Synthesis of a dehydroabietyl derivative bearing a 2-(2'-hydroxyphenyl) benzimidazole unit and its selective Cu2+ chemosensing

Article abstract of DOI:10.3390/molecules16010100

A dehydroabietyl derivative 2 bearing a 2-(2'-hydroxyphenyl)benzimidazole unit was synthesized and its sensing behaviors toward metal ions were investigated by UV-Vis and fluorescence spectroscopy methods. In THF solution, compound 2 exhibited excellent s

Full text of DOI:10.3390/molecules16010100

Molecules 2011, 16, 100-106; doi:10.3390/molecules16010100
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis of a Dehydroabietyl Derivative Bearing a
2-(2′-Hydroxyphenyl) Benzimidazole Unit and Its Selective
Cu2+ Chemosensing
Ying-Chun Wang 1, Lu-Zhi Liu 2, Ying-Ming Pan 2 and Heng-Shan Wang 2,*
1
College of Chemistry and Chemical Engineering, Jishou University, Jishou 416000, Hunan
Province, China; E-Mail: wang_yingchun2010@yahoo.com.cn (Y-C.W.)
2
Key Laboratory of Medicinal Chemical Resources and Molecular Engineering, College of Chemistry
and Chemical Engineering, Guangxi Normal University, Guilin 541004, Guangxi Province, China;
E-Mails: liuluzhi2010@hotmail.com (L.-Z.L.); panym2004@yahoo.com.cn (Y.-M.P.)
* Author to whom correspondence should be addressed; E-Mail: wang_hengshan@yahoo.com.cn;
Tel.: +86-773-212-0958; Fax: +86-773-212-0958
Received: 17 November 2010; in revised form: 9 December 2010 / Accepted: 16 December 2010 /
Published: 28 December 2010
Abstract: A dehydroabietyl derivative 2 bearing a 2-(2′-hydroxyphenyl)benzimidazole
unit was synthesized and its sensing behaviors toward metal ions were investigated by
UV-Vis and fluorescence spectroscopy methods. In THF solution, compound 2 exhibited
excellent selectivity for Cu(II) over miscellaneous other metal ions including Cr(II),
Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Al(III), Mg(II), Pb(II), Hg(II), Na(I), Li(I) and K(I)
evidenced through the quenching of the fluorescence of the benzimidazole fragment. The
reaction between 2 and Cu2+ was found to be stoichiometric with the formation of a
1:1 complex.
Keywords: dehydroabietic acid; absorption; fluorescence; copper; metal-ion recognition
1. Introduction
Recently, the design and synthesis of fluorescent organic compounds for chemical sensors has been
an active research area because of their possible applications in the environmental and bio-analytical
fields [1-4]. 2-(2′-Hydroxyphenyl)benzimidazole and its derivatives are attractive fluorescent
compound candidates that display large Stokes shifts and high fluorescence quantum yields, and

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present great photophysical stability ascribed to an excited state intramolecular proton transfer (ESIPT)
mechanism [5,6]. They have been studied as laser dyes [7], polymer UV-light stabilizers [8] and
fluorescent probes for labelling proteins [9]. Moreover, they are excellent photoluminescent materials.
In recent years, more attention has been paid to their complexes and those of other similar N,O-donor
ligands with metal ions and non-metal ions due to their potential role as electroluminescent materials
in organic light emitting diodes [10,11]. Nevertheless, there have been few reports on the use of the
2-(2′-hydroxyphenyl)benzimidazole unit as a fluorophore and recognition site for metal-ion
recognition in organic media. Here we report the design and synthesis of a dehydroabietyl derivative 2
bearing a 2-(2′-hydroxyphenyl)benzimidazole unit that shows high selectivity towards Cu2+ by the
quenching of the fluorescence of the benzimidazole fragment. The influence of different metal cations
on the fluorescence intensity of compound 2 in THF is also discussed.
2. Results and Discussion
Compound 2 was synthesized as outlined in Scheme 1. The starting material, dehydroabietic acid,
was obtained from commercially disproportionated rosin and purified by repeated crystallisation of the
corresponding ethanolamine salt [12]. The intermediate 12-bromo-13,14-diaminodeisopropyl-
dehydroabietate methyl ester (1), was prepared by sequential esterification, bromation, dinitration and
reduction according to literature procedures [13]. Compound 2, a white powder which has low solubility
in methanol and is readily soluble in chloroform, was prepared in good yields (74.4%) by coupling/
oxidation of 1 with salicylaldehyde in dioxane using air as the oxidant. The salient features of the
coupling/oxidation include a simple procedure, mild conditions, no need for a commercial oxidant/
additive, no waste produced (the only by-product being water) and easy purification. The structure of 2
was confirmed by 1H-NMR, 13C-NMR, MS, IR and elemental analysis.
Scheme 1. Outline of the synthesis route for compound 2.
The influence of different metal cations on the fluorescence intensity of compound 2 was
investigated in THF. As shown in Figure 1, free 2-(2′-hydroxyphenyl)benzimidazole 2 exhibits three
emissions centered at 355, 384 and 448 nm. The fluorescence intensity of the 448 nm band was
significantly quenched and a new peak at 540 nm was observed in the presence of Cu2+. A
fluorescence enhancement was observed in the presence of Mg2+, Co2+, Ni2+, Zn2+ and Cd2+,

