Conformation, defects, and dynamics of a discotic liquid crystal and their influence on charge transport.

Future applications of discotic liquid crystals (DLCs) in electronic devices depend on a marked improvement of their conductivity properties. We present a study of 2,3,6,7,10,11-hexakishexyloxytriphenylene (HAT6) and show how local conformation, structural defects, and thermal motions on the picosecond time scale strongly affect the efficient charge transport in DLCs. A direct and successful comparison of classical molecular dynamics (MD) simulations with both neutron powder diffraction and quasielastic neutron scattering (QENS) give a full insight into the structural and dynamical properties of HAT6. The local conformation of HAT6 molecules is characterized by a mutual rotation (twist) angle of about 37° and typically a mutual aromatic-core distance of 3.4 Å instead of the average distance of 3.65 Å usually quoted. We show that a considerable number of structural traps is present in HAT6, which persist at the picosecond time scale. We find that the high disorder in the mutual positions of the aromatic cores is an important factor contributing to the limited conductivity of HAT6 compared to larger DLCs.

[1]  J. Brédas,et al.  Charge transport properties in discotic liquid crystals: a quantum-chemical insight into structure-property relationships. , 2004, Journal of the American Chemical Society.

[2]  Wojciech Pisula,et al.  Discotic liquid crystals: a new generation of organic semiconductors. , 2007, Chemical Society reviews.

[3]  Huai Sun,et al.  Computer simulations of poly(ethylene oxide): force field, pvt diagram and cyclization behaviour , 1997 .

[4]  Jenny Nelson,et al.  Solar Cells by Self-Assembly? , 2001, Science.

[5]  R. Bushby,et al.  Mechanism of charge transport in discotic liquid crystals. , 1995, Physical review. B, Condensed matter.

[6]  J. Nelson,et al.  Charge transport parameters of HBC at different temperatures , 2008 .

[7]  Takashi Kato,et al.  Functional liquid-crystalline assemblies: self-organized soft materials. , 2005, Angewandte Chemie.

[8]  Kurt Kremer,et al.  Charge mobility of discotic mesophases: a multiscale quantum and classical study. , 2007, Physical review letters.

[9]  Huai Sun,et al.  Development and validation of COMPASS force field parameters for molecules with aliphatic azide chains , 2004, J. Comput. Chem..

[10]  Gerald R. Kneller,et al.  nMOLDYN: A program package for a neutron scattering oriented analysis of Molecular Dynamics simulations , 1995 .

[11]  Dongwook Kim,et al.  Understanding of assembly phenomena by aromatic-aromatic interactions: benzene dimer and the substituted systems. , 2007, The journal of physical chemistry. A.

[12]  G. Cinacchi,et al.  Atomistic molecular dynamics simulation of hexakis(pentyloxy)triphenylene: Structure and translational dynamics of its columnar state , 2004 .

[13]  Kurt Kremer,et al.  Columnar mesophases of hexabenzocoronene derivatives. II. Charge carrier mobility. , 2008, The Journal of chemical physics.

[14]  G. Kearley,et al.  Molecular Modelling of Ground- and Excited-States Vibrations in Organic Conducting Devices: Hexakis(n-hexyloxy)triphenylene (HAT6) as Case Study , 2010 .

[15]  B. Montgomery Pettitt,et al.  Structural and energetic effects of truncating long ranged interactions in ionic and polar fluids , 1985 .

[16]  S. Tsuzuki,et al.  Origin of attraction and directionality of the pi/pi interaction: model chemistry calculations of benzene dimer interaction. , 2002, Journal of the American Chemical Society.

[17]  Takuma Yasuda,et al.  Self-assembly of functional columnar liquid crystals. , 2009, Chemical communications.

[18]  G. Kearley,et al.  Structure and dynamics of a discotic liquid-crystalline charge-transfer complex. , 2007, Chemphyschem : a European journal of chemical physics and physical chemistry.

[19]  H. Sun,et al.  COMPASS: An ab Initio Force-Field Optimized for Condensed-Phase ApplicationsOverview with Details on Alkane and Benzene Compounds , 1998 .

