Switching of intramolecular charge transfer in cruciforms: Metal ion sensing

Article abstract of DOI:10.1021/ja050017n

Three different cruciforms containing the 1,4-bis-4′-pyridylethynyl-2,5-distyrylbenzene or the 1,4-bis-phenylethynyl-2,5-distyrylbenzene unit were synthesized and investigated with respect to their metal sensing properties. Upon addition of metal cations

Full text of DOI:10.1021/ja050017n

Published on Web 03/02/2005
Switching of Intramolecular Charge Transfer in Cruciforms: Metal Ion
Sensing
James N. Wilson and Uwe H. F. Bunz*
School of Chemistry and Biochemistry, Georgia Institute of Technology, 770 State Street, Atlanta, Georgia 30332
Received January 3, 2005; E-mail: uwe.bunz@chemistry.gatech.edu
In most, but not all, chromophores, HOMO and LUMO are
spatially superimposed. However, adding donor and/or acceptor
substituents to cross-shaped chromophores at suitable positions can
lead to independent electronic shifts in HOMO and LUMO into
opposite directions.1-4 In the cruciforms 2-4, the HOMO is located
on the distyrylbenzene branch, while the LUMO is situated on the
bisarylethynyl axis of the molecule.4 As a consequence, the
intramolecular charge-transfer emission and absorption of such
cruciforms is dependent upon solvent polarity, etc. Herein, we
demonstrate dramatic blue and/or red shifts of emission and
absorption upon coordination of metal cations to cruciforms 2-4;
the cruciforms should be valuable functional scaffolds for dif-
ferential metal sensor arrays.5-7
The synthesis of 1 and 2 is described elsewhere,4 while details
of the preparation of 3 and 4 are presented in the Supporting
Information. The general route to 1-4 starts from 1,4-bis-
(diethyloxyphosphonylmethyl)-2,5-diiodobenzene, which is sub-
jected to a Wittig-Horner reaction8 followed by a Sonogashira
coupling to an arylalkyne9 to give the desired cruciforms in good
yields and purity after crystallization. In a first experiment, 4 was
exposed to Zn2+ ions in dichloromethane. We selected Zn2+ due
to its importance as an active ion in biological systems.10,11 Upon
coordination of 0.81 equiv of Zn2+, the emission of 4 (Figure 1,
570 nm) is blue shifted (420 nm). Upon addition of a total of 4
equiv of Zn2+, the emission red shifts back again to 530 nm. This
behavior was surprising and needed explanation. Compounds 2 and
3 are similar to 4 but are lacking either the pyridine (2) or the
dibutylamino (3) groups. Their emission spectra in dichloromethane
and upon addition of zinc triflate are likewise shown in Figure 1.
Addition of zinc ions to 2 leads to a blue shift (527 f 430 nm),
while addition of zinc to 3 leads to a red shift (457 f 564 nm) of
the observed emission. Addition of more Zn2+ ions to either 2 or
3 does not lead to any further spectral changes.
Figure 1. Emission spectra of 2, 3, and 4 upon addition of zinc
trifluoromethylsulfonate in dichloromethane.
a lowering of the HOMO due to the electron-withdrawing effect
of the positive charge brought about by the zinc coordination to
the dibutylamino groups. The LUMO will not be greatly affected
because it is situated at the noncoordinating alkyne branch of the
cruciform. The reverse is true for 3. Upon coordination of the
pyridine nitrogen to a metal cation, the LUMO and, therefore, the
excited state of 3, should be stabilized, while the HOMO should
be virtually unaffected. Quantum chemical calculations (B3LYP,
6-31G**) on donor-acceptor cruciforms (-NMe2 instead of -NBu2
groups) supported the intuitive notion of an intramolecular charge-
transfer band.4b,d A ZINDO computation using B3LYP-6-31G**
geometry with (correctly) planarized -NMe2 groups demonstrates
that the observed lowest energy transitions, indeed, involve the
branch-wise separated HOMO and LUMO.12,13 The exposure of 4
to trifluoroacetic acid was investigated. The results of protonation
were similar to those obtained for the complexation of 4 with Zn2+
(Table 1). The emission spectra of 2-4 show large shifts upon
protonation or complexation, and the absorption spectra of 2 and 4
show blue shifts. In 3, the changes in absorption upon zinc
coordination are somewhat more difficult to assess because the
lowest absorption maximum in the complexed and free 3 are only
weak shoulders, probably forbidden CT transitions. The donor-
acceptor system 4 shows the largest oVerall ratiometric shifts upon
The blue shift in 2 and the red shift in 3 upon addition of zinc
ions can be explained. In 2, addition of the zinc ions will lead to
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J. AM. CHEM. SOC. 2005, 127, 4124-4125
10.1021/ja050017n CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Optical Data for Metal Coordination of Compounds 1-4
(1 is included as a reference)
The addition of recognition elements such as crown ethers linked
onto the aniline groups and/or electron acceptors or bipyridine
motifs onto the bis(pyridinylethynyl)benzene unit will lead to
cruciforms with further differentiated response capabilities. Such
cruciforms could be made by adding different aldehydes and alkyne
building blocks to the central bisphosphonate core. The creation
of potent sensory platform arrays that can perform ratiometric
sensing of two or more metal cations at the same time is a winning
proposition with this versatile system.
