Deciphering the Multifarious Charge-Transport Behaviour of Crystalline Propeller-Shaped Triphenylamine Analogues

Article abstract of DOI:10.1002/chem.201804620

A collection of para-substituted propeller-shaped triphenylamine (TPA) derivatives have been computationally investigated for charge-transport characteristics exhibited by the derivatives by using the Marcus–Hush formalism. The various substituents chosen herein, with features that range from electron withdrawing to electron donating in nature, play a key role in defining the reorganisation energy and electronic coupling properties of the TPA derivatives. The TPA moiety is expected to possess weak electronic coupling on the basis of poor orbital overlap upon aggregation, owing to the restriction imposed by the propeller shape of the TPA core. However, the substituent groups attached to the TPA core can significantly dictate the crystal-packing motif of the TPA derivatives, wherein the variety of noncovalent intermolecular interactions subsequently generated drive the packing arrangement and influence electronic coupling between the neighbouring orbitals. Intermolecular interactions in the crystalline architecture of TPA derivatives were probed by using Hirshfeld and quantum theory of atoms-in-molecules techniques. Furthermore, symmetry-adapted perturbation theory analysis of the TPA analogues has revealed that a periodic arrangement of energetically stable dimers with significant electronic coupling is essential to contribute high charge-carrier mobility to the overall crystal.

Full text of DOI:10.1002/chem.201804620

A Journal of
Accepted Article
Title: Deciphering the multifarious charge transport behaviour of
crystalline propeller-shaped triphenylamine analogues
Authors: Ambili R. V., Devika Sasikumar, Hridya P., and Mahesh
Hariharan
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10.1002/chem.201804620
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Deciphering the multifarious charge transport behaviour of
crystalline propeller-shaped triphenylamine analogues
Ambili R. V.,† Devika Sasikumar,† Hridya P.† and Mahesh Hariharan*[a]
ordering and the packing arrangement of the molecules in
crystals play a huge role in modulating the charge transport
abilities.[9] The crystalline architectures that encompass higher
degrees of intermolecular orbital overlap enhance the mobility of
the charges throughout the organic charge transporters.[10–15]
The propeller-shaped triphenylamine (TPA) core has gained
immense interest as hole transporting materials in OLEDs owing
to the good thermal stability, high glass transition temperature,
and the efficient hole transporting ability coupled with low
ionization potential.[8,16–20] Functionalization of the TPA molecule
would result in diverse crystalline packing arrangements,[21]
which in turn can fine-tune the optoelectronic properties of the
core TPA moiety by enhancing the  interorbital interaction
between the phenyl rings.[9] Thus, a thorough understanding of
the interconnection between the molecular structure, packing
arrangement and transport properties is indispensable in
designing efficient and economical organic materials that are
pertinent for optoelectronic devices.[22,23]
In the current work, we have attempted a comprehensive
computational evaluation of the charge transport characteristics
of a collection of crystalline para-substituted triphenylamine
analogues. The different substituents that vary from electron
withdrawing to electron donating in character when attached to
the TPA moiety at the para positions brought about multifarious
packing motifs and charge transport abilities among the
derivatives. In general, the carrier transport properties of organic
molecules are majorly evaluated using the band theory
(coherent mechanism) suitable for low-temperature conditions
and the hopping mechanism (incoherent mechanism) for high-
temperature conditions.[20,24] Herein, we have computed the non-
adiabatic charge transfer between two neighbouring molecules
in the TPA analogues in the hopping regime. The rate of charge
transfer evaluated in the hopping regime predominantly depends
upon the reorganization energy  and the charge transfer
integral, V (refer experimental section).[25][26] Hence, a complete
evaluation of both the reorganization energy and the electronic
coupling between the neighbouring molecules in organic crystals
is essential to have a comprehensive outlook towards the
charge transport properties of the molecules.[27] Although there
have been reports in the past about the correlation of
reorganization energy with the overall charge transport abilities
of various TPA derivatives using computationally generated
geometries,[8] a complete assessment by utilizing the existent
crystalline TPA motifs to correlate the reorganization energy,
electronic coupling and transport properties still demand
attention.
Abstract: A collection of para-substituted propeller-shaped
triphenylamine (TPA) derivatives have been computationally
investigated for the charge transport characteristics exhibited by
the derivatives using the Marcus-Hush formalism. The various
substituents chosen in the current work, having features that
range from electron withdrawing to electron donating nature,
play a key role in defining the reorganization energy and
electronic coupling properties of the TPA derivatives. TPA
moiety is expected to possess weak electronic coupling on the
basis of poor orbital overlap upon aggregation owing to the
restriction imposed by the propeller shape of the triphenylamine
core. However, the substituent groups attached to the TPA core
can significantly dictate the crystal packing motif of the TPA
derivatives, wherein the variety of noncovalent intermolecular
interactions subsequently generated drive the packing
arrangement and influence the electronic coupling between the
neighbouring orbitals. The intermolecular interactions in the
crystalline architecture of TPA derivatives were probed using
Hirshfeld and QTAIM techniques. Furthermore, SAPT analysis
of the TPA analogues has revealed that a periodic arrangement
of energetically stable dimers having significant electronic
coupling is essential in order to contribute a high charge carrier
mobility to the overall crystal.
Introduction
The interest towards the field of organic semiconductors has
burgeoned over the past few decades with regard to their
potential applications as organic displays, organic light emitting
diodes (OLEDs),[1,2] organic field effect transistors (OFETs)[3,4]
and photodetectors.[5,6] The potential use of organic
semiconductors instead of inorganic materials has globally
attracted attention since the past decade. A basic OLED design
includes an anode, a hole transporting material (HTM), emitting
layer, an electron transporting material (ETM) and cathode.[7,8]
The electric and optical performances of the optoelectronic
devices rely on the efficient charge transporting capabilities of
the carrier organic materials contained in the devices. High hole
mobility of HTM and electron mobility of ETM layers are
preferred for the fabrication of high performance OLEDs. The
process of crystallization is one method for regulating the charge
carrier characteristics of organic materials since molecular
[a] Ambili R. V., Devika Sasikumar, Hridya P. and Mahesh Hariharan
School of Chemistry, Indian Institute of Science Education and
Research Thiruvananthapuram (IISER-TVM), Maruthamala P.O.,
In this regard, we have attempted to study the effect of
regulation of substituent group attached to the para position of
each of the phenyl rings in the TPA moiety, on the crystalline
packing and the overall charge mobility values of TPA
derivatives. Figure 1 shows the TPA derivatives evaluated in the
current work. The TPA analogue containing BO2C6H12 as the
Vithura,
Thiruvananthapuram,
Kerala,
69551
(India)
E-mail: mahesh@iisertvm.ac.in
[†] These authors contributed equally to this work.
