Understanding the molecular orientation growth on a nanometer scale and adjustable electron transition performance of a terpyridyl derivative under different external environments

Article abstract of DOI:10.1039/c8ce01940a

In this study, a terpyridyl derivative 2-(6-(pyridin-2-yl)-4-(4-((E)-2-(pyridin-4-yl)-vinyl)phenyl)pyridin-2-yl)pyridine (abbreviated as PYTPY) has been designed and prepared with an A-π-A architecture (A = electron acceptor), whose spontaneous aggregatio

Full text of DOI:10.1039/c8ce01940a

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HIGHLIGHT
Tiddo J. Mooibroek, Antonio Frontera et al.
Towards design strategies for anion–π interactions in crystal engineering
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DOI: 10.1039/C8CE01940A
CrystEngComm
ARTICLE
Understanding the molecular orientation growth at nanometer
scale and adjustable electron transition performance of a
terpyridyl derivative under different external environments
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
DOI: 10.1039/x0xx00000x
Hai-Yan Wang,† Lin Kong,† * Shu-Juan Zhu, Hong-Ping Zhou, * Jia-Xiang Yang
www.rsc.org/
In this study,
a terpyridyl derivative 2-(6-(pyridin-2-yl)-4-(4-((E)-2-(pyridin-4-yl)-vinyl)phenyl)pyridin-2-yl)pyridine
(abbreviated as PYTPY) has been designed and prepared with A-π-A architecture (A = electron acceptor), whose
spontaneous aggregation style can be triggered by different external conditions. Mathematical modelling though time
dependent density functional theory (TD-DFT) provides a quantitative understanding of the equilibrium molecular
geometries at aggregated state between adjacent PYTPY molecules under different additives, such as the addition of
+
+
water, H , and Ag ion. The vertical electronic excitations and the related UV-vis absorbing character of PYTPY under
different conditions were also forecasted, which was verified by the examined absorption spectra. Our method of
calculating intermolecular interaction between adjacent molecule, predicting spontaneous aggregation style and
absorbing performance under external environment is generally applicable to design and create other functional organic
materials.
forefront of investigations.
1
. Introduction
2,2’:6’,2’’-terpyridyl derivatives have been demonstrated to
be promising candidates for this study because of their unique
In the field of materials science, the crystal engineering is an
electronic/optical character and its easily modified
structure/performance due to the extensive electronic
delocalization [12-13]. Terpyridyl derivatives have gained
much interests in the fields of supramolecular, coordination
chemistry and material science in the past decade, which show
a wide range of potential applications covering light-to-
electricity conversion [14], light-emitting electrochemical cells
important part, which depends on the understanding of
structure-property relationship [1-3]. One of the most relevant
issues associated to structure-property relationship study of an
organic molecule is the control of self-assembly process, which
is required to create functional materials for specific
applications. Self-assembly upon spontaneous aggregation of
organic molecules turns out to be a very powerful tool that is
able to generate supramolecular ordered functionalized
structures, formed by spontaneous assembling based on non-
covalent interactions. There are two factors that can take
effect of a material, one is the intrinsic performance of it [4-5],
the other is the conformation of an organic molecule in the
bulk scale under different conditions [6]. In this regard,
changing the external factors such as pH [7], heat [8], pressure,
the introduction of metal ion or guest compound [9], is an
attractive prospect to alter the aggregation style and/or
properties of an organic material [10-11]. Thus, the studies of
external environment on the self-assembly and the related
performance of a functional organic compound are at the
(LECs) [15], (electro)luminescent systems (e.g. organic light-
emitting diodes, OLED), nonlinear optical devices [16] and/or
life science. The chemical-physical performance of terpyridyl
derivatives can be tuned through a careful choice of peripheral
substituent groups [17], acid-base medium [18], and/or
coordinated metal [19], which are responsible for their
aggregation state. Thus, well-designed supramolecular
architectures upon terpyridine derivative can be easily realized
due to strong metal-terpyridine connectivities [20] and strong
hydrogen bond interactions between pyridine and acid
medium [21], which open avenues to smart materials with the
opportunity of switching the physical and/or chemical
characters depending on parameters such as pH value or the
existence of metal ions [10]. The self-assembly and the related
aggregate style are two main factors that can change the
performance of it. Despite tremendous progress in controlling
the self-assembly process and the related aggregated
structures by electrostatic interactions or stoichiometry
a.Department of Chemistry, Key Laboratory of Functional Inorganic Materials of
Anhui Province, Anhui University, Hefei 230039, P. R. China.
†
†
The two authors contributed equally to this work.
† *corresponding author, E-mail: Kong_lin2009@126.com, zhpzhp@263.net
Electronic Supplementary Information (ESI) available: [preparation procedure of
intermediates 1 and 2; Crystal data of PYTPY; Molecular orbital diagrams of PYTPY method, spatial control of the self-assembly process of such
monomer and/or dimer at different conditions; Characterization of PYTPY-Ag]. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
x0xx00000x
type of terpyridine derivative is still difficult to achieve.
This journal is © The Royal Society of Chemistry 20xx
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In this contribution, this study aims to provide a detailed 2.1 Preparation of PYTPY
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DOI: 10.1039/C8CE01940A
step towards a better understanding of the structure-property
The synthesis procedure of intermediates
1 and 2 were
relationship between the external condition and self-assembly
process of the target terpyridine derivative PYTPY (Figure 1).
Pyridine-substituted styrene unit is linked at 4-bit of
listed in ESI. To prepare the target compound PYTPY,
intermediate (0.42 g), 4-pyridine formaldehyde (2.00 g) and
2
potassium t-butoxide (t-BuOK) (0.50 g) were mixed together
and grinded for 30 min. The reaction was monitored by thin-
layer chromatography (TLC) to ensure complete reaction.
