We report the synthesis of three fully π-conjugated diblock copolymers containing selenophene- and thiophene-based repeating units. All of these diblock copolymers undergo phase separation, and by ...systematically changing the compatibility of the two blocks through side chain modification, we are able to access different thin film morphologies. Introducing a bulky 2-ethylhexyl side chain increases solubility while retaining crystallinity of the selenophene block. While poly(3-hexylselenophene)-b-poly(3-hexylthiophene) and poly(3-(2-ethylhexyl)selenophene-b-poly(3-(2-ethylhexyl)thiophene) form more disordered fibrillar structures, poly(3-hexylthiophene)-b-poly(3-(2-ethylhexyl)selenophene) forms long (1–2 μm) solid state fibrillar structures that are reminiscent of the lamellae that are formed by nonconjugated block copolymers. We further use electron energy loss spectroscopy to visualize thiophene- and selenophene-rich domains at the nanometer scale in each of these examples. By studying new polymer compositions and relating them to solid state structure, we further our understanding of heterocycle induced phase separation and phase separation in general.
Selenophene−thiophene block copolymers were synthesized and studied. The properties of these novel block copolymers are distinct from those of statistical copolymers prepared from the same monomers ...with a similar composition. Specifically, the block copolymers exhibit broad and red-shifted absorbance features and phase-separated domains in the solid state. Scanning transmission electron microscopy and topographic elemental mapping confirmed that the domains are either rich in selenophene or thiophene, indicating that the blocks of distinct heterocycles preferentially associate with one another in the solid state. This preference is surprising in view of the chemical similarities between repeat units. The overall results demonstrate a phase separation that is controlled by elemental differences. As a result of this phase separation, these novel conjugated block copolymers should find utility in a variety of studies and optoelectronics uses.
Density functional theory (DFT) calculations are useful to model orbital energies of conjugated polymers, yet discrepancy between theory and experiment exist. Here we evaluate a series of relatively ...straightforward calculation methods using the standard Gaussian 09 software package. Five calculations were performed on 22 different conjugated polymer model compounds at the B3LYP and CAM-B3LYP levels of theory and results compared with experiment. Chain length saturation occurs at approximately 6 and 4 repeat units for homo- and donor–acceptor type conjugated polymers, respectively. The frontier orbital energies are better approximated using B3LYP than CAM-B3LYP, and the HOMO energy can be reasonably correlated with experiment mean signed error (MSE) = 0.22 eV. The LUMO energies, however are poorly correlated (MSE = 0.59 eV), and we show that the molecular orbital energy of the triplet state gives a much better estimate of the experimentally determined LUMO level (MSE = −0.13 eV).
Selenophene Electronics Hollinger, Jon; Gao, Dong; Seferos, Dwight S.
Israel journal of chemistry,
June 2014, Letnik:
54, Številka:
5-6
Journal Article
Recenzirano
Substituting individual heavier (or lighter) atoms would appear to be an extremely straightforward method of controlling the optoelectronic properties of π‐conjugated molecules that has minimal ...impact on solid‐state materials properties, yet things are never as they appear. Selenophenes are a distinct class of material separate from thiophenes. Their HOMOLUMO gap is often narrow, which is attractive for photonic applications. Intermolecular SeSe interactions increase ordering on a molecular scale and lead to distinct solid‐state organization, which often leads to excellent charge‐transport properties. The crystallization of selenophene is distinct from thiophenes, and thus, composites of selenophenes and other organic materials have distinct nano‐ and microscale morphologies. Some of the best organic optoelectronic devices use selenophene‐containing materials, yet early results were less encouraging. The most recent syntheses, structure determination, and electronic device properties of π‐conjugated selenophene‐based materials are reviewed. Significantly more work is justified and this review sets the stage for those studies.
Grignard metathesis polymerization was used to synthesize a series of poly(3-hexylselenophene)-block-poly(3-hexylthiophene) copolymers with two different molecular weights and varying selenophene ...content. These polymers were characterized by optical absorption spectroscopy (film and solution), differential scanning calorimetry, powder X-ray diffraction, variable temperature absorption spectroscopy, and atomic force microscopy (on self-assembled polymer nanofibers). The selenophene to thiophene ratio has a large influence on optical properties, and absorption is tunable across the range of both homochromophores. We observe phase separation in the solid state in both pristine and annealed samples. When allowed to slowly assemble in solution, high molecular weight copolymers have a very sharp transition from the molecularly dissolved to the aggregated state. Most interestingly, increasing polyselenophene content induces the polymer to assemble more readily (at a higher temperature) but also appears to hinder the degree of ordered assembly when the thiophene block is not sufficiently long. This study furthers the understanding of the differences between these structurally similar conjugated polymer building blocks and provides insight into the factors that control heterocycle-induced phase separation.
