Vol. 44 No. 5
SCIENCE IN CHINA (Series B)
October 2001
π-π interactions in the self-assembly of melamine and
barbituric acid derivatives
YANG Wensheng (M!›), JIANG Yueshun (ÛA), ZHUANG Jiaqi (l¥),
LÜ Nan (Ç ]), CHEN Siguang (/›) & LI Tiejin (9­)
Department of Chemistry, Jilin University, Changchun 130023, China
Correspondence should be addressed to Yang Wensheng (email: wsyang@mail.jlu.edu.cn)
Received April 9, 2001
Abstract Self-assembly of a pair of complementary molecular components, 5-(4-dodecyloxybenzylidene)-(1H,3H)-2,4,6-pyrimidinetrione (PB12) and 4-amino-2,6-didodecylamino-1, 3, 5-triazine
(M12) was studied by cyclic voltammogram, surface photovoltage spectroscopy, fluorescence
spectroscopy, FTIR and X-ray diffraction. It is found that after mixing equimolar amount of PB12
and M12 at room temperature, not only triply complementary hydrogen bonds are formed between
PB12 and M12 but also further self-assembly of the supermolecules based on network of hydrogen
bonds occurs via π-π interactions. During the self-assembly of the supermolecules, π-π interactions
are induced by delocalized interactions between the HOMO of M12 and the LUMO of PB12, resulting in the formation of a supramolecular nanotube with a layered structure bearing a d value of
0.41 nm and PB12 and M12 are arranged alternatively between adjacent supermolecules.
Keywords: self-assembly, barbituric acid derivative, melamine derivative, π-π interactions.
Molecular self-assembly based on intermolecular non-covalent interactions is general in biological system and has been widely employed by chemists as one of the most effective ways to
fabricate nanostructured materials. From the viewpoint of thermodynamics, the enthalpic changes
brought by noncovalent interactions are much smaller than those by covalent interactions, therefore, there exists delicious balance between the enthalpy and entropy in the self-assembly. Among
the noncovalent interactions, hydrogen bonds have been widely used to fabricate nanostructures
due to its directional and strength property. Whereas, it is still difficult to use weaker interactions
which are general in biological system, such as π-π and hydrophobic, in construction of organized
nanostructures in artificial system[1,2]. To overcome the confinement of structure and dimension
brought by directional and saturation properties of hydrogen bonds, Whitesides et al.[3,4] realized
assembly of two hexamers into a sphere-like complex structure by preorganizing the molecular
components by “hub” and “spokes” which could decrease the entropic change of the self-assembly
process. We have realized the self-assembly of hydrogen bonded cyclic hexamer into nanotube by
adjusting the electron pushing and drawing ability of melamine and barbituric acid derivatives
which can increase π-π interactions between the resulting hexamers[5]. When treated by polar solvent, the nanotube can further assembly into supercoil structure[6]. To design nanostructures rationally, here we study the π-π interactions between hydrogen bonded cyclic hexamers from
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π-π INTERACTION IN SELF-ASSEMBLY
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melamine and barbituric acid derivatives by fluorescence, FTIR, electrochemical, surface photovoltage and X-ray diffraction.
1 Experimental
1.1
Instruments
Cyclic voltograms were measured with 8511B Potentio (produced by Yanbian Yongheng
Electrochemical Instrument Factory) and X-Y recorder. Three electrodes were used in the meas
urement. Working electrode and counter electrode were Pt plate and saturated SCE was used as
reference electrode. Supporting electrolyte was TBAP, and CH2Cl2 (analytical grade) was used as
solvent and refluxed in argon atmosphere in CaH2. The solvent was dried using 0.5 nm molecular
sieve before use. Before the electrochemical measurement, the solution was degassed with argon
(99.99%) for 15 min. Fluorescence spectra were taken on a Shimadzu RF-5000 photofluorimeter.
FTIR spectra were carried on Nicolet 5DX FTIR spectrometer. KBr pellets were used for meas
urements. X-ray diffraction patterns were taken on Rigaku D/max γA X-ray diffractor. Surface
photovoltage spectra (SPS) were recorded on a homebuilt apparatus. Indium tin oxide (ITO) glass
was used as electrodes and ITO/sample/ITO sandwiched structure was used for measurements[7].
