Solvent dependent competition between fluorescence resonance energy transfer and through bond energy transfer in rhodamine appended hexaphenylbenzene derivatives for sensing of Hg2+ ions

Article abstract of DOI:10.1039/c2dt32803h

Hexaphenylbenzene (HPB) derivatives 5 and 7 having rhodamine B moieties have been designed and synthesized, and have been shown to display solvent dependent. Fluorescence resonance energy transfer (FRET) and through bond energy transfer (TBET) in the pres

Full text of DOI:10.1039/c2dt32803h

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Solvent dependent competition between fluorescence
resonance energy transfer and through bond energy
transfer in rhodamine appended hexaphenylbenzene
derivatives for sensing of Hg2+ ions†
Cite this: Dalton Trans., 2013, 42, 4456
Vandana Bhalla,* Varun Vij, Ruchi Tejpal, Gopal Singh and Manoj Kumar*
Hexaphenylbenzene (HPB) derivatives 5 and 7 having rhodamine B moieties have been designed and
synthesized, and have been shown to display solvent dependent. Fluorescence resonance energy transfer
(FRET) and through bond energy transfer (TBET) in the presence of Hg2+ ions among the various cations
(Cu2+, Pb2+, Zn2+, Ni2+, Cd2+, Ag+, Ba2+, Mg2+, K+, Na+, and Li+) have been tested. Derivative 5
displays quite high through bond energy transfer efficiency in the presence of Hg2+ ions in methanol
whereas derivative 7 exhibits better FRET efficiency in the presence of Hg2+ ions in THF and CH3CN than
derivative 5.
Received 23rd November 2012,
Accepted 3rd January 2013
DOI: 10.1039/c2dt32803h
www.rsc.org/dalton
donors connected to acceptors via linkers and energy transfer
(ET) in such systems occurs through space and through
Introduction
Among various heavy metal ion pollutants, mercury contami- bonds.13 The efficiency of FRET is controlled by the distance
nation is widespread with distinct toxicological profiles. between the energy donor and energy acceptor fluorophores
Mercury is found in different forms in many products like and the spectral overlap between the emission spectrum of the
paints, electronic products and batteries, which enhance the energy donor and the absorption spectrum of the energy
severe effects on human health and environment.1 Given these acceptor,14 whereas the TBET15 systems are not limited by the
health and environmental concerns, efforts are being made for constraint of such spectral overlap between the donor emis-
detection and quantification of Hg2+ ions in various environ- sion and the acceptor absorption. Furthermore, high energy
mental and biological samples. In this context, development transfer efficiencies, fast energy transfer rates and large
of mercury selective fluorescent chemosensors2 has attracted pseudo-Stokes’ shift enable applications of TBET systems as
considerable research interest due to their high sensitivity, optical materials,16 photosynthetic models,17 in biotechnol-
selectivity and simplicity over other traditional methods ogy18 and as chemosensors. Recently, from our laboratory, we
including atom absorption spectroscopy,3 induced coupled reported naphthalimide–rhodamine fluorescent dyad and penta-
plasma spectroscopy,4 X-ray fluorescence spectrometry,5 and quinone–rhodamine dyad and triad which undergo TBET in
anodic stripping voltammetry.6 Several fluorescent chemosen- the presence of Hg2+ ions in mixed aqueous media.19 Now, in
sors involving different photophysical processes like photo- the present manuscript we have designed, synthesized and
induced electron/energy transfer,7 metal–ligand charge transfer evaluated new rhodamine appended HPB derivatives in which
(MLCT),8 intramolecular charge transfer (ICT),9 excimer/exci- the HPB unit acts as a donor and rhodamine as an acceptor.
plex formation,10 imine isomerization,11 chelation enhanced Synthesis of HPB derivatives has attracted great attention
fluorescence (CHEF)12 have been reported.
because of their potential applications in supramolecular and
Recently, the labeling of organic molecules with fluorescent material chemistry.20 HPB core has been used for the prep-
tags has attracted attention due to potential applications of aration of graphitic-like, dendritic and photoconductive poly-
such systems in biochemical experiments. Such systems have cyclic aromatic hydrocarbons or as a scaffold for a starlike array
of functional materials such as porphyrin which have potential
applications in the field of nanotechnology and molecular elec-
Department of Chemistry, UGC Sponsored-Centre for Advanced Studies-I, Guru
tronics. The unique propeller-shaped arrangement of six peri-
pheral aryl groups around a central benzene ring in various HPB
derivatives limit conjugation and disfavor extensive intermole-
cular π–π interactions. Keeping this in view, we designed and
synthesized HPB-based derivatives 5 and 7 having rhodamine
Nanak Dev University, Amritsar-143005, Punjab, India.
