Bichromophoric rhodamine-iridium(III) sensory system: Modulation of the energy-transfer process through a selective sensing behavior

Article abstract of DOI:10.1021/ic2006315

A newly designed bichromophoric rhodamine-iridium(III) system was found to show color/electronic absorption spectral changes and modulation of the energy-transfer process from rhodamine 6G to the iridium(III) luminophore upon selective Hg^II ion binding.

Full text of DOI:10.1021/ic2006315

COMMUNICATION
pubs.acs.org/IC
Bichromophoric RhodamineꢀIridium(III) Sensory System: Modulation
of the Energy-Transfer Process through a Selective Sensing Behavior
Chunyan Wang and Keith Man-Chung Wong*
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, People's Republic of China
S Supporting Information
b
6G hydrazide11b and rhodamine 6G ethylamine,9c respec-
ABSTRACT: A newly designed bichromophoric rhodamineꢀ
tively. The reaction of L1 and L2 with 0.5 equiv of a dinuclear
iridium(III) system was found to show color/electronic
absorption spectral changes and modulation of the energy-
transfer process from rhodamine 6G to the iridium(III)
luminophore upon selective HgII ion binding.
iridium(III) precursor complex, Ir2(dpqx)4Cl2 (dpqx = 2,3-
diphenylquinoxaline),14b followed by metathesis with NH4PF6
afforded the bichromophoric rhodamineꢀiridium(III) derivatives,
[Ir(dpqx)2(L1)]PF6 (1) and [Ir(dpqx)2(L2)]PF6 (2), in reason-
able yield (Scheme 1).
The electronic absorption spectra of 1 and 2 in methanol at
298 K exhibit intense high-energy absorption bands at 298 and
366 nm and a less intense low-energy absorption band at 472 nm
(Figure S1 in the Supporting Information). With reference to the
spectroscopic studies of the other related iridium(III) 2,3-
quinoxalinato complexes,14 the low-energy absorption band is
ascribed to a spin-allowed metal-to-ligand charge-transfer
(MLCT) [dπ(Ir) f π*(dpqx)] transition. On the other hand,
the weak shoulder that occurred beyond 500 nm is tentatively
ince the first preparation of rhodamine in 1905,1 rhodamine-
S
based dyes have been widely used as fluorophoric and
chromophoric probes in the past several decades.2ꢀ5 Many
derivatives of rhodamine are known to exist in two constitutional
isomers with completely different spectroscopic properties in
equilibrium,5 in which the ring-closed spirolactam form is color-
less and nonfluorescent while the ring-opened amide form
displays an intense pink to red color and strong fluorescence.
From the inspiration of the pioneering work of Czarnik and co-
workers,6 who utilize the ring-opening process of a rhodamine B
hydrazide as a fluorescent chemosensor to selectively sense Cu2þ
cation, various rhodamine-based turn-on fluorescent probes for
metal cations or analytes of interest have been reported in the
past few years.7ꢀ12
Although there are a few reports on the combination of the
rhodamine sensing unit with another organic chromophore or
luminophore, such as dansyl,12a,b pyrene,12c BODIPY,12d,e and 1,8-
naphthalimide10d moieties, in a single molecule, to the best of our
knowledge, the related construction of a bichromophoric array
through the incorporation of a luminescent transition-metal com-
plex into a rhodamine derivative with sensory functionality is
relatively unexplored. The cyclometalated iridium(III) or poly-
pyridyliridium(III) system has gradually become an important
class of luminescent transition-metal complexes in recent
years,13,14 because of their wide tunability of the emission energy,
less thermally accessible 3MC state, high electrochemical reversi-
bility, synthetic versatility, photostability, and chemical stability. As
an extension of our interest in the design of luminescent
chemosensors based on various transition-metal complexes,15,16
herein we report the design and synthesis of a novel class of
bichromophoric sensory assemblies, 1 and 2 (Scheme 1), from
the combination of a rhodamine sensing derivative and lumines-
cent cyclometalated iridium(III) complex. Their photophysical
selective metal cation sensing behaviors and energy-transfer
properties have also been investigated.
3
assigned as a spin-forbidden MLCT transition, resulting from
the spinꢀorbit coupling of an iridium(III) heavy atom. Upon
excitation at 365 nm, complexes 1 and 2 in methanol at 298 K are
found to show an intense emission band at 675 nm (Figure S1 in
the Supporting Information), which is ascribed to the triplet
excited state of MLCT [dπ(Ir) f π*(dpqx)] origin. Similar
emission bands were also observed in the related iridium(III) 2,3-
quinoxalinato complexes14 (622ꢀ670 nm) and the rhodamine
derivative-free model complex, [Ir(dpqx)2(4-carboxaldehyde-40-
methyl-2,20-bipyridine)]PF6 (675 nm; Figure S2 in the Supporting
Information). The observation of a rather weak emission at 550 nm
characteristic of rhodamine 6G in 1 is probably due to the existence
of the open form of rhodamine 6G in a trace amount, as a result of
equilibrium or the sensitivity of the imine linker to hydrolysis.
