Selective Hg2+ sensing behaviors of rhodamine derivatives with extended conjugation based on two successive ring-opening processes

Article abstract of DOI:10.1021/ic401810x

A novel class of rhodamine derivatives with two spirolactam moieties have been synthesized, and their two stereoisomers of cis- and trans-forms have been successfully separated and isolated, as well as structurally characterized by X-ray crystallography. Attributed to the successive ring-openings of two spirolactam moieties, different solution color, electronic absorption and emission responses were exhibited upon addition of various concentrations of mercury(II) ion. Arising from two successive ring-opening processes in the presence of various concentration of Hg(II) ion, two reporting states with different spectroscopic properties were suggested, that is, the first state showing pink color (absorption maximum at 496 nm) but no emission, while the second state giving purple color (absorption maximum at 593 nm) and red emission (emission maximum at 620 nm). The mechanism of such different spectroscopic responses was also proposed and has also been supported by computational studies. An extension of the present work to the study of the corresponding chemodosimeters from the compounds with two spirolactam groups has been made, in which a stoichiometric and irreversible Hg(II)-promoted reaction of thiosemicarbazides was utilized to form 1,3,4-oxadiazoles.

Full text of DOI:10.1021/ic401810x

Article
pubs.acs.org/IC
Selective Hg2+ Sensing Behaviors of Rhodamine Derivatives with
Extended Conjugation Based on Two Successive Ring-Opening
Processes
Chunyan Wang†,‡ and Keith Man-Chung Wong*,†,‡
†Department of Chemistry, South University of Science and Technology of China, No. 1088, Tangchang Boulevard, Nanshan
District, Shenzhen, Guangdong, P.R. China
‡Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China
S
* Supporting Information
ABSTRACT: A novel class of rhodamine derivatives with two
spirolactam moieties have been synthesized, and their two
stereoisomers of cis- and trans-forms have been successfully
separated and isolated, as well as structurally characterized by
X-ray crystallography. Attributed to the successive ring-
openings of two spirolactam moieties, different solution
color, electronic absorption and emission responses were
exhibited upon addition of various concentrations of mercury-
(II) ion. Arising from two successive ring-opening processes in the presence of various concentration of Hg(II) ion, two
reporting states with different spectroscopic properties were suggested, that is, the first state showing pink color (absorption
maximum at 496 nm) but no emission, while the second state giving purple color (absorption maximum at 593 nm) and red
emission (emission maximum at 620 nm). The mechanism of such different spectroscopic responses was also proposed and has
also been supported by computational studies. An extension of the present work to the study of the corresponding
chemodosimeters from the compounds with two spirolactam groups has been made, in which a stoichiometric and irreversible
Hg(II)-promoted reaction of thiosemicarbazides was utilized to form 1,3,4-oxadiazoles.
ring-closed spirolactam form is colorless and nonfluorescent,
while the ring-opened amide form displays intense pink to red
color and strong fluorescence. Various rhodamine-based turn-
on fluorescent probes for metal cations or oxidative species
have been reported in the past few years.6−9 Depending on the
nature of functional groups attached to the spirolactam, such
fluorescent probes were found to selectively bind to various
metal cations, in particular for the sensing of mercury(II) ion.
In general, the sensing mechanism of these probes is based on
the structural modulation from the spirolactam to the open-
cycle form. Upon addition of analytes, a drastic color change
from colorless to intense color with a concomitant appearance
of strong fluorescence will result from the spirocycle opening
process through a reversible coordination or an irreversible
chemical reaction with the rhodamine probe. Because of the
resonance structure of the rhodamine, in which the two amino
groups are partially positive-charged, any emission quenching
through the possible photoinduced electron transfer (PET)
would not be possible (Scheme 1). As a result, the ring-opening
would lead to the intense absorption band and emission in the
visible region.
