A mild and catalyst-free conversion of solid phase benzylidenemalononitrile/benzylidenemalonate to N-benzylidene-amine and its application for fluorescence detection of primary alkyl amine vapor

Article abstract of DOI:10.1039/c3cc48299e

A new rapid and catalyst-free solid/vapor reaction between benzylidenemalonate/benzylidenemalononitrile and primary alkyl amines was found. With these as sensory units of fluorescent polymers, probes for primary amine vapor with high sensitivity and selectivity were developed. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc48299e

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A mild and catalyst-free conversion of solid phase
benzylidenemalononitrile/benzylidenemalonate
to N-benzylidene-amine and its application
for fluorescence detection of primary alkyl
amine vapor†
Cite this: Chem. Commun., 2014,
50, 872
Received 30th October 2013,
Accepted 12th November 2013
DOI: 10.1039/c3cc48299e
Liqi Shi,ab Yanyan Fu,a Chao He,a Defeng Zhu,a Yixun Gao,ab Yuerong Wang,ab
Qingguo He,*a Huimin Caoa and Jiangong Cheng*a
www.rsc.org/chemcomm
A new rapid and catalyst-free solid/vapor reaction between benzyl-
idenemalonate/benzylidenemalononitrile and primary alkyl amines was
found. With these as sensory units of fluorescent polymers, probes for
primary amine vapor with high sensitivity and selectivity were developed.
As important intermediates in chemical industry, amines such as
amphetamine derivatives have received great attention for use in
chemical leakage, food security and drug abuse. Methods for
detection of amines have been developed using electrochemistry,1,2
chromatography3 and colorimetry.4–7
Fig. 1 Molecular structures of M1, M2 and the target molecules.
Fluorescent sensors could be easily integrated as very sensitive
In this work, one unusual reaction between the benzyl-
portable devices for real-time vapor-phase detection, which have idenemalonate (BZMA)/benzylidenemalononitrile (BZMN) derivative
been attracting broad interest these days. Vapor detection of solid film and primary amine vapor was found. Within five minutes,
amines by fluorescence methods has been reported via bonding BZMA/BZMN could be converted to benzylidene-propylamine without
interaction8–10 and photoinduced electron transfer.11,12 However, any catalyst (Fig. 1) at room temperature. With these two units as the
many sensory materials tend to respond to all kinds of amines.13 sensing units, fluorescent conjugated copolymers were designed and
Our group developed some methods for detection of secondary synthesized for primary amine vapor sensing. The sensing behavior
amines (for example methamphetamine) and aniline with high was investigated systematically (Fig. 1).
sensitivity and selectivity.14 But until now, no study has been
The reaction was found accidentally when the drop-casted film of
reported that could efficiently differentiate the primary amines dibromophenylamino-benzylidenemalonate (M1) was put aside in
from secondary and tertiary amines by a specific chemical reac- an atmosphere of n-propylamine vapor. 5 min later, the color of the
tion.10 To solve the problem, two aspects will be highly favorable. film changed from bright yellow to pale yellow, and the emission
On the one hand, developing a reaction with high specificity color changed from green to bluish-green. To determine the reason
within a very short reaction time and under mild conditions will behind this phenomenon, the amine exposed film was vacuum
be highly preferred especially for in situ detection. On the other dried to remove the volatile compounds, then the 1H-NMR spectrum
hand, for fluorescence detection, a molecular wire effect and a of the film species dissolved in CDCl3 was recorded. It is observed
large spectral change will be favored for sensitive detection, that the peak attributed to the ethyl units on the ester disappeared
which will be advantageous for the detection by preventing the with the formation of new n-propyl-groups, which is different from
background noise interference.
that of n-propylamine, and the characteristic singlet peak of a proton
located on ethylene shifted from 7.6 to 8.1 ppm reflecting a different
conjugation that reduced the electron density on this H-atom
(Fig. 2). The high resolution mass spectrum (HR-MS) exhibits a
molecular ion peak at 469.9995, which combined with the 13C-NMR
(Fig. S3, ESI†) results proved the formation of the N-benzylidene-
propylamine structure. It is found that only primary alkyl amines
could react with M1. Other amines including secondary amines,
tertiary amines or aniline derivatives were tested and no interaction
a State Key Lab of Transducer Technology, Shanghai Institute of Microsystem and
Information Technology, Chinese Academy of Sciences, Changning Road 865,
Shanghai 200050, China. E-mail: hqg@mail.sim.ac.cn, jgcheng@mail.sim.ac.cn
b Graduate School of the Chinese Academy of Sciences, Yuquan Road 19,
Beijing 100039, China
† Electronic supplementary information (ESI) available: Details of the synthesis
of the monomers and polymers and the spectroscopic properties. See DOI:
10.1039/c3cc48299e
872 | Chem. Commun., 2014, 50, 872--874
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Scheme 1 Syntheses of P0, P1 and P2.
Fig. 2 1H-NMR of monomers M1 and M2, before and after treated with
n-propylamine vapor.
was found between the secondary amines, tertiary amines or aniline
derivatives and M1 under the same conditions within 30 minutes
(Table S2, ESI†).
Fig. 3 SEM images of porous films of polymers P1 and P2.
More interestingly, the same phenomenon was found when M1 was
replaced by dibromophenyl-aminobenzylidenemalononitrile (M2).
Both P1 and P2 tend to form a porous film on a quartz
Upon exposure to n-propylamine, the color of the film changed from substrate by drop-casting its toluene solution onto the quartz
