Microwave-assisted synthesis and luminescent properties of triphenylamine substituted mono- and di- branched benzimidazole derivatives

Article abstract of DOI:10.1007/s11696-020-01400-1

Abstract: In the present work, the synthesis of the target products using sodium metasulfite (Na₂S₂O₅) and p-toluenesulfonic acid (PTSA) separately as catalysts was studied. Herein, the liquid phase microwave method was ch

Full text of DOI:10.1007/s11696-020-01400-1

Chemical Papers
https://doi.org/10.1007/s11696-020-01400-1
ORIGINAL PAPER
Microwave‑assisted synthesis and luminescent properties
of triphenylamine substituted mono‑ and di‑ branched benzimidazole
derivatives
Jiaxu Fu1 · Liuqing Yan1 · Shuang Wang1 · Hongying Song1 · Qiang Gu1 · Yumin Zhang1
Received: 23 August 2020 / Accepted: 16 October 2020
© Institute of Chemistry, Slovak Academy of Sciences 2020
Abstract
In the present work, the synthesis of the target products using sodium metasulfte (Na2S2O5) and p-toluenesulfonic acid
(PTSA) separately as catalysts was studied. Herein, the liquid phase microwave method was chosen to synthesize triph-
enylamine substituted mono- and di-branched benzimidazole derivatives compared with the solid phase microwave method,
and the reaction conditions were optimized using Na2S2O5 as a catalyst in N,N-dimethylformamide (DMF) solvent. A possible
reaction mechanism is discussed. Ten new triphenylamine-benzimidazole derivatives were successfully synthesized. On this
basis, PTSA using a catalyst was introduced into the reaction, the yields of the target products were evidently increased (the
yield was enhanced 5‒22% using PTSA as a catalyst). It is found that PTSA only acted as a catalyst, while Na2S2O5 acted as
both a catalyst and an oxidant, and PTSA could efectively catalyze the synthesis of benzimidazoles. Further, the luminescent
properties of the synthesized compounds were comparatively studied after the structures of the synthesized compounds were
confrmed. The results showed that the fuorescence quantum yield and the intensity of the synthesized compounds were
enhanced with the increase in the number of substituted benzimidazole on triphenylamine, and the diferent substituents on
5-position of benzimidazole also have signifcant efect on the luminescent properties of the compound.
Graphic Abstract
Keywords Triphenylamine-benzimidazole · Catalysis · Microwave chemistry · Reaction mechanisms · Luminescent
property
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11696-020-01400-1) contains
supplementary material, which is available to authorized users.
Extended author information available on the last page of the article
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Chemical Papers
efcient catalyst and oxidant used in the synthesis of ben-
zimidazole derivatives under microwave-assisted condition
(Bui et al. 2016; Gabriel et al. 2006). And another catalyst,
PTSA as an acid catalyst was widely applied in the feld of
organic synthesis because it is an efcient, non-toxic, and
inexpensive solid acid. Although PTSA has been used to
synthesize simple structure 2-arylsubstituted benzimida-
zole, quinoline, quinoxaline and pyrimidine from amine
and aldehyde condensation (Han et al. 2007; Chan et al.
2020; Shi et al. 2008; Jin et al. 2002), the synthesis of
benzimidazole derivatives using PTSA as a catalyst in
microwave radiation has not been reported. Especially,
the synthesis of the larger steric hindrance triphenylamine
substituted mono- and di- branched benzimidazole deriva-
tives has not been reported.
Introduction
In recent years, benzimidazole derivatives as fuorescent
organic small molecules have attracted intensive attention
because of their value in technological applications refer-
ring to fuorescent probes for detecting metal ion (Saluja
et al. 2012; Wang et al. 2013; Jayabharathi et al. 2012),
pH probes (Sevinoç et al. 2014), electrochromic devices
(Sydam et al. 2013), sensors (Wannalerse et al. 2008) and
organic semiconductor materials (Lai et al. 2008) and
so on. Compared with metal and inorganic compounds,
organic compounds were easily modifed with various
functional structures. Organic fluorescent molecules
such as benzimidazole derivatives are a group of hetero-
cyclic organic compounds consist of benzene/imidazole
ring structure. According to the comprehensive reports
from Yamamoto et al. on n-type π-conjugated units,
benzimidazole is a potentially acceptor unit, displayed
good electrochemical and optical features (Tanimoto and
Yamamoto 2006; Ahn et al. 2001). Also, benzimidazole
derivatives showed high electron transporting ability due
to the electron-withdrawing imine (C=N) bonds on their
molecular skeletons (Akpınar et al. 2010; Newkome et al.
1982; Tanimoto and Yamamoto 2004). Triphenylamine is
a non-planar molecule having a larger conjugated structure
(Janic and Kakas 1984). triphenylamine derivatives are
widely used in organic optoelectronic functional materials
(Salbeck et al. 1997; Tokito et al. 1998), OLEDs (Liu et al.
2014; Xia et al. 2009; Nguyen et al. 2014), organic dyes
(Shang et al. 2016), solar cells (Chen et al. 2013; Le et al.
2018) and so on. Herein, we designed and synthesized
novel D-A type organic small molecule compounds using
triphenylamine as electron donor and benzimidazole-based
moiety as electron acceptors, whose luminescence prop-
erty were further studied.
