Lowest-Energy Crystalline Polymorphs of P3HT

We systematically study low-energy crystalline polymorphs of the archetypal conjugated polymer, regioregular poly-3-hexylthiophene (rr-P3HT) using the best available density functional theory methods benchmarked against the ab initio coupled cluster method. A comprehensive conformational search is performed for two-dimensional π-stacks being the most rigid structural unit of bulk P3HT. We have identified a number of nearly isoenergetic polymorphs below the energy level of room-temperature amorphous structures and well below the energy of optimized best-fit experimental models. Classical molecular dynamics simulations show that these crystals retain their structure at least at 200 K. At room temperature, although the conjugated backbone of the π-stack remains ordered, aliphatic side chains are melted, transforming from low-energy folded conformations to high-entropy fully unfolded structures. Our study shows that P3HT is a statistically frustrated system with multiple competing interactions, which complicates fabrication of highly ordered bulk forms but gives structural flexibility of glasses

Lowest-Energy Crystalline Polymorphs of P3HT

We systematically study low-energy crystalline polymorphs of the archetypal conjugated polymer, regioregular poly-3-hexylthiophene (rr-P3HT) using the best available density functional theory methods benchmarked against the ab initio coupled cluster method. A comprehensive conformational search is performed for two-dimensional π-stacks being the most rigid structural unit of bulk P3HT. We have identified a number of nearly isoenergetic polymorphs below the energy level of room-temperature amorphous structures and well below the energy of optimized best-fit experimental models. Classical molecular dynamics simulations show that these crystals retain their structure at least at 200 K. At room temperature, although the conjugated backbone of the π-stack remains ordered, aliphatic side chains are melted, transforming from low-energy folded conformations to high-entropy fully unfolded structures. Our study shows that P3HT is a statistically frustrated system with multiple competing interactions, which complicates fabrication of highly ordered bulk forms but gives structural flexibility of glasses

Ab Initio Study of a Molecular Crystal for Photovoltaics: Light Absorption, Exciton and Charge Carrier Transport

Using ab initio methods we examine the molecular and solid-state electronic properties of a recently synthesized small-molecule donor, <i>p</i>-DTS­(PTTh₂)₂, which belongs to the dithienosilole-pyridylthiadiazole family of chromophores. In combination with the PC₇₀BM acceptor, <i>p</i>-DTS­(PTTh₂)₂ can be used to fabricate high-efficiency bulk heterojunction organic solar cells. A precise picture of molecular structure and interchromophore packing is provided via a single-crystal X-ray diffraction study; such details cannot be easily obtained with donor materials based on conjugated polymers. In first-principles approaches we are limited to a single-crystallite scale. At this scale, according to our investigation, the principal properties responsible for the high efficiency are strong low-energy light absorption by individual molecules, large exciton diffusion length, and fast disorder-resistant hole transport along π-stacks in the crystallite. The calculated exciton diffusion length is substantially larger than the average crystallite size in previously characterized device active layers, and the calculated hole mobility is 2 orders of magnitude higher than the measured device-scale mobility, meaning that the power conversion “losses” on a single-crystallite scale are minimal

Ab Initio Study of a Molecular Crystal for Photovoltaics: Light Absorption, Exciton and Charge Carrier Transport

Using ab initio methods we examine the molecular and solid-state electronic properties of a recently synthesized small-molecule donor, <i>p</i>-DTS­(PTTh₂)₂, which belongs to the dithienosilole-pyridylthiadiazole family of chromophores. In combination with the PC₇₀BM acceptor, <i>p</i>-DTS­(PTTh₂)₂ can be used to fabricate high-efficiency bulk heterojunction organic solar cells. A precise picture of molecular structure and interchromophore packing is provided via a single-crystal X-ray diffraction study; such details cannot be easily obtained with donor materials based on conjugated polymers. In first-principles approaches we are limited to a single-crystallite scale. At this scale, according to our investigation, the principal properties responsible for the high efficiency are strong low-energy light absorption by individual molecules, large exciton diffusion length, and fast disorder-resistant hole transport along π-stacks in the crystallite. The calculated exciton diffusion length is substantially larger than the average crystallite size in previously characterized device active layers, and the calculated hole mobility is 2 orders of magnitude higher than the measured device-scale mobility, meaning that the power conversion “losses” on a single-crystallite scale are minimal

Crystal Structure and Li-Ion Transport in Li₂CoPO₄F High-Voltage Cathode Material for Li-Ion Batteries

In this work, we provide a structural and computational investigation of the Li₂CoPO₄F high-voltage cathode material by means of neutron powder diffraction (SG <i>Pnma</i>, <i>a</i> = 10.4528(2) Å, <i>b</i> = 6.38667(10) Å, <i>c</i> = 10.8764(2) Å, <i>R</i>_F = 0.0145), crystal chemistry approaches (Voronoi–Dirichlet partitioning and bond valence sums mapping), and density functional theory. The material reveals low energy barriers (0.12–0.43 eV) of Li hopping and a possible 3D channel system for Li-ion migration. It is found that only one Li per formula unit can be extracted within the potential stability window of the commercially available electrolytes. The interrelation between dimensionality, topology and energetics of Li-ion diffusion and peculiarities of the Li₂CoPO₄F crystal structure are discussed in detail

Polymorphism of Crystalline Molecular Donors for Solution-Processed Organic Photovoltaics

