Introduction
Organic semiconductors are best canditate materials for organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic photovoltaic cells (OPVCs) since they have enormous properties such as mechanical flexibility, light weight, bulk chemical synthesis and low-cost organic electronic devices [1–4]. Ability to adjust the properties of the organic semiconductors including mostly π-conjugated molecules through the chemical tailoring of the molecular structure is their most important advantage for application of the optoelectronic devices[5–8]. To create semiconducting materials with various properties in organic electronic devices, subtle modification such as incorporation various heterotams to organic compounds can be performed. Therefore, understanding the structure-property relationship which is one of the most fundemental researchwork in organic semiconductors today has vital role in the optic-electronic field that it is becoming increasingly needed area in order to improve the properties of the functional materials in the best way for their applications [9, 10]. For this purpose, single crystal x-ray diffraction (SCXRD) is the most strongest method for analysing the structure-property relationship of the π-conjugated molecules in terms of their molecular arrangements in aggregation phase and charge transport properties[11–13]. A detailed analysis on the between solid-state molecular arrangements (molecular packing) and charge transport characteristics in molecular crystals bring a solution if and how the materials might eventually be used in device fabrication, and how the transporting properties of materials can be designed in order to improve their electronic applications.
The organic single crystal structure of the π-conjugated molecules can provide detailed information on the 3D molecular structure, molecular arrangements, and noncovalent interactions including intermolecular and intramolecular nonclassical hydrogen bonds, C\(-H\cdots\pi\), X\(-Y\cdots\pi\), π\(\cdots\)π stacking and strong short interactions in aggregate phase [12, 14–18]. The determination of these features gives excellent information about the functional material properties which are performed by the whole collective of organic fragments not only of single molecules themselves and these collective arrangements offer intirinsic properties with different molecular packing created by noncovalent interactions [18–22]. Therefore, to perform the optimized optical and electronic properties in molecular and supramolecular levels, manipulation of the molecular packing and intermolecular interactions are arguably a main issue since molecular arrangements between the neighboring molecules in solid phase has considerably effect on charge transport properties of the molecules [23–26]. They can form different solid state packing geometry in different directions which results in different transport properties and anisotropies.
π\(\cdots\)π stacking interactions including types edge-to-face T-shape, parallel or antiparallel displaced (J type), and cofacial parallel or antiparallel stacked (H type) are one of the most important noncovalent interactions between the molecular orbitals in supramolecular level to elucidate the charge transport feature based on the aggregation phase of the neighboring molecules[27–31]. The principle account of the stacking interactions is that the electron-withdrawing substituents tend to enhance the π\(\cdots\)π stacking interaction while electron-donating substituents reduce. Thus, co-facial π\(\cdots\)π stacking is generally not preferred for aromatic compounds because of quadrupole repulsion. In terms of the electronic model of the molecules, π\(\cdots\)π interactions between the HOMO and LUMO orbitals which created antiparallel stacking modes J or H type more benefical for the efficient charge transport than HOMO-HOMO or LUMO-LUMO orbital overlaps. Due to the fact that charge pathways between the adjacent rings in solid phase created by the π\(\cdots\)π stacking interactions, specifying the perpendicular distance (d) between the rings, pitch and roll distances and angles are important issue to determine whether the interactions are strong and their stacking types. The stronger interaction formed by the small d value with the favourable stacking type (antiparallel H and J) results in more efficient charge transport in solid phase[32–34].
Beside the molecular packing of the molecules, the charge transfer rate which relates to the parameters of reorganization energy and charge transfer integral is important parameter to set up the structure-property relationship in optoelectronic device performance[35–38]. Charge transfer integral is related to intensity of electronic coupling constant which associated with geometry of the stacking interactions including perpendicular distance between the rings, pitch and roll distances and angles, and stacking type of the molecules in solid phase[39–42]. According to experimental and theoretical studies more overlap between the adjacent rings with the favourable stacking type result in high electronic coupling therefore high charge transfer integral which is desired to be high for efficient charge transfer rate verified by Marcus Electron Theory[43]. Therefore, along with examining the single crystal structure and electronic interactions of the molecules, it is necessary to carry out the theoretical investigations to predict the charge transport rate of the practical materials. The reorganization energy determine the change in energy of the molecule because of the presence of excess charge and the surrounding medium [44]. Minimization of reorganization energy and maximization of transfer integral are considered suitable for molecular design.
In this study, to fully understanding the structure-property relationships and to predict the charge carrier mobility in terms of microscopic point of view, the effects of the molecular packing, noncovalent interactions and the crystal geometry on the charge transport properties by considering the reorganization energy of the molecules were investigated through crystallographic and theoretical studies of the typical two pyrazole derivatives. The molecules have electron-rich pyrazole moiety which attains conjugation by donating electron, so it behaves as a donor-acceptor type of functional molecules. Therefore, pyrazole derivatives generally have been used as hole transport materials. In the same way we have found that, both molecules show hole tranport properties due to effect of the heteroatom brom and electron withdrawing nitro groups. The 3D molecular structures, reorganization energy, electronic properties including absorption spectra, frontier orbitals, ionization potential (IP) and electronic affinity (EA), crystal packing modes of both compounds were calculated to analyze the relationship between structures and properties and therefore to lead the way on designing new pyrazole derivative organic semiconductors with good charge transport rate.