Below is our first video, which introduces conjugated polymers and explains why they’re useful.

To learn more about conjugated polymers and how CCI scientists study them, check back here soon for more videos. We’ll post them as they’re completed.

Journal: J. Chem. Theory Comp. 2012, submitted. Check back here for a link to the full-text article when it’s published.

Abstract: The modeling of conjugated polymers entails a unique challenge for classical force fields. Conjugation imposes strong restraints upon bond rotation; planar configurations are favored, but the concomitantly shortened bond lengths result in moieties being brought into closer proximity than usual. The ensuing steric repulsions are particularly severe in the presence of side-chains, straining angles and stretching bonds to a degree infrequently found in non-conjugated systems. We herein demonstrate the resulting inaccuracies by comparing the LMP2-calculated inter-ring torsion potentials for a series of substituted stilbenes and bithiophenes to those calculated using standard classical force fields. We then implement adjustments to the OPLS-2005 force field in order to improve its ability to model such systems. Finally, we show the impact of these changes on the dihedral angle distributions, persistence lengths, and conjugation length distributions observed during molecular dynamics simulations of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) and poly 3-hexylthiophene (P3HT), two of the most widely-used conjugated polymers.

Journal: J. Am. Chem. Soc. 2012, submitted. Check back here for a link to the full-text article when it’s published.

Abstract: Raman spectra were recorded experimentally and calculated theoretically for bithiophene, terthiophene, and quaterthiophene samples as a function of excitation polarization. Distinct spectral signatures were assigned and correlated to the molecular/unit cell orientation as determined by X-ray diffraction. The ability to predict molecular/unit cell orientation within organic crystals using polarized Raman spectroscopy was evaluated by predicting the unit cell orientation in a simulated terthiophene crystal given a random set of simulated polarized Raman spectra. Polarized Raman microscopy offers a promising tool to quickly and economically determine the unit cell orientation in known organic crystals and crystalline thin films. Implications of our methodologies for studying individual molecule conformations are discussed.

Journal: Macromolecules 2012, published online 2/23/2012.

Link to full-text article: http://pubs.acs.org/doi/abs/10.1021/ma300013e

Abstract: The Ni-catalyzed Kumada catalyst transfer polycondensation of a novel biaryl monomer, 2-(4-bromo-2,5-bis(2-ethylhexyloxy)phenyl)-5-chloromagnesiothiophene (2), afforded the respective π-conjugated alternating copolymer poly(thiophene-alt-p-phenylene) (PTPP). Under optimized conditions, the polystyrene-equivalent number-average molecular weight (Mn) of the copolymers prepared using this method were varied between 6.4 and 39 kDa by adjusting the initial monomer-to-catalyst ratios and, in all cases, the resulting materials exhibited narrow polydispersity indices (PDIs ≤ 1.33). Moreover, the Mn of the copolymers produced were found to increase linearly with monomer conversion. The ability of PTPP to be utilized as a macroinitiator for further copolymerization was confirmed through a series of chain extension experiments as well as block copolymerizations involving 2-bromo-5-chloromagnesio-3-hexylthiophene. MALDI-MS analysis showed that the major population of the PTPP prepared using the aforementioned method contained H/Br end groups, as would be expected for efficient catalyst transfer during the polymerization with minimal occurrence of chain termination. Collectively, these results were consistent with a controlled polymerization reaction and constituted the first such example in which a conjugated polymer with an alternating repeat unit was produced via a chain-growth process.

Journal: Macromolecules 2012, submitted. Check back here for a link to the full-text article when it’s published.

Abstract: A series of alternating copolymers containing oligomeric bis(2-ethylhexyl)-p-phenylene vinylene (BEH-PPV) chromophores and conformational-flexible n-decyl or tetraethylene glycol chains were prepared. The polymerization was carried out using Sonogashira coupling conditions between monomers composed of an iodo-terminated PPV oligomer (trimer, pentamer, or septamer) and a bis(phenylacetylene)-containing flexible chain. Polymers containing the n-decyl chain attained higher molecular weights compared to the tetraethylene glycol-containing polymers most likely due to the more rigid conformation of the n-alkyl chain. 4-ethynylanisole-capped oligomers (trimer, pentamer, or septamer) were prepared and their solution photophysical properties were compared to the analogous polymeric materials. The absorption and emission peaks, fluorescence quantum yields and fluorescence lifetimes of the polymers were primarily determined by chromophore length of the constituent oligomers.

Last year more than 500 kids, families, students and teachers made their own Snow in Texas with the CCI. Join us again for more snowy fun!

Date: Saturday, March 3, 2012

Time: 11:00 am – 3:00 pm

Location: Parking lot next to the Larry R. Faulkner Nanoscience and Technology Building (FNT) on the University of Texas at Austin campus

Journal: Nano Lett. 2012, published online 2/21/2012.

