N-Substitution of acridone with electron-donating groups: Crystal packing, intramolecular charge transfer and tuneable aggregation induced emission

Article abstract of DOI:10.1039/c9ra10615d

Acridone derivatives with electron-rich triphenylamine functionalized at the amino position were synthesized and their properties were experimentally and computationally investigated. The single crystal structure analysis revealed that the π-π interaction of acridone and the formation of hydrogen bonds of carbonyl and the hydrogen atoms of the pending phenyl ring were crucial in the determination of molecular packing in the crystalline state. An intramolecular charge transfer (ICT) process was observed between acridone and triphenylamine even with reduced conjugation by the nitrogen atom of acridone. Tuneable aggregation induced emissions with blue and green fluorescence were found due to the different aggregation state and particle size, which varied according to the water content in THF. Furthermore, the size of the spacer between acridone and the appended amine was also important in adjusting the property of aggregation induced emission or aggregation caused quenching in the solid state.

Full text of DOI:10.1039/c9ra10615d

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N-Substitution of acridone with electron-donating
groups: crystal packing, intramolecular charge
transfer and tuneable aggregation induced
emission†
Cite this: RSC Adv., 2020, 10, 7092
Renfei Liu, Guanxing Zhu and Gang Zhang
*
Acridone derivatives with electron-rich triphenylamine functionalized at the amino position were
synthesized and their properties were experimentally and computationally investigated. The single crystal
structure analysis revealed that the p–p interaction of acridone and the formation of hydrogen bonds of
carbonyl and the hydrogen atoms of the pending phenyl ring were crucial in the determination of
molecular packing in the crystalline state. An intramolecular charge transfer (ICT) process was observed
between acridone and triphenylamine even with reduced conjugation by the nitrogen atom of acridone.
Tuneable aggregation induced emissions with blue and green fluorescence were found due to the
different aggregation state and particle size, which varied according to the water content in THF.
Furthermore, the size of the spacer between acridone and the appended amine was also important in
adjusting the property of aggregation induced emission or aggregation caused quenching in the solid state.
Received 17th December 2019
Accepted 12th February 2020
DOI: 10.1039/c9ra10615d
rsc.li/rsc-advances
1
4–16
stimuli due to the conformational change.
The introduc-
Introduction
tion of biphenyl carbazole or tetraphenylethylene to the amino
Acridone is a basic structural unit of natural alkaloids, which position of acridone can also supply molecules with inter-
1
has received considerable attention for its good bioactivity.
esting electroluminescence and piezochromic luminescence
1
7–19
The tricyclic acridone contains an electron-donating amine properties.
and an electron-withdrawing carbonyl in the central ring and
Molecules containing electron-donating groups and
demonstrates excellent photoluminescence and wonderful electron-withdrawing groups in the chain can form a donor–
2
photostability, which makes it a good uorescence signalling acceptor system with lower energy gaps and are good candidates
3
–7
unit in sensors. Various organic functional materials with as photoelectric materials or as uorescence dyes for bioimag-
20–24
interesting properties have been prepared by the functionali- ing.
When acridone is used to construct a donor–acceptor
zation of acridone at the positions of the amine, carbonyl and system, it can serve as an acceptor once the electron-donating
the phenyl ring. The connection of functional groups to the groups are linked to the phenyl ring to ensure an efficient ICT
12
phenyl ring of acridone is among the most studied owing to process through the phenyl ring to the carbonyl. The
the ready halogenation followed by the well-developed cross- condensation of carbonyl of acridone and cyanoacetic esters
coupling reactions. For example, the introduction of amine can also generate an acridone based donor–acceptor system,
or carbazole to the phenyl ring of acridone would give hole- which lowers the rotational barrier with controllable rotational
25
transporting materials or host materials for OLED applica- rate around the formal C–C double bond. Recently we found
8
–12
tions.
at the phenyl ring to form acridone oligomers with tuneable withdrawing anthraquinone was connected to the amino posi-
The acridone units could also couple with each other that acridone can be used as a donor when the electron-
1
3
energy levels. The functionalization of carbonyl of acridone tion of acridone to generate a molecule with interesting ther-
26
usually involved the formation of C–C double bonds with mally activated delayed uorescence properties. However, the
another functional group, such as malononitrile, or uorene, presence of a nitrogen atom in acridone makes it difficult to
to furnish molecules with variable colour upon external build a donor–acceptor system with acridone as an acceptor via
N-substitution due to the reduced conjugation. To further study
the inuence of N-substitution on the properties of acridone,
herein we report the synthesis and properties of electron-rich
Co-Innovation Center for Efficient Processing and Utilization of Forest Products,
College of Chemical Engineering, Nanjing Forestry University, Nanjing, 210037, P.
