FRET-induced nanomolar detection of Fe2+ based on cinnamaldehyde-rhodamine derivative

Article abstract of DOI:10.1016/j.tetlet.2011.06.044

A new dimethylaminocinnamaldehyde linked rhodamine based fluorescence receptor 3 is synthesized which shows fluorescence resonance energy transfer in the presence of Fe²⁺ ions, thus enhancing rational partition in between donor and acceptor emi

Full text of DOI:10.1016/j.tetlet.2011.06.044

Tetrahedron Letters 52 (2011) 4333–4336
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
FRET-induced nanomolar detection of Fe2+ based
on cinnamaldehyde-rhodamine derivative
⇑
Manoj Kumar , Naresh Kumar, Vandana Bhalla
Department of Chemistry, UGC center for Advance Studies-1, Guru Nanak Dev University, Amritsar 143005, Punjab, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new dimethylaminocinnamaldehyde linked rhodamine based fluorescence receptor 3 is synthesized
which shows fluorescence resonance energy transfer in the presence of Fe2+ ions, thus enhancing rational
partition in between donor and acceptor emissions and permitting separated measurement of emissions
of both fluorophores.
Received 6 May 2011
Revised 8 June 2011
Accepted 10 June 2011
Available online 17 June 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Rhodamine
Cinnamaldehyde
FRET
Fe2+
The development of fluorescent chemosensors for the recogni-
tion of environmentally and biologically important cations has been
a research area of particular interest. Among different metal ions,
iron is a primary metal ion in numerous physiological processes
involving electron transfer and oxidation.1 Iron, being both useful
and cytotoxic2, deficiency throughout the developmental phases
may lead to permanent loss of motor skills.3 Accretion of iron in
the central nervous system has been involved in a number of dis-
eases, such as Parkinson’s, Huntington’s, and Alzheimer’s disease,
associated with an increased quantity of iron.2a Keeping in view
the roles played by iron in day to day life the development of tech-
niques for selective determination of iron is in immense demand.
Fluorescence spectroscopy has been a very useful technique for
selective and sensitive recognition of metal ions. In most of the fluo-
rescent sensors the cation binding involves photophysical changes
like photoinduced electron transfer (PET),4 photoinduced charge
transfer (PCT),5 formation of monomer/excimer,6 energy transfer7,
and more recently fluorescence resonance energy transfer (FRET)
where the excitation energy from one fluorophore (donor) is trans-
ferred to another fluorophore (acceptor) without the emission of a
photon.8 FRET is an active field in supramolecular chemistry due
to its potential practical benefits in cell physiology, optical therapy,
as well as selective and sensitive sensing toward targeted molecular
or ionic species.9 FRET induced by guest binding is a proficient ap-
proach to design ratiometric fluorescence probes, as they can emit
at two different wavelengths with a single excitation source.10
Our research work involves the design, synthesis, and evalua-
tion of fluorogenic receptors for selective sensing of soft metal ions,
anions and evaluation of their switching behavior.11 Recently, we
reported a naphthalimide appended rhodamine based fluorescent
chemosensor which showed thorough bond energy transfer in
the presence of Hg2+ ions.12 In continuation of this work, we have
now designed and synthesized a dimethylaminocinnamaldehyde
linked rhodamine based chemosensor 3 which undergoes fluores-
cence resonance energy transfer in the presence of Fe2+ ions. Rho-
damine and its derivatives which absorb in the range of 450–
600 nm (Fig. 1) are well known for their desirable properties,
including good photostability, high extinction coefficient, and high
fluorescent quantum yield.13 On the other hand, dialkylaminocin-
namaldehyde derivatives emit in the range of 400–550 nm
(Fig. 1). Hence, the spectral overlap between the emission spectra
of dimethylaminocinnamaldehyde and absorption spectra of the
ring-opened form of rhodamine is significant (Fig. 1). On the basis
of this, we envisaged that the attachment of dialkylaminocinnam-
aldehyde moiety with rhodamine would give a system which may
exhibit the phenomenon of fluorescence resonance energy trans-
fer. Thus, chemosensor 3 was synthesized in which rhodamine is
linked to dimethylaminocinnamaldehyde moiety. Compound 3
undergoes fluorescence resonance energy transfer in the presence
of Fe2+ ions enhancing rational partition in between donor and
acceptor emissions permitting separated measurement of the
emissions of both fluorophores. To the best of our knowledge, this
is the first report where fluorescence resonance energy transfer
phenomenon is observed between dimethylaminocinnamaldehyde
and rhodamine moieties on the addition of Fe2+ ions.
