Blue organic light-emitting supramolecular microfibers: The self-assembly of a 1,2,4-triazolo[1,5-a]pyrimidine derivative

Article abstract of DOI:10.3390/molecules13040855

The design and synthesis of 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine-2- thioaceto-(2-fluorobenzyl) hydrazone (TPTH), which self-assembled into supramolecular microfibers with blue organic light-emitting properties, is reported. This is the first occur

Full text of DOI:10.3390/molecules13040855

Molecules 2008, 13, 855-863
molecules
ISSN 1420-3049
© 2008 by MDPI
www.mdpi.org/molecules
Full Paper
Blue Organic Light-emitting Supramolecular Microfibers: The
Self-assembly of a 1,2,4-Triazolo[1,5-a]pyrimidine Derivative
Zu-Ming Liu, Hong-Bo Wang, Xiao-Lei Zhu, Hai-Yang Tu * and Guang-Fu Yang
Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of Chemistry,
Central China Normal University, Wuhan 430079, P. R. China
* Author to whom correspondence should be addressed; E-mail: haiytu@mail.ccnu.edu.cn;
http://klpcb.ccnu.edu.cn/
Received: 6 March 2008; in revised form: 7 April 2008 / Accepted: 12 April 2008 / Published: 12
April 2008
Abstract: The design and synthesis of 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine-2-
thioaceto-(2-fluorobenzyl) hydrazone (TPTH), which self-assembled into supramolecular
microfibers with blue organic light-emitting properties, is reported. This is the first
occurrence of the molecular self-assembly of 1,2,4-triazolo[1,5-a]pyrimidine derivatives.
Keywords: 1,2,4-Triazolo[1,5-a]pyrimidine; acetohydrazone; synthesis; self-assembly;
supramolecular
Introduction
Recently, self-assembly of organic small molecules into supramolecules has attracted a great deal
of attention [1]. These supramolecules always have three-dimensional fibrous network structures and
might have potential applications in drug delivery, coatings, lithography, catalyst supporters, scaffolds
for tissue engineering, the engineering of nanostructural materials and as self-supporting porous
materials in the novel separation for macro-molecules, and so forth [2]. Although it is well known that
the self-assembly processes is mediated by cooperative weak and noncovalent bonds, such as
hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals interactions, and water-
mediated hydrogen bonds, there is still no sufficient understanding about the general rules of the self-
assembly mechanism [3-5]. Therefore, significant efforts have been devoted to identify novel system

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with a three-dimensional interconnecting self-organized network structure with the aim to enrich the
content of supramolecular chemistry and obtain new functional materials with desirable properties [6-
9].
In last decades, 1,2,4-triazolo[1,5-a]pyrimidine derivatives have been extensively studied for their
applications in medical chemistry and agrochemistry [10-12]. Additionally, a few examples of 1,2,4-
triazolo[1,5-a]pyrimidines have also been applied in the field of photosensitive materials [13, 14], and
some triazole-based compounds were also reported to be useful as electron transporting materials in
organic light-emitting diodes [15, 16]. However, so far there have been there are no reports concerning
the self-assembly of 1,2,4-triazolo[1,5-a]pyrimidine derivatives, to the best of our knowledge.
Bearing in mind that the triazolopyrimidinyl moiety is a large π-conjugated system, it might
display some unique photonic properties when linked with another π-conjugated system through a
nonconjugated bridge (-SCH2CONHN=CH-) containing hydrogen bond acceptors or donors. A
synthetic route to such systems using aryl aldehydes as starting materials is shown in Scheme 1 [17].
We hoped these compounds might form a superstructure via self-assembly by the cooperative π-π
stacking interactions and hydrogen bond interactions. Interestingly, one compound, 5,7-dimethyl-
[1,2,4]triazolo[1,5-a]pyrimidine-2-thioaceto-(2-fluorobenzyl)hydrazone (4a), was indeed found to self-
assemble into supramolecular microfibers in the solid state and displayed a unique photonics property.
To our knowledge, this is the first reported occurrence of the molecular self-assembly of 1,2,4-
triazolo[1,5-a]pyrimidine derivatives.
Results and Discussion
As shown in Scheme 1, 2-mercapto-5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine and ethyl
α-bromoacetate were employed as starting materials to obtain 5,7-dimethyl-[1,2,4]triazolo[1,5-
a]pyrimidine-2-thioacetohydrazones (TPTH) by a three-step synthetic sequence. Thus 2-
ethoxyacetylthio-5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (2) was synthesized in 94% yield by the
reaction of 1,2-mercapto-5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine with ethyl α-bromoacetate in
the presence of sodium hydroxide. Compound 2 which was then reacted with hydrazine to give 2-
hydrazinoacetylthio-5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (3). Compounds 4 (TPTH) were
generated by the reaction between aldehydes and the intermediate 3.
Firstly, we examined the absorption spectra of 4a in solution and in the solid state, as displayed in
Figure 1. In DMF solution, there are two broad absorption bands at 250 nm and 350 nm for the phenyl
and triazolo[1,5-a]pryrimidinyl chromophores, respectively. The broad band reflects the existence of
electron transfer between the phenyl and triazolo[1,5-a]pyrimidinyl. This broadening effect might
originate from extensive excitation coupling, and the extent of the broadening would strongly correlate
with the degree of coupling. This depends on several factors, such as the closeness of the adjacent
molecule, the tilting angle, the overlap position, the substituted groups, and the extinction. The
experiment showing the relationship between the absorbance and concentration indicated that 4a is not
aggregated in the solution. However, a narrow band with a maximum absorption at around 320 nm was
observed in the the solid state absorption spectra of 4a. Compared with that in the DMF solution, the
absorption spectra in the solid state gave a considerable red-shift, which might be attributed to the

