In Silico Stark Effect: Determination of Excited-State Polarizabilities of Squaraine Dyes.

The static electric polarizabilities (α) of a quadrupolar squaraine dye are investigated in silico, either as the excess polarizability, i.e., the change from the ground to the lowest excited state, Δα, or as those of the two states separately, depending on the approach. The polarizabilities are worked out by making use of the energy and dipole moment Taylor expansions as a function of the electric field (E), in which α is represented by the quadratic and linear terms, respectively, and also by means of the linear response approach. Dipoles and energies are computed at a few values of the electric field, with different strategies that consider the geometry of the molecule either frozen in the ground state or relaxed at each E value. From a physical standpoint, the most appropriate approach to describing the molecular polarizability depends on the processes in which the molecule is involved: for example, fluorescence methods provide information about relaxed excited states, and absorption methods are used to determine the polarizability changes of excited states frozen in the ground-state conformation. We show that the excited-state polarizability does not strongly differ from the ground-state polarizability when the field is applied along the main axis of the squaraine. In contrast, remarkable differences are found when the field is applied perpendicular to the molecular plane due to a large geometrical distortion of the molecular backbone.

Wavelength dispersion of the local field intensity in silver-gold nanocages.

This study provides a combined theoretical and experimental analysis of the far-field (extinction) and of the near-field (SERS enhancement) spectral distribution in hollow nanoparticles, that is, silver-gold nanocages (NCs). Chitosan protected NCs have been synthesized by a galvanic replacement-based procedure: their morphological properties and chemical composition have been characterized by TEM, STEM and ICP. NCs were then functionalized with a thiolated organic dye prior to carrying out SERS measurements. Finite Element Method simulations of a single NC have shown that the field enhancement at the excitation wavelength follows the same spectral dependence as the extinction spectrum and, consequently, the SERS enhancement profile, as a function of the excitation wavelength, peaks at higher energy with respect to extinction. The simulated extinction is remarkably narrower than the experimental spectrum of NCs in solution, indicating that the colloidal sample is substantially polydispersed. However, a simple qualitative model that we have developed would suggest that the SERS enhancement profile is blue-shifted with respect to the extinction in the presence of polydispersivity as well. In addition, NC dimers have been simulated: both their extinction and near field-spectra shift to the red when the size of the gap is reduced analogous to what happens with dimers of filled spherical nanoparticles (NPs). In addition, simulations also revealed that a NC dimer is only slightly more efficient in amplifying the field with respect to the isolated NC, and this behavior is peculiar to NCs. In fact, filled spherical NP dimers exhibit a remarkably stronger field enhancement with respect to the isolated NP. By means of Wavelength Scanned SERS, we measured the spectral distribution of the local field in a dispersion of NCs. We observed experimentally that the local field is distributed in the same spectral region as the extinction and that the absolute value of the SERS enhancement factor maintains a low value throughout the range explored (568-800 nm). We propose that the observed correlation between the SERS profile and the extinction is accidental and originates from the limited increase in amplification provided by NC aggregates with respect to isolated NCs.

Growth and optical properties of silver nanostructures obtained on connected anodic aluminum oxide templates.

Ag nanostructures are grown by AC electrodeposition on anodic alumina oxide (AAO) connected membranes acting as templates. Depending on the thickness of the template and on the voltage applied during the growth process, different Ag nanostructures with different optical properties are obtained. When AAO membranes about 1 µm thick are used, the Ag nanostructures consist in Ag nanorods, at the bottom of the pores, and Ag nanotubes departing from the nanorods and filling the pores almost for the whole length. When AAO membranes about 3 µm thick are used, the nanostructures are Ag spheroids, at the bottom of the pores, and Ag nanowires that do not reach the upper part of the alumina pores. The samples are characterized by angle resolved x-ray photoelectron spectroscopy, scanning electron microscopy and UV-vis and Raman spectroscopies. A simple NaOH etching procedure, followed by sonication in ethanol, allows one to obtain an exposed ordered array of Ag nanorods, suitable for surface-enhanced Raman spectroscopy, while in the other case (3 µm thick AAO membranes) the sample can be used in localized surface plasmon resonance sensing.

Multipolar symmetric squaraines with large two-photon absorption cross-sections in the NIR region.

