Malika Jeffries-EL

Organic−Inorganic Nanocomposites by Placing Conjugated Polymers in Intimate Contact with Quantum Rods

Recent advances in the synthesis [ 1 ] and assembly [ 2 ] of nanocrystals (NCs) provide unique opportunities to exploit NCs for the development of next generation organic/inorganic hybrid solar cells as one of the most promising alternatives to Si solar cells to deliver effi cient energy conversion with inexpensive fabrication. [ 1 , 2 ] These conjugated polymer-based photovoltaic devices capitalize on the advantages peculiar to conjugated polymers (CPs), such as light weight, fl exibility, processability, roll-to-roll production, low cost, and large area, in conjunction with the high electron mobility and tunable optical properties of inorganic NCs. Of the organic/inorganic hybrids, poly(3-hexylthiopene) (P3HT) is one of the most extensively utilized CPs due to its excellent solution processability, environmental stability, high charge carrier mobility, and tailorable electrochemical properties. [ 3 , 4 ] CdSe quantum dots (QDs) are the most commonly investigated NCs because of their quantum-confi ned nature and well-matched energy level with P3HT. [ 5–11 ]

Conducting Block Copolymer Nanowires Containing Regioregular Poly(3‐Hexylthiophene) and Polystyrene

Grignard Metathesis polymerization (GRIM) for the synthesis of regioregular poly(3‐alkylthiophenes) proceeds via a “living” chain growth mechanism. Due to the “living” nature of this polymerization regioregular poly(3‐alkylthiophenes) with predetermined molecular weight, narrow molecular weight distributions and desired chain end functionality are now readily available. Allyl terminated poly(3‐hexylthiophene) was successfully used as a precursor for the synthesis of di‐block copolymers containing polystyrene. The addition of “living” poly(styryl)lithium to the allyl terminated regioregular poly(3‐hexylthiophene) generated the di‐block copolymer. Poly(3‐hexylthiophene)‐b‐polystyrene was also synthesized by atom transfer radical polymerization. Integration of poly(3‐hexylthiophene) in di‐block copolymers with polystyrene leads to the formation of nanowire morphology and self‐ordered conducting nanostructured materials.

Controlled evaporative self-assembly of hierarchically structured regioregular conjugated polymers

A toluene solution of the semiconducting conjugated polymer regioregular poly(3-hexylthiophene) (rr-P3HT) was confined in a sphere-on-flat geometry, forming an axially symmetric, capillary-held microfluid, from which the consecutive “stick–slip” motion of the contact line of the solution viasolvent evaporation was effectively regulated. As a result, hierarchical “snake-skin” like structures of high regularity were obtained where each microscopic ellipsoid within the “snake-skin” was composed of bundles of rr-P3HT nanofibers. This facile, one-step deposition technique based on controlled evaporative self-assembly opens up a new avenue for organizing semicrystalline conjugated polymers into two-dimensional ordered patterns in a simple, cost-effective, and controllable manner.

An Efficient Synthesis of 2,6-Disubstituted Benzobisoxazoles: New Building Blocks for Organic Semiconductors

2,6-Disubstituted benzobisoxazoles have been synthesized by a highly efficient reaction of diaminobenzene diols with various orthoesters. The scope of this new reaction for the synthesis of substituted benzobisoxazoles has been investigated using four different orthoesters. The utility of these compounds as building blocks for the synthesis of conjugated polymers is demonstrated.

Facile synthesis of 2,6-disubstituted benzobisthiazoles: functional monomers for the design of organic semiconductors.

The synthesis of several synthetically useful 2,6-disubstituted benzobisthiazoles is described. The method is based on the Lewis acid-catalyzed ring-closing reaction between substituted orthoesters and diamino benzene dithiol. The resulting benzobisthiazoles are obtained cleanly and in good yields. These materials are of interest for the development of new organic semiconductors.

Tuning the Optical and Electronic Properties of 4,8-Disubstituted Benzobisoxazoles via Alkyne Substitution

In an effort to design new electron-deficient building blocks for the synthesis of conjugated materials, a series of new trans-benzobisoxazoles bearing halogen or alkynyl substituents at the 4,8-positions was synthesized. Additionally, the impact of these modifications on the optical and electronic properties was investigated. Theoretical calculations predicted that the incorporation of various alkynes can be used to tune the energy levels and band gaps of these small molecules. The targeted 4,8-disubstituted benzobisoxazoles were easily prepared in good yields using a two-step reaction sequence: Lewis acid catalyzed orthoester cyclization followed by Sonogashira cross-coupling. The experimentally determined HOMO values for these 4,8-disubstituted benzobisoxazoles ranged from -4.97 to -6.20 eV and showed reasonable correlation to the theoretically predicted values, with a percent deviation that ranged from 2.4-12.8%. However, the deviation between actual and predicted HOMO values was reduced to less than 3.5% when the theoretical values were extrapolated to the long-chain limit and compared to copolymers containing the 4,8-disubstituted benzobisoxazoles. Collectively, these results indicate that these 4,8-disubstituted trans-benzobisoxazoles can be used for the synthesis of new conjugated materials with electronic properties that are variable and predictable.