Molecules 2008, 13
857
intermolecular interaction of 4a. In addition, the narrow band shows that the electron transfer does not
occur because of the intermolecular π-π stacking and nonclassical hydrogen bond interactions.
Scheme 1. Synthesis of 5,7-dimethyl-[1,2,4]triazolo[1,5-a]pyrimidine-2-thioacetohydrazones (TPTH).
CH3
CH3
N
N
N
N
N
BrCH2COOEt
NH2NH2.H2O
N
SCH2COOEt
CH3
SH
H3C
N
H3C
N
2
1
CH3
N
N
N
N
RCHO
SCH2CONHNH2
SCH2CONHN CHR
N
N
H3C
N
H3C
N
3
4
N
4c, R =
4a, R =
4e, R =
4d, R =
4b, R =
O
S
N
F
Figure 1. UV absorption spectra of 4a in (1) the solid state and (2) DMF (1×10-5M).
Secondly, we also examined the emission spectra of 4a in solution and in the solid state (Figure 2).
When excited at 302 nm, two emission peaks at 335 nm and 352 nm can be observed in the solution
spectrum. The luminescence quantum yield of 4a is 0.45 (relative to the standard coumarin). The
emission behavior in DMF solution can be simply explained by the conformational changes of
chromophores within the 4a molecule. However, an identical peak at 462 nm with lower intensity was
observed in the solid state, which might result from the aggregation of 4a. These results are consistent
with that of UV-vis experiment. There are two isolated π-conjugated system in the 4a molecule, which
display individual optical properties both in the UV-vis and fluorescence spectra in a diluted solution,
but the molecule can self-assemble in the solid state via noncovalent interactions.
Subsequently, an atypical field emission scanning electronic microscopy (Field-emission SEM)
image of the synthesized microfibers was obtained, in order to examine the configuration of the
aggregate. As shown in Figure 3a, the 4a sample consists of rather regular microfibers with lengths up

Molecules 2008, 13
858
to the micrometer range, and many microfibers are in an orderly arrayed pattern and each of these
microfibers is almost uniform in size. In addition, there are also some branched or short microfibers in
the fibrous networks. As seen from the magnified image of a single one (Figure 3b), the microfiber is
smooth and clean with a diameter of 278 nm. It suggests a possible formation mechanism by which 4a
initially self-organizes and then assembles into shorter microfibers, after which it self-assembles into
longer microfibers.
Figure 2. Emission spectrum of the 4a in (1) DMF solution (1×10-5M; excited at 302
nm) and (2) solid state (excited at 315nm).
(1)
(2)
Figure 3. Field-emission SEM images of 4a supramolecular aggregates: a) overall sample
morphology; b) a detailed view on a single microfiber.
To visualize the molecular dispositions of the self-assembled suprastructures of 4a, molecular
simulations were performed using the Insight II software. Figure 4 illustrates the molecular modeling
results of the self-assembled system. As shown, the 1,2,4-triazolo[1,5-a]pyrimidinyl of one molecule
can pack with that of the adjacent molecule through π-π stacking interactions, while the phenyl of the
two adjacent molecules can pack with each other very well. Additionally, the F atom of one molecule
forms two hydrogen bonds with the H atom of the N=CH group of the adjacent molecule, while the
oxygen atom of the C=O group forms a hydrogen bond with the 6-H of the triazolopyrimidinyl of the

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adjacent molecule, and the hydrogen bond between the 6-H and 3-N atom of two adjacent
triazolopyrimidinyl was also observed.
Figure 4. A possible mechanism for the self-assembly of 4a. The hydrogen bond
distances are: F…H, 2.79Å, 2.74 Å; O…H, 2.26Å; N…H, 2.62Å.
Conclusions
In conclusion, we successfully designed and synthesized a 1,2,4-triazolo[1,5-a]pyrimidine
derivative containing two independent π-conjugated systems and hydrogen bond accptors and donors.
The experimental results demonstrated that 4a was a non-planar intermolecular photoinduced charge
transfer compound [18], which could self-assemble in the solid state and displayed a special
fluorescence spectrum, compared with that obtained in dilute solution. In addition, the microfibers
obtained by the self-assembly of TPTH showed blue photoluminescence properties. Molecular
simulation results indicated that π-π stacking interactions and hydrogen bonds might be the important
driving forces for the formation of supra-molecular microfibers. To our knowledge, this work provides
a completely new supramolecular system for producing new potential blue organic light-emitting
material.

