Nanoscopic management of molecular packing and orientation of small molecules by a combination of linear and branched alkyl side chains

Minwoo Jung, Youngwoon Yoon, Jae Hoon Park, Wonsuk Cha, Ajeong Kim, Jinback Kang, Sanjeev Gautam, Dongkyun Seo, Jeong Ho Cho, Hyunjung Kim, Jong Yong Choi, Keun Hwa Chae, Kyungwon Kwak, Hae Jung Son, Min Jae Ko, Honggon Kim, Doh Kwon Lee, Jin Young Kim, Dong Hoon Choi, Bongsoo Kim

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34 Citations (Scopus)

Abstract

We synthesized a series of acceptor-donor-acceptor-type small molecules (SIDPP-EE, SIDPP-EO, SIDPP-OE, and SIDPP-OO) consisting of a dithienosilole (SI) electron-donating moiety and two diketopyrrolopyrrole (DPP) electron-withdrawing moieties each bearing linear n-octyl (O) and/or branched 2-ethylhexyl (E) alkyl side chains. X-ray diffraction patterns revealed that SIDPP-EE and SIDPP-EO films were highly crystalline with pronounced edge-on orientation, whereas SIDPP-OE and SIDPP-OO films were less crystalline with a radial distribution of molecular orientations. Near-edge X-ray absorption fine structure spectroscopy disclosed an edge-on orientation with a molecular backbone tilt angle of ∼22°for both SIDPP-EE and SIDPP-EO. Our analysis of the molecular packing and orientation indicated that the shorter 2-ethylhexyl groups on the SI core promote tight π-π stacking of the molecular backbone, whereas n-octyl groups on the SI core hinder close π-π stacking to some degree. Conversely, the longer linear n-octyl groups on the DPP arms facilitate close intermolecular packing via octyl-octyl interdigitation. Quantum mechanics/molecular mechanics molecular dynamics simulations determined the optimal three-dimensional positions of the flexible alkyl side chains of the SI and DPP units, which elucidates the structural cause of the molecular packing and orientation explicitly. The alkyl-chain-dependent molecular stacking significantly affected the electrical properties of the molecular films. The edge-on oriented molecules showed high hole mobilities in organic field-effect transistors, while the radially oriented molecules exhibited high photovoltaic properties in organic photovoltaic cells. These results demonstrate that appropriate positioning of alkyl side chains can modulate crystallinity and molecular orientation in SIDPP films, which ultimately have a profound impact on carrier transport and photovoltaic performance.

Original languageEnglish
Pages (from-to)5988-6003
Number of pages16
JournalACS Nano
Volume8
Issue number6
DOIs
Publication statusPublished - 2014 Jun 24

ASJC Scopus subject areas

  • Engineering(all)
  • Materials Science(all)
  • Physics and Astronomy(all)

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    Jung, M., Yoon, Y., Park, J. H., Cha, W., Kim, A., Kang, J., Gautam, S., Seo, D., Cho, J. H., Kim, H., Choi, J. Y., Chae, K. H., Kwak, K., Son, H. J., Ko, M. J., Kim, H., Lee, D. K., Kim, J. Y., Choi, D. H., & Kim, B. (2014). Nanoscopic management of molecular packing and orientation of small molecules by a combination of linear and branched alkyl side chains. ACS Nano, 8(6), 5988-6003. https://doi.org/10.1021/nn501133y