Cu2+-induced blue shift of the pyrene excimer emission: A new signal transduction mode of pyrene probes

Article abstract of DOI:10.1021/ol015524y

matrix presented A pentiptycene-bispyrenyl system (1) has been synthesized and investigated as a fluorescent chemosensor for metal ions. A novel blue shift along with an intensity enhancement of the pyrene excimer emission is observed for 1 in the presenc

Full text of DOI:10.1021/ol015524y

ORGANIC
LETTERS
Cu2+-Induced Blue Shift of the Pyrene
Excimer Emission: A New Signal
Transduction Mode of Pyrene Probes
2001
Vol. 3, No. 6
889-892
Jye-Shane Yang,* Che-Sheng Lin, and Chung-Yu Hwang
Department of Chemistry, National Central UniVersity, Chung-Li, Taiwan 32054
jsyang@rs250.ncu.edu.tw
Received January 7, 2001
ABSTRACT
A pentiptycene-bispyrenyl system (1) has been synthesized and investigated as a fluorescent chemosensor for metal ions. A novel blue shift
along with an intensity enhancement of the pyrene excimer emission is observed for 1 in the presence of Cu2+. Such a new signal transduction
mode of pyrene probes results from the formation of a static pyrene excimer that has very different characteristics from its dynamic counterpart.
A key feature rendering pyrene so attractive and useful as a
fluorescent probe is its relatively efficient excimer formation
and emission in comparison to other polyaromatic fluoro-
phores.1 Two informative parameters associated with the
pyrene excimer are the intensity ratio of the excimer to the
monomer emission (IE/IM) and the wavelength corresponding
to the maximum of excimer emission (λE). While the IE/IM
parameter is sensitive to the structure of pyrene-labeled
systems and thus has been very useful in the biological,
polymeric, and sensory materials chemistry,2-4 the corre-
sponding pyrene λE is much less variable and generally
locates at 475-485 nm. We report herein a pentiptycene-
derived bispyrenyl system (1) that displays not only the IE/
IM but also the λE variations depending on the metal ion
present: namely, Ca2+ and Cd2+ decrease the IE/IM ratio
without changing the λE, whereas Cu2+ shifts the λE toward
blue (λE ) 440 nm) along with an enhanced IE/IM. Excitation
spectra suggest that the mechanisms of the initial and the
blue-shifted excimer formation are dynamic and static,
respectively. In conjunction with the results from the
corresponding investigation of compound 2, molecular
models that might account for our observations are proposed.
Compound 1 was designed by following our continued
interests in the supramolecular chemistry of iptycene deriva-
tives5,6 to investigate its interactions with metal ions by the
(4) Selected examples of recently developed pyrene probes and sensors:
(a) Lewis, F. D.; Zhang, Y.; Letsinger, R. L. J. Am. Chem. Soc. 1997, 119,
5451-5452. (b) Lou, J.; Hatton, T. A.; Laibinis, P. E. Anal. Chem. 1997,
69, 1262-1264. (c) Reis e Sousa, A. T.; Castanheira, E. M. S.; Fedorov,
A. Martinho, J. M. G. J. Phys. Chem. A 1998, 102, 6406-6411 (d) Suzuki,
Y.; Morozumi, T.; Nakamura, H.; Shimomura, M.; Hayashita, T.; Bartsh,
R. A. J. Phys. Chem. B 1998, 102, 7910-7917. (e) Monahan, C.; Bien, J.
T.; Smith, B. D. Chem. Commun. 1998, 431-432. (f) Matsui, J.; Mitsuishi,
M.; Miyashita, T. Macromolecules 1999, 32, 381-386. (g) Nishizawa, S.;
Kato, Y.; Teramae, N. J. Am. Chem. Soc. 1999, 121, 9463-9464. (h)
Principi, T.; Goh, C. C. E.; Liu, R. C. W.; Winnik, F. M. Macromolecules
2000, 33, 2958-2966. (i) Sahoo, D.; Narayanaswami, V.; Kay, C. M.; Ryan,
R. O. Biochemistry 2000, 39, 6594-6601. (j) Krauss, R.; Weinig, H.-G.;
Seydack, M.; Bendig, J.; Koert, U. Angew. Chem., Int. Ed. 2000, 39, 1835-
1837.
