Effect of Bulky Functional Groups on Donor and Acceptor Interactions and their Photoluminescence Properties

Article abstract of DOI:10.1002/cphc.202000612

Material designs that use donor and acceptor units are often found in organic optoelectronic devices. Molecular level insight into the interactions between donors and acceptors are crucial for understanding how such interactions can modify the optical pro

Full text of DOI:10.1002/cphc.202000612

Accepted Article
Title: Effect of Bulky Side Group on Donor and Acceptor Interactions
and its Photoemission
Authors: Kai Lin Woon, S. A. S. Mustapa, Nor Shafiq Mohd Jamel, V.
S. Lee, M. Z. Zakaria, and A. Ariffin
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: ChemPhysChem 10.1002/cphc.202000612
Link to VoR: https://doi.org/10.1002/cphc.202000612
01/2020

10.1002/cphc.202000612
ChemPhysChem
ARTICLE
Effect of Bulky Side Group on Donor and Acceptor Interactions
and its Photoemission
K.L. Woon,[a]*, S. A. S. Mustapa[a] , N. S. MohdJamel[b], V. S. Lee[b], M. Z. Zakaria[b] A. Ariffin[b]
[a]
Low Dimensional Material Research Centre,
Department of Physics,
University Malaya,
Kuala Lumpur,
Malaysia
* Corresponding authors E-mail: ph7klw76@um.edu.my
[b]
Department of Chemistry,
Faculty of Science,
University Malaya,
Kuala Lumpur,
Malaysia
Supporting information for this article is given via a link at the end of the document.
Abstract: Material designs that use donors and acceptors are often
found in organic optoelectronic devices. Molecular level insight into
the interactions between donors and acceptors are crucial for
understanding how such interactions can modify the optical properties
of the organic optoelectronic materials. In this paper, tris(4-(tert-
butyl)phenyl)amine (pTPA) was synthesized as a donor in order to
compare with unmodified triphenylamine (TPA) in donor-acceptor
system by having 2,4,6-triphenyl-1,3,5-triazine (TRZ) as an acceptor.
Dimerization of donors and acceptors occurred in solvent when the
concentration of solute is high. At 0 K, using polarizable continuum
model, the nitrogen atom of TPA is found to stack on top of the center
of triazine of TRZ while such alignment is offset in pTPA and TRZ. We
attributed such alignment in TPA-TRZ as the result of attractive
interaction between partial localization of 2pz electrons at the nitrogen
atom of TPA and π deficient of triazine in TPA-TRZ. By taking account
random motions of solvent effect at 300 K in quantum molecular
dynamics and classical molecular dynamics simulations to interpret
the marked difference in emission spectra between TPA-TRZ and
pTPA-TRZ, it was revealed that the attractive interaction between
pTPA and TRZ in toluene is weaker than TPA and TRZ. Because of
the weaker attractive interaction between pTPA and TRZ in toluene,
the dimers adopted numerous ground state conformations resulting in
broad emission superimposed with multiple small Gaussian peaks.
This is in contrast to TPA-TRZ which has only one dominant dimer
conformation. This study demonstrates that the strength of
intermolecular interactions between donors and acceptors should be
taken into consideration in designing supramolecular structure.
the molecules [4,5]. When such interactions are strong, self-
assembled supramolecules can be formed [6,7] as seen in
deoxyribonucleic acid [8] and liquid crystals [9]. Lone-pair-π
interactions have been speculated to occur in supramolecular
structure in the biological systems [10-11]. Some heteroaromatic
molecules are intrinsically π deficient such as 1,3,5-triazine which
is speculated to give rise to lone-pair-π interactions with water
molecules and amine [12] and formation of new supramolecular
networks and crystal structures [13,14]. Intrinsically π deficient
conjugated systems along with electron rich molecules are often
used in organic photovoltaics [15,16] and organic light emitting
diodes that made use of thermally activated delayed fluorescence
molecules (TADF) [17,18]. Both are important classes of
photoactive materials. However, whether such interactions can
modify the optical properties of the supramolecules are not well
studied. It is reported that lone-pair-π interactions in 1-D
naphthalene diimide can modify the photochromic behaviors [19].
Recently, through space charge transfer in TADF molecules have
gained interests [20-22]. A study of how interactions between
spatially separated donor and acceptor influence the optical
properties of the system can shield light on the characters of
heteroaromatic molecules and supramolecules required for their
design. Hence, we use triphenylamine (TPA) as a donor and
2,4,6-triphenyl-1,3,5-triazine (TRZ) as an acceptor to be the
model molecules. We functionalized the TPA with tert-butyl group
as a bulky side group (pTPA) to weaken the interactions with the
assumption that it will increase the intermolecular distance. We
mixed TRZ and TPA or pTPA at molar ratio in a solvent and we
observed that emissions from the TPA-TRZ and pTPA-TRZ are
drastically different. Through quantum molecular dynamic
simulations, we found that pTPA-TRZ adopted several broad
molecular conformations compared with TPA-TRZ resulting in
different photoluminescence spectra.