In the present work, we focus on the synthesis and lumi-
nescence property of new 5- substituted 4-(1H-benzoimi-
dazole-2-yl)-N,N-diphenylaniline (triphenylamine substi-
tuted mono- branched benzimidazole) and 5- substituted
N-(4-(1H-benzoimidazole-2-yl)phenyl)-4-(1H-benzoimi-
dazole-2-yl)-N-phenylanline (triphenylamine substituted
di- branched benzimidazole) bearing diferent electron-with-
drawing and electron-donating substituents at 5- position of
benzimidazole. Herein, Na2S2O5 is used as a catalyst and
oxidant to explore the synthesis of 2-substituted benzimida-
zole staring from high steric resistance triphenylamine alde-
hydes and substituted o-phenylenediamine by solid phase
and liquid phase microwave-assisted method, respectively.
On the basis of the obtained optimum synthesis process,
the catalyst PTSA was introduced into the reaction, and the
catalytic efects of Na2S2O5 and PTSA were compared. The
structures of the synthesized compounds were characterized
1
by HRMS, FT-IR, H NMR and 13C NMR spectroscopy.
The luminescence property was studied by detecting their
absorption and fuorescence spectrum. The results suggested
that the catalysis of PTSA was superior to that of Na2S2O5,
and the maximum absorption wavelength and fuorescence
quantum yield of the synthesized compounds are respec-
tively red-shifted and obviously enhanced with increasing
the number of substituted benzimidazole on triphenylamine.
So far, the developed synthesis of substituted benzi-
midazoles generally started from o-phenylenediamine or
o-nitroaniline and carboxylic/aldehyde derivatives using
diverse catalytic/oxidation system. The mainly used oxida-
tion system including p-toluenesulfonic acid/air (Han et al.
2007), Oxone (Beaulieu et al. 2003), molecular iodine
(Gogoi et al. 2006), bisulfte adduct (Weidner-Wells et al.
2001), FeCl3·H2O (Singh et al. 2000), air (Lin et al. 2005),
sodium dithionite (Romero et al. 2013) and so on.
Results and discussion
Synthesis condition optimization
Moreover, microwave technique was widely applied in
organic synthesis in recent years. The principal advantages
of various reported microwave-assisted synthesis are sim-
ple work up procedure, fast reaction rate, high yield, well
selectivity, environmentally friendly (Raner et al.1995;
Gedye et al. 1986; Dariusz 1998; Srikrishna and Naga-
raju 1992; Shi et al. 2019a). Sodium pyrosulfte (Na2S2O5)
is a non-toxic food additive, has been reported to be an
It is known that Vilsmeier–Haack reaction is widely used
to formylation reaction (Chakradhar et al. 2009). In gen-
eral, N,N-dimethylformamide (DMF) and phosphorus oxy-
chloride (POCl3) are used to introducing an aldehyde group
on the activated aromatic ring. In our present work, two
kinds of target products were successfully prepared using
triphenylamine (TPA) as a raw material by controlling the
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Chemical Papers
Table 1 The synthesis of triphenylamine aldehydes
feed mole ratio of POCl3 and DMF (Table 1). As shown in
Table 1, triphenylamine’s monoaldehyde (1a), dialdehyde
(1b) were respectively synthesized in 82% and 62% yields.
Also, with the increase of the number of substituted alde-
hyde groups on triphenylamine, the mole ratio of POCl3 to
DMF was increased from 1:3 to 2:3, the reaction time was
also increased from 2 to 3 h. However, the yield decreased
from 82.3% (1a) to 62.4% (1b). It indicated that the formyla-
tion reaction become more difcult and melting point (M.P.)
of the product enhanced with extending the substituted alde-
hyde group (Table 1).
Entry Compound n(POCl3)/n(DMF) Reaction Yield (%) M.P. (°C)
time (h)
1
2
1a
1b
1:3
2:3
2
3
82
62
132–134
144–146
In our previous work, we successfully synthesized
1,2,4,5-tetrasubstituted imidazoles (Shi et al. 2019a) and
pyrazolone derivatives containing substituted isoxazole ring
compounds by solvent-free microwave-assisted method in
moderate to good yield (Zhang et al. 2016; Yan et al. 2017;
Mi et al. 2018; Li et al. 2012). Herein, on the basis of our
previous work (Shi et al. 2019b), the synthesis possibility
of the desired products 2-N,N-diphenylaniline substituted
benzimidazoles were investigated starting from 4-substi-
tutied o-diphenylaniline and triphenylamine aldehydes using
Na2S2O5 as a catalyst and oxidant under solvent-free and
microwave irradiation conditions (Table 2 and Scheme 1).
As shown in Table 2, four of the desired compounds were
synthesized in 11‒68% yield. However, the target product
2c was not obtained. Also, the yields of the obtained 2d and
2e were very low. The possible reason was that silica gel
Table 2 The synthesis of compounds 2a‒2e under solvent- free and
microwave irradiation conditions
Entry Compound R
nCatalyst
(mmol)
Time (min) Yielda (%)
1
2
3
4
5
2a
2b
2c
2d
2e
H
Cl
Br
CH3
1.0
1.0
1.0
1.0
20
20
25
25
25
68
51
–
36
11
COOH 1.0
aIsolated yield
Scheme 1 Synthetic routes to compound 2a‒2j
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Chemical Papers
is good at dispersing the reactants while preventing them
from touching each other and the catalytic oxidation of the
catalyst could not be fully exploited. Additionally, the prod-
ucts were difcult to be separated resulting from the similar
polarity and reciprocity between product and by-product
mono-Schif base. It was verifed by HPLC–MS analysis of
the reactants when the reaction for synthesizing product 2c
was over (Fig. 1a). Therefore, it was necessary to choose a
solvent that could inhibit the interaction of mono-Schif base
and the product and promote the reactant dissolution.
the crude product at the end point of the reaction under the
same chromatographic conditions with Fig. 1a was meas-
ured (Fig. 1b). The result showed that only a few mono-
Schif base (m/e=441) was produced, and the cross between
product and mono-Schif base was not formed. It suggested
that the reactant solubility in DMF efectively inhibited the
formation of by-products. Therefore, the liquid microwave
method was more suitable for the reaction system. The reac-
tion mechanism was inferred from the above HPLC–MS
analysis and the simple tracking experiments (Scheme 2).
