X-ray crystallographic and photophysical properties of rhodamine-based chemosensor for Fe3+

Article abstract of DOI:10.1016/j.saa.2009.02.045

A fluorescent sensor for Fe³⁺, 2-[3′,6′-bis(ethylamino)-2′,7′-dimethyl-3-oxospiro[1H-isoindole-1,9′-[9H]xanthen]-2-((2-aminoethyl) methyl)]phenol, has been synthesized and characterized by ¹H NMR, ¹³C NMR and X-ray crystal

Full text of DOI:10.1016/j.saa.2009.02.045

Spectrochimica Acta Part A 73 (2009) 398–402
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
X-ray crystallographic and photophysical properties of rhodamine-based
3+
chemosensor for Fe
Lizhu Zhanga,b, Jiangli Fan , Xiaojun Peng
a
a,∗
a
State Key Laboratory of Fine Chemicals, Dalian University of Technology, 158 Zhongshan Road, Dalian 116012, PR China
Yunnan Nationalities University, 134 Yieryi Street, Kunming 650031, PR China
b
a r t i c l e i n f o
a b s t r a c t
fluorescent sensor for Fe3+
,
2-[3 ,6 -bis(ethylamino)-2 ,7 -dimethyl-3-oxospiro[1H-isoindole-1,9 -
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Article history:
A
Received 12 November 2007
Received in revised form 11 August 2008
Accepted 26 February 2009
1
[
9H]xanthen]-2-((2-aminoethyl) methyl)]phenol, has been synthesized and characterized by H NMR,
C NMR and X-ray crystallography. The photophysical processes of the Fe -sensor have been carefully
investigated by fluorescence and absorbance spectra. Upon addition of Fe , the sensor shows strong flu-
orescence change with absorbance and emission peaks at 530 and 554 nm, respectively. Other metal ions
have no such response.
13
3+
3+
Keywords:
X-ray crystallography
Photophysical properties
Rhodamine 6G
©
2009 Elsevier B.V. All rights reserved.
Chemosensor
3+
Fe
1. Introduction
2. Experimental
Rhodamine dyes are widely used in fluorescent labels and other
present biological techniques [1,2]. Although so many rhodamine
compounds and derivatives have been obtained, there are a few
single-crystal reports about the rhodamine derivatives bearing the
lactam moiety [3,4]. Detailed information on their molecular and
crystal structures is necessary to understand their photophysical
and photochemical properties. On the other hand, Fe3 ions as tran-
sition metal ions plays a crucial role in the growth and development
of living systems. Numerous enzymes use iron as a catalyst for
oxygen metabolism, electron transfer, and DNA and RNA synthesis
2.1. Materials and instruments
Mass spectral determinations were made on a HP1100 API-
ES mass spectrometer. NMR spectra were recorded on a Varian
400 MHz NMR spectrometer. UV–vis and fluorescence spectra were
performed on PerkinElmer LS55 Luminescence spectrometer. The
purification of the dye was performed by recrystallization or
conventional column chromatography. All other chemicals and sol-
vents involved were analytical reagents. Redistilled water was used
for stock solutions. The pH was recorded by PHS-501 instrument
(Pengshun Instruments Co. Ltd., Shanghai, China). The synthetic
route is shown in Scheme 1. X-ray crystallographic analysis was car-
ried out on a BRUKER-SMART-APEX. Imaging Plate diffractometer
with Mo K␣ radiation (ꢀ = 0.71073 Å).
+
[
5–7]. Many excellent fluorescent indicators of transition metal ions
2+
2+
2+
2+
have been reported, such as Cu , Pb , Zn , Hg and so on [8–13].
However, there are a few Fe3 -induced fluorescent sensors reported
+
[
14–16]. It might be due to the paramagnetic nature of ionic iron.
Some receptors, such as analogues of ferrichromes or siderophores,
are often signaled by fluorescence quenching. Thus, there is an
urgent need to develop selective fluorescent chemosensors for Fe3+
with fluorescence enhancement. Herein, we report a fluorescent
chemosensor, compound 2, based on rhodamine 6G derivatives,
which exhibits considerable fluorescence enhancement induced by
2
.2. Syntheses and crystal growth
2.2.1. Synthesis of compounds
2.2.1.1. Synthesis of compound 1. Rhodamine 6G (1 g, 2.09 mmol)
was dissolved in 20 mL of methanol, followed by addition of
ethylenediamine (0.67 mL, 10.05 mmol). The reaction mixture was
refluxed for 3 h until the color becomes to light yellow. The reac-
tion was cooled to room temperature, and the precipitate was
collected and washed with 30 mL of water. Crude product was
purified by recrystallization from acetonitrile to give 749 mg com-
3+
Fe
.
∗ Corresponding author. Tel.: +86 411 39893899; fax: +86 411 39893906.
E-mail address: pengxj@dlut.edu.cn (X. Peng).
pound 1 (white solid) in 78.4% yield. 1H NMR (CDCl ): ı 7.93 (d,
J = 2.4 Hz, 1H), 7.45 (t, J = 1.6 Hz, 2H), 7.05 (d, J = 2.4 Hz, 1H), 6.34
3
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.02.045