Introduction
Supramolecules are molecular complexes where their physical
properties go beyond the mere sum of the constituent molecules
depending on how the constituent molecules interact. These
interactions can be categorized into π-π, CH-π, cation-π, OH or
ON-π and lastly lone-pair-π interactions [1]. Such interactions can
modify the structure-property relationship in supramolecules. For
example, π-π stacking interactions can result in formation of
triplet excimers [2,3] and lower the singlet and triplet energies of
Results and Discussion
Figure 1 shows the structure of the molecules used in this study.
1,3,5-triazine ring can both act as π-acceptor and σ-donor. The
σ-donor originated from the sp2 lone pair electrons located at the
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10.1002/cphc.202000612
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nitrogen atom in the plane of the ring. The π-acceptor originated
from strong electronegativity difference between carbon and
nitrogen atoms. However, in TRZ, the phenyl moieties attached to
the triazine effectively shield it from interactions with other
molecules leaving only the π-deficient acceptor.
deficient. The full list of ESP energy for corresponding surface
maxima and minima can be found in supplementary information
Figure S1.
Figure 3(a) shows the optimized structure of TPA-TRZ in toluene
along with their combined ESP. It is clear that the center of 1,3,5-
triazine ring in TRZ is stacked on top of the nitrogen atom of the
TPA. One of the local maxima as indicated as a big red dot of TRZ
is adjacent with the one of the local minima in TPA (big green dot).
The ESP energy of the electron deficient triazine is now +1.59
kcal/mol and the nitrogen atom in TPA has an ESP energy of -
12.2 kcal/mol. The presence of the minimum potential at nitrogen
atom in TPA indicates a degree of localization of 2pz electrons.
Because of the local minima and maxima locations, we suspected
that in Fig 3(a), the structure is stabilized by interaction between
the partial localization of 2pz electrons in TPA and the π deficient
ring in TRZ. Interaction energy is obtained by calculating the total
energy of the dimer subtracted by energy of each molecules.
Interaction energy of -1.95 kcal/mol is obtained between TPA and
TRZ. The interaction energies between various organic
compounds with water stabilized by lone pair-π deficient ring have
been found to be around ~2 kcal/mol [12]. We also need to
consider the possibility of Fig 3(a) is a ground state electron-
Figure 1(a): Molecular structures of (a) 2,4,6-triphenyl-1,3,5-triazine (TRZ) (b)
triphenylamine (TPA) (c) tris-(4-tert-butylphenyl)amine (pTPA)
The more negative the electrostatic surface potential (ESP) over
the centre of the aromatic ring, the stronger will be the interaction.
In TPA, the central nitrogen atom formed three co-planar N-C
bonds via sp2 -hybridization leaving a valence lone pair electrons
on the 2pz orbital. The 3 benzene rings of TPA are not coplanar.
It is twisted by ~42o from the plane leaving lone pair 2pz electrons
at the nitrogen atom. Because of the lack of co-planarity, the 2pz
lone pair of electrons cannot be fully delocalized into the π orbitals
of the benzene rings resulting in 1.72 occupancy as analysed
using natural bond orbitals at B3LYP/cc-pVDZ level for both TPA
and pTPA. Functionalizing TPA with tris-(4-tert-butyl) has no
influence on the lone pair occupancy. The ESP on the molecular
surface is an effective means to analyse and predict noncovalent
interactions. In TRZ, there is a region of negative surface potential
above and below the ring, arising from the π deficient electrons.
donor-acceptor complex where
a fraction of charge has
transferred from the TPA to TRZ. In order to calculate the fraction
of electronic charge transfer at the ground state, we used atomic
dipole moment corrected Hirshfeld (ADCH) population method
[25]. The complex (TPA-TRZ) is divided into TPA and TRZ. The
ADCH charge in TPA and TRZ alone is compared with the
complex. From the analysis, we can see that TPA has acquired
0.053 |e| while TRZ has lost in equal amount in the complex. This
is far lower than the typical value (~0.2|e|) usually found in donor-
acceptor complexes [26]. The charge transfer interaction will be
minimal.
Figure 2: The colour-filled ESP on van der Waals surface based on the output
of Multiwfn.
Figure 2 shows the colour-filled ESP on van der Waals surface
based on the output of Multiwfn [23,24]. The grid spacing was set
to 0.2 Bohr with isosurface of q = 0.001 e/bohr3. The red and the
green dots represent the local surface minima and maxima
respectively. The sp2 lone pair of triazine create a negative
electrostatic potential with ESP energy of -17.2 kcal/mol (blue
mesh-contour). One of the local maxima is located at π deficient
triazine with an ESP value of + 8.6 kcal/mol. Also note that local
maxima can be found in the center of phenyl rings, but the ESP
energy is about -10.1 kcal/mol which is far from being electron
Figure 3. (a) The optimized structure of TPA-TRZ in toluene obtained at 0K
along with their combined ESP. (b) The intermolecular distance is defined as
distance between the centre of the triazine ring to the nitrogen atom in the TPA.