E-mail: vanmanan@yahoo.co.in, mksharmaa@yahoo.co.in
†Electronic supplementary information (ESI) available: 1H, 13C, mass spectra,
UV-vis and fluorescence spectra in different alcohols, equation for quantum
yield. See DOI: 10.1039/c2dt32803h
4456 | Dalton Trans., 2013, 42, 4456–4463
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moieties. For a ‘through bond energy transfer’ process to come respectively, each through a conjugated spacer is presented in
into effect there should be a low degree of conjugation Schemes 1 and 2, respectively. Suzuki–Miyaura cross coupling
between energy donor and energy acceptor and low degree of of compound 1 with boronic ester 2 furnished compound 3 in
planarity. We envisioned that the HPB derivatives 5 and 7 53% yield. The 1H NMR spectrum of compound 3 showed
having two/six rotors rotating around their own axis could three doublets (4H each) and one multiplet (24H) correspond-
impede the electron conjugation between donor and acceptor ing to aryl protons (ESI, S3†). The mass spectrum of compound
moieties thus, fulfilling the requirements for TBET to occur. 3 showed parent ion peak at 717.4 corresponding to diamine 3
Interestingly, derivatives 5 and 7 exhibit energy transfer by fol- (ESI, S5†). A similar approach using a six-fold Suzuki–Miyaura
lowing both the mechanisms i.e. through space and through coupling reaction of boronic ester 2 with hexakis(4-bromophe-
bond in the presence of Hg2+ ions and, interestingly, process nyl)benzene yielded derivative 6 in 70% yields.21
of energy transfer is solvent dependent. To the best of our
Furthermore, the reaction of derivatives 3 and 6 with rhoda-
knowledge, this is the first report where donor–acceptor mine acid chloride 422 in dichloromethane and N,N′-dimethyl-
systems display solvent dependent switching of energy transfer formamide furnished compounds 5 and 7 in 63% and
mechanism in the presence of Hg2+ ions. In THF and CH3CN 50% yields, respectively (Schemes 1 and 2). The structures
through space energy transfer mechanism is operative whereas of compounds 5 and 7 were corroborated from their spectro-
in protic solvent such as MeOH through bond energy transfer scopic and analytical data (ESI, S6–S11†). The 1H NMR spec-
is operative.
trum of compound 5 showed one triplet (24H), one quartet
(16H), three doublets (4H, 2H, 2H), two multiplets (6H, 36H),
and two broad signals (4H, 2H). The 1H NMR spectrum of
compound 7 showed one doublet (6H), five multiplets (72H,
Results and discussion
The synthetic route for derivatives 5 and 7, in which two and 48H, 18H, 18H and 18H) and two broad singlets (24H, 6H).
six rhodamine moieties are connected to the HPB core, The corresponding mass spectra showed parent ion peaks at
Scheme 1 Synthetic scheme of compound 5.
Scheme 2 The synthetic scheme of compound 7.
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Scheme 3 The synthetic scheme of compound 11.
1565.5 (M+) and 3630.8 (M + 1+) corresponding to the conden- opened amide conformation that facilitates the complexation.
sation products 5 and 7, respectively. This spectroscopic data Thus, in the presence of mercury ions, compound 5 shows the
corroborates the structures of derivatives 5 and 7. We also syn- absorption characteristics of both donor and acceptor com-
thesized a model compound 11 in 44% yield in which one rho- ponents. No such observation was found in the presence of
damine unit is attached to HPB core without any spacer by other metal ions except for Fe2+ where a slight colour change
reaction of rhodamine acid chloride 4 with compound 10 was observed on adding 20 equiv. of Fe2+ ions. Similar behav-
(Scheme 3) which was synthesized conveniently in 60% yield iour was observed in the case of compound 7 under the same
by Diels–Alder cycloaddition of tetraphenylcyclopentadienone experimental conditions as used for compound
5 (ESI,
8 and compound 9. The structures of compounds 10 and 11 Fig. S1†). However, no variation in the absorption spectrum of
were corroborated from their spectroscopic and analytical data derivative 7 was observed in the presence of other metal ions
(ESI, S12–S17†).
