Blue emitting organic semiconductors under high pressure:status and outlook

The microstructure of ZK40, ZK40 with 2 wt% of Nd and Gd (ZK40-2Nd and ZK40-2Gd, respectively) were investigated with optical, scanning and transmission electron microscopy, X-ray diffraction and Scanning Kelvin Probe Force Microscopy. The mechanical properties and the corrosion behaviour were correlated with the microstructure. The 2 wt% Gd addition enhanced the ductility, while the Nd addition resulted in deterioration in mechanical properties. The corrosion behaviour was also enhanced with the addition of Gd

Enhanced Piezoresponse and Nonlinear Optical Properties of Fluorinated Self-Assembled Peptide Nanotubes

Self-assembled L,L-diphenylalanine (FF) nanostructures offer an attractive platform for photonics and nonlinear optics. The nonlinear optical (NLO) coefficients of FF nanotubes depend on the diameter of the tube [S. Khanra et al. Phys. Chem. Chem. Phys. 19(4), 3084-3093 (2017)]. To further enhance the NLO properties of FF, we search for structural modifications. Here, we report on the synthesis of fluorinated FF dipeptides by replacing one ortho-hydrogen atom in each of the phenyl groups of FF by a fluorine atom. Density-functional theoretical calculations yield insights into minimum energy conformers of fluorinated FF (Fl-FF). Fl-FF self-assembles akin to FF into micron-length tubes. The effects of fluorination are evaluated on the piezoelectric response and nonlinear optical properties. The piezoelectric d15 coefficient of Fl-FF is found to be more than 10 times higher than that of FF nanotubes, and the intensity of second harmonic generation (SHG) polarimetry from individual Fl-FF nanotubes is more than 20 times that of individual FF nanotubes. Furthermore, we obtain SHG images to compare the intensities of FF and Fl-FF tubes. This work demonstrates the potential of fluorine substitution in other self-assembled biomimetic peptides for enhancing nonlinear optical response and piezoelectricity

Correlating charge transport to structure in deconstructed diketopyrrolopyrrole oligomers: A case study of a monomer in field-effect transistors

Copolymers based on diketopyrrolopyrrole (DPP) cores have attracted a lot of attention because of their high p-type as well as n-type carrier mobilities in organic field-effect transistors (FETs) and high power conversion efficiencies in solar cell structures. We report the structural and charge transport properties of n-dialkyl side-chain-substituted thiophene DPP end-capped with a phenyl group (Ph-TDPP-Ph) monomer in FETs which were fabricated by vacuum deposition and solvent coating. Grazing-incidence X-ray diffraction (GIXRD) from bottom-gate, bottom-contact FET architectures was measured with and without biasing. Ph-TDPP-Ph reveals a polymorphic structure with pi-conjugated stacking direction oriented in-plane. The unit cell comprises either one monomer with a = 20.89 angstrom, b = 13.02 angstrom, c = 5.85 angstrom, alpha = 101.4 degrees, beta = 90.6 degrees, and gamma = 94.7 degrees for one phase (TR1) or two monomers with a = 24.92 angstrom, b = 25.59 angstrom, c = 5.42 angstrom, alpha = 80.3 degrees, beta = 83.5 degrees, and gamma = 111.8 degrees for the second phase (TR2). The TR2 phase thus signals a shift from a coplanar to herringbone orientation of the molecules. The device performance is sensitive to the ratio of the two triclinic phases found in the film. Some of the best FET performances with p-type carrier mobilities of 0.1 cm(2)/V s and an on/off ratio of 10(6)are for films that comprise mainly the TR1 phase. GIXRD from in operando FETs demonstrates the crystalline stability of Ph-TDPP-Ph

Second order phase transition and stabilizing CH···H and CH···S interactions in naphthyl end-capped bithiophene at 3.5 GPa

The semiconducting bithiophene systems are largely unexplored structurally at high-pressures. Here, we characterize the high-pressure structure and phase behavior of 5,5′-bis(naphth-2-yl)-2,2′-bithiophene (NaT2) by single crystal X-ray diffraction, photoluminescence and Raman compression measurements in the diamond anvil cell to 7.6 GPa. Experimental results are supported by empirical interaction energy calculations. NaT2 maintains its ambient pressure structure (P2$_1$/c) on compression but undergoes a subtle second-order phase transition at ca. 3.5 GPa. The newly formed phase at 4.0 GPa is characterized by the formation of a new S···H interaction between thiophene moieties in adjacent molecules, as well as by the emergence of a more favorable compression pathway in its long-chain direction. NaT2 also undergoes a band gap closure manifested as a distinct yellow to red color change on compression that is fully reversible on decompression

Visualisation of charge-transfer excitations in donor-acceptor molecules using the particle-hole map: a case study

Charge-transfer (CT) excitations are essential for photovoltaic phenomena in organic solar cells. Owing to the complexity of molecular geometries and orbital coupling, a detailed analysis and spatial visualisation of CT processes can be challenging. In this paper, a new detail-oriented visualisation scheme, the particle-hole map (PHM), is applied and explained for the purpose of spatial analysis of excitations in organic molecules. The PHM can be obtained from the output of a time-dependent density-functional theory calculation with negligible additional computational cost, and provides a useful physical picture for understanding the origins and destinations of electrons and holes during an excitation process. As an example, we consider intramolecular CT excitations in Diketopyrrolopyrrole-based molecules, and relate our findings to experimental results

Tuning Intermolecular Interactions: A Study of the Structural and Vibrational Properties of p-Hexaphenyl under Pressure

Hydrostatic pressure is used to modulate the intermolecular interactions in the conjugated oligophenyl, parahexaphenyl. These interactions affect the structural properties and also cause changes in the molecular geometry that directly alter the electronic properties. We use Raman spectroscopy to investigate the nature of the structural changes. Our Raman studies in the temperature range of 12 K to 300 K, under pressures up to 70 kbar, indicate that the potential energy of two neighboring phenyl rings as a function of the torsional angle is W -shaped. The libration of the phenyl rings between the two minima of the W -shaped potential can be modulated by either promoting the molecule to a higher energy state (activation energy of 0.045 eV) by raising the temperature or by decreasing the intermolecular separation, which makes the potential more U -shaped. Both these situations make the molecule seem more planar. We infer the shape of the potential from the relative intensity of the inter-ring C-C stretch Raman mode at 1280 cm-1 to the C-H bending mode at 1220 cm-1 (I1280/I1220). These results are interpreted within the framework of ab initio electronic and vibrational spectra calculations of a biphenyl molecule. We have also conducted X-ray studies to check the sample purity

High Pressure Studies on the Planarity of Para-hexaphenyl

We present experimental and theoretical findings on the geometry of para-hexaphenyl (PHP) molecules in polycrystalline powder. A new method to assess the planarity of PHP via Raman spectroscopy is presented. Based on this method we describe a W-shaped potential energy curve which governs the torsional motion between neighboring phenyl rings. We determine the activation energy to promote PHP from a non-planar to a planar state to be 0.04 eV, in good agreement with our quantum chemical calculations. Finally we are able to experimentally planarize the molecules by the application of hydrostatic pressure, which modifies the W-shaped potential energy curve to a U-shaped one