assemblies and interfaces has created a demand for new chemical and physical techniques with which to characterize them. For macroscopic samples/interfaces, coherent second harmonic generation (SHG) has proven itself a useful characterization tool due, at least in part, to metal-based plasmon enhancement. In an effort to defeat or bypass the size restrictions inherent to SHG, we have utilized a related incoherent methodology, hyper-Rayleigh scattering (HRS), to interrogate aqueous colloidal suspensions of 13 nm diameter gold particles. The nanoscale particles have proven to be remarkably efficient scatterers; when evaluated in terms of the first hyperpolarizability ( ), HRS signals from the gold particles substantially surpass those observable from the best available molecular chromophores. Moreover, the present experiments indicate that is highly sensitive to colloid aggregation and imply that HRS is an effective tool for the characterization of symmetry-reducing perturbations of nanoscale interfaces.The intentional miniaturization of metal/solution interfaces to the point where colloidal stabilization of suspensions of solvated metal particles occurs has been possible, at least at a rudimentary level, for more than a century. Arguably, however, only over the past decade has the concept generated widespread excitement among chemists and materials scientists. Fueling the contemporary interest are demonstrated and emerging applications involving medical screening, 1 chemical sensing, 2 singleelectron conduction (Coulomb blockade behavior), 3 optical frequency tripling, 4 and new materials-assembly schemes. [5][6][7] Accompanying the new applications chemistry is a need for interface characterization. Among the more attractive methodologies for macroscopic metal/solution interface characterization is optical second harmonic generation (SHG). 8 For otherwise centrosymmetric metal structures, interface formation creates an asymmetry providing for interface localized signal generation. SHG has been utilized to interrogate surface symmetry, 9,10 surface charge, 11 adsorbate coverage, 12 and/or adsorbate orientation 13 on gold surfaces as well as to interrogate gold particles at the liquid/air interface. 14 However, the coherent nature of conventional SHG and the necessity of utilizing a finite wavelength of radiation place a practical lower limit on sample size. 15 On the other hand, implementation of incoherent second harmonic generation or hyper-Rayleigh scattering (HRS) methods 16 should circumvent the interface size limitation (albeit at the expense of much smaller signal intensity). 17,18 On the basis of a successful application of HRS to nanoscale silica/water interfaces (surface acidity characterization) 19 and on the basis of a brief report of HRS from silver colloid samples (molecular adsorption effects), 20 we reasoned that the methodology might be applicable to nanoscale gold/water interfaces...

Electroabsorption or Stark effect spectroscopy has been applied to a pair of porphyrin-based chromophores with the aim of deriving a detailed understanding of the origin of the remarkable first hyperpolarizability, β, of one of the chromophores:  [5-[[4‘-(dimethylamino)phenyl]ethynyl]-15-[(4‘‘-nitrophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II). The measurements show that significant changes in molecular dipole moment, Δμ, accompany excitations of x-polarized transitions of the aminophenyl (donor)/nitrophenyl (acceptor) functionalized chromophore, but are absent for a related chromophore lacking the donor and acceptor groups. For linear chromophores, changes in dipole moment are a prerequisite to effective molecular first hyperpolarization and incident light frequency doubling behavior. A more detailed consideration of the available |Δμ| data, within the context of a conventional two-level model, has yielded the following:  (a) specific information about the roles of each of seven electronic and vibronic excited states in defining β, (b) a semiquantitative explanation for the apparent frequency independence of β found in previous hyper-Rayleigh scattering experiments performed at 830 and 1064 nm, and (c) an explanation for the contrasting frequency dependence of the nonlinear optical response for the analogous Cu(II)-containing chromophore. Finally, the experimental findings are in generally good agreement with published ZINDO calculations which had pointed toward the exceptional effectiveness of yne linkages in coupling donor and acceptor moieties to the highly polarizable porphyrin core assembly.

No abstract

Polarized hyper-Rayleigh scattering (HRS) has been used to interrogate a putative octupolar “super” chromophore Ru(trans-4,4‘-diethylaminostyryl-2,2‘-bipyridine)3 2+. While the hyperpolarizability (β) is impressive, it is apparent from polarized HRS measurements that the symmetry of the excited state, and thus the hyperpolarizability tensor, is best described as dipolar in nature. Electroabsorption (electronic Stark effect) experiments support these findings, implying that changes in dipole moment (Δμ) accompany optical excitation. The measured |Δμ| contributions from both metal-to-ligand charge transfer and intraligand transitions have been used to model the wavelength-dependent hyperpolarizability; reasonable qualitative agreement between experimental results and a simple three-state, two-level dipolar model is obtained.