Link to full-text article: http://pubs.acs.org/doi/abs/10.1021/nl2045815

Abstract: Electronic factors in molecules such as quantum interference and cross-conjugation can lead to dramatic modulation and suppression of conductance in single-molecule junctions. Probing such effects at the single-molecule level requires simultaneous measurements of independent junction properties, as conductance alone cannot provide conclusive evidence of junction formation for molecules with low conductivity. Here, we compare the mechanics of the conducting para-terminated 4,4′-di(methylthio)stilbene and moderately conducting 1,2-bis(4-(methylthio)phenyl)ethane to that of insulating meta-terminated 3,3′-di(methylthio)stilbene single-molecule junctions. We simultaneously measure force and conductance across single-molecule junctions and use force signatures to obtain independent evidence of junction formation and rupture in the meta-linked cross-conjugated molecule even when no clear low-bias conductance is measured. By separately quantifying conductance and mechanics, we identify the formation of atypical 3,3′-di(methylthio)stilbene molecular junctions that are mechanically stable but electronically decoupled. While theoretical studies have envisaged many plausible systems where quantum interference might be observed, our experiments provide the first direct quantitative study of the interplay between contact mechanics and the distinctively quantum mechanical nature of electronic transport in single-molecule junctions.

Journal: Chem. Sci. 2012, published online January 2012.

Link to full-text article: http://pubs.rsc.org/en/Content/ArticleLanding/2012/SC/c2sc00770c

Abstract: We describe the synthesis and characterization of a new class of cyano-functionalized oligoenes and their derivatives. We have made the vinylogous series of α,ω-diphenyl-µ,ν-dicyano-oligoenes (DPDCn) comprised of each odd-numbered member from 3 to 13 linear conjugated olefins. Installing cyano groups onto the oligoene backbone lowers HOMO and LUMO energies by up to ~0.7 eV, thereby stabilizing the molecule with respect to oxidative decomposition; this exemplifies a new approach to the stabilization of conjugated oligoenes. UV-vis absorption spectra and redox potentials across the DPDCn series reveal that the molecular band gap ranges from 2.80 to 1.75 eV. This gap can be further tuned by the facile installation of a variety of aryl end-groups. The choice of end-groups also greatly affects the physical properties such as solubility and the solid-state packing. We also present the longest oligoene crystal structure reported to date. Moreover, we find that the prototypical linear structure makes oligoenes suitable as molecular wires and connectors in the bottom-up construction of nanoscale architectures. As a proof of concept, carboxylic acid terminated oligoenes were used to position 10-nm Fe₃O₄ nanoparticles on a GaAs (100) substrate. ]]> http://www.cci.nano.utexas.edu/publications/functionalizing-molecular-wires-a-tunable-class-of-aw-diphenyl-mn-dicyano-oligoenes/feed/ 0 http://www.cci.nano.utexas.edu/just-because-flowers/explore-ut-2012/ http://www.cci.nano.utexas.edu/just-because-flowers/explore-ut-2012/#comments Tue, 03 Jan 2012 22:38:06 +0000 lyon@mail.utexas.edu http://www.cci.nano.utexas.edu/just-because-flowers/explore-ut-2012/ Title: Explore UT 2012
Location: Larry R. Faulkner Nanoscience and Technology Building, UT-Austin campus
Description: Visit the CCI at Explore UT 2012!
Start Time: 11:00
Date: 2012-03-03
End Time: 15:00

Figure 1. (top) Wide-field fluorescence images of MEH-PPV (a) before and (b) after solvent-vapor annealing. Each bright spot in (b) corresponds to an aggregate of MEH-PPV chains. (bottom) Polarization anisotropy histograms for (c) pristine, (d) para-terphenyl and (e) ortho-terphenyl MEH-PPV measured for 150-300 aggregates. The parameter M indicates the order of the aggregates’ nanodomains, with complete order corresponding to M = 1.

In a paper published in ACS Nano, the CCI team used a controlled solvent vapor annealing process to assemble single chains of phenylene-vinylene conjugated polymers into aggregates that were investigated spectroscopically. The conjugated polymers were modified to include structure-directing features, termed “morphons” by the CCI. By varying the type of morphon used, the ordering of nanoscale domains in the aggregates could be controlled. For example, single aggregate polarization anisotropy measurements revealed that the nanodomains of MEH-PPV aggregates containing rigid para-terphenyl groups were nearly completely aligned (ordered), whereas those in aggregates containing hinge-like ortho-terphenyl groups were nearly completely isotropic (disordered) (Figure 1).

This alignment or isotropy of the aggregates respectively facilitated or inhibited interchain coupling of MEH-PPV. The CCI team observed that the para-terphenyl MEH-PPV exhibited fluorescence spectral features similar to those of pristine MEH-PPV, indicative of extensive interchain electronic interactions. In contrast, the ortho-terphenyl MEH-PPV lacked such spectral features, indicating that the ortho-terphenyl morphons effectively prevented extensive interchain interactions (Figure 2).

Both types of morphon-included MEH-PPV chains hold promise for next-generation opto-electronic applications. Highly aligned polymer nanodomains have been demonstrated to permit long-range energy transfer, and rigid morphons such as para-terphenyl groups could be used to promote long-range alignment and thus long-range energy transfer in polymers used for plastic solar cells. Meanwhile, nanodomain disorder and minimized interchain interactions have been shown to increase electroluminescence efficiency in organic light-emitting diodes (OLEDs), so bent morphons such as ortho-terphenyl groups could be used to optimize OLED materials.

Figure 2. Representative single-molecule fluorescence spectra for (a) pristine, (b) para-terphenyl and (c) ortho-terphenyl MEH-PPV.

These results also provide exciting new insight regarding the formation of low-energy traps in polymer films. Single-chain fluorescence lifetime measurements revealed that both para- and ortho-terphenyl morphons disrupted phenylene-vinylene conjugation in MEH-PPV. These results, when considered in conjunction with the fluorescence spectra, indicate that energy trapping sites in MEH-PPV result from coupling interactions between neighboring polymer chains.