Abstract
AbstractA key challenge in the field of organic electronics is predicting how chemical structure at the molecular scale determines nature and dynamics of excited states, as well as the macroscopic optoelectronic properties in thin film. Here, the donor–acceptor dyes 4,7‐bis[5‐[4‐(3‐ethylheptyl)‐2,3‐difluorophenyl]‐2‐thienyl]‐2,1,3‐benzothiadiazole (2,3‐FFPTB) and 4,7‐bis[5‐[4‐(3‐ethylheptyl)‐2,6‐difluorophenyl]‐2‐thienyl]‐2,1,3‐benzothiadiazole (2,6‐FFPTB) are synthesized, which only differ in the position of one fluorine substitution. It is observed that this variation in chemical structure does not influence the energetic position of the molecular frontier orbitals or the ultrafast dynamics on the FFPTB backbone. However, it does result in differences at the macroscale, specifically regarding structural and electrical properties of the FFPTB films. Both FFPTB molecules form crystalline films at room temperature, whereas 2,3‐FFPTB has two ordered smectic phases at elevated temperatures, and 2,6‐FFPTB does not display any liquid crystalline phases. It is demonstrated that the altered location of the fluorine substitution allows to control the electrostatic potential along the molecular backbone without impacting molecular energetics or ultrafast dynamics. Such a design strategy succeeds in controlling molecular interactions in liquid crystalline phase, and it is shown that the associated molecular order, or rather disorder, can be exploited to achieve ambipolar transport in FFPTB films.
Highlights
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We investigated the influence of a minor molecular modification, i.e., 2,3 versus 2,6 F-substitution in FFPTB donor– acceptor dyes, on several key material properties of general importance in organic semiconductor devices, namely optical absorption and emission, ultrafast excited-state dynamics, ultrafast vibronic response, intermolecular packing, and electron and hole mobilities
With density functional theory (DFT) calculations and absorption measurements in solution, we demonstrate that the variation in the position of the F-substitution does not influence the energetic position or spatial distribution of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels
Summary
We synthesized the F-substituted BT derivatives shown in Figure 1a, 4,7-bis[5-[4-(3-ethylheptyl)-2,3-difluorophenyl]-2-thienyl]2,1,3-benzothiadiazole (2,3-FFPTB), and Figure 1b, 4,7-bis[5-[4-(3ethylheptyl)-2,6-difluorophenyl]-2-thienyl]-2,1,3-benzothiadiazole (2,6-FFPTB). The variation in the electrostatic potential along the 2,6-FFPTB backbone is less pronounced These differences in local electronic density of both LC dyes lead to stark differences in the molecular packing. Both dyes form crystalline films at room temperature and isotropic melts at temperatures above 130 °C (see Table S1 in the Supporting Information). From the perspective of electronic applications, such soluble small molecules that form crystalline films at room temperature, but display LC properties at elevated temperatures, are interesting because they combine the advantages of good chemical purity, molecular self-assembly and self-healing, high carrier mobility, and excellent structural and thermal durability.[7,51,52,53]
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