In conclusion, two new cruciform chromophores, 3 and 4, have
been synthesized, and the set of 2-4 has been investigated with
respect to the change of their optical properties upon metal ion
binding. Depending upon the binding preference of the metal, the
optical properties of the cruciforms change via switching of the
intramolecular charge-transfer band. The almost complete spatial
separation of HOMO and LUMO makes it possible to address
desired spectral features independently.
1
2
3
4
abs, λmax
330
nda
nda
92000
421, 442
nda
340, 446
328
373 sh
353
335, 440
+Zn2+ CH2Cl2
331
350
456 sh
+Η+ MeOH
ꢀ, CH2Cl2
Em, λmax
nda
48000
530
nda
342 (pH 0.34)
53800
571
418 (1 equiv)
530 (4 equiv)
435 pH 2.7
543 pH 0.34
0.08
83700
456
564
+Zn2+
430
+Η+ MeOH
nda
nda
nda
Φ CH2Cl2
Φ Zn2+
0.83
nda
0.31
0.66
0.49
0.01
0.10 (1 equiv)
0.10 (4 equiv)
a Not determined (nd).
Acknowledgment. We thank the National Science Foundation
for support (DMR 0138948) PI UB). We thank Professor Dr.
Christoph Fahrni (Gatech) for stimulating discussions and creative
input.
Supporting Information Available: Synthesis and characterization
of 3 and 4. This material is available free of charge via the Internet at
http://pubs.acs.org.
References
(1) (a) Marsden, J. A.; Miller, J. J.; Shirtcliff, L. D.; Haley, M. M. J. Am.
Chem. Soc. 2005, 127, ASAP. (b) Marsden, J. A.; O’Connor, M. J.; Haley,
M. M. Org. Lett. 2004, 6, 2385-2388. (c) Pak, J. J.; Weakley, T. J. R.;
Haley, M. M. J. Am. Chem. Soc. 1999, 121, 8182-8192.
(2) (a) Tykwinski, R. R.; Diederich, F. Liebigs Ann. Recl. 1997, 649-661.
(b) Gisselbrecht, J. P.; Moonen, N. N. P.; Boudon, C.; Nielsen, M. B.;
Diederich, F.; Gross, M. Eur. J. Org. Chem. 2004, 2959-2972.
(3) (a) Zhao, Y. M.; Tykwinski, R. R. J. Am. Chem. Soc. 1999, 121, 458-
459. (b) Eisler, S.; Tykwinski, R. R. J. Am. Chem. Soc. 2000, 122, 10736-
10737. (c) Zhao, Y. M.; Ciulei, S. C.; Tykwinski, R. R. Tetrahedron Lett.
2001, 42, 7721-7723.
(4) (a) Wilson, J. N.; Windscheif, P. M.; Evans, U.; Myrick, M. L.; Bunz, U.
H. F. Macromolecules 2002, 35, 8681-8683. (b) Wilson, J. N.; Josowicz,
M.; Wang, Y. Q.; Bunz, U. H. F. Chem. Commun. 2003, 2962-2963. (c)
Wilson, J. N.; Smith, M. D.; Enkelmann, V.; Bunz, U. H. F. Chem.
