Efficient Suppression of Dynamic Disorder in Organic Semiconductors via Electron-Donating and -Withdrawing Substituents

Charge transport in high-mobility organic semiconductors is considerably hindered by dynamic disorder, so strategies for its suppression are in great demand. To formulate them, hidden relationships among the molecular structure, crystal structure, and dynamic disorder are to be uncovered. In this work, we show that a crossover from a layered packing into a brickwork arrangement, induced by an electron-donating substituent in the molecular structure of a conjugated oligomer, results in a significant decrease in the low-frequency Raman signal. In light of the recently suggested hypothesis about the relation between the low-frequency Raman signal and dynamic disorder, this drop is a marker of dynamic disorder suppression. Periodic (solid-state) DFT calculations confirm that the dynamic disorder decreases and reveal the suppressed contributions to it from virtually all low-frequency vibrational modes. A similar decrease in the low-frequency Raman intensity upon electron-donating/-withdrawing substitution followed by the crossover to brickwork packing is observed in two other pairs of conjugated oligomers and a pair of annulated compounds, for which our calculations also indicate a dramatic suppression of the dynamic disorder. We anticipate that the revealed relationship among the molecular structure, crystal structure, and dynamic disorder and the suggested disorder suppression mechanism will facilitate the rational design of high-mobility organic semiconductors.