(1) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Inter-
science: London, 1970.
(2) Winnik, F. M. Chem. ReV. 1993, 93, 587-614.
(3) De Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A.
J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97,
1515-1566.
(5) Yang, J.-S.; Lee, C.-C.; Yau, S.-L.; Chang, C.-C.; Lee, C.-C.; Leu,
J.-M. J. Org. Chem. 2000, 65, 871-877.
(6) Yang, J.-S.; Liu, C.-P.; Lee, G.-H. Tetrahedron Lett. 2000, 41, 7911-
7915.
10.1021/ol015524y CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/16/2001

fluorescent probe of pyrenes. We reasoned that a hybrid of
the potential noncyclic polyether ligand4d and the three-
dimensional aromatic pentiptycene group5-7 might form a
selective ionophore, where the cation-π interactions8 might
contribute to the ion recognition of 1. The synthesis of
compound 1 was performed starting with the precursors of
pentiptycene hydroquinone5 3 and 1-bromomethylpyrene9 5
based on the route shown in Scheme 1. The same procedures
Scheme 1
Figure 1. Fluorescence spectra of 1 in CH2Cl2 (1 × 10-5 M,
excitation at 335 nm) in the presence of 0, 0.1, 0.2, 0.5, 0.8, 1.0,
1.2, 1.4, 1.6, 1.8, and 2.0 equiv of Ca(ClO4)2‚4H2O predissolved
in MeCN (0.005 M) and the corresponding change in the IE/IM ratio
represented by the intensities at 475 (I475) and 375 (I375) nm (inset).
that shown in Figure 1. Binding constants, expressed as log
K, determined by either fluorimetry or absorption spectros-
copy11 assuming 1:1 complex formation are consistent, and
the values are 4.6 and 4.8 for Ca2+ and Cd2+, respectively.
Surprisingly, a pronounced blue shift along with an
intensity enhancement of the pyrene excimer emission was
observed for 1 in the presence of Cu2+ (Figure 2A). An
intensity maximum is reached at [Cu2+]/[1] ) 10-13, where
the λE shifts as much as 35 nm (from 475 to 440 nm). The
corresponding absorption spectra are not shifted but broaden-
ing, resulting in isosbestic points at 325, 331, 339, and 348
nm (Figure 2B), and suggest a binding constant of log K )
4.4 for the formation of 1:1 complexes of 1 and Cu2+.11
Further introduction of Cu2+ does not result in a larger shift
of the λE but decreases the intensity of the 440 nm emission.
The reduced intensity could be attributed to the increased
content of acetonitrile that competes with 1 for Cu2+ and/or
the nature of Cu2+ as a moderate fluorescence quencher12,13
(vide infra).
were also employed in the formation of compound 2 from
hydroquinone.
Although dichloromethane is such a poor solvent in
solvating ionic species that one might expect many metal
ions will interact with 1, the other 12 metal ions10 studied in
this work cause little or no fluorescence changes. When
Compound 1 in dichloromethane displays selective and
sensitive fluorescence responses to metal ions.10 As is shown
in Figure 1, a decrease of the excimer with an increase of
the monomer emission of 1 is selectively induced by Ca2+
among six alkali and alkaline earth metal ions. Selectivity
is also found for 1 among nine transition metal ions, where
only Cd2+ leads to a reduced IE/IM in a manner similar to
(11) Connors, K. A. Binding Constants-The Measurement of Molecular
Complex Stability; John Wiley & Sons: New York, 1987; Chapter 4.
(12) Varnes, A. W.; Dodson, R. B.; Wehry, E. L. J. Am. Chem. Soc.
1972, 94, 946-950.
(13) (a) Sasaki, D. Y.; Shnek, D. R.; Pack, D. W.; Arnold, F. H. Angew.