Then, the organic component was dissolved in
terpyridine unit to construct A-
Scheme 1), which is used to revolute the
essential for the molecular engineering. Moreover, extending
the -spacer is useful to enhance the photocurrent for
efficient charge transfer [22]. Self-assembly strategies towards
-conjugated terpyridine into advanced supramolecular
architectures are evaluated under different external conditions
π
-A type molecule PYTPY
(
π
-bridge unit that is
π
2 2
dichloromethane (CH Cl , 200 mL), washed with saturated salt
water for three times, dried overnight with anhydrous
magnesium sulfate, removed the organic solvent under rotary
evaporation. The residue was purified through column
chromatography on silica gel with ethyl acetate used as the
π
+
+
such as water media, the existence of H and/or Ag ions. We
start with the aggregate style of PYTPY in dilute
eluent to get the target compound as a light-yellow solid.
tetrahydrofuran (THF) solution under evaporation process
1
Yield: 30%. H NMR (400 MHz, deuterated chloroform CDCl
3
) δ
+
(
Figure 1). Then different equivalents of H ions are added to
(ppm): 8.78 ~8.73 (m, 4H), 8.69 (d, J = 8.0 Hz, 2H), 8.60 (d, J =
change the intermolecular interactions between adjacent
molecules, which will then change the orientation aggregate
along different directions. Very recently, Spitzer used proton
diffusion model to study the assembly process of a dendritic
peptide conjugate at a solid-liquid interface [23]. Here, in this
study we fix the proton at proper location, N atom at pyridine
6
2
1
1
1
.0 Hz, 2H), 7.97 (d, J = 8.2 Hz, 2H), 7.89 (td, J = 7.8, 1.32 Hz,
H), 7.68 (d, J = 8.2 Hz, 2H), 7.42~7.30(m, 5H), 7.11(d, J =
13
6.3Hz, 1H). C NMR (CDCl
3
, 100 MHz): δ = 156.2, 156.0,
50.3, 149.4, 149.2, 144.4, 138.6, 136.9, 136.8, 132.4, 127.8,
-1
27.6, 126.7, 123.9, 121.4, 120.9, 118.0. FI-IR (KBr, cm ): 3054
(m), 1581 (C
=
N, s), 1457 (C
=
C, s), 1379 (s), 1040 (m), 800 (s),
+
cycles. Further, Ag ions are introduced into the system to
+
7
24 (s), 658 (m). Ms. Cal: 412.50, Found: 413.25 (M+H ).
study the changeable intermolecular interactions under strong
coordination effect. Based on TD-DFT computations with the
B3LYP or BP86 functional (see computational details), its single
point energy, structural, electronic and intramolecular charge
transfer were successively studied, the computational
proposal of which are verified by experimental data. The
combined theoretical-experimental approach shows that
investigating the self-assembly process is key in manipulating
growth direction of aggregated organic structure, and
regulating/optimizing the optical performance as well. Our
strategy is applicable to many pH/metal ion responsive
Light yellow crystals of PYTPY suitable for X-ray diffraction
were obtained by slow evaporation of PYTPY-THF solution at
room temperature for about two weeks.
systems for the preparation of organic aggregates at Scheme 1 The preparation procedure of the target PYTPY, four pyridine units
were signed as
①, ②, ③, and ④.
micro/macro size.
2
.2 Self-assembly of PYTPY in different external conditions
To construct PYTPY aggregates and study the effect of outer
condition on the formation, PYTPY was added into water
medium with and without acid. Firstly, PYTPY-THF solution was
added into water with different water volume ratio (f
detail, f = 90% was described as an example, 200 L of PYTPY-
THF solution (1.0 10 mol/L) was added into 1.8 mL distilled
w
). For
w
µ
-
3
×
water under violently stirring for 10 min, then the solution was
left undisturbed overnight to let the crystals grow naturally.
After that the solution was ultrasonic dispersed for 10 seconds
to study the morphology and the UV-vis absorption. Similar
experimental methods were used to prepare other samples
with the external environment regulated.
w
Next, a proper f was chosen when different proportions of
Figure 1 Illustration of the weak intermolecular interactions and self-assembly
structure of PYTPY under different external conditions. At the center: Single
crystal structure of PYTPY with the atom numbering.
hydrochloric acid was added to study the influence of acidity
on self-assembly process. Upon lowering the pH, the pyridine
units were protonated, which in turn induced a monomer to
supramolecular aggregation transition.
To study the influence of the coordination effect on the self-
assembly, PYTPY-THF solution was added into water with silver
2
. Experimental Section
2
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ARTICLE
ion, the aggregated morphology and optical performance of N1-C16-C15-C14-C22) of adjacent molecule withVtiehweArdticisletOannlcinee
which were examined at room temperature. PYTPY@Ag being 3.8036 Å are contributed to molecuDlOaIr: 1a0g.1g0r3e9g/aCt8ioCEn0a19lo4n0Ag
composite was prepared through PYTPY and AgNO in N,N- b axis as shown in Figure S8. All these weak interactions are
3
dimethylformamide (DMF) solution at room temperature supposed to be the driving forces for the self-assembly to
under stirring. The PYTPY-Ag complex was prepared in DMF construct into bulk-size and/or micro/nano-size structures.
o
solution at high temperature (140 C) as well. The two were
used as the controlled samples.
Table 1 Crystal data and structure’s refinement for PYTPY single crystal
Compound
Formula
Formula weight
PYTPY
V [Å3]
Z
2102.5(12)
4
1.303
2
.3 TD-DFT calculation
C
28
H
20
N
4
Calculations were carried out via TD-DFT method (Gaussian
-3
412.48
Dcalcd [g·cm ]
-
1
0
9). Non-hydrogen atoms were fixed using Cartesian Temperature (K)
296(2)
μ [mm ]
0.079
coordinates, and hydrogen atoms were optimized at their
most suitable coordinates. Single point energy was calculated
at M06/B3LYP method, the detailed level basis for each model
was described in the main text. The assembling energy
-18<=h<=18
-15<=k<=15
Crystal system
Space group
a [Å]
Monoclinic
Cc
Limiting indices
-
13<=l<=13
F (000)
Completeness
to theta =
864
between two relative fragments was defined as Einteraction
=
15.827(5)
13.169(5)
10.544(4)
100.0 %
2
4.99
Edimer - 2Emolecule-free [24]. The electron cloud density distribution
b [Å]
θ range [°]
Reflections
collected /
unique
2.05-24.99
7243/3228
[R(int) =
of each energy level and the corresponding electron transition
were also calculated with bp86 and/or B3LYP method, which
was described in the main text along with the level basis.
c [Å]
0.0459]
Data /
α [deg]
90.000
restraints /
parameters
Final R indices
3228 / 2 / 290
3
. Results and discussions
R1 = 0.0446,
wR2 = 0.1002
β [deg]
106.918(6)
[
I>2sigma(I)]
3
.1 Crystal structure of PYTPY
0
0.12
.13 × 0.12×
R indices (all
data)
GOF
R
1
= 0.0668,
wR = 0.1130
1.038
Crystal size (mm)
PYTPY crystallizes in a monoclinic form with space group Cc as
shown in the center of Figure 1 with the cell length a =
1
2
o
5.8267, b = 13.1694, c = 10.5443 and cell angle
β
= 106.918 .