⇑
Corresponding author. Tel.: +91 183 2258802 9; fax: +91 183 2258820.
E-mail address: mksharmaa@yahoo.co.in (M. Kumar).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.06.044

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M. Kumar et al. / Tetrahedron Letters 52 (2011) 4333–4336
Figure 2. UV–vis spectra of 3 (5 l
M) in the presence of Fe2+ ions (20 equiv) in THF;
Figure 1. Spectral overlap between donor (4) emission (red) and ring opened
rhodamine B absorption (blue).
Inset showing the color change before and after the addition of Fe2+ ions.
The condensation of rhodamine hydrazide 214 with N,N-dim-
ethylaminocinnamaldehyde 1 furnished compound 3 in 78% yield
(Scheme 1).15 We also synthesized the model compound 4 by the
condensation of 1 with aniline.16 The structure of compound 3
was confirmed from its spectroscopic and analytical data. The IR
spectrum of compound 3 showed stretching band at 1677 cmÀ1
corresponding to the C@N group. There is no absorption band cor-
responding to free aldehyde and amino groups, which indicates
that condensation has taken place. The 1H NMR spectrum (Supple-
mentary data S6) of compound 3 showed a triplet (12H) at
1.15 ppm corresponding to the methyl protons, a singlet (6H) at
2.94 ppm corresponding to the methyl protons, a quartet (8H) at
3.28–3.58 ppm corresponding to the methylene protons, five mul-
tiplets (2, 9, 1, 2 and 1H respectively) at 6.23–6.27, 6.42–6.67,
7.03–7.04, 7.39–7.43 and 7.95–7.96 ppm corresponding to aro-
matic and CH protons, two doublets (1H each) at 7.23 and
8.09 ppm corresponding to aromatic and HC@N protons respec-
tively. A parent ion peak at m/z 614 [M+H]+ corresponding to the
condensation product 3 was observed in TOF MS ES+ spectrum
(Supplementary data S8). These spectroscopic data corroborate
the structure 3 for this compound.
380 nm decreases while a new absorption band appears at 554 nm
corresponding to the ring opened form of the rhodamine (Fig. 2).
The addition of other metal ions like Hg2+, Fe3+, Cu2+, Ni2+, and
Ag+ also results in the appearance of the characteristic absorption
band of rhodamine moiety (Supplementary data S2). This indicates
that compound 3 is interacting with different metal ions in the
ground state.
The UV–vis behavior of compound 3 in the presence of different
metal ions is attributed to the alteration in electronic properties of
3 that is, increased internal charge transfer (ICT) on metal ion com-
plexation, while characteristic absorbance of rhodamine is attrib-
uted to the opening of spirolactam ring to its amide form along
with a color change from colorless to pink. The fluorescence spec-
trum of compound 3 (1.0 lM) exhibits a strong emission at 470 nm
attributed to dimethylaminocinnamaldehyde moiety when excited
at 380 nm (Fig. 3). The rhodamine moiety in 3 remains in a closed,
non-fluorescent spirolactam form indicating weak spectral overlap
between dimethylaminocinnamaldehyde (energy donor) emission
and rhodamine (energy acceptor) absorption. As a result the fluo-
rescence resonance energy transfer (FRET) is suppressed and the
emission due to the dimethylaminocinnamaldehyde moiety is ob-
served at 470 nm. In the presence of Fe2+ ions compound 3 shows a
ratiometric response, the fluorescence spectrum shifted to 582 nm,
the region of rhodamine emission with the gradual decrease in do-
nor emission at 470 nm (Fig. 3). This is attributed to the binding of
Fe2+ ions with spirolactam ring of rhodamine resulting in its open-
ing to amide form. Efficient overlap between energy donor (dim-
ethylaminocinnamaldehyde moiety) and energy acceptor
(rhodamine moiety) is thus possible as the energy gap between en-
ergy donor and energy acceptor is greatly reduced enhancing the
intramolecular FRET.
