The different recognition mechanism of Carbazole derivatives for biological important anion: Theory and experiment

Article abstract of DOI:10.4067/S0717-97072013000300015

Two fluorescent anion sensors bearing phenol, carbazole-NH and -NO ₂ group were designed and synthesized. They both exhibited highly binding ability for H₂PO₄? among the anions tested. In the presence of H₂PO₄?, the fluorescence of the sensors underwent a dramatic enhancement, while the presence of other anions such as F?, AcO?, Cl?, Br?, and I?, had weak or no effect on the fluorescence. The determination limit of two sensors (1 and 2) toward H₂PO₄? were 3.0×10?7 and 5.0×10?7 molL-1, respectively. Theoretical investigation indicated it was the highest HOMO to induce the red-shift phenomena of two sensors.

Full text of DOI:10.4067/S0717-97072013000300015

J. Chil. Chem. Soc., 58, Nº 3 (2013)
THE DIFFERENT RECOGNITION MECHANISM OF CARBAZOLE DERIVATIVES FOR BIOLOGICAL
IMPORTANT ANION: THEORY AND EXPERIMENT
XUEFANG SHANGa*, HONGWEI WUa, SHENYU JIAb, JIE HANb, YUN LIUb, ZHENYA YINb, XIAOJIAO QINb,
JINLIAN ZHANGb, XIUFANG XUc
aDepartment of Chemistry, Xinxiang Medical University, Jinsui Road 601, Xinxiang, Henan 453003 China
bSchool of Pharmacy, Xinxiang Medical University, Jinsui Road 601, Xinxiang, Henan 453003 China
cDepartment of Chemistry, Nankai University, Tianjin 300071 China
(Received: September 10, 2012 - Accepted: April 14, 2013)
ABSTRACT
Two fluorescent anion sensors bearing phenol, carbazole-NH and –NO2 group were designed and synthesized. They both exhibited highly binding ability for
H2PO4− among the anions tested. In the presence of H2PO4−, the fluorescence of the sensors underwent a dramatic enhancement, while the presence of other anions
such as F−, AcO−, Cl−, Br−, and I−, had weak or no effect on the fluorescence. The determination limit of two sensors (1 and 2) toward H2PO4− were 3.0×10−7 and
5.0×10−7 mol·L-1, respectively. Theoretical investigation indicated it was the highest HOMO to induce the red-shift phenomena of two sensors.
Keywords: anion sensor; determination limit; fluorescence; theoretical investigation
1. INTRODUCTION
The development of highly selective and sensitive chemosensors for
biological important anions is a very active research field in supramolecular
chemistry 1-10. In particular, H2PO4−-sensitive systems have attracted much
interest, as the H PO4− ion plays critical roles in a variety of fundamental
processes such as 2energy transduction, signal processing, genetic information
storage, and membrane transport 11. In addition, phosphates can be found
in many chemotherapeutic and antiviral drugs. It is for sure that phosphate
originating from the over use of agricultural fertilizers can also lead to
eutrophication in inland waterways. Thus, the rational design and synthesis of
efficient sensors to selectively recognize H2PO4- analytes is a fundamental goal
for supramolecular chemistry.
Accordingly, a variety of sensing systems, involving amide, urea (thiourea),
hydrazine, calix[n]arenes, hydroxyl derivatives have been employed for the
selective detection of H2PO4- 12-15. While phenol commonly plays an important
role in the anion binding site of biological system, very little investigation
has been carried out to develop H2PO4− sensors bearing phenol and carbazole
groups.
To our knowledge, the determination limit of the receptor towards anions
Scheme 1. Molecular structure of compound 1 and 2
were rarely reported although it has been widely researched in the cation
recognition 16, 17. While it is essential to discuss the determination limit of the
receptor towards anions which could provide quantitative basis for the further
application in environment, medicine and biology.
