English

Effects of Alkyl-Chain Engineering on the Thermodynamic Properties of Amphiphilic Organic Semiconductors

• Li Mingliang ^1,2, ,
• Li Shuo ^1,2, ,
• Wang Guozhi ^1,2, ,
• Guo Xuefeng ^{3,4, ,}

• 1.

  GRIMAT Engineering Institute Co., Ltd., Beijing 101407, P. R. China

• 2.

  State Key Laboratory of Advanced Materials for Smart Sensing, General Research Institute for Nonferrous Metals, Beijing 100088, P. R. China

• 3.

  State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China

• 4.

  Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China

Corresponding author: Guo Xuefeng, guoxf@pku.edu.cn

• Received Date: 29 August 2019
  Accepted Date: 12 September 2019
  Revised Date: 12 September 2019
  Available Online: 15 November 2020
• Fund Project:

  Natural Science Foundation of Beijing, China Z181100004418003

  National Natural Science Foundation of China 21727806

  the National Key R & D Program of China 2017YFA0204901

  The project was supported by the National Key R & D Program of China (2017YFA0204901), National Natural Science Foundation of China (21727806) and Natural Science Foundation of Beijing, China (Z181100004418003)

Abstract: Due to their special polar structure, amphiphilic molecules are simple to process, low in cost and excellent in material properties. Thus, they can be widely applied in the preparation of functional film materials and bionics related to cell membranes. Therefore, amphiphilic organic semiconductor materials are receiving increasing attention in research and industrial fields. The structure of organic amphiphilic semiconductor molecules usually consists of three functional parts: a hydrophilic group, a hydrophobic group, and a linking group between them. The adjustment of their correlation to achieve the target performance is particularly important and needs experimental discussion regarding synthetic methodologies. In this work, we focused on the engineering of a substituent alkyl-chain, and an amphiphilic functional molecule (benzo[b]benzo[4, 5] thieno[2, 3-d]thiophene, named C_nPA-BTBT, n = 3–11) was proposed and synthesized. This molecule links the hydrophobic semiconductor backbone and hydrophilic polar group through alkyl chains of different lengths. Fundamental properties were investigated by nuclear magnetic resonance (NMR) and ultraviolet-visible spectroscopy (UV-Vis) to conform the structure and the band gap properties of the designed organic semiconductor. Thermodynamic features were investigated by thermogravimetric analysis (TGA) and corresponding differential thermal gravity (DTG), which indicate that the functional molecule C_nPA-BTBT (n = 3–11) has a great stability in ambient conditions. Moreover, the results show that the binding ability of the amphiphilic molecule to water molecules was regulated by the odd-even alternating effect of the alkyl chain and the intramolecular coupling with BTBT. Furthermore, differential scanning calorimetry (DSC) and polarized optical microscopy (POM) were used to study the material properties in detail. As the length of the alkyl chain increased, the functional molecule C_nPA-BTBT (n = 3–11) gradually changed from "hard" species with no thermodynamic changes to a transition one with a pair of thermodynamic peaks, and eventually to a "soft" one as a typical liquid crystal with clear observation of Maltese-cross spherulites. The cooling and freezing points were further studied, and the values and trends of their enthalpy and corresponding temperature fluctuated and alternated due to the volume effect, odd-even alternating effect, flexibility, and other functions of the alkyl chain. Three molecular models were proposed according to the thermodynamic study results, namely the brick-like model, transition model, and liquid crystal model. This work presents in-depth discussion on material structure and corresponding thermodynamic properties, and it is an experimental basis for the design, synthesis, optimization, and screening of target performance materials.

Key words:
• Functional amphiphilic semiconductor
•  /  Alkyl-chain engineering
•  /  Thermodynamics
•  /  DSC

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