L. Zhang et al. / Spectrochimica Acta Part A 73 (2009) 398–402
399
Scheme 1. The synthetic route of compound 2.
(
2
(
9
1
4
s, 2H), 6.23 (s, 2H), 3.22 (m, J = 5.2 Hz, 4H), 3.15 (t, J = 6.4 Hz, 2H)
.35 (t, J = 6.4 Hz, 2H), 1.90 (s, 6H), 1.32 (t, J = 7.2 Hz, 6H). 13C NMR
400 MHz, CDCl ) ı: 14.910, 16.891, 38.525, 40.946, 44.127, 65.176,
shown in Fig. 1. The main skeleton of the molecule is formed
from a xanthene ring and a spirolactam-ring [3,4]. The atoms
of the xanthene ring or the spirolactam-ring are almost copla-
nar. The r.m.s. deviations is only 0.0248 Å for the xanthene ring
and 0.0231 Å for the spirolactam-ring, respectively. The dihedral
3
6.670, 106.28, 118.17, 122.96, 124, 128.23, 128.48, 131.37, 132.66,
47.61, 151.83, 153.7, 168.8 Q-TOFMS: calculated for: C28H32N O :
4
2
+
◦
57.2604, found: m/z = 457.2589 [M+H] .
angle between the two planes is 85.7 . Compared with other
reported models, the lactam moiety is not absolutely orthogo-
nal to the xanthene moiety. It may be the effect of phenol part.
The phenol part of the molecule (C30 benzene ring plus O3)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2
.2.1.2. 2-[3 ,6 -Bis(ethylamino)-2 ,7 -dimethyl-3-oxospiro[1H-
isoindole-1,9 -[9H]xanthen]-2-((2-aminoethyl)
methyl)]phenol
is close to being planar, with an r.m.s. deviation for the fitted
(
2
(
compound 2). Compound 1 (500 mg, 1.2 mmol) was dissolved in
0 mL of methanol, followed by addition of 2-hydroxybenzaldehyde
146 mg, 1.2 mmol). The yellow solution was mixed for 1 h at room
temperature, and then added NaBH until the reactant disappeared
atoms of 0.0065 Å. The dihedral angles of the phenol plane with
the planes of the xanthene ring and the spirolactam-ring are
◦ ◦
09.4 and 107.8 , respectively. As shown in Table 1, the com-
1
4
pound 2 crystallizes in triclinic space group P-1 with unit cell
parameters a = 11.6453(5) Å, b = 11.8588(3) Å, c = 12.9822(3) Å and
˛ = 116.321(10), ˇ = 103.173920, ꢂ = 98.337(2).
on TLC. The solvent was removed by rotatory evaporation. Crude
product was purified by column chromatography to give 317 mg
compound 2 (white solid) in 47.1% yield. 1H NMR (CDCl ): ı 7.93
3
(
(
q, J = 2.0 Hz, 1H), 7.46 (t, J = 3.6 Hz, 2H), 7.09 (d, J = 7.2 Hz, 1H), 7.05
q, J = 2.0 Hz, 1H), 6.83 (d, J = 7.2 Hz, 1H), 6.72 (d, J = 8.0 Hz, 1H),
6
.69 (t, J = 7.6 Hz, 1H), 6.33 (s, 2H), 6.21 (s, 2H), 3.65 (s, 2H), 3.28
(
(
1
1
t, J = 5.2 Hz, 2H), 3.19 (q, J = 6.8 Hz, 4H), 2.49 (t, J = 5.6 Hz, 2H), 1.84
s, 6H), 1.31 (t, J = 7.2 Hz, 6H). 13C NMR (CDCl , 100 MHz) ı: 14.95,
Table 1
3
Crystal data and structure refinement for compound 2.
6.67, 39.03, 43.05, 51.82, 65.38, 96.68, 106.05, 113.07, 118.56,
22.92, 124.03, 127.68, 128.26, 128.43, 128.59, 131.28, 132.89,
47.65, 151.79, 153.66, 158.77, 169.30 ESI-MS positive: m/z = 563.3
2
1
Empirical formula
Formula weight
Temperature
C35H36N4O3
560.68
273(2) K
+
[
M+H] .
Wavelength
Crystal system
Space group
0.71073 Å
Triclinic
P-1
2
.2.2. Crystal growth and conditions
White cubic single-crystal of compound 2 was obtained from
hexane–dichloromethane (1:1, v/v) solution by slow evaporation
at room temperature after 20 days, and then mounted it on the
goniometer of single-crystal diffractometer. The crystal data has
been collected by using Mo K␣ radiation (ꢀ = 0.71073) in the ꢁ range
Unit cell dimensions
a = 11.6453(5) Å, ˛ = 116.3210(10)
b = 11.8588(3) Å, ˇ = 103.173(2)
c = 12.9822(3) Å, ꢂ = 98.337(2)
3
Volume
Z
1501.21(8) Å
2
◦
of 2.66–25 by using ω/2ꢁ scan mode. A total of 5018 reflections
Density (calculated)
Absorption coefficient
F(0 0 0)
Theta range for data collection
Index ranges
1.240 Mg/m3
−
1
were recorded. The structure determination of the compound 2 has
been carried out by direct methods using SHELXS software. The
goodness-of-fit was 1.012.
0.08 mm
596
2.66–25.00
−13 ≤ h ≤ 12, −14 ≤ k ≤ 13, −15 ≤ l ≤ 15
Reflections collected
Independent reflections
Refinement method
Data/restraints/parameters
9518
5018
3. Results and discussion
Full-matrix least-squares on F2
5018/0/383
3.1. Crystal structure
2
Goodness-of-fit on F
0.941
Final R indices [I > 2sigma(I)]
R indices (all data)
Largest diff. peak and hole
R1 = 0.0508, wR2 = 0.1429
R1 = 0.0716, wR2 = 0.1569
0.553 and −0.391e−3
The crystallographic data of compound 2 are summarized
in Table 1 and the ORTEP structure with atom-numbering is