[*] Corresponding author
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10.1002/chem.201804620
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nature of the substituents contained in the molecules. The role
of substituents in controlling the crystalline packing motif of
organic molecules is well established. For instance, an earlier
report from our group has highlighted the addition of electron
withdrawing groups to the aromatic cores resulting in
enhancement of face-to-face arrangement of molecules in the
crystals.[28] Substitution dictates the molecular arrangement in
crystals, and the generated packing motif defines the
optoelectronic properties of molecules.
The crystallographic information files (.cif) of propeller-
shaped triphenylamine derivatives, except TB, were extracted
from CSD version 5.38 using ConQuest 1.19 as the search
engine. Qualitative single crystal analysis of TB, which was
synthesized and crystallized in our laboratory, revealed an
orthorhombic crystal system with Pbca space group (Table S1
and S2, SI). Subtle non-covalent interactions such as C-H•••,
, H•••H, dihalogen and hydrogen bonding interactions
predominantly directed the crystalline packing of TPA derivatives,
thereby leading to the eclectic range of properties exhibited by
the crystals. In addition, TPA derivatives possessed many other
remarkable interactions, which resulted in the crystal packing
arrangement of the series of TPA derivatives analyzed here.
In detail, the crystal packing of TCN and TNO2 crystals
comprised of a wave-like arrangement of molecules extending
along the a-axis as shown in figure 2. While C-H••• interaction
(dC-H•••π = 2.79 Å) alone contributed to the wave-like packing in
TNO2, an interplay of both C-H••• and N-C••• (dC-H•••π = 2.85 Å
& dN-C•••π = 3.27 Å) interactions directed the crystal packing of
TCN. In addition, weak C-H•••O hydrogen bonding interaction
(dC-H•••O = 2.50, 2.56 & 2.60 Å) also influenced the crystalline
Figure 1: Derivatives of TPA: a) TCN, b) TNO2, c) TPh, d) TCOOH, e) TCOMe,
f) TI, g) TBr, h) TH, i) TCH3, j) TCH2OH and k) TB.
substituent moiety was synthesized and crystallized in our
laboratory while the crystal structure of the remaining TPA
derivatives were accessed from Cambridge Structural Database
(CSD).
Results and Discussion
The single molecule of triphenylamine (TPA) being C3
symmetrical and propeller-shaped owing to the sp3 nitrogen core
is predicted to have a low carrier mobility due to the less extent
of orbital overlap expected between the non-planar phenyl rings
of TPA upon aggregation. However, despite the non-planar
shape, the intermolecular interactions in the TPA crystals can
drive the packing of molecules in such a way that the aromatic
cores undergo sufficient overlap to promote the charge carrier
transport in the crystalline state. The substituent groups present
in the TPA moiety can contribute to a wide range of interactions
in the crystals eventually leading to different packing motifs,
which have a major role in tuning the charge carrier mobility of a
crystal along with the reorganization energy of the molecule.
Gescheidt and coworkers[8] have paved a way to understand the
substitution effect on the reorganization energy of TPA
packing of TNO2 crystal. In TCOMe crystal, C-H••• (dC-H•••π
=
2.71, 2.82 & 2.85 Å) in combination with C-H•••O (dC-H•••O = 2.67
Å) interactions resulted in a wave-like packing arrangement
which is directed along the c-axis (Figure 2).[29] Crystal structure
of halogen substituted TPA derivatives; TBr and TI were marked
by a wave extending along the b- and c-axis respectively (Figure
2 and S1, SI). C-H••• interactions (dC-H•••π = 2.87 & 2.82 Å)
contributed to the crystal packing in TI whereas diverse
interactions such as C-H••• (dC-H•••π = 2.80, 2.68 & 2.80 Å), C-
Br••• (dC-Br•••π = 3.46 Å), Br•••H (dBr•••H = 2.96 Å) along with a
Br•••Br dihalogen interaction (dBr•••Br = 3.52 Å) aided in the TBr
crystal packing. The intermolecular interactions that governed
the crystalline packing of TCH3 and TH include C-H••• (dC-H•••π
=
2.88 & 2.87 Å) and H•••H interactions (ranging from 2.34 Å to
2.60 Å) whereas C-H••• noncovalent interaction at distances of
2.79 & 2.87 Å facilitated the crystal packing arrangement in TPh
(Figure S1, SI).[30] The presence of boron atoms introduced
intermolecular B•••H interactions in TB crystal (dB•••H = 2.95 Å)
that influenced the crystal packing of the derivative TB. C-H•••
and H•••H interactions along with hydrogen bonding interaction
operating at distances of 1.87, 2.72 & 2.34 Å respectively
directed the TCH2OH crystal packing (Figure S1, SI).[31]
Interestingly, the crystal structure analysis of TCOOH revealed
the presence of voids (approximate volume 600 Å3) in the crystal
(shown in figure S2, SI); an effect of strong cyclic hydrogen
bonding interaction (dO•••H = 1.77 Å) which resulted in a network
arrangement.[32] The cyclic hydrogen bonding interaction was
found to be a characteristic property of the TCOOH crystal and
possibly the absence of such cyclic hydrogen bonding
interactions in the TCH2OH derivative might have resulted in the
lack of voids of large volume in the TCH2OH crystal.
analogues by undertaking
a
theoretical analysis using
computationally generated structures of the various TPA
derivatives. However, according to Marcus theory, the mobility of
charge carriers in organic crystals rely on both intramolecular
reorganization energy () as well as the charge transfer integral
(electronic coupling, V) between the molecules. So, herein we
are attempting to expound the substituent effect on the charge
carrier ability of crystalline TPA derivatives by probing both the
reorganization energy and the electronic coupling of molecules
in their crystalline geometry. The substituents were so selected
such that the different kinds of intermolecular interactions such
as hydrogen bonding, halogen bonding,  interactions, C-
H••• interactions, etc. can be included in the quest to
understand the relevance of each interaction on packing
arrangement and thereby on the charge carrier mobility of TPA
crystals. Moreover, the reorganization energy of organic
molecules that dictate the carrier mobility is influenced by the
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Figure 2: Wave-like crystalline packing arrangements of a) TCN, b) TNO2, c) TBr and d) TCOMe.