INTRODUCTION
■
Since the pioneering works of Pedersen, Lehn, and Cram1 on
the design and syntheses of cation-selective crown ethers,
cryptands, and spherands, studies on the host−guest
interactions of known chemical composition and structure are
indeed highly interesting and useful. Conventional approaches
to chemosensors have commonly made use of a “lock-and-key”
design, wherein a specific receptor is synthesized to strongly
and highly selectively bind the analyte of interest.2 The
development of optical signaling systems for various chemo-
sensors to detect different types of analyte has received
considerable attention in the past few decades. Design of
sensors based on changes in luminescence is a promising field
and appears to be particularly attractive because of their
simplicity, high sensitivity, high selectivity, and instantaneous
response by simple luminescence enhancement or quenching.3
Numerous excellent studies focus on the development of
luminescent chemosensors for various analyses of transition- or
heavy-metal ions such as Hg2+, Pb2+, Zn2+, and Cu2+ owing to
their fundamental role in a wide range of chemical, biological,
and environmental processes.4
For the ordinary rhodamine derivatives having only one
spirolactam group for the ring-opening process, their
Rhodamine-based dyes have been widely used in the past
several decades since the first report on the preparation of
rhodamine in 1905.5a It is well-known that many derivatives of
rhodamine are in equilibrium5b between two constitutional
isomers with completely different spectroscopic properties. The
Received: July 14, 2013
Published: November 11, 2013
© 2013 American Chemical Society
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Scheme 1. Drastic Color Change and Emission Enhancement Arising from the Ring-Opening of Spirolactam Group
Scheme 2. Synthetic Route of 1−3
sulfuric acid (10 mL) dropwise at 0 °C. The resulting suspension was
heated at 100 °C for 3 h. After the mixture was cooled down to room
temperature and poured into ice water (100 mL) with vigorous
stirring, the pH of the mixture was adjusted to ∼7. The mixture was
extracted with dichloromethane (20 mL) three times. The organic
layers were dried over anhydrous magnesium sulfate and evaporated to
give the crude product of the mixture of cis-1 and trans-1 (1.43 g,
54%). The purification and separation of the stereoisomer were
achieved by silica column chromatography eluting with dichloro-
methane and methanol (150:1) to give the pure form of cis-1 and
trans-1 as a white solid in the ratio of about 1:1. Subsequent
recrystallization of the respective compounds by diffusion of diethyl
ether vapor into a solution of the product in dichloromethane afforded
absorption and emission colors are mainly dependent on the
substituents on the amino groups with limited energy
variations. Herein we report the design and synthesis of a
novel class of rhodamine derivatives, 1 and 2, in which there are
two spirolactam groups with five fused six-membered rings
(Scheme 2). Two stereoisomers of cis- and trans-forms have
been successfully separated, isolated, as well as structurally
characterized by X-ray crystallography. Mercury(II) ion was
found to selectively induce ring-opening of the spirolactam
groups. In view of the different extent of the π-conjugation
upon successive ring-opening processes, different solution
color, electronic absorption, and emission responses were
exhibited in the presence of various concentrations of
mercury(II) ions. An extension of the present work to the
study of the corresponding chemodosimeters from the
compounds with two spirolactam groups has been made, cis-3
(Scheme 2), in which an stoichiometric and irreversible Hg(II)-
promoted reaction of thiosemicarbazides was utilized to form
1,3,4-oxadiazoles.
1
cis-1 and trans-1 as white crystals, respectively. cis-1: H NMR (300
MHz, CDCl3) δ 7.83−7.80 (2H, m), 7.46−7.39 (4H, m), 7.13 (1H, s),
6.93−6.90 (2H, m), 6.48 (2H, d, J = 8.9 Hz), 6.48 (2H, d, J = 2.5 Hz),
6.32 (2H, dd, J = 2.5 Hz, J = 8.9 Hz), 5.98 (1H, s), 3.35 (8H, q, J = 7.0
Hz), 1.17 (12H, t, J = 7.0 Hz). 13C NMR (100 MHz, CDCl3), δ 169.0,
153.4, 153.2, 152.2, 149.8, 134.2, 129.4, 128.8, 128.6, 127.4, 125.0,
123.5, 116.4, 108.6. 105.2, 104.4, 97.8, 83.4, 44.6, 12.6. HRMS (EI) for
C42H36N2O6 (M+): calcd 664.2573, Found: 664.2550. Trans-1: 1H
NMR (300 MHz, CDCl3), δ 7.79−7.77 (2H, d, J = 7.6 Hz), 7.63−7.58
(2H, m), 7.53−7.48 (2H, m), 7.14−7.11 (3H, m), 6.50−6.47 (4H, m),
6.32 (2H, dd, J = 2.4 Hz, J = 8.9 Hz), 6.03 (1H, s), 3.36 (8H, q, J = 7.0
Hz), 1.17 (12H, t, J = 7.0 Hz). 13C NMR (75 MHz, CDCl3), δ 169.3,
153.3, 153.0, 152.1, 149.8, 135.1, 129.8, 128.8, 127.8, 126.8, 124.6,
124.1, 116.4, 108.6, 105.1, 104.2, 97.8, 83.6, 44.6, 12.6. HRMS (EI) for
C42H36N2O6 (M+): calcd 664.2573, Found: 664.2564.
Synthesis of cis-2 and trans-2. To a suspension of crude product
of the mixture of cis-1 and trans-1 (0.5 g, 0.75 mmol) in ethanol (20
mL) was added hydrazine (1.2 mL). The resulting suspension was
refluxed overnight. After the resulting suspension was cooled to room
temperature, the solvent was removed. To the residue was added
deionized water (10 mL). The aqueous phase was extracted with
dichloromethane (20 mL) three times. The organic phase was dried by
anhydrous magnesium sulfate. The solvent was removed to give the
crude product of cis-2 and trans-2 (0.493 g, 94%) in the ratio of about
1:1. The purification and separation of the stereoisomer were achieved
by silica column chromatography eluting with dichloromethane and
methanol (80:1) to give the pure form of cis-2 and trans-2 as a white
EXPERIMENTAL SECTION
■
Materials and Reagents. All the solvents for synthesis were of
analytical grade. Methanol for analysis was of HPLC grade. 3-
(Diethylamino)phenol, phthalic anhydride, hydrazine monohydrate,
and resorcinol were purchased from Aldrich Chemical Co.