The crystallography data are summarized in Table 1. Selected
bond lengths and bond angles are listed in Table S1. In the
molecule, the bond lengths of the benzene and pyridine ring
are all of aromatic character. The linkage bond length between
3
3
.2 Orientation growth and optical performance in water media
.2.1 Theoretical calculation for orientation growth
It is known that the aggregation structure of a material can
the benzene ring and the pyridine unit is quite conjugated with be influenced by the external environment of the growth
C6-C7 being 1.339(4) Å and C13-C14 being 1.492(4) Å. The C6- process. However, the intermolecular forces between adjacent
C7 bond is nearly coplanar to the adjacent benzene and molecules is the main cause of the accumulation of a material
pyridine ring with the torsion angle between C5–C6 and C7–C8 [25]. Thus, in order to explain the aggregation of PYTPY
o
o
being -176.5(3) that is very close to 180 . The adjacent molecules, firstly, thermodynamic principles are used through
pyridine rings are also nearly coplanar as the torsion angle computational calculation of weak intermolecular interactions
o
between C15-C16 bond and C17-C18 bond is 170.3(3) . Similar between adjacent molecules by varying the relative position of
result is also observed between C22-C23 bond and C24-C25 adjacent molecules and regulation of packing models along a,
bond, with the torsion angle being -173.4(3). The structural b and c axes [26]. The selected fragments were cut out directly
features reveals that non-hydrogen atoms are highly from the CIF data, which are listed in Figure 2. The single point
conjugated and nearly coplanar, which would favor the energy of free molecule was calculated through b3lyp/6-31g
electronic delocalization in the whole molecule.
method, which was calculated through b3lyp/6-31g(d) for the
There is a C2-H2⋅⋅⋅N4 intermolecular hydrogen bond with dimer. The total energy of the selected fragment and
o
the H2⋅⋅⋅N4 distance of 2.723 Å (
∠
C2-H2⋅⋅⋅N4 = 156.95(22) ) molecule-molecule interaction energy are listed in Table 2. The
along a axis as shown in Figure S6a. The adjacent molecules results shows that the dimer is two times lower in energy than
are also stacked through C18-H18⋅⋅⋅N2 weak interactions along monomer. The energy of intermolecular forces between
-
1
c axis at a short intermolecular distance of 2.333 Å with the adjacent molecules along a axis is 10.66 kJ
⋅
mol , which is 1.83
mol along c axis, respectively.
interaction with the energy being
⋅mol . The relative interaction force along a, b, c axis is
o
-1
-1
∠
C18-H18⋅⋅⋅N2 being 159.143(192) (Figure S6b). These two kJ
⋅mol along b axis and 6.40 kJ⋅
types of weak hydrogen bonds are the main driving forces to Also, there exits weak
form one-dimensional (1D) structures (Figure S6). There also 1.80 kJ
π-π
-1
exists C3-H3⋅⋅⋅C27 weak intermolecular interactions with the 1.00: 0.17: 0.60. The results reveal that the interactions along
o
distance being 2.861 Å and
∠
C3-H3⋅⋅⋅C27 being 137.561(234) , a axis are stronger than that along the other two, and the
which is very important in crystal packing to form two- interactions along a and c directions are much stronger than
dimensional sheets in the bc plane as shown in Figure S7. that along b axis, which suggests that the interactions between
Further, weak
π-π
interactions between pyridine ring (C4-C5- adjacent molecules might lead to one-dimensional (1-D,
C2-C1-N1-C3) of one molecule and another pyridine ring (C23- possibly grew along a axis due to C2-H2⋅⋅⋅N4 intermolecular
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hydrogen bond) and/or semi two-dimensional (2-D, possibly at and the width ranges from 65 nm to 130 nm. TVhieew ALr/ticDle Orantlinioe
ab plane) orientation growth in nature (both in micro and ranges from 9 to ~19, especially in theDrOedI:-1m0.1a0r3k9e/dC8cCirEc0l1e9s40inA
macro size). The result is reasonable that there exists multi- Figure 3b. The TEM image of a typical rod is listed in Figure 3c.
w
types of weak interactions along a axis compared to that along The results mean that f change do not influence the intrinsic
the other two, which means that the weak interactions structure, but influences the aggregation degree. Thus, it can
between adjacent molecules along this direction are be concluded from the calculation results and the
somewhat stronger than that along the other two. The experimental observations that the intermolecular interactions
stronger interaction will lead to spatially oriented growth. between adjacent molecules control the aggregation.
Thus, understanding the formational mechanisms of assembly
process can be challenging due to these weak interactions at
different directions, which generally determines growth
directionality so crucial for the rational design and
construction of spontaneous aggregation assembles into both
micro and macro sizes. The calculation results give us
confidence to predict aggregation style in other external
environments.
Figure 3 Morphology of PYTPY nanostructures obtained from THF-H
2
O mixture at
10- mol/L: (a) SEM image of PYTPY with f
being
0%, (b) SEM image of PYTPY as f being 99%, (c) TEM image of a single nanorod
as shown in (b). (d) UV-vis absorption and (e) fluorescent emission spectra of
PYTPY in THF-H O with tunable water volume fraction
5
the concentration being 1.0
×
w
9
w
2
3
.2.3 Calculated and experimental electron transition
performance at monomer state and dimer
The electron transition performance of a compound is dependent
on the charge separation between the ground and excited state,
which can be influenced by aggregated style [10]. The electron
transition can be estimated through the charge-hole distance dhe
between the ground and excited state [29]. To understand the
Figure 2 Fragments selected for calculation of weak interactions along (a) a axis,
b) b axis, (c) c axis, and (d) for very weak π-π interactions.