2. MATERIAL AND METHODS
Most of the starting materials were obtained commercially and all
reagents and solvents used were of analytical grade. All anions, in the form of
tetrabutylammonium salts [such as (n-C4H9)4NF, (n-C4H )4NCl, (n-C H9)4NBr,
(n-C H9)4NI, (n-C4H9)4NAcO, (n-C4H9) NH2PO4], were p9urchased fro4m Sigma-
Aldr4ich Chemical Co., and stored in 4a desiccator under vacuum, and used
without any further purification. Tetra-n-butylammonium salts were dried for
24 h in vacuum with P O5 at 333 K. Dimethyl sulfoxide (DMSO) was distilled
in vacuum after dried 2with CaH2. C, H, N elemental analyses were made on
Vanio-EL. 1H NMR spectra were recorded on a Varian UNITY Plus-400 MHz
Spectrometer. ESI-MS was performed with a MARINER apparatus. UV-vis
titration experiments were made on a Shimadzu UV2550 Spectrophotometer at
298 K. Fluoremetric titrations were performed on a Cary Eclipse Fluorescence
Spectrophotometer at 298 K. The binding constant, K , was obtained by non-
linear least squares calculation method for data fitting.s
Here, we reported the synthesis, anion binding ability and determination
limit of fluorescent sensors (1 and 2) containing phenol and carbazole groups
(Scheme 1). The strategy employed in the design of sensors was as following.
Firstly, we introduced carbazole and phenol groups, as a fluophore and binding
site, into the same sensor molecule which aroused strong fluorescent response.
Secondly, to achieve “naked-eye” recognition upon binding of H2PO4−, we
introduced nitro group as a chromophore.
3, 6-Dichloro-1, 8-diaminocarbazole was synthesized according to
literature 18
.
N, N’-Di(1’-methylimino-2’-hydroxyl-3’-nitrobenzene)-3, 6-dichloro-1,8-
diamino carbazole (1)
3, 6-Dichloro-1, 8-diaminocarbazole (1 mmol, 265 mg) and
3-nitrosalicylaldehyde (2 mmol, 334 mg) were suspended in 35 mL ethanol.
The mixture was heated under refluxing for 4 h. The yellow precipitate was
separated by filtration. The solid was washed with diethyl ether and dried under
vacuum. 1H NMR (DMSO-d6): δ 13.16 (s, 2H, OH), 12.09 (s, 1H, NH), 9.21(s,
2H), 8.32 (d, 3H), 8.16 (dd, 3H), 7.58 (s, 2H), 7.21 (t, 2H). Elemental analysis:
e-mail: xuefangshang@126.com
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J. Chil. Chem. Soc., 58, Nº 3 (2013)
Calc. for C26H15Cl2N5O6: C, 55.34; H, 2.68; N, 12.41%; Found: C, 55.42; H,
2.77; N, 12.09%. ESI-MS (m/z): 562.2.
N, N’-Di(1’-methylimino-2’-hydroxyl-5’-nitrobenzene)-3, 6-dichloro-1,8-
diamino carbazole (2)
It was synthesized according to the above procedure. 1H NMR (DMSO-d6):
δ 13.06 (s, 2H, OH), 12.01 (s, 1H, NH), 9.20 (s, 2H), 8.81 (s, 2H), 8.31 (d, 4H),
7.51 (s, 2H), 7.22 (d, 2H). Elemental analysis: Calc. for C26H15Cl2N5O6: C,
55.34; H, 2.68; N, 12.41%; Found: C, 55.18; H, 2.96; N, 12.74%. ESI-MS
(m/z): 562.1.
3. RESULTS AND DISCUSSION
3.1 UV-vis titration
The anion sensing ability of compound 1 was investigated in DMSO
through UV–vis spectroscopy. The changes in UV–vis spectra of compound 1
(4.0×10-5 mol·L-1) upon the titration with H PO4− were shown in Figure 1. The
absorption spectrum of free 1 exhibited two2broad absorption bands centered at
325 nm and 425 nm, which could be assigned to the excitation of π electrons in
the carbazole system and the intramolecular charge-transfer transitions within
the whole structure of compound 1, respectively 19. With the stepwise addition
of H PO4− ion to the solution of compound 1, the absorbance intensity at 325 nm
was 2reduced and the absorbance band at 425 nm was increased. Simultaneously
the absorbance band at 425 nm shifted to the long wavelength at 500 nm
(Δλ=75 nm), and the color of the solution changed from yellow to orange-
red. After the interation between compounds and phosphate, a small quantity
of water was added to the complexation. Interestingly, the addition of protic
solvent (such as H2O or ethanol) to the solution of complexation (1-H PO −)
made the orange-red solution change back to the yellow, which suggest2ed t4he
interaction between compound 1 and H2PO4− was hydrogen bond in essence 20,
21. Particularly, the presence of F− and AcO− induced similar changes in UV-vis
spectrum of 1 compared with H2PO4−, but the addition of excess equiv of Cl−,
Br− and I− ions resulted in slight changes in the UV–vis spectrum of compound
1 (Figure 2), which indicated compound 1 showed different binding ability
with F−, H2PO4−, almost no or very weak binding ability with Cl−, Br− and I−.