Syntheses of PB12 and M12
5-(4-dodecyloxybenzzykidene)-(1H,3H)-2,4,6 pyrimidinetrione (PB12) and 4-amino-2,6-didodecylamino-1,3,5-triazine (M12) were synthesized according to ref. [8] and their structures were
identified by elemental analysis, NMR, IR and UV-visible spectroscopies. For the self-assembly,
equimolar amount of PB12 and M12 was mixed in chloroform and kept at room temperature. The
self-assembly process was followed by fluorescence spectroscopy. After the self-assembly reached
the equilibrium, the solvent was removed in vacuum and the remaining yellow powder was used
for surface photovoltage, FTIR and X-ray diffraction measurements. The molecular structures of
PB12 and M12 are shown in scheme 1.
1.2
Scheme 1
2
2.1
Results and discussion
Fluorescence spectral study
PB12 shows intramolecular charge transfer (ICT) band at 394 nm (ε =2.5×104) and locally ex-
cited (LE) at 250 nm (ε =6.8×103) in chloroform solution. M12 shows π π∗ transition band at 230
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nm (ε =2.2×105). The fluorescence spectra of PB12 at different concentrations in chloroform solution show ICT emission at 450 nm, TICT emission at 500 nm and LE emission around 350 nm[9].
After equimolar PB12 and M12 were mixed in chloroform at room temperature, the intensity of the
ICT emission of PB12 was weakened as time passed while the intensity of LE band was enhanced.
It took about 1200 h for the self-assembly process to reach the equilibrium. At this time, the ICT
emission nearly disappeared and the LE band experienced a red shift from 350 to 365 nm (fig. 1).
These results indicate that there are strong intermolecular interactions between PB12 and M12 during the self-assembly process, resulting in the disappearance of the ICT emission.
2.2
Surface photovoltage spectra
The SPS spectra of PB12 powder and the self-assembly (M12 PB12) were taken out in order to
study the changes in the self-assembly. Both of them show strong photovoltage responses (fig. 2).
PB12 shows two photovoltage bands, the band at 390 nm corresponding to the ICT band of
monomer, while the red shifted band at 420 nm corresponding to the ICT band of J-aggregate.
Such D-π-A molecules are arranged in head-to-tail manner in the J-aggregate. The SPS of the
self-assembly was entirely different from that of PB12. For the self-assembly, the photovoltage
response appeared at 330 nm. This blue-shifted band also indicates that strong intermolecular interactions occur during the process of self-assembly. According to the principle of SPS, photovoltage response results from the charge separation after excitation. Generally, D-π-A molecules
can generate efficiency charge separation and show strong photovoltage responses. Therefore, the
above SPS results indicate that the conjugation degree of PB12 has descended, resulting in the
Fig. 1. Emission spectra of PB12 and M12 at different times
after mixing. The concentration is 5×10−5 mol/L. Excitation
wavelength is at 307 nm. 1, Before mixing; 2, 5 min; 3, 48 h;
4, 192 h; 5, 664 h and 6, 1240 h after mixing.
Fig. 2. Photovoltage spectra of PB12 and the self-assembly of PB12 and M12. 1, PB12; 2, the self-assembly of PB12
and M12.
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disappearance of the ICT band.
2.3
Electrochemical behavior
To better understand the intermolecular
interactions during the self-assembly, we studied the electrochemical behavior of PB12 and
M12 by using cyclic voltammetry. Fig. 3 gives
the cyclic voltograms of PB12 and M12 at a
scanning rate of 100 mV/s. PB12 shows an irreversible reduction wave at Epc = −0.520 V,
but no wave in positive potential region, indicating that PB12 is a D-π-A molecule bearing
weak electron pushing and strong drawing
substituents[10]. The cyclic voltogram of M12
shows no redox peak in the range of −0.8
Fig. 3. The cyclic voltograms of PB12 and M12. The scanning range of curve 1 is –0.8 1.3 V and that of curve 2 is
–0.8 1.8 V.
+1.3 V. When the scanning range was extended to 2.0 V, an irreversible oxidation wave appears at
Epa= +1.597 V and an irreversible reductive wave at +0.20 V related to the resulted species of the
above oxidative process. The oxidation wave of M12 observed at positive potential indicates that
the three strong electron pushing substituents (amine groups) introduced on the triazine ring
should act as electron donors.
