One of the most effective ways to tune the electronic properties of conjugated polymers is to dope them with small-molecule oxidizing agents, creating holes on the polymer and molecular anions. Undesirably, strong electrostatic attraction from the anions of most dopants localize the holes created on the polymer, reducing their mobility. Here, we employ a new strategy utilizing a substituted boron cluster as a molecular dopant for conjugated polymers. By designing the cluster to have a high redox potential and steric protection of the corelocalized electron density, we obtain highly delocalized polarons with mobilities equivalent to films doped with no anions present. AC Hall effect measurements show that P3HT films doped with our boron clusters have conductivities and polaron mobilities roughly an order of magnitude higher than films doped with F 4 TCNQ, even though the boron-cluster-doped films have poor crystallinity. Moreover, the number of free carriers approximately matches the number of boron clusters, yielding a doping efficiency of ∼100%. These results suggest that shielding the polaron from the anion is a critically important aspect for producing high carrier mobility, and that the high polymer crystallinity required with dopants such as F 4 TCNQ is primarily to keep the counterions far from the polymer backbone.
This Viewpoint Article describes major advances pertaining to perfunctionalized boron clusters in synthesis and their respective applications. The first portion of this work highlights key synthetic methods allowing one to access a wide range of polyhedral boranes (B4 and B6 – B12 cluster cores) that contain exhaustively functionalized vertices. The second portion of this Viewpoint showcases the historical developments in using these molecules for applications ranging from materials science to medicine. Lastly, we suggest potential new directions for these clusters as they apply to both synthetic methods and applications.
Carrier mobility in doped conjugated polymers is limited by Coulomb interactions with dopant counterions. This complicates studying the effect of the dopant's oxidation potential on carrier generation because different dopants have different Coulomb interactions with polarons on the polymer backbone. Here, dodecaborane (DDB)-based dopants are used, which electrostatically shield counterions from carriers and have tunable redox potentials at constant size and shape. DDB dopants produce mobile carriers due to spatial separation of the counterion, and those with greater energetic offsets produce more carriers. Neutron reflectometry indicates that dopant infiltration into conjugated polymer films is redox-potential-driven. Remarkably, X-ray scattering shows that despite their large 2-nm size, DDBs intercalate into the crystalline polymer lamellae like small molecules, indicating that this is the preferred location for dopants of any size. These findings elucidate why doping conjugated polymers usually produces integer, rather than partial charge transfer: dopant counterions effectively intercalate into the lamellae, far from the polarons on the polymer backbone. Finally, it is shown that the IR spectrum provides a simple way to determine polaron mobility. Overall, higher oxidation potentials lead to higher doping efficiencies, with values reaching 100% for driving forces sufficient to dope poorly crystalline regions of the film.
We report the stereospecific ring-opening metathesis polymerization (ROMP) of endodicyclopentadiene (DCPD) by various well-defined molybdenum-based and tungsten-based alkylidene initiators. Tungsten MAP (MonoAryloxide Pyrrolide) initiators with the general formula W(X)(CHCMe 2 Ph)(Me 2 Pyr)(OAr) (X = arylimido, alkylimido, or oxo; Me 2 Pyr = 2,5dimethylpyrrolide; OAr = an aryloxide) yield cis,syndiotactic-poly(DCPD), while biphenolate alkylidene complexes with the general formula M(NR)(CHCMe 2 Ph)(biphen) (M = Mo or W; R = alkyl or aryl, biphen = (e.g.) 3,3′-(t-Bu) 2 -5,5′-6,6′-(CH 3 ) 4 -1,1′-biphenyl-2,2′-diolate) yield cis,isotactic-poly(DCPD). Subtle changes in the initiator can greatly alter the structure of the poly(DCPD)s that are formed. Cis,syndiotactic or cis,isotactic poly(DCPD)s (made with 50-1000 equiv of DCPD) are accessible within seconds to minutes in dichloromethane at room temperature. No isomerization or cross-linking reactions are observed, and addition of a chain transfer reagent (1-hexene) or the use of THF as a solvent does not decrease the stereospecificity of the polymerizations. Cis,syndiotactic and cis,isotactic poly(DCPD)s can be distinguished readily from each other by 13 C NMR spectroscopy. Hydrogenation of each stereoregular poly(DCPD) produces crystalline H-poly(DCPD)s that have melting points between 270 and 290 °C.
We have developed a fast and efficient route to obtain perfunctionalized ether-linked alkyl and benzyl derivatives of the closo-[B12(OH)12]2− icosahedral dodecaborate cluster via microwave-assisted synthesis. These icosahedral boron clusters exhibit three-dimensional delocalization of the cage-bonding electrons, tunable photophysical properties, and a high degree of stability in air in both solid and solution states. A series of closo-[B12(OR)12]2−, hypocloso-[B12(OR)12]1− and hypercloso-[B12(OR)12]0 clusters have been prepared with reaction times ranging from hours to several minutes. This method is superior to previously reported protocols since it dramatically decreases the reaction times required and eliminates the need for inert atmosphere conditions. The generality of the new microwave-based method has been further demonstrated through the synthesis of several new derivatives, which feature redox potentials up to 0.6 V more positive than previously known B12(OR)12 cluster compounds. We further show how this method can be applied to a one-pot synthesis of hybrid, vertex-differentiated species B12(OR)11(OR) that was formerly accessible only via multi-step reaction sequence.
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