(c) The normal of the plane created by the two-line vectors between adjacent
nitrogen atoms in the triazine ring (shaded area) and the direction of 2pz orbital
of the nitrogen atom in TPA. (d) The optimized structure of pTPA-TRZ in toluene
along with their combined ESP.
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The intermolecular distance is defined as the distance between
the center of the triazine ring in TRZ to the nitrogen atom of
TPA/pTPA as shown in Figure 3(b). The intermolecular distance
between TPA and TRZ is 4.99 Å. The value we obtained is at the
lower end of the limit. The angle is defined as an enclosed angle
between normal vectors formed by the planes created by the two-
line vectors between adjacent nitrogen atoms in the triazine ring
and the direction of 2pz orbital of the nitrogen atom in TPA/pTPA
as shown in Figure 3(c). The angle for TPA-TRZ is 0.35o.
However, for pTPA-TRZ, the center of triazine ring of TRZ is
located at one of the phenyl of TPA with distance of 3.82 Å and
interaction energy of -3.01 kcal/mol. The angle for pTPA-TRZ is
4.1o. Supposedly, by functioning TPA with bulky side group, the
intermolecular distance between pTPA and TRZ should be further
away, however, this is not the case here. The Highest Occupied
Molecular Orbital (HOMO) of TPA is -5.13 eV while the pTPA is -
4.90 eV indicating that the bulky side group acts as an electron
donating group and the change is too small to result in stronger
donor and acceptor interaction compared with TRZ with HOMO of
-6.80 eV which is very deep. ADCH analysis also indicated
negligible charge transfer at the ground state for pTPA-TRZ.
However, the off-set alignment of pTPA with respect of triazine
ring indicates that other forces of attraction might contribute the
decrease in interaction energy. It is important to note that
interaction energy consists of various contributions including van
der Waals, π-π stacking, Pauli repulsion, electrostatics in origin
and just to name a few. The electrostatics can be originated from
electron rich and electron deficient part of the molecules such as
in hydrogen bonding and even lone-pair – π interactions.
Although the bulky side group of TPA exerts ‘steric repulsion’ or
‘van der Waals repulsion’ and can affect how the pTPA stacks on
top of TRZ, the decrease of interaction energy must be attractive
in nature. Hence, one possible force is van der Waals forces of
attraction since there is an increase in surface area of interaction
between pTPA and TRZ as the bulky side groups from TPA
appears to interact with the phenyl group in TRZ as seen in Figure
3(d).
between 1,3,5-triazine-phenyl and tris-4-tert-butyl-phenyl
interactions. One thing is certain, the partial localization of 2pz
electrons-π deficient interaction has disappeared. This is due
energetic unfavorable positioning/packing of the bulky side group
when the nitrogen atom in pTPA is stacked on top of π deficient
triazine.
Figure 5(a) shows the photoluminescence spectra of the solutions
of pTPA-TRZ in toluene at different concentrations. We can see
that as concentration of pTPA-TRZ increases, the lower emission
spectra are broadened superposed with multiple Gaussian peaks.
In Figure 5(b), we compare the spectra between pTPA-TRZ and
TPA-TRZ at 15 mM. The first peak below 400 nm is contributed
by emission from TPA or pTPA while the lower peak at 500 nm
for TPA-TRZ is originated from the dimerization of TPA and TRZ
[28]. TRZ is a non-emissive molecule even excited at 300 nm.
Figure 5. (a) The photoluminescence spectra of pTPA-TRZ at different
concentrations in toluene as a solvent. (b) The spectra difference between
pTPA-TRZ and TPA-TRZ at 15 mM.
Functionalizing the TPA with tris-4-tert-butyl red-shifted the
emission peak from 354 nm to 374 nm as expected as tris-4-tert-
butyl is slightly electron donating. Using cyclohexane as a solvent,
multiple Gaussian peaks can also be seen in emission spectra. A
new weak absorption peak can be seen at 390 nm and 425 nm in
cyclohexane and toluene respectively (see supplementary
information Figure S2). At first glance, the multiple Gaussian
peaks as seen in pTPA-TRZ seem to display similar patterns with
vibrionic progression. Since we know that the emission is
contributed by the dimerization of pTPA and TRZ, there is little
reason to suggest the lack of vibronic progression in TPA-TRZ.
However, we know that organic molecules can adopt various
conformations in solvents in particular the weakly bounded dimer
where solvent collisions can have sufficient energy to weaken or
destroy the bounded state. Figure 3 represents the optimized
structure of the molecules at 0K under implicit solvent model. It is
well-known that solvent temperature can affect the structural
properties of bio-molecules by controlling their flexibility [29].
Sufficient knowledge regarding the intermolecular forces are
needed to predict and accurately estimate their configurations of
dimer in solvent. In this paper, the relative populations of various
conformers with the accuracy of quantum molecular dynamics
(QMD) and inclusion of solvent under polarizable continuum
model using density functional theory (DFT) are performed.