R. China. E-mail: gangzhang@njfu.edu.cn
triphenylamine functionalized acridone at the amino position.
†
Electronic supplementary information (ESI) available. CCDC 1972073–1972075. For a purpose of comparison, the N-substitution of acridone
For ESI and crystallographic data in CIF or other electronic format see DOI:
with methoxyphenyl is also involved.
1
0.1039/c9ra10615d
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Results and discussion
The synthesis of N-substituted acridone derivatives could be
easily achieved via copper-catalyzed Ullmann amination of
acridone and the related aryl bromide with 2,2,6,6-
tetramethylheptane-3,5-dione as ligand to give the target
22
compounds in 77–83% yields (Scheme 1). All the compounds
were fully characterized by NMR, IR and HRMS.
Single crystals that are suitable for X-ray diffraction analysis
were obtained by slow evaporation of ethanol/dichloromethane
solution (2a and 2b) or vapor diffusion of hexane into the
toluene solution (2c), which allows a detailed structural inves-
tigation on the conjugation and intermolecular interactions in
the solid state (Fig. 1). In the acridone moiety of these three
compounds, the shortest C–C bonds with the distances 1.36–
˚
1
.37 A are found at the a and c positions, indicating the largest
olen character. Meanwhile, the bond length of carbonyl is 1.24
A˚ , which is longer than that of the relevant C–O double bond.
The bond alternation indicates the presence of ICT process in
13,26
acridone, as veried in other reports.
The C–N bond between
Fig. 1 Crystal structures of 2a–2c. (a) Crystal structural information;
crystal packing and hydrogen bond lengths around carbonyl (b) 2a, (c)
˚
acridone and the pending phenyl has a length of 1.45 A. The
dihedral angles of the acridone plane and the pending phenyl
2b and (d) 2c.
ꢀ
ring are in the range 75–85 due to the steric hinderance. The
bond lengths and dihedral angles are in good agreement with
27
those of the non-substituted N-phenylacridone. The dihedral
angles in the triphenylamine moiety of 2b and 2c are in the
range 63–82 . The dihedral angle of the two methoxyphenyl
rings is larger than the other ones around the nitrogen atom. It
is worth mentioning that the three N–C bond lengths are
informations of hydrogen bonds are consistent with the crys-
tallographic evidence for the presence of C–H/O hydrogen
ꢀ
28
bonds. It is worthwhile to note that the C–H/O hydrogen
bond plays an important role in directing the molecular over-
lapped packing of acridone. As also found in the crystal packing
different at the triphenylamine moiety with the shortest one
27
of N-phenylacridone, this observation can be served as
˚
(
1.41 A) at the pending phenyl that connects to the acridone,
a guideline in the design of acridone based crystalline materials
and other analogous molecules, such as 5-oxophenothiazine
and 5,5-dioxophenothiazine.
while the other two are slightly longer with the distances 1.42–
˚
1
.43 A. The bond length difference demonstrated the delocal-
ization of electron from the triphenylamine towards the acri-
done unit. The crystal analysis also revealed the p–p
interactions between the acridone moiety with the distances
The inuence of the substituents on the electronic proper-
ties of acridone derivatives was investigated by the density
functional theory (DFT) calculations at B3LYP/6-311+G(2d,p)
˚
29
3
.42–3.61 A. The hydrogen bonds between the oxygen atom of
level of theory in vacuo (Fig. 2). The LUMOs are located at the
carbonyl and the hydrogen atom of pending phenyl ring with
acridone moiety with the energy levels in the range 1.78–1.86 eV.
The distributions of HOMOs are substituent dependent. With
the methoxyphenyl as the substituent, the HOMO mainly
occupies the acridone backbone in 2a with the energy level of
˚
the distances in the scale 2.30–2.58 A are all observed in these
ꢀ
crystal structures. The :C–H/O angles are 140–169 . These
ꢁ
5.81 eV. However, the HOMO locations of 2b and 2c are at the
pending triphenylamine, which is separated from the LUMO
positions and is different from the previously reported donor–
acceptor system with amine connecting to the phenyl ring of
acridone, in which the HOMO spreads over the whole mole-
12
cule. The HOMO energy level of 2b is elevated to ꢁ5.36 eV due
to the inuence of the electron donating dimethoxyphenyl-
amine. The HOMO energy level of 2c decreases to ꢁ5.71 eV, and
no HOMO is observed in the acridone moiety, attributable to
the extended conjugation of biphenyl.
