Effects of the Polarity and Bulkiness of End-Functionalized Side Chains on the Charge Transport of Dicyanovinyl-End-Capped Diketopyrrolopyrrole-Based n-Type Small Molecules

Abstract

When designing organic semiconductors, side-chain engineering is as important as modifying the conjugated backbone, which has a significant impact on molecular ordering, morphology, and thus electronic device performance. We have developed three dicyanovinyl-end-capped donor–acceptor diketopyrrolopyrrole-based n-type small molecules (C2C9CN, SiC4CN, and EH4PCN) bearing an identical length of alkyl spacer yet different end-functionalized side chains (i.e., alkyl-, siloxane-, and phosphonate-end pendants). The effects of the end-functionalized side chains on the intrinsic molecular properties, microstructure, and charge transport of the small-molecule series were investigated. In comparison with the alkyl-end side chains, incorporating siloxane-end side chains into the backbone facilitates 2D edge-on oriented high intergrain connectivity/crystallinity and compatibility with the substrate surface, whereas the phosphonate-end analogues have an adverse effect on the film-forming quality due to high polarity. Thereby, an organic field-effect transistor fabricated by SiC4CN shows the best electron mobility up to 1.59 × 10–1 cm2 V–1 s–1 along with a high current on/off ratio >105. This study contributes to our understanding of the role of the end-functionalized side chains (e.g., the effects of polarity and bulkiness of the end groups) for the development of high-performance semiconductors.

Usefulness of Polar and Bulky Phosphonate Chain-End Solubilizing Groups in Polymeric Semiconductors

“…Phosphonate chain-end functionalized DPP copolymers were prepared by the synthesis procedures outlined in Scheme . Briefly, starting with DPP ( 1 ), bis­(2-ethylhexyl)­(4-bromobutyl)­phosphonate ( 2 ) prepared by the Michaelis–Arbuzov reaction was attached to the DPP core via base (potassium carbonate)-promoted N -alkylation, followed by dibromination with N -bromosuccinimide, yielding highly soluble DPP monomer with phosphonate chain-end solubilizing groups ( 4 ) (two-step overall yield = 23.5%). Polymerization via a palladium-catalyzed Stille coupling was performed by using 4 , bis-stannylated comonomers (Th-tin, Se-tin, TT-tin, and BT-tin in Scheme ), tris­(dibenzylidene­acetone)­dipalladium(0) [Pd 2 (dba) 3 ], and tri­( o -tolyl)­phosphine [P­( o -tolyl) 3 ] in anhydrous chlorobenzene under microwave heating at 140 °C for 3 h, thereby affording the desired copolymers EH4P-Th, EH4P-Se, EH4P-TT, and EH4P-BT as dark green solids.…”

“…Recently, we examined the effects of polarity and bulkiness induced by phosphonate-end side chains on the optical and electrochemical properties, film microstructure, and charge-transport characteristics in a diketopyrrolopyrrole (DPP)-based small molecule platform in line with side-chain engineering considerations . Phosphonates are useful functional groups with a high polarity and bulkiness, which show a high compatibility with other organic functional groups.…”

“…Recently, we examined the effects of polarity and bulkiness induced by phosphonate-end side chains on the optical and electrochemical properties, film microstructure, and chargetransport characteristics in a diketopyrrolopyrrole (DPP)based small molecule platform in line with side-chain engineering considerations. 139 Phosphonates are useful functional groups with a high polarity and bulkiness, which show a high compatibility with other organic functional groups. They have been used for a substrate modification or functionality tuning of bioactive molecules, supported catalysts, and photoactive and electroactive molecules to enhance the interfacial compatibility and enable easier solution processing using many solvents including water.…”

The solvent effect on the morphology and molecular ordering of benzothiadiazole-based small molecule for inkjet-printed thin-film transistors

“…To address these issues, many researchers have investigated various small molecules with highly ordered structures and strong intermolecular interaction by controlling the ordering of the molecules, thereby enhancing the charge transport between them. [30][31][32][33][34] By controlling the size and position of the alkyl chains and utilizing donor or acceptor moieties with high planarity and strong intermolecular interaction, molecular ordering between molecules can be improved.…”

Structural Study of Diketopyrrolopyrrole Derivative Thin Films: Influence of Deposition Method, Substrate Surface, and Aging

“…Among the OSC molecules, much interest has been placed on thiophenes. ,,, One group of molecules with thiophene repeats, which has attracted much attention due to their high carrier mobilities and stable performance in OFETs and OPVs, is diketopyrrolopyrrole (DPP) derivatives. ,− A less investigated DPP derivative is the n -dialkyl side-chain-substituted thiophene DPP end-capped with a phenyl group (Ph-TDPP-Ph), which has a broadband absorption spectrum up to ∼650 nm and can be both solution-processed and vacuum-deposited, making it interesting for devices that require absorption over most of the visible spectrum. , One of the few studies on Ph-TDPP-Ph by Pickett et al investigated the correlation between the microstructure of Ph-DPP-Ph and charge transport in Ph-TDPP-Ph OFETs. The authors found that Ph-TDPP-Ph adopts two different crystalline phases in thin-film form, with one phase having significantly higher charge carrier mobility (by 2–3 orders of magnitude); however, it was not clear how to process the thin film in order to obtain the phase with the higher carrier mobility.…”