(
Table 2 Total, assembling energies of PYTPY molecules along different directions at the electronic structure and electron transition of PYTPY in diluted THF
M06/6-31+g(d) level
solution and aggregated state, molecular orbital and energy level
distribution calculations of TD-DFT were performed through
bp86/cc-pvtz level basis set for both molecule monomer and dimer.
The molecular geometry used for the calculation was obtained from
X-ray diffraction crystallographic data just as that used for single
point energy calculation. As for the dimer, the molecular geometry
possessing the lowest energy was chosen just as shown in Figure
axis
Energy (Hatree)
binding
energy ΔE
relative
binding
energy
1.00
0.17
0.60
(
dimer)
(molecule-free)
(
kJ/mol)
-10.66
-1.83
a
b
c
-2594.52824098
-2594.52346614
-2594.52661375
-2594.52485592
-1297.26208380
-6.40
-1.80
π-π
0.17
3
a. The theoretical spectral characteristics (detailed information is
ΔE = Einteraction = Edimer – 2Emolecule-free
,
listed in Table 3) showed two main transitions as λabs = 325.96 nm
with oscillator strength f being 0.2516, and λabs = 279.65 nm with f
being 0.2020, respectively. The lower energy band is transited from
the HOMO-6 orbit to the LUMO with major contributors C(HOMO-6)-
LUMO being 0.56068. The higher energy band is from the HOMO-10
orbit to the LUMO with C(HOMO-10)-LUMO being 0.43994. In the HOMO-
3
.2.2 Morphology of aggregated structure formed from THF-H
mixed solvent
In order to verify the influence of intermolecular
2
O
interactions on molecular aggregation, PYTPY-THF solution was
added into water media. Then the aggregated morphology was
checked which is shown in Figure 3. It is well known that the
self-assembly morphology is related to the solubilization of a
compound in a solvent [27]. That is to say, when the
compound in a good solvent is added into a bad solvent, a
possible aggregated self-assembly structure will be observed,
at micro and/or macro sizes [28]. When the concentration of
PYTPY is 1.0
well distributed nanorods with length of about 1 μm can be
captured in Figure 3a. When the f value is increased to 99%,
the rods gather together to form bundle-like structure as
shown in Figure 3b. The length of the rod is about 0.7~1.4 μm
6
and HOMO-10 orbits, the electrons are primarily concentrated on
terpyridine unit (Figure S11). In the LUMO orbit, the electrons are
primarily concentrated on benzene unit, C=C double bond and
pyridine unit ①. The results mean that the electron transition is
accompanied by the intramolecular charge transfer performance.
Although the PYTPY is a typical A-π-A type molecule, the electronic
cloud density of terpyridine unit is much higher than that of
pyridine unit ① at the other end [30]. When the whole molecule is
at the ground state, it is reasonable that the electron cloud may be
concentrated at the terpyridine unit and away from the pyridine
unit ①. When it is excited, the electron cloud density will be
-5
w
× 10 mol/L with water volume fraction f = 90%,
w
4
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redistributed. Thus, a weak intramolecular charge transfer from THF-H
ARTICLE
2
O mixed solution is reasonable that can be uViseewdArttioclesOtnuldinye
other organic materials in other solvents. DOI: 10.1039/C8CE01940A
terpyridine unit to pyridine unit ① can be observed when the
molecule is excited. As for the fluorescence emission, there exists a
main fluorescence band with λFL = 380.27 nm, which results from
the transition between the LUMO orbit and the HOMO with the
3
.3 The influence of acidity on the aggregation, morphology and
electron transition
In 2018, Valverde et al. reported the self-assembly kinetics of π-
f
LUMO-HOMO being 0.4657 and CLUMO-HOMO being 0.50516. The conjugated amino acids at micro and/or nanoscale. They found that
calculated results are verified by the experimental observation. this type of molecule can self-assemble into long fibers upon
protonation in an acidic environment, due to the protonation-
induced nucleation by monomeric addition followed by subsequent
stages of aggregation and elongation [31]. Here, we consider that
PYTPY molecule possesses four pyridine rings that can interact with
From the experimental data, the main absorption bands of PYTPY in
THF solution was captured at 327 nm and 277 nm as shown in
Figure 3d. The optimal emission wavelength appeared at 383 nm as
shown in Figure 3e. The experimental results fit well with that of
the calculated data, which mean that our calculated method for
predicting the electron transition performance in monomer state is
reasonable.
The electron absorption property of PYTPY in dimer state is also
calculated. The regulation of packing model with the lowest single
point energy was selected from the results mentioned above. The
+
H , which will then influence the intermolecular interactions and
the related aggregation process. Thus, the influence of acidic
environment on the self-aggregation style and the related
aggregated morphology of PYTPY was studied.
+
3
.3.1 The effect of 1 and/or 3 equivalents of H on the aggregation
forces
Considering the reactive activity of different pyridine units and the
calculated results is also listed in Table 3, which shows a main spatial resistance effect, the selected molecular model for 1 positive
charge was added at N atom of the pyridine unit (cycle ① in
Scheme 1), which is shown in Figure 4a and optimized through
b3lyp/6-31g level basis. The dimer’s molecular models along
different directions were also constructed from the CIF data (Figure
transition band centered at λabs = 367.37 nm that is from HOMO-9
orbit to LUMO+1 orbit with the f(HOMO-9)-(LUMO+1) being 0.0014 and
C(HOMO-9)-(LUMO+1) being 0.66732. Another transition band is
calculated at λabs = 298.60 nm with the f value being 0.0227, the
major contributors of which are quite complicated that is listed in
Table 3. The electron cloud distributions of the dimer at different
energy level are listed in Figure S12. In the HOMO-9 orbit, the
electron is mainly located at the terpyridine unit of the calculated
2
) and was optimized through b3lyp/6-31g(d) method. The
optimized model are showed in Figure 4b-d. The very first driving
force in acid medium for self-aggregation is attributable to
coulombic forces between positive charged pyridine group and the
opposite charged ions. Exploiting these properties, the possibility to
fragment 1 as shown in Figure 2a, which is mainly located form aggregates of free positive charged PYTPY molecules has been
+
throughout the whole molecule of fragment 1 as for LUMO+1 orbit demonstrated, which is responsive to pH variation or H equivalent.