Molecules 2008, 13
Experimental
General
860
Melting points were measured with a Buchi melting point apparatus and are uncorrected. TLC was
performed on Merck 60 F254 silica gel-coated aluminum sheets, and spots were detected by UV light
1
(254 nm). The H-NMR spectra were recorded on a Vario MERUCY-PLUS 400 instrument using
DMSO-d6 as a solvent and TMS as an internal standard. MS spectra were performed on a Hewlett-
Packard 5988A instrument. IR spectra were recorded as KBr pellets on a Perkin-Elmer Fourier
transform infrared spectrophotometer. Elemental analyses were performed on a Vario El III CHNS
instrument. The compound 2-mercapto-5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (1) was
synthesized according to [19]. The other chemicals were obtained from commercially available
sources and used without further.purification.
Syntheses
Preparation of 2-ethoxyacetylthio-5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (2)
To a stirred mixture of 2-mercapto-5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (1, 18 g, 0.1 mol),
sodium hydroxide (4 g, 0.1 mol) and water (250 mL), a mixture of BrCH2CO2Et (16.7 g, 0.1 mol) in
methanol (50 mL) was added dropwise at room temperature. After stirring the reaction mixture for 2
hours, the precipitate formed was filtered off and recrystallized from methanol to afford 2 as a white
needle-like crystals (24 g, 94% yield); mp 142-143oC; 1H-NMR δ: 1.24 (t, 3H, CH2CH3), 2.57 (s, 3H,
5-CH3), 2.67 (s, 3H,7-CH3), 4.07 (s, 2H, SCH2), 4.16-4.19 (q, 2H, CH2CH3), 6.69 (s, 1H, 6-H); MS
m/e (relative intensity) 266 (M+, 34.6), 193 (100), 149 (23.9), 108 (80.5), 107 (32.6).
Preparation of 2-hydrazinoacetylthio-5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (3)
2-Ethoxyacetylthio-5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (2, 13.3g, 0.05 mol), hydrazine
monohydrate (20 mL, 0.35 mol) in ethanol (200 mL) were refluxed for 4 hours. After cooling to room
temperature, the precipitate was filtered and washed twice with ethanol. The solid was recrystallized
from methanol to give 3 as white crystals (11.7 g, 93% yield); mp 202-203 oC; 1H-NMR δ: 2.55 (s, 3H,
5-CH3), 2.65 (s, 3H, 7-CH3), .9.5 (s, 2H, SCH2), 4.35 (bs, 2H, NH2), 7.10 (s, 1H, 6-H), 9.40 (s, 1H,
NH).
General Procedure for Preparation of
5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine-2-thioaceto-hydrazones 4a-e
2-Hydrazinoacetylthio-5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (3, 1.4 g, 5 mmol), the
appropriate aldehyde (6 mmol) and acetic acid (0.5 mL) were refluxed in ethanol (50mL) until the
starting materials disappeared. After cooling to room temperature, the preciptate was filtered and