Herein, gas produced in the reaction progress was monitored
by the wet extensive pH indicator paper. The wet extensive
pH indicator paper turned red frst and then red disappear-
ing, which is an evidence of SO2 formation. The diference
between this reaction and the previous synthesis of the small
steric hindrance benzimidazoles is the production of no bi-
Schif base.
It is known that DMSO and DMF are all-purpose solvent.
At the beginning of the study, DMSO and DMF were chosen
as solvents to investigate the synthesis of product 2c starting
from 1a and 4-bromobenzene-1, 2-diamine. It is disappoint-
ing that the obtained 2c in DMSO solvent was still hard to
be separated efciently. As a result, DMF as a solvent was
introduced to the reaction in the presence of 1 equivalent
Na2S2O5. The desired product 2c was frstly obtained in 37%
yield (Table 4, entry 3). Further, the HPLC–MS analysis of
As shown in Scheme 2, the lone pair electron on a nitro-
gen atom of 4-substitutied o-diphenylaniline attacked the
Fig. 1 The HRMS analysis of the obtained compound 2c under solvent-free (a) and DMF as a solvent (b) microwave radialization conditions
Scheme 2 The inferred reaction mechanism to synthesize compounds 2a–2e
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carbonyl carbon of triphenylamine aldehyde by nucleo-
philic addition to form transitional state product A, further
occurred H+ transfer to obtain secondary alcohol amine
B. Sequentially dehydrating to produce mono-Schif base
(See Fig. 1b). Then, the lone pair electrons on the other
amino nitrogen atom continued to attack the imine carbon
of mono-Schif base, further occurred H transfer to cyclize
to 4-(5-substitute-2,3-dihydro-1H-benzo[d]imidazol-2-yl)-
N,N-diphenylaniline. Finally, 4-(5-substitute-2 ,3-dihydro-
1H-benzo[d]imidazol-2-yl)-N,N-diphenylaniline was oxi-
dized to the target product by SO2 produced via the thermal
decomposition of Na2S2O5 (Romero et al. 2013).
However, it was found that the reactants could not be reacted
completely and the reaction time was prolonged when the
reaction temperature was decreased to 75 °C. Therefore,
1:1.05 molar ratios of the reactants, reaction time 20 min
and reaction temperature 98 °C were chosen to carry out the
liquid phase microwave reaction.
The application of the optimum conditions
Subsequently, the optimized reaction condition was extended
to synthesize the designed compounds. Five desired prod-
ucts 2a‒2e were successfully synthesized in 25–86% yield
at 98 °C using DMF as a solvent and under microwave
irradiation condition (Table 4, entries 1–5, method A).
The yields were markedly improved compared to the solid
phase microwave method (Table 2). Further, the liquid
microwave-assisted method was extended to the synthesis
of di-branched triphenylamine-benzimidazole derivatives,
and fve target products 2f–2j were successfully prepared
in 20–84% yield (Table 4, entries 6–10, method A). The
polarity of 2e, 2j is too large, or the solubility of 2b and 2g
in the eluent is too small to be hardly separated by column
chromatography, further resulting in low yield and very long
separation time (Table 4, entries 5 and 10, 2 and 7).
To get a high yield, the synthesis conditions were inves-
tigated by employing 4-methylbenzene-1,2-diamine(I) and
4-(diphenylamino)benzaldehyde (II) using 1 equivalent
Na2S2O5 as a catalyst and oxidant under microwave irra-
diation condition in 15 mL DMF, and the reaction was
monitored by TLC. The results are summarized in Table 3.
As shown in Table 3, the yield of the obtained target prod-
uct (2a) increased frstly and remained almost unchanged
and then decreased with the molar ratio of I and II (nI:nII)
decreasing from 1:1 to 1:1.05. I could be completely con-
sumed when nI:nII was controlled at 1:1.05, and the yield
of the obtained target product (2a) was the highest (86%).
It is known that the frst benzimidazole compound was
synthesized by Hoebrecker under the strong acidic condi-
tions in 1872. (Lu et al. 2002) and (Boufatah et al. 2004)
also synthesized benzimidazole derivatives respectively
using polyphosphoric acid and hydrochloric acid as the
catalyst. By chance, when 0.1 equivalent PTSA replaced 1
equivalent Na2S2O5 and was added to the reaction, the yield
of product 2d was signifcantly increased (Na2S2O5 56%;
PTSA 77%) (Table 4, entry 4). Similarly, the catalyst PTSA
was expanded to catalyze the synthesis of the rest of ben-
zimidazole derivatives using DMF as a solvent and under
Table 3 Optimization of reaction conditions
Entry
nI:nII
Reaction time Reaction tem-
Yielda (%)
(min)
perature (oC)
1
2
3
4
1:1
20
20
20
25
98
98
98
75
84%
86%
85%
69%
1:1.05
1:1.1
1:1.05
aIsolated yield
Table 4 The synthesized
compounds 2a‒2j under
microwave irradiation and
solvent as DMF
Entry
Compound
R
Method Aa
Time (min)
Method Bb
Time (min)
M.P. (oC)
Yieldc (%)
Yieldc (%)
1
2
3
4
5
6
7
8
9
10
2a
2b
2c
2d
2e
2f
H
Cl
Br
CH3
COOH
H
Cl
Br
20
20
20
20
30
20
20
20
20
30
86
84
37
56
25
84
80
49
58
20
15
15
15
15
20
15
15
15
15
20
91
89
57
77
47
94
86
68
79
41
>280
216–218
242–244
245–247
184–186
>280
210–212
242–244
>280
2g
2h
2i
CH3
COOH
2j
>280
aAll the compounds were synthesized using Na2S2O5 as a catalyst at 98 °C under microwave irradiation
bAll the compounds were synthesized using PTSA as a catalyst at 120 °C under microwave irradiation
cIsolated yield
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Chemical Papers
microwave irradiation condition, the desired products were
successfully synthesized at 120 °C in 41–94% yield (Table 4,
entries 1–3 and 5–10, method B). The results showed that
the yields of the obtained target products were improved
and the reaction times were reduced comparing with using
Na2S2O5 as a catalyst. This result may be due to the proto-
nation of the carbonyl group of triphenylamine aldehyde in
the acid system enhanced the electropositivity of the car-
bonyl carbon which improved the activity of the carbonyl
group and promoting the nucleophile attack of the amine. It
is hypothesized that the formation of triphenylamine-benzi-
midazole could be occurred with the followed mechanism
(Scheme 3).