+
and partly located at the C=C double bond of fragment 2. The Here, different H equivalents (from 1 to 3 equiv.) are chosen to
simplify calculation model and/or reduce calculation time.
results also reveal the intramolecular transfer process when the
aggregated dimer is excited. The experimental result (Figure 3d-e)
obtained in THF-H
2
O mixed solution shows two main absorption
bands centered at 368 nm and 296 nm, which are very close to that
of 367.37 nm and 298.60 nm, respectively.
The calculation results show that the single point energy of one
positive charged PYTPY molecule is -1297.61779989 a.u. The
+
energies of PYTPY dimer under 1 equiv. of H existence along three
different directions are -2595.44411806, -2595.40870835 and -
2
1
595.43649565 a.u., respectively. The relative binding energy is
.00: 0.83: 0.96. That is to say, the assembling energies along
Table
3 Some of calculated excitation energies (E), oscillator strengths (f),
corresponding wavelengths (λabs) and major contributors for molecular monomer and
dimer based on bp86/cc-pvtz.
different directions are similar to each other with the minimum
difference being 4%. The results mean that PYTPY molecules have
tendency to aggregate into three-dimensional structures in 1
E (eV)
λ (nm)
f
composition (C)
+
equivalent of H .
+
absorption
for monomer
3.8037*
325.96
279.65
380.27
0.2516
0.2020
0.4657
102(H
1
-6)→109(L
1
)
)
(0.56068)
(0.43994)
(0.50516)
The effect of 2 equivalents H on the PYTPY aggregation is shown
in ESI. The selected molecular model for 2 positive charge was
added at N atom of the pyridine unit ① and ④ according to
spatial resistance effect as shown in Figure S15. In the following
4.4336*
98(H -10)→109(L
1
1
fluorescence
for monomer
3.2604*
3.3749*
109(L
1
)→108(H
1
)
+
part, the effect of 3 equivalents H will be discussed in detail.
Considering the spatial resistance effect and the repulsive
interaction between charged molecules, the selected molecular
model for 3 positive charge was added at N atom of the pyridine
unit ①, ② and ③, which is shown in Figure 4e. The 3 positive
charged dimer’s models were optimized through b3lyp/6-31g(d)
method. The optimized structures of the dimer are listed in Figure
207(H
2
-9)→218(L
2
+1) (0.66732)
absorption
for dimer
367.37
298.60
0.0014
0.0227
1
97(H
97(H
2
-19)→217(L
-19)→218(L
2
)
(0.13123)
+1) (0.43135)
4.1522
1
2
2
2 2
198(H -18)→218(L +1) (0.15319)
2 2
211(H -5)→223(L +6) (0.40032)
4
e-h. The detailed construction process of the models is as follows.
The interaction between adjacent protonated tripyridine units is
As discussed above, the experimental data for PYTPY in monomer used to construct the first dimer’s model as shown in Figure 4f. The
*
main transition
and dimer states fit well with the calculated results, which also interaction between the benzene unit and one of the pyridine unit
reveals that this simply calculation method to forecast the of the protonated tripyridine unit (② or ③) is used to construct
intramolecular charge transfer performance in dilute THF and/or
the second model as shown in Figure 4g. The interaction between
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+
pyridine unit ④ and one of the pyridine unit of the protonated similar to each other. When more dosage of H is added, the force
View Article Online
tripyridine unit (② or ③) is used to construct the third model along one direction will increase, which wiDllObI:e10b.1e0n3e9f/iCci8aCl Ef0o1r9410-DA
(
Figure 4h). These three types of dimer include the basic types of aggregation. To make sure the reasonableness of such predictions,
interaction between adjacent positive charged acidified the aggregated morphology was checked from scanning electron
3
molecules. The single point energy of 3 positive charged molecule microscope (SEM) images.
monomer and the dimers was also calculated through b3lyp/6- 3.3.2 The morphology under different equivalents of H+
31g(d) method. The calculated single point energy of 3 positive
Indeed, the theoretical calculation results are well validated by
charged molecular monomer is -1298.92678021 a.u., which is - experiments. When PYTPY-THF solution was added with different
equivalents of hydrochloric acid, the morphology of PYTPY assembly
changed violently as shown in Figure 5. The equivalents of
hydrochloric acid was changed from 1 to 4.
2
598.35840314, -2598.53703945 and -2598.04008762 a.u. for the
dimers at three directions, respectively. The relative binding energy
is calculated to be 2.71: 3.66: 1.00. The relative binding energy
along the direction of Figure 4g is higher than that of the other two.
+
The results mean that when 3 equivalents of H are added into
PYTPY solution, the aggregated morphology may be one-
dimensional structure, possibly grows according to the staking style
as shown in Figure 4g. The results are reasonable that the exclusion
effect between protonated pyridine units may decrease the
interaction strength between adjacent protonated molecules, while
the attraction effect between protonated pyridine unit and multi-
electron conjugated benzene unit is the main character, which lead
to higher interaction force of model Figure 4g than that of 4f.
Furthermore, the molecules in model 4h exhibits a twisted
Figure 5 SEM images of PYTPY aggregates formed under different equivalent of
+
H . (a): 1, (b): 2, (c): 3 and (d): 4 equivalents. (e) UV-vis spectra of PYTPY in THF
+
interaction structure that will decrease the interaction strength. solution when different equivalents of H were added
That is to say, the interactions between adjacent molecules in
model 4g show the strongest strength.