The two-photon absorption (TPA) properties of two extended symmetric squaraine dyes are thoroughly characterized from the experimental and quantum-chemical point of view. The two molecules are specially engineered to have a multipolar structure with increasing complexity, D-π-A-π-D and A'-π-D-π-A-π-D-π-A', respectively. The experimental TPA spectra, measured by means of the Z-scan technique in the femtoseconds regime, display considerably high values of TPA cross sections (σ(TPA)) for both molecules. In particular, the squaraine with the more extended structure shows the highest value of σ(TPA) ever reported for this class of molecules. CIS and TDDFT calculations of the one and two-photon absorption properties are carried out to clarify the origin of the observed TPA properties and fully characterize the electronic properties of these compounds. The calculations, in good agreement with the experimental data, suggest that the origin of this exceptionally high σ(TPA) can be ascribed to the presence of a peripheral A' group, that increases the density of excited states involved in the TPA process.

Nonlinear infrared and optical responses of a Holstein-Peirls-Hubbard dimer.

A simple model based on the Holstein-Peierls-Hubbard Hamiltonian has been used to calculate the nonlinear responses at infrared and optical frequencies. The model is applied to a molecular ion radical dimer in order to account for the contribution to the nonlinear responses arising from the intermolecular charge transfer excitations and from their coupling to both intramolecular and intermolecular phonons. A similar calculation has been performed on a model quadrupolar conjugated molecule characterized by intramolecular charge transfer excitations. The calculations are performed according to a collective electronic oscillator scheme by solving the Liouville equation for the bielectronic density matrix. Such a choice allows us to retain the ability, inherent in the Hubbard type models, to fully account for the on-site electron correlation effects. Calculated spectra are reported for one-photon and two-photon absorption (the latter in the form of the imaginary part of the Kerr susceptibility) and for third harmonic generation. Narrow resonances are observed in the infrared, related mostly to the coupling with intramolecular modes. The off-resonant contribution to the nonlinear susceptibility arising from the electron-phonon interactions appears to be marginal (in the order of 1%) in all cases.

Polyphosphazenes as biomaterials: surface modification of poly(bis(trifluoroethoxy)phosphazene) with polyethylene glycols.

Investigations were carried out on the metathetical exchange reaction between the -O-CH2CF3 moieties of poly(bis(trifluoroethoxy)phosphazene) (PTFP), in the state of slightly swollen films, and the alkoxide ions derived from methoxypolyethylene glycol (MPEG) of molecular mass ranging from 350 to 5000 g/mol. The substitution of these hydrophilic chains, mostly confined to thin surface layers, was revealed by means of optical microscopy and scanning electron microscopy observations, surface elemental analysis by energy-dispersive X-ray analysis (EDXA), FTIR-ATR analysis and water contact angle measurements. The surface biocompatibility was enhanced in all cases, whilst the mechanical properties of the original PTFP films were substantially retained in the modified samples exhibiting low substitutions. Such samples were obtained especially when the metathetical reaction was carried out with MPEG5000.

Excited state absorption of fullerenes measured by the photoacoustic calorimetry technique.

Photoacoustic calorimetry (PAC) is used to determine the excited state absorption cross sections in a molecular system showing reverse saturable absorption behavior. PAC experiments on fullerene and fulleropyrrolidine in toluene solutions are performed at 532 nm and 690 nm, with a ns laser source. The PAC signal amplitude displays a superlinear increase when the energy of the applied laser source is increased. This behavior is ascribed to a process of enhanced absorption due to molecules populating the excited electronic states. The PAC signal observed for these chromophores is simulated numerically. The simulations rely on a description of the absorbing molecule as a six-level system, whose molecular parameters (i.e. absorption cross sections and lifetimes) are the ones for a reverse saturable absorber. The time-dependent population in the different energy levels is described through a rate equation system. This kind of model has been widely used by us to reproduce other experimental data such as nonlinear transmittance and Z-scan data. The PAC signal amplitude is the sum of the different contributions to non-radiative relaxation which arise from molecules populating different energy levels. The absorption cross sections for the singlet and triplet excited states of fullerene and fulleropyrrolidine are derived from the simulated PAC signal amplitudes. The values obtained are in good agreement with literature data measured with different techniques.