The relative populations are then estimated from the resulting
residence times such that probability density mapping of the TPA-
TRZ and pTPA-TRZ in terms of radial and the angular distribution
can be plotted as shown in Figure 6. In comparison of distribution
of energy obtained in Figure 6(a) and Figure 6(b), the energy for
the structure in Figure 3(a) and Figure 3(b) are at the very low end
Figure 4. (a) The four regions of attractive intermolecular interactions for TPA-
TRZ. (b) The interactions are more complicated consisting of interaction
between 1,3,5-triazine-phenyl and tris-4-tert-butyl-phenyl interaction.
Reduced density gradient can be used to visualize the weak
interaction region [27]. For TPA-TRZ, there are four regions of
attractive intermolecular interactions as shown in Figure 4(a).
Three of the regions correspond to off-alignment phenyl-phenyl
interactions (a type of π-π interaction) as indicated in black circle
while the red circle is the partial localization of 2pz electrons -π
deficient interaction. In Figure 4(b), we can see that the
interactions are more complicated consisting of interaction
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of the distributions which are ~ 2.3 eV and 1.5 eV lower than the
mean energy of the respective populations (see supplementary
information Figure S3). As we can be seen in Figure 6(a), the
ground state of TPA-TRZs tend to concentrate at about 7 Å at an
angle of ~ 55o in contrast to pTPA-TRZ (Figure 6(b)) which,
surprisingly, has a shorter intermolecular distance ~ 5.5 Å and an
angle of ~ 10o (see supplementary information Figure S4 for a list
of molecular configurations).
sampled preferential molecular configurations performed in
ground state QMD earlier and subjected the molecules to first
excited state QMD. Figure 6(c) and Figure 6(d) show the excited
state TPA-TRZ and pTPA-TRZ probability density distribution
versus radial angle respectively for molecular initial configurations
of (7 Å , 55o) for TPA-TRZ and (5.5 Å, 15o) for pTPA-TRZ. Firstly,
we noticed that the intermolecular distance is reduced in both
TPA-TRZ and pTPA-TRZ. Furthermore, the excited complex
adopted more confined molecular configurations. TPA-TRZ
exhibits two preferential orientations. Closer examination of the
dynamics revealed that TRZ can rotate around its axis (see
electronic media ptpa-trz.mpeg). In excited state, the electron is
located at TRZ and hole is located in TPA/pTPA. The coulombic
interaction between hole and electron enhances the confinement
of the dimers resulting a shorter intermolecular distance. Due to
stronger coulombic interaction which act as a restoring force, any
external jostling such as collisions from solvent molecules, will
eventually bring the dimers in equilibrium positions. Excited state
QMDs are also carried out with other less dominant ground state
molecular configurations. We found out that excited state pTPA-
TPZ can exhibit a wide range of intermolecular distance from 3.8
Å to 5.3 Å (see supplementary information Figure S5 and S6)
Since, the energy of the lowest charge transfer excited state is a
function of intermolecular distance [31], in pTPA-TRZ, the
emission is broadened modulated with peaks corresponding to
dominant conformational distribution.
Figure 6. (a) The ground state TPA-TRZ probability density distribution versus
radial angle at 300K for molecule having initial configuration of (7 Å, 55o). (b)
The ground state of pTPA-TRZ probability density distribution versus radial
angle for molecular initial configurations of (5.5 Å, 15o). (c) The excited state of
TPA-TRZ probability density distribution versus radial angle for molecular initial
configurations (7 Å, 55o). (d) The excited state of pTPA-TRZ probability density
distribution versus radial for molecular initial configurations (5.5 Å, 15o)
The complexes also tend to spread into a wider distribution with
two lower preferences at ~6.5 Å and ~7.5 Å with both at an angle
of ~25o. A molecular configuration obtained from the QMD
representing each preferential orientation in ground state is
sampled for interaction energy calculations. The interaction
energy for TPA-TRZ is ~ -1.21 kcal/mol which is ~0.74 kcal/mol
lower than the 0K TPA-TRZ optimized ground state. For pTPA-
TRZ, the interaction energies at the three different preferential
orientations are -0.13 kcal/mol (~5.5 Å, ~15o), -0.06 kcal/mol (~6.5
Å, ~25o), -0.04 kcal/mol (~7.5 Å, ~25o) far lower than the optimized
ground state at 0K. This could explain why pTPA-TRZ molecules
have a wider distribution than TPA-TRZ. As in the earlier
discussion, we suggested that the interaction between pTPA-TRZ
is mainly van der Waals in origin. The contribution of van der
Waals forces can be modelled using Buckingham potential or
Figure 7. The photoluminescence spectra and the 0-0 transition energies
probability distributions for (a) TPA-TRZ (b) pTPA-TRZ. The molecular
structures of 0-0 transition at the peak of the distributions for (c) TPA-TRZ (d)
pTPA-TRZ along with their intermolecular distances and angles
The 0-0 transition energy can be calculated by subtracting the
excited state energy of the molecule with the ground state of that
molecule. Each energy obtained from the excited state of TPA-
TRZ in QMD in Figure 6(c) are subtracted by list of energies
obtained from the ground state in Figure 6(a). From this, we
generated 10 million possible 0-0 transition energies and a
probability density can be plotted. The structures that contribute
to the peak of distribution can be extracted out. The same method
can be repeated for pTPA-TRZ. Figure 7(a) and (b) shows the
photoluminescence spectra and the distributions of 0-0 transition
energies for TPA-TRZ and pTPA-TRZ. Note that current DFT
theory tends to underestimate the transition energies [32] and
-6
Lennard-Jones potential [30] and these potentials have an r
dependent. Van der Waals force of attraction becomes far weaker
when intermolecular distance increases but becomes stronger
when the area of interaction is large. However, columbic
interaction has a potential of r -1 dependent and hence, the partial
localization of 2pz electrons -π deficient interaction remains intact.
