Hyperbranched polythiophenes were synthesized by potentiodynamic electropolymerization of 2,2';3',2''-terthiophene and 5'-(2-thienyl)-2,2';3',2''-terthiophene. The molecular architecture, i.e., the extent of branching of the resulting polymers, could be adjusted by varying the switching potentials. We compare these systems to hyperbranched polythiophenes which we obtained via a simple one-pot synthesis route based on FeCl(3) oxidative polymerization of the monomers. Interestingly, we find that the properties of the electropolymerized materials obtained with high switching potentials are comparable to those of the chemically synthesized polythiophenes. A detailed optical and electrochemical characterization of these systems is performed showing the high potential of this material class for optoelectronic applications. Cyclic voltammetry coupled with in situ conductance measurements further reveal reversible doping upon oxidation (p-doping) and reduction (n-doping) and comparable values for the conductance for the chemically and electrochemically synthesized materials.
Hyperbranched polythiophenes were prepared via a simple one-pot synthesis approach based on oxidative coupling of branched conjugated monomers. Only small variations in the building unit and architecture lead to large differences of absorption and photoluminescence properties. Interestingly, soluble hyperbranched polythiophenes with relatively small molecular weights show enhanced absorption at low and high wavelengths compared to linear analogues, such as poly(3-hexyl thiophenes) with high molecular weights. With this versatile approach we present a method to design tailor made, functional materials with potential applications in optoelectronics.
A comprehensive treatment of light propagation through intact leaves based on the theories of radiative transfer and absorption statistics was used to calculate the theoretical absorption spectra of the chlorophyll-containing particles under conditions of multiple scattering and pigment spatial distribution equivalent to those in a leaf. These spectra were compared with the experimental in vivo spectra of leaves and in vitro spectra of chlorophyll-protein complexes extracted form these leaves. We conclude that the main discrepancies between the in vivo and in vitro spectra are apparently due to the optical artifacts specific for light propagation in leaves-multiple scattering and distributional error. Alterations of the pigment properties upon extraction significantly contribute to these discrepancies. The method has an estimated accuracy of about 10% and can be applied to derive the intrinsic optical properties of the photosynthetic mechanism in a leaf, as well as for the systematic study of their changes in the course of light adaptation.
Fullerene-based acceptors have dominated organic solar cells for almost two decades. It is only within the last few years that alternative acceptors rival their dominance, introducing much more flexibility in the optoelectronic properties of these material blends. However, a fundamental physical understanding of the processes that drive charge separation at organic heterojunctions is still missing but urgently needed to direct further material improvements. Here we use a combined experimental and theoretical approach to understand the intimate mechanisms by which molecular structure contributes to exciton dissociation, charge separation, and charge recombination at the donor-acceptor (D-A) interface. We use model systems comprised of polythiophene-based donor and rylene diimide-based acceptor polymers and perform a detailed density functional theory (DFT) investigation. The results point to the roles that geometric deformations and direct-contact intermolecular polarization play in establishing a driving force (energy gradient) for the optoelectronic processes taking place at the interface. A substantial impact for this driving force is found to stem from polymer deformations at the interface, a finding that can clearly lead to new design approaches in the development of the next generation of conjugated polymers and small molecules.
We present the synthesis of regioregular polythiophenes with alkylthiophene side chains P3TC16 prepared by Ni-catalyzed polymerization from the branched, thiophene-based monomer 5-bromo-5″-hexadecyl-[2,2′;3′,2″]terthiophene. The optical properties in solution and thin films of the polymer were investigated in situ as a function of temperature and compared to the low regioregularity analogue FeP3TC6 synthesized by Fe(III) mediated oxidative polymerization of 5″-hexyl-[2,2′;3′,2″]terthiophene. It was found that due to the regioregular structure, P3TC16 tends to strong aggregation in solution, which is ascribed to π−π interactions. The bandgap in thin films of 1.88 eV is slightly smaller than the bandgap of the reference polymer poly(3-hexylthiophene) (P3HT, 1.91 eV). Interestingly, it was found that the HOMO and LUMO levels of P3TC16 are shifted to significantly lower values as compared to P3HT. First results regarding the application of P3TC16 in FETs are shown and mobilities of up to 3.1 × 10 −2 cm 2 /(V s) were achieved. Open circuit voltages of up to 710 mV in combination with PC[60]BM in organic solar cells were found, which is about 30% higher than for P3HT, which can be attributed to the low HOMO energy level.
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