[20]  E. Sudhölter,et al.  Multiple glass transitions in the plastic crystal phase of triphenylene derivatives , 2005 .

[21]  K. Kremer,et al.  Structure–charge mobility relation for hexabenzocoronene derivatives , 2008 .

[22]  John M. Warman,et al.  Record Charge Carrier Mobility in a Room‐Temperature Discotic Liquid‐Crystalline Derivative of Hexabenzocoronene , 1999 .

[23]  K. Müllen,et al.  Triangle-shaped polycyclic aromatic hydrocarbons. , 2007, Angewandte Chemie.

[24]  K. Senthilkumar,et al.  Charge transport in columnar stacked triphenylenes: Effects of conformational fluctuations on charge transfer integrals and site energies , 2003 .

[25]  Ralf Blossey,et al.  Can the hybrid meta GGA and DFT‐D methods describe the stacking interactions in conjugated polymers? , 2009, J. Comput. Chem..

[26]  F. Krebs,et al.  Crystal structures of 2,3,6,7,10,11-oxytriphenylenes. Implications for columnar discotic mesophases , 2000 .

[27]  G. Kearley,et al.  Density functional calculations of potential energy surface and charge transfer integrals in molecular triphenylene derivative HAT6 , 2010 .

[28]  Fontes,et al.  Liquid-crystalline and helical order in a discotic mesophase. , 1988, Physical review letters.

[29]  P. Schuhmacher,et al.  Fast photoconduction in the highly ordered columnar phase of a discotic liquid crystal , 1994, Nature.

[30]  William L. Jorgensen,et al.  Aromatic-aromatic interactions: free energy profiles for the benzene dimer in water, chloroform, and liquid benzene , 1990 .

[31]  Alessandro Troisi,et al.  Charge transport in semiconductors with multiscale conformational dynamics. , 2009, Physical review letters.

[32]  G. Kearley,et al.  Dynamics of a triphenylene discotic molecule, HAT6, in the columnar and isotropic liquid phases. , 2003, Journal of the American Chemical Society.

[33]  Atomistic simulation of structure and dynamics of columnar phases of hexabenzocoronene derivatives. , 2006, The Journal of chemical physics.

[34]  N. Boden,et al.  ORIENTATIONAL ORDERING AND DYNAMICS IN THE COLUMNAR PHASE OF A DISCOTIC LIQUID CRYSTAL STUDIED BY DEUTERON NMR SPECTROSCOPY , 1998 .

[35]  R. Friend,et al.  Self-organized discotic liquid crystals for high-efficiency organic photovoltaics. , 2001, Science.

[36]  J. Warman,et al.  Charge carrier mobilities in the crystalline solid and discotic mesophases of hexakis-hexylthio and hexakis-hexyloxy triphenylene , 1997 .

[37]  K. Kremer,et al.  Columnar mesophases of hexabenzocoronene derivatives. I. Phase transitions. , 2008, The Journal of chemical physics.

[38]  H. Berendsen,et al.  Molecular dynamics with coupling to an external bath , 1984 .

[39]  Rudolph A. Marcus,et al.  Electron transfer reactions in chemistry. Theory and experiment , 1993 .

[40]  Kurt Kremer,et al.  Towards high charge-carrier mobilities by rational design of the shape and periphery of discotics. , 2009, Nature materials.

[41]  Edward F. Valeev,et al.  Estimates of the Ab Initio Limit for π−π Interactions: The Benzene Dimer , 2002 .

[42]  S. Kelly,et al.  Liquid Crystals for Charge Transport, Luminescence, and Photonics , 2003 .

[43]  John M. Warman,et al.  MECHANISM OF CHARGE TRANSPORT ALONG COLUMNAR STACKS OF A TRIPHENYLENE DIMER , 1998 .

[44]  Christopher A. Hunter,et al.  The nature of .pi.-.pi. interactions , 1990 .

[45]  K. A. Suresh,et al.  Liquid crystals of disc-like molecules , 1977, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.