In photoexcitation in toluene, a wider range of conformational
configurations of pTPA-TRZ are excited by the photons compared
with TPA-TRZ.
The next question is what happen to the molecular conformation
in excited states in the solvent? In order to answer this question,
we procced to carry out excited state QMD. In order to do that, we
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hence, the upper y-axes in Figure 7(a) and 7(b) are referred to the
energy obtained from the simulations while the bottom y-axes are
the experimental values. The full width half maximum (FWHM) for
the simulated curve for Figure 7(a) is ~0.8 eV while for Figure 7(b)
the FWHM is ~1.1 eV. Such broadening is related to the disorder
in the molecular conformations [33]. The molecular conformations
that are responsible for the 0-0 transitions are given in Figure 7(c)
and Figure 7(d) for TPA-TRZ and pTPA-TRZ respectively. As we
charge transfer states, the oscillatory strength depends on certain
degree of electronic wavefunction overlap. Besides, the
intermolecular distance plays an important role, the degree of
face-to face overlapping is also important for excitation of spatially
separated charge transfer states. Hence, the T-shaped and end-
to-end for donor and acceptor pair are unlikely to participate in
excited charge transfer states leaving with off-aligned TPA-TRZ
with certain degree of π-π stacking interactions to be the main
can see, because of the weaker interaction for pTPA-TRZ at 300K, excited charge transfer state conformer. For pTPA-TRZ, the
molecules can adopt different preferential orientations at the
ground state. A, B and C are obtained from subtracting excited
energies from Figure S5(c), Figure 6(d) and Figure S5(d)
respectively with the ground state energies in Figure 6(b). This is
selected to illustrate that different conformations of molecular
complexes can result in different emission wavelengths. The
peaks of 0-0 transitions depend on the excited geometry as
shown in Figure 7(d). The 0-0 transition energies decrease with
decreasing excited state intermolecular distance as shown in
Figure 7(d).
staggered conformation was not observed due to the methyl
group in pTPA. We also observed that there are more toluene
molecules acting as an inter-linkages or bridges involved in the
interaction for TPA-TRZ than pTPA-TRZ system. We found no
distinct conformer for pTPA-TRZ system. Since QMD treats the
interactions intrinsically while MD uses parameterized force field,
the two methods are unlikely to give identical results, however,
both methods showed the lack of distinct conformers for pTPA-
TRZ.
Another question is, as revealed in QMD in the TPA-TRZ dimer,
why the radial angle between TPA and TRZ is not near zero in
contrast to ground state optimized DFT calculations? It is also
important to note that in the mixture of TPA and TRZ with equal
molar ratio, we expect not all TPA will be dimerized with TRZ.
Most likely, only a small fraction of them will. We carried out
classical molecular dynamics (MD) by explicitly including the
solvent molecules which cannot be done using QMD due to very
high computational cost. Starting with the initial conditions which
are well-dispersed, the final optimized MD structures for TPA
(cyan)-TRZ (orange) and TPA(cyan)-pTRZ(orange) are shown in
Figure 8(a) and Figure 8(b) respectively with the embedded
toluene (yellow) molecules. It appeared that it has aggregated
with size between 30 Å - 60 Å but the solvent molecules are also
appeared inside the aggregate. Toluene plays an important role
by affecting the intermolecular orientation of supramolecules
TPA-TRZ and pTPA-TRZ through a solvation process between
toluene and compound, which may reflect and induce specific
interactions involved [34]. As for toluene naturally is a nonpolar/
apolar organic solvent, it does affect the morphology of
compounds TPA-TRZ and pTPA-TRZ through co-assembly
happened to form an aggregate as shown in Figure 8(a) and 8(b).