Magnesium(II) perchlorate, calcium(II) perchlorate tetrahydrate,
barium(II) perchlorate, lithium perchlorate, sodium perchlorate,
potassium perchlorate, iron(II) perchlorate, cobalt(II) perchlorate,
nickel(II) perchlorate, mecurry(II) perchlorate, silver(I) perchlorate,
and lead(II) perchlorate hydrate were purchased from Aldrich
Chemical Co. with purity over 99.0% and were used as received.
Synthesis. Synthesis of cis-1 and trans-1. To a solution of 3-
diethylaminophenol (2.0 g, 12 mmol) in toluene (60 mL) was added
phthalic anhydride (1.8 g, 12 mmol). The suspension was heated to
reflux for 6 h. The resulting residue was washed with methanol to
provide the intermediate S1 (2.1 g, 55%). To a mixture of S1 (2.5 g,
8.0 mmol) and resorcinol (0.44 g, 4.0 mmol) was added concentrated
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solid after removal of solvent. Subsequent recrystallization of the
respective compounds by diffusion of diethyl ether vapor into a
solution of the product in dichloromethane afforded cis-2 and trans-2
spectra at room temperature were recorded on a Spex Fluorolog-3
Model FL3-211 fluorescence spectrofluorometer equipped with a
R2658P PMT detector. The solution samples for the electronic
absorption and emission titration experiments were in the
concentration range from 9.57 × 10−6 to 1.79 × 10−5 M. Elemental
analyses of complexes were performed on a Flash EA 1112 elemental
analyzer at the Institute of Chemistry, Chinese Academy of Sciences.
Theoretical and Computational Methodology. All computa-
tions were done with the Gaussian09 program package10 with different
parameters for structure optimizations and vibrational analysis. The
singlet ground-state geometries were fully optimized with Becke’s
three-parameter exchange functional along with the Lee−Yang−Parr
correlation functional with the restricted (B3LYP) at the standard split
valence plus polarization function 6-31G(d) basis set.8i The fully
optimized stationary points were further characterized by harmonic
vibrational frequency analysis to ensure that real local minima had
been found that had no imaginary vibrational frequencies. The
electronic absorption spectra were calculated using the time-depend-
ent density functional theory (TD-DFT) method with B3LYP/6-
31G(d) at the optimized ground state structures. The polarizable
continuum model (PCM) was adopted in the TD-DFT calculation of
the absorption spectra in the solvent (methanol for the partially and
fully ring-opened cis-1 and acetonitrile for 4 and rhodamine
6G_oxadiazole).
1
as a white crystal respectively. cis-2: H NMR (400 MHz, CDCl3), δ
7.82−7.80 (2H, m), 7.44−7.42 (4H, m), 7.09 (1H, s), 7.01−6.99 (2H,
m), 6.44 (2H, d, J = 2.5 Hz), 6.34 (2H, d, J = 8.9 Hz), 6.27 (2H, dd, J
= 2.5 Hz, J = 8.9 Hz), 5.97 (1H, s), 3.46 (4H, s), 3.33 (8H, q, J = 7.0
Hz), 1.16 (12H, t, J = 7.0 Hz). 13C NMR (100 MHz, CDCl3), δ 166.5,
153.4, 150.8, 149.1, 133.0, 129.3, 128.9, 127.7, 126.3, 123.5, 123.3,
115.9, 108.6, 104.9, 104.4, 98.2, 65.4, 44.5, 12.7. HRMS (EI) for
C42H40N6O4 (M+): calcd 692.3111, Found: 692.3122. trans-2: 1H
NMR (400 MHz), δ 7.74 (2H, d, J = 7.5 Hz), 7.31−7.29 (2H, m),
7.27−7.25 (2H, m), 7.07 (1H, s), 6.81 (2H, d, J = 7.5 Hz), 6.45 (2H,
d, J = 2.5 Hz), 6.33 (2H, d, J = 8.8 Hz), 6.27 (2H, d, J = 2.5 Hz, J = 8.8
Hz), 5.87 (1H, s), 3.66 (4H, s), 3.34 (8H, q, J = 7.0 Hz), 1.16 (12H, t,
J = 7.0 Hz). 13C NMR (100 MHz, CDCl3), δ 166.2, 153.6, 153.4,
150.5, 149.0, 132.2, 129.8, 128.2, 127.9, 127.5, 123.3, 123.0, 115.9,
108.5, 104.6, 104.3, 98.0, 65.3, 44.4, 12.7. HRMS (EI) for C42H40N6O4
(M+): calcd 692.3111, Found: 692.3110.
Synthesis of cis-3. The mixture of 2 (346 mg, 0.5 mmol) and
phenyl isothiocyanate (300 μL, 2.47 mmol) in dimethylformamide
(DMF, 3 mL) was stirred at room temperature for 24 h under
nitrogen. The solvent was removed, and the crude product was
purified by silica column chromatography eluting with dichloro-
methane and methanol (100:1) to give the crude product as a light
purple solid after removal of solvent. Subsequent recrystallization of
the compound by diffusion of diethyl ether vapor into a solution of the
crude product in dichloromethane afforded pure cis-3 (221 mg, yield
46%). 1H NMR (400 MHz, CDCl3), δ 7.78−7.76 (2H, m), 7.52−7.47
(4H, m), 7.24 (2H, s), 7.19 (1H, s), 7.19−7.13 (4H, m), 7.11−7.08
(2H, m), 6.96−6.94 (4H, m), 6.81 (2H, m), 6.64 (2H, s), 6.58 (2H, d,
J = 2.6 Hz), 6.42 (2H, d, J = 8.8 Hz), 6.29 (2H, dd, J = 2.6 and 8.9
Hz), 5.69 (1H, s), 3.40−3.30 (8H, m), 1.18−1.15 (12H, m). 13C NMR
(100 MHz, CDCl3) δ 183.0, 166.5, 154.3, 154.0, 149.8, 148.4, 137.5,
135.0, 129.7, 128.7, 128.6, 127.6, 126.6, 125.5, 124.3, 123.8, 115.8,
109.2, 105.6, 103.2, 98.5, 66.5, 44.6, 12.6. FAB-MS: 963 [M + H]+.