such as Cu2+, Fe2+, Fe3+, Co2+, Pb2+, Zn2+, Ni2+, Cd2+, Ag+, Ba2+,
The binding behaviour of compounds 5 and 7 toward Mg2+, K+, Na+, and Li+.
different cations (Cu2+, Hg2+, Fe2+, Fe3+, Co2+, Pb2+, Zn2+, Ni2+,
In the fluorescence spectrum, receptor 5 exhibited fluor-
Cd2+, Ag+, Ba2+, Mg2+, K+, Na+, and Li+) as their perchlorate escence emission at 506 nm in CH3CN when excited at 290 nm
salts was investigated by UV-Vis and fluorescence spectroscopy. which is attributed to the typical band of HPB moiety (Fig. 2).
The absorption spectrum of 5 (5 μM) exhibits two bands at 242 The rhodamine moiety in 5 remains in a closed, non-fluor-
and 278 nm in THF (Fig. 1). The absence of any absorption escent spirolactam form indicating weak spectral overlap
transition at 400–600 region and appearance of colourless sol- between hexaphenylbenzene (energy donor) emission and rho-
ution indicates lactonized conformation of rhodamine in the damine (energy acceptor) absorption. As a result, the emission
compound. However, upon addition of Hg2+ ions (0.1–20 due to the HPB moiety is observed at 506 nm. In the presence
equiv.), the intensity of absorption bands at 242 nm and of Hg2+ ions (50 equiv.), the emission band at 506 nm
278 nm increased and a new band appeared at 554 nm (Fig. 1). decreases along with the formation of new band characteristic
These changes are accompanied by a gradual change of colour of the acceptor component (rhodamine) at 585 nm (ϕ = 0.48)
from colourless to pink, visible to the naked eye (inset, Fig. 1). which is attributed to the opening of the spirolactam ring of
The formation of a new band at 554 nm is attributed to the rhodamine to an amide form (Scheme 4). Since the excitation
interaction of Hg2+ ions with the receptor 5 leading to the wavelength of compound is 290 nm, a double frequency peak
opening of spirolactam ring of rhodamine moiety to its ring at 580 nm is anticipated which interferes with the emission
Fig. 1 UV-vis spectra of receptor 5 (5 μM) in the presence of Hg2+ ions (0–20
equiv.) in THF. Inset shows the change in color of 5 (5 µM) on addition of Hg2+
ions.
Fig. 2 Fluorescence response of receptor 5 (1 µM) on addition of Hg2+ (0–50
equiv.) in CH3CN, λex = 290 nm. Inset shows the change in the fluorescence on
the addition of Hg2+ ions.
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Scheme 4 Hg2+ induced FRET off–on in aprotic solvents.
band at 585 nm due to ring opened rhodamine moiety. The
Interestingly, derivative 7 exhibits better FRET efficiency in
discrimination between rhodamine band and double fre- THF (ESI, Fig. S3†) and in CH3CN (Fig. 3) than that of deriva-
quency peak is done on the basis of the change in colour of tive 5 as shown in Table 1. We propose that in derivative 7,
fluorescence emission from blue to orange which indicates steric congestion caused by six rhodamine moieties restrict the
that the band at 585 nm is due to the spirolactam ring rotation of the phenyl rings around their own axis and the con-
opening of rhodamine group (Fig. 2, inset). The emission spec- jugation across the donor and acceptor moieties is more facili-
trum of donor (HPB moiety) group and absorption spectrum tated, thus, through space energy transfer (FRET) is more
of acceptor (rhodamine moiety) group show a spectral overlap facilitated in derivative 7 in comparison to derivative 5 where
(ESI, Fig. S4 and S5†) which results in fluorescence resonance the steric congestion is relatively less.
energy transfer with large pseudo-stokes shift of 295 nm.