C-H•••O non-classical hydrogen bonding (dC-H•••O = 2.54 Å), C-
H••• (dC-H•••π = 2.74 Å) and O-H•••C (dO-H•••C = 2.67 Å) are the
other major interactions contributing to the overall packing of
TCOOH crystal. No discernible wave-like pattern could be
observed in the crystals of hydrogen bonded (TCOOH &
TCH2OH) and hydrocarbon substituted (TCH3 & TPh) TPA
derivatives in addition to TB and TH crystals. The prominent
wave–like pattern of molecules in TPA crystals implicate a well
ordered and patterned arrangement of nonplanar TPA
derivatives.
Hirshfeld surface (HS) analysis, a remarkable method to
explore the packing modes and intermolecular interactions in
molecular crystals using partitioning of crystal space was carried
out to quantify the weak intermolecular interactions present in
the triphenylamine derivatives (Figure 3, Table S3, SI).[33–35] In
HS analysis, the nature and type of intermolecular interactions
experienced by a molecule in the crystal is quantitatively
summarized using a two-dimensional mapping approach. The 2-
D plot, which is unique for a given crystal structure or a
polymorph, maps the fraction of points on the Hirshfeld surface
as a function of the closest distances from the corresponding
point to nuclei inside and outside the surface. The packing
modes adopted by the TPA crystals were then identified based
on the value of ρ, which is the ratio of %C•••H to %C•••C
interactions (Figure 4, Figure S3 and Table S4, SI). A high ρ
value prevail due to an edge to face packing arrangement
facilitated by a higher percentage of C•••H interaction while a
concomitant decrease in the ρ value indicates increase in the
contribution of C•••C interaction that eventually results in face to
face arrangement of the molecules in the crystal. The different
packing modes of a molecular crystal as defined by the ρ values
values in the range of ca. 8-73 which is indicative of a
herringbone packing motif (face-to-edge type packing
arrangement). TCN (ρ = 1.80) and TNO2 (ρ = 2.03) derivatives
have ρ values pointing out to gamma packing arrangement (face
to face packing arrangement) while the ρ value of TCOMe (ρ =
4.42) fall in the range expected for a sandwich herringbone type
packing motif. An increase in the percentage of C•••C contacts
resulting in a concomitant transformation from herringbone to
gamma type packing was observed with a corresponding
increase in the electron withdrawing nature of the substituent
group attached to the TPA moiety. The observed increment in
C•••C contact could be attributed to decreased electron density
on the phenyl rings of TPA derivatives substituted with electron
withdrawing groups, which in turn enhances the scope of face-
to-face arrangement ( stacking) in the crystals. The improved
π stacking with an increase in %C•••C contacts is evident in
TCN and TNO2 derivatives wherein one out of the three phenyl
rings of each TPA moiety were aligned face to face with another
phenyl ring of the adjacent molecule. The separations between
the phenyl rings so aligned are 4.70 & 4.89 Å in TCN and TNO2
derivatives respectively. N•••H interactions (41.2%) along with
C•••H (19.9%), C•••C (11.1%), and H•••H (25.9%) interactions
dictated the packing of TCN, while TNO2 crystal packing was
determined by C•••C (5.6%), C•••H (11.4%), H•••H (16.3%),
C•••N (2.7%), N•••H (3.5%), C•••O (3.6%), and N•••O (1.9%)
interactions (Figure 5). The acetyl group is less electron
withdrawing in nature when compared to cyano and nitro groups,
so that the ρ value of TCOMe crystal has increased to 4.42 with
a decrease in %C•••C interactions when compared to TCN and
TNO2 crystals. Additionally, H•••H (51%) and C-O•••H (23.6%)
interactions also contributed to the total HS of TCOMe.
Fingerprint plots of TH, TPh, TB & TCH3 revealed the presence
of C•••C, C-H•••, and H•••H interactions in the range of 0.6-
1.1%, 13.1-41.6%, and 57.8-75.8% respectively. Moreover B•••H
(1.9%) and O•••H (8.7%) interactions also contributed to the HS
in HS analysis include herringbone (ρ
> 4.5), sandwich
herringbone (3.2 < ρ < 4.0), gamma (γ) (1.2 < ρ < 2.7), and β
(0.46 < ρ < 1.0) type packing arrangements.[36] The studied TPA
derivatives with the exception of TCN, TNO2 and TCOMe have ρ
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Figure 3: Stack bar plot of Hirshfeld surface analyses and  values of derivatives of TPA calculated from analysis of 2-D fingerprint plots.
of TB. HS analysis indicated the presence of Br•••Br and I•••I
dihalogen interactions in TBr and TI crystals respectively.
Furthermore, C•••C (2.7% & 0.4%), C•••H (21.6% & 13.7%),
H•••H (28% & 33.6%) and X•••H (X = Br & I, 38.9% & 34.6%)
interactions also influenced the crystalline packing of TBr and TI
derivatives. TCOOH with 2.9% C•••C contacts and 15.7% C•••H
contacts resulted in a ρ value of 5.41. Hydrogen bonding
interactions contributed to 37.3% of the total HS in TCOOH and
14.8% in TCH2OH. Interestingly the analysis of TCH2OH crystal
revealed no C•••C contacts, hence the ρ value converged to
infinity.