brown-red to pale yellow, and the emission color changed from orange surface after evaporation of a natural solvent, which was
to bluish-green. Similarly, M2 could also only react selectively with observed by SEM (Fig. 3). The bright area is the polymer, and
primary alkyl amines but could not react with secondary, tertiary and the dark region is the substrate. Such a surface morphology led
aromatic amines. The identical 1H-NMR and 13C-NMR data were to enhanced gas permeability and a large surface area to
observed as that of M1. HRMS results further confirm that the same volume ratio, which are advantageous for vapor-phase sensing.
product was produced. It was really unexpected that the carbon–carbon In comparison, under the same conditions, no such porous
double bonds linked with diester or dicyno units were broken to form film could be found for P0, indicating that BZMA/BZMN units
such carbon–nitrogen double bonds of Schiff’s base upon exposure to are critical for the formation of the porous structure.
primary amine vapor under such mild conditions. Such reactions
P1 showed maximum emission at 531 nm corresponding to
usually occur in different ways15,16 and should be conducted in solution yellowish-green fluorescence and P2 showed maximum emission
in the presence of a catalyst and/or under heating conditions in at 573 nm corresponding to orange fluorescence (Fig. S1 and
addition to longer reaction time. The proposed reaction mechanism Table S1, ESI†). Stokes shifts of P1 and P2 are at 151 and 197 nm
is presented in Fig. S2 (ESI†). At present, the detailed mechanism of (52 nm for P0), respectively. A larger Stokes shift could greatly
CQC breakage and the formation of CQN is still unknown.
Actually, for both M1 and M2, there exists intramolecular its larger signal-to-noise ratio and low self-absorption.
charge transfer (ICT) with triphenylamines as electron-donors, To investigate the sensory character of P1 and P2 films, we
improve the sensitivity to trace quantities of analytes because of
and malonates or malononitriles as electron-acceptors. But after tested the changes in emission spectra and time-course fluores-
the reaction with primary amines, malonates or malononitriles cence response in benzylamine (BZA) vapor. BZA was selected
were converted to N-benzylidene-propylamine, which significantly because of its relatively low saturated vapor pressure (672 ppm,
decreased the electron-withdrawing capacity of the acceptors. Such a 15 1C),17 and it could be easily diluted and also prevents the
larger ICT change means a large emission change, which could be liquefaction on polymer films. As shown in Fig. 4, the emission
used for primary amine probe design. Therefore, if M1 or M2 could
be used as sensing units to construct a fluorescent conjugated
polymer probe, it will result in a greater fluorescence signal change
and higher sensitivity due to the molecular wire effect. Therefore,
probe P1/P2 were designed and synthesized by alternative copoly-
merization of M1/M2 with the fluorene unit via a palladium
catalyzed Suzuki coupling reaction (Scheme 1). The synthesis of
the polymers and the characterization data are provided in the ESI.†
P0, with no malonate or malononitrile units, was also synthesized for
comparison. In addition, the reason to choose the fluorene unit is its
well known highly efficient fluorescence and stability.
Fig. 4 Fluorescence spectrum change in P1 (left) and P2 (right) after
exposure to benzylamine and n-propylamine.
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A highly efficient new solid/vapor reaction between BZMA/BZMN
and primary alkyl amines under catalyst free and mild conditions was
found. Two fluorescent copolymer probes for primary amine vapor
sensing were constructed using the two units as sensing units. Both
polymers tend to form a porous film, which is beneficial for vapor
penetration. The polymer with BZMN could trigger a much bigger
emission wavelength change (from orange to bluish-green). The
fluorescence spectra change at low concentration of BZA vapor clearly
presented the chemical reaction kinetics. The special solid/vapor
reaction that triggered large spectrum-shift and fluorescence turn-on
detection represents an efficient way for the design of a highly
Fig. 5 (a) Fluorescent spectrum change of P2 to 100 ppm benzylamine.
1 min interval for each spectrum obtained. (b) Time-course fluorescent
responses of P2 to 200 ppm, 100 ppm, 50 ppm BZA vapor.
peak of P1 at 531 nm blue-shifted to 522 nm and the maximum sensitive and selective fluorescent sensor. And a highly efficient
intensity increased by 45% upon exposure to saturated BZA vapor catalyst-free solid/vapor phase reaction also provided a green and
for 300 s. In comparison, the emission peak of P2 at 573 nm blue economic way to synthesize such Schiff’s base compounds.
shifted to 522 nm and achieved a 7-fold enhanced emission signal
This work was supported by funds from the National Nature
at this wavelength. P2 experienced a much larger emission peak Sciences Foundation of China (No. 51003118, 21273267, and
shift, and the emission color changed from orange to bluish-green, 61325001), the research programs from the Ministry of Science
while that of P1 transformed from green to bluish-green (Fig. 4). and Technology of China (No. 2012BAK06B03), and the Shanghai
The bigger emission wavelength and intensity change could be Science and Technology Committee (No. 11JC1414700).
easily detected by the naked eye (Fig. S4, ESI†) either under
sunlight or UV irradiation, which means P2 would be a much
better probe for primary amine vapor sensing.
Since saturated BZA vapor could trigger a very fast emission
change from P2 to the resulting polymer with Schiff’s base units, to
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874 | Chem. Commun., 2014, 50, 872--874
This journal is ©The Royal Society of Chemistry 2014