The binding behavior of compound 3 was studied toward differ-
ent metal cations (Hg2+, Fe2+, Fe3+, Pb2+, Cd2+, Cu2+, Zn2+, Ni2+, Ag+,
Co2+, Mn2+, Mg2+, Ba2+, Li+, Na+, and K+) as their perchlorate salts in
THF by UV–vis and fluorescence spectroscopy. The absorption
spectrum of compound 3 (5.0 lM) shows two absorption bands
(Fig. 2) at 308 and 380 nm due to the dimethylaminocinnamalde-
hyde moiety, assigned to the transition from S0 to S2 and S1 states,
respectively.17 Low energy absorption at 380 nm indicates that this
is a
p–
p⁄ type of transition. On the addition of Fe2+ ions the band at
Under the same conditions as used above for compound 3, we
also carried out fluorescence studies of model compound 4 which
N
O
NH2
N
O
N
N
O
2
N
N
N
O
O
N
3 (78%)
EtOH:DCM
Reflux, 12 h
N
1
N
Aniline
N
Figure 3. Fluorescence spectra of 3 (1.0
(20 M) in THF; kex = 380 nm Inset showing the fluorescence change before and
after the addition of Fe2+ ions.
l
M) in response to the presence of Fe2+ ions
4 (80%)
l
Scheme 1.

M. Kumar et al. / Tetrahedron Letters 52 (2011) 4333–4336
4335
lacks acceptor unit in the presence of Fe2+ ions (Supplementary
tive experiments in the presence of Fe2+ at 20
l
M mixed with
data S5). It was observed that no energy transfer takes place in
the case of compound 4 in the presence of Fe2+ ions. Thus, the
advantage of the FRET system for energy transfer is obvious in
compound 3 where two moieties are linked together. Thus, in case
of compound 3 the addition of Fe2+ ions ‘trigger’ the FRET process
from the energy donor to energy acceptor upon excitation at donor
absorption wavelength. The energy transfer efficiency18 from do-
nor to acceptor was calculated to be 89.7% (Supplementary data
S4). This type of ratiometric fluorescence behavior is not induced
by the addition of any other metal ions investigated except Fe3+
which also induces ratiometric behavior but to small extent
(Fig. 5A). Other metal ions like Hg2+, Cu2+, Ag+ show unusual fluo-
rescence behavior (Fig. 4). The addition of Hg2+ ions to the solution
of 3 results in the 100% linear increase in emission at 470 nm. The
reason for increase in the fluorescence emission at 470 nm on the
addition of Hg2+ ions is due to the coordination of Hg2+ ions with
nitrogen atom of imino moiety of receptor 3 which decreases the
electron density on nitrogen and suppresses the electron transfer
from nitrogen to dialkylaminocinnamaldehyde moiety. The addi-
tion of Cu2+ ions leads to small decrease (22%) in emission which
may be attributed to the photo-induced electron transfer from
the dialkylaminocinn-amaldehyde moiety to the Cu2+ ions. While
in the presence of Ag+ ions there is red shift to 518 nm with
enhancement in emission which is due to the photo-induced
charge transfer (ICT) process operating in the presence of Ag+ ions.