Chem., Int. Ed. Engl. 1994, 34, 905-907. (b) De Santis, G.; Fabbrizzi, L.;
Licchelli, M.; Mangano, C.; Sacchi, D.; Sardone, N. Inorg. Chim. Acta 1997,
257, 69-76. (c) Yoon, J.; Ohler, N. E.; Vance, D. H.; Aumiller, W. D.;
Czarnik, A. W. Tetrahedron Lett. 1997, 38, 3845-3848. (d) Torrado, A.;
Walkup, G. K.; Imperiali, B. J. Am. Chem. Soc. 1998, 120, 609-610. (e)
Bolletta, F.; Costa, I.; Fabbrizzi, L.; Licchelli, M.; Montalti, M.; Pallavicini,
P.; Prodi, L.; Zaccheroni, N. J. Chem. Soc., Dalton Trans. 1999, 1381-
1385. (f) Bodenant, B.; Weil, T.; Businelli-Pourcel, M.; Fages, F.; Barbe,
B.; Pianet, I.; Laguerre, M. J. Org. Chem. 1999, 64, 7034-7039. (g)
Grandini, P.; Mancin, F.; Tecilla, P.; Scrimin, P.; Tonellato, U. Angew.
Chem., Int. Ed. 1999, 38, 3061-3064.
(7) (a) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 5321-
5322. (b) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864-
11873.
(8) Ma, J. C.; Dougherty, D. A. Chem. ReV. 1997, 97, 1303-1324.
(9) Akiyama, S.; Nakasuji, K.; Nakagawa, M. Bull. Chem. Soc. Jpn. 1971,
44, 2231-2236.
(10) Fifteen metal ions (perchlorate salts unless otherwise noted) were
studied in this work, including Li+, Na+, K+, Mg2+, Ca2+, Ba2+, Mn2+
-
(acetate), Co2+, Ni2+, Cu2+, Zn2+, Y3+(nitrate), Ag+, Cd2+(chloride), and
Hg2+(acetate).
890
Org. Lett., Vol. 3, No. 6, 2001

recorded at both the monomer (375 nm) and the excimer
(475 nm) regions for 1 in metal-free solutions are the same,
indicating a dynamic nature for the 475 nm emission. In the
presence of Ca2+ or Cd2+, the dynamic character is main-
tained for the residual pyrene excimer. Thus, the decrease
of IE/IM in Figure 1 simply suggests that the pyrene groups
in the ion-bound complexes cannot adopt an overlapping
geometry. On the other hand, the excitation spectrum of the
Cu2+-induced 440 nm emission is broadened and largely red-
shifted (e.g., 11 nm for the 0-0 band) in comparison to that
recorded at 375 nm (Figure 2A, inset). Evidently, the 440
nm emission is from a static excimer. The same conclusions
can be drawn on the basis of the fluorescence decay times.2,15
In metal-free solutions, both the pyrene monomer (294 ns,
65%) and excimer (102 ns, 100%) emissions have long decay
times (τ and preexponential). In addition, a rising time of
36 ns was determined for the 475 nm excimer emission,
corresponding to the short component of monomer emission
(32 ns, 35%). By contrast, the Cu2+-induced 440 nm emission
has a relatively short lifetime (6.3 ns, 96%; 46.5 ns, 4%)
and shows no rising component. The 16-fold reduction in
fluorescence lifetime for the 440 nm vs 475 nm excimer
emission suggests a larger radiative decay rate constant for
the static than the dynamic pyrene excimers.16
The pyrene excimer is proposed to have a symmetrical
sandwich-like structure, and the blue-shifted form is generally
attributed to partially overlapping pyrene dimers.2,17 For most
pyrenyl systems, emission from the latter is generally weak
or negligible and often obscured by the strong emission from
the former. As a consequence, methods such as spectral
deconvolution analysis and time-resolved spectrometry are
required for the identification of the blue-shifted excimer.2,17
It is known that a locally excited and partially overlapped
pyrene dimer can undergo a rapid structural relaxation to
the lower energy pyrene excimer,17c which might account
for the weak emission of blue-shifted pyrene excimers and
the poor sensitivity of the pyrene λE to the ground-state
structures. Accordingly, the binding of Cu2+ in 1 not only
brings the pyrene groups together in the ground state leading
to the formation of static excimers but also prohibits or
Figure 2. (A) Fluorescence and (B) absorption spectra of 1 in
CH2Cl2 (1 × 10-5 M, excitation at 335 nm) in the presence of (a)
0, (b) 1.0, (c) 5.0, (d) 7.0, (e) 9.0, (f) 11.0, and (g) 15 equiv of
Cu(ClO4)2‚6H2O predissolved in MeCN (0.005 M) and the corre-
sponding change in the excitation spectra (normalized) monitored
at 375 (dash) and 440 (full) nm (inset).