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respectively, while no significant spectral changes of 2 occurred in the presence of other cations such
as Li+, Na+, K+, Al3+, Mn2+, Hg2+ and Pb2+. Also, based on the relative fluorescence intensities shown
in Figure 2, it can be concluded that Cu2+ is selectively recognized by 2 among the studied metal ions.
Figure 1. Fluorescence spectra for compound 2 excited at 350 nm in the absence and
presence of different metal cations in THF. The concentrations of 2 and metal ion were
both 1×10−5 mol/L.
120
Co2+, Zn2+, Mg2+, Cd2+, Ni2+
100
2 only, Li+, Na+, K+,
80
Al3+
,
Mn2+, Hg2+, Pb2+
60
40
20
0
Cu2+
350
400
450
500
550
600
wavelength(nm)
Figure 2. Relative fluorescence intensity at the metal ion to compound 2 mole ratio of 2.0
(almost the same even at 1.0) in the titration of 2 with different metal ions.
1.8
1.5
1.2
0.9
0.6
0.3
0.0
+
2+
Na K+
Cu2+
Cd
Zn2+ Co
2+
2+
2
4
6
8
None
Mg2+
Ni
The changes in fluorescence intensity of 2 upon addition of Cu2+ in THF solution were recorded.
Figure 3 shows the fluorescence spectral changes of 2 (λex = 350 nm) upon the addition of Cu2+ in THF
at 298 K. As mentioned, this addition resulted in a remarkable decrease of the fluorescence signal
corresponding to the benzimidazole moiety at 448 nm, and the appearance of a new emission peak at
540 nm attributed to the formation of a benzimidazole-copper compound. Figure 4 shows the kinetic
characteristics of the reaction in THF solution. When the reaction time of Cu2+ (1.00 × 10−5 M) with 2
(1.00 × 10−5 M) in THF was longer than 10 min, a stable fluorescence signal was observed and it
remained constant for at least 12 h. Higher concentrations of either Cu2+ or 2 will be of benefit to
shorter reaction times. These results prove the formation of a complex of 2 with Cu2+. The
fluorescence quenching data are analyzed by the Stern-Volmer equation [14]:
(1)
F0 / F = 1 + Ksv c(Q)

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103
where F0 and F denote the fluorescence intensities before and after the addition of the quencher,
respectively, c(Q) denotes the concentration of quencher, and Ksv is the Stern-Volmer quenching
constant. Figure 5 displays the Stern-Volmer plots of 2-Cu2+ system at 298 K. The plots showed that
within the investigated concentrations, the results agreed with the Stern-Volmer Eq. (1). And the
quenching constant Ksv of 2 to Cu2+ obtained from the Stern-Volmer equation is 4.44 × 105 L mol−1.
Figure 3. Fluorescence spectral changes of 2 (λex = 350 nm) upon the addition of Cu2+ in
THF at 298 K.
100
80
60
40
20
0
350
400
450
500
550
600
wavelength(nm)
Figure 4. Kinetic behavior of the fluorescence of the 2-Cu2+ reaction system in THF
solution. [2] = 1.00 × 10−5 M, [Cu2+] = 1.00 × 10−5 M.
50
45
40
35
30
25
0
5
10
15
20
Time (min)
Figure 5. The Stern-Volmer plots of 2-Cu2+ system at 298K.
1.80
1.65
1.50
1.35
0.2
0.4
0.6
0.8
1.0
[Cu2+]×106 M-1