Elemental analyses calcd for C56H50N8O4S2·1/2CH2Cl2·1/2H2O
(found): C 66.88 (67.00), H 5.17 (4.94), N 11.04 (11.34).
RESULT AND DISCUSSIONS
■
Syntheses and Characterizations. The synthetic method-
ology is shown in Scheme 2. Reaction of resorcinol and 2-(4-
diethylamino-2-hydroxybenzoyl)benzoic acid in concentrated
sulfuric acid under reflux condition afforded compound 1.11
Formation of 2 was achieved from the subsequent reaction of 1
and hydrazine. As revealed from their 1H NMR spectra of both
1 and 2, the existence of two diastereomers of cis- and trans-
forms in about 1:1 ratio were found, which can be successfully
separated and isolated by column chromatogrphy. A mixture of
cis-2 and trans-2 in 1:1 ratio was obtained even from pure
isomer of cis-1 or trans-1, indicating that the transformation of
ester to amide group may involve the ring-opening of the
spirolactam moiety. On the other hand, there are two
enantiomers, R,R and S,S, for each of the diastereomer of
trans-form (Scheme 3). No attempt has been made to separate
Synthesis of 4. The mixture of 3 (50 mg, 0.052 mmol) and
Hg(ClO4)2 (44 mg, 0.11 mmol) in acetonitrile (15 mL) was stirred at
room temperature for 20 min. The color of the solution turned to
purple-blue. The solvent was removed, and the crude product was
purified by silica column chromatography eluting with dichloro-
methane and methanol (50:1) to afford the desired product as a blue
solid (42 mg, 90%) after removal of solvent. Subsequent
recrystallization of the compound by diffusion of diethyl ether vapor
into a solution of the crude product in dichloromethane afforded pure
Scheme 3. Diastereomers and Enantiomers of 1 and 2
1
product. H NMR (400 MHz, acetone-d6) δ 9.49 (2H, s), 8.04 (2H,
dd, J = 0.8 and 8.0 Hz), 8.02 (1H, s), 7.84 (2H, td, J = 1.2 and 7.6 Hz),
7.77 (2H, td, J = 1.2 and 7.6 Hz), 7.54−7.49 (8H, m), 7.40−7.36 (4H,
m), 7.30−7.26 (4H, m), 7.03−6.98 (3H, m), 4.08 (4H, q, J = 7.2 Hz),
4.00 (4H, q, J = 7.2 Hz), 1.50 (6H, t, J = 7.2 Hz), 1.37 (6H, t, J = 7.2
Hz). 13C NMR (100 MHz, acetone-d6) δ 160.8, 160.4, 159.8, 157.7,
157.0, 155.9, 139.1, 134.4, 132.3, 132.1, 131.9, 130.6, 129.9, 129.5,
128.8, 123.9, 123.3, 120.9, 120.7, 118.2, 106.1, 99.6, 48.8, 48.6, 13.8,
13.0. FAB-MS: 448.3 [M]2+, 895.3 [M − H]+. Elemental analyses calcd
for C56H48Cl2N8O12·CH2Cl2·H2O (found): C 57.10 (57.32), H 4.34
(4.07), N 9.35 (9.56).
Physical Measurements and Instrumentation. 1H and 13C
NMR spectra were recorded at room temperature on a Bruker
1
AVANCE DPX 300 (300 MHz for H and 75 MHz for 13C) or 400
1
(400 MHz for H and 100 MHz for 13C) Fourier-transform NMR
1
spectrometers. H NMR chemical shifts were reported in ppm using
CDCl3 (δH 7.26 ppm) or (CD3)2CO (δH 2.05 ppm) as internal
standard. 13C NMR chemical shifts were reported in ppm relative to
CDCl3 (δC 77.16 ppm) or (CD3)2CO (δC 29.8 ppm). High resolution
EI mass spectra were recorded on a Thermo Scientific DFS High
Resolution Magnetic Sector mass spectrometer. The electronic
absorption spectra were obtained using a Hewlett-Packard 8452A
diode array spectrophotometer. Steady state excitation and emission
such two enantiomers. All of them (cis-1, trans-1, cis-2, and
1
trans-2) have been characterized by H NMR, 13C NMR, and
HR (EI) mass spectrometry. Single crystals of cis-1, trans-1, cis-
2, and trans-2 suitable for X-ray crystallographic studies were
obtained by the slow diffusion of diethyl ether vapor into a
concentrated dichloromethane solution of the corresponding
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compounds. The molecular structures of two pairs of
steroisomers have also been confirmed by X-ray crystallography
(Supporting Information, Figure S1).12 With the aid of 2D
1H-1H COSY NMR results, the assignment of all resonance
signals of individual compounds has been established, and their
corresponding 1H NMR spectra showing the aromatic region in
CDCl3 are depicted in Figure 1. The chemical shifts for the
Figure 2. Selectivity study of cis-2 (conc. = 1.0 × 10−5 M) in MeOH
upon addition of various metal ions (8 mol equiv).