Figure 2. Changes in intensity of the absorption band (500 nm) in
compound 1 (4.0×10-5 mol·L-1) in presence of anions (8.0×10-4 mol·L-1) tested.
As for free compound 2, the strong absorption at 325 nm and 425 nm
appeared (Figure 1). With the addition of H2PO4− ion, the absorption intensity
at 325 nm weakened and a small sharp absorption peak at 375 nm developed
gradually. At the same time the absorption intensity at 425 nm strengthened
and slightly shifted to the long wavelength (450 nm, ∆λ=20 nm). Clearly, red-
shift phenomenon occurred between compound 2 and H2PO −. In addition, one
isosbestic point at 360 nm appeared indicating the stable c4omplex formed
.
22
The presence of AcO− and F− induced similar changes in UV-vis spectrum
of 2, but the additions of excess equiv of Cl−, Br− and I− ions resulted in
slight changes in the UV–vis spectrum of 2, which indicated the very weak
interaction and could be ignored.
3.2 Fluorescence titration
The photophysical responses of compound 1 toward the addition of
anions tested was also investigated in DMSO. Just as Figure 3 showed, upon
the addition of H2PO − ion (0-10 equiv) to the solution of compound 1, the
fluorescence centered4 at 390 nm when excited at 355 nm got enhanced. To
account for such fluorescence enhancement, the reason was followed: Firstly,
before coordination with H2PO4− ion, the hydrogen atoms of the –OH group
of free 1 could form an intramolecular hydrogen bond with the oxygen atom
of NO2, which resulted in a photoinduced electron transfer (PET), and the
de-excitation of the resulting tautomer occurred mainly via a nonradiative
pathway. These processes consequently led to the weak fluorescence of 1.
Therefore, an enhancement of the fluorescence emission of 1 was observed.
Secondly, the configuration of compound 1 was flexible and could rotate
freely. Upon complexation with H2PO − ion, the host molecule 1 was rigidified,
which gave birth to a large increase in4emission intensity because of inhibiting
vibrational and rotational relaxation modes of nonradiative decay. In addition,
the addition of F− and AcO− resulted in similar fluorescent changes compared to
that of H2PO4−. Nevertheless, fluorescence emission of sensor 1 was insensitive
to the addition of excess equiv of H PO4−, Cl−, Br− and I− ions (Figure 4), which
indicated the very weak interaction2and could be ignored.
Figure 1. UV-vis spectral changes of compounds (4.0×10-5 mol·L-1) upon
the addition of H2PO4− (0~100×10-5mol·L-1), a) 1; b) 2. Arrows indicated the
increased direction of H2PO4− concentration.
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J. Chil. Chem. Soc., 58, Nº 3 (2013)
binding sites 27. As expected from their basicity, H PO4−, AcO−, and F− will
bind more strongly than the other anions studied; in2addition, the tetrahedron
configuration of H2PO4− ion may well match two compounds in terms of shape
and could form multiple hydrogen bonding interaction. Consequently, H PO4−
ion can be selectively recognized from other anions based on binding con2stant.
For the same anion, the binding ability of compound 1 containing 3-NO2 was
stronger than that of compound 2 containing 5-NO2. The reason may be the
intramolecular hydrogen bond could improve the acidity of receptor and the
anion binding ability could be strengthened. Moreover, the binding constants
obtained by UV-vis data were in the same order of magnitude with it obtained
by fluorescence data, which indicated the results of binding constants obtained
by UV-vis data were proved by fluorescence.
Table 1. Binding constants of compound 1 and 2 with various anions.
Aniona
Ks(1)
Ks(2)
Absorption
Emission
(6.15±0.19)×104
(7.58±0.21)×104
(2.52±0.72)×104
(3.36±0.45)×104
(1.99±0.64)×104
(2.11±0.32)×104
(3.13±0.68)×104
(4.54±0.21)×104
(2.19±0.75)×104
(2.47±0.33)×104
(1.22±0.56)×104
(0.97±0.46)×104
H2PO4−
Figure 3. Changes in the emission spectra of compounds (4.0×10-5
mol·L-1) in absence and presence of H PO4− (0~80×10-5mol·L-1), a) compound
1, λex=355 nm; b) compound 2, λex=3255 nm. Arrows indicated the increased
direction of H2PO4− concentration.