The charge transport characteristics of the TPA crystals
were investigated by employing first principle quantum
mechanical calculation via the method established by Goddard
and Deng.[37] Marcus theory expresses the rate of charge
transfer between molecules in terms of the coupling matrix
element and reorganization energy of the molecule. Electronic
coupling between the molecular orbitals and the reorganization
energy of individual molecules are the major factors affecting the
charge transport properties. The functional groups on the TPA
molecule alter molecular vibrations so that the reorganization
energy will get affected.[8,38] Electrostatic surface potential (ESP)
was generated at the B3LYP/6-311G+(d,p) level of theory to
gain further insight into the effect of substituent groups by
qualitatively providing
a picture of the electron density
distribution in the TPA derivatives (Figure S4, SI).
Electropositive and electronegative regions of the ESP surfaces
are represented in blue and red colours respectively in the ESP
plots. The transformation of phenyl rings from electron rich to
electron deficient nature was observed with increase in the
electron withdrawing ability of substituent groups attached to the
TPA moiety. For instance, the phenyl rings of TCN are in blue
colour whereas the TPh phenyl rings are in red colour as
represented in figure S4, SI.
The extent of coupling between the intermolecular orbitals
depends on the packing of molecules in the crystal. Gamma
type packing arrangement likely enhances the ease of charge
transport through the TPA crystals owing to greater orbital
overlap between the phenyl rings of TPA moiety. The different
kinds of packing arrangement of molecules in the crystals are
expected to be the reason behind the range of optoelectronic
properties exhibited by the TPA crystals. Therefore, we have
focused on the variation of charge transport properties with
different geometrical arrangement of molecules in the TPA
crystals. In the following studies, a detailed analysis of the
correlation between crystalline packing arrangement and charge
transport properties are provided in order to explain the wide-
ranged carrier mobilities showcased by the TPA crystals.
Quantum theory of atoms-in-molecules (QTAIM) analysis offered
further insights into the noteworthy intermolecular interactions
that defined the packing arrangement of molecules in the TPA
crystals (Figure 5, Figure S5 and Table S5, SI). TPA derivatives
that displayed distinct intermolecular interactions in the crystal
packing arrangement (obtained from HS analysis) were chosen
for the QTAIM analysis. Bond critical points (BCP) and bond
paths (BP) were determined while performing QTAIM
calculations and the positive electron density at BCP confirmed
the existence of short contacts among interacting atoms in the
TPA molecules. TCOMe displayed several intermolecular C-
H•••O short contacts in the range 2.67-6.56 Å with varying
electron densities. QTAIM analysis of TCOOH demonstrated the
existence of an O•••C interaction at a distance of 3.11 Å and an
O•••O interaction at a distance of 3.24 Å. To probe the nature of
the O•••O interaction in TCOOH, Pendas’ interacting quantum
Figure 4: 2-D fingerprint plots of a) TCN, b) TNO2, c) TCH2OH and d) TCOOH
obtained from HS analysis.
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Figure 5: QTAIM images of a) D1 of TCN, b) D of TCOOH and c) TCH3.
atoms (IQA) analysis[39–41] was performed on the derivative. The
net interaction energy EIQA obtained from the IQA analysis
determines whether the interaction is repulsive or attractive. The
EIQA obtained from the IQA analysis of TCOOH revealed the
destabilising nature of the O•••O interaction (Table S6, SI).[42]
Strong hydrogen bonding between oxygen and hydrogen atoms
at a distance of 1.77 Å with high electron density value of 0.25
a.u. at BCP was identified in TCOOH whereas weaker acyclic
hydrogen bonding was found to exist in TCH2OH crystal, at a
distance of 1.87 Å with a relatively reduced electron density of
0.18 a.u. at BCP. QTAIM analysis of TBr crystal revealed a
Br•••Br dihalogen interaction at 3.52 Å. The Br•••Br interaction
was found to be attractive in terms of EIQA energy (Table S6, SI)
and was observed to constitute type II dihalogen bonding.
Dihalogen interactions are classified into type I (when θ1 = θ2)
and type II (when θ1 = 90 & θ2 = 180) classes based upon the
interacting angles, θ1 and θ2, and generally the dihalogen
interactions falling into the type II category are rarely observed
among the halogen containing crystals.[43,44] The ESP surface
generated for TBr unveiled a sigma hole on bromine, which
further substantiated the presence of type II halogen bonding
interaction in the TBr crystal (Figure S4, SI). C-H•••Br weak
hydrogen bonding at 3.07 Å and •••Br halogen bonding
interaction at 3.66 Å are the other interactions observed in TBr
crystal. In contrast, the TI crystal did not exhibit any halogen
bonding interaction in spite of a prominent sigma hole being
observed on iodine in the ESP surface.
whereas the intermolecular - interaction was absent in D2.
The presence of intermolecular  interaction in D1
substantiated the considerable contribution of D1 in the
establishment of gamma packing in TCN crystal. The crystal
structure analyses revealed that, out of all the diverse
intermolecular interactions that govern the packing arrangement
of TPA crystals,  non-covalent interaction predominantly
facilitated the characteristic gamma packing motif of TCN and
TNO2 crystals among the TPA derivatives. The key role played
by the  interaction majorly in TCN and TNO2 can be
attributed to the high electron withdrawing nature of the
substituent CN and NO2 moieties when compared to the
remaining units contained in the TPA derivatives. The gamma
packing arrangement thus preferred in TCN and TNO2 has
profusely affected the charge transport characteristics of TCN
and TNO2 among all the TPA derivatives owing to the improved
overlap of molecular orbitals contained in the phenyl rings.