MeOH upon addition of various metal ions. Such selective
color change suggests that 1 or 2 can serve as a “naked-eye”
indicator for Hg(II) ion. It is interesting to note that the
solution color of 1 or 2 changes from colorless to pink without
observable emission in the presence of a small amount of
Hg(II) ion. Upon further addition of excess amount of Hg(II)
ion, the pink solution changes to purple color and an intense
red-color emission was observed. Figure 3 shows the solution
1
Figure 1. H NMR spectra (right) of cis-1 (a), trans-1 (b), cis-2 (c),
and trans-2 (d) in CDCl3 showing the assignment of aromatic
resonances.
same proton in their cis- and trans-forms are different,
suggestive of the nature of chemically nonequivalent environ-
ments imparted by the change of the relative spatial orientation
in the stereoisomers. Compound cis-3 was obtained by the
reaction of 2 (mixture of trans-2 and cis-2) and isothiocyana-
tobenzene. It is interesting to note that reaction from the pure
form of trans-2 or cis-2 as the starting material also afforded
compound cis-3 in only one configuration of cis-form. For
characteristic peaks of the quaternary carbon of spirolactam in
cis-1 (83.4 ppm), trans-1 (83.6 ppm), cis-2 (65.4 ppm), trans-2
(65.3 ppm), cis-3 (66.5 ppm) as shown in 13C NMR spectra
suggested the existence of closed forms in the solution.
Figure 3. Photographs showing the color changes (left) and emission
enhancement (right) in the absence of Hg(II) ion for cis-2 (a) and
trans-2 (d), and in the presence of 1.5 equiv of Hg(II) ion for cis-2 (b)
and trans-2 (e) and in the presence of excess amount of Hg(II) ion for
cis-2 (c) and trans-2 (f) in MeOH.
color and emission changes of cis-2 and trans-2 with Hg(II) ion
as the representative example, while the corresponding solution
color and emission changes of trans-1 are shown in Supporting
Information, Figure S2. Both 1 and 2 were found to exhibit the
corresponding responses of color and emission changes
instantaneously upon the addition of Hg(II) ion.
Because of the centrosymmetric space groups of Pbcn and
P21/n, two enantiomers of trans-1 and trans-2 were cocrystal-
lized as a racemic mixture, respectively. In general, the five six-
membered rings consisting of alternating three benzene and
two pyran rings fuse together to form a planar structure, which
is the analogue to a pentacene molecule. The planes of 3H-
isobenzofuran-1-one in 1 and 2-amino-2,3-dihydro-1H-isoin-
dol-1-one in 2 are orthogonal to the plane of the five fused ring
with the dihedral angles of 85.497−89.441°. In the trans-form,
the two 3H-isobenzofuran-1-one (in 1) or 2-amino-2,3-
dihydro-1H-isoindol-1-one (in 2) groups face opposite
directions with the interplanar angles of 11.405−14.056°. On
the other hand, the two planes in the cis-form are pointing to
the same direction with the interplanar angles of 5.549−9.728°.
Hg(II) Ion Sensing. Compounds 1 and 2 are colorless and
nonemissive in their solution state. No observable electronic
absorption band could be found in the visible region, indicative
of the ring closure nature in the spirolactam groups. Selective
sensing behaviors of 1 and 2 were investigated for various alkali
and alkaline-earth as well as transition metal cations. Solutions
of 1 or 2 in methanol exhibit drastic color changes selectively in
the presence of Hg2+ with no observable color change for K+,
Na+, Li+, Ca2+, Mg2+, Ba2+, Pb2+, Cd2+, Zn2+, Co2+, Ni2+, Ag+,
Fe2+, Cu2+. Figure 2 shows the selectivity study of cis-2 in
Spectroscopic Titrations for Hg(II) Ion. Electronic
absorption and emission titration studies of 1 and 2 (both in
cis- and trans-forms) with various concentrations of Hg(II) ion
were carried out to investigate the profiles of the spectroscopic
changes. Both the cis-1 and trans-1 exhibit essentially the same
sepectroscopic responses upon addition of Hg(II) ion,
indicating that the ring-opening behaviors of stereoisomers of
1 are almost the same. The corresponding electronic absorption
spectral changes of cis-1 in methanol are illustrated in Figure 4
(left) (Supporting Information, Figure S3 for trans-1). Initial
addition of Hg(II) ion resulted in an emergence of the
absorption bands at 464, 496, and 530 nm, which is responsible
for the pink color change. Two well-defined isosbestic points at
288 and 332 nm are observed, indicative of a clean conversion.
