Phenyl- versus cyclohexyl-terminated substituents: comparative study on aggregated structures and electron-transport properties in n-type organic semiconductors

Substituent engineering is a key route to high-performance functional molecular materials in the same way as the development of a pi-electron core for organic (opto-)electronics. Here we demonstrate a comparative study between aromatic phenyl- and aliphatic cyclohexyl-terminated side-chain substituents on an electron-deficient pi-electron core, 3,4,9,10-benzo[de]isoquinolino[1,8-gh]quinolinetetracarboxylic diimide (BQQDI), to get insights into the impact of intermolecular interactions between the substituents in the solid state on high-performance electron-transport properties. In the BQQDI system, both phenyl- and cyclohexyl-terminated ethyl substituents show similar packing structures, demonstrating the unobvious impact of terminal groups. However, solution-processed single-crystal transistor studies revealed a relatively low electron mobility of cyclohexyl-terminated BQQDI. Based on molecular dynamics simulations, we attribute this discrepancy to dynamic molecular motions coupled with electronic coupling in the solid state. While phenyl groups in the phenylethyl substituent show intermolecular C-H & ctdot;pi interactions which lead to less dynamic motions, the cyclohexyl counterpart does not show any specific intermolecular interactions. Hence, a low-dynamic feature thanks to inter-side-chain interactions is promising for excellent charge-transport properties. The present findings underline the crucial role of interactions between substituents in the development of organic materials via side-chain-engineered control of the solid-state dynamic motions.