When 1 equivalent of H+ is added, PYTPY molecules tend to
aggregate into blocky structure (Figure 5a), the size of which ranges
from 200 nm to 300 nm. Compared to the results got from THF-H
2
O
solution (Figure 3), the blocky structure in this condition clearly
+
indicates the strong influence of additional H ion on the self-
assembly process of PYTPY. As free PYTPY molecules tend to form 1-
+
D structure (as discussed in Part 3.2), when H is added, the pyridine
ring near the C=C double bond will be acidulated firstly, and
increases the electron acceptor strength of pyridine unit ① and
decrease the intramolecular charge transfer strength of the whole
PYTPY molecule, which then decrease the dipole-dipole interactions
between adjacent acidulated PYTPY molecules. Thus, the 1-D
growth dynamic decrease and the as-observed rod-like structure
(
Figure 3) disappears. The relative enhanced interactions along the
other two dimensions make acidulated PYTPY molecules self-
assembly into 3-D blocky structure in assistance of 1 equivalents of
+
+
H . Further, when the equivalent of H is increased, the 3-D blocky
structures are connected together to form 1-D wires. Because the
+
reactivity of N atom at cycle ① with H is higher than that at cycle
②
and/or ③, blocky structures may always appear when more
+
equivalent of H is added. Indeed, some blocks appear on the nodes
of the wire as shown in Figure 5b-d. Thus, it can be concluded that
the formation mechanism of 1-D wires observed in acid solution
was different from the 1-D rods obtained in neutral water medium.
Combined with theoretical calculations, it is thought that 1-D wires
in acid solution is constructed due to homologous π-π interactions
as shown in Figure 4g and 2d, and that C2-H2⋅⋅⋅N4 intermolecular
hydrogen bond can lead molecules grow along a axis to form 1-D
Figure 4 (a-d) Fragments selected from simulated structure after acidification by
+
1
equivalent of H ion for calculation of (a) single molecule and weak interactions rods in pure water.
along different directions. (e-h) Fragments selected from simulated structure
after acidification by 3 equivalents of H ion for calculation of (e) simulated single
molecule and weak interactions.
3
.3.3 The calculated and experimental UV-vis absorption under
+
different equivalents of H+
The electronic structure and transition performance of PYTPY
Form the discussions mentioned before, different equivalents of
H+ may bring about different aggregation forces. When 1
equivalents of H is added, the forces along different directions may
+
under 1 or 3 equivalents of H was investigated at the b3lyp/cc-pvtz
level basis set, both for acidified monomer and dimer. The
molecular geometry used for the acidified dimer was the same as
+
6
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that used for the lowest single point energy calculation (Figure 4b properties such as UV-vis absorption will be tuned along with the
View Article Online
+
DOI: 10.1039/C8CE01940A
and 4g). As for PYTPY-1H monomer, the theoretical result shows changeable morphology. In the past decade, terpyridine derivatives
two main transitions as λabs = 367.71 nm and 277.55 nm, have generated a wide-range of supramolecular architectures
+
respectively. As for the PYTPY-1H dimer, there exists an absorption through coordinating effect with some types of metal ions, such as
2+
2+
2+
band at 368.14 nm from HOMO-2 orbit to LUMO+3 orbit with f Zn , Cd , or Ru [33-34]. In this part, silver ion was chosen
value being 0.1571. At the same time, the f value at 324.55 nm is through the hard-soft-acid-base theory, as its common coordination
calculated as 0.0008, which means that the absorption band at this number is four which make silver ion simply link two PYTPY
2
energy gap is very weak. Moreover, a new absorption band molecules in one complex cell [35]. THF-H O was chosen as the
centered at 403.20 nm appeares which is transited from the mixed solvent because silver ion cannot be reduced by THF.
+
HOMO-2 orbit to the LUMO+1 with C(HOMO-2)-(LUMO+1) being 0.69974.
The optimized dimer under Ag ion existence was shown in Figure
The new band is considered as coming from the increased electron 6, which was optimized through b3lyp/6-31g* method. The detailed
+
accepting strength of the protonated pyridine unit and the modelling was described in the labelling of Figure 6. Ag ion was
corresponding enlarged π-conjugated structure, which may also located at the position to link two pyridine groups from adjacent
+
appear when the equivalents of H is increased. Further, as the molecules as Figure 6a, link pyridine group of one molecule and
protonated saturation of pyridine unit is one, this absorption terpyridine group of adjacent molecule as Figure 6b, link terpyridine
wavelength will be kept at around 410 nm when the equivalents of group of one molecule and π-conjugated electrons of styrene unit
+
H is more than one. Indeed, when we simulated the equivalents of at adjacent molecule as Figure 6c. Similar to the construction of 3
+
+
H
as three, the transition at λabs = 411.22 nm exists for the positive charged dimers, the location of Ag ion covered all of the
aggregated dimer. possible ways of weak interaction. The related interaction energies
The calculated results are verified by the experimental were calculated through b3lyp/6-31g* method. The single point
measurements to check the feasibility of the calculation method energies of the dimers shown in Figure 6a-c was -2595.93115778, -
and the accuracy of the modelling structures for the protonated 2595.95707570 and -2595.72817679 a.u., respectively. The relative
PYTPY. In THF solution, when different equivalents of hydrochloric binding energy along different directions were calculated as 0.98:
acid are added, the absorption bands at 323 nm and 282 nm shows 1.00: 0.84. The results were reasonable that the four coordination
+
an increase trend firstly and then a decrease trend (Figure 5e). number of Ag ion can make it link the three N atoms of terpyridine
Meanwhile three new bands centred at 364 nm, 384 nm and 411 unit and one N atom of pyridine unit. Also, there existed
+
nm appeare (Figure 5e), which gradually increase and exhibit coordination effect between Ag ion and conjugate electrons with
slightly change only after the equivalent is larger than 7.0. The moderate strength. The results also meant that the aggregated
+
experimental conclusions are consistent with the predicted results, PYTPY molecules under Ag existence may construct three
which are reasonable that when hydrochloric acid is added into the dimensional structure at macro scale. Considering the related larger
+
+
THF solution of PYTPY, the pyridine unit of cycle ① is protonated volume of Ag ion than that of H ion, the aggregated structure may
firstly and changed the π-conjugated molecule. At the same time, also exist as zero-dimensional structure at nanoscale, as spatial
+
the terpyridine group at the other end of the structure will be steric hindrance effect of Ag ion can obstruct the effective
protonated after 1.0 equivalent, which may cause a stronger aggregation of PYTPY molecules. As shown in Figure 7a, when 1
+
electron acceptor strength. The result is the appearance of equivalent of Ag ion was added, the morphology of PYTPY under
+
absorption bands at longer wavelength (from 360 nm to 410 nm). Ag circumstance showed curly nanowires with length of about
When the equivalent is larger than 2.0, the repulsive force between dozens of micrometers and width of about dozens of nanometers,
adjacent protonated-PYTPY molecules will be strong and prevent which did not match with the calculated results. So we check the
further protonation of the molecule, which may lead the subtle morphology through TEM image as shown in Figure 7b. It can be
changes of absorption band and/or absorbance value at around observed that the curly nanowires were composed of some tiny
3
60-410 nm. Also, we must realize that when the first drop of nanoparticles with the size of about 3-5 nm, that is to say PYTPY
hydrochloric acid is added into the studied system, the protonation molecules tended to form zero-dimensional structure. To preclude
process of both terpyridine and pyridine units will occour at the the possible existence of Ag nanoparticles, XRD pattern was
same time. So the decrease of absorbance at 323/282 nm and the examined which did not support existence of Ag(0). So the observed
+
increase at 360-410 nm exhibit gradual change.
