PBDTBO, a conjugated polymer comprising benzo[1,2-b:4,5-b']dithiophene (BDT) and 5,6-bis(octyloxy)benzo[c][1,2,5]oxadiazole (BO) units, exhibits a deep HOMO energy level of -5.27 eV and excellent solubility. A device incorporating PBDTBO and [6,6]-phenyl-C(61)-butyric acid methyl ester (1:1, w/w) exhibited a power conversion efficiency of 5.7%.

We have used Stille coupling polymerization to synthesize a series of new crystalline low–band gap conjugated polymersPTHBO, PBTTBO, and PTTTBOconstituting mainly electron-rich thiophene (TH), 2,2′-bithiophene (BT), and thieno[3,2-b]thiophene (TT) units in conjugation with electron-deficient 2,1,3-benzooxadiazole (BO) moieties. All of these polymers exhibited (i) sufficient energy offsets with respect to those of fullerenes to allow efficient charge transfer and (ii) low-lying highest occupied molecular orbital (HOMO, −5.47 eV). These polymers exhibited excellent thermal stability, high crystallinity, and broad spectral absorptions. As a result, bulk heterojunction photovoltaic devices derived from these polymers and fullerenes provided open-circuit voltages (V oc) as high as 1.02 V. In particular, the photovoltaic device comprising the PTTTBO/PC61BM (1:1) blend system and 1,8-diiodooctane (DIO, 0.5 vol %) as an additive exhibited excellent performance, under AM 1.5 G irradiation (100 mW cm–2), with a value of V oc of 0.85 V, a short-circuit current density (J sc) of 11.6 mA cm–2, a fill factor (FF) of 0.54, and a promising power conversion efficiency (PCE) of 5.3%.

No abstract

Helix-coil equilibrium studies are important for understanding helix formation in protein folding, and for helical foldamer design. The quantitative description of a helix using statistical mechanical models is based on experimentally derived helix propensities and the assumption that helix propensity is position-independent. To investigate this assumption, we studied a series of 19-residue Ala-based peptides, to measure the helix propensity for Leu, Phe, and Pff at positions 6, 11, and 16. Circular dichroism spectroscopy revealed that substituting Ala with a given amino acid (Leu, Phe, or Pff) resulted in the following fraction helix trend: KXaa16 > KXaa6 > KXaa11. Helix propensities for Leu, Phe, and Pff at the different positions were derived from the CD data. For the same amino acid, helix propensities were similar at positions 6 and 11, but much higher at position 16 (close to the C-terminus). A survey of protein helices revealed that Leu/Phe-Lys (i, i + 3) sequence patterns frequently occur in two structural patterns involving the helix C-terminus; however, these cases include a left-handed conformation residue. Furthermore, no Leu/Phe-Lys interaction was found except for the Lys-Phe cation-π interaction in two cases of Phe-Ala-Ala-Lys. The apparent high helix propensity at position 16 may be due to helix capping, adoption of a 3₁₀-helix near the C-terminus perhaps with Xaa-Lys (i, i + 3) interactions, or proximity to the peptide chain terminus. Accordingly, helix propensity is generally position-independent except in the presence of alternative structures or in the proximity of either chain terminus. These results should facilitate the design of helical peptides, proteins, and foldamers.

No abstract