Further, by considering the ratio of the fluorescence intensity of
energy acceptor at 582 nm (I582) to that of energy donor at 470 nm
(I470), we observed 9.5-fold fluorescence increase in the case of 3-
Fe2+ complex. To check the practical ability of compound 3 as a Fe2+
selective ratiometric fluorescent sensor, we carried out competi-
Hg2+, Fe3+, Pb2+, Cd2+, Cu2+, Zn2+, Ni2+, Ag+, Co2+, Mn2+, Mg2+, Ba2+
,
Li+, Na+, and K+ at 20
lM. As shown in Figure 5B no significant var-
iation in ratiometric emission was observed by comparison with or
without the other metal ions. It was found that 3 has a detection
limit of 60 Â 10À9 mol LÀ1 for Fe2+ which is sufficiently low for
the detection of submillimolar concentration range of Fe2+ ions
found in many chemical systems. Fitting the changes in the fluo-
rescence spectra of compound 1 with Fe2+ ions using the nonlinear
regression analysis program SPECFIT19 gave a good fit and demon-
strated that 1:1 stoichiometry (host:guest) was the most stable
species in the solution with a binding constant (log b) = 5.45 with
0.32 error. The method of continuous variation (Job’s plot) was
also used to prove the 1:1 stoichiometry (Supplementary data
S4).20
In conclusion, we have synthesized a new fluorescence reso-
nance energy transfer ‘turn-on’ dimethylamino-cinnamaldehyde
linked rhodamine based ratiometric fluorescent chemosensor 3
which is used for the selective recognition of Fe2+ ions over other
chemically close metal ions with a detection limit up to the nano-
molar range. The design of such systems which involve irradiation
of different fluorescent labels with single excitation source has
importance that the dye which has to emit at a longer wavelength
absorbs at the excitation source more effectively and hence results
in no net loss in fluorescence intensity which is an important issue
in those cases where detection of low levels of fluorescence is
involved.
Acknowledgments
We are thankful to the DST (New Delhi) (Ref. No. SR/FTP/CS-10/
2006), CSIR (New Delhi) (Ref. No. 01 (2326)/09/EMR-II) for finan-
cial support and Guru Nanak Dev University for laboratory
facilities.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.06.044.
References and notes
1. Silvia, J.; Williams, R. The Biological Chemistry of the Elements: The Inorganic
Chemistry of Life; Claredon Press: Oxford, 1991.
2. (a) Burdo, J. R.; Connor, J. R. Biometals 2003, 16, 63; (b) Connor, J. R.; Menzies, S.
L.; St. Martin, S. M.; Mufson, E. J. J. Neurosci. Res. 1990, 27, 595; (c) Crichton, R.
R.; Wilmet, S.; Legssyer, R.; Ward, R. J. J. Inorg. Biochem. 2002, 91, 9; (d)
Eisenstein, R. S. Annu. Rev. Nutr. 2000, 20, 627.
Figure 4. Fluorescence spectra of 3 (1.0
(20 M each) in THF; kex = 380 nm.
lM) in the presence of different metal ions
l
3. (a) Felt, B. T.; Lozoff, B. J. Nutr. 1996, 126, 693; (b) Earley, C. J.; Connor, J. R.;
Beard, J. L.; Malecki, E. A.; Epstein, D. K.; Allen, R. P. Neurology 2000, 54, 1698.
4. (a) Akoi, I.; Sakaki, T.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1992, 730; (b) Bu,
J. H.; Zheng, Q. Y.; Chen, C. F.; Huang, Z. T. Org. Lett. 2004, 6, 3301.
5. (a) Kim, S. K.; Bok, J. H.; Bartsch, R. A.; Lee, J. Y.; Kim, J. S. Org. Lett. 2005, 7, 4839;
(b) Böhmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713.
6. (a) Jin, T.; Ichikawa, K.; Koyana, T. Chem. Soc., Chem. Commun. 1992, 499; (b)
Schazmann, B.; Alhashimy, N.; Diamond, D. J. Am. Chem. Soc. 2006, 128, 8607.
