A furan-based synthetic biopolymer composed of a bifuran monomer and ethylene glycol was synthesized through melt polycondensation, and the resulting polyester was found to have promising thermal and mechanical properties. The bifuran monomer, dimethyl 2,2′-bifuran-5,5′-dicarboxylate, was prepared using a palladium-catalyzed, phosphine ligand-free direct coupling protocol. A titanium-catalyzed polycondensation procedure was found effective at polymerizing the bifuran monomer with ethylene glycol. The prepared bifuran polyester exhibited several intriguing properties including high tensile modulus. In addition, the bifuran monomer furnished the polyester with a relatively high glass transition temperature. Films prepared from the new polyester also had excellent oxygen and water barrier properties, which were found to be superior to those of poly(ethylene terephthalate). Moreover, the novel polyester also has good ultraviolet radiation blocking properties.

Two homopolyesters and a series of novel random copolyesters were synthesized from two bio-based diacid esters, dimethyl 2,5-furandicarboxylate, a well-known renewable monomer, and dimethyl 2,2′-bifuran-5,5′-dicarboxylate, a more uncommon diacid based on biochemical furfural. Compared to homopolyesters poly­(butylene furanoate) (PBF) and poly­(butylene bifuranoate) (PBBf), their random copolyesters differed dramatically in that their melting temperatures were either lowered significantly or they showed no crystallinity at all. However, the thermal stabilities of the homopolyesters and the copolyesters were comparable. Based on tensile tests from amorphous film specimens, it was concluded that the elastic moduli, tensile strengths, and elongation at break values for all copolyesters were similar as well, irrespective of the furan:bifuran molar ratio. Tensile moduli of approximately 2 GPa and tensile strengths up to 66 MPa were observed for amorphous film specimens prepared from the copolyesters. However, copolymerizing bifuran units into PBF allowed the glass transition temperature to be increased by increasing the amount of bifuran units. Besides enhancing the glass transition temperatures, the bifuran units also conferred the copolyesters with significant UV absorbance. This combined with the highly amorphous nature of the copolyesters allowed them to be melt-pressed into highly transparent films with very low ultraviolet light transmission. It was also found that furan–bifuran copolyesters could be as effective, or better, oxygen barrier materials as neat PBF or PBBf, which themselves were found superior to common barrier polyesters such as PET.

It is known that hydrogen stabilizes the growing diamond surface under chemical vapor deposition conditions, but a detailed, stepwise, atomic level understanding of how hydrogen atoms function in the growth mechanism is still missing. In the present work a b initio molecular orbital theory is used to address the structures and energetics of hydrogen atoms on dimer-reconstructed diamond (100)2x 1 surfaces. These surfaces are modeled with clusters consisting of nine carbon atoms in four separate layers, which form the basic structural unit of the diamond lattice. Lattice constraints are modeled in the clusters in two different ways in order to determine lower and upper bounds for HC-CH, HC--C' and C=C dimer bond lengths, carbon-hydrogen bond dissociation energies, n bond strengths, and dehydrogenation energies on diamond (100)2 x 1 surfaces. Calculated lower and upper bounds for the dimer bond lengths are 1.58-1.71 8, on the monohydrogenated (100)2x 1 surface, 1.55-1.68 A on the surface comprising HC--C' radicals, and 1.38-1.44 8, on the clean (100)2x 1 surface. The strength of the first C-H bond (413 and 418 kJ/mol) is essentially independent of the treatment of lattice constraints. The second C-H bond is weaker than the first by 44-82 kJ/mol, due to formation of a weak n bond on the clean surface whose strength is sensitive to the lattice constraints. The energy for the molecular desorption of hydrogen is predicted to lie in between 308 and 356 kJ/mol on diamond (100)2x 1. Inclusion of the correlation energy correction is necessary for describing the energetics correctly.The calculated n bond strength on vacant surface dimers implies a substantial driving force for pairing of hydrogen atoms and dangling bonds separately on dimers on the (100)2x 1 surface, and suggests that most dangling bonds are paired under diamond growth conditions.

In the quest for renewable energy, an increasing amount of attention is devoted to the use of organic conjugated materials as active components in photovoltaic devices. Because the primary photoexcitations in conjugated molecules and polymers are strongly bound electron-hole pairs (excitons), efficient charge generation only takes place at the heterojunction between a low-ionization-potential (electron-donor) material and a high-electron-affinity (electron-acceptor) material in multicomponent architectures. This provides the energy mismatch between the frontier molecular orbitals required to overcome the exciton binding energy, which is on the order of 0.4 eV in conjugated polymers. [1][2][3][4] The highest quantum yields for charge generation in organic cells to date have been reported for polymer-fullerene blends. [5,6] C 60 is an efficient electron acceptor but features reduced absorption cross section in the visible spectral region due to symmetry-forbidden optical transitions (note that these symmetry constraints are slightly relaxed in the soluble C 60 derivatives such as [6,6]-phenyl C 61 -butyric acid methyl ester (PCBM) [7] ). Thus, in the simplest scenario, photoinduced charge generation in polymer-fullerene cells involves the formation of a local excited state on the conjugated polymer, which acts both as electron donor and sensitizer, followed by electron transfer to C 60 . A different picture was proposed on the basis of time-resolved transient [8] and photoluminescence quenching [9] experiments performed in solutions of model dyad and triad compounds including oligo(phenylenevinylene) (OPV n ) as donor and N-methylfulleropyrrolidine (MPC 60 ) as acceptor. [8][9][10] The solution data suggest a two-step mechanism with resonant energy transfer (RET) from the photoexcited conjugated chain to the covalently linked fullerene derivative prior to hole migration from MPC 60 to the OPV n moiety. In films, the direct photoinduced electron-transfer reaction is much faster than in solution as a result of more favorable donor-acceptor interactions, and the dynamics of direct electron transfer versus RET can no longer be distinguished using femtosecond pump-probe spectroscopy. [11,12] Thus, the two competitive channels (electron transfer versus RET followed by hole transfer) likely contribute to charge generation in solid-state photovoltaic devices. We note that energy transfer has also been demonstrated to compete with charge generation in other C 60 -based molecular dyads. [13,14] Two questions arise at this stage. First, can one take advantage of RET to design more efficient polymer-based photovoltaic cells? A threefold increase in photocurrent has been recently reported by McGehee and co-workers on inserting a thin layer of a low-bandgap polymer at the interface between a polythiophene derivative (donor) and TiO 2 (acceptor). [15] The enhanced charge generation quantum yield was ascribed to RET to the low-bandgap material and the resulting improved harvesting of electronic excitations at the donor-acceptor he...