A novel nitrogen-containing compound with D-π-A structure: Theoretical and experimental study

Article abstract of DOI:10.1139/cjc-2013-0437

2-(2-Hydroxy-styryl)-4,6-dimethyl-1,3,5-triazine (P1), a novel compound with D-π-A structure, possesses a variety of isomers. Density functional theory calculations were employed to explore the isomerization mechanisms of its various isomers at the 6-311+G* level. The calculated results show that the phenol configurations of compound P1 are more stable than the quinoid configurations, whether they exist as monomers, complexes, with methanol or dimers. The most stable structure of compound P1 was used to calculate the UV-vis and fluorescence spectra. To validate the calculated results, the title compound was synthesized and characterized by ¹H NMR and ¹³C NMR techniques. The measured absorption and fluorescence spectra of P1 in methanol and N,N-dimethylformamide are in good agreement with the calculated ones.

Full text of DOI:10.1139/cjc-2013-0437

411
ARTICLE
A novel nitrogen-containing compound with D-␲-A structure:
theoretical and experimental study
Xiaoting Gong, Ziyan Zhou, Shuguang Zhang, Shuping Zhuo, Xiurong Huang, Siquan Jiang,
and Zhongmin Su
Abstract: 2-(2-Hydroxy-styryl)-4,6-dimethyl-1,3,5-triazine (P1), a novel compound with D-␲-A structure, possesses a variety of
isomers. Density functional theory calculations were employed to explore the isomerization mechanisms of its various isomers
at the 6-311+G** level. The calculated results show that the phenol configurations of compound P1 are more stable than the
quinoid configurations, whether they exist as monomers, complexes, with methanol or dimers. The most stable structure of
compound P1 was used to calculate the UV-vis and fluorescence spectra. To validate the calculated results, the title compound
was synthesized and characterized by 1H NMR and 13C NMR techniques. The measured absorption and fluorescence spectra of P1
in methanol and N,N-dimethylformamide are in good agreement with the calculated ones.
Key words: 1;3;5-triazine, synthesis, isomerization mechanism, proton transfer, density functional theory.
Résumé : La 2-(2-hydroxy-styryl)-4,6-diméthyl-1,3,5-triazine (P1), un nouveau composé comportant une structure D-␲-A, possède
une grande variété d'isomères. Les calculs de la théorie de la fonctionnelle de la densité ont été utilisés pour explorer les
mécanismes d'isomérisation de des différents isomères de ce composé au niveau 6-311+G**. Les résultats des calculs montrent que
les structures phénoliques du composé P1 sont plus stables que les structures quinoïdiques, qu'elles se présentent sous forme de
monomères, de complexes de méthanol ou de dimères. La configuration la plus stable du composé P1 a servi a` calculer les
spectres UV-vis et de fluorescence. Pour valider les résultats des calculs, le composé P1 a été synthétisé et caractérisé a` l'aide des
techniques de RMN 1H et RMN 13C. Les spectres d'absorption et de fluorescence de P1 mesurés dans du méthanol et du
N,N-diméthylformamide coïncident bien avec ceux qui avaient été calculés. [Traduit par la Rédaction]
Mots-clés : 1;3;5-triazine, synthèse, mécanisme d'isomérisation, transfert de proton, théorie de la fonctionnelle de la densité.
makes sense to study the isomerization process via proton trans-
fer from the chemistry and molecular biology viewpoint. The
Introduction
Molecular design aided by density functional theory (DFT)
compound that we designed and synthesized containing nitrogen
has been verified to be an effective methodology to devise
and −OH, readily forming complexes with metal ions, is expected
functionality-oriented compounds.1 In this way, the properties of
to be applied in the field of metal ion detection or ion sensors.