7. (a) Jin, T. Chem. Commun. 1999, 2491; (b) Castle-lano, R. K.; Craig, S. L.;
Nuckolls, C.; Rebek, J., Jr. J. Am. Chem. Soc. 2000, 122, 7876.
8. (a) Othman, A. B.; Lee, J. W.; Wu, J.-S.; Kim, J. S.; Abidi, R.; Thuery, P.; Strub, J.
M.; Drosselaer, A. V.; Vicens, J. J. Org. Chem. 2007, 72, 7634; (b) Zhou, Z.; Yu, M.;
Yang, H.; Huang, K.; Li, F.; Yi, T.; Huang, C. Chem. Commun. 2008, 3387; (c) Lee,
M. H.; Kim, H. J.; Yoon, S.; Park, N.; Kim, J. S. Org. Lett. 2008, 10, 213; (d) Jisha, V.
S.; Thomas, A. J.; Ramaiah, D. J. Org. Chem. 2009, 74, 6667; (e) Xu, M.; Wu, S.;
Zeng, F.; Yu, C. Langmuir 2010, 26, 4529; (f) Kaewtong, C.; Noiseephum, J.; Upaa,
Y.; Morakot, N.; Morakot, N.; Wanno, B.; Tuntulani, T.; Pulpoka, B. New J. Chem.
2010, 34, 1104.
9. (a) Ji, H.-F.; Brown, G. M.; Dabestani, R. Chem. Commun. 1999, 609; (b) Leray, I.;
O’Reilly, F.; Habib Jiwan, J.-L.; Soumillion, J.-Ph.; Valeur, B. Chem. Commun.
1999, 795; (c) Leray, I.; Lefevre, J.-P.; Delouis, J.-F.; Delaire, J.; Valeur, B. Chem.
Eur. J. 2001, 7, 4590.
10. van der Meer, B. W.; Coker, G.; Simon, S. Y. Resonance Energy Transfer, Theory
and Data; VCH: Weinheim, 1994; (b) Adams, S. R.; Harootunian, A. T.; Buechler,
Y. J.; Taylor, S. S.; Tsien, R. Y. Nature 1991, 349, 694.
Figure 5. Fluorescence response of 3 (1.0 lM) to various cations (20 lM each) in
THF; kex = 380 nm. Black bars represent selectivity (I582/I470) of 3 upon addition of
different metal ions; gray bars represent competitive selectivity of receptor 3
toward Fe2+ ions (20
lM) in the presence of other metal ions (20 lM).

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M. Kumar et al. / Tetrahedron Letters 52 (2011) 4333–4336
11. (a) Dhir, A.; Bhalla, V.; Kumar, M. Org. Lett. 2008, 10, 4891; (b) Kumar, M.; Dhir,
A.; Bhalla, V. Org. Lett. 2009, 11, 2567; (c) Kumar, M.; Kumar, R.; Bhalla, V.
Chem. Commun. 2009, 7384; (d) Kumar, M.; Dhir, A.; Bhalla, V. Chem. Commun.
2010, 46, 6744; (e) Kumar, M.; Kumar, N.; Bhalla, V. Dalton Trans. 2011, 40,
5170.
150.54, 152.68 ppm. MS ES+, m/z: = 614 [M+H]+. C39H43N5O2: calcd C 76.32, H
7.06, N 11.41. Found C 76.09, H 7.28, N 11.17.