4
00
L. Zhang et al. / Spectrochimica Acta Part A 73 (2009) 398–402
Fig. 1. The crystal structure of 2. Thermal ellipsoids are scaled as 30% probability level.
or transition metal ions, do not interfere with the Fe3+-induced
fluorescence increase, indicating that the compound 2 has are
remarkable selectivity for Fe3+
3
.2. Fluorescence and absorbance spectra
When the FeCl3 was added into the colorless solution of 2 in
.
ethanol, a strong fluorescence appeared. The maximum emission
wavelength of the Fe –2 complex are at 554 nm. Different from
The changes of the UV–vis spectra with the addition of various
ions are shown in Fig. 3. The colorless compound 2 reveals almost
no absorption peak in the visible wavelength range (>400 nm). But
3
+
3
+
2+
2+
2+
2+
+
2+
Fe , other metal ions such as Mg , Ca , Ni , Mn , K , Zn
,
Cu , Cr , Fe , Pb , Co , Cd , Ba and Hg2+, did not induce any
spectral response under identical conditions, even at high concen-
trations (Fig. 2). It is probably due to several combined influences
cooperating to achieve the unique selectivity for the Fe3 ion, such
as the suitable coordination geometry conformation of the recep-
2
+
3+
2+
2+
2+
2+
2+
3+
addition of Fe to the ethanol solution of compound 2 showed an
obvious pink color with absorption peak at 530 nm. The change is
readily detected visually for Fe3+. Compared with Fe , other metal
ions, even at higher concentration, show only light change. The non-
linear fitting of the titration curve indicates a 1:1 stoichiometry for
3+
+
3
+
3+
tor, the radius of the Fe ion and the amide deprotonation ability
of the Fe ion. Competition experiments of Fe with other metal
ions did not show a reduced selectivity for Fe , only Ni , Co
Mg and Ba very lightly decreased the fluorescence intensity.
Some environmentally relevant metal ions, such as alkaline-earth
the 1–Fe complex. This binding mode was supported by the data
3
+
3+
3+
of Job’s plots evaluated from the absorption spectra of 2 and Fe
3+
2+
2+
,
with a total concentration of 100 ␮M (inset in Fig. 3). According to
2
+
2+
the 1:1 model and the fluorescent titration, a stability constant (K )
a
3+
4
of 1.1 × 10 was obtained for the compound 2 binding with Fe
(Fig. 4).
The pH titration control experiments revealed that the solution
of 2 did not emit any obvious fluorescence in the pH range from
Fig. 2. The fluorescence response of 2 (10 ␮M) to various cations in ethanol. The
light bars represent the integrated emission of 2 in the presence of 16 equiv. of the
cations of interest (Cr(ClO4)3, Hg(ClO4)2, Pb(ClO4)2, Zn(ClO4)2, Cu(ClO4)2, Cd(ClO4)2,
MgCl2, CaCl2, NiCl2, MnCl2, KCl, CoCl2, BaCl2 and FeCl2); the black bars represent the
changes in integrated emission that occur upon subsequent addition of 8 equiv. of
Fe(ClO4)3 to solutions containing 2 and 16 equiv. of the cations of interest. Excitation
was provided at 510 nm and emission was integrated from 515 to 800 nm.
Fig. 3. UV–vis change of compound 2 in ethanol upon the addition of different
amounts of Fe3+. Inset: Job’s plots of compound 2 added with various concentration
of Fe3+.