The redox properties of 2a–2c were studied by measuring
cyclic voltammetry (CV) in dichloromethane for oxidation and
in THF for reduction. An irreversible oxidation wave at E1/2
¼
1.11 V and a reversible reduction wave at E1/2 ¼ ꢁ2.57 V are
Scheme 1 Synthesis of acridone derivatives with electron rich groups observed for 2a (Table 1, ESI Fig. S24†). However, a reversible
at the amino position.
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Fig. 3 UV/vis absorption (solid line) and emission (dot line) spectra of
2a, 2b and 2c in dichloromethane at the concentration 10 mM.
3.01 eV for 2b and 3.04 eV for 2c, respectively. The electron
transitions were further evaluated by the time-dependent
density functional theory (TD-DFT) calculations at the APFD/6-
311G+(2d,p) level of theory in dichloromethane solvent (ESI
Fig. S29–S31†). The absorption band at 395 nm of 2a is attrib-
uted to the HOMO / LUMO transition. However, multiple
electron transitions are found for the absorptions of 2b and 2c
at the long wavelength (ESI Tables S7–S9†). The UV/vis
absorption spectra were also measured in other solvents. Each
compound shares similar UV/vis absorption patterns and the
shi of band positions is negligible (ESI Fig. S7–S9†).
Fig. 2 DFT calculations of the molecular frontier orbitals and energy
levels of acridone derivatives.
and an irreversible oxidation waves are found for 2b and 2c. The
reversible oxidation wave with smaller oxidative potential
should originate from the oxidation of triphenylamine, which
possesses electron-rich amine with lone pair electrons on the
nitrogen atom, thus making the oxidation easier and rising the
HOMO energy levels. A reversible reduction wave is seen for 2b
and 2c, and the half-wave potentials are close to that of 2a,
indicating the similar LUMO energy levels. The electrochemical
analysis is in good agreement with the computational results.
The photophysical properties of the compounds 2a–2c were
characterized by UV/vis absorption and uorescence emission
in dichloromethane (Fig. 3). The UV/vis absorption spectra
shows that two absorption bands in the long wavelength area
are at 376–378 nm and 395–397 nm, which are assigned to the
The uorescence spectra that were measured in dichloro-
methane at room temperature are shown in Fig. 3. Two emis-
sion maxima at 406 nm and 427 nm with an absolute quantum
yield of 35% are observed for compound 2a. The uorescence
time is 1.9 ns and the corresponding radiative and non-radiative
7
ꢁ1
7
ꢁ1
rates are 19 ꢂ 10 s and 36 ꢂ 10 s , respectively (ESI Table
S4†). Interestingly, dual uorescence emission is detected for 2b
and 2c in dichloromethane. One emission maximum is in the
range 400–464 nm for both of them, which are at the same
position to that of 2a and should originate from the locally
excited (LE) state of the acridone moiety. Another emission
maximum is very broad and is located at 571 nm for 2b and
2,26
p–p* transitions.
The molar absorbances of 2a and 2b are
6
27 nm for 2c, which should be the results of ICT process
quite close in the band range 395–397 nm, but smaller than that
of 2c due to the extended conjugation. The optical energy gaps
estimated from the onsets of the absorptions are 3.05 eV for 2a,
between the electron-accepting acridone and the electron-
donating triphenylamine. Although the methoxyl group is an
Table 1 Summary of photophysical and electrochemical properties of 2a–2c
a
b
c
c
F
d
d
e
f
g
h
Compd
l
max (nm)
E
g,opt (eV)
l
em (nm)
F
(%)
l
em (nm)
F
F
(%)
E
ox1,1/2 (V)
E
red1,1/2 (V)
E
HOMO (eV)
E
LUMO (eV)
Eg,electro. (eV)
2
2
2
a
b
c
395
397
395
3.05
3.01
3.04
406, 427
426, 565
407, 425, 627 1.2
35
1.1
464
457
496
13
17
2
1.11
0.35
0.32
ꢁ2.57
ꢁ2.55
ꢁ2.50
ꢁ5.73
ꢁ5.06
ꢁ5.02
ꢁ2.49
ꢁ2.39
ꢁ2.44
3.24
2.67
2.58
a
c
b
Absorption at longest wavelength in dichloromethane at the concentration 10 mM. Optical energy gap calculated from the absorption onset.
