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DDC: 600 × Subject: spectroscopy ×

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    Benchmark of Simplified Time-Dependent Density Functional Theory for UV–Vis Spectral Properties of Porphyrinoids
    (Weinheim : Wiley-VCH Verlag, 2019) Batra, Kamal; Zahn, Stefan; Heine, Thomas
    Time-dependent density functional theory is thoroughly benchmarked for the predictive calculation of UV–vis spectra of porphyrin derivatives. With the aim to provide an approach that is computationally feasible for large-scale applications such as biological systems or molecular framework materials, albeit performing with high accuracy for the Q-bands, the results given by various computational protocols, including basis sets, density-functionals (including gradient corrected local functionals, hybrids, double hybrids and range-separated functionals), and various variants of time-dependent density functional theory, including the simplified Tamm–Dancoff approximation, are compared. An excellent choice for these calculations is the range-separated functional CAM-B3LYP in combination with the simplified Tamm–Dancoff approximation and a basis set of double-ζ quality def2-SVP (mean absolute error [MAE] of ≈0.05 eV). This is not surpassed by more expensive approaches, not even by double hybrid functionals, and solely systematic excitation energy scaling slightly improves the results (MAE ≈0.04 eV).
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    Spatially resolved spectroscopic differentiation of hydrophilic and hydrophobic domains on individual insulin amyloid fibrils
    ([London] : Macmillan Publishers Limited, part of Springer Nature, 2016) Deckert-Gaudig, Tanja; Kurouski, Dmitry; Hedegaard, Martin A. B.; Singh, Pushkar; Lednev, Igor K.; Deckert, Volker
    The formation of insoluble β-sheet-rich protein structures known as amyloid fibrils is associated with numerous neurodegenerative diseases, such as Alzheimer’s and Parkinson’s disease. A detailed understanding of the molecular structure of the fibril surface is of interest as the first contact with the physiological environment in vivo and plays a decisive role in biological activity and associated toxicity. Recent studies reveal that the inherent sensitivity and specificity of tip-enhanced Raman scattering (TERS) renders this technique a compelling method for fibril surface analysis at the single-particle level. Here, the reproducibility of TERS is demonstrated, indicating its relevance for detecting molecular variations. Consequently, individual fibrils are systematically investigated at nanometer spatial resolution. Spectral parameters were obtained by band-fitting, particularly focusing on the identification of the secondary structure via the amide III band and the differentiation of hydrophobic and hydrophilic domains on the surface. In addition multivariate data analysis, specifically the N-FINDR procedure, was employed to generate structure-specific maps. The ability of TERS to localize specific structural domains on fibril surfaces shows promise to the development of new fibril dissection strategies and can be generally applied to any (bio)chemical surface when structural variations at the nanometer level are of interest.
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    Population difference gratings created on vibrational transitions by nonoverlapping subcycle THz pulses
    ([London] : Macmillan Publishers Limited, part of Springer Nature, 2021) Arkhipov, Rostislav; Pakhomov, Anton; Arkhipov, Mikhail; Babushkin, Ihar; Demircan, Ayhan; Morgner, Uwe; Rosanov, Nikolay
    We study theoretically a possibility of creation and ultrafast control (erasing, spatial frequency multiplication) of population density gratings in a multi-level resonant medium having a resonance transition frequency in the THz range. These gratings are produced by subcycle THz pulses coherently interacting with a nonlinear medium, without any need for pulses to overlap, thereby utilizing an indirect pulse interaction via an induced coherent polarization grating. High values of dipole moments of the transitions in the THz range facilitate low field strength of the needed THz excitation. Our results clearly show this possibility in multi-level resonant media. Our theoretical approach is based on an approximate analytical solution of time-dependent Schrödinger equation (TDSE) using perturbation theory. Remarkably, as we show here, quasi-unipolar subcycle pulses allow more efficient excitation of higher quantum levels, leading to gratings with a stronger modulation depth. Numerical simulations, performed for THz resonances of the H20 molecule using Bloch equations for density matrix elements, are in agreement with analytical results in the perturbative regime. In the strong-field non-perturbative regime, the spatial shape of the gratings becomes non-harmonic. A possibility of THz radiation control using such gratings is discussed. The predicted phenomena open novel avenues in THz spectroscopy of molecules with unipolar and quasi-unipolar THz light bursts and allow for better control of ultra-short THz pulses.
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    Tailoring optical properties and stimulated emission in nanostructured polythiophene
    ([London] : Macmillan Publishers Limited, part of Springer Nature, 2019) Portone, Alberto; Ganzer, Lucia; Branchi, Federico; Ramos, Rodrigo; Caldas, Marília J.; Pisignano, Dario; Molinari, Elisa; Cerullo, Giulio; Persano, Luana; Prezzi, Deborah; Virgili, Tersilla
    Polythiophenes are the most widely utilized semiconducting polymers in organic electronics, but they are scarcely exploited in photonics due to their high photo-induced absorption caused by interchain polaron pairs, which prevents the establishment of a window of net optical gain. Here we study the photophysics of poly(3-hexylthiophene) configured with different degrees of supramolecular ordering, spin-coated thin films and templated nanowires, and find marked differences in their optical properties. Transient absorption measurements evidence a partially-polarized stimulated emission band in the nanowire samples, in contrast with the photo-induced absorption band observed in spin-coated thin films. In combination with theoretical modeling, our experimental results reveal the origin of the primary photoexcitations dominating the dynamics for different supramolecular ordering, with singlet excitons in the nanostructured samples superseding the presence of polaron pairs, which are present in the disordered films. Our approach demonstrates a viable strategy to direct optical properties through structural control, and the observation of optical gain opens the possibility to the use of polythiophene nanostructures as building blocks of organic optical amplifiers and active photonic devices. © 2019, The Author(s).