2a was considered as the model compound to investigate
the efects of diferent electron donating (2b and 2g) and
electron withdrawing (2c‒2e and 2f‒2j) substituents on ben-
zimidazole luminescence properties.
Figure 2 shows that the absorption spectra of compounds
2a‒2e and 2f‒2j in ethanol solution and their maximum
absorption wavelengths are summarized in Table 5. Two
primary bands, the absorption of these compounds around
284‒305 nm (2a‒2e) and 330‒340 nm (2f‒2j) was attrib-
uted to the locally excited π–π* transition centered on triph-
enylamine, while another absorption of longer wavelength
around 345–368 nm (2a‒2e) nm and 369–377 nm (2f‒2j)
could be associated with the charge transfer of the π–π*
transition from the HOMO of the electron-donating triph-
enylamine moiety to the LUMO of the electron-accepting
benzimidazole moiety (Ge et al. 2008; Gong et al. 2010;
Pina et al. 2013). Besides, the band of the di-branched com-
pounds 2f‒2j red-shifted 12–19 nm (Fig. 2 and Table 5)
comparing with mono-branched compounds 2a‒2e. This
might be because the interaction of electron-donor and elec-
tron-acceptor upon excitation enhanced π-electron delocali-
zation (Lin et al. 2004; Wang et al. 2010). The synthesized
mono-branched compounds 2b‒2e (Table 5: entries 2–5)
and di-branched compounds 2g‒2i (Table 5, entries 7–10)
containing diferent substituents except for 2g had a slight
red shift (1–6 nm) respectively comparing to their model
compounds 2a (Table 5, entry 1) and 2f (Table 5, entry 6).
It is likely that the introduction of –CH3, –COOH and halo-
gen groups slightly increased the degree of conjugation of
the molecule, and further enhanced the conjugated system
electron delocalized.
It was seen from Table 4, the better yields were obtained
when no substituents was at 5-position of o-phenylenedi-
amine (Table 4, entries 1 and 6), and it was higher than that
of the bearing electron-withdrawing substituent at 4-position
of o-phenylenediamine (Table 4, entries 2 and 7, 3 and 8, 5
and 10). The result may be due to the fact that the electron-
withdrawing substituent on o-phenylenediamine reduced the
electron cloud density of amino N, which was not conducive
to amino N nucleophilic attack on carbonyl carbon. Moreo-
ver, the yield of the target product might also be controlled
thermodynamics. The structures of the obtained compounds
1
2a‒2j were confrmed by FT-IR, H and 13C NMR and
HRMS spectra analysis. The –C–NH–C– protons of imida-
zole ring exhibited resonances at δ 12.58–13.18 ppm using
DMSO as the solvent. The resonances for carbon directly
attached to 2-position carbon atom attached to NH on imi-
dazole ring and carbon bearing on triphenylamine were
respective observed peaks at δ 148.43–152.93 ppm and δ
146.54–149.55 ppm.
With respect to the emission spectrum, the maximum
emission wavelength and fuorescence quantum yield of
compounds 2a‒2e are shown in Fig. 3 and Table 5. The com-
pound bearing electron-withdrawing substituent (Table 5,
entries 2, 3 and 5) displayed a red-shift about 10 nm at a
maximum emission wavelength comparing to that of com-
pound 2a (Table 5, entry 1). However, when substituent was
replaced with electron-donating group (–CH3) (Table 5,
entry 4), a negligible blue-shift was observed. The main
reason was that the electron-withdrawing groups (–Cl, –Br,
–COOH) enhanced the electron-pulling strength of ben-
zimidazole-based moiety acceptor (Lin et al. 2004). The
Luminescence properties
Further, the absorbance and fuorescence properties of com-
pounds 2a‒2f (1.0×10–6 mol/L) were evaluated in ethanol
solution. It is known that substituents had signifcant efects
in spectroscopic shift of both absorption and fuorescence.
In general, electron-donating groups could cause an increase
molar absorption coefcient and enhance fuorescence ef-
ciency, while electron-withdrawing groups reduced fuores-
cence quantum yield (Giri et al. 1988). Herein, compound
Scheme 3 The possible reaction mechanism to synthesis of compounds 2a‒2e using PTSA as a catalyst
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Fig. 2 The absorption spectrum of 2a‒2e and 2f‒2j (1.0×10−6 mol/L) containing diferent substituents in ethanol solvent (X: Quinine sulfate)
Table 5 Spectroscopic data (Absorption λmax, fuorescence emission
max, and fuorescence quantum yield Φ) of compound 2a‒2j in etha-
nol solvent [1.0×10−6 mol/L (λex =310 nm)]
could be seen from Fig. 3 and Table 5 that the fuores-
cence intensity and Φ of di-branched compounds 2f‒2j
(Table 5, entry 6–10) was higher than that of the corre-
sponding mono-branched compound 2a‒2e (Table 5, entry
1–5) bearing the same substitute. It is likely because the
increases of the systematic conjugation and the coplanarity
of the molecular structure were enhanced as the number
of substituted benzimidazole in the structure increased.