morphology of PYTPY in Ag addition confirmed the calculation
+
The results examined and discussed in acid media give us results. Maybe, the existence of Ag ions at the side of PYTPY
confidence to study the aggregation style and the related optical molecule obstructed aggregation of PYTPY, but can link the zero-
performance in coordination environment when metal ions are dimensional PYTPY aggregation particles together, then form
+
added into the system.
prolonged structure. This speculation was reasonable that Ag ions
existed all around the studied system and the nanowires appeared
all around the field of vision.
3
.4 The influence of silver ion on the self-aggregation
+
Pyridine derivatives are reliable candidates to employ molecular
The possible aggregated style of PYTPY under Ag existence was
building blocks with self-assembly structures due to supramolecular used to calculate the absorbing performance through bp86/6-311g*
scaffolds through either non-covalently or covalently interactions method, the results are listed in Table 4. The main transition of
+
[
32]. As there exists terpyridine group with strong coordination PYTPY dimer under Ag ion existence appears at 3.6688 eV,
ability, PYTPY will be coordinated with metal ions, which will bring corresponding to 337.94 nm, which come from the electron
about violent changeable morphology, the related optical transition between HOMO-8 orbit and LUMO+3 orbit. At HOMO-8
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orbit of PYTPY dimer under Ag ion existence, the electron cloud calculation results as shown in Figure 6d, in which Ag+ ion is
+
View Article Online
DOI: 10.1039/C8CE01940A
density is mainly concentrated at the terpyridine unit at one
attached to terpyridine unit of one PYTPY molecule and pyridine
unit ① of another molecule through coordination bond to construct
PYTPY-Ag crystal cell. The UV-vis absorption band and the related
electron cloud density distribution of PYTPY-Ag complex for
controlled analysis are analyzed through b3lyp method. The metal
center Ag(I) has been described by the lanl2dz effective core
potential (ECP) basis set while non-metal atoms by the cc-pvtz basis
set.
Figure
6 Fragments selected from simulated structure at existence of 1
+
equivalent Ag+ ion for calculation weak interactions when Ag linked (a) two
pyridine groups from adjacent molecules, (b) pyridine group of one molecule and
terpyridine group of adjacent molecule, (c) terpyridine group of one molecule
and π-conjugated electrons of styrene unit at adjacent molecule, (d) simulated
structure of PYTPY-Ag complex upon calculation results
molecule and the pyridine unit at adjacent molecule. While for
LUMO+3 orbit, the electron cloud is dispersed at the whole
conjugated structure of one PYTPY molecule. The experimental data
shows a main UV-vis absorption band at 335 nm (Figure 7e),
matches well with the calculated result.
In the experiment, we noticed that the redox properties of the
solvent influences the morphology. For example, when DMF with
+
reducing performance was used as the solvent, Ag ions can be
0
reduced to Ag and form Ag nanoparticles. The phenomenon also
+
+
reveals the effect of Ag ion on the morphology of PYTPY. When Ag
ion is existed in the system, the morphology of PYTPY changes
violently from nanorod to nanowire composed of zero-dimensional
+
0
tiny particles. When Ag ion is reduced to Ag by DMF, the
morphology of PYTPY still showed rod-like morphology with only
slight change on the size. Moreover, it can be observed that some
Ag particles appears at the surface of PYTPY nanorods as shown in
Figure 7c. Furthermore, when temperature is increased to boiling
Figure 7 (a-d) Morphology of samples prepared under different conditions when
AgNO
when 1 equivalent of AgNO
of PYTPY samples when 2 equivalents of AgNO
solution, (c-d) SEM image of PYTPY samples when 3 equivalents of AgNO
3
was added into the studied system. (a) SEM image of PYTPY samples
was added into PYTPY-THF solution, (b) TEM image
were added into PYTPY-THF
were
3
o
3
point of DMF (~153 C), and the mixed solution is refluxed
3
overnight, the PYTPY-Ag complex may formed. The related
morphology of PYTPY-Ag complex is shown in Figure 7d, some thin
spindle structure is observed, with the length of ~2 µm, the width
of hundreds of nanometers and the thickness being dozens of
nanometers. Also, there do not exist any tiny Ag nanoparticles that
o
added into PYTPY-DMF solution (c) under violently stirring at 80 C for 30 min, (d)
refluxed overnight. (e-f) Normalized UV-vis absorption and fluorescence spectra
+
of: (e) PYTPY aggregates when 1 equivalent of Ag ion was added, (f) PYTPY-Ag
complex obtained from refluxed DMF solution.