The selective binding properties of 1 and 2 were investigated
for various alkali and alkaline-earth as well as transition-metal
cations, such as Kþ, Naþ, Liþ, Ca2þ, Mg2þ, Ba2þ, Pb2þ, Cd2þ
,
Zn2þ, Co2þ, Ni2þ, Agþ, Fe2þ, Cu2þ, and Hg2þ. Solutions of 1 and
2 in methanol exhibit drastic color changes from pale yellow to
magenta selectively in the presence of a HgII ion, while only a pale-
orange color was observed for the CuII ion (Figure 1), indicating
that 1 and 2 can serve as a “naked-eye” indicator for the HgII ion.
Electronic absorption titration studies of 1 and 2 showed that a
new absorption band at 530 nm emerges with an increase in the
concentration of HgII ions in a methanol solution. The electronic
absorption spectral changes of 1 in methanol upon the addition
of a HgII ion are illustrated in Figure 2 (left) (Figure S3 in the
Supporting Information for 2), and the emergence of the
absorption band at 530 nm is responsible for the magenta color
The rhodamine derivatives containing bidentate ligands, L1
and L2, were prepared from the condensation reaction of
4-carboxaldehyde-40-methyl-2,20-bipyridine with rhodamine
Received: March 28, 2011
Published: May 20, 2011
r
2011 American Chemical Society
5333
dx.doi.org/10.1021/ic2006315 Inorg. Chem. 2011, 50, 5333–5335
|

Inorganic Chemistry
COMMUNICATION
Scheme 1. Synthetic Route of 1 and 2
Figure 2. Electronic absorption (left) and corrected emission spectra of
1 in methanol (concentration = 8.12 μM) at 298 K upon the addition of
various concentrations of Hg2þ. Insets show the plot of the absorbance
at 530 nm (left) and the emission intensity (right) as a function of the
concentration of Hg2þ with a theoretical fit.
It is interesting to note that complex 1 with the open form of
rhodamine 6G exhibited an about 52-fold enhancement of
iridium(III) emission at 675 nm (Figure S9 in the Supporting
Information) upon excitation at 520 nm. In view of the fact that
the rhodamine 6G donor absorbs strongly at 520 nm upon ring
opening, the huge emission enhancement is mainly due to an
efficient intramolecular energy-transfer process from rhodamine
6G to the cyclometalated iridium(III) moiety. Upon ring open-
ing of rhodamine 6G, an intense excitation band at 530 nm in the
excitation spectrum of 1, monitored at the iridium(III) emission
of 675 nm, emerged that was found to closely resemble the
characteristic rhodamine 6G absorption band (Figure S8 in the
Supporting Information, inset). This result further supports the
occurrence of an efficient intramolecular energy-transfer process
from rhodamine 6G to the iridium(III) luminophore upon ring
opening of the rhodamine derivative. The observation of a less
efficient energy transfer upon HgII ion binding, relative to the
case in protonation, suggests the possibility of triplet relaxation,
resulting from the heavy-atom effect upon HgII ion binding.
Complex 1 represents the first example with efficient energy
transfer from a rhodamine donor to a luminescent transition-
metal complex and such an energy-transfer process could be
modulated by the ring opening of rhodamine 6G through the
selective binding of HgII ions. In the present study, the
luminescent cyclometalated iridium(III) chromophore in 1
is employed as an energy acceptor to report the sensing signal
transmitted from the rhodamine derivative, which serves as a
selective receptor as well as a latent energy donor. Further-
more, rhodamine 6G absorbs strongly at about 530 nm,
resulting in a shift of the excitation of the iridium(III)
luminophore to lower energy being possible (Figure S9 in
the Supporting Information). No observable color or spectral
changes of 1 were obtained upon the addition of a HgII ion in
a solution mixture of EtOHꢀH2O (1:1, v/v) or in a
MeCNꢀH2O mixture (10 mM HEPS, pH 7.0, 50% MeCN),
indicative of the difficulty for the operation in the aqueous
and/or aqueousꢀorganic media.