To get more insight into role of solvent in metal–receptor
Derivative 5 exhibits FRET efficiency of 67% and 10.8 folds of complexation, we carried out the fluorescence studies of com-
fluorescence enhancement at λem of 585 nm in acetonitrile. pound 5 and 7 in protic solvents such as MeOH, EtOH,
However in THF, compound 5 shows 7 fold fluorescence emis- n-PrOH and n-BuOH. On increasing the amounts of methanol
sion enhancement on addition of incremental amounts of in THF solution of compound 5, the fluorescence of the com-
Hg2+ ions (0.1–50 equiv.) with 43% FRET efficiency (ESI, pound 5 starts quenching with a considerable red shift (ESI,
Fig. S2†). Under the same experimental conditions as used for Fig. S6†) and is quenched completely in pure methanol. This
compound 5, compound 7 (in CH3CN) exhibits 12.9 fold fluor-
escence emission enhancement on addition of Hg2+ ions
(0.1–50 equiv.) with 85% resonance energy transfer efficiency
and 8.3 folds fluorescence emission enhancement in presence
of Hg2+ ions (0.1–50 equiv.) with 60% resonance energy transfer
efficiency in THF. These results suggest that the donor–acceptor
systems 5 and 7 in the presence of Hg2+ ions undergo more
emission enhancement and better resonance energy transfer
efficiency in CH3CN than that in THF (Table 1), thus, indicating
possibility of solvent molecules playing the crucial role in
metal–receptor complexation. We believe that CH3CN being a
coordinating solvent,23 actively indulges in the complexation of
Fig. 3 Fluorescence response of receptor 7 (1 µM) on addition of Hg2+ (0–50
Hg2+ ions with HPB derivatives 5 and 7 through solvent-assisted
equiv.) in CH3CN, λex = 290 nm. Inset shows the change in fluorescence upon of
Hg2+ ions.
coordination apart from the receptors binding to the Hg2+ ions.
Table 1 Comparison of fluorescence efficiency, quantum yields and fluorescence enhancement factor of 5 and 7
FRET
Acetonitrile
Tetrahydrofuran
TBET (methanol)
Derivative
E
ϕ at λ = 585 nm
FEF
E
ϕ
FEF
E
ϕ at λ = 585 nm
FEF
5
0.67
0.85
0.02
0.48
0.012
0.36
10.8 fold
12.9 fold
0.43
0.60
0.09
0.58
0.01
0.40
7 fold
8.3 fold
0.98
0.92
4 × 10−3
0.68
197 fold
130 fold
5–Hg2+
7
3.4 × 10−3
0.46
7–Hg2+
E = efficiency, ϕ = quantum yield, FEF = fluorescence enhancement factor.
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derivative 6 (donor) and rhodamine B (acceptor) in methanol.
The fluorescence spectrum exhibits the individual emission
band at 426 nm, corresponding to derivative 6 only. No fluor-
escence corresponding to rhodamine group was observed at
λex = 290 nm (ESI, Fig. S21†) which shows that there is no inter-
molecular resonance energy transfer between hexaphenylben-
zene (donor) and rhodamine (acceptor). Thus, the advantage
of the TBET system for energy transfer is obvious.
Furthermore, the operation of through bond energy transfer
mechanism in the presence of Hg2+ ions in derivative 5 in
EtOH, n-PrOH and n-BuOH and the increase in fluorescence
emission enhancement in the same order as the proton donat-
ing ability of these solvents i.e. n-butanol (42 fold) (ESI,
Fig. S9†), n-propanol (68 fold) (ESI, Fig. S10†), ethanol (75 fold)
(ESI, Fig. S11†) and methanol (197 fold), confirm above assump-
tion regarding solvent assisted TBET process. The fluorescence
enhancement factor for derivative 5 in presence of Hg2+ ions
in MeOH (197 folds) is higher than enhancement factors
(Table 1) of derivative 5 in THF and CH3CN in presence of
Hg2+ ions, thus, confirming the 98% energy transfer efficiency
of cassette 5 in MeOH in presence of Hg2+ ions. Derivative 7
also exhibits through bond energy transfer in the presence of
Hg2+ ions in methanol (130 folds) (Fig. 5), n-butanol (7.5 folds)
(ESI, Fig. S12†), n-propanol (10 folds) (ESI, Fig. S13†) and ethanol
(25 folds) (ESI, Fig. S14†). The solution of compound 7 in
MeOH is non-fluorescent (ϕ = 3.4 × 10−3) and addition of
incremental amounts of Hg2+ions (0.1–150 equiv.) to the sol-
ution of receptor 7 leads to appearance of an emission band
due to rhodamine (acceptor) moiety with 130 fold emission
enhancement at 585 nm (ϕ = 0.46) along with the bright yellow
fluorescence visible to the naked eye (inset, Fig. 5). The fluor-
escence efficiency of compound 7 in methanol comes out to
be E = 0.92. However, in comparison to derivative 5, through
bond energy transfer in the case of derivative 7 in protic media
is 92%. It is proposed that in derivative 7, conjugation across
the donor and acceptor moieties is more facilitated due to
presence of six rhodamine moieties at the periphery which
restricts rotation of the phenyl rings around their own axis,
thus, making derivative 7 a weaker candidate for TBET in com-
parison to derivative 5. The detection limits of compound 5 and
7 as fluorescent sensors for the analysis of Hg2+ ions were found
Fig. 4 Fluorescence response of receptor 5 (1 µM) on addition of Hg2+ (0.1–20
equiv.) in methanol, λex = 290 nm. Inset shows the ‘turned on’ fluorescence of
compound 5 on addition of Hg2+ ions.
