Topological Design of Highly Anisotropic Aligned Hole Transporting Molecular Bottlebrushes for Solution-Processed OLEDs.

Polyvinyl polymers bearing pendant hole transport functionalities have been extensively explored for solution-processed hole transport layer (HTL) technologies, yet there are only rare examples of high anisotropic packing of the HT moieties of these polymers into substrate-parallel orientations within HTL films. For small molecules, substrate-parallel alignment of HT moieties is a well-established approach to improve overall device performance. To address the longstanding challenge of extension from vapor-deposited small molecules to solution-processable polymer systems, a fundamental chemistry tactic is reported here, involving the positioning of HT side chains within macromolecular frameworks by the construction of HT polymers having bottlebrush topologies. Applying state-of-the-art polymer synthetic techniques, various functional subunits, including triphenylamine (TPA) for hole transport and adhesion to the substrate, and perfluoro alkyl-substituted benzyloxy styrene for migration to the air interface, were organized with exquisite control over the composition and placement throughout the bottlebrush topology. Upon assembling the HT bottlebrush (HTB) polymers into monolayered HTL films on various substrates through spin-casting and thermal annealing, the backbones of HTBs were vertically aligned while the grafts with pendant TPAs were extended parallel to the substrate. The overall design realized high TPA π-stacking along the out-of-plane direction of the substrate in the HTLs, which doubled the efficiency of organic light-emitting diodes compared with linear poly(vinyl triphenylamine)s.

Design and development of multifunctional polyphosphoester-based nanoparticles for ultrahigh paclitaxel dual loading.

Multifunctional polyphosphoester-based nanoparticles capable of loading paclitaxel (PTX) both chemically and physically were prepared, achieving an ultrahigh equivalent PTX aqueous concentration of 25.30 mg mL-1. The dual-loaded nanoparticles were effective in killing cancer cells, which has the potential to minimize the amount of nanocarriers needed for clinical applications, due to their ultrahigh loading capacity.

Magnetically-active Pickering emulsions stabilized by hybrid inorganic/organic networks.

Magnetically-active hybrid networks (MHNs) are complex inorganic/organic composite materials that have been synthesized from the coupling of amine-functionalized iron oxide nanoparticles (amine-IONs) and pre-assembled shell crosslinked knedel-like (SCK) polymeric nanoconstructs. The intricate structure of these materials is composed of several inter-connected bundles of SCKs covalently bound to amine-IONs, which afford them magnetic responsivity. The MHNs were originally designed to sequester complex hydrocarbons from water; however, they have displayed a remarkable ability to form stable Pickering emulsions between organic solvents and water, upon mechanical stimulus. Two methods of emulsification, vortex and probe sonication, have been utilized to yield magnetically-active toluene-in-water and dodecane-in-water emulsions, which are stable for up to two months in the presence of the MHNs. A detailed study of the effect of the water-to-oil (W : O) volume ratio and the MHN concentration on the droplet size of the emulsions revealed that the smallest droplet size, and narrowest dispersity were obtained at a W : O = 3 : 1, for all conditions tested. Additionally, concentrations of MHNs as low as 1 mg mL-1 and 1.5 mg mL-1, for emulsions prepared via vortex and probe sonication, respectively, were sufficient to yield the smallest droplets and narrowest distributions. Furthermore, the oil droplets stabilized by the MHNs exhibited magnetic character, and could be manipulated with an external magnetic field.

Exciton delocalization drives rapid singlet fission in nanoparticles of acene derivatives.