Measured in dichloromethane at the concentration 10 mM, excitation wavelength: 377 nm for 2a, 379 nm for 2b and 376 nm for 2c.
Measured in solid state, excitation wavelength: 365 nm for 2a–2c. Measured in 0.1 M Bu
d
e
n
4
NPF
6
in CH
2
Cl
2
at room temperature with a scan
Cl
ꢁ1
f
speed of 0.1 V s and ferrocene as internal reference. Measured in THF, the other parameters are same to the measurements in CH
2
2
.
g
h
E
HOMO ¼ ꢁ(4.8 + Eox1,onset) eV.
ELUMO ¼ ꢁ(4.8 + Ered1,onset) eV.
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electron-donating group, it is not strong enough to achieve the
separation of molecular frontier orbitals, hence only locally
excited uorescence is observed in 2a. But dimethoxyphenyl-
amine is a stronger electron-donating group and forms the
donor–acceptor system with acridone, thus resulting the ICT
process. The second emission maximum of 2c is more red-
shied than that of 2b due to a longer spacer caused larger
dipole moment in the excited state. The absolute quantum yield
of 2b in dichloromethane from the LE state is 1.7%, which is
lower than that from the ICT state (3.3%). But the lifetime from
the LE state is 3.9 ns, which is longer than that from the ICT
state (0.69 ns). Both the radiative and non-radiative rates of 2b
in dichloromethane from the LE state are lower than these from
the ICT state. For compound 2c, the absolute quantum yield
(
1.7%) and lifetime (4.7 ns) in dichloromethane from the LE
Fig. 4 Fluorescence spectra of 2a–2c measured in solid state. Inset:
the powder emissions (top to down: 2a, 2b and 2c) under 365 nm UV
state are close to these of 2b. A quantum yield of 1.2% and
lifetime of 4.4 ns for 2c is found from its ICT state, which leads light.
to a comparable radiative and non-radiative rates to those from
the LE state, but smaller than those from the ICT state of 2b.
These results are reasonable considering the relatively longer (AIEE). The lifetimes of 2a and 2b in the solid state are 3.9 ns
spacer between the donor and acceptor in 2c.
and 3.2 ns, respectively, leading to the close radiative (3.3–5.3 ꢂ
The uorescence property was further investigated by 107
ꢁ1 7 ꢁ1
s
) and non-radiative rates (22.3–25.9 ꢂ 10
s ). The
measuring the spectra in different solvents. The emission lifetime of 2c in the solid state is 37.9 ns, which is longer than
maximum of compound 2a shis from 391 nm in the non-polar those of 2a and 2b. The corresponding radiative and non-
solvent cyclohexane with low quantum yield (4.4%) and short radiative rates are 0.06 ꢂ 10 s and 2.6 ꢂ 10 s , respec-
lifetime (0.014 ns) to 405 nm in the polar solvent acetonitrile tively. The non-radiative rate is about 47 times faster than the
with relatively higher quantum yield (23%) and longer lifetime radiative rate. Thus, the non-radiative transitions consume
7
ꢁ1
7
ꢁ1
(2.0 ns). No emissions from the ICT process was observed. But most of the energy in the excited states and lead to a weak
for compound 2b, dual emissions from both LE and ICT states uorescence of 2c in the solid state.
are observed in other solvents. The emission maximum from
the LE state shows a red-shi with the increase of solvent hole-transporting materials for solar cell.