As shown in Fig. 3 and Table 5, the fuorescence inten-
sity and Φ of 2b and 2g were higher than that of 2c and
2h when the hydrogen on benzene ring was respectively
replaced with chlorine and bromine. It resulted from an
introduction of a heavy atom (Cl and Br) into a molecule
enhance the rate of S1-T1 spin-forbidden process (Chen
et al. 2014; Chandra et al. 1978). Moreover, the fuores-
cence intensity of the compounds 2d and 2i bearing ‒CH3
on 5-position of benzimidazole was the strongest, but Φ
was the lowest. This possible reason was that the interac-
tion between methyl group and triphenylanilyl group on
benzimidazole in ethanol solvent increased the character-
istic absorption peak intensity of coupounds 2d and 2i.
That is, this interaction caused the absorbance of com-
pounds 2d and 2i to increase (Fig. 2), which further led to
Φ decreasing (Table 5, entries 4 and 9). It is notable that
although the synthesized compound 2f, 2h, 2j had good
fuorescence properties, the overall fuorescence quantum
yield not reach the expectation. This maybe lied in the fact
that the C–C bond from triphenylamine molecule to the
imidazole ring increases the degree of rotational freedom
of the molecule, resulting in triphenylamine twist out of
benzimidazole plane (Zhao et al. 2006). Therefore, the
poor coplanarity of benzimidazole ring and triphenylamine
might decrease the degree of the conjugation of the mol-
ecule and prevent the circulation of π-electrons.
λ
Entry
Compound
λmax (nm)
λemmax (nm)
Φa
1
2
3
4
5
6
7
8
9
2a
2b
2c
2d
2e
2f
352
357
358
353
358
371
369
375
373
377
424
435
434
421
434
415
421
422
414
432
0.235
0.258
0.226
0.132
0.209
0.562
0.413
0.393
0.243
0.437
2g
2h
2i
10
2j
aΦ
was
calculated
by
means
of
the
equation,
n2
Astd
Ax
FX
x
namely,Φx =
×
×
× Φstd, where x, represents sample to
n2
Fstd
std
be tested, std represents standard sample (quinine sulfate), n repre-
sents the refractive index, A represents the absorbance, and F repre-
sents the fuorescence intensity
di-branched compounds 2f‒2j also followed the same rule
as mono-branched compounds 2a‒2e. However, the emis-
sion wavelengths of compounds 2f‒2j were blue-shifted
compared with that of the corresponding compounds 2a‒2e.
This result may be due to the planarity of the synthesized
compounds.
Besides, the fuorescence quantum yield (Φ) was meas-
ured and calculated using quinine sulfate as a standard
according to the literatures (Gill et al. 1969; Fletcher
1969). The obtained fluorescence quantum yields of
the products are listed in Table 5. The Φ of compound
2f (Table 5, entry 6) was highest (Φ = 0.562) among the
synthesized compound 2a‒2j, and the Φ of the rest com-
pounds were in the range of 0.132–0.437. Similarly, it
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Chemical Papers
Fig. 3 The emission spectra of 2a‒2e and 2f‒2j in ethanol solvent (1.0×10−6 mol/L (λex= 310 nm))
Shanghai Aladdin Biochemical Technology Co., Ltd, and
was not further purifed before being used. The other sol-
vents and reagents used were supplied by Tianjin Tiantai
Chemical Co. Ltd (China) and Sinopharm Chemical Rea-
gent Co., Ltd. All melting points were determined on an
XT-4 melting point apparatus (China) and were uncorrected.
HRMS was obtained using an US Agilent1290–micrOTOF
Q II spectrometer. 1H and 13C NMR spectra were recorded
using a Bruker AVANCE-500 or 600 NMR spectrom-
eter (Germany) and with TMS as an internal standard.
FT-IR spectra were measured, KBr pellets as a reference,
using a Shimadzu IRAfnity-1 instrument in the range of
500–4000 cm–1. A XO-50 N microwave reactor with a ther-
mocouple thermometer purchased from Nanjing Xianou
instruments Manufacture Company was used to synthesize
the target products. Fluorescence spectra were measured
with a FLS920 spectrofuorimeter (Edinburgh Instruments.
UK). The absorption spectra were measured by Shimadzu
UV-3600 within the wavelength range from 270 to 450 nm.
Conclusions
In summary, the synthesis of triphenylamine substituted
mono- and di-branched benzimidazole derivatives was
investigated and compared under microwave irradiation
solvent-free and solvent (DMF) conditions in the present of
1 equivalent Na2S2O5. The results showed that the micro-
wave-assisted liquid phase method had better selectivity
and yield. Then, ten desired triphenylamine–benzimidazole
derivatives were successfully synthesized in 20–86% yield.