novel compounds are predicted by calculating their microscopic
structural and electronic parameters and validated through experi-
Theoretical calculation details
mental study. Thus, chemists can reduce the blindness and enhance
the predictability in the synthesis of the desired compounds.2
1,3,5-Triazine is an important six-membered nitrogen-containing
heterocyclic compound whose isomers and derivatives are exten-
sively present in organisms, acting as pivotal building blocks of
many natural compounds that manipulate biological functions.3
Moreover, it is also an important chemical intermediate with a
variety of applications in the pharmaceutical, textile, and plastic
industries. It is also used to synthesize pesticides, dyes, explosives,
surfactants, organic photoelectric materials, and so on.4–9 In the
past few years, much effort has been devoted to theoretical
and experimental investigations on 1,3,5-triazine and its deriva-
tives.10–16 In this paper, the compound 2-(2-hydroxy-styryl)-4,6-
dimethyl-1,3,5-triazine (P1) with a D-␲-A structure composed of the
electron-donating phenol ring and the electron-withdrawing 1,3,
5-triazine ring, as shown in Fig. 1, was devised by molecular design
theory and synthesized experimentally for the purpose of valida-
tion. P1 has many isomers that transform from one to another via
chemical bond rotation and proton transfer.17–20 Therefore, it
Models and methods
All calculations reported here were carried out using the Gauss-
ian 09Wsuite package.21 All geometric configurations of the iso-
mers, the involved transition states, and intermediates, i.e., all of
the isomers of the P1 monomer, complexes of P1 with methanol,
and dimers of P1, were optimized at the DFT-B3LYP/6-311+G** level.
Their vibrational frequencies were computed so that the stable
structures and the first-order transition states could be deter-
mined, and further confirmation of the transition states was done
by intrinsic reaction coordinate analysis. The TDDFT theory22 and
the polarizable continuum model were employed to study the
UV-vis absorption spectrum and the fluorescence spectrum of P1
in its most stable configuration at the B3LYP/6-311+G** level.
Results and discussion
Figure 1 illustrates all isomers of compound P1, and three prob-
able isomerization pathways were assumed: (i) intramolecular
proton transfer in the P1 monomer, (ii) intermolecular proton
Received 21 September 2013. Accepted 27 February 2014.
X. Gong, Z. Zhou, S. Zhang, S. Zhuo, X. Huang, and S. Jiang. College of Chemical Engineering, Shandong University of Technology, Zibo, Shandong,
255049, China.
Z. Su. Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun, Jilin, 130024, China.
Corresponding author: Ziyan Zhou (e-mail: zyzhou@sdut.edu.cn).
Can. J. Chem. 92: 411–416 (2014) dx.doi.org/10.1139/cjc-2013-0437
Published at www.nrcresearchpress.com/cjc on 5 March 2014.

412
Can. J. Chem. Vol. 92, 2014
Fig. 1. Optimized geometric structures of various species in the isomerziation pathways of intramolecular prototropy.
Table 1. Zero-point energies Ezp, total energies E, Gibbs free energies G, and imaginary frequencies ␯
of various species in the reaction pathways of intramolecular prototropy.
Species
E
zp (a.u.)
E (a.u.)
G (a.u.)
v (cm−1
)
⌬E (kJ mol−1
)
M1
M2
M3
M4
M5
M6
M7
Ts1-2
Ts2-3
Ts3-4
Ts4-5
Ts5-6
Ts5-7
0.237090
0.237558
0.235875
0.237267
0.236738
0.237867
0.236732
0.236683
0.230195
0.232024
0.232382
0.231800
0.232053
−742.8996017
−742.8996553
−742.8735773
−742.8738016
−742.8489239
−742.8976648
−742.8678067
−742.8939787
−742.7885634
−742.8515562
−742.836420
−742.7829346
−742.832482
−742.711503
−742.707768
−742.685715
−742.682627
−742.657307
−742.704114
−742.676907
−742.702387
−742.603610
−742.664657
−742.682627
−742.595354
−742.644352
0.14
0
68.5
67.9
133.2
5.2
83.6
14.9
−78.6
−1881.0
−1537.1
−982.6
−1761.4
−1391.7
126.3
291.7
166.0
306.5
176.4
Fig. 2. Schematic diagram of potential energy surfaces describing
Fig. 3. Frontier molecular orbitals of M1 and M2.
intramolecular proton transfer (the energy of M2 is taken as zero).