Figure 8(c) and Figure 8(d) are the conformers in stick model from
MDs with solvent molecules removed with the color scheme
corresponding to the nearest pair of interactions. For clarity, the
interaction pairs are given in Figure 8(e) and Figure 8(f). Figure
8(e) shows a ring staggered (yellow bar in Figure 8(e)), T-shaped
(blue bar), end-to-end for donor and acceptor pair (purple bar) and
off aligned face to face (green bar) for TPA-TRZ. None has near
zero radial angle. As previously shown in Figure 4(a), π-π
interactions can be seen in the three phenyl rings of TPA with TRZ.
However, in MD, we can see that only one of the phenyl rings from
TPA is involved in π-π interactions. This is also similar with QMD
where the center of triazine ring doesn’t stack on top of the
nitrogen atom of TPA, similar to the explicit solvent in MD, we
observed more pi-pi interaction of phenyl group in TPA and TRZ.
With combination of attractive force from the partial localization of
2pz electrons -π deficient interaction and π-π interactions, the
dimer is unlikely to stack completely face to face with each other
when there is a greater freedom of motion. The formation of
excited charge transfer state depends on the mutual orientation
and distance of the molecules [35]. For spatially separated excited
Figure 8. Final optimized MD structures in ball model TRZ (orange) and toluene
(yellow) and (a) TPA , (b) pTPA (cyan), (c) conformers in stick model for (c)
TPA-TRZ (d) pTPA-TRZ in toluene (not shown) (e) list of conformers with the
nearest molecules with corresponding colors in (c) and (d) for (e) TPA-TRZ and
(f) pTPA-TRZ
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dielectric constant of 2.38, typical values for a toluene. Suitable molecules
from ground state QMD are sampled to perform first excited state QMD
using time dependent LC-ωPBE at same basis level. In order to establish
the relevance to the experimental observations, the vertical absorption
oscillatory strengths along with the probability densities of that particular
orientations needed to be taken account. Hence, the mapping of Figure
6(a) is reduced from 50 x 50 matrix to 10 x 10 matrix and the products of
vertical absorption oscillatory strengths and the probability densities, p,
were calculated (see Table S2-S5 in supplementary information). The
molecules with the highest product, pmax and up to 0.1pmax were selected
for excited state QMD. Time-reversible integrator with dissipation is also
implemented. The relevant data points are then extracted from the output
files so that intermolecular distance and the angle formed by the two
normal vectors defined in the manuscript can be calculated using Python
3.5 with the help of numpy and sympy modules. For initial configuration for
molecular dynamics simulations of 10:10:60 molecules of
TPA:TRZ:toluene and pTPA:TRZ:toluene were randomly distributed in the
simulation box of 80.6 x 84.9 x 77.3 Å3 using a Packmol Program [40].
CHARMM force field and conjugate gradient algorithm were applied to
optimize the energy until the convergence of 0.01 kcal/(Å mol) was
obtained using HyperChem software. Series of molecular dynamics
simulations were done using NVT ensemble. In the heating phase of 200
ps with 1 fs time step, the temperature of the system is increased from an
initial value of 0 K to a final value of 300 K with temperature step of 10 K.
Then, each system was kept at 300 K for another 400 ps using time step
of 2 fs and the final complex structure was obtained for interaction analysis.
Conclusion
The formation of heterodimer between TPA and TRZ is
encouraged by the interaction between partial localization of 2pz
electrons in the nitrogen atom of TPA and π electron deficient in
triazine and the π-π stacking interactions between phenyl groups.
However, such interactions can be destroyed by introducing tert-
butyl group as a bulky side groups at para- position in TPA. The
weakly bounded heterodimers formed between pTPA and TRZ
adopt numerous conformational configurations in solvent in
contrast to TPA-TRZ, with only one dominant conformer is found
in QMD. As a result, emission from pTPA-TRZ heterodimers
consist of numerous superimposed Gaussian peaks. One of the
important implications that we can infer from such system,
because of strong coulombic interaction between hole and
electron in donor-acceptor system in the excited state,
intermolecular distance is expected to reduce when there is
sufficient face to face stacking between donors and acceptors.
This will be accompanied by geometry relaxation after de-
excitation contributing the loss of energy via molecular
reorganizations in particularly if flexible bonds are connected
between them.
Experimental Section
Conflicts of interest
Characterization and measurements. Materials were characterized by
means of 1H and 13C NMR spectroscopy using BRUKER AVN III 400 MHz
FT-NMR spectrometer using deuterated chloroform as the solvent and
TMS as internal standard.
There are no conflicts to declare
Acknowledgements
Material Synthesis. Tris-(4-tert-butylphenyl)amine was synthesized using
Friedel-Crafts alkylation reaction according to literature procedure [36].