The overall charge transport characteristics of the TPA
crystals were evaluated by employing the Marcus-Hush
formalism and the results are presented in table 1. Generally,
TPA crystals are classified as hole transporters[16–18,30] and
among the series of TPA derivatives considered, TCN, TCOOH
and TCH3 exhibited significant hole transport characteristics
(refer table 1 & figure S7, SI). The electron mobility values of the
TPA derivatives studied were found to be low when compared to
the hole mobility. The halogenated TPA derivatives, TBr and TI,
have high hole reorganization energy that when coupled with a
herringbone packing arrangement proved detrimental for an
efficient hole transport property. Furthermore, TB, TPh, and
TCH2OH showcase comparatively low hole mobility due to the
high hole reorganization energy along with a low intermolecular
coupling ability. Hence regardless of having a wide variety of
intermolecular interactions, suitable crystal packing for the
efficient hole transporting nature could not be achieved by the
above mentioned TPA derivatives (TBr, TI, TB, TPh and
TCH2OH) and hence show insignificant hole mobility when
compared to TCN, TCOOH and TCH3. Despite having a
favorable sandwich herringbone and gamma-type packing
arrangement, TCOMe and TNO2 crystals respectively
showcased low hole transporting capability as a result of a
significantly high hole reorganization energy. TCN possessed
the highest hole mobility among the TPA derivatives which could
be a consequence of both the gamma type packing arrangement
and very low value for hole reorganization energy (0.073 eV).
Generally, the functional groups attached to the parent molecule
increase the reorganization energy by providing a greater
QTAIM analysis of TCN revealed the existence of
intramolecular C•••C interactions between two phenyl rings of
TCN molecule at a short distance of 2.98 Å, with a substantial
positive electron density of 0.077 a.u. at BCP. Gamma type
packing arrangement established two different kinds of dimers
which characterize the crystal packing in both TCN (Figure S6,
SI) and TNO2. The presence of O•••N short contacts (dN•••O
=
3.47 Å) with an electron density of 0.024 a.u. and N•••C
interactions at a distance of 3.26 Å (ρ = 0.034 a.u.) were
identified in TNO2 crystal. Moreover C-H•••N weak hydrogen
bonding was also observed in the TNO2 crystal operating at a
distance of 2.60 Å and with an electron density of 0.043 a.u.
QTAIM analysis was performed separately on the two different
kind of dimers of TCN with an intermolecular distance of 9.35
(D1) and 7.11 Å (D2) between the centroids of monomers
(Figure 6).[45] D1 and D2 of TCN derivatives both contained C-
N•••π, C-H•••N interactions at distances of 3.43-3.90 Å and 2.77-
3.88
Å
respectively. QTAIM analyses revealed
a

intermolecular interaction at a distance of 3.76 Å in D1 (Figure 5)
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Figure 6: a) Shows the dimer 1 (D1) and dimer 2 (D2) that constitutes the crystalline packing motif of TCN; Overlap between the phenyl rings of b) D1 and c) D2.
number of molecular vibrational modes.[46,47] However, earlier
theoretical works have demonstrated that the substitution of
cyano group is an exception, and considerably reduces the
internal reorganization energy of a molecule.[38,48] In addition,
cyanation increased the electronic coupling between
neighbouring molecules by promoting -stacking between them.
Major contributing dimers of TCN for charge transport are shown
in figure 6. Interestingly a 1-D hole transporting pathway with
exceptional hole mobility of 15.3 cm2V-1s-1 was observed for
TCN by extending the dimer, D1 (explained in QTAIM analysis)
along
the
c-axis
as
shown
in
figure
7.
Table 1: Results obtained from mobility calculations performed on the TPA derivatives based on Marcus theory of charge transfer rates.

h
(eV)
e
(eV)
Vh
(eV)
Ve
(eV)
kh
ke
h
e
Molecule
(s-1, ×1013
)
(s-1, ×1013
)
(cm2 V-1 s-1)
(cm2 V-1 s-1)
6.02×10-1
6.47×10-1
1.84×10-1
2.53×10-1
1.09×10-11
6.85×10-2
2.05×10-5
5.49×10-2
4.23×10-1
1.38×10-1
2.01×10-1
TCN
0.073
0.125
0.138
0.150
0.129
0.123
0.114
0.179
0.125
0.099
0.383
0.174
0.290
0.323
0.315
2.74
0.054
0.037
0.031
0.084
0.066
0.057
0.014
0.001
0.007
0.009
0.016
0.042
0.045
0.041
0.083
0.150
0.067
0.095
0.052
0.059
0.028
0.026
9.081
2.176
1.154
4.844
5.842
3.852
0.378
0.002
0.078
1.398
0.019
2.291
0.503
0.198
1.015
0.001
0.385
0.002
0.316
0.922
0.040
0.629
4.67
TNO2
TCOMe
TCOOH
TBr
7.93×10-1
4.65×10-1
3.29
6.66×10-1
2.54
TCH3
TI
0.338
1.18
7.70×10-2
1.77×10-3
2.04×10-2
5.43×10-1
6.01×10-3
TB
0.368
0.262
0.109
0.164
TPh
TH
TCH2OH

h= hole reorganization energy; e = electron reorganization energy, Vh = highest HOMO charge transfer coupling; Ve = highest LUMO charge transfer coupling, kh
= highest hole hopping rate; ke= highest electron hopping rate, h = hole mobility; e = electron mobility (Refer to experimental section for more details).
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Table 2: Results from SAPT(0) analyses.
d(Å)
9.3
Ec
Ei
Ed
Eex
Et
D1
D2
-22.1
-9.9
-7.4
-5.8
-34.5
-49.5
19.1
30.6
-43.8
-34.5
7.1
d = Dimer separation; Ee = Electrostatic energy in kJ/mol; Ei = Induction
energy in kJ/mol; Ed = Dispersion energy in kJ/mol; Eex = Exchange energy in
kJ/mol; Et = Total SAPT(0) energy in kJ/mol.
Figure 7: a) 1-D hole transporting pathway of TCN crystal; b) HOMO and c)
LUMO orbitals of TCN (Surfaces are at isovalue of 0.02 e/Å3).
the presence of  intermolecular interaction in D1 as obtained
from QTAIM analysis.[54] Because of the small separation
between the monomers of D2, the contribution of dispersion
energy dominated in stabilizing D2. Thus, the dimer contributing
more to gamma packing and having more charge transfer
coupling, D1, was found to be more stable.
Attaining a highly conducting 1-D charge transport pathway is
favorable in the development of efficient optoelectronic devices.