The electron rich triphenylamine is oen used to synthesize
32–34
However, it is also
polarity, similar to that of 2a. No regularity on the positions of an ideal building block in the construction of molecules with
35–39
emission maximum from the ICT state is found in the non-polar AIEE property due to the presence of rotatable C–N bonds.
solvent cyclohexane and toluene, but a red shi in the emission Hence, the photophysical properties of compounds 2a–2c in the
from the ICT state with the increase of solvent polarity is aggregation state was further investigated by measuring their
observed in the polar solvents. The uorescence of compound spectra in the water/THF mixture. The absorption maxima at
2
c from the ICT state is gradually isolated from its LE state and the longest wavelength is red-shied with the increase of water
the emission is red-shied from 414 nm in cyclohexane to fraction (ESI Fig. S10–S12†). However, the absorption intensity
27 nm in dichloromethane and cannot be found in the polar demonstrates a dramatic loss when water contents are 95% for
6
acetonitrile. Thus, the emission of acridone–triphenylamine 2a, 80% for 2b and 75% for 2c, indicating that the aggregation
system can be modulated by either adjusting the spacer or happens at these points. For compounds 2b and 2c, the
changing the polarity of the solvent.
absorption gradually shis back to its original pattern with the
The uorescence spectra of 2a–2c were also measured in the increased water contents.
solid state (Fig. 4). Compounds 2a and 2b are blue uorescence
The uorescence spectra of 2a in water/THF demonstrates
with the emission maxima at 464 nm and 457 nm, respectively. an interesting variation depending on the water content (ESI
Whereas compound 2c demonstrates a weak pale yellow uo- Fig. S19†). The emission maxima is red-shied from 399 nm in
rescence with broad emission maxima in the range 458–504 nm. pure THF to 409 nm with 10% water fraction accompanied by
All the emissions in the solid state are red-shied compared to a rise in the uorescence intensity due to the water addition
these from the locally excited states in the solutions due to the caused increase in solvent polarity, which is consistent with the
30
close intermolecular contacts caused the excimer coupling.
uorescence spectra measured in different solvents. The uo-
The absolute quantum yields are 13% for 2a, 17% for 2b and 2% rescence intensity starts to fall when more water is added and
for 2c, respectively (ESI Table S5†). The loss of emission inten- the emission maxima shis to 425 nm when the water content
sity in solid state is common for the excited states of the is 90%. A new emission maximum appears at 431 nm when the
aggregates oen decay or relax to the ground state in the non- water fraction is 95% and reaches its highest intensity with 99%
31
radiative manner. But compound 2b in solid state exhibits water. This new emission should be the aggregation induced
a much higher quantum yield than that in the solution, uorescence, as proofed by the UV/vis absorption analysis. The
demonstrating an aggregation induced enhanced emission
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average particle size is 337 nm in 99% water (ESI Fig. S25†). The a sol in 95% water without light scattering. The emission decay
absolute quantum yield is 7.3% in 99% water, which is lower of 2b in 85% water can be tted in a double-exponential manner
than that in pure THF (10%) and in the solid state (ESI Table with a lifetime of 3.4 ns, whereas the lifetime of 2b in 95% water
S6†). Considering the lifetimes, both the radiative and non- is much longer (89 ns) with quadruple-exponential tting,
radiative rates gradually decrease from the solution to aggre- unravelling the different decay pathway depending on the
gation state and to the solid state.
aggregation state.
Although compound 2b and 2c share similar structure, their
Compound 2c in pure THF presents a bluish green uores-

uorescence spectra in water/THF are quite different. cence with the emission maxima at 473 nm and an absolute
Compound 2b in pure THF shows very weak blue uorescence quantum yield of 10% (ESI Fig. S22†). The emission intensity
with an absolute quantum yield of 3.5%. With the increase of reduces due to the increased solvent polarity when water is
water contents in the solvent, a blue uorescence with the added. With the increase of water content, a weak yellow uo-
emission maxima at 433 nm starts to appear with 80% water rescence appears in 85% water, indicating the aggregation
content and reaches its highest intensity (21% quantum yield) induced emission, which is consistent with the results of UV/vis
in 85% water (Fig. 5). The uorescence intensity decreases with absorption. Compound 2c gives the strongest yellow uores-
the increase of water content and the emission maxima is red- cence in 90% water with the emission maxima at 538 nm and an
shied to 525 nm. The green uorescence shows its highest absolute quantum yield of 1.5%, but less than that in pure THF.
intensity (1.9% quantum yield) in 99% water. This tuneable Compound 2c in 90% water is a sol with an average particle size
dual uorescence emission in aggregation state is rarely re- of 80 nm, smaller than that of 2b in 95% water (ESI Fig. S28†).
ported and it should originate from the different water contents Similar to the sol state aggregation of 2b, the emission decay of
caused variable aggregation states and particle size of 2c in 90% water is tted in a quadruple-exponential way with
40–42
compound 2b in THF.