Such growth of absorption bands reaches a saturation at about
1.7 equiv of Hg(II) ion (Figure 4 (left, inset). On the basis of
the occurrence of similar absorption energy for rhodamine
derivatives containing three fused rings of a xanthene moiety,
the observed absorption bands are ascribed to the ring-opening
of one spirolactam group. Further addition of Hg(II) ion
resulted in additional absorption bands at 546 and 593 nm, and
the absorbance of this 593 nm absorption band increases
drastically with the saturation at 80 equiv of Hg(II) ion (Figure
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Figure 4. Electronic absorption (left) and emission (right) spectral changes of cis-1 (conc. = 1.19 × 10−5 M) in MeOH in the presence of various
concentrations of Hg(II) ion. Insets show the plot of absorbance or emission intensity as a function of the concentration of Hg(II) ion.
Scheme 4. Proposed Mechanism for the Color Changes and Emission Behaviors of 1 and 2
Figure 5. Electronic absorption (left) and emission (right) spectral changes of cis-2 (conc. = 1.01 × 10−5 M) upon addition of various concentrations
of Hg(II) ion in MeOH. Insets show the plot of absorbance or emission intensity as a function of the concentration of Hg(II) ion.
4 (left)). The observation of additional new absorption bands
at 546 and 593 nm is reasonably attributed to another ring-
opening process of the second spirolactam group since an
extension of the π-conjugation across the fused-structure of five
rings is anticipated to shift the absorption to the red.
It is noteworthy that an intense vibronic-structured emission
at 620 and 670 nm was switched on at high concentration of
Hg(II) ion (Figure 4 (right)). The close resemblance of the
emission intensity changes at 620 nm (Figure 4 (right, inset))
and the absorbance changes at 593 nm (Figure 4, (left, inset))
as a function of Hg(II) ion concentrations is suggestive of the
same origin, arising as a result of the second ring-opening
process. This was further confirmed by the observation of
excitation bands at about 546 and 593 nm in the excitation
spectrum. The vibronic-structured emission showed a mirror
image as the absorption band (Supporting Information, Figure
S4), typical of fluorescence nature as observed in other organic
fluorophores, such as BODIPY, rhodamine, and fluorescein.
Arising from two successive ring-opening processes in the
presence of various concentrations of Hg(II) ion, two reporting
states with different spectroscopic properties were suggested,
that is, the first state showing pink color (absorption maximum
at 496 nm) but no emission, while the second state showing
purple color (absorption maximum at 593 nm) and red
emission (emission maximum at 620 nm). The mechanism of
such different spectroscopic responses is proposed in Scheme 4
(See Computational Studies). In the first reporting state with
only one ring-opened spirolactam group, there is a π-
conjugation across of three fused rings of a xanthene moiety
and a phenyl group, which is responsible for the pink color and
the absorption bands at 464, 496, and 530 nm. One would
anticipate observing the corresponding emission attributed to
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Figure 6. Electronic absorption (left) and emission (right) spectral changes of trans-2 (conc. = 9.57 × 10−6 M) upon addition of various
concentrations of Hg(II) ion in MeOH. Insets show the plot of absorbance or emission intensity as a function of the concentration of Hg(II) ion.
such π-conjugated structure, similar to that of the ordinary
rhodamine derivatives. However, no detectable emission was
observed in this reporting state, probably because of the PET
process from another diethylamino group. In the second
reporting state with two ring-opened spirolactam groups in the
presence of a higher concentration of Hg(II) ions, a higher
extended π-conjugation across the fused-structure of five rings
and two phenyl groups is suggested to give the purple color and
the absorption bands at 546 and 593 nm. An intense vibronic-
structured emission at 620 and 670 nm emerged, which is
attributed to this higher extension of the π-conjugated
structure. In this reporting state, the aforementioned PET
process was blocked with no available lone pair electron since
both of the two electron-donating diethylamino groups were
converted into electron-deficient ammonium groups.
The spectroscopic changes of 2 upon addition of Hg(II) ion
have also been investigated and are basically similar to those of
1. The corresponding electronic absorption spectral changes of
cis-2 in methanol are illustrated in Figure 5 (left). In view of the
observation of the 593 nm absorption band (Figure 5 (left))
and 620 nm emission peak (Figure 5 (right)), successive ring-
opening processes upon addition of Hg(II) ion were also
demonstrated (Scheme 4). The growth of absorbance or
emission intensity exhibits a sigmoid response curve and
reaches saturation at relatively low concentration of Hg(II) ion
of about 7 mol equiv. It was suggested that the two amide
groups in cis-orientation would cooperatively bind one Hg(II)
ion leading to the ease of the second ring-opening process.