The electrochemical oxidation occurred at the site with the largest HOMO density, generating
a π cationic radical. The electrochemical reduction occurred at the site with the largest LUMO
density, generating a π anionic radical. Comparing the chemical activity of radicals, tertiary radical is more stable than the primary and secondary radicals. So the irreversible curve indicates that
the π radical should be a primary or secondary radical[10]. The active π anionic radical can be generated while the electrochemical reduction of PB12 occurred at the double bond of vinyl, where the
LUMO is mainly located. The electrochemical oxidation of M12 should occur at the electron
pushing substitute that has the largest HOMO density. Because the electron pushing ability of
NHR is stronger than that of NH2 and the HOMO of NHR is mainly located at the N atom, so the
electrochemical oxidation can generate π cationic radical. According to these analyses, the electrode reactions of PB12 and M12 are proposed in scheme 2.
There may exist several resonance structures of PB12 and M12. Compared PB12 with PB −2 • ,
PB12 is a D-π-A molecule, while the π bond of vinyl in PB −2 • has been broken and the structure
+•
, the triazine ring and C-CHR
of PB −2 • is a D-π conjugated system. Compared M12 with M 12
+•
have changed.
bond in M 12
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SCIENCE IN CHINA (Series B)
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Scheme 2
M12
From the electrochemical results, it can be concluded that electrons can be transferred from
to PB12 during the self-assembly process, and generate charge transfer complex
+•
[ M 12
- PB −2 • ]. If such electron transfer took place, it would damage the D-π-A structure of PB12
−•
and lead to the disappearance of the ICT band. The final product, PB12
, only shows the π π*
band corresponding to the D-π structure. This is consistent with the emission at 365 nm and the
photovoltage response at 330 nm observed in the experiments, suggesting that the electron transfer
from M12 to PB12 may occur during the self-assembly process.
2.4 FTIR
In order to further affirm possibility of the charge transfer during the self-assembly, we studied the IR spectra of PB12 and M12 in the self-assembly. The NH stretch of imide of PB12 at 3228
cm−1 and several weak NH stretches of M12 around 3500
3000 cm−1 disappear in the self-as-
sembly. A new NH stretch band corresponding to the self-assembly occurs at 3318 cm−1. Before
the assembly, both PB12 and M12 can form self-complementary hydrogen bonds, so the changes of
N-H stretches indicate the formation of new triply hydrogen bonds between them[11]. The vibrations of triazine ring of M12 at 1581 (s), 1520 (s), 1467 (s) (overlapped with the deformation of
CH2), 1450 (sh), and 1421 cm−1 (w), disappear after the assembly (see fig. 4)[12], also indicating
that the conjugated structures of both M12 and PB12 are destroyed after the assembly. The new
band of the self-assembly at 1606 cm−1 (s) may be contributed by C
+
N and C
N
stretches of
−1
(M12) . In addition, the band at 1539 (vs) and 1675 cm (s) may also contain the stretch of C N
and C
N . Comparing the IR spectra of the imide bond of PB12 before and after the assembly,
the stretch of 2-C==O is observed at 1754 cm−1 (s), and those of 4, 6-C == O at 1695 (m) and 1667
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cm−1(vs) before the assembly[11], after the assembly, stretch vibration of 2-C == O shifts to
1737 and 1721 cm-1, and the stretches of
4,6-C == O shift to 1675 cm−1 and become one
weak band. In the charge transfer complex
+•
- PB −2 • ], the 4- or 6-C== O changes to
[ M 12
C
O−, so only one band can be observed after
the assembly. After assembly, the pyrimidine
tritone ring gets an electron, so the 2-C == O
stretch shifts to longer wavelength. Before the
assembly, the spectrum of PB12 shows II and
III bands of the imide at 1550 (s) and 1192
cm−1 (s) respectively, corresponding to the
CNH vibration in the resonance structure of
1,6 imide or 3,4 imide. The resonance structure
of the imide is proposed as follows:
Fig. 4. IR spectra of PB12, M12 and the self-assembly of
PB12 and M12. 1, PB12; 2, M12; 3, the self-assembly of PB12
and M12.
After the assembly the II and III bands of the two imides disappear, and the new bands are
observed at 1539 (vs) and 1176 cm-1 (s). These bands are assigned to the II and III bands of 1, 2 or
2, 3 imide. This further suggests that the conjugated structure of PB12 is destroyed in the
self-assembly. Comparing the CH2 stretch vibrations, the bands of PB12 are observed at 2920 and
2854 cm−1, those of M12 at 2920 and 2853 cm−1, while in the self-assembly the CH2 stretches appear at 2920 and 2581 cm−1, indicating that the arrangement of the long alkyl chains has become
ordered in the self-assembly. Comparing the deformation of CH2 before and after the assembly, it
is observed at 1473 (w) and 1467 cm−1 (s) respectively for PB12 and M12. After the assembly there
are two peaks at 1468 and 1464 cm−1, meaning that the long alkyl chains are arranged in an orthorhombic subcell packing in the self-assembly[11]. The ordered arrangement of the alkyl chains
suggests that van der Waals interactions also contribute to the self-assembly process.