The average particle sizes of a longer lifetime (31.5 ns) than in the solution state (2.9 ns), but
compound 2b in 85% and 95% water are 532 nm and 108 nm, shorter than that of 2b.
respectively (ESI Fig. S26 and S27†). This reveals that the bigger
It is of interest to note that 2b and 2c share similar struc-
particle size would lead to a comparable emission to that in its ture, but demonstrate different luminescence in solution and
solid state. Indeed, the radiative and non-radiative rates in solid in solid state. In solution, compound 2b shows weak uores-
state and in 85% water are quite close to each other, indicating cence, but the emission of 2c is stronger. Compound 2c
a similar emission manner. The different aggregation state possesses more rotatable joints than that of 2b, thus the
could also be identied by naked eyes when the concentrations intramolecular rotation should not be the main reason of weak
of 2b in 85% and 95% water are increased. Irradiated with uorescence for 2b. The lower quantum yield of 2b should be
a laser pointer, compound 2b in 85% water is a suspension with due to the ICT process. Compound 2b has a short phenyl
scattered light beam due to the big particle size, whereas it is spacer, which allows a more efficient ICT process from dime-
thoxyphenylamine to acridone. The uorescence spectrum of
2
b also shows the luminescence derived from ICT process is
the main fraction in the dual uorescence emission (Fig. 2).
With slightly long spacer biphenyl, the ICT process in 2c is not
that so efficient. Together with the reduced conjugation by the
nitrogen atom of acridone, it resulted in an improved locally
excited emission with stronger intensity than that of 2b. In the
solid state, compound 2b shows AIEE phenomenon due to the
restricted intramolecular rotation, whereas compound 2c
demonstrates aggregation caused emission quenching. It is
found that the acridone part favours the formation of p–p
stacking from the crystal structural analysis. The distance of
˚
neighbouring acridone facets is 3.54 A for 2b, which is farther
˚
than that of 2c (3.42 A). Moreover, compared to 2c, the dihe-
dral angles of triphenylamine, the acridone and the pending
phenyl of 2b are larger, which can facilitate the isolation of the
molecules from each other, thus minimizing the potential
non-radiative transitions and improving the luminescence of
2
b in the solid state. Although the rotation around C–N and
C–C bonds in 2c are restricted in solid state, the tight inter-
molecular contacts caused the electronic coupling makes itself
less emissive. The comparison of radiative and non-radiative
rates also revealed that the non-radiative transitions is the
main reason in the loss of uorescence intensity for 2c in solid
state.
Fig. 5 The fluorescence spectra of 2b in the THF/water mixtures with
different water fractions (concentration: 10 mM, excitation wavelength:
3
74 nm). Bottom: the photographs of 2b in 85% and 95% water
(
(
concentration: 50 mM) under room light (left two), 365 nm UV light
middle two) and irradiation with a laser pointer (right two).
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1
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pending amine plays an important role in the determination of
China: Chem., 2018, 61, 418.
AIEE or aggregation caused emission quenching. This work will 23 C. Zhang and X. Zhu, Acc. Chem. Res., 2017, 50, 1342.
contribute to the design of novel AIEE materials and explore the 24 M. Kivala and F. Diederich, Acc. Chem. Res., 2009, 42(2), 235.

eld of acridone functionalization.
25 M. Krick, J. J. Holstein, A. Wuttke, R. A. Mata and
G. H. Clever, Eur. J. Org. Chem., 2017, 5141.
2
6 R. Liu, H. Gao, L. Zhou, Y. Ji and G. Zhang, ChemistrySelect,
Conflicts of interest
2019, 4, 7797.
There are no conicts to declare.
27 V. E. Zavodnik, L. A. Chetkina and G. A. Val'kova,
Kristallograya, 1981, 26, 392.
2
2
8 R. Taylor and O. Kennard, J. Am. Chem. Soc., 1982, 104, 5063.
9 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato,
A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts,
B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov,
J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini,
F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson,
D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega,
G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, T. Vreven, K. Throssell,
J. A. Montgomery Jr, J. E. Peralta, F. Ogliaro,
M. J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin,
V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand,
K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo,
R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma,
O. Farkas, J. B. Foresman and D. J. Fox, Gaussian 16,
Revision A.03, Gaussian, Inc., Wallingford CT, 2016.
Acknowledgements
We are grateful to the Jiangsu Specially Appointed Professor
Plan for nancial support.
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