dichloromethane is replaced by the more polar acetonitrile,
no more fluorescence variations could be induced by Ca2+
or Cd2+, whereas the spectral features of blue-shifted and
intensity-enhanced pyrene excimer emission are retained,
albeit to a lesser extent and requiring a higher [Cu2+]/[1]
ratio (log K ) 3.6), in the presence of Cu2+.14 An intensity
maximum also occurs, where the [Cu2+]/[1] ratio is ∼60 and
the λE is 449 nm. Unlike the case in dichloromethane, the
λE continues to shift toward blue upon further addition of
Cu2+ (e.g., λE ) 444 nm at [Cu2+]/[1] ∼180). Apparently,
the interactions between 1 and metal ions are weak, and this
might be related to its high ion selectivity even in dichloro-
methane solutions.
To understand the different fluorescence responses of 1
in dichloromethane to Ca2+ or Cd2+ vs Cu2+, the nature of
the blue-shifted 440 nm emission vs the normal 475 nm
excimer should be characterized. One of the powerful
methods for the differentiation of a dynamic excimer from
a static one is the excitation spectrum.2 Excitation spectra
(15) Decay times were determined at room temperature by means of an
Edinburgh photon counting apparatus (OB900-14A) with λex ) 335 nm
for all measurements. The goodness of fit was judged by the reduced ø2
value (<1.10 in all cases), the randomness of the residuals, and the
autocorrelation function.
(16) (a) On the basis of the deconvolution analysis of emission spectra
and the assumption of preexponentials ) fraction of emitters (i.e. dynamic
excimer ) 35% in a metal free dichloromethane solution), the quantum
efficiency (ΦF) of the dynamic excimer of 1 in dichloromethane is estimated
to be ca. 0.25.16b Due to the complication of monomer excitation and
emission and fluorescence quenching by the unbound Cu2+, the determi-
nation of the quantum yield of the static pyrene excimer in the presence of
Cu2+ is difficult. However, an estimation based on Figure 2 suggests a
lower limit of 0.26 for the ΦF value of the Cu2+-bound static excimer in
dichloromethane solutions. This would lead to radiative rate constants of
∼2.5 ×106 and 4.1 ×107 s-1 for the dynamic and static excimers,
respectively. (b) Anthracene (ΦF ) 0.27 in hexane)1 was used as the
actinometer.
(17) (a) De Schryver, F. C.; Collart, P.; Vandendriessche, J.; Goedeweeck,
R.; Swinnen, A.; Van der Auweraer, M. Acc. Chem. Res. 1987, 20, 159-
166. (b) Matsui, A.; Mizuno, K.-I.; Tamai, N.; Yamazaki, I. Chem. Phys.
1987, 113, 111-117. (c) Saigusa, H.; Lim, E. C. Acc. Chem. Res. 1996,
29, 171-178. (d) Zimerman, O. E.; Weiss, R. G. J. Phys. Chem. A 1998,
102, 5364-5374.
(14) Figures are shown as the Supporting Information.
Org. Lett., Vol. 3, No. 6, 2001
891

minimizes such a relaxation event in the excited state leading
to a blue-shifted excimer emission.
The overall fluorescence enhancement of 1 upon the
binding of Cu2+ is also interesting, because the opposite is
more common in previously reported Cu2+ chemosensors.13
While Cu2+-induced fluorescence enhancement was also
observed in a few cases,18 the mechanism is different from
that in 1: namely, the binding of Cu2+ turns off certain
nonradiative decay processes (e.g., photoinduced electron
transfer) of the fluorophores in earlier cases, but it results in
the formation of new emitting species (static excimer) having
a stronger fluorescence character than its origins (monomer
and dynamic excimer) in 1. Nonetheless, the unbound Cu2+
appears to behave as a fluorescence quencher and to be
responsible for the reduced fluorescence at the stages of Cu2+
addition at the beginning (low [Cu2+]/[1] ratio) and after the
intensity maximum (high [Cu2+]/[1] ratio).19 In both condi-
tions, the net increase of the stronger fluorescent Cu2+-1
complexes is too low to compensate for the fluorescence
quenching by the additional unbound Cu2+. Thus, the
observed fluorescence enhancement should be considered as
a lower limit.16
Figure 3. Molecular models showing the proposed structures of 1
in dichloromethane in the presence of (a) Ca2+ or Cd2+and of (b)
Cu2+
.