Figure 1. Photograph of 1 (8.38 μM in MeOH) showing the color
change in the presence of various cations (40 equiv).
change. An interference experiment has been performed to
examine the selectivity of 1. Almost the same response was
observed for the solution containing the same concentration of
HgII ions and other metal cations (Figure S4 in the Supporting
Information), indicative of the high selectivity of 1 toward the
HgII ion. A similar absorption band was also observed upon the
addition of acid to 1 and 2 (Figures S5 and S6 in the Supporting
Information). In view of the fact that ring opening of the
spirolactam of rhodamine derivatives is commonly induced by
protonation,7a the observation of similar electronic absorption
spectral changes clearly suggests conversion of the spirolactam
form to the corresponding ring-opened amide upon the addition
of acid and a HgII ion. The close resemblance of the experimental
data to the theoretical fit is supportive of the binding mode of 1:1
stoichiometry [Figure 2 (left, inset)]. This 1:1 complexation
mode has been further confirmed by the method of continuous
variation showing a break point at a [1]/([1] þ [Hg2þ]) mole
fraction of 0.5 (Figure S7 in the Supporting Information). The
log Ks values for HgII ion binding to 1 and 2 were found to be
4.55 ((0.09) and 3.63 ((0.04), respectively. The addition of an
equimolar concentration of ethylenediaminetetraacetic acid to
the solution mixture of 1 or 2 and HgII ion in methanol resulted
in a color change from magenta back to the original pale-yellow
color, suggesting that the HgII ion-induced ring-opening process
of the rhodamine derivative is reversible in the present chemo-
sensing system.
Emission titration experiments with a HgII ion or acid in
methanol at room temperature were performed to investigate the
change in the emission response of 1 to the ring opening of the
rhodamine derivative. Upon excitation at the isosbestic wave-
length of 365 nm, the emission intensity ratio of iridium(III)-
based MLCT at 675 nm to rhodamine 6G at 555 nm was
increased upon an increase in the concentration of HgII ions
[Figure 2 (right)] or acid (Figure S8 in the Supporting In-
formation). A log Ks value of 4.58 ((0.16) was obtained for
binding of a HgII ion to 1 [Figure 2 (right, inset)]. The good
agreement of this value with that obtained from the electronic
absorption titration studies suggests that the emission enhance-
ment of the iridium(III) luminophore is ascribed to the ring
opening of the rhodamine derivative, similar to the electronic
absorption spectral changes.
In contrast, the emission spectra of 2 upon the addition of acid
showed that the emission of rhodamine 6G was substantially
enhanced, with no observable change in the emission intensity of
the iridium(III) luminophore (Figure S9 in the Supporting
Information), indicative of insignificant energy transfer from
rhodamine 6G. Such an inefficient energy transfer in 2 is ascribed
to the introduction of an ethyl group as a nonconjugated linker
between rhodamine 6G and the iridium(III) luminophore. The
discrepancy between the log Ks values obtained in 1 and 2 in the
electronic absorption titration studies is ascribed to the difference
5334
dx.doi.org/10.1021/ic2006315 |Inorg. Chem. 2011, 50, 5333–5335

Inorganic Chemistry
COMMUNICATION
in the separation distance of the amide group from the nitrogen
atom on the imine unit, which is anticipated to participate in the
binding process of the HgII ion. A higher concentration of HgII ions
is required in 2 (Figure S3 in the Supporting Information), relative
to 1, to reach saturation in the electronic absorption titration studies,
which is indicative of the lower binding affinity of 2 for the HgII ion,
in accordance with the lower log Ks value obtained in 2. Although 1
and 2 showed similar color and electronic absorption spectral
changes upon ring opening of the same rhodamine derivative, the
difference in their emission responses demonstrates that such an
energy-transfer process could be regulated by the judicious design of
the rhodamine-containing ligand.
In conclusion, a novel system of a bichromophoric chemo-
sensor, with hybridization of the organic fluorophoric rhodamine
sensing derivative and the luminescent cyclometalated iridium-
(III) complex, has been designed and synthesized. Selective HgII
ion-sensing properties have been studied to show color/electro-
nic absorption spectral changes. More importantly, such HgII ion
binding has been found to modulate the energy-transfer process
from rhodamine 6G to the iridium(III) luminophore upon ring
opening of rhodamine 6G. As a “proof-of-principle” concept, the
present result may open up new avenues for the fundamental
understanding of the photophysical and selective sensory beha-
viors and the design of a new chemosensor system with
hybridization of the organic fluorophoric rhodamine and the
luminescent transition-metal complex.
(5) Valeur, B. Molecular Fluorescence: Principles and Applications;
Wiley-VCH Verlag GmbH: New York, 2001; Chapter 10.