The possible reaction mechanism was proposed. And on this
basis, the catalyst PTSA (0.1 equivalent) was introduced to
catalyze the reaction. Not only the yields of the obtained
target products were signifcantly increased (yield: 41–94%),
but also the reaction time was obviously reduced. It sug-
gested that PTSA was a highly efcient and environmentally
friendly catalyst. Further, the luminescent properties of the
synthesized compounds were compared. The results showed
that di-branched compounds 2f‒2i were marked red-shifted
12–19 nm in absorption spectrum and displayed a signifcant
enhancement (maximum increase is 0.327) for Φ when com-
pared with mono-branched compounds 2a‒2e. It suggested
that the successful introduction of multiple benzimidazole
rings enhance the degree of conjugate of the molecule and
improved fuorescence performance. Moreover, the design,
synthesis and performance measurement of other benzimi-
dazole derivatives are going.
Synthesis
General synthesis approach for triphenylamine’s
monoaldehyde, dialdehyde and trialdehyde (1a, 1b)
Compounds 1a, 1b were prepared referring to literature
(Chakradhar et al. 2009). Firstly, Phosphorus oxychloride
(15 mL 0.16 mol) was added dropwise to N,N-dimeth-
ylformamide (DMF) according to a certain proportion
(n(POCl3)/n(DMF) = 1:3, 2:3) under an ice-water bath condi-
tion and continuous magnetic stirring. Then, triphenylamine
(5.00 g 0.02 mol) was added to the above reaction mixture,
and was stirred for 1 h at room temperature. Sequentially, the
reaction mixture was then heated to 98 °C in oil bath until
TLC indicated the reaction end. The residue was cooled to
Experimental section
Materials and methods
Triphenylamine and various substituted o-phenylenediamine
were from Shanghai Darui Chemical Co., Ltd, China and
1 3

Chemical Papers
room temperature and was poured slowly into iced water
(150 mL). A large amount of solid particles were precip-
itated when the obtained mixture was adjust pH= 7 with
sodium hydroxide solution. Finally, the crude products
were fltered, washed, dried and then was further purifed
with column chromatography (silica gel, 200–300 mesh) to
generate 4-(diphenylamino)benzaldehyde (1a, yield: 82%),
4,4′-(phenylazanediyl)dibenzaldehyde (1b, yield: 62%),
respectively.
crystallize. Finally, the desired products 2a‒2j were obtained
by purifed with column chromatography (silica gel, 200–300
mesh, the eluant: petroleum ether/ethyl acetate=3/1 (except
for carboxyl substituted product, frstly, removal of impurities
with the eluant: petroleum ether/ethyl acetate=1/1. Then, the
target product, the eluant: chloroform/methanol=10/1.).
Characterization data of synthesized compounds
4‑(1H‑benzo[d]imidazol‑2‑yl)‑N,N‑diphenylaniline (2a)
General synthesis approach
for triphenylamine‑benzimidazole under solvent‑free
and microwave irradiation conditions.
1H NMR (500 MHz, DMSO-d6) δ (ppm) 12.79 (s, 1H;
N–H), 8.05 (d, J=8.7 Hz, 2H; Ar–H), 7.55 (s, 2H; Ar–H),
7.37 (m, 4H; Ar–H), 7.17 (dd, J=5.9, 3.1 Hz, 2H; Ar–H),
7.15–7.11 (m, 6H; Ar–H), 7.05 (d, J=8.7 Hz, 2H; Ar–H);
13C NMR (126 MHz, DMSO-d6) δ (ppm) 151.62, 149.24,
147.08, 130.23, 128.14, 125.40, 124.43, 123.62, 122.31,
122.04, 40.00. IR (KBr) (ν/cm–1) 3137, 3029, 1612, 1593,
1454, 1403, 1325, 1277, 839, 748, 694. HRMS (EI) m/z
[M+H]+ Calcd for C25H19N3: 362.1657, found: 362.1695.
A mixture of dry silica gel (1.00 g), 3-(diphenylamino)ben-
zaldehyde (1.05 mmol), substituted o-phenylenediamine
(1.00 mmol), Na2S2O5 (1.00 mmol) was fully ground in a
mortar, then transferred to a 50 mL dried round-bottomed
fask and heated with microwave irradiation for 20–25 min
(the reaction heating power was 300 W). The reaction
progress was monitored by TLC. When the microwave-
assisted reaction was over, the residue was cooled to room
temperature and purifed by column chromatography (silica
gel, 200–300 mesh, the eluant: petroleum ether/ethyl ace-
tate = 8/1 (except for carboxyl substituted product, frstly,
removal of impurities with the eluant: petroleum ether/ethyl
acetate = 1/1. Then, the target product, the eluant: dichlo-
romethane/methanol=10/1.) to generate 4-(6-substituted-
1H-benzo[d]imidazol-2-yl)-N,N-diphenylanilines.
4‑(5‑chloro‑1H‑benzo[d]imidazol‑2‑yl)‑N,N‑diphenylaniline
(2b)
1H NMR (300 MHz, DMSO-d6) δ (ppm) 12.93 (s, 1H;
N–H), 8.04 (d, J=8.8 Hz, 2H; Ar–H), 7.66 (s, 1H; Ar–H),
7.51 (s, 1H; Ar–H), 7.37 (m, 4H; Ar–H), 7.21–7.10 (m, 7H;
Ar–H), 7.04 (d, J=8.8 Hz, 2H; Ar–H); 13C NMR (75 MHz,
DMSO-d6) δ (ppm) 149.07, 146.54, 129.75, 125.04, 122.67,
121.36, 39.59. IR (KBr) (ν/cm–1) 3290, 3152, 3030, 1590,
1490, 1446, 1328, 1271, 1183, 1109, 835, 802, 750, 693.
HRMS (EI): Calcd for C25H18ClN3 [M + H]+ 396.1268,
found 396.1285.