of Ca2+, Cd2+, or Cu2+ are not unique among the 15 metal
ions investigated.10,20 In addition, the flanking benzene rings
of the pentiptycene group in 1 vs 2 not only provide a venue
for cation-π interactions but also perturb the conformation
of polyether chains. Both effects might contribute to the
recognition of Ca2+ and Cd2+ by 1 in dichloromethane, but
their relative contributions cannot be justified, mainly due
to the overall weak interactions between 1 and metal ions.
Furthermore, the detailed structures of Figure 3, particularly
the partially overlapping pyrene dimer, remain to be estab-
lished. Further studies designed toward a better understanding
of these aspects are in progress.
In conclusion, our studies on the pentiptycene-bispyrenyl
hybrid 1 have revealed the potential of the three-dimensional
pentiptycene group in the formation of new metal ion sensors.
In particular, a novel blue shift and intensity enhancement
of the pyrene excimer emission has been observed for 1 in
the presence of the Cu2+ ion. This is attributed to the
conversion of a sandwich-like dynamic excimer to a partially
overlapping static excimer. Such a new signal transduction
mode of pyrene probes might be useful in the design of new
pyrene-based sensory materials.
To gain an insight into the role of the pentiptycene groups
of 1 in ion recognitions, compound 2 was also investigated.
Results show that substitution of the pentiptycene group by
a phenyl ring no longer leads to fluorescence responses to
Ca2+ or Cd2+ in either dichloromethane or acetonitrile
solutions. Fluorescence changes due to the presence of Cu2+
can be observed for 2,14 but the extent of blue shift and
intensity enhancement of pyrene excimer is greatly reduced
in both dichloromethane (log K ) 4.2) and acetonitrile (log
K ) 1.4) solutions in comparison to the case of 1.
Apparently, the flanking benzene rings of the pentiptycene
groups play an important role in the recognition of Ca2+,
Cd2+, and Cu2+. In conjunction with the fact that the radii
of Ca2+ (1.00 Å) and Cd2+ (0.95 Å) ions are comparable
and that of Cu2+ is smaller (0.57 Å),20 molecular models21
that might account for the fluorescence behavior of 1 are
proposed in Figure 3. It should be noted that it is the nature
rather than the size of metal ions that is responsible for the
spectral changes and ion selectivity of 1, because the sizes
Acknowledgment. Financial support for this research was
provided by the National Science Council (NSC 89-2113-
M-008-013). We greatly appreciate Professor Kuo Chu
Hwang at National Tsing Hua University for obtaining the
lifetime data.
(18) (a) Ghosh, P.; Bharadwaj, P. K. J. Am. Chem. Soc. 1996, 118,
10579-10587. (b) Ramachandram, B.; Samanta, A. J. Phys. Chem. A 1998,
102, 10579-10587. (c) Mitchell, K. A.; Brown, R. G.; Yuan, D.; Chang,
S.-C.; Utecht, R. E.; Lewis, D. E. J. Photochem. Photobiol. A: Chem. 1998,
115, 157-161. (d) Rurack, K.; Kollmannsberger, M.; Resch-Genger, U.;
Daub, J. J. Am. Chem. Soc. 2000, 122, 968-969.
(19) The little differences of optical density upon the introduction of
Cu2+ at the wavelengths used for excitation cannot account for the dramatic
excimer intensity variations at low and high [Cu2+]/[1] ratios.
Supporting Information Available: Experimental pro-
cedures and characterization data for compounds 1, 2, and
4 and figures of the absorption and/or fluorescence spectra
of 1 in acetonitrile and 2 in dichloromethane and acetonitrile
in the presence of Cu2+. This material is available free of
charge via the Internet at http://pubs.acs.org.
(20) Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC Press:
Boca Raton, 1997. The coordination number is assumed to be six for Ca2+
and Cd2+ and four for Cu2+
.
(21) The structures of 1 were optimized in the absence of metal ions
using the MM2 force field supplied by Chem 3D Plus, a product of
Cambridge Scientific Computing, Inc.
OL015524Y
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Org. Lett., Vol. 3, No. 6, 2001