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In this work we address a specific experimental and theoretical question regarding the influence of a conformational population in the modeling of photoelectron circular dichroism (PECD) spectroscopy. In the past two decades, PECD has revealed a rich and complex phenomenology in molecular processes with unprecedented insight, especially in molecular geometry sensitivity. Since the early development of this spectroscopy, theory has pointed out the importance of conformer influence on PECD; in particular, the rotation of methyl groups was surprisingly found to be responsible for strong modulation of the PECD signal. Here, to advance understanding of the effect of rotations, we have chosen to study norcamphor, a single-conformer molecule, as a benchmark for a PECD comparison between experiment and theory at the density functional theory (DFT) and time-dependent density functional theory (TDDFT) levels. The excellent agreement between experimental data and theory for norcamphor sheds light on the influence of rotations and gives a solid explanation for the reasonable qualitative agreement in the PECD of camphor, where three methyl groups are added to the same molecular structure.

A combination of experimental and quantum mechanical investigations is applied to the study of the optical features of 4,4'-bis(2-benzoxazolyl)stilbene (BBS) dissolved in solution or in a poly(l-lactic acid) (PLA) thermoplastic matrix at different concentrations. The experimental analyses allow the characterization of BBS solutions and dispersions in terms of absorption and emission features, along with the collection of some key parameters such as fluorescence quantum yield, anisotropy and lifetime, while the computational approach gives a detailed description of the photophysical behavior of BBS in the different environments. For the 10 -5 M BBS solution, the fluorescence spectra show the expected peaks at 425 and 455 nm of the non-interacting BBS molecules with a single fluorescence lifetime of 0.85 ns without revealing any aggregation phenomena, prevented by the short lifetime and fast diffusion rate of the monomer. Moreover, the calculated spectra are in excellent agreement with the experiments, thus showing the reliability of the computational approach. In time-resolved emission experiments (TRES) on more concentrated solutions (10 -4 M) and on BBS crystals, the presence of an excimer is revealed by the appearance of a broad peak around 540 nm, followed by the disappearance of the two main peaks at 460 nm on a time scale of about 10 ns. The computational analysis attributes this behavior to the formation of aggregates of different geometries. The BBS dispersions in PLA reveal the presence of different BBS architectures depending on the fluorophore content. Even at low concentrations, BBS is mainly dispersed as a monomer in the matrix, spheroid aggregates of about 800-900 nm in diameter are also present and the relevant fluorescence spectra arise from the combination of monomer and aggregate contributions. At higher concentrations, BBS starts forming crystals of a peculiar helicoidal shape, with a diameter of about 2 μm, variable length up to several hundreds of μm and emission spectra similar to those of isolated BBS crystals.

We present an in-depth investigation of Au nanoparticles self-assembled on a zigzag-like TiO(x)/Pt(111) ultrathin polar film, whose structure is known in great detail. The peculiar pattern of defects (picoholes) templates a linear array of size-selected (ca. 1 nm) Au nanoparticles without disruption of the titania layer, as observed by scanning tunneling microscopy. Their structure and electronic properties have been investigated by several large-area spectroscopic tools, i.e. high-resolution core and valence level photoemission and angle-scanned and energy-scanned photoelectron diffraction. The comparison between experimental data and density functional theoretical calculations indicates that the Au atoms landing on the oxide film are rather mobile, and that the picoholes can act as effective trapping and nucleation centers for the growth of the Au nanoparticles. All the experimental results are in concord in indicating that the Au NPs are flat islands with a maximum thickness of 2-3 layers exposing the (111) surface.