Fluorinated organic molecules exhibit a series of unique features such as great thermal and oxidative stability, [1] elevated resistance to degradation, [2] enhanced hydrophobicity, high lipophobicity of perfluorinated substances, [3] and inverted charge density distribution in fluorinated aromatic compounds.[4] These special features are related to the unique properties of the fluorine atom:[5] a) fluorine is the most electronegative element, with a Pauling electronegativity of 4.0, which is much larger than that of hydrogen (2.2); b) fluorine is the smallest electron-withdrawing group (van der Waals radius, r = 1.35 , only slightly larger than hydrogen, r = 1.2 ). Furthermore, these fluorine atoms often have a great influence on inter-and intramolecular interactions through C-F···H, F···S, and C-F···p F interactions. [2,6] As a result, fluorinated conjugated materials have been explored for their applications in organic field-effect transistors (OFET) [7] and organic light-emitting diodes (OLED). [4,8] However, there are only a few examples of applying fluorinated compounds in organic photovoltaics, [9] especially as p-type semiconductors in bulk heterojunction (BHJ) polymer solar cells.Since the fluorine atom is a strong electron-withdrawing substituent, the introduction of F into the conjugated backbone would lower both the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of the conjugated polymers, as demonstrated by Heeger and BrØdas in a theoretical study of poly(phenylene vinylene) having various substituents. [10] Experimentally, Yu et al. confirmed the electronic effect of the fluorine substituent in their study of a series of benzodithiophene thieno[3,4-b]thiophene copolymers.[9b] After one fluorine atom was substituted onto the thieno [3,4-b]thiophene unit, the copolymer exhibited decreased LUMO and HOMO energy levels, but with a similar band gap, as compared with those of the nonfluorinated analogue. A larger open-circuit voltage (V oc ) was observed from the BHJ device based on the F-substituted polymer, and this difference is largely because of the lower HOMO energy level. Moreover, the short-circuit current (J sc ) and the fill factor (FF) were noticeably increased by judicious selection of solvent and additives, [11] possibly because of an optimized film morphology facilitated by these F atoms. A similar enhancement on the morphology by employing F atoms was observed by Kim et al. in their study of poly(3-hexylthiophene) (P3HT) having various end-groups.[9a] The CF 3 end-group-modified P3HT showed significant improvement in both the J sc and FF values for its BHJ devices, thus leading to a 40 % increase in the efficiency (h). The much improved morphology of the polymer/PC 61 BM blend was attributed to the decreased surface energy of the fluorine-containing polymer. However, there has been no precedent study on the photovoltaic properties of F-containing low-band-gap polymers constructed using the donor-acceptor strategy, [12] which is a comm...

Three structurally identical polymers, except for the number of fluorine substitutions (0, 1, or 2) on the repeat unit (BnDT-DTBT), are investigated in detail, to further understand the impact of these fluorine atoms on open circuit voltage (V(oc)), short circuit current (J(sc)), and fill factor (FF) of related solar cells. While the enhanced V(oc) can be ascribed to a lower HOMO level of the polymer by adding more fluorine substituents, the improvement in J(sc) and FF are likely due to suppressed charge recombination. While the reduced bimolecular recombination with raising fluorine concentration is confirmed by variable light intensity studies, a plausibly suppressed geminate recombination is implied by the significantly increased change of dipole moment between the ground and excited states (Δμ(ge)) for these polymers as the number of fluorine substituents increases. Moreover, the 2F polymer (PBnDT-DTffBT) exhibits significantly more scattering in the in-plane lamellar stacking and out-of-plane π-π stacking directions, observed with GIWAXS. This indicates that the addition of fluorine leads to a more face-on polymer crystallite orientation with respect to the substrate, which could contribute to the suppressed charge recombination. R-SoXS also reveals that PBnDT-DTffBT has larger and purer polymer/fullerene domains. The higher domain purity is correlated with an observed decrease in PCBM miscibility in polymer, which drops from 21% (PBnDT-DTBT) to 12% (PBnDT-DTffBT). The disclosed "fluorine" impact not only explains the efficiency increase from 4% of PBnDT-DTBT (0F) to 7% with PBnDT-DTffBT (2F) but also suggests fluorine substitution should be generally considered in the future design of new polymers.

Simply mixing a Cu(II) salt and 6,6'-dihydroxy-2,2'-bipyridine (H2L) in a basic aqueous solution afforded a highly active water oxidation catalyst (WOC). Cyclic voltammetry of the solution at pH = 12-14 shows irreversible catalytic current with an onset potential of ~0.8 V versus NHE. Catalytic oxygen evolution takes place in controlled potential electrolysis at a relatively low overpotential of 640 mV. Experimental and computational studies suggest that the L ligand participates in electron transfer processes to facilitate the oxidation of the Cu center to lead to an active WOC with low overpotential, akin to the use of the tyrosine radical by Photosystem II to oxidize the CaMn4 center for water oxidation.

The concepts of steric energy, steric potential, and steric charge are introduced within the density functional theory framework. The steric energy, representing a hypothetical state with all electrons packed into the lowest orbital and other effects entirely excluded, is a measure of the intrinsic space occupied by an electronic system. It is exclusive, repulsive, and extensive, and it vanishes for homogeneous electron gas. When Bader's zero-flux boundary condition is adopted, atoms in molecules are found to achieve balanced steric repulsion among one another with vanished steric energy density interfaces. A few molecular systems involving conformation changes and chemical reactions have been investigated to examine the relative contribution of the steric and other effects, providing insights for a few controversial topics from a different perspective.

Three metal-organic frameworks (MOFs) of the UiO-68 network topology were prepared using the amino-TPDC or TPDC bridging ligands containing orthogonal phosphorylurea groups (TPDC is p,p 0terphenyldicarboxylic acid), and investigated for sorption of uranium from water and artificial seawater.The stable and porous phosphorylurea-derived MOFs were shown to be highly efficient in sorbing uranyl ions, with saturation sorption capacities as high as 217 mg U g À1 which is equivalent to binding one uranyl ion for every two sorbent groups. Coordination modes between uranyl groups and simplified phosphorylurea motifs were investigated by DFT calculations, revealing a thermodynamically favorable monodentate binding of two phosphorylurea ligands to one uranyl ion. Convergent orientation of phosphorylurea groups at appropriate distances inside the MOF cavities is believed to facilitate their cooperative binding with uranyl ions. This work represents the first application of MOFs as novel sorbents to extract actinide elements from aqueous media.