Abstract
4-hydroxycyanobenzene (4HCB) is a dipolar molecule formed of an aromatic substituted benzene ring with the CN and OH functional groups at the 1 and 4 positions. In the crystalline state, it forms spiral chains via hydrogen bonding, which pack together through interactions. The direct stacking of benzene rings down the a- and b-axes and its interactions throughout the structure gives rise to its semiconductor properties. Here, high-pressure studies are conducted on 4HCB in order to investigate how the packing and intermolecular interactions, related to its semiconductor properties, are affected. High-pressure single-crystal X-ray diffraction was performed with helium and neon as the pressure-transmitting mediums up to 26 and 15 GPa, respectively. The pressure-dependent behaviour of 4HCB in He was dominated by the insertion of He into the structure after 2.4 GPa, giving rise to two phase transitions, and alterations in the interactions above 4 GPa. 4HCB compressed in Ne displayed two phase transitions associated with changes in the orientation of the 4HCB molecules, giving rise to twice as many face-to-face packing of the benzene rings down the b-axis, which could allow for greater charge mobility. In the He loading, the hydrogen bonding interactions steadily decrease without any large deviations, while in the Ne loading, the change in 4HCB orientation causes an increase in the hydrogen bonding interaction distance. Our study highlights how the molecular packing and interactions evolve with pressure as well as with He insertion.
Highlights
Organic semiconductors are promising materials for optoelectronic applications due to their tunable electronic properties, flexibility, and low cost [1,2,3]
The pressure-dependent behaviour of 4HCB in He was dominated by the insertion of He into the structure after 2.4 GPa, giving rise to two phase transitions, and alterations in the π − π interactions above 4 GPa. 4HCB compressed in Ne displayed two phase transitions associated with changes in the orientation of the 4HCB molecules, giving rise to twice as many face-to-face packing of the benzene rings down the b-axis, which could allow for greater charge mobility
In the He loading, the hydrogen bonding interactions steadily decrease without any large deviations, while in the Ne loading, the change in 4HCB orientation causes an increase in the hydrogen bonding interaction distance
Summary
Organic semiconductors are promising materials for optoelectronic applications due to their tunable electronic properties, flexibility, and low cost [1,2,3]. Organic semiconductors usually contain π-conjugated motifs, allowing the formation of π-bonding orbitals with small energy gaps between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) that facilitate charge transport [1]. The charge mobility will depend on the packing of the π systems, determined through their intermolecular interactions. In this regard, structure–property relationships on single-crystal molecular semiconductors (avoiding grain boundary effects) is important for understanding the structural motifs that influence the charge mobility properties, allowing the design of improved semiconductors [1,2]. The lower charge carrier mobility along the c-axis can be explained from the lack of Molecules 2019, 24, 1759; doi:10.3390/molecules24091759 www.mdpi.com/journal/molecules
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