The fabrication of an extended system constituted by the
monomer D1 has resulted in the development of a 1-D hole
transporting pathway thereby rendering significant transport
property to the TCN crystal.[49] In organic semiconductors, the
charge transport characteristics are affected by the extent of
overlap between the corresponding molecular orbitals of
neighbouring molecules. The hole mobility across an organic
crystal is determined by the degree of overlap between the
HOMO orbitals of adjacent molecules whereas the overlap of
corresponding LUMO orbitals profoundly affected the electron
mobility of the crystal.[50,51]
Hence, in order to better explain charge carrier abilities of
the TCN molecule, frontier molecular orbital (FMO) analysis of
the TPA derivative was performed using B3LYP/6-311G+(d,p)
basis set. The FMO analysis revealed a uniform distribution of
electron density over the three phenyl rings of TPA moiety in the
HOMO orbital of TCN derivative (Figure 7). Consequently, upon
extending the TCN molecule along the c-axis, efficient overlap of
the HOMO molecular orbitals between the phenyl rings of
adjacent molecules was observed which has possibly played a
key role in generating a 1-D hole transport pathway of high hole
mobility. However, the LUMO orbitals are constituted by an
asymmetric distribution of electrons, and the electron density is
concentrated on only one of the three phenyl rings of TPA
moiety in TCN. Thus, in contrast to the hole transporting ability
of TCN, the electron transport characteristic is comparatively
inefficient as the asymmetric distribution of electron density over
the phenyl rings in the LUMO could ensure quality overlap with
only one of the immediate neighbours. Hence the extension of
D1 or D2 cannot generate a 1-D electron transporting pathway
in the TCN derivative. The dimers, D1 and D2 of TCN that
resulted from the gamma packing motif are different in terms of
the contribution towards the charge transfer coupling. To
understand the relation between the stability of the dimers and
the observed mobility of the constituent dimers of TCN,
symmetry-adapted perturbation theory (SAPT) analysis[52,53] was
employed (Table 2, details are provided in the experimental
Since the TCN derivative is possessing high hole mobility
among all the TPA derivatives with a gamma type packing
arrangement, anisotropic mobility calculation[55–57] was
performed on TCN to understand the angular variation of
mobility (Figure S8, SI). Figure 8 depicts the anisotropic mobility
curves for both hole and electron along a*c* plane of TCN
derivative. The calculated anisotropic hole mobility reaches a
maximum of 6.284 cm2V-1s-1 along the directions, Ф = 160and
3whereas the minimum hole mobility value of 0.818 cm2V-1s-1
was obtained at a direction of Ф = 50and 250. Hence the TCN
crystal has an anisotropy (ratio of highest to lowest hole mobility)
of 7.68 along ac plane, which implies that the effective mobility
displayed by the crystal when used in OFETs would vary
significantly with the orientation of the crystal with the transistor
channel. The maximum hole mobility is observed along
crystallographic c-axis which corresponds to the direction of
extension of dimer D1. Due to the additional hole mobility
contributions from neighbouring molecules oriented in other
directions to the D1 extension channel, the maximum anisotropic
mobility value is not comparable with the obtained 1-D hole
mobility value. The minimum value has been observed along the
a-axis which is orthogonal to the axis of reference (c-axis). The
highest mobility value along c-axis is a result of better orbital
overlap and larger charge transfer integral of D1. The maximum
hole mobility directed along c-axis further substantiated the
presence of an efficient 1-D hole transport pathway which can
be exploited for use in electronic applications necessitating high
mobility. Electronic mobility is not significant when compared to
the hole transport ability of TCN and is represented by the red
coloured plot in figure 8. Surprisingly, TCOOH and TCH3 crystals
exhibited significant charge transport characteristics even
though they lack efficiently organized arrangement of molecules
in the crystal state. Presence of voids in TCOOH, established by
cyclic hydrogen bonding interaction attributes to the high hole
carrier mobility in TCOOH crystal even though the individual
TCOOH molecule is having a high hole reorganization energy of
0.15 eV. The periodic organization of the voids in TCOOH
section). SAPT analysis offered
a quantitative picture by
crystal (Figure S2, SI) as highlighted in the work executed by
determining the total interaction energy of dimers along with the
physical contributions in stabilizing the dimers such as
electrostatics, induction, dispersion and exchange energies
(unlike the qualitative data obtained from ESP). A more negative
SAPT(0) energy obtained for D1 indicated the higher stability of
D1 over D2 in TCN. Electrostatic contribution to stability was
much more pronounced in D1 than D2 which is consistent with
[32]
Vaidhyanathan and coworkers
resulted in a 1-D hole
transporting channel constituted by the extension of dimer D
along the a-axis.
The dimer D (Figure 5) majorly contributed to the high hole
transporting ability of the TCOOH crystal. The individual units of
dimer (D) are separated by a large distance of 8.89 Å, yet the
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general trend of mobility of TPA crystals decreased with respect
to the electron withdrawing effect of the substituents attached at
the para position of the phenyl ring of TPA molecule, provided,
no other traits such as the void in TCOOH and the dense
packing of TCH3 crystal rendered exceptional abilities to the
crystals.
Conclusions
In summary, we have calculated the carrier mobility
values of designated TPA crystals by quantum chemical
calculations. In addition, we have inspected the structure-
property correlation of TPA derivatives by exploring the
intermolecular interactions that fabricated the crystalline
packing arrangements of the studied TPA derivatives. HS
analysis and QTAIM calculations were employed in order to
quantify the intermolecular interactions characteristic of the
TPA crystals analyzed. The electron withdrawing nature of the
substituent group governed the face-to-face packing
arrangement of molecules in the TPA crystals with a methodical
transition to herringbone packing motif in the presence of
electron donating substituents. From the detailed analyses of
charge transporting abilities, it can be inferred that the TPA
derivatives generally act as hole transporters rather than as
electron transporting species. The substituent groups attached
to the TPA core can alter the vibrational modes of individual
molecule of the TPA derivatives thereby varying the hole
reorganization energy of the parent TPA molecule with respect
to the substituent group present.[8] The general hole
transporting characteristic of the TPA crystals was found to be
exceptionally high in TCN owing to the fact that TCN has lower
reorganization energy and highly ordered gamma packing
arrangement. The stability of the dimers obtained from the
gamma packing arrangement, contributing efficiently to the hole
mobility of TCN, D1 and D2 were examined using SAPT(0)
calculation. Thus, we have traversed the mobility-stability
correlation through TCN dimers wherein D1 has higher stability
because of the high electrostatic contribution obtained from the
efficient stacking of monomer units of dimer D1. The
organized arrangement of molecules (D1 dimer) in TCN crystal
contributed to an efficient 1-D hole conducting pathway with a
high hole mobility. In addition, TCH3 and TCOOH also had
comparably high hole transport values with respect to the
selected TPA derivatives. The presence of voids resulting in 1-
D hole transporting channel and the extra stability driven by
dense packing of molecules accounted for the large mobility
values of TCOOH and TCH3 crystals respectively. The
comprehensive investigations done so far in the present work
imparts a proposition to tune the hole transporting properties of
a series of TPA derivatives by virtue of the attached substituent
groups, even though, the propeller shape of the triphenylamine
core can appear as a hindrance for efficient molecular packing.
