Recent Publications

Kohn-Sham orbitals from our DFT calculations corresponding to the LUMO and LUMO+1 of the isolated fullerene molecules: a) PCBM, and b) 4-tBu.

Kohn-Sham orbitals from our DFT calculations corresponding to the LUMO and LUMO+1 of the isolated fullerene molecules: a) PCBM, and b) 4-tBu.

Abstract:  The efficiency of bulk heterojunction (BHJ) organic photovoltaics is sensitive to the morphology of the fullerene network that transports electrons through the device. This sensitivity makes it difficult to distinguish the contrasting roles of local electron mobility (how easily electrons can transfer between neighboring fullerene molecules) and macroscopic electron mobility (how well-connected is the fullerene network on device length scales) in solar cell performance. In this work, a combination of density functional theory (DFT) calculations, flash-photolysis time-resolved microwave conductivity (TRMC) experiments, and space-charge-limit current (SCLC) mobility estimates are used to examine the roles of local and macroscopic electron mobility in conjugated polymer/fullerene BHJ photovoltaics. The local mobility of different pentaaryl fullerene derivatives (so-called ‘shuttlecock’ molecules) is similar, so that differences in solar cell efficiency and SCLC mobilities result directly from the different propensities of these molecules to self-assemble on macroscopic length scales. These experiments and calculations also demonstrate that the local mobility of phenyl-C60 butyl methyl ester (PCBM) is an order of magnitude higher than that of other fullerene derivatives, explaining why PCBM has been the acceptor of choice for conjugated polymer BHJ devices even though it does not form an optimal macroscopic network. The DFT calculations indicate that PCBM’s superior local mobility comes from the near-spherical nature of its molecular orbitals, which allow strong electronic coupling between adjacent molecules. In combination, DFT and TRMC techniques provide a tool for screening new fullerene derivatives for good local mobility when designing new molecules that can improve on the macroscopic electron mobility offered by PCBM.

PFT general structure self-assembly color-coded

Abstract:  In an effort to favor the formation of straight polymer chains without crystalline grain boundaries, we have synthesized an amphiphilic conjugated polyelectrolyte, poly(fluorene-alt-thiophene) (PFT), which self-assembles in aqueous solutions to form cylindrical micelles. In contrast to many diblock copolymer assemblies, the semiconducting backbone runs parallel, not perpendicular, to the long axis of the cylindrical micelle. Solution-phase micelle formation is observed by X-ray and visible light scattering. The micelles can be cast as thin films, and the cylindrical morphology is preserved in the solid state. The effects of self-assembly are also observed through spectral shifts in optical absorption and photoluminescence. Solutions of higher-molecular-weight PFT micelles form gel networks at sufficiently high aqueous concentrations. Rheological characterization of the PFT gels reveals solid-like behavior and strain hardening below the yield point, properties similar to those found in entangled gels formed from surfactant-based micelles. Finally, electrical measurements on diode test structures indicate that, despite a complete lack of crystallinity in these self-assembled polymers, they effectively conduct electricity.

Abstract:  A family of pentaaryl[C59N]azafullerenes and their mono- and dihydro precursors was isolated by acid catalyzed reaction of a C59N precursor with electron-rich aromatic compounds. Single-crystal X-ray diffraction on two compounds reveals characteristic packing motifs; the triaryl dihydro derivative has a pseudo-stacked, feather-in-cavity arrangement shown at left.

Abstract: The relationship between the size of the substituents of aryl groups in a series of fifteen 6,9,12,15,18-pentaaryl-1-hydro[60]fullerenes and the solid state structures and packing motifs of these compounds has been analyzed. Pentaarylfullerenes have a characteristic ‘badminton shuttlecock’ shape which causes several derivatives to crystallize into columnar stacks. However, many pentaarylfullerenes form non-stacked structures with, for example, dimeric, layered, diamondoid, or feather-in-cavity relationships between molecules. Computational modeling gave a qualitative estimate of the best shape match between the ball and socket surfaces of each pentaarylfullerene. The best match was for pentaarylfullerenes having large, spherically shaped para-substituents on the aryl groups. The series of pentaarylfullerenes was characterized by single-crystal X-ray diffraction. A total of 34 crystal structures were obtained as various solvates and were categorized by their packing motifs.

Abstract:  The reaction of tetrazine 1 with Sc3C2@C80 exclusively affords the open-cage derivative 2 instead of the expected C2-inserted derivative 3 bearing a four-membered ring, as previously obtained for C60. The structure of 2 has been firmly established by NMR spectroscopy and theoretical calculations. EPR spectroscopy shows that a single Sc atom of the Sc3C2 cluster gets located within the bulge created by the bridging addend, which is a first step toward release of the internal metal atoms.