transfer catalyzed by methanol, and (iii) intermolecular diproton
transfer resulting from P1 monomer dimerization. The calculated
results show that all frequencies of the isomers are positive, re-
flecting that they are in stable configurations. Each transition
state has one and only one imaginary vibrational frequency, indi-
cating that they are all first-order transition states located in the
respective potential energy surface. Such a type of imaginary vi-
brational frequency means that there is a tendency to isomerize
from one isomer to another. In addition, it was further confirmed
by intrinsic reaction coordinate analysis that the isomers involved
in each transition state were in stable configurations. As a conse-
quence, the proposed transitions states are reasonable.
Published by NRC Research Press

Gong et al.
413
Fig. 4. Geometric structures of various species in the reaction pathways of methanol−P1 complexes and the P1 dimers via intermolecular
prototropy obtained from DFT-B3LYP/6-311+G** calculations (interatomic distances are in nm).
Intramolecular proton transfer
backward activation energies are 126.3 and 57.8 kJ mol−1, respec-
Figure 1 illustrates the structures of the isomers and the transi-
tion states that resulted from intramolecular proton transfer, and
Table 1 lists the zero-point energies, the total energies, the Gibbs
free energies, and the vibrational frequencies of all of the isomers
and transitions states. Figure 2 depicts reaction pathways and the
energy barriers of the isomerizations.
tively. Such high activation energies bause the isomerization
hardly to occur. The energy of M3 is close to that of M4, whereas
the energy barrier between them is as high as 223.2 kJ mol−1. It is
obvious that the isomerization of M3 to M4 is difficult to occur. In
a word, P1 predominantly presents in the forms of M2 and M1 at
room temperature, as it hardly transforms from phenol to qui-
noid through intramolecular rotation and proton transfer.
The variation of key geometric parameters in the isomerization
pathway between M3 (M5) and M4 (M6) is shown in Fig. S1A
(Fig. S1C) (see the “Supplementary material” section. Proton trans-
fer is done through a four-membered ring transition. Similarly,
Fig. S1B (Fig. S1D) shows the variation of geometric parameters in
the isomerization pathway between M4 (M5) and M5 (M7). Proton
transfer is done through a six-membered ring transition.
The frontier molecular orbitals of M1 and M2 are shown in
Fig. 3. The square of the frontier molecular orbital can represent
its electron cloud density. Therefore, according to the frontier
molecular orbital diagram, we can acquire the electron cloud dis-
tribution. One can see that the electron cloud of the HOMOs pre-
dominantly centralizes on the phenol ring, while that of the
LUMOs mainly resides on the triazine ring. This indicates that
compound P1 possesses a D-␲-A structure.
The P1 monomer has seven stable isomers (M1−M7) and six tran-
sition states, as shown in Fig. 1. M1, M2, and M6 are phenols with
dihedral angles between the benzene ring and the triazine ring of
26.5°, 28.8°, and 22.9°, respectively. M1 and M2 are trans-isomers,
while M6 is in the cis configuration. Theoretically, the trans con-
figuration is more stable than the cis configuration. In contrast,
M3, M4, and M7 are quinoid in the trans configuration with dihe-
dral angles between the benzene ring and the triazine ring of
0.45°, 0.03°, and 0.02°, respectively. The benzene ring and the
triazine ring are almost situated in the same plane. Although M5
is also a phenol configuration, the plane angle between the ben-
zene ring and the triazine ring is 88.4°, approaching the orthogo-
nal. M5 is the least stable structure, since the bond angle C=C=C is
164.8°, possibly due to the poor conjugative effect. From Fig. 1, we
can see that only the isomerization between M1 and M2 is caused
by chemical bond rotation, while others are via intramolecular
proton transfer.