Absorption
Emission
AcO−
F−
Absorption
Emission
Cl− (Br− or I−)
NDb
ND
a
All anions were added in the form of tetra-n-butylammonium (TBA)
salts.
b The binding constant could not be determined.
3.4 Interference experiment
The binding ability of compound 1 with H PO4− ion was the strongest
among the studied anions according to the bi2nding constant. The above
results derived from the condition that only one anion existed. We conduct the
interference experiment whether the binding ability of H PO4− was influenced
by other anions (Figure 5). In Fig. 5, the concentratio2n of compound 1 is
4.0×10-5 mol·L-1. According to UV-vis experimental data, when various anions
(F-, AcO- and H2PO4-, 8.0×10-5 mol·L-1 ) were added seperately, the intensity of
spetral response of compound 1 were different. As shown in Fig. 5, the spectral
response of 1 upon the addition of the mixed anions was almost as the same
as the addition of H2PO4− which indicated the binding ability of H2PO4− with
compound 1 was not interfered by the existence of other anions. Similarly, the
same result also existed in the interaction of compound 2 with H2PO4−. This
point has experimental and practical importance.
Figure 4. Changes in intensity of the emission band at 392 nm in compound
1 (4.0×10-5 mol·L-1) in presence of anions (4.0×10-4 mol·L-1) tested.
For compound 2, the spectral changes induced by anions were clearly
different from compound 1 (Figure 3). Two emission peaks at 392 nm and
460 nm appeared for free compound 2. As the concention of H2PO4− was 0.5
equiv, the intensity of emission peak at 392 nm increased to maximum and
decreased to minimum for the emission peak at 460 nm. With the concentration
of H2PO − increased stepwise (1-10 equiv), the intensity of emission peak at
392 nm 4decreased and the intensity of emission peak at 460 nm increased
gradually. To account for such fluorescence phenomenon, the ICT mechanism
was exploited. Upon interaction with H2PO4−, the reduction potential of
compound 2 was enhanced, namely the electron transfer from the electron
rich phenol moiety bonded with H2PO4− to the electron deficient -NO2 moiety
became more feasible. Upon further addition of H2PO4−, it appeared that the
deprotonated species LH−, being more electron rich compared to the hydrogen-
23
bonded complex with H2PO4−, activated the ICT process more efficiently
.
Compared with compound 1, the different interacted mechanism was induced
by the different site which the nitro group existed. There were similar changes
in the fluorescence emission of 2 induced by addition of AcO− and F− ions
with those of H2PO4−. Nevertheless, fluorescence emission of compound 2 was
insensitive to addition of excess equiv Cl−, Br− and I− ions.
3.3 Binding constant
The job-plot analysis indicated the spectral change could be ascribed
to the formation of 1:1 host-guest complexation. The obtained binding
constants were listed in Table 1 using the method of non-linear least squares
calculation according to the UV–vis and fluorescence data 24-26. The selectivity
trend of binding ability of two compounds to anions followed the order of:
H2PO − > AcO− > F− >> Cl− ~ Br− ~ I−. It was apparent that the selectivity
for sp4ecific anions can be rationalized on the basis of the anion’s basicity and
the interactions between the host and the anionic guests. However, multiple
hydrogen-bond interactions were also necessary in high-affinity anion
Figure 5. UV-vis spectral changes of compound 1 (4.0×10-5 mol·L-1) upon
the addition of anions (8.0×10-5 mol·L-1).
3.5 Determination limit
In cation recognition, determination limit was reported by many literatures
16, 17. According to these reported literatures, the intensity in absorption apectra
of compounds is lineal at low cationic concentration. While, determination
limit of anion was almost reported few. Thus, the determination limit of
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J. Chil. Chem. Soc., 58, Nº 3 (2013)
compound 1 to H2PO4− was studied (Figure 6). The interaction of compound
1 with H2PO4− could be detected down to at least concentration of 3.0 × 10−7
mol·L-1, at this time the concentration of H PO − ion was 2.3 × 10−7 mol·L-1.
And the corresponding absorption intensi2ty i4ncrease to 1.3 times, which
showed compound 1 could potentially be used as a sensor for monitoring
H2PO4− levels in physiological and environmental systems. Similar experiment
indicated the interaction of compound 2 with H2PO4− could be detected down
to at least concentration of 5.0 × 10−7 mol·L-1, at this time the concentration of
H2PO4− ion was 2.1 × 10−7 mol·L-1.