We compare the singlet fission dynamics of five pentacene derivatives precipitated to form nanoparticles. Two nanoparticle types were distinguished by differences in their solid-state order and kinetics of triplet formation. Nanoparticles that comprise primarily weakly coupled chromophores lack the bulk structural order of the single crystal and exhibit nonexponential triplet formation kinetics (Type I), while nanoparticles that comprise primarily more strongly coupled chromophores exhibit order resembling that of the bulk crystal and triplet formation kinetics associated with the intrinsic singlet fission rates (Type II). In the highly ordered nanoparticles, singlet fission occurs most rapidly. We relate the molecular packing arrangement derived from the crystal structure of the pentacene derivatives to their singlet fission dynamics and find that slip stacking leads to rapid, subpicosecond singlet fission. We present evidence that exciton delocalization, coincident with an increased relative admixture of charge-transfer configurations in the description of the exciton wave function, facilitates rapid triplet pair formation in the case of single-step singlet fission. We extend the study to include two hexacene derivatives and find that these conclusions are generally applicable. This work highlights acene derivatives as versatile singlet fission chromophores and shows how chemical functionalization affects both solid-state order and exciton interactions and how these attributes in turn affect the rate of singlet fission.

Evidence for the rapid conversion of primary photoexcitations to triplet states in seleno- and telluro- analogues of poly(3-hexylthiophene).

Broadband pump-probe spectroscopy is used to examine the ultrafast photophysics of the π-conjugated polymers poly(3-hexylselenophene) (P3HS) and poly(3-hexyltellurophene) (P3HTe) in solution. An excited-state absorption feature that we attribute to a transition in the triplet manifold appears on the picosecond time scale. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations support this assignment. The formation of triplets is consistent with significant fluorescence quenching observed in solutions of the neat polymers. Triplet formation occurs in ~26 and ~1.8 ps (upper limit) for P3HS and P3HTe, respectively. The successive decrease in fluorescence quantum efficiency and triplet formation time are consistent with intersystem crossing facilitated by the heavier selenium and tellurium atoms. These results strongly suggest that primary photoexcitations are rapidly converted into triplet states in P3HS and P3HTe.

Poly(3-alkyltellurophene)s are solution-processable polyheterocycles.

The synthesis and characterization of a series of poly(3-alkyltellurophene)s are described. Polymers are prepared by both electrochemical and Kumada catalyst transfer polymerization methods. These polymers have reasonably high molecular weights (M(n) = 5.4-11.3 kDa) and can be processed in a manner analogous to that of their lighter atom analogues. All examples exhibit red-shifted optical absorption, as well as solid-state organization, as evidenced by absorption spectroscopy and atomic force microscopy. Overall, the synthesis and characterization of these materials open up a wide range of future studies involving tellurium-based polyheterocycles.

Tellurophenes with delocalized π-systems and their extended valence adducts.

The π-conjugated 2,5-substituted tellurophene compounds 2,5-bis(2-(9,9-dihexylfluorene))tellurophene (1) and 2,5-diphenyltellurophene (3) were synthesized through ring closing reactions of 1,4-substituted butadiyne. The oxidative addition of Br(2) to tellurophene compounds 1 and 3 was studied through absorption spectroscopy, NMR, electrochemistry, X-ray crystallography, and density functional theory (DFT) calculations. When Br(2) adds to the tellurium center the absorption spectrum shifts to a lower energy. From electrochemistry and DFT calculations we show that this is caused by lowering the lowest unoccupied orbital. The highest occupied orbital is also lowered, but to a lesser extent. This shift in absorption spectrum and lowering of the oxidation potential can provide a method to modify tellurophene containing materials. The two-electron oxidative addition is promising for catalyzing energy storage reactions.

Polytellurophenes.

Will polytellurophenes bridge the gap between conjugated polymer and inorganic solid-state semiconductors? Polytellurophenes are a virtually unexplored class of conjugated polymer. In this paper, the synthetic methodologies that have been used to prepare polytellurophenes are chronicled. The properties of the resulting polymers are discussed and their potential for use as electronic materials is evaluated. It is far too early to know if these materials will lead to a useful class of thin-film semiconductors, however some key challenges associated with their synthesis and implementation are outlined. These challenges will need to be addressed as the conjugated polymer research community begins to utilize this area of the periodic table.

Controlling phase separation and optical properties in conjugated polymers through selenophene-thiophene copolymerization.

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.