General synthesis approach
for triphenylamine‑benzimidazole in DMF solvent
under microwave irradiation
A mixture of 4-(diphenylamino)benzaldehyde (1.05 mmol)
or 4,4′-(phenylazanediyl)dibenzaldehyde (1.05 mmol), and
substituted o-phenylenediamine (1.00 mmol or 2.00 mmol) in
DMF solvent (15 mL) was fully transferred to a 250 mL micro-
wave reaction bottle equipped with a magnetic stir bar, refux
condenser and thermocouple thermo element. Then, Na2S2O5
(1 mmol) was added to the above mixture for synthesizing
compounds 2a‒2e while 2 mmol Na2S2O5 was required for
synthesizing compounds 2f‒2j and heated with microwaved
at 98 °C for 20–30 min. However, when PTSA is used as a
catalyst, only 0.1 mmol PTSA was added to the above mixture
for synthesizing compounds 2a‒2j and heated with micro-
wave irradiation at 120 °C for 15–25 min. The reaction was
monitored by TLC. The residue was cooled to room tempera-
ture, and then poured into beaker flled with 80–100 mL of
ice water to gain crude product until it was completely pre-
cipitated. Further, the crude product was collected by vacuum
fltration, washed with water (3×25 mL) and dried. The slurry
was extracted with ethyl acetate if the crude product didn’t
4‑(5‑bromo‑1H‑benzo[d]imidazol‑2‑yl)‑N,N‑diphenylaniline
(2c)
1H NMR (600 MHz, DMSO-d6) δ (ppm) 12.94 (s, 1H; N–H),
8.04 (d, J=8.8 Hz, 2H; Ar–H), 7.72 (dd, J=5.7, 3.3 Hz, 1H;
Ar–H), 7.67 (dd, J=5.7, 3.3 Hz, 1H; Ar–H), 7.40–7.34 (m,
4H; Ar–H), 7.30 (dd, J=8.5, 1.8 Hz, 1H; Ar–H), 7.17–7.07
(m, 6H; Ar–H), 7.04 (d, J=8.8 Hz, 2H; Ar–H); 13C NMR
(151 MHz, DMSO-d6) δ (ppm) 152.93, 149.55, 146.98,
130.26, 129.12, 128.31, 125.54, 125.06, 124.57, 122.98,
121.78. IR (KBr) (ν/cm−1) 3322, 3112, 3031, 1587, 1484,
1325, 1287, 1183, 1119, 846, 753, 687, 510. HRMS (EI):
Calcd for C25H18BrN3 [M+H]+ 440.0762, found 440.0793.
4‑(5‑methyl‑1H‑benzo[d]imidazol‑2‑yl)‑N,N‑diphenylaniline
(2d)
1H NMR (600 MHz, DMSO-d6) δ (ppm) 12.58 (s, 1H;
N–H), 8.03 (d, J= 8.8 Hz, 2H; Ar–H), 7.49–7.27 (m, 6H;
1 3

Chemical Papers
Ar–H), 7.15–7.07 (m, 6H; Ar–H), 7.06–7.02 (m, 2H; Ar–H),
6.99 (d, J=8.2 Hz, 1H; Ar–H), 2.41 (s, 3H; CH3); 13C NMR
(151 MHz, DMSO-d6) δ (ppm) 151.38, 149.01, 147.13,
130.20, 127.98, 125.28, 124.32, 124.07, 122.23, 40.14,
21.80. IR (KBr) (ν/cm–1) 3146, 3034, 2961, 2870, 1587,
1487, 1443, 1394, 1325, 1277, 1192, 1110, 845, 743, 687.
HRMS (EI): Calcd for C26H21N3[M+H]+ 376.1814, found
376.1839.
4‑(5‑bromo‑1H‑benzo[d]
imidazol‑2‑yl)‑N‑(4‑(6‑bromo‑1H‑benzo[d]imidazol‑2‑yl)
phenyl)‑N‑phenylaniline (2h)
1H NMR (500 MHz, DMSO-d6) δ (ppm) 13.01 (s, 2H;
N–H), 8.11 (d, J= 8.7 Hz, 4H; Ar–H), 7.75 (s, 2H; Ar–H),
7.53 (d, J = 8.5 Hz, 2H; Ar–H), 7.43 (t, J = 7.9 Hz, 2H;
Ar–H), 7.34–7.30 (m, 2H; Ar–H), 7.24–7.17 (m, 7H;
Ar–H); 13C NMR (126 MHz, DMSO-d6) δ (ppm) 152.71,
148.88, 146.50, 130.49, 128.51, 126.25, 125.34, 124.22,
123.64, 123.25, 121.96, 114.55. IR (KBr) (ν/cm−1) 3328,
3137, 3029, 1594, 1487, 1324, 1271, 1178, 1125, 840, 743,
694, 509. HRMS (EI): Calcd for C32H21Br2N5 [M + H]+
634.0242, found 634.0230.
2‑(4‑(diphenylamino)phenyl)‑1H‑benzo[d]
imidazole‑5‑carboxylicacid (2e)
1H NMR (600 MHz, DMSO-d6) δ (ppm) 13.18 (s, 1H;
N–H), 12.25 (s, 1H; COOH) 8.09 (d, J = 8.8 Hz, 2H),
7.82 (d, J=8.5 Hz, 1H), 7.59 (s, 1H), 7.40–7.36 (m, 3H),
7.19–7.10 (m, 6H), 7.09–6.98 (m, 3H), 6.49 (d, J=8.1 Hz,
1H); 13C NMR (151 MHz, DMSO-d6) δ (ppm) 168.59,
149.63, 146.99, 140.56, 134.13, 130.28, 128.46, 125.58,
124.60, 123.09, 121.73, 120.79, 115.78, 113.09. IR (KBr)
(ν/cm−1) 3132, 3019, 1677, 1614, 1587, 1487, 1327, 1287,
1193, 1147, 949, 830, 753, 697. HRMS (EI): Calcd for
C26H19N3O2 [M+H]+ 406.1556, found 406.1590.