(6) Dujols, V.; Ford, F.; Czarnik, A. W. J. Am. Chem. Soc. 1997, 119, 7386.
(7) For some reviews and examples, see: (a) Kim, H. N.; Lee, M. H.;
Kim, H. J.; Kim, J. S.; Yoon, J. Chem. Soc. Rev. 2008, 37, 1465.
(b) Beija, M.; Afonso, C. A. M.; Martinho, J. M. G. Chem. Soc. Rev.
2009, 38, 2410. (c) Quang, D. T.; Kim, J. S. Chem. Rev. 2010, 110, 6280.
(8) Kwon, J. Y.; Jang, Y. J.; Lee, Y. J.; Kim, K. M.; Seo, M. S.; Nam,
W.; Yoon, J. J. Am. Chem. Soc. 2005, 127, 10107.
(9) (a) Yang, Y. K.; Yook, K. J.; Tae, J. J. Am. Chem. Soc. 2005,
127, 16760. (b) Zheng, H.; Qian, Z.-H.; Xu, L.; Yuan, F.-F.; Lan, L.-D.;
Xu, J.-G. Org. Lett. 2006, 8, 859. (c) Soh, J. H.; Swamy, K. M. K.; Kim,
S. K.; Kim, S.; Lee, S. H.; Yoon, J. Tetrahedron Lett. 2007, 48, 5966.
(d) Suresh, M.; Shrivastav, A.; Mishra, S.; Suresh, E.; Das, A. Org. Lett.
2008, 10, 3013. (e) Mao, J.; Wang, L.; Dou, W.; Tang, X.; Yan, Y.; Liu,
W. Org. Lett. 2007, 9, 4567. (f) Atanu, J.; Kim, J. S.; Jung, H. S.; Parimal,
K. B. Chem. Commun. 2009, 4417.
(10) (a) Bae, S.; Tae, J. Tetrahedron Lett. 2007, 48, 5389. (b) Lee,
M. H.; Kim, H. J.; Yoon, S.; Park, N.; Kim, J. S. Org. Lett. 2008, 10, 213.
(c) He, G.; Guo, D.; He, C.; Zhang, X.; Zhao, X.; Duan, C. Angew. Chem.,
Int. Ed. 2009, 48, 6132. (d) Jou, M. J.; Chen, Z.; Swamy, K. M. K.; Kim,
H. N.; Kim, H.-J.; Lee, S.-G.; Yoon, J. Chem. Commun. 2009, 7218. (e)
Huang, W.; Wu, D.; Guo, D.; Zhu, X.; He, C.; Meng, Q.; Duan, C. Dalton
Trans. 2009, 2081.
(11) (a) Franzini, R. M.; Kool, E. T. Org. Lett. 2008, 10, 2935.
(b) Yang, X.-F.; Guo, X.-Q.; Zhao, Y.-B. Talanta 2002, 57, 883. (c)
Zhang, W.; Tang, B.; Liu, X.; Liu, Y.; Xu, K.; Ma, J.; Tong, L.; Yang, G.
Analyst 2009, 134, 367.
(12) (a) Lee, M. H.; Kim, H. J.; Yoon, S.; Park, N.; Kim, J. S. Org. Lett.
2008, 10, 213. (b) Suresh, M.; Mishra, S.; Mishra, S. K.; Suresh, E.;
Mandal, A. K.; Shrivastav, A.; Das, A. Org. Lett. 2009, 11, 2740.
(c) Othman, A. B.; Lee, J. W.; Wu, J.-S.; Kim, J. S.; Abidi, P. T.; Strub,
J. M.; Dorsselaer, A. V.; Vicens, J. J. Org. Chem. 2007, 72, 7634. (d) Yuan,
M.; Zhou, W.; Liu, X.; Zhu, M.; Li, J.; Yin, X.; Zheng, H.; Zuo, Z.;
Ouyang, C.; Liu, H.; Li, Y.; Zhu, D. J. Org. Chem. 2008, 73, 5008.
(e) Zhang, X.; Xiao, Y.; Qian, X. Angew. Chem., Int. Ed. 2008, 47, 8025.
(13) For some reviews and examples, see: (a) King, K. A.;
Spellane, P. J.; Watts, R. J. J. Am. Chem. Soc. 1985, 107, 1431.
