Improved Force Field Parameters for Morphological Modeling of Organic Semi-transistors

1. Improved Force Field Parameters for Morphological Modeling of Organic Semi-transistors Shanshan Wu July 2013 Laboratory of Computational Molecular Design (LCMD) École Polytechnique Fédérale de Lausanne EPFL

2. Group Introduction

5. Introduction

6. Organic Field-effect Semi-transistors • Introduce aromatic and conjugated polymers as active semi-conducting layer; • Aligned by non-covalent forces (e.g. vdW, electrostatic); • Biodegradable, low-cost, handy (comparing to metal-oxide type).

7. Current Research Highlights • New approach for synthesis • Relations between morphology and properties (Computational Studies Actively Involved) • Process Control

8. Relations Between Morphology and Properties Different Morphology may lead to different properties. Charge Mobility* ~0.0038 cm2V-1S-1 ~0.0001 cm2V-1S-1 Morphology investigations play a key role in the properties tuning and the design of organic field effect semi-transistors (OFET). *Reference: Zhang, L.; Colella, N.; Liu, F.; et. al J. Am. Chem. Soc. 2013, 135, 844-854

9. Computational Approaches in Revealing Relations between Morphology and Properties Computational Experimental Electronic Properties UV, IR, etc. Quantum Mechanical ( TD-DFT) Atomistic Morphology Not Applicable Atomistic MD simulation Mesoscopic Morphology Coarse Grained MD simulation AFM, TEM, etc. So far, Molecular Mechanical MD simulations is indispensable to reveal detailed morphology at atomistic level.

10. Force field in Modeling OFET Two Main Features of OFET materials Extended Conjugation Intermolecular Interactions Planar Geometry Flexible Conformation Characteristic Herringbone Arrangement

11. Reference Code: LIWRAK (Cambridge Structure Database)

12. Which force field parameters affect the most? Partial Charges, vdW Inter-ring Torsion Parameters • Close steric interactions • Conjugation length • Polarization effects • Molecular packing i) Conformational Variation ii) Folding, Self-assembly iii) Electronic Behaviors

13. Force Field Parameterization and MD Simulations for Thiophene-derived OFET Polymers

14. Thiophene-derived OFET Materials Self-assembled oligothiophene-oligopeptide hybrids Promising nanofibril for the application in OFET Regio-regular P3HT aggregates in crystal Potential Crystallized OFET

15. Force Field Parameterization Problems of General Amber Force Field (GAFF) Parameters: GAFF(AM1-BCC) No Good Parameters for Extensive Conjugation B3LYP/6-31G* inappropriate modeling of geometry B97xD/6-31G*//PBE-dDSC/6-31G*: EAS= EAA+6.19kcal/mol GAFF(AM1-BCC): EAS= EAA+26.05 kcal/mol AA AS inaccurate relative energies (particular for stacking structures)

16. Atomistic Charge Scheme Extensive Conjugation Pi-Pi Stacking Significant Multi-pole Interactions X AM1-BCC* • Charges derived from atomic orbitals with AM1 method; • BCC corrections only based on bond types; • Non-bonded multi-pole corrections are not available. Alternatives: ESP derived Charges • Charges directly derived from electrostatic potentials; • Giving relatively accurate description for the long-range multi-pole interactions. *Reference: [1] Jakalian, A.; Jack, D. B.; Bayly, C. I., J. Comput. Chem. 2002, 23,1623-1641 [2] Mobley, D. L.; Dumont, E.; Chodera, J. D.; Dill, K. A., J. Phys. Chem. B 2011, 115,1329-1332

17. Torsional Parameters Inter-ring Bond Alkyl-chain Linker Formulas

18. Validation C-C-C-C θ

19. C-C-C-C φ

20. all_syn syn_end all_anti 2syn_end syn_mid

21. AA AS SE SM 2SE

22. MD Simulations In situ simulation problem and adjustment Fox and Kollman, J. Phys. Chem. B, 1998, 102, 8070 Chloroform Model in GAFF Very Recent Testing of Chloroform Model in GAFF non-polarized model [1] Y. Marcus, The Properties of Solvents, Wiley (1998) [2]Carl Caleman, Paul J. van Maaren, Minyan Hong, Jochen S. Hub, Luciano T. Costa and David van der Spoel, Force Field Benchmark of Organic Liquids: Density, Enthalpy of Vaporization, Heat Capacities, Surface Tension, Isothermal Compressibility, Volumetric Expansion Coefficient, and Dielectric Constant, J. Chem. Theor. Comput., 8, 61-74 (2012) [3] D. R. Lide, CRC Handbook of Chemistry and Physics 90th edition, CRC Press: Cleveland, Ohio (2009)

23. Chloroform model in GAFF cannot well-simulate the real experimental solvation effect. Most importantly, the GAFF model neglects the rather high polarizability of chloroform, which may increase the inaccuracy of modeling particularly for some organic molecules with polarizability. thiophene ring Permanent Dipole Problems May Occur: 1) Wrong orientation effect in solute-solvent interface; 2) More chaotic structure; 3) Errors in solvation free energy estimation. Major issue for “coarse” morphological modelling Adjustment and Attempts to avoid the Possible Problem: I) Reduce the freedom of thiophene rings; II) Constraints put on the linkers with thiophene. Using high-constrained inter-ring torsional parameters suggested from Case's group to “frozen” the inter-ring rotation. Re-parameterized the parameters on linkers to increase the constraint of chain rotation.

24. “Coarse” Morphological Simulation Results Computational Results

25. Conclusions 1) The partial charge scheme is important to simulate the multi-pole interactions in self-assembled nanofibril; 2) Due to inaccurate chloroform parameters in GAFF, only rough morphological in situ modelling can be made on the condition of appropriate charge scheme and high-constrained inter-ring torsional parameters; 3) The rough morphological modelling of oligothiophene nanofibril displays a relatively good trend for twisting, which is in consistent with the experimental results.

26. Future Outlook 1) Development of MD methods to obtain detailed conformational variation (such as REMD,MD sampling approach); 2) Improvement on solvent parameters; 3) Property studies from MD simulations in solid-state crystals.

27. Acknowledgements • Laboratory for Computational Molecular Design (LCMD), EPFL • ERC Starting Grants • EPFL Supercomputer Applications Prof. Clémence Corminboeuf (EPFL) for advice on work and also for providing the slides of group introduction.

28. Thanks for Attention! Questions?