Molecules 2011, 16
104
The UV-Vis absorption spectra of 2 upon addition of Cu2+ is shown in Figure 6. Two absorption peaks
of 2 were observed at 267 and 337 nm with molar extinction coefficients ε values (104 M−1 cm−1) in
agreement with π-π* transitions. Upon gradual addition of increasing concentrations of Cu2+ (0~2 equiv.)
to the solution of 2, the absorption peak at 276 nm was dramatically increased and shifted to longer
wavelength (17 nm), whereas no significant change was observed at the 338 nm absorption peak. The
stoichiometry of the complex between 2 and Cu2+ was established by Job’s plots as shown in Figure 6
(inset). The maximum for complexation of 2 with Cu2+ at a mole fraction of 0.5 indicates 1:1 complex
formation.
Figure 6. UV-Vis spectral changes of 2 upon the addition of Cu2+ ion. [2] = 1.00 × 10−5 M,
[Cu2+] = 0~2 × 10−5 M. Inset shows the job’s plots for complexation of 2 with Cu2+
determined by UV-Vis spectroscopy in THF. [2] + [Cu2+] = 1.0 × 10−5 M at 298 K.
1.5
0.25
1.2
0.20
0.15
0.10
0.9
0.05
0.00
0.0
0.2
0.4
0.6
0.8
1.0
[Cu(Ⅱ)]/[Cu(Ⅱ)]+[2]
0.6
0.3
0.0
250
300
350
400
wavelength(nm)
3. Experimental
3.1. General
Fluorescence spectra measurements were acquired on a RF-5301PC spectrophotometer，equipped
with a xenon discharge lamp and a 1 cm quartz cell. The UV-Vis absorption spectra were acquired
using a CARY 100 spectrophotometer. NMR spectra were recorded in CDCl3 on a Bruker AVANCE-
500 MHz NMR spectrometer with TMS as internal standard. Elemental analyses were performed on a
Carlo Erba model 1106 elemental analyzer. IR spectra were acquired on IR spectra were recorded on a
Nicolet ESP 360 FT-IR instrument. Mass spectra were obtained on a Bruker Esquire HCT
spectrometer. All of the chemicals used in this work were of analytical grade purchased from
commercial suppliers and used without further purification. The solutions of the metal ions were
prepared from their corresponding nitrate or chloride salts. The stock solution of compound 2
(1.00 × 10−4 M) was prepared in a 100 mL volumetric flask, by dissolving 2 (4.83 mg) in THF, and
diluting to the mark with THF, and the required concentration then being obtained by accurate dilution.
The metal ions stock solutions (1.00 × 10−2 M) were prepared in THF.

Molecules 2011, 16
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3.2. Preparation of Methyl 2′-hydroxyphenyl-12-bromo-13,14-imidazolyl-deisopropyldehydroabietate (2).
12-Bromo-13,14-diaminodeisopropyldehydroabietate methyl ester (190 mg, 0.5 mmol) and
salicylaldehyde (61 mg, 0.5 mmol) were dissolved in dioxane (15 mL). After refluxing for 5 h, a white
precipitate was formed. The solid was filtered and purified with re-crystallization from methanol to
1
give white needle-shaped crystals of 2 (0.18 g, 74.4%), m.p. 172-172.8 °C; H-NMR (CDCl3) δ: 1.29
(s, 3H), 1.34 (s, 3H), 1.52-1.59 (m, 2H), 1.71-1.81 (m, 5H), 2.23-2.32 (m, 2H), 2.74-2.75 (m, 2H), 3.03
(m, 1H), 3.70 (s, 3H), 6.94 (s, 1H), 7.00-7.02 (m, 1H), 7.07 (d, J = 8.55Hz, 1H), 7.44-7.47 (m, 2H),
13
8.45 (s, 1H), 12.71 (s, 1H ); C-NMR (CDCl3) δ: 16.6, 18.5, 24.5, 25.2, 29.3, 36.6, 37.8, 38.2, 44.1,
47.6, 52.3, 117.5, 119.2, 119.5, 124.1, 128.8, 126.9, 132.8, 134.0, 147.7, 150.4, 152.1, 161.0, 164.0,
178.7; IR (KBr) υmax: 2936, 1724, 1635, 1361, 1248, 1114, 742 cm−1; Anal. Calcd. for C25H27BrN2O3,
C 62.12, H 5.63, N 5.80; found: C 62.24, H 5.58, N 5.75; positive ACPIMS m/z (relative intensity)
484.2 [M + H]+ (100).
4. Conclusions
A dehydroabietyl derivative 2 bearing a 2-(2′-hydroxyphenyl)benzimidazole unit was synthesized
using methyl dehydroabietate as a starting material via several steps involving bromation, dinitration,
1
13
reduction and coupling/oxidation. The structure of compound 2 was characterized by H-NMR, C-
NMR, IR, MS and elemental analysis. Its sensing behaviors toward metal ions (Li+, Na+, K+, Mg2+,
Co2+, Ni2+, Zn2+, Cu2+, Cd2+, Al3+, Mn2+, Hg2+ and Pb2+) were investigated by a fluorescence
spectroscopy method, and the fluorescence intensity was only quenched by Cu2+. The results show that
compound 2 may be applicable as a fluorescent chemosensor for Cu2+.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No.20762001), and
the open fund of Key Laboratory of Development & Application of Forest Chemicals of Guangxi
(GXFC08-05), and the Project of Ten, Hundred, Thousand Distinguished Talents in New Century of
Guangxi (No.2007228).
Supplementary Materials
Supplementary Materials can be found at http://www.mdpi.com/1420-3049/16/1/100/s1.
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Sample Availability: Samples of the compounds are available from the authors.
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