16. Synthesis of 4. A mixture of N,N-dimethylcinnamaldehyde (0.50 g, 2.85 mmol)
and aniline (0.39 g, 4.28 mmol) in a 1:1 mixture of dry dichloromethane and
absolute ethanol was refluxed for 12 h. After the completion of the reaction
solvent was evaporated and the residue left was crystallized from CHCl3/
CH3OH to give compound 3 in 80% yield; mp 136 °C. 1H NMR (CDCl3, 300 MHz):
d = 3.02 (s, J = 6H, CH3), 6.69 (d, J = 6H, 2H, Ar-H), 6.89–6.97 (m, 1H, CH), 7.05 (s,
1H, Ar-H), 7.14–7.20 (m, 3H, Ar-H and CH), 7.33–7.44 (m, 4H, Ar-H), 8.21 (d,
J = 9H, HC@N) ppm. 13C NMR (CDCl3, 300 MHz): d = 40.20, 112.01, 120.92,
123.50, 123.92, 125.50, 128.99, 145.05, 151.29, 152.13, 162.36 ppm. C17H18N2:
calcd C 81.56, H 7.25, N 11.19. Found C 81.30, H 7.50, N 11.56.
12. Kumar, M.; Kumar, N.; Bhalla, V.; Singh, H.; Sharma, P. R.; Kaur, T. Org. Lett.
2011, 13, 1422.
13. (a) Haugland, R. P. The Handbook: a guide to fluorescent probes and labeling
technologies, the tenth edition, molecular probes; Invitrogen Corp: Karlsbad, CA,
2005; (b) Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2005, 127, 10464; (c) Kim,
H. A.; Lee, M. H.; Kim, H. J.; Kim, J. S. Chem. Soc. Rew. 2008, 37, 1465.
14. Dujols, V.; Ford, F.; Cazarnik, A. W. J. Am. Chem. Soc. 1997, 119, 7386.
15. Synthesis of 3.
A
mixture of compound
1
(0.10 g, 0.21 mmol) and N,N-
1:1 mixture of dry
17. (a) Chakraborty, A.; Kar, S.; Guchhait, N. Chem. Phys. 2006, 324, 733; (b)
Chakraborty, A.; Kar, S.; Guchhait, J. Photochem. Photobiol. A: Chem. 2006, 181,
246.
18. (a) Lakowicz, J. R. Topics in Fluorescence Spectroscopy Volume 2: Principles;
Kluwer Academic Publishers: New York, 2002. p. 130; (b) Seth, D.;
Chakraborty, A.; Setua, P.; Chakrabarty, D.; Sarkar, N. J. Phys. Chem. B. 2005,
109, 12080; (c) Gilat, S. L.; Adronov, A.; Fréchet, J. M. J. Angew. Chem., Int. Ed.
1999, 38, 1422; (d) Lee, M. H.; Quang, D. T.; Jung, H. S.; Yoon, J.; Lee, C. H.; Kim,
J. S. J. Org. Chem. 2007, 72, 4242.
dimethylcinnamaldehyde (0.04 g, 0.26 mmol) in
a
dichloromethane and absolute ethanol was refluxed for 12 h. After the
completion of the reaction solvent was evaporated and the residue left was
crystallized from CHCl3/CH3OH to give compound 3 in 78% yield; mp 212 °C. IR
(KBr) m ;
max = 1677 cmÀ1 1H NMR (CDCl3, 300 MHz): d = 1.15 (t, J = 6 Hz, 12H,
CH3), 2.94 (s, 6H, CH3), 3.28–3.58 (q, 8H, CH2), 6.23–6.27 (m, 2H, Ar-H), 6.42–
6.67 (m, 9H, Ar-H and CH), 7.03–7.04 (m, 1H, CH), 7.23 (d, 1H, CH), 7.39–7.43
(m, 2H, Ar-H), 7.95–7.96 (m, 1H, Ar-H), 8.09 (d, J = 9H, HC@N) ppm. 13C NMR
(CDCl3, 300 MHz): d = 12.76, 40.26, 44.29, 65.47, 98.07, 105.79, 108.17, 11.95,
122.70, 122.87, 123.45, 124.59, 127.88, 128.29, 133.21, 139.71, 148.98, 150.30,
19. Gampp, H.; Maeder, M.; Meyer, C. J.; Zhuberbulher, A. D. Talanta 1985, 32, 95.
20. Job, P. Ann. Chim. 1928, 9, 113.