This study of trilateral relationship (structure-property-stability)
of TPA crystals by tuning the substituents provide a strategy to
Figure 8: The predicted anisotropic hole and electron mobility for TCN crystal.
dimer possessed high electronic coupling (0.08 eV) between the
constituent monomers which in turn resulted in the high charge
carrier transport ability of dimer D. In spite of having a value of
21.54, TCH3 crystal exhibited appreciable hole mobility with a
drift mobility value of 2.54 cm2V-1s-1.has been calculated for
all the TPA crystals, and the high values () are
indicative of herringbone packing motif. In TCH3 molecule (that
has a methyl substituent on the phenyl ring), a part of C-H•••
contacts were contributed by the interactions between methyl
hydrogens and the carbon on the phenyl ring of TCH3. In detail,
TCH3 showed high hole mobility because of the following
reasons: (1) Contrary to C-H••• contacts, wherein both the
participating atoms are from the phenyl rings, C-H•••
interactions between methyl hydrogens and phenyl carbons in
TCH3 promoted face-to-face packing via -interactions rather
than edge-to-face packing (Figure 5). Since the  value
calculation might have overestimated the herringbone character
of crystal packing in TCH3, the value correlation with charge
transport properties cannot validate the high mobility value of
TCH3; (2) In addition, TCH3 crystal is the most densely packed
crystal among all the TPA derivatives analyzed here. The
densely packed structure increased the number of dimers
contributing to the overall crystal mobility and improved the
orbital coupling between the neighbouring molecules.[58] Both (1)
and (2) factors could together explain the increase in the overall
mobility of TCH3 crystal. IQA analysis performed on the TCH3
crystal confirmed that C•••H interaction is attractive in nature
whereas the -interaction was found to be repulsive in nature
(Table S6, SI). Thus, the stabilizing nature of C•••H interaction
promoted the face-to-face packing arrangement of the phenyl
rings in TCH3 rather than the-interaction, thereby generating
a comparatively high hole-transport value in TCH3.
In contrast to TCN, the higher hole reorganization energy
and lower charge transfer coupling values together resulted in
the considerable reduction in the efficiency of hole transport in
TNO2 even though the crystals of TCN and TNO2 share a
common gamma type packing arrangement. As we can observe,
the charge transport properties of TCN is unique owing to the
low hole reorganization energy of the molecule when compared
to the other TPA derivatives studied. On an overall note, the
improve the charge transport characteristic of
a widely
employed material, TPA chromophore, for OLED applications.
Experimental Section
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X-ray Crystallography: High quality single crystals were used for X-ray
diffraction experiments. Crystals were mounted on glass fiber using oil
(infineum V8512). X-ray source used is monochromatic Mo Kα radiation
of wavelength (λ = 0.710 Å at 298 K) and the diffraction spots collected.
Bruker APEXII was used for processing the data. Structures were solved
by direct methods and expanded using Fourier techniques. Hydrogen
atom included in the idealized position but not refined while non-
hydrogen atom was refined anisotropically. HFIX command in SHELXL-
97 was used to constrain hydrogen atom position relative to their parent
position. The CIF files were validated using Check CIF. 3-D structure
visualization and crystal packing explored using Mercury 3.8.
where 푉퐴퐵 is the repulsion between nuclei of A and B, 푉퐴퐵 is the
푛푛
푒푒
repulsion between electrons of A and B and 푉퐴퐵 & 푉퐴퐵 corresponds to
푛푒
푒푛
the attraction between nucleus of one atom and the electrons of the other
atom. IQA helps to understand the nature of intermolecular interactions
from the interatomic energy contribution values.
Material Science Suite: Charge carrier mobility was calculated using
Jaguar[39] DFT model from Schrodinger Materials Science Suite 2016-
2.[61] Calculations were carried out at the B3LYP/6-311G** level of theory
using geometry obtained from crystal structure. The functional and basis
set used for the charge transport calculations in this work were selected
by following the previously reported theoretical works on various
triphenylamine derivatives and other organic chromophores wherein, the
charge transport characteristics were analysed solitary from the
calculated reorganisation energy values (refer to Table S7, SI for more
details).
Computational Details
Hirshfeld Analysis:[34–36] Hirshfeld surface was generated using Crystal
explorer 3.1 to study various intermolecular interactions present in the
volume where the contribution to total electron density at any point
exceeds that of all of its neighbouring molecules. The Hirshfeld surface is
mapped by normalized contact distance dnorm which is a function of both
di and de, evaluated by the equation
Marcus theory for charge transfer rates considers electron/hole hopping
as a non-adiabatic process at high temperatures. The rate of electron or
hole transfer (푘) is calculated using Marcus-Hush formalism.[62]
1
푉2
휋
푘 =
(
)2 푒−휆
⁄
4푘
푇
퐵
푑푖 − 푟
푑푒 − 푟
푖
푒
ℏ
휆푘퐵푇
푑푛표푟푚
=
+
푟
푟
푒
where 푘퐵 is the Boltzmann constant, T is the temperature in kelvin, λ is
the reorganisation energy and V is the coupling matrix constant.