Figure 7. Partial 1H NMR spectrum of compound 1 and 2 upon the
addition of F−.
3.7 Theoretical investigation
The geometries of compound 1 and 2 were optimized (Fig. 8) using
density functional theory at B3LYP/3-21G level with Gaussian03 program
30. From Fig. 8, The interacted sites existed in the form of hydroxyl groups
of compound 1. However, one hydroxyl group in compound 2 existed in the
form of ketone. The structural difference of two compounds determined the
different mechanism in anion recognition. The intramolecular hydrogen bonds
also existed between carbozale-NH and hydroxyl group in benzene ring or
hydroxyl group and the nitrogen atom.
Figure 6. UV-vis intensity of compound 1 (3.0×10−7 mol·L-1) as a function
of concentrations of F− (0-30 μmol·L-1).
3.6 1H NMR titration
To further shed light on the nature of the interaction between two
1
compounds and anions, H NMR spectral changes upon the addition of F−
as their tetrabutylammonium salts to the DMSO-d6 solution of 1, 2 (1×10−2
mol·L−1) were investigated. Obviously observed from Fig. 7, the peak of
compound 1 at 13.16 ppm, which was assigned to −OH, broadened, exhibited
a slightly downfield shift and thoroughly disappeared with the increase of F−
ion. At the same time, the peak at 12.09 ppm which were assigned to −NH,
broadened gradually and other chemical shifts of proton peaks almost did not
shift indicating the strong hydrogen-bonding interactions occurred between
compound 1 and F− ion 28. As for compound 2, the peak of –OH and –NH
occurred at 13.06 and 12.01 ppm, respectively. With the stepwise addition
of F–, the peak of –OH broadened, exhibited a slightly downfield shift and
thoroughly disappeared which was the same as compound 1. However, the
peaks of –NH and other protons shifted to the upfield direction gradually. The
reason may be as followed: In compound 2, one hydroxyl group formed the
structure of ketone and could formed intramolecular hydrogen bonds with –NH
and other hydroxyl group which can be proved by theoretical investigation.
With the addition of 0.5 equiv of F– ion, intramolecular hydrogen bond was
broken which induced deshielding effect and the proton peak of –NH group
shifted to the upfield direction. With the further addition of F– ion, hydroxyl
moiety was deprotonated due to the formation of FHF–. The proton peak of
FHF– did not appear which may be related with the small quantity water in
DMSO-d6 29. Other protons exhibited a slight upfield shift which indicated the
increase of the electron density on them owing to the through-bond effects. The
possible binding mode in the solution was showed in Scheme 2.
Figure 8. Optimized geometries of compounds 1 and 2.
In addition, selected frontier orbitals for compound 1 and 2 were shown
in Fig. 9. We introduced molecular frontier orbital in order to explain UV-
vis absorption spectra in the interacted process of host-guest induced electron
transition of frontier orbital. The highest HOMO density in compound 1 and
2 were mainly localized on the carbazole moiety. While, the highest LUMO
density was mainly localized on the one phenyl ring moiety and carbazole
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J. Chil. Chem. Soc., 58, Nº 3 (2013)
moiety which demonstrated it was the electron transition of the highest HOMO
to arouse the red-shift phenomenon in UV-vis spectra of host-guest.
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4. CONCLUSION
In conclusion, two fluorescent anion sensors bearing phenol, carbazole-
NH and –NO group were successfully prepared. The addition of H2PO4− anion
elicited red s2hift phenomenon in UV-vis spectra and a visible enhancement
in the fluorescence emission intensity of sensor 1 owing to a quenching PET
process from the –OH to -NO2 group. However, the ICT mechanism of sensor 2
binding anions was exploited. The different interacted mechanism was induced
by the different site which the nitro group existed. Theoretical investigation
indicated the electron transition of the highest HOMO aroused the red-
shift phenomenon in UV-vis spectra of host-anion. Particularly, the novel
design strategy that the carbazole moiety is incorporated into the fluorescent
p-conjugated system would help to extend the development of fluorescent
anion sensors with the hydroxyl group as anion binding site.
ACKNOWLEDGEMENT
This work was supported by the Nature Science Fund of Henan Province
(2010B150024, 112300410104), Young Teacher Fund of Henan Province
(2011GGJS-126) and Doctorial Starting Fund of Xinxiang Medical University.
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