quenching in fluorescence is due to the photoinduced electron
transfer (PET) from nitrogen atom of the spirolactam ring to
the photoexcited hexaphenylbenzene unit. Since, photoin-
duced electron transfer (PET) phenomenon is more operative
in polar solvents than in non-polar solvents,24 PET becomes
more effective in methanol which leads to fluorescence
quenching. On addition of incremental amounts of Hg2+ ions
(0.1–20 equiv.) to the solution of receptor 5 in methanol,
PET is blocked accompanied by the opening of spirolactam
ring of rhodamine group which leads to the donor group (hexa-
phenylbenzene) to emit fluorescence energy which is rapidly
transferred to rhodamine group which emit fluorescence at
585 nm with 197 fold emission enhancement and fluorescence
efficiency of E = 0.98 (Fig. 4). The ‘turn on’ emission observed
on subsequent addition of Hg2+ ions can also be seen by
the naked eye (Fig. 4, inset). This result corroborates the idea
of through bond energy transfer. Thus, the mechanism of
energy transfer from HPB based donor to rhodamine acceptors
in presence of Hg2+ ions is strongly dependent upon the
nature of solvent. We believe that ring opening of the spiro-
lactam structure is solvent assisted in protic environ-
ment, and hydrogen bonding between solvent and acceptor
moiety prevents donor and acceptor fragments from becom-
ing planar, and thus, the TBET process is facilitated
(Scheme 5).
Under the same set of conditions as used for compounds 5
and 7, we recorded the fluorescence of equimolar solution of
Scheme 5 Hg2+ induced TBET off–on in protic solvents.
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emission was observed by comparison with or without the
other metal ions (ESI, Fig. S8†).
To elucidate the binding mode of receptor 5 with Hg2+ ions,
the 1H NMR spectrum of its complex with mercury perchlorate
was also recorded. The downfield shifts of 0.24 and 0.12 ppm
corresponding to the protons of NCH2CH3 and NCH2CH3,
respectively, and the aromatic protons of the rhodamine moi-
eties of receptor 5 in the presence of 1.0/2.0 equiv. of Hg2+
ions (ESI, Fig. S18†) indicate the transformation of non-fluor-
escent spirocyclic form of rhodamine moiety in receptor 5 to
the fluorescent ring opened amide form (Scheme 4). Thus
from this NMR study, we may conclude that mercury is inter-
acting with receptor 5 as supported by fluorescence studies.
The 1 : 2 stoichiometry between compound 5 and Hg2+ ions
was confirmed by the Job’s plot (ESI, Fig. S19†). The binding
constant (log β) was found to be 9.58 0.05, inferred from the
nonlinear regression analysis program SPECFIT (global analy-
sis system V3.0 for 32-bit Windows system).
Fig. 5 Fluorescence response of receptor 7 (1 µM) on addition of Hg2+ (1–60
equiv.) in methanol, λex = 290 nm and the emission intensity of receptor 7
(1 µM) at 585 nm as a function of Hg2+ ions. Inset shows the ‘turned on’ fluor-
escence of compound 7 on addition of Hg2+ ions.
to be 50 × 10−9 M and 10 × 10−8 M, respectively (ESI, Fig. S15†)
which are sufficiently low for the detection of nano-molar con-
centrations of Hg2+ ions as found in many chemical systems.
Furthermore, the fluorescence spectra of model compound
11 in THF shows very weak emission band at 455 nm due to
the HPB group when excited at 290 nm and addition of incre-
mental amounts of Hg2+ ions (0.1–100 µM) to the solution of
receptor 5 in THF leads to slight decrease in intensity of the
emission band at 455 nm, suggesting no energy transfer from
donor HPB unit to acceptor rhodamine moiety (ESI, Fig. S22†).