Molecules 2008, 13
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washed twice with ethanol. The solid was recrystallized from DMF or DMF- acetonitrile mixture to
afford 4 as white crystals.
5,7-Dimethyl-1,2,4-triazolo[1,5-a]pyrimidine-2-thioaceto-(2-fluorobenzyl)hydrazone (4a). Yield:
1
94%; mp 196-197 oC; H-NMR δ: 2.49, 2.52 (2s, 3H, 5-CH3), 2.62, 2.64 (2s, 3H, 7-CH3), 4.12, 4.54
(2s, 2H, SCH2), 7.06 (s, 1H, 6-H), 7.30-7.80 (m, 4H, Ar-H), 8.23 (2s, 1H, CH), 11.8 (2S, 1H, NH); MS
m/e (relative intensity) 358 (M+, 6.14), 221 (42.7), 194 (52.4), 193 (52.9), 180 (100), 149 (37.6), 108
(79.4), 67 (43.7); FTIR: ν(C=O) at 1677 cm-1; Anal. Calcd. for C16H15FN6OS: C, 53.62%; H, 4.22%; N,
23.45%. Found: C, 53.75%; H, 4.29%; N, 23.31%.
5,7-Dimethyl-1,2,4-triazolo[1,5-a]pyrimidine-2-thioaceto-(2-pyridinyl)hydrazone (4b). Yield: 91%;
1
mp 205-206 oC; H-NMR δ: 2.55 (s, 3H, 5-CH3), 2.65, 2.67 (2s, 3H, 7-CH3), 4.15, 4.58 (2s, 2H,
SCH2), 7.11, 7.12 (2s, 1H, 6-H), 7.41-7.43 (d, 1H, 3-pyridine), 7.86-7.96 (m, 2H, 4,5-pyridine), 8.08,
8.24 (2s, 1H, CH), 8.61 (d, 1H, 6-pyridine), 11.84, 12.00(2S, 1H, NH); MS m/e (relative intensity) 342
(M+1, 13.3), 221 (67.7), 194 (100), 193 (79.8), 180 (68.9), 149 (63.4), 108 (34.6), 65 (35.8); FTIR:
ν(C=O) at 1693 cm-1; Anal. Calcd. for C15H15N7OS: C, 52.77%; H, 4.43%; N, 28.72%; Found: C,
52.95%; H, 4.51%; N, 28.21%.
5,7-Dimethyl-1,2,4-triazolo[1,5-a]pyrimidine-2-thioaceto-(4-pyridinyl)hydrazone (4c). Yield: 98%;
mp 251-252oC; 1H-NMR δ: 2.55 (s, 3H, 5-CH3), 2.65, 2.67 (2s, 3H, 7-CH3), 4.16, 4.58 (2s, 2H, SCH2),
7.11, 7.12 (2s, 1H, 6-H), 7.64, 7.65 (2s, 2H, 3,5-pyridine), 8.02, 8.23 (2s, 1H, CH), 8.62, 8.63 (2s, 2H,
2,6-pyridine) 11.92, 12.05 (2s, 1H, NH); MS m/e (relative intensity) 342 (M+1, 100), 221 (29.0), 194
(13.0), 193 (52.3), 180 (51.6), 149 (18.3), 108 (16.0); FTIR: ν(C=O) at 1705 cm-1; Anal. Calcd. for
C15H15N7OS: C, 52.77%; H, 4.43%; N,2 8.72%; Found: C, 53.02%; H, 4.53%; N, 28.28%.
5,7-Dimethyl-1,2,4-triazolo[1,5-a]pyrimidine-2-thioaceto-(2-thiophenyl)hydrazone (4d). Yield: 99%;
mp 184-185 oC; 1H-NMR δ: 2.55, 2.57 (2s, 3H, 5-CH3), 2.66, 2.67 (2s, 3H, 7-CH3), 4.11, 4.45 (2s, 2H,
SCH2), 7.11-7.13 (m, 2H, 6-H, 3-thiophene), 7.44-7.47 (m, 1H, 4-thiophene), 7.63-7.66 (m, 1H, 5-
thiophene), 8.22, 8.44 (2s, 1H, CH), 11.62, 11.72 (2s, 1H, NH); MS m/e (relative intensity) 347 (M+1
46.9), 346 (M+ 56.7), 221 (100), 194 (30.2), 193 (75.0), 180 (42.8), 149 (25.9), 108 (31.1), 67 (12.9);
FTIR: ν(C=O) at 1675 cm-1; Anal. Calcd. for C14H14N6OS2: C, 48.54%; H, 4.07%; N, 24.26%; Found: C,
48.32%; H, 4.03%; N, 24.28%.
5,7-Dimethyl-1,2,4-triazolo[1,5-a]pyrimidine-2-thioaceto-(2-furyl)hydrazone (4e). Yield: 99%; mp
1
190-192oC; H-NMR δ: 2.55 (s, 3H, 5-CH3), 2.66, 2.67 (2s, 3H, 7-CH3), 4.11, 4.50 (2s, 2H, SCH2),
6.62 (s, 1H, 4-furan), 6.90, 6.91(d, 1H, 3-furan), 7.11, 7.12 (2s, 1H, 6-H), 7.82 (s, 1H, 5-furan), 7.93,
8.11 (2s, 1H, CH), 11.60, 11.71(2S, 1H, NH); MS m/e (relative intensity) 331 (M+1 94.5), 330 (M+
100), 221 (32.2), 193 (41.6), 180 (17.3), 149 (14.7), 108 (26.2), 52(42.4); FTIR: ν(C=O) at 1674 cm-1;
Anal. Calcd. for C14H14N7O2S: C, 50.90%; H, 4.27%; N, 25.44%; Found: C, 50.82%; H, 4.33%; N,
25.28%.

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Acknowledgements
This work was supported by National Natural Science Foundation of China (No. 20572030,
20432010, 20528201, and 20672044), Key Project of the Ministry of Education (Nos. 103116 and
104205), Program for New Century Excellent Talents in University of China and Program for
Excellent Research Group of Hubei Province (No. 2004ABC002).
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