It can be seen from Table 1 that among M1−M7, M2 is the lowest
in energy, whereas M5 is the highest. According to the minimum
energy principle, M2 is the most stable structure and M5 is the
least stable. Isomerization from M2 to M1 is slightly endothermic
(0.14 kJ mol−1). The forward and backward activation energies are
14.90 and 14.76 kJ mol−1, respectively, revealing that the reaction
can readily occur even at room temperature. Isomerization from
M2 to M3 is also endothermic (68.5 kJ mol−1). Its forward and
Intermolecular proton transfer
Although M2 (M4) is the most stable phenol (quinoid) isomer of
P1, the transformation from M2 to M4 belongs to the intramolec-
ular proton transfer reaction with a very high activation energy
(Fig. 2). For this reason, direct isomerization of M2 to M4 hardly
takes place at room temperature. In this context, we proposed two
mechanisms to accelerate the reaction: (1) methanol-assisted in-
Published by NRC Research Press

414
Can. J. Chem. Vol. 92, 2014
Table 2. Zero-point energies Ezp, total energies E, Gibbs free energies G, and imaginary frequencies ␯
of various species in reaction pathways of methanol−P1 complexes and P1 dimers obtained from
DFT-B3LYP/6-311+G** calculations.
Species
E
zp (a.u.)
E (a.u.)
G (a.u.)
v (cm−1
)
⌬E (kJ mol−1
)
M2−CH3OH
TS(2-4)−CH3OH
M4−CH3OH
M2−2CH3OH
TS(2-4)−2CH3OH
M4−2CH3OH
M2−M2
0.290905
0.284915
0.291200
0.344302
0.337481
0.344676
0.477268
0.469856
0.475175
−858.6778161
−858.6458564
−858.6619423
−974.4610791
−974.4360795
−974.451754
−1485.8257012
−1485.8009429
−1485.8019609
−858.439111
−858.409705
−858.422265
−974.176034
−974.154043
−974.164786
−1485.418140
−1485.397227
−1485.393217
0.0
83.9
41.7
0.0
65.6
24.5
0.0
−1137.0
−771.0
−692.1
TS(2-4)2
M4−M4
65.0
62.3
Fig. 5. Schematic comparison of potential energy surfaces of the
three reaction pathways (the energies of the reactants are assumed
to be the zero point).
termolecular proton transfer and (2) intermolecular diproton
transfer.
Figure 4 illustrates the pathways of the methanol-assisted inter-
molecular proton transfer and intermolecular diproton transfer
reactions as well as the bond length parameters. The zero-point
energies, total energies, Gibbs free energies, and vibrational
frequencies of various species in the reaction pathways of the
methanol−P1 complexes and P1 dimers are summarized in Table 2.
There are three kinds of hydrogen bonds involved in these
isomerization mechanisms, namely O–H···O, O–H···N, and N–H···O.
For the methanol-assisted intermolecular proton transfer, a
10-membered ring can be formed in the transition state via hy-
drogen bonding between hydrogen and oxygen atoms when one
methanol molecule participates in the reaction. When two meth-
anol molecules participate in the reaction simultaneously, how-
ever, a 12-membered ring can be formed. For the intermolecular
diproton transfer mechanism, a 16-membered ring can be formed.
The calculated results for the three kinds of hydrogen bonds dem-
onstrate that the shorter the bond length, the more the bond
angle approaches 180°, suggesting the formation of a stronger
hydrogen bond.23 The hydrogen bond lengths O–H···N and
O···H–N in the process of dimer isomerization via intermolecular
diproton transfer are shorter than those in the methanol-assisted
intermolecular proton transfer. Furthermore, the hydrogen bond
angles of the former case approach 180°. Therefore, the formed
hydrogen bonds in the former case are stronger than those in the
latter case. The bimolecular methanol−P1 complex is also proba-
bly formed by means of methanol-assisted intermolecular proton
transfer. The resulting hydrogen bonds O−H···N and O···H−N are
shorter and stronger than those of the methanol complex. The
lengths (vibration wave numbers) of O–H bonds in M2−CH3OH,
M2−2CH3OH, and M2−M2 are 0.0975 nm (3622.6 cm−1), 0.0981 nm
(3527.1 cm−1), and 0.0990 nm (3327.0 cm−1), respectively, slightly
longer (lower) than that in the M2 monomer, 0.0965 nm
(3779.3 cm−1). These mean that a red shift of the O–H stretching
vibration mode takes place during the intermolecular proton
transfer process. Likewise, the bond length (vibration wave num-
bers) of the N–H bond in M4 is 0.1012 nm (3586.2 cm−1), while those
in M4−CH3OH, M4−2CH3OH, and M4−M4 increase (decline) to
0.1031 nm (3274.6 cm−1), 0.1044 nm (3019.6 cm−1), and 0.1082 nm
(2433.5 cm−1), respectively. Therefore, we also can conclude that a red
shift occurs on the N−H stretching vibration mode.