Reorganization energy is given by
푖
where, 푑푖 is the distance from a point on the surface to the nearest
nucleus inside the surface, 푑푒 is the distance from a point on the surface
to the nearest nucleus outside the surface, 푟 and 푟 are the van der
Waals radius of atoms involved in the exterior and interior to the surface
at a point respectively. The 2-D fingerprint plots obtained from the
Hirshfeld surface analysis presents the fraction of surface points
corresponding to each ( 푑푖 , 푑푒
intermolecular interactions present in the crystal and their % contribution
to Hirshfeld surface area.
∗
∗
푒
푖
(
)
λ± = λ1 + λ2 = E0 − E0 + (E± − E±)
where EO and E± correspond to the energy of neutral and cationic/anionic
species in the optimized geometry. EO∗ is the energy of a neutral molecule
in cationic/anionic geometry, E±∗ is the energy of cation/anion in the
neutral geometry for vertical transition during electron transfer. Coupling
matrix element V, quantifies the extent of HOMO-HOMO overlap in hole
transfer and LUMO-LUMO overlap in case of electron transfer, is
obtained from Marcus – Hush two-state model.
) pair. It helps to find the various
QTAIM: The wave functions of triphenylamine crystal structures were
generated by employing B3LYP functional and 6-311++G** basis set at
the DFT level of theory using Gaussian 09 suite.[59] The generated
wavefunction were used for QTAIM analysis using PROAIM in AIM 2000
software package.[60] Quantum theory of atoms in molecules analysis
helps in determining and characterizing the interatomic interactions in a
crystal and are presented in the form of molecular graphs. QTAIM is a
generalization of quantum mechanics to open the quantum system. The
molecular space is partitioned to atomic basins, and the expectation
value of any molecular property is expressed as sum of averages over
the atomic basins. The QTAIM defined BCP (bond critical point) of a
bond, as a point along the bond where the electron density is minimum. It
also corresponds to the point where the bond path interacts the zero flux
surface separating the atoms. The electron density and energy density at
BCP is indicative of strength and nature of the bond. The Laplacian of
electron density at BCP (∇2휌) when lower in magnitude (∇2휌<0) defines a
covalent bond while closed interactions like van der Waals interactions,
electrostatic interactions and hydrogen bonds have ∇2휌 > 0.
Diffusion coefficient evaluated from the hopping value is given by
1
퐷 =
∑푟2 푘푖푃
푖
푖
2푁
푖
where N is the dimensionality of distance, 푃 is the probability of
푖
electron/hole transfer to the ith neighbour and 푘푖 is the rate of
hole/electron hopping to the ith neighbour. 푃 is given by
푖
푘푖
푃 =
푖
∑
푘푖
Charge carrier mobility 휇 is evaluated from Einstein relation
푒
휇 =
푘퐵푇
where 푒 is the electronic charge.
The orientation dependent charge carrier mobility of molecules in crystals
with respect to the conducting channel was determined by the following
equation.
푒
µ휑
=
∑푟2푘푖푃 퐶푂푆2 훾푖 퐶푂푆2 (휃푖 − 휑)
푖
2푘퐵푇
푖
where 훾푖 is the angle between the charge-hopping pathway and the
plane of interest and(휃푖 − 휑) is the angle between the hopping pathway
and the conducting channel.
IQA:[40,41] Interacting quantum atoms method was used to determine the
energy of molecular system and to analyze the electron density of
stabilizing interactions obtained from the QTAIM formalism. This was
done using AIMALL software. The energy of molecular system is defined
Symmetry-Adapted Perturbation Theory (SAPT):[52] SAPT was
employed to determine the non-covalent interaction energies of dimer
molecules. SAPT is a form of energy decomposition analysis (EDA)
where the total interaction energy was calculated by subtracting the
monomer energies from the dimer energy. SAPT(0) calculations provide
the contributing components of interaction energy. The results obtaining
as
∑
1 ∑퐴 ∑
퐸 = 퐴 퐸푖퐴푛푡푟푎
+
퐸
퐵≠퐴
푖푛푡푒푟
퐴퐵
2
where the summation run over all the atoms. Intra atomic energy, 퐸푖퐴푛푡푟푎
,
is defined as
퐸푖퐴푛푡푟푎 = 푇퐴 + 푉퐴 + 푉퐴
from SAPT(0) analysis is
a second order perturbation expansion
푛푒
푒푒
constituting first order electrostatic and exchange energy parts & second
order dispersion, induction and their exchange counterparts as the
perturbation terms.
where TA is the kinetic energy of electrons in an atom, 푉푛퐴푒 is the energy
corresponding to the attraction between electrons and nucleus of an
atom and 푉퐴 is the energy corresponding to the repulsion between
푒푒
푆퐴푃푇(0)
(2)
(2)
퐸
= 퐸푒(1푙푐) + 퐸(1) + 퐸푖(푛2푑) + 퐸푖(푛2푑)−푒푥 + 퐸 + 퐸
푒푥
푑푖푠 푑푖푠−푒푥
푖푛푡
electrons in an atom. Interatomic energy between A and B atoms is
SAPT can theoretically explain the interactions and can practically
quantify the interaction energies. SAPT analysis was performed using
jun-cc-pvdz basis set.
calculated as
퐴퐵
퐸
= 푉퐴퐵 + 푉퐴퐵 + 푉퐴퐵 + 푉퐴퐵
푛푛 푛푒 푒푛 푒푒
푖푛푡푒푟
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10.1002/chem.201804620
Chemistry - A European Journal
FULL PAPER
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10.1002/chem.201804620
Chemistry - A European Journal
FULL PAPER
FULL PAPER
Tuning
attached
the
to
substituent
the non-planar
group
Ambili R. V.,† Devika Sasikumar,†
Hridya P.† and Mahesh Hariharan*
triphenylamine (TPA) chromophore
plays a vital role in modulating the
charge transporting capabilities of
TPA crystals by significantly affecting
the internal reorganization energy of
the molecule, crystal packing and
thus the intermolecular electronic
coupling efficiency.
[†]
These authors contributed equally to this
work.
Page No. – Page No.
Deciphering the multifarious charge
transport behaviour of crystalline
propeller-shaped triphenylamine
analogues
This article is protected by copyright. All rights reserved.