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.