Table 3. UV-vis absorption spectral data of M1 and M2.
Transition
␭
calcd/nm
ƒ
energy (eV) Transition nature
Coefficient
0.66919
M1 353.58
308.04
M2 351.80
300.96
0.3902 3.5065
0.3283 4.0249
0.3126 3.5243
0.4565 4.1196
HOMO ¡ LUMO
HOMO − 1 ¡ LUMO 0.54232
HOMO ¡ LUMO 0.67917
HOMO − 1 ¡ LUMO 0.61996
Scheme 1. Synthesis of P1.
It can be seen from Figs. 2 and 5 that all of the isomerization
reactions from phenol configurations to quinoid configurations
are endothermic reactions. As mentioned above, M2 ¡ M4 be-
longs to intramolecular proton transfer that requires overcoming
a high energy barrier and thus hardly occurs at room temp-
erature. This is attributed to the formation of an unstable four-
membered ring transition complex without hydrogen bonding.
With the aid of methanol, the activation energy can be signifi-
cantly decreased. The activation energies are 70.28 and 52.72 kJ mol−1
for the monomolecular methanol assisted and bimolecular meth-
anol assisted reaction pathways, respectively. The latter pathway
is superior to the former, since a 12-membered ring complex and
a 10-membered ring complex were formed as their respective
transition states. These reaction pathways are in accordance with
the water-assisted process.24–26 Besides, the formation of O−H···O,
Table 2 shows the calculated values for zero-point energies,
total energies, Gibbs free energies, and imaginary frequencies of
various isomers and transition state complexes in the reaction
pathways of the intermolecular proton transfer process. Whether
in the methanol complex, bimolecular methanol complex, or
dimer, the energy of phenol is always lower than that of the
corresponding quinoid, strongly suggesting that the phenol con-
figuration is more stable than the quinoid one. Figure 5 depicts
the potential energy surface curves of the three reaction path-
ways.
Published by NRC Research Press

Gong et al.
415
Fig. 6. (A) UV-vis absorption spectra of P1 (5 ␮mol L−1) in methanol and DMF; (B) fluorescence spectra of P1 (5 ␮mol L−1) in methanol.
O−H···N, and O···H−N hydrogen bonds is also involved in these
processes. In comparison with the two reactions above, the calcu-
lated activation energy in the dimer for the intermolecular dipro-
ton transfer mechanism is 44.84 kJ mol−1, being close to the value
(52.72 kJ mol−1) in the bimolecular methanol assisted process. This
results from a stable 16-membered ring complex as the transition
state. Owing to the hydrogen bond effects of O−H···N and O···H−N,
the intermolecular diproton transfer has a lower activation en-
ergy. According to the total energies of both the monomer and
dimer, the calculated bond energies of O–H···N and O···H–N27 are
69.3 and 142.7 kJ mol−1, respectively, suggesting that the dimer is
superior to the monomer in stability.
the reaction mixture was neutralized to weak acidity with
20% HCl solution. The yellow precipitate was collected and recrystal-
lized with ethanol. Yield 30.1%. IR (KBr, cm−1): 3074, 1589, 1458,
1415, 1300, 1041, and 983. 1H NMR (400 MHz, DMSO) ␦: 8.47 (d, J =
16 Hz, 1H), 7.70 (d, J = 7.6, 1H), 7.28 (dd, J = 7.6, J = 7.2, 1H), 7.19 (d, J =
16, 1H), 6.87 (dd, J = 7.6, J = 7.2, 1H), 2.56 (s, 6H). 13C NMR (400 MHz,
DMSO) ␦: 175.3, 171.4, 157.6, 138.8, 132.1, 129.1, 124.6, 122.0, 119.9,
116.8, 25.2. Anal. calcd. for C13H13N3O (%): H 5.77, C 68.70, N 18.49;
found: H 5.91, C 68.14, N 18.71.