Abstract:  We have synthesized a series of pentaarylfullerene derivatives and incorporated them into bulk-heterojunction (BHJ) solar cells using poly(3-hexylthiophene) as the electron donating material. Through a combination of grazing incidence X-ray diffraction and atomic force microscopy we see two distinct nanoscale morphologies emerge across the series. Investigating the device physics using JV characterization, light intensity dependence, and fluorescence quenching studies, we are able to correlate the morphology to the device physics. This has afforded the opportunity to clearly see how altering the nanoscale phase segregation can change the distinct physical processes occurring within a BHJ solar cell. We observe an order of magnitude increase in the efficiency of devices that utilize self-assembling fullerene derivatives over their nonassembled counterparts. More importantly, however, these results allow us to shed light on the mechanism behind this differential phase segregation using a fullerene self-assembly model. On the basis of the data, we propose new design rules for the structure of future generations of fullerene electron accepting materials for use in BHJ solar cells.

Abstract: Gold(I), silver(I), and copper(I) phosphine complexes of 6,9,12,15,18-pentaaryl[60]fullerides 1a and 1b, namely, [(4-MeC6H4)5C60]Au(PPh3) (2a), [(4-t-BuC6H4)5C60]Au(PPh3) (2b), [(4-MeC6H4)5C60]Ag(PCy3) (3a), [(4-t-BuC6H4)5C60]Ag(PPh3) (3b), [(4-t-BuC6H4)5C60]Ag(PCy3) (3c), [(4-MeC6H4)5C60]Cu(PPh3) (4a), and [(4-t-BuC6H4)5C60]Cu(PPh3) (4b), have been synthesized and characterized spectroscopically. All complexes except for 3c were also characterized by single-crystal X-ray diffraction. Several coordination modes between the cyclopentadienyl ring embedded in the fullerene and the metal centers are observed, ranging from η1 with a slight distortion toward η3 in the case of gold(I), to η23for silver(I), and η5 for copper(I). Silver complexes 3a and 3b are rare examples of crystallographically characterized Ag(I) cyclopentadienyls whose preparation was possible thanks to the steric shielding provided by fullerides 1a and 1b, which stabilizes these complexes. Silver complexes3a and 3b both display unexpected coordination of the cyclopentadienyl portion of the fulleride anion with Ag(I). DFT calculations on the model systems (H5C60)M(PH3) and CpMPH3 (M = Au, Ag, or Cu) were carried out to probe the geometries and electronic structures of these metal complexes.

Abstract: Dehydro[24]annulene 1c adopts an unusual planarized conformation in the crystal. A multilayered stack of hydrogen-bonded grids delineates tightly packed nanotubular channels. The related macrocycles 1a and 1b, on the other hand, have the expected puckered conformations in the crystal.

Abstract: Two gold(I) complexes, (Ph3P)Au[C60(4-MeC6H4)5] (1) and (Ph3P)Au[C60(4-t-BuC6H4)5] (2), were prepared in excellent yield and characterized by single-crystal X-ray diffraction. Complex 1, grown from two solvent systems, has different coordination modes of the fullerene-embedded Cp ring to the (PPh3)Au fragment. For 1 3 (ODCB)2, the cyclopentadienyl ring coordinates to the (Ph3P)Au fragment in an η1 fashion distorted toward η3 geometry, while in 1 3 (CHCl3)2(CS2), crystal packing forces produce enough distor- tion to give near η2 coordination.

Abstract: A triphenylamine-derived pentaaryl fullerene undergoes an unusual oxidative dearylation under basic conditions to give tetraarylated epoxy fullerene in high yield. The structure of the product was confirmed by single crystal X-ray diffraction. A mechanism is proposed to account for the loss of the addend and the subsequent formation of the epoxide group.

Abstract: The fullerene adducts 1a and 1b, whose molecular shapes either promote or hinder the formation of 1-D stacks, have been examined for their potential to form 1-D wire-like domains in bulk-heterojunction organic solar cells. The photovoltaic efficiency of solar cells based on blends of the stacking fullerene 1a with regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) is greatly enhanced compared to nonstacking model fullerene 1b.

  • Sonmez, G; Shen, C. K.-F.; Rubin, Y; Wudl, F. Zero Band Gap and Intrinsic Conductivity in Conjugated PolymersPolym. Mat. Sci. Engin. 200490, 232-233.

Abstract: Solid-state 1H experiments were performed an open-cage fullerene hosting molecular hydrogen. The anisotropy of the molecular hydrogen rotation was studied by double-quantum magic-angle-spinning NMR. The time scale of the molecular hydrogen rotation was estimated by spin-lattice relaxation measurements as a function of temperature.

Rubin Insertion of H2 into an Open Fullerene - Cover Picture

Abstract: A “molecular surgery” approach has been used to create an opening within a fullerene cage that is large enough to allow atoms and small molecules to pass through. The thermodynamics for the insertion of He and H2 into the open fullerene (left and right pictures, respectively) as well as their escape have been studied by NMR spectroscopy and theoretical methods.

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