For the simulated PYTPY-Ag complex, the main transition appears
are stuck at the surface of the thin spindles. The XRD pattern do not at 3.2207 eV, corresponding to 384.96 nm, which come from the
0
show the existence of Ag for this sample (Figure S23). The results electron transition between HOMO-4 orbit and LUMO+1 orbit. At
also reveal the different reaction speed and reaction mechanism. HOMO-4 orbit (Figure S20), the electron cloud is mainly
o
When the temperature was low (80 C e.g.) and the reaction time concentrated at the whole conjugated structure of one PYTPY
+
0
+
was short (30 min), DMF reduced Ag to Ag [36]. While at higher molecule and partly located at the Ag ion. For the LUMO+1 orbit,
+
temperature (boiling point), the reaction speed of PYTPY with Ag
the electron cloud is mainly concentrated at pyridine unit of the
to construct PYTPY-Ag complex was faster than that of reducing same PYTPY molecule at HOMO-4 orbit, the C=C double bond and
0
reaction to Ag , so PYTPY-Ag complex was formed at higher connected benzene unit. There also exists some electron cloud
temperature [37]. However, this reaction need longer time, so >12 distribution at the Ag
hours was taken.
excited process of PYTPY-Ag complex is also companied by the
+
ion. The results mean that the electron
In the following part, the model of PYTPY-Ag complex was used electron cloud re-distribution, which can adjust intramolecular
for controlled calculation. The experimental and calculated results charge transfer here. The experimental UV-vis absorption data of
+
for Ag induced PYTPY molecules’ aggregation reveal the possible PYTPY-Ag complex exhibits a wide band from 300 nm to 550 nm,
coordination style of PYTPY-Ag complex. The main possible the main absorption band centers at 347 nm and a shoulder band
coordination mode may exist at pyridine group and terpyridine appears at 388 nm band (Figure 7f). The 347 nm absorption band
group. Thus, the structure of PYTPY-Ag complex is simulated upon may come from the characteristic absorption of PYTPY, which
8
| J. Name., 2012, 00, 1-3
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‡
1
The crystallographic data of PYTPY: CCDC 1870772V.iew Article Online
shows an obviously red-shift by 20 nm from 327 nm of free PYTPY
molecule (Figure3d), and 12 nm red-shift from 335 nm of PYTPY
R. Zou, J. Zhang, S. Z. Hu, F. Hu, H. Y. Zhang, Z. Y. Fu,
DOI: 10.1039/C8CE01940A
CrystEngComm, 2017, 19, 6259;
Q. Zhang, J. Su, D. Feng, Z. Wei, X. Zou and H. C. Zhou, J. Am.
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Q. Benito, I. Maurin, M. Poggi, C. Martineau-Corcos, T.
Gacoin, J. P. Boilot, S. Perruchas, J. Mater. Chem. C, 2016, 4,
+
under Ag ion existence (Figure7e). The coordination effect
2
3
+
between Ag ion and PYTPY may enlarge the π-conjugated degree
and results in the red-shifted absorption. Furthermore, the 388 nm
band is considered as coming from the intrinsic absorption of the
complex, which is consistent with theoretical calculations.
1
1231
4
5
6
7
8
9
1
M. R. Elmorsy, R, Su, A. A. Fadda, H. A. Etman, E. H. Tawfik,
A. El-Shafei, Dye Pigm., 2018, 158, 121;
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4 Some of calculated excitation energies (E), oscillator strengths (f),
X. J. Zhang, X. H. Zhang, K. Zou, C. S. Lee and S. T. Lee, J. Am.
Chem. Soc., 2007, 129, 3527
corresponding wavelengths (λabs) and major contributors for PYTPY in dilute THF
+
solution when 1 equivalent Ag ion was added
J. C Bolinger, M. C. Traub, J. Brazard, T. Adachi, P. F. Barbara,
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Langmuir, 2018, 34, 7346
E (eV)
λ (nm)
f
composition (C)
absorbance for
PYTPY when Ag
existed
3.6688*
337.94
0.0454
208(H
7
-8) → 220(L
7
+3)
+2)
+
(0.58221)
M. R. Molla, M. A. Rub, A. Ahmed, M. A. Hoque, J. Mole. Liq.,
2
017, 238, 62
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absorbance
PYTPY-Ag complex
for
3.5801
346.31
384.96
0.0009
0.6292
220(H
8
-5) → 228(L
8
(0.69082)
3.2207*
221(H
8
8
-4)→227(L +1)
11 H. X. Liu, J. Lu, T. Zheng, D. M. Liu, F. Y. Cui, Environ. Eng. Sci.,
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1
1
1
1
1
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main transition
4
Conclusion
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In summary, this study used the computational calculation
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the intermolecular interaction between adjacent molecules of
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the target terpyridyl derivative PYTPY. The simulated structure
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of PYTPY under the existence of water, Ag ion and different 18 Y. Z. Zhang, H. W. Yang, S. Guan, Z. H. Liu, L. Y. Guo, J. W. Xie,
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J. B. Zhang, N. N. Zhang, J. Song, B. Zhang, Y. Q. Feng, Dyes
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equi. of H ion brought about different types of intermolecular
interactions that resulted in changeable morphology and
tunable absorption band. The calculated aggregation state and
absorbing performance fit well with that captured at
experimental results. This simply calculation method can give
us an opportunity to understand the weak intermolecular
interactions, the possible aggregate style and morphology,
which affords insight into tailoring the self-assemble process,
and can also study the main transitions between the ground
states and the excited states of an organic compound under
different external conditions. Based on such results, the
possible optical performance of an organic compound can be
premastered to guide the design, preparation, experiments
and applications.
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2
9 T. T. Wang, J. L. Zhang, H. M. Hu, Y. Cheng, L. L. Xue, X. F.
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Acknowledgements
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Dr. Kun Wang and Dr. Hai-Zhu Yu at AnHui University for
helpful suggestions and discussions. This work was supported
by the Natural Science Foundation of Anhui Province of China
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View Article Online
DOI: 10.1039/C8CE01940A
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CrystEngComm
View Article Online
DOI: 10.1039/C8CE01940A
Graphical abstract for：
Understanding the molecular orientation growth at nanometer scale and
adjustable electron transition performance of a terpyridyl derivative under
different external environments
Hai-Yan Wang, Lin Kong, Shu-Juan Zhu, Hong-Ping Zhou, Jia-Xiang Yang
TD-DFT calculation is used to forecast the aggregation style of TPYPY under different
external additions, predict the related morphology and electron transition performance.