Trifluoroacetic acid can be used for activation of tert-butyl alcohol. A
mixture of triphenylamine (0.5 g), 2-methylpropane-2-ol (2 mL), and
trifluoroacetic acid (10 mL) was stirred and refluxed for 24h. Then, the
mixture was poured in 20% NaOH solution and the aqueous layer was
extracted with dichloromethane. The organic layer was dried, filtered and
the solvent was removed under vacuum. The precipitated was subjected
to column chromatography using 100% heptane. The resulting white solid
collected has a mass of 0.5865g (69 % yield). 1H NMR (400 MHz, CDCl3):
δ 1.31 (s, 27H; C-(CH3)3), 7.01 (d, J = 8.8 Hz, 6H; Ar-H), 7.24 (d, J = 8.8
Hz, 6H; Ar-H), 13C NMR (100 MHz, CDCl3): σ=31.7 [(C-(CH3)3], 34.4 (C),
123.6 (Ar-CH), 126.1 (Ar-CH), 145.2 (Ar-C), 145.5 (Ar-C).
The authors gratefully acknowledge the funding by the University
Malaya Research University Grant-Faculty Program (GPF046B-
2018) and Dr. Hadieh Monajemi and Kheng Ghee Ng from Data
Intensive Computing Centre of University Malaya for rentlessly
maintaining the GPU servers. Also, the authors would like to thank
Dr. Muhammad Faisal Bin Khyasudeen for the technical
discussion.
Keywords: dimer• donor • acceptor • TADF • conformer
References
Photophysical properties. The UV-visible absorption spectra of the
compounds in solutions of toluene at a concentration of 100 µM was
observed using a Shimadzu UV-Vis spectrophotometer (UV-2600). The
photoluminescence emission spectra of the compounds in solutions of
cyclohexane and toluene at room temperature were characterized using a
Perkin Elmer luminescence spectrophotometer with 325nm excitation
wavelength.
[1]
N. Hayashi, A. Kanda, H. Higuchi, K. Ninomiya in Topics in Heterocyclic
Chemistry, Vol. 18 (Eds.: K. Matsumoto, N. Hayashi), Springer, 2009,
pp. 1-35.
[2]
[3]
Y. S. Wang, J. Yang, Y. Tian, M. M. Fang, Q. Y. Liao, L. W. Wang, W.
P. Hu, B. Tang, Z. Li, Chem Sci 2020, 11, 833-838.
M. K. Etherington, N. A. Kukhta, H. F. Higginbotham, A. Danos, A. N.
Bismillah, D. R. Graves, P. R. McGonigal, N. Haase, A. Morherr, A.
Batsanov, C. Pflumm, V. Bhalla, M. R. Bryce, A. P. Monkman, J Phys
Chem C 2019, 123, 11109-11117.
Computational calculations. All molecular geometries are optimized
using density functional calculations performed in Terachem 1.9 [37] using
the optimally tuned range-separated [38] LC-ωPBE functional at cc-pVDZ
basis level under polarizable continuum model (PCM) with toluene as a
solvent [39]. The calculations were performed using a GPU server that had
64 GB RAM installed to support eight Tesla K10 graphic cards. The
optimized geometries are then subjected to quantum molecular dynamics
(QMD) simulations using the same functional theory except at 6-31g basis
set. The initial temperature is set at 550K with equilibrium temperature of
300K using Langevin as a thermostat. Langevin is chosen to simulate the
effect of jostling of solvents. The QMDs are carried out with 1 femtosecond
(fs) time step with 100000 steps. The solvent radius is set to be 3.48 Å and
[4]
K. L. Woon, Z. A. Hasan, B. K. Ong, A. Ariffin, R. Griniene, S.
Grigalevicius, S. A. Chen, Rsc Adv 2015, 5, 59960-59969.
V. Jankus, A. P. Monkman, Adv Funct Mater 2011, 21, 3350-3356.
B. Feringan, P. Romero, J. L. Serrano, C. L. Folcia, J. Etxebarria, J.
Ortega, R. Termine, A. Golemme, R. Gimenez, T. Sierra, J Am Chem
Soc 2016, 138, 12511-12518.
[5]
[6]
[7]
[8]
M. Carini, M. Marongiu, K. Strutynski, A. Saeki, M. Melle-Franco, A.
Mateo-Alonso, Angew Chem Int Edit 2019, 58, 15788-15792.
Z. H. Zong, P. Z. Li, A. Y. Hao, P. Y. Xing, J Phys Chem Lett 2020, 11,
4147-4155.
6
This article is protected by copyright. All rights reserved.

10.1002/cphc.202000612
ChemPhysChem
ARTICLE
[9]
C. Tschierske, Angew Chem Int Edit 2013, 52, 8828-8878.
[10] C. D. Jia, H. H. Miao, B. P. Hay, Cryst Growth Des 2019, 19, 6806-
6821.
[11] J. Novotny, S. Bazzi, R. Marek, J. Kozelka, Phys Chem Chem Phys
2016, 18, 19472-19481.
[12] A. Reyes, L. Fomina, L. Rumsh, S. Fomine, Int J Quantum Chem 2005,
104, 335-341.
[13] T. J. Mooibroek, P. Gamez, Inorg Chim Acta 2007, 360, 381-404.
[14] J. S. Costa, A. G. Castro, R. Pievo, O. Roubeau, B. Modec, B.
Kozlevcar, S. J. Teat, P. Gamez, J. Reedijk, Crystengcomm 2010, 12,
3057-3064.