Besides, no orange colour fluorescence was observed with the
naked eye (ESI, Fig. S22,† inset) on addition of Hg2+ ions. This
indicates that the band at 580 nm corresponds to double fre-
quency peak. This result shows that derivative 11 having a rho-
damine unit linked to HPB core without any spacer behaves as
one planar conjugated molecule.
Cyclic voltammogram of 5 [CH2Cl2, c = 1 × 10−3 M,
(n-Bu)4NClO4 as supporting electrolyte using a glassy carbon
working electrode, a (Ag/Ag+) reference electrode, and a Pt wire
counter electrode] exhibits three electrochemical oxidation
waves at E1/2 = −1.60 V, 0.74 V and 1.53 V (ESI, Fig. 20A†). On
addition of 2 equiv. of Hg2+ ions, the cyclic voltammogram
exhibits a shift in these oxidation waves to −1.414 V, 0.50 V
and 1.42 V, respectively (ESI, Fig. 20B†). These shifts in oxi-
dation waves toward lower potential in presence of Hg2+ ions
indicate the decrease in oxidation potential of derivative 5 due
to the formation of complex between derivative 5 and Hg2+ ions.
Conclusion
To test if the proposed complexation of compound 5 and 7
with Hg2+ ions could be reversed we also carried out a reversi-
bility experiment. The addition of tetrabutylammonium iodide
(TBAI) to the solutions of 5–Hg2+ (1.0 µM in methanol) and
7–Hg2+ complexes (1.0 µM in methanol) resulted in quenching
of their respective fluorescence intensities. The quenching
of fluorescence is due to the strong affinity of iodide ions for
the Hg2+ ions which is responsible for decomplexation of
receptor–Hg2+ complex i.e. Hg2+ ions are not available for
binding with receptor. Further addition of Hg2+ ions revives the
respective fluorescence emission indicating the reversible
behaviour of both derivatives 5 and 7 for Hg2+ ions (ESI,
Fig. S16 and S17†).
In conclusion, we synthesized hexaphenylbenzene derivatives
5 and 7 incorporating rhodamine moieties. Derivatives 5 and 7
exhibit fluorescence resonance energy transfer through space
(FRET) in aprotic solvents and through bond energy transfer
(TBET) in protic solvents only in the presence of Hg2+ ions
among various metal ions such as Cu2+, Fe2+, Fe3+, Co2+, Pb2+,
Zn2+, Ni2+, Cd2+, Ag+, Ba2+, Mg2+, K+, Na+, and Li+. Derivative 7
exhibits more efficient FRET efficiency in the presence of Hg2+
ions in THF and CH3CN than that of derivative 5 whereas
derivative 5 displays nearly perfect through bond energy trans-
fer efficiency in the presence of Hg2+ions in MeOH.
We also tested the fluorescence response of 5 and 7 to the
other metal ions such as Fe3+, Fe2+, Pb2+, Cd2+, Cu2+, Zn2+,
Ni2+, Ag+, Co2+, Mg2+, Li+, Na+, and K+ in THF, however, no sig-
nificant variation in the fluorescence spectra of 5 and 7 was
observed with any other metal ion except Fe3+ and Fe2+ which
also induce similar fluorescence emission but to a small
extent (ESI, Fig. S7†). To check the practical ability of com-
pound 5 and 7 as a Hg2+ selective fluorescent sensor, we
carried out competitive experiments in the presence of Hg2+ at
50 equiv. and 100 equiv., respectively, mixed with Fe3+, Fe2+,
Pb2+, Cd2+, Cu2+, Zn2+, Ni2+, Ag+, Co2+, Mg2+, Li+, Na+, and K+
(100 equiv. each). No significant variation in the fluorescence
Experimental
General experimental methods
All metal perchlorates were purchased from Aldrich and were
used without further purification. Potassium carbonate,
ethanol and tetrabutylammonium salts of anions were purchased
from S.D. Fine Chemicals. THF was dried over sodium metal
and benzophenone before it was used for analytical studies.