4‑(5‑methyl‑1H‑benzo[d]
imidazol‑2‑yl)‑N‑(4‑(6‑methyl‑1H‑benzo[d]imidazol‑2‑yl)
phenyl)‑N‑phenylaniline (2i)
1H NMR (600 MHz, DMSO-d6) δ (ppm) 12.65 (s, 2H;
N–H), 8.11–8.08 (m, 4H; Ar–H), 7.51–7.30 (m, 6H;
Ar–H), 7.21–7.13 (m, 7H; Ar–H), 7.01 (d, J = 8.2 Hz, 2H;
Ar–H), 2.43 (s, 6H; CH3); 13C NMR (151 MHz, DMSO-
d6) δ (ppm) 148.43, 146.73, 130.40, 128.13, 125.91,
125.11, 124.97, 123.68, 39.99, 21.80. IR (KBr) (ν/cm−1)
3298, 3029, 2961, 2857, 1609, 1590, 1481, 1443, 1389,
1320, 1277, 1184, 1119, 837, 753, 697. HRMS (EI): Calcd
for C34H27N5 [M + H]+ 505.2266, found 505.2287.
N‑(4‑(1H‑benzo[d]imidazol‑2‑yl)phenyl)‑4‑(1H‑benzo[d]
imidazol‑2‑yl)‑N‑phenylaniline (2f)
1H NMR (500 MHz, DMSO-d6) δ (ppm) 12.84 (s, 2H;
N–H), 8.16–8.10 (m, 4H; Ar–H), 7.61–7.54 (m, 4H; Ar–H),
7.43 (t, J=7.9 Hz, 2H; Ar–H), 7.22–7.17 (m, 11H; Ar–H);
13C NMR (126 MHz, DMSO-d6) δ (ppm) 151.49, 148.61,
146.67, 130.44, 129.12, 128.30, 126.06, 125.11, 124.85,
123.67, 122.38. IR (KBr) (ν/cm−1) 3131, 3029, 1590, 1492,
1398, 1321, 1276, 1168, 1114, 835, 742, 678. HRMS (EI):
Calcd for C21H15NO3 [M+H]+ 478.2032, found 478.5335.
2‑(4‑((4‑(6‑carboxy‑1H‑benzo[d]imidazol‑2‑yl)
phenyl)(phenyl)amino)phenyl)‑1H‑benzo[d]
imidazole‑5‑carboxylicacid (2j)
1H NMR (500 MHz, DMSO-d6) δ (ppm) 8.17 (d,
J = 8.5 Hz, 6H; Ar–H), 7.83 (d, J = 8.0 Hz, 2H; Ar–H),
7.55 (d, J = 8.3 Hz, 2H; Ar–H), 7.43 (t, J = 7.8 Hz,
2H; Ar–H), 7.20 (t, J = 7.5 Hz, 7H; Ar–H); 13C NMR
(126 MHz, DMSO-d6) δ (ppm) 153.12, 148.72, 146.62,
130.45, 128.51, 126.14, 125.18, 124.76, 123.95, 123.62.
IR (KBr) (ν/cm−1) 3152, 1594, 1551, 1487, 1400, 1326,
1277, 1184, 1119, 959, 837, 781, 678. HRMS (EI): Calcd
for C34H23N5O4 [M + H]+ 566.1828, found 566.1801.
4‑(5‑chloro‑1H‑benzo[d]
imidazol‑2‑yl)‑N‑(4‑(6‑chloro‑1H‑benzo[d]imidazol‑2‑yl)
phenyl)‑N‑phenylaniline (2g)
1H NMR (500 MHz, DMSO-d6) δ (ppm) 12.93 (s, 2H;
N–H), 8.11 (d, J=8.7 Hz, 2H; Ar–H), 8.02 (d, J=8.8 Hz,
1H; Ar–H), 7.72 (dd, J=5.6, 3.4 Hz, 1H; Ar–H), 7.67 (dd,
J=5.6, 3.4 Hz, 1H; Ar–H), 7.60–7.52 (m, 2H; Ar–H), 7.43
(t, J=7.8 Hz, 1H; Ar–H), 7.36 (d, J=4.6 Hz, 2H; Ar–H),
7.20 (td, J=5.7, 3.3 Hz, 5H; Ar–H), 7.10 (t, J=7.8 Hz, 2H;
Ar–H), 7.01 (d, J=8.8 Hz, 2H; Ar–H); 13C NMR (151 MHz,
DMSO-d6) δ (ppm) 152.91, 148.84, 146.53, 132.47, 130.47,
128.46, 126.66, 125.29, 124.64, 124.36, 123.63, 122.57,
122.02. IR (KBr) (ν/cm−1) 3333, 3147, 3030, 1595, 1492,
1325, 1277, 1183, 1120, 840, 796, 752, 694. HRMS (EI):
Calcd for C32H21Cl2N5 [M+H]+ 546.1252, found 546.1258.
Acknowledgements We are grateful to Dr. Ch. Y. Wang for NMR
spectra, Dr. Zh. L. Wei for Ms spectrum and Dr. J. G. Cao for fuores-
cence spectra.
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Afliations
Jiaxu Fu1 · Liuqing Yan1 · Shuang Wang1 · Hongying Song1 · Qiang Gu1 · Yumin Zhang1
1
* Yumin Zhang
College of Chemistry, Jilin University, Changchun 130012,
People’s Republic of China
zhang_ym@jlu.edu.cn
1 3