UV-vis absorption spectra and fluorescence spectra
The UV-vis absorption spectra data of P1 in both methanol and
DMF were acquired on a spectrophotometer (UV-3400), as shown
in Fig. 6A. The absorption peak around 300 nm is assigned to the
triazine ring’s absorption and the absorption around 350 nm is
caused by the conjugative effect of P1. The excited peak and lumi-
nescence peak of the fluorescence spectra for compound P1 in
methanol are at about 460 and 545 nm, respectively, as shown in
Fig. 6B; hence, the Stokes shift is 80–88 nm, reflecting that intra-
molecular proton transfer occurs and compound P1 possesses the
D-␲-A structure.
The variation of geometry parameters for the monomolecu-
lar methanol assisted intermolecular proton transfer and the
P1 dimerization intermolecular proton transfer isomerization is
shown in Fig. S2.
UV-vis absorption spectra and fluorescence spectra
By means of the TDDFT//B3LYP method, the configurations of
M1 and M2 in excited states were optimized with the 6-311+G**
basis set. The polarizable continuum model was employed to
study the UV-vis absorption spectra and fluorescence spectra of M1
and M2 in methanol. The calculated data are summarized in
Table 3 and Fig. 3.
Conclusion
The calculated results show that the phenol configurations of
compound P1 (2-(2-hydroxy-styryl)-4,6-dimethyl-1,3,5-triazine) are
more stable than the quinoid configurations, whether they exist
as monomers, complexes with methanol, or dimers. P1 predomi-
nantly presents in the forms of M1 and M2 at room temperature.
The investigation on three proposed reaction mechanisms dem-
onstrates that the activation energies of intermolecular proton
transfer in methanol−P1 complexes and in the P1 dimer are lower
than those of the intramolecular proton transfer mechanism. The
intermolecular proton transfer could markedly decrease the acti-
vation energies due to the formation of three hydrogen bonds.
The measured UV-vis absorption spectra and fluorescence spectra
of the P1 monomer in methanol are in good agreement with the
calculated ones. Hence, the experiments further verify the calcu-
lated results.
Experimental section
2,4,6-Trimethyl-1,3,5-triazine was synthesized as described in
the literature.28 All chemicals were analytical reagents and used
without further purification. The H and 13C NMR spectra were
1
recorded at 298 K on an AVANCZ 400 spectrometer with tetramethyl-
silane as the internal standard. Elemental analyses (carbon, hy-
drogen, and nitrogen) were performed on a PerkinElmer 240C
elemental analyzer. The Fourier transform infrared spectra were
acquired on a PerkinElmer spectrometer in the range of 4000–
400 cm−1. UV-vis spectra were obtained with a Shimadzu UV-3400
spectrometer. Fluorescence spectra were measured on a Shi-
madzu RF-5301pc spectrometer with a slit width of 3 nm for both
excitation and emission.
Synthesis of P1
Supplementary material
The synthesis of P1 is described in Scheme 1. 2,4,6-Trimethyl-
1,3,5-triazine (6.15 g, 0.05 mol) was added to a solution of KOH
(2.8 g, 0.05 mol) in 40 mL of methanol. The mixture was heated to
reflux under vigorous stirring followed by dropwise addition of
6.1 g of salicylaldehyde in 20 mL of methanol solution. After that,
the mixture was subjected to reaction under refluxing tempera-
ture for 30 h. During the reaction, TLC was used to track the
reaction progress at intervals. When the reaction was completed,
Supplementary material for this paper is available on the jour-
nal web site at http://nrcresearchpress.com/doi/suppl/10.1139/cjc-
2013-0437.
Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (20703008) and the Natural Science Foundation of
Published by NRC Research Press

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Can. J. Chem. Vol. 92, 2014
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