L. Zhang et al. / Spectrochimica Acta Part A 73 (2009) 398–402
401
Fig. 4. Fluorescent titration of compound 2 (a) and stability constant (Ka) of compound 2 binding with Fe3+ (b).
Scheme 2. The proposed mechanism of Fe3+-induced compound 2 ring opening and fluorescent enhancement.
5
.0 to 10.0. Although in acid media (pH 2–4), there is a large flu-
the following facts: (1) upon the addition of excess ethylenediamine
to the pink colored complex solution, it turns to the original color-
less state; (2) this colorless solution was identified as compound 2
by TLC and MS.
orescence background. However, the pH effect is reversible: when
pH is returned to neutral, the fluorescence background declines to
the non-fluorescence state. It is suggested that compound 2 was
suitable in the cases of pH >5 (Fig. 5).
4. Conclusion
3.3. Mechanism
In summary, based on rhodamine 6G, the fluorescent sensor
for Fe3+, compound 2, was prepared and structurally characterized.
The possible mechanism for these fluorescent changes is shown
3+
in Scheme 2. According to some rhodamine-based chemosensors
reported in literatures, it may be that Fe3 coordinates with the cor-
responding part of compound 2 and induces the opening of the spiro
ring. This coordination is a reversible process, which is supported by
This sensor displays highly selectivity for Fe over other metal ions.
Due to the efficiency and simplicity of fluorescence measurement,
compound 2 may find its applications in a variety of fields requiring
rapid Fe3+ ion analysis.
+
Acknowledgments
This work was supported by the National Science Foundation of
China (20706008 and 20705621), Ministry of Education of China
(Program for Changjiang Scholars and Innovative Research Team in
University, IRT0711 and Cultivation Fund of the Key Scientific and
Technical Innovation Project, 707016).
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