Acetonitrile (HPLC grade) and methanol (HPLC grade) were used
for analytical studies. All the fluorescence spectra were recorded
on SHIMADZU 5301 PC spectrofluorimeter. UV spectra were
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recorded on Shimadzu UV-2450PC spectrophotometer with a (300 MHz, CDCl3) δ 1.13 (t, 24H, J = 6.6 Hz, NCH2CH3), 3.29 (q,
quartz cuvette (path length: 1 cm). The cell holder was thermo- 16H, J = 6.0 Hz, NCH2CH3), 6.19–6.27 (m, 6H, ArH), 6.60 (d,
statted at 25 °C. Elemental analysis was done using Flash EA 4H, J = 8.4 Hz, ArH), 6.79–6.81 (m, 36H, ArH), 6.99 (d, 2H, J =
1112 CHNS/O analyzer of Thermo Electron Corporation. 7.8 Hz, ArH), 7.16 (d, 2H, J = 7.8 Hz, ArH), 7.46 (br, 4H, ArH),
1H and 13C NMR spectra were recorded on JEOL-FT NMR-AL 7.97 (br, 2H); 13C NMR (75 MHz, CDCl3) 13.03, 44.72, 98.34,
300 MHz spectrophotometer using CDCl3 and DMSO-d6 as 106.89, 108.53, 123.69, 124.34, 125.44, 125.53, 126.00, 126.93,
solvent and TMS as internal standards. Data are reported as 127.01, 127.18, 128.42, 129.14, 131.25, 132.13, 133.15, 136.07,
follows: chemical shifts in parts per million (δ), multiplicity 137.39, 138.89, 140.23, 140.80, 140.90, 140.98, 141.07, 149.18,
(s = singlet, br = broad signal, d = doublet, m = multiplet), 153.44, 153.93, 169.12; MALDI-MS: 1565 (M+). Anal. Calcd for
coupling constants (Hz), integration, and interpretation. All
C110H96N6O4: C, 84.37; H, 6.18; N, 5.37. Found: C, 84.54; H,
spectrophotometric titration curves were fitted with SPECFIT 6.22; N, 5.45.
32 software.
Synthesis of compound 7. To the stirred solution of acid
chloride 4 (0.1 g, 0.22 mmol) in DMF (HPLC) (10 mL) was
above solution was added the solution of hexamine 6 (0.07 g,
Experimental details of determining detection limit
To determine the detection limit, the fluorescence titration of 0.09 mmol) in DMF and triethylamine. The reaction mixture
compound 5 with Hg2+ ions was carried out by adding aliquots was stirred overnight at room temperature. The mixture so
of mercury solution of micromolar concentration and the flu- obtained was treated with water, extracted with dichloro-
orescence intensity as a function of Hg2+ ions added was then methane, and dried over anhydrous Na2SO4. The organic layer
plotted. From this graph the concentration at which there was was evaporated under reduced pressure and the crude product
a sharp change in the fluorescence intensity multiplied with was purified by column chromatography (EtOAc) followed by
the concentration of receptor 5 gave the detection limit.
the recrystallization from methanol to give 135 mg of 7 (yield
50%): mp >260 °C; H NMR (300 MHz, CDCl3) δ 1.13–1.11 (m,
72H, NCH2CH3), 3.30–3.28 (m, 48H, NCH2CH3), 6.21–6.24 (m,
1
Experimental details
Synthesis of compounds 1, 2 and 6: Compounds 1 and 2 were 18H, ArH), 6.62–6.57 (m, 18H, ArH), 6.74 (br, 24H, ArH), 6.98
synthesized according to the literature procedures. Compound (br, 12H, ArH), 7.25–7.14 (m, 18H, ArH), 7.47 (d, 12H, ArH),
6 was synthesized according to the procedure previously devel- 7.98 (br, 6H, ArH). 13C NMR (100 MHz, CDCl3: DMSO-d6)
oped in our lab.
12.24, 43.70, 97.19, 105.55, 107.82, 110.38, 122.8, 124.00,
Synthesis of compound 3. To a solution of 1 (0.3 g, 125.87, 126.28, 128.22, 129.91, 131.44, 135.46, 139.92, 148.16,
0.43 mmol) and 2 (0.23 g, 1.08 mmol) in THF were added 152.24, 166.85, 170.02. MALDI-MS: 3630.8 (M + 2+). Anal.
