Breaking Down the Problem: Optical Transitions, Electronic Structure, and Photoconductivity in Conjugated Polymer PCDTBT and in Its Separate Building Blocks

Conjugated polymers with alternating electron-withdrawing and electron-donating groups along their backbone (donor–acceptor copolymers) have recently attracted attention due to high power conversion efficiency in bulk heterojunction solar cells. In an effort to understand how the bandgap in a typical donor–acceptor copolymer is reduced by internal charge transfer character and what the implications of this charge transfer are, we have synthesized the isolated repeat unit (CDTBT) of the photovoltaically highly successful PCDTBT polymer. We compare here the spectroscopic and electrochemical properties of the polymer, the repeat unit, and the separate carbazole donor and dithienylbenzothiadiazole acceptor moieties (CB and dTBT, respectively) in the solid state and in solutions of various polarity. The results are interpreted with the help of time-dependent density functional theory (TD-DFT) calculations. We identify the dominant electronic transitions responsible for the first two absorption bands in the “camel back” spectrum of PCDTBT as partial charge transfer transitions with significant delocalization in the directly excited states. The low bandgap, overall shape, and partial charge transfer character of the PCDTBT absorption spectrum originate from transitions in the dTBT unit. The attached CB moiety extends the conjugation length in CDTBT, rather than acting as a localized donor. Further electronic delocalization, leading to a relatively small reduction in bandgap, occurs upon polymerization. We use our finding of higher delocalization following excitation in the second absorption band to explain the increased yield of photogenerated charges from this band in PCDTBT solid thin films. Moreover, we point out the importance of initial delocalization in the functioning of bulk heterojunction solar cells. The results presented here are therefore not only highly important for a better understanding of donor–acceptor copolymers in general but can also potentially guide the strategic development of future photovoltaic materials

Structure and analysis of the binding energy of the copper carbonyl ion (CuCO+) complex: an ab initio study

Using a large, polarized basis set, SCF and MP4 calculations of the previous termstructure and binding energy of the coppernext term(I) - carbon monoxide complex have been calculated. Both carbonyl (Cu+-CO) and isocarbonyl (Cu+-OC) coordination modes have been found to correspond to minima on the potential previous termenergynext term profiles, the Cu+-CO species being more stable at the MP4 level by 15.7 kcal/mol. A decomposition of the interaction previous termenergiesnext term according to the procedure suggested by Morokuma allows to conclude that Cu+-CO is a typical polarization complex, whereas Cu+-OC has a predominant electrostatic character

Solvation Free Energies of Amides and Amines: Disagreement between Free Energy Calculations and Experiment

We present molecular dynamidfree energy calculations on the molecules acetamide, N-methylacetamide, N,N-dimethylacetamide, ammonia, methylamine, dimethylamine, and trimethylamine. Unlike the experimental data, which suggest a very non-additive solvation free energy (N-methylacetamide and methylamine having the most negative free energy of solvation), the calculations all find that the free energy of solvation monotonically increases as a function of methyl addition. The disagreement with experiment is surprising, given the very good agreement (within 0.5 kcaYmo1) with experiment for calculation of the solvation free energy of methane, ethane, propane, water, methanol, and dimethyl ether

Molecular graphics investigation of the addition of nucleophiles to (η4:butadiene) M(CO)3 complexes (M=Fe,Co+)

The addition reactions of nucleophiles to (η4-butadiene)M(CO)3 complexes (M = Fe, Co+) were investigated using a theoretical model recently developed in the framework of the extended Hückel method and allowing evaluation of the intermolecular interaction energy Eint between the organometallic substrate and the hydride ion, chosen as a model reactant. When taking account of both electrostatic and charge-transfer components of Eint, it is seen that this model is able to rationalize the remarkable difference in regioselectivity exhibited by these two complexes towards nucleophilic attack by an identical reactant: the iron complex slightly favours approach to the internal carbon atom of the diene, but the cobalt complex exhibits a marked preference for attack on the external carbon atom. This is in complete agreement with both experimental observations and the results of a previous second order perturbational analysis