[15] X. J. Wan, C. X. Li, M. T. Zhang, Y. S. Chen, Chem Soc Rev 2020, 49,
2828-2842.
[16] A. Wadsworth, M. Moser, A. Marks, M. S. Little, N. Gasparini, C. J.
Brabec, D. Baran, I. McCulloch, Chem Soc Rev 2019, 48, 1596-1625.
[17] N. Bunzmann,S. Weissenseel, L. Kudriashova, J. Gruene, B.
Krugmann, J. V. Grazulevicius, A. Sperlich , V. Dyakonov, Mater.
Horiz., 2020,7, 1126-1137
[18] Y. C. Liu, C. S. Li, Z. J. Ren, S. K. Yan, M. R. Bryce, Nat Rev Mater
2018, 3.
[19] J. J. Liu, D. Z. Lu, Y. F. Chen, Y. T. Dong, Z. Yang, Dalton Trans.,
2015, 13, 5957-5960.
[20] Y. K. Wang, C. C. Huang, H. Ye, C. Zhong, A. Khan, S. Y. Yang, M. K.
Fung, Z. Q. Jiang, C. Adachi, L. S. Liao, Adv Opt Mater 2020, 8,
1901150.
[21] E. Spuling, N. Sharma, I. D. W. Samuel, E. Zysman-Colman, S. Brase,
Chem Commun 2018, 54, 9278-9281.
[22] M. Auffray, D. H. Kim, J. U. Kim, F. Bencheikh, D. Kreher, Q. S. Zhang,
A. D'Aleo, J. C. Ribierre, F. Mathevet, C. Adachi, Chem-Asian J 2019,
14, 1921-1925.
[23] T. Lu, F. W. Chen, J Comput Chem 2012, 33, 580-592.
[24] T. Lu, F. W. Chen, J Mol Graph Model 2012, 38, 314-323.
[25] T. Lu, F. W. Chen, J. Theor. Comput Chem,2012,11,163-183
[26] H. Méndez, G. Heimel, S. Winkler, J. Frisch, A. Opitz, K. Sauer,
B. Wegner, M. Oehzelt,C. Röthel, S. Duhm, D. Többens, N. Koch,
I. Nat. Commun., 6 ,2015, 8560
[27] E. R. Johnson,S. Keinan, P. Mori-Sꢀnchez, J. Contreras-García, A. J.
Cohen, W. Yang, J. Am.Chem. Soc.2010,132, 6498−6506
[28] K. L. Woon, C. L. Yi, K. C. Pan, M. K. Etherington, C. C. Wu, K. T.
Wong, A. P. Monkman, J Phys Chem C 2019, 123, 12400-12410.
[29] N. Berger, L.J.B. Wollny, P. Sokkar, S. Mittal, J. Mieres-Perez, R.
Stoll,W. Sander, E. Sanchez-Garcia, Chemphyschem, 20, 2019,1664-
1670
[30] T. C. Lim, Z Naturforsch A 2009, 64, 200-204.
[31] Y. Fang, S. L. Gao, X. Yang, Z. Shuai, D. Beljonne, J. L. Bredas,
Synthetic Met 2004, 141, 43-49
[32] T.J. Penfold, Dias F.B., A. P. Monkman, Chem Commun 54,2018
3926-3935
[33] Serevicius T, Skaisgiris R , Dodonova J , Kazlauskas K, Jursenas S ,
Tumkevicius S Phys. Chem. Chem. Phys., 22, 2020,265-272
[34] S. Xue, P. Xing, J. Zhang, Y. Zeng, Y. Zhao, Chemistry–A European
Journal. 2019, 25, 31, 7426-7437
[35] A. Aster, G. Licari, F. Zinna, E. Brun, T. Kumpulainen, E. Tajkhorshid,
J. Lacour, E. Vauthey, Chem. Sci. 2019,10, 10629-10639
[36] D. Vak, J. Jo, J. Ghim, C. Chun, B. Lim, A. J. Heeger andD.-Y.
Kim,Macromolecules, 2006,39, 6433–6439.
[37] I.S. Ufimtsev, T. J. Martínez, J. Chem. Theo. Comp., 2009,5, 2619
[38] H. T. Sun, C. Zhong, J. L. Bredas, J Chem Theory Comput 2015, 11,
3851-3858.
[39] F. Liu, N. Luehr, H.J. Kulik, T.J. Martinez , J. Chem. Theory Comput.
2015, 11, 7, 3131–3144
[40] L. Martínez, R. Andrade, E.G. Birgin, J. M. Martínez, J Comput Chem.
2009, 2157-2164
7
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ARTICLE
Entry for the Table of Contents
Functionalizing TPA with tris-(4-(tert-butyl) as tris(4-(tert-butyl)phenyl)amine (pTPA) decreases the strength of attractive interactions in
toluene. The weakly bounded pTPA and TRZ adopt numerous conformations resulting in emissions from different excited charge-
transfer conformers.
8
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