Using ab initio calculations and classical molecular dynamics simulations coupled to complementary experimental characterization, four molecular semiconductors were investigated in vacuum, solution, and crystalline form. Independently, the molecules can be described as nearly isostructural, yet in crystalline form, two distinct crystal systems are observed with characteristic molecular geometries. The minor structural variations provide a platform to investigate the subtlety of simple substitutions, with particular focus on polymorphism and rotational isomerism. Resolved crystal structures offer an exact description of intermolecular ordering in the solid state. This enables evaluation of molecular binding energy in various crystallographic configurations to fully rationalize observed crystal packing on a basis of first-principle calculations of intermolecular interactions

A Combined Experimental and Theoretical Study of Conformational Preferences of Molecular Semiconductors

Structural modules used for assembling molecular semiconductors have typically been chosen to give desirable optical and electronic properties. Growing evidence shows that chemical functionalities should be considered for controlling molecular shape, which is important for function because of its influence on polymer secondary structure, lattice arrangements in crystals, and crystallization tendencies. Using density functional theory (DFT) calculations, followed by a natural bond orbital (NBO) analysis, we examine eight molecular semiconductors with resolved single crystal X-ray structures to understand the features that dominate molecular conformations and ultimately develop practical rules that govern these preferences. All molecules can be described by a D′–A–D–A–D′ architecture and have a 4,4-dimethyl-4<i>H</i>-silolo­[3,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (DTS) donor (D) core unit, with [1,2,5]­thiadiazolo­[3,4-<i>c</i>]­pyridine (PT), 5-fluorobenzo­[<i>c</i>]­[1,2,5]­thiadiazole (FBT), or benzo­[1,2,5]­thiadiazole (BT) electron acceptor (A) units, and either thiophene, 5-hexyl-2,2′-bithiophene, or benzofuran electron-donating end-caps (D′). The NBO analysis shows that the energy difference between the two alternative conformations, or rotamers, (Δ<i>E</i>_rot) is a delicate balance of multiple competing nonbonding interactions that are distributed among many atoms. These interactions include attractive “donor–acceptor” electron sharing, steric repulsion, and electrostatic stabilization or destabilization. A proper grouping of these interactions reveals two primary factors determining <i>Δ<i>E</i></i>_rot. The first concerns heteroatoms adjacent to the bonds connecting the structural units, wherein the asymmetric distribution of π-electron density across the link joining the units results in stabilization of one of two rotamers. The second factor arises from electrostatic interactions between close-contact atoms, which may also shift the <i>Δ<i>E</i></i>_rot of the two rotamers. When all these constituent interactions cooperate, the dihedral angle is “locked” in a planar conformation with a negligible population of alternative rotamers

Crystal Structure and Li-Ion Transport in Li₂CoPO₄F High-Voltage Cathode Material for Li-Ion Batteries

In this work, we provide a structural and computational investigation of the Li₂CoPO₄F high-voltage cathode material by means of neutron powder diffraction (SG <i>Pnma</i>, <i>a</i> = 10.4528(2) Å, <i>b</i> = 6.38667(10) Å, <i>c</i> = 10.8764(2) Å, <i>R</i>_F = 0.0145), crystal chemistry approaches (Voronoi–Dirichlet partitioning and bond valence sums mapping), and density functional theory. The material reveals low energy barriers (0.12–0.43 eV) of Li hopping and a possible 3D channel system for Li-ion migration. It is found that only one Li per formula unit can be extracted within the potential stability window of the commercially available electrolytes. The interrelation between dimensionality, topology and energetics of Li-ion diffusion and peculiarities of the Li₂CoPO₄F crystal structure are discussed in detail

Tailored Electronic Structure and Optical Properties of Conjugated Systems through Aggregates and Dipole–Dipole Interactions

A series of PPVO (<i>p</i>-phenylene vinylene oligomer) derivatives with functional groups of varying electronegativity were synthesized via the Horner–Wadsworth–Emmons reaction. Subtle changes in the end group functionality significantly impact the molecular electronic and optical properties of the PPVOs, resulting in broadly tunable and efficient UV absorption and photoluminescence spectra. Of particular interest is the NO₂-substituted PPVO which exhibits photoluminescence color ranging from the blue to the red, thus encompassing the entire visible spectrum. Our experimental study and electronic structure calculations suggest that the formation of aggregates and strong dipole–dipole solute–solvent interactions are responsible for the observed strong solvatochromism. Experimental and theoretical results for the NH₂-, H-, and NO₂-substituted PPVOs suggest that the stabilization of ground or excited state dipoles leads to the blue or red shift of the optical spectra. The electroluminescence (EL) spectra of H-, COOH-, and NO₂-PPVO have maxima at 487, 518, and 587 nm, respectively, in the OLED device. This trend in the EL spectra is in excellent agreement with the end group-dependent PL spectra of the PPVO thin-films

A Critical Assessment of the Therapeutic Potential of Resveratrol Supplements for Treating Mitochondrial Disorders

In human cells, mitochondria provide the largest part of cellular energy in the form of adenosine triphosphate generated by the process of oxidative phosphorylation (OXPHOS). Impaired OXPHOS activity leads to a heterogeneous group of inherited diseases for which therapeutic options today remain very limited. Potential innovative strategies aim to ameliorate mitochondrial function by increasing the total mitochondrial load of tissues and/or to scavenge the excess of reactive oxygen species generated by OXPHOS malfunctioning. In this respect, resveratrol, a compound that conveniently combines mitogenetic with antioxidant activities and, as a bonus, possesses anti-apoptotic properties, has come forward as a promising nutraceutical. We review the scientific evidence gathered so far through experiments in both in vitro and in vivo systems, evaluating the therapeutic effect that resveratrol is expected to generate in mitochondrial patients. The obtained results are encouraging, but clearly show that achieving normalization of OXPHOS function with this strategy alone could prove to be an unattainable goal