K2CO3 (0.48 mg, 3.5 mmol), distilled H2O (3 mL), and [Pd- Calcd for C246H228N18O12: C, 81.43; H, 6.33; N, 6.95. Found: C,
(Cl)2(PPh3)2] (0.12 g, 0.17 mmol) under argon and the reaction 81.13; H, 6.40; N, 7.16.
mixture was refluxed overnight. The THF was then removed
Synthesis of compound 10. Tetraphenylcyclopentadienone 8
under vacuum and the residue so obtained was treated with (0.9 g, 2.35 mmol) and phenyl acetylene 9 (0.5 g, 2.59 mmol)
water, extracted with dichloromethane, and dried over anhy- were suspended in minimal amount of diphenylether and
drous Na2SO4. The organic layer was evaporated and the com- refluxed overnight under nitrogen atmosphere. The reaction
pound was purified by column chromatography using ethyl mixture so obtained was cooled to room temperature, poured
acetate as an eluent to give compound 3 which was further into methanol and the solid hence obtained was filtered. The
recrystallized from methanol to provide 0.58 g of white solid crude product was purified by column chromatography (chloro-
(yield 53%). mp: 220 °C. 1H NMR: δ 6.64 (d, 4H, J = 8 Hz, ArH), form–hexane, 1 : 4) to afford 0.56 g of light yellow solid 10
6.81–6.85 (m, 24H, ArH), 7.05 (d, 4H, J = 8 Hz), 7.24 (d, 4H, J = (yield = 60%) which was further recrystallized in methanol.
1
7.5 Hz, ArH); 13C NMR (75 MHz, CDCl3): 80.51, 118.52, 124.77, Mp >260 °C; H NMR (300 MHz, CDCl3) δ 3.32 (s, 2H, NH2),
125.14, 125.20, 126.54, 126.64, 127.15, 127.20, 131.42, 131.84, 6.19 (d, 2H, J = 9 Hz, ArH), 6.57 (d, 2H, J = 9 Hz, ArH), 6.79–6.86
135.47, 136.87, 137.25, 139.35, 139.58, 140.42, 140.57, 140.62, (m, 25H, ArH). 13C NMR (75 MHz, CDCl3) δ 85.90, 87.82, 90.48,
152.61. FAB-MS: 717 (M + 1)+; Anal. Calcd for C54H40N2: C, 96.82, 114.17, 118.29, 125.40, 125.49, 126.92, 126.97, 131.84,
90.47; H, 5.62; N, 3.91. Found: C, 90.32; H, 5.69; N, 3.99.
131.89, 132.68, 136.25, 140.65, 141.99, 143.73, 154.05, 156.70.
Synthesis of compound 5. The acid chloride 4 (0.1 g, MALDI-MS: 549.2769 (M+). Anal. Calcd for C42H31N: C, 91.77;
0.21 mmol) was dissolved in dry dichloromethane (10 mL). To H, 5.68; N, 2.55 Found: C, 91.43; H, 5.71; N, 2.48.
the above solution was added the solution of compound 3
Synthesis of compound 11. The acid chloride (0.07 g,
(0.07 g, 0.09 mmol) in dichloromethane and triethylamine. 0.14 mmol) was dissolved in dry dichloromethane (10 mL). To
The reaction mixture was stirred overnight at room tempera- the above solution was added the solution of compound 10
ture. The mixture so obtained was treated with water, extracted (0.07 g, 0.14 mmol) in dichloromethane and triethylamine.
with dichloromethane, and dried over anhydrous Na2SO4. The The reaction mixture was stirred overnight at room tempera-
organic layer was evaporated under reduced pressure and the ture. The mixture so obtained was treated with water, extracted
crude product was purified by column chromatography with dichloromethane, and dried over anhydrous Na2SO4. The
(EtOAc : hexane, 7 : 3) and recrystallised from methanol to give organic layer was evaporated under reduced pressure and the
0.081 g of white solid 5 (yield 63%): mp >260 °C; 1H NMR crude product was purified by column chromatography
4462 | Dalton Trans., 2013, 42, 4456–4463
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(hexane : EtOAc, 95 : 5) and recrystallised from methanol to 11 J.-S. Wu, W.-M. Liu, X.-Q. Zhuang, F. Wang, P.-F. Wang,
give 60 mg of off-white solid 11 (yield 44%): mp >260 °C;
1H NMR (300 MHz, CDCl3) δ 7.88 (br, 1H, ArH), 7.40–7.43 (m,
S.-L. Tao, X.-H. Zhang, S.-K. Wu and S.-T. Lee, Org. Lett.,
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6.47–6.52 (m, 4H, ArH), 6.21–6.25 (m, 6H, ArH), 3.33 (q, 8H, J =
8 Hz, NCH2CH3), 1.17 (t, 12H, J = 7.5 Hz, NCH2CH3). 13C NMR
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1999.
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Acknowledgements
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