Upon addition of Hg(II) ion into the solution of trans-2, the
change of solution color and emission intensity is slightly
different from those of cis-2. In addition to the absorption
bands at 496 and 593 nm, the electronic absorption spectra of
trans-2 with excess Hg(II) ion revealed that there is another
low-energy absorption at 688 nm (Figure 6, (left)), which may
account for the slight difference in the solution color. On the
other hand, an emission band at 623 nm was also observed
upon the ring-opening of two spirolactam groups (Figure 6,
(right)). The emission intensity profile is consistent with the
plot of the 593 nm absorbance to the function of Hg(II) ion
concentration, which reaches the saturation at 25 equiv of
Hg(II) ion. The discrepancies in their sensitivity and
spectroscopic response toward Hg(II) ion between cis-2 and
trans-2 indicate that the different conformations of stereo-
isomers in 2 would influence their ring-opening behaviors,
which is in contrast to the case of 1. As ring-opening of the
spirolactam of rhodamine derivatives is commonly induced by
protonation,6 successive ring-opening processes with the similar
spectroscopic changes were also observed for cis-1 (Supporting
Information, Figure S5) or trans-1 (Supporting Information,
Figure S6) upon addition of acid. Unlike 1, addition of small
amount of acid resulted in no observable solution color and
spectroscopic changes for 2, mainly because of the presence of
two other NH2 groups. This result shows the advantage of
compound 2 to selectively sense Hg(II) ion without the
interference of acid.
Chemodosimeter Studies. The research of selective
chemosensors for toxic heavy metal ions for the detection of
environmentally and biologically species has been developed
largely in recent years.3b,c,6c,8a A rhodamine-based chemo-
dosimeter for Hg(II) ion was first reported by Tae and co-
workers in 2005,8a in which a stoichiometric and irreversible
Hg(II)-promoted reaction of thiosemicarbazides was utilized to
form 1,3,4-oxadiazoles leading to the concept of selective
Hg(II) ion colorimetric and fluorescent chemodosimeters.9 To
extend the work to study the corresponding chemodosimeters
from the present compound with two spirolactam groups, a
related compound cis-3 was synthesized (Scheme 2). An
optimized condition of acetonitrile-buffer (1:1, v/v), in which
the buffer was HEPES buffer solution (pH = 7.0, 10 mmol) in
water, as the solvent was used. Upon addition of Hg(II) ion
into the solution of cis-3, an instantaneous color change from
colorless to purple was observed, indicating that the Hg(II) ion
mediated desulfurization reaction proceeded rapidly at room
temperature. No such color change or only very small color
change was observed upon addition of various ions, including
Li+, Na+, K+, Ca2+, Mg2+, Ba2+, Zn2+, Cd2+, Ag+, Cu2+, Pb2+, F−,
Cl−, and I−, suggestive of the high selectivity toward Hg(II) ion
(Supporting Information, Figure S7). In view of the presence of
two thiosemicarbazides undergoing desulfurization reaction in
cis-3, the time required for the complete reaction may be longer
than the related simple rhodamine compounds. It was found
that the electronic absorption spectrum of cis-3 remained the
same after 30 min upon addition of about 1.8 equiv of Hg(II)
ion. A 30-min reaction time was therefore selected in the
subsequent spectroscopic experiments. Electronic absorption
titration studies of cis-3 with various concentrations of Hg(II)
ion were also carried out. Similar to the cases in 1 and 2 (both
in trans- and cis-forms), two successive ring-opening processes
were indicated from the spectroscopic profile (Figure 7). The
absorbance at 602 nm, which corresponds to the extended π-
conjugated structure after the second ring-opening of
spirolactam groups, emerged, and the formation of 4 (Scheme
5) in this solution upon addition of Hg(II) ion was identified
by mass spectroscopy showing prominent peaks at m/z = 448.3
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xanthene moiety. The energy level of the HOMO was found
between those of the lowest unoccupied molecular orbital
(LUMO) with the ring-opened xanthene character and the
HOMO−1 with the five fused rings character, indicating that
the diethylamino group could act as potential electron donor of
the proposed PET process for the emission originated from
HOMO−1 and LUMO excited state, accounting for the
nonemissive behavior of partial ring-opened cis-1 (cis-1-p). In
contrast, both of the HOMO and LUMO of the full ring-
opened cis-1 (cis-1-f) were found to locate at the five fused rings
of the luminophore (Figure 8 (right)), which demonstrates that
there is no electron donor for the PET process, and red
fluorescence was therefore observed. Similar PET from a
deprotonated tertiary amine N-oxide moiety was suggested by
DFT calculations for the nonradiative deactivation of the
emitting process of the chromophore in the related rhodamine-
based fluorescent probe.8i The DFT calculation results for
trans-1 were similar to that for cis-1 (Supporting Information,
Figure S10). TD-DFT calculations on the absorption spectra in
methanol were also carried out to further understand the
electronic transitions. As listed in Supporting Information,
Table S2 and shown in Supporting Information, Figure S11, an
intense absorption peak at 471 nm (f = 1.0346) for the partial
ring-opened cis-1 (cis-1-p) and at around 535 nm (f = 1.3279)
for the full ring-opened cis-1 (cis-1-f) were calculated, which
were ascribed to the strong xanthene-centered n→π* transition
(HOMO−1→LUMO) and the n→π* transition centered at
the five fused rings (HOMO→LUMO), respectively. The
drastic red-shift in the absorption band from 471 to 535 nm
was responsible for the color change from pink to purple in the
solution of cis-1 upon further addition of Hg(II) ions. Such red-
shift in the absorption band was in agreement with the
experimental results in electronic absorption spectroscopy and
ascribed to the increase in the extended π-conjugation from
xanthene to five fused rings.