Based on the above experimental results, the self-assembly process can be illustrated as follows: first PB12 and M12 form supermolecule through triply complementary intermolecular hydrogen bonds. It has been well documented that melamine and barbituric acid derivatives can form
three kinds of supermolecules through triple hydrogen bonds, such as linear, crinkled, and cyclic
hexamer, controlled by the steric demands of the substituents. Because of the large substituents
introduced in PB12 and M12, the most reasonable tape they adapt should be cyclic hexamer. The
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SCIENCE IN CHINA (Series B)
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hydrogen bonding directed self-assembly
process should be a fast one, and our
NMR results show that the triple hydrogen bonds between PB12 and M12 can be
formed within 5 min. So it can be concluded from fig. 1 that there are further
interactions in the π system of the subunits after formation of the hexamer. The
charge transfer between PB12 and M12
also mainly occurs during the assembly of
the hexamers. If the assembly process
goes on, the subunits will be stacked into
nanotube. This assumption was confirmed
by our TEM study by which nanotubes
with diameter of about 6 nm were ob[5]
served . Molecular mechanics calculations show that the diameter of the hexamer subunit is 5.6
nm, which is consistent with the TEM result. Fig. 5 shows the schematic representation of a hydrogen bonded cyclic hexamer.
Fig. 5. Schematic representation for the cyclic hexamer after
π-π interactions.
2.5 X-ray diffraction pattern
The X-ray diffraction pattern of PB12
powder shows peaks at 2θ = 9.0 , 9.9 ,
15.1 , 19.5 , 24.4 , 25.3 , 28.8 , and
40.3 . The pattern of M12 shows peaks at 2θ
= 4.7 , 7.0 , 8.9 , 11.0 , 12.9 , 18.7 ,
21.3 , 22.5 , 24.9 , 29.3 , and 39.1 .
Those indicate that PB12 and M12 have fine
crystallinity. While the self-assembly of PB12
and M12 shows only one intense peak at 2θ
=21.6 (see fig. 6), and the corresponding d
Fig. 6. X-ray diffraction pattern of the self-assembly of PB12
and M12.
value is 0.41 nm, representing the distance
[6]
between the two hexamer layers . Whitesides et al.[4] have realized the assembly of two hexamers into a sphere-like complex structure by
preorganizing the molecular components by “hub” and “spokes” in their system with a layer dis-
tance between two hexamers of 0.48 nm. So in our system, there are stronger π-π interactions between the hexamers[13]. The stronger π-π interactions can overcome the unfavorable entropic
change during the process of assembling the hexamer subunits into nanotube. During this process,
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M12 and PB12 form charge transfer complex through delocalized interactions between the HOMO
of M12 and the LUMO of PB12. The d value of 0.41 nm obtained from X-ray diffraction pattern
+•
corresponds to the distance between two hydrogen bonded hexamers formed from three M 12
and
three PB −2 • . In the self-assembly, PB12 and M12 are arranged alternatively between adjacent supermolecules.
3 Conclusion
The self-assembly of a pair of complementary molecular components, 5-(4-dodecyloxybenzylidene)-(1H,3H)-2,4,6-pyrimidinetrione (PB12) and 4-amino-2,6-didodecylamino-1,3,5-triazine
(M12) was studied. It was found that after mixing equimolar amount of PB12 and M12 at room
temperature, hydrogen bonded supramolecular hexamer was formed through triply complementary hydrogen bonds between them, then the further self-assembly of the supermolecules via delocalized π-π interactions between the HOMO of M12 and the LUMO of PB12 results in the formation of a supramolecular nanotube with a layered structure bearing a d value of 0.41 nm. In the
nanotube, PB12 and M12 were arranged alternatively between adjacent supermolecules. Such a
study will provide new illumination for the rational design and construction of organic nanostructured materials.
Acknowledgements
69890226).
This work was supported by the National Natural Science Foundation of China (Grant No.
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