(b) Dixon, I. M.; Collin, J.-P.; Sauvage, J.-P.; Flamigni, L.; Encinas,
S.; Barigelletti, F. Chem. Soc. Rev. 2000, 29, 385. (c) Lo, K. K. W.; Hui,
W. K.; Chung, C. K.; Tsang, K. H. K.; Ng, D. C. M.; Zhu, N.; Cheung,
K. K. Coord. Chem. Rev. 2005, 29, 1434. (d) Lowry, M. S.; Bernhard,
S. Chem.—Eur. J. 2006, 12, 7970. (e) Flamigni, L.; Barbieri, A.;
Sabatini, C.; Ventura, B.; Barigelletti, F. Top. Curr. Chem. 2007,
281, 143. (f) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq,
F.; Kwong, R.; Tsyba, Z.; Bortz, M.; Mui, B.; Bau, R.; Thompson,
M. E. Inorg. Chem. 2001, 40, 1704. (g) Zhao, Q.; Li, F.; Huang, C.
Chem. Soc. Rev. 2010, 39, 3007. (h) Zhao, Q.; Li, F.; Liu, S.; Yu, M.;
Liu, Z.; Yi, T.; Huang, C. Inorg. Chem. 2008, 47, 9256. (i) Zhao, N.;
Wu, Y.-H.; Wen, H.-M.; Zhang, X.; Chen, Z.-N. Organometallics
2009, 28, 5603. (j) Zhao, N.; Wu, Y.-H.; Shi, L.-X.; Lin, Q.-P.; Chen,
Z.-N. Dalton Trans. 2010, 39, 8288.
(14) (a) Hwang, F.-M.; Chen, H.-Y.; Chen, P.-S.; Liu, C.-S.; Chi, Y.;
Shu, C.-F.; Wu, F.-I.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H. Inorg. Chem.
2005, 44, 1344. (b) Tani, K.; Fujii, H.; Mao, L.; Sakurai, H.; Hirao, T.
Bull. Chem. Soc. Jpn. 2007, 80, 783.
(15) (a) Yam, V. W. W.; Wong, K. M. C.; Zhu, N. J. Am. Chem. Soc.
2002, 124, 6506. (b) Yu, C.; Wong, K. M. C.; Chan, K. H. Y.; Yam,
V. W. W. Angew. Chem., Int. Ed. 2005, 44, 791. (c) Yu, C.; Chan, K. H. Y.;
Wong, K. M. C.; Yam, V. W. W. Proc. Natl. Acad. Sci. U.S.A. 2006,
103, 19657. (d) Yu, C.; Chan, K. H. Y.; Wong, K. M. C.; Yam, V. W. W.
Chem.—Eur. J. 2008, 14, 4577.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details include
b
syntheses and characterizations, photophysical spectra of 1, and
other electronic absorption and emission spectral changes of 1 and
2 upon the addition of a HgII ion and acid. Thismaterialisavailable
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: wongmc@hku.hk.
’ ACKNOWLEDGMENT
K.M.-C.W. acknowledges support from The University of
Hong Kong. The work described in this paper has been
supported by a GRF grant from the Research Grants Council
of Hong Kong Special Administrative Region, China (Project
HKU 7051/07P). We gratefully acknowledge Professor Vivian
Wing-Wah Yam for access to the equipment for photophysical
measurements and for her helpful discussion.
’ REFERENCES
(1) Noelting, E.; Dziewonsky, K. Ber. Dtsch. Chem. Ges. 1905, 38,
3516.
(2) (a) Haugland, R. P. Handbook of Fluorescent Probes and Research
Chemicals, 6th ed.; Molecular Probes, Inc.: Eugene, OR, 1996; p 22.
(b) Beija, M.; Carlos, A. M. A.; Josꢀe, M. G. M. Chem. Soc. Rev. 2009, 38,
2410.
(16) (a) Wong, K. M. C.; Tang, W. S.; Lu, X. X.; Zhu, N.; Yam,
V. W. W. Inorg. Chem. 2005, 44, 1492. (b) Tang, W. S.; Lu, X. X.; Wong,
K. M. C.; Yam, V. W. W. J. Mater. Chem. 2005, 15, 2714. (c) Tang,
R. P. L.; Wong, K. M. C.; Zhu, N.; Yam, V. W. W. Dalton Trans.
2009, 3911.
(3) (a) Goddard, J. P.; Reymond, J. L. Trends Biotechnol. 2004,
22, 363. (b) Li, L. T.; Kracht, J.; Peng, S. Q. Bioorg. Med. Chem. Lett.
2003, 13, 1245. (c) Kruger, R. G.; Dostal, P.; McCafferty, D. G. Chem.
Commun. 2002, 2092.
(4) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.;
Springer: New York, 2006; pp 67ꢀ69.
5335
dx.doi.org/10.1021/ic2006315 |Inorg. Chem. 2011, 50, 5333–5335