Figure 7. Electronic absorption spectral changes of cis-3 (conc. = 12 ×
10−6 M) upon addition of increasing concentrations of Hg(II) ion in
acetonitrile-buffer (1:1, v/v) solution. (buffer = HEPES buffer solution
(pH = 7.0, 10 mmol) in water).
[M]2+ and 895.3 [M − H]+. The formation of 4 from cis-3 and
Hg(II) ion was further confirmed by comparing the electronic
absorption spectra of cis-3 and Hg(II) ion mixture, and pure 4
(Supporting Information, Figure S8) prepared independently.
Such absorbance at 602 nm reached to a saturation in 2 equiv
of Hg(II) ion revealing the Hg(II) ion chemodosimeter in 1:2
stoichiometry. However, no detectable emission was observed,
probably because of the PET process from the −(C2N2O)-
NH(C6H5) groups (Scheme 5) (See Computational Studies).
A similar PET process leading to emission quenching behavior
has also been reported.3b,6c Selectivity and interference studies
were also performed by monitoring the absorbance at 602 nm
in the solutions of cis-3 containing various ions (Supporting
Information, Figure S9). Apart from the remarkably high
selectivity toward Hg(II) ion, such colorimetric response will
not be influenced by subsequent addition of other metal ions.
When the resulting colored solution was subsequently treated
with 0.5 mol/L EDTA solution, the absorbance signal at 602
nm remained almost unchanged, indicative of the occurrence of
irreversible desulfurization reaction.
Computational Studies. To explore the feasibility of PET
in the present system, density functional theory (DFT)
calculations were performed, on the basis of the proposed
structures of the partial and full ring-opened form of 1, with
one and two ring-opened spirolactam group(s) on the
carboxylic acid, respectively. As shown in Figure 8 (left), the
highest occupied molecular orbital (HOMO) of the partial
ring-opened cis-1 (cis-1-p) was localized on the diethylamino
group and the benzene ring linked directly to the ring-closed
In contrast to the similar structure of the full ring-opened
forms of 1 and 2, it is interesting to note that compound 4, with
two ring-opened spirolactam groups, was found to show
nonemissive behavior. The frontier molecular orbital energies
and electron density distributions of 4 were shown in Figure 9
(left). Five molecular orbitals (from HOMO to HOMO−4)
located at one of the −(C2N2O)NH(C6H5) groups or their
phenyl were found between the frontier molecular orbitals of
the five fused rings chromophore (HOMO−5 and LUMO).
The −(C2N2O)NH(C6H5) groups were suggested to act as
electron donors of the proposed PET process, responsible for
nonradiative deactivation of the excited state of 4. On the other
hand, a closely related simple rhodamine 6G_oxadiazole was
reported by Tae and co-workers to give intense fluorescence at
Scheme 5. Proposed Mechanism for the Color Changes and Emission Behaviors of cis-3 and 4
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Figure 8. Optimized ground state geometries, frontier molecular orbital energies and electron density distributions of cis-1-p (left) and cis-1-f
(right).
Figure 9. Optimized ground state geometries, frontier molecular orbital energies, and electron density distributions of 4 (left) and rhodamine
6G_oxadiazole (right).
around 555 nm in water−methanol (4/1 v/v, pH = 7).8a DFT
calculations were also performed on such system, and the
molecular orbitals with −(C2N2O)NH(C6H5) character were
lower in energy than the HOMO with xanthene character
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(Figure 9 (right)). The absence of an electron donor for the
fluorescence quenching in the PET process elucidated the
intense fluorescence in rhodamine 6G_oxadiazole. TD-DFT
calculations on the absorption spectra of 4 and rhodamine
6G_oxadiazole were also performed, as shown in Supporting
Information, Figure S11 and Table S2.
the equipment for photophysical measurements and for her
helpful discussions.
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ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data of cis-1, trans-1, cis-2, and trans-2 in
CIF format. Structures of cis-1, trans-1, cis-2, and trans-2,
titration spectral changes with Hg(II) ion and acid, and other
related DFT and TDDFT results. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: keithwongmc@sustc.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
K.M.C.W. acknowledges the receipt of “Young Thousand
Talents Program” award and the start-up fund administrated by
the Organisation Department of CPC Central Committee and
South University of Science and Technology of China,
respectively. We gratefully thank Dr. L. Szeto and Prof. W.T.
Wong for X-ray crystal structure determinations, Dr. J. Weng
for computational studies, and Prof. V.W.W. Yam for access to
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Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.;
Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador,
P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; , and Fox, D. J. Gaussian 09,
Revision A.01; Gaussian, Inc.: Wallingford, CT, 2009.
(11) Compound 1 has previously been reported without successful
separation and isolation of the diastereomers. Kamino, S.; Horio, Y.;
Komeda, S.; Minoura, K.; Ichikawa, H.; Horigome, J.; Tatsumi, A.;
Kaji, S.; Yamaguchi, T.; Usami, Y.; Hirota, S.; Emomoto, S.; Fujita, Y.
Chem. Commun. 2010, 46, 9013.
(12) Although some of the refinement R-factors are large resulting
from the low completeness of data collection probably because of the
poor quality of single crystals, the assignment of stereoisomers for cis-
and trans-forms could be definitely confirmed.
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