Nanoscopic Management of Molecular Packing and Orientation of Small Molecules by a Combination of Linear and Branched Alkyl Side Chains

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.

ε‐Branched Flexible Side Chain Substituted Diketopyrrolopyrrole‐Containing Polymers Designed for High Hole and Electron Mobilities

“…[ 3a-c,4 ] Therefore, side-chain engineering can be as important as manipulating the conjugated building blocks in the backbones when designing high-performance conjugated polymers. [ 5 ] In this work, we report the substantially enhanced charge-transport characteristics of a series of DPP-based polymers showing vastly superior FET performance (hole mobilities ( µ h ) of 12.25 cm 2 V −1 s −1 and I on / I off ≥ 10 6 together with electron mobilities ( µ e ) larger than 2 cm 2 V −1 s −1 ). These have been achieved by simply modulating the side-chain branching position (i.e., replacing the commonly used 2-octyldodecyl solubilizing group as the β -branched chain of the DPPbased polymers with the 5-octylpentadecyl chain ( ε -branched chain)).…”

Increasing H-aggregation of p-DTS(FBTTh2)2 to improve photovoltaic efficiency by solvent vapor annealing

“…In particular, the peak ratio decreases with bandwidth for H-aggregation and increases with bandwidth for J-aggregation [38e40]. The other was that red-shift absorption peak (compared to absorption peak of monomer) represented J-aggregation and the blue-shift one represented H-aggregation [22,26,41,42]. Each of the two viewpoints has its own merits.…”

Influence of alkyl substitution on the supramolecular organization of thiophene- and dioxine-based oligomers

“…4 The supramolecular organization can also be controlled by the shape of the molecular core and alkyl substituents. 5,6 Side-chain engineering is a powerful technique to improve the properties and solubility of semiconducting materials, but can affect the molecular organization as well. 7 While the length of alkyl chains solely impacts the intermolecular distance, the geometry of the substituents, e.g.…”