Poly(3‐heptylselenophene)‐stat‐poly(3‐hexylthiophene) is synthesized and characterized in terms of its crystallinity and performance in an organic photovoltaic (OPV) cell. Despite the random ...distribution of units along the polymer main chain, the material is semi‐crystalline, as demonstrated by differential scanning calorimetry and wide‐angle X‐ray diffraction. Thin‐film absorption suggests an increased compatibility than seen with 3‐hexylselenophene monomer. Optoelectronic properties are an average of the two homopolymers, and OPV performance is enhanced by a broadened absorption profile and a favorable morphology.
Poly(3‐heptylselenophene)‐stat‐poly(3‐hexylthiophene) is synthesized, characterized, and fabricated into photovoltaic devices. The copolymers are crystalline despite the random distribution of units along the backbone. Optoelectronic properties of the copolymer are an approximate average of the corresponding homopolymers, while the morphological properties are distinct.
We report the linear and nonlinear regions of the relationship between number average molecular weight determined by gel permeation chromatography (GPC) and 1H NMR end-group analysis for a series of ...o-tolyl-initiated poly(3-hexylthiophene)s (P3HTs). For conjugated polymers with chains that are 39–138 repeat units in length (6.5–23 kDa), GPC systematically overestimates the number average molecular weight (M n) by a factor of 1.3 ± 0.1 (standard error), and GPC and 1H NMR end-group analysis correlate in a linear manner. For chains 138–1130 (23–188 kDa) repeat units in length, we observe a nonlinear relationship between GPC and end-group analysis. Static light-scattering experiments confirm that at high molecular weight (>70 kDa) decreasing the catalyst loading does not appreciably increase the polymer chain length. Thus, we conclude that there is a molecular weight limit in the synthesis of externally initiated polythiophenes and a propensity for the growth of nonexternally initiated chains which increases as a function of M n. This is significant as external initiation has been reported to result in nearly 100% externally initiated chains as well as reduce the possibility of chain–chain coupling in a typical synthesis. Our data show that 100% external initiation only holds true for polymers that are less than 40 kDa and encourages caution when determining M n by NMR using this synthetic methodology at high molecular weights.
Two diketopyrrolopyrrole (DPP)-carbazole (Cz) based pi -conjugated copolymers, PDBTCz-H (P1) and PDBTCz-Me (P2), were designed and synthesized to study the effects of N-substitution of the carbazole ...unit on the molecular ordering, main chain conjugation, and charge transport properties of these polymers. It was found that the existence of hydrogen bonding interaction between the N-H group in the carbazole unit and the C&z.dbd; O group in the DPP unit has a significant impact on the UV absorption, crystallinity, thin film morphology, as well as charge transport characteristics of P1. The hydrogen bonding is a very competitive force with the pi - pi stacking interaction, leading to the more twisted backbone structure and poorer molecular ordering of P1 in the solid state. Although the crystallinity of the P1 thin films could be somewhat improved by thermal annealing, the polymer main chains of P1 remain rather twisted and less conjugated in comparison with P2. The poorer main chain conjugation of P1 caused by the hydrogen bonding led to a dramatic drop in charge transport performance in organic thin film transistors (OTFTs). The highest hole mobility achieved for P1 is 8.9 10 super(-3) cm super(2) V super(-1) s super(-1), which is almost two orders of magnitude lower than that of P2 (0.53 cm super(2) V super(-1) s super(-1)).
The nanostructure morphology and electron donor performance of a poly(3-hexylselenophene)-block-poly(3-hexylthiophene) (P3HS-b-P3HT) copolymer was studied in a photovoltaic device with a 6,6-phenyl ...C61 butyric acid methyl ester (PCBM) acceptor. P3HS-b-P3HT forms fiberlike nanostructures spontaneously, which leads to an initial optimal device performance. Furthermore the nanostructure morphology is not greatly affected by annealing, which leads to a device stability that outperforms P3HT, P3HS, or a P3HS/P3HT mixture under identical conditions. External quantum efficiency, hole mobility, and current–voltage measurements show that the block copolymer also outperforms a ternary blend that consists of a physical mixture of P3HS, P3HT, and PCBM with the same overall composition. Overall, the observation of optimal device performance and morphology without annealing as well as enhanced thermal stability demonstrates the advantage of fully conjugated diblock copolymers in nanostructured devices.
