Correlating Ultrafast Dynamics, Liquid Crystalline Phases, and Ambipolar Transport in Fluorinated Benzothiadiazole Dyes

Abstract

organic semiconductors (OSCs) that can be easily processed with low-cost, largethroughput fabrication techniques. While high carrier mobilities are generally desirable, optoelectronic devices, such as organic light-emitting diodes [1] and organic photovoltaics, [2] specifically profit from ambipolar transport, i.e., balanced electron and hole transport. However, the majority of OSCs are predominantly unipolar, and demonstrate higher hole mobilities than electron mobilities. [3] Therefore, device fabrication typically requires the deposition of multiple unipolar OSCs to achieve the desired electrical properties, thereby complicating the fabrication protocols.Soluble small-molecule semiconductors that form crystalline films have the potential to combine the advantages of high chemical purity and superior (opto)electronic properties with good processability. However, crystalline organic films formed from soluble small molecules often display poor structural and thermal integrity, as well as lower carrier mobilities compared to nonsoluble small-molecule derivatives. In other words, processability generally comes at the expense of both reduced thermal and mechanical stability, as well as electrical transport.In this study, we synthesized soluble small-molecule dyes that form crystalline films at room temperature, but A 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.

Crystals of Diphenyl-Benzothiadiazole and Its Derivative with Terminal Trimethylsilyl Substituents: Growth from Solutions, Structure, and Fluorescence Properties

“…Linear conjugated molecules with a central benzothiadiazole ( BTD ) fragment are well-known as stable organic luminophores with a large Stokes shift and high fluorescence quantum yield. − Crystals based on these compounds, due to a wide range of remarkable properties, are attractive for the development of optoelectronic technologies based on them. − Of great interest are well-crystallized substances capable of forming large single crystals with a size of the order of 1 cm or more, which is a huge advantage for a detailed study of the relationship between their structure and properties and has high potential for their application as optical single crystal elements obtained by growth methods from solutions or melt. Good candidates for this task are rather short linear conjugated molecules with a sufficiently high solubility in standard organic solvents as well as stable over a wide operating temperature range.…”

Crystals of 4,7-Di-2-thienyl-2,1,3-benzothiadiazole and Its Derivative with Terminal Trimethylsilyl Substituents: Synthesis, Growth, Structure, and Optical-Fluorescent Properties

“…Linear conjugated molecules with a central benzo[c] [1,2,3]thiadiazole (BTD) fragment and a symmetric arrangement of peripheral aromatic groups constitute a young family of organic semiconducting luminophores. The BTD group is a commonly used heterocyclic acceptor for developing donor-acceptor materials for organic electronics and photonics [1][2][3][4][5][6][7][8][9]. Solutions and crystals based on these materials often exhibit high efficiency in optoelectronic fluorescence and high thermal stability and photostability [7,10].…”

“…photonics [1][2][3][4][5][6][7][8][9]. Solutions and crystals based on these materials often exhibit high efficiency in optoelectronic fluorescence and high thermal stability and photostability [7,10].…”