Crossed 2D versus Slipped 1D π-Stacking in Polymorphs of Crystalline Organic Thin Films: Impact on the Electronic and Optical Response

María José Aliaga-Gosalvez, Nicola Demitri, Michael Dohr, Juan Carlos Roldao, Sang Kyu Park, Sangyoon Oh, Shinto Varghese, Soo Young Park*, Yoann Olivier, Begoña Milián-Medina, Roland Resel, Johannes Gierschner

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


Polymorphs of organic semiconductors are of great interest as they shed light to structure-property relationships. The full X-ray thin film structure analysis of two polymorphs (B, G) of an important n-type semiconducting dicyano-distyrylbenzene based small molecule (CN-TFPA) is reported. Drastically different structures of the monotropic phases are revealed, that is an uncommon 2D crossed π-stacked arrangement for the B-phase versus a 1D slipped π-stack for G. Both phases exhibit a layered structure in the (100) plane with high structural integrity, driven by the hydrophobic contacts of the terminal CF3 groups; as (100) coincides with the film surface, this allows for exfoliation by scotch tape. An in-depth time-dependent density functional theory (TD-DFT) based quantum mechanics/molecular mechanics (QM/MM) study reveals all subsequent significantly differing optical and electronic responses which result from the different arrangements: the B film shows little excitonic interaction with strong blue fluorescence, amplified spontaneous emission (ASE), and good 2D n-type transport. The G film forms H-aggregates with strong green fluorescence, no ASE, and 1D n-type charge transport. The established structure-property relationships are seen as a crucial step for computer-aided device analysis.

Original languageEnglish
Article number1900749
JournalAdvanced Optical Materials
Issue number21
Publication statusPublished - 1 Nov 2019


  • fluorescent materials
  • organic optoelectronics
  • organic semiconductor
  • photophysics
  • thin film polymorphism
  • two-dimensional (2D) charge transport

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Atomic and Molecular Physics, and Optics

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