Fluorescent polymer guest:small molecule host solution-processed OLEDs

Solution-processed OLEDs with polymer hosts and polymer or small-molecule guests have been studied extensively. More recently, efficient solution-processed OLEDs with small molecule hosts and small molecule guests were also reported. However, small molecule hosts of polymer guests in solution-processed fluorescent OLEDs have not been investigated. In this work guest:host systems consisting of the small molecule 4,4′-bis(9-carbazolyl)-biphenyl (CBP) as host to polymer guests such as novel benzobisoxazole (BBO)-containing copolymers and well-known poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) are compared to those with poly(N-vinyl carbazole) (PVK) host, which previously yielded highly efficient phosphorescent OLEDs. In the case of MEH-PPV, guest:host OLEDs are also compared to those with a neat MEH-PPV emitting layer. It is found that replacing the polymer host PVK with the small molecule host CBP improves efficiencies by up to 100%. A blue emissive BBO-polymer:CBP device reaches a luminous efficiency (ηL,max) of 3.4 cd A−1 (external quantum efficiency ηext = 2.4%), while the PVK-based device exhibits ηL,max = 1.7 cd A−1 (ηext = 1.2%). A green emissive BBO:CBP OLED exhibits ηL,max = 5.7 cd A−1 (ηext = 2.1%), while that in the PVK host is 3.1 cd A−1 (ηext = 1.1%). For MEH-PPV:CBP these values are 3.7 cd A−1 (ηext = 1.4%), compared to 2.9 cd A−1 (ηext = 1.0%) for MEH-PPV:PVK and 0.7 cd A−1 (ηext = 0.4%) for the neat MEH-PPV device. Possible origins of the improvement are discussed, including increased charge mobility, smoother film morphology, and the potential effect of multiple non-coiling host small molecules (in contrast to the likely coiled PVK) surrounding a polymer guest.

Efficient synthesis of benzobisazole terpolymers containing thiophene and fluorene

We report the synthesis and lumuminescence properties of three novel polymers composed of 9,9-dioctylfluorene and a donor–acceptor-donor (D–A-D) triad of a benzobisazole moiety sandwiched between two octylthiophenes. The requiste monomers, 2,6-bis(5-bromo-3-octylthiophen-2-yl)-benzobisazoles were obtained efficiently via the Lewis acid catalyzed cyclization of 2-bromo-3-octyl-5-(triethoxymethyl)thiophene and the corresponding diamino diols or dithiols. The polymers were synthesized in excellent yield by the Suzuki coupling reaction between the D–A-D benzobisazole monomers and 9,9-dioctylfluorene bisboronic acid. Alkyl side chains provided the polymers with solubility in common organic solvents, enabling characterization using gel permeation chromatography, 1H NMR, UV-Vis and fluorescence spectroscopy. The polymers have optical bandgaps of 2.43–2.63 eV and HOMO levels at −5.54 to −5.65 eV relative to vacuum as determined by UV visible and photoelectron spectroscopy respectively. Light-emitting diodes using blends of the copolymers in a poly(N-vinyl carbazole) matrix yielded blue-green emission with luminous efficiencies of 0.86 Cd/A at ∼505 nm. This efficient and high-yielding route is a promising approach for the synthesis of polymers containing benzobisazole moieties.

Volatile organic compound discrimination using nanostructured polythiophene sensors

New synthesis methods have allowed us to make many semiconducting polythiophenes polymers with different side and end groups. Also, co-polymers combining a polythiophene chain attached to another polymer chain were synthesized. This design freedom brings a new dimensionality to the sensing properties of the materials. Single chip micro sensor resistor arrays, utilizing multiple polymers, were fabricated and then tested in an automated system. The sensors demonstrated ppm level sensitivity to various volatile organic compounds (VOCs) including both polar and non-polar materials. Polymers with different chemical structures show strong selectivity to different VOCs. By applying pattern recognition algorithms, the sensor response clearly discriminates between the tested VOCs allowing us to conjecture as to the role molecular modification have in determining response to specific VOCs

Conductive polymer "molecular wires" for neuro-robotic interfaces

Direct electrical communication between living nervous systems and external devices would allow a wide variety of clinical and engineering applications. This neuro-robotic interface has not yet been fully achieved because existing interface systems cannot establish sufficiently close contact between neurons and communicating electrodes. We present here the synthesis and testing of a new class of conductive polymer that can be used to coat metal electrodes and achieve the necessary close contact. The polymer coatings can be made to incorporate biological adhesion proteins to maximize biocompatibility. Neurons extracted from mouse and rat brains were able to attach to the coatings, survive beyond five days, and grow out long, communicating processes. These new polymer films offer the potential for a new level of communication between robotic devices and the nervous system and may eventually make a high-bandwidth interface a reality.