Commun. 2004, 1700-1701. (d) Wilson, J. N.; Hardcastle, K. I.; Josowicz,
M.; Bunz, U. H. F. Tetrahedron 2004, 60, 7157-7167. (e) For the original
use of the word cruciform, see: Klare, E.; Tulevski, G. S.; Sugo, K.; de
Picciotto, A.; White, K. A.; Nuckolls, C. J. Am. Chem. Soc. 2003, 125,
6030-6031. (f) Woo, H. Y.; Hong, J. W.; Liu, B.; Mikhailovsky, A.;
Korystov, D.; Bazan, G. C. J. Am. Chem. Soc. 2005, 127, 820-821.
(5) (a) Lavigne, J. J.; Anslyn, E. V. Angew. Chem. 1999, 38, 3666-3669.
(b) Wiskur, S. L.; Ait-Haddou, H.; Lavigne, J. J.; Anslyn, E. V. Acc.
Chem. Res. 2001, 34, 963-972. (c) Lavigne, J. J.; Savoy, S.; Clevenger,
M. B.; Ritchie, J. E.; McDoniel, B.; Yoo, S. J.; Anslyn, E. V.; McDevitt,
J. T.; Shear, J. B.; Neikirk, D. S. J. Am. Chem. Soc. 1998, 120, 6429-
6430.
(6) (a) Albert, K. J.; Lewis, N. S.; Schauer, C. L.; Sotzing, G. A.; Stitzel, S.
E.; Vaid, T. P.; Walt, D. R. Chem. ReV. 2000, 100, 2595-2626. (b)
Dickinson, T. A.; White, J.; Kauer, J. S.; Walt, D. R. Nature 1996, 382,
697-700.
(7) (a) Sen, J.; Suslick, K. S. J. Am. Chem. Soc. 2000, 122, 11565-11566.
(b) Suslick, K. S.; Rakow, N. A.; Sen, A. Tetrahedron 2004, 60, 11133-
11138.
Figure 2. Emission of 2, 3, and 4 upon addition of different metal salts in
dichloromethane.
complexation with zinc. A noteworthy conclusion that we draw is
that the aniline units in 4 are better ligands for zinc cations than
the pyridine ones.
With these results in hand, we examined the influence of other
metal cations on the emission of 2-4 in dichloromethane. Three
different responses can occur: (a) no change in emission, (b)
bathochromic or hypsochromic shift (i.e., a ratiometric response),
and (c) quenching of fluorescence. Figure 2 shows that 2-4 are
nonresponsive to Na+ and K+. However, Li+ shows a blue shift
with 2 and 4, but it does not influence the emission of 3. Ca2+
,
Hg2+, and Zn2+ can be distinguished from each other, but Mg2+
and Zn2+ give identical results with all three cruciforms. For
eventual biological applications, it is interesting to note that Ca2+
and Mg2+ give different results for all three dyes and are, therefore,
easily discerned by the cruciforms. Ag+ seems to quench fluores-
cence when coordinated to the pyridine end of the cruciform, but
not to the aniline moiety. Overall, the observed capability of these
sensing platforms to differentially discern different metal cations
is impressive.
(8) Horner, L.; Klink, W. Tetrahedron Lett. 1964, 36, 2467-2470.
(9) (a) Bunz, U. H. F. Chem. ReV. 2000, 100, 1603-1645. (b) Marsden, J.
A.; Haley, M. M. In Metal-Catalyzed Cross-Coupling Reactions, 2nd ed.;
deMeijere, A., Diederich, F., Eds.; Wiley-VCH: Weinheim, Germany,
2004; pp 317-394.
(10) Henary, M. M.; Wu, Y. G.; Fahrni, C. J. Chem.sEur. J. 2004, 10, 3015-
3025.
(11) (a) Walkup, G. K.; Burdette, S. C.; Lippard, S. J.; Tsien, R. Y. J. Am.
Chem. Soc. 2000, 122, 5644-5645. (b) Burdette, S. C.; Lippard, S. J.
Coord. Chem. ReV. 2001, 216, 333-361. (c) Budde, T.; Minta, A.; White,
J. A.; Kay, A. R. Neuroscience 1997, 79, 347-358.
Cruciforms 2-4 can be seen as members of a differential sensor
array.5-7 Single members of such an array do not recognize a
specific analyte, but an ensemble of different ones will; the modular
assembly of the cruciforms by standard organic synthesis is facile.
(12) McFarland, S. A.; Finney, N. S. J. Am. Chem. Soc. 2002, 124, 1178-
1179.
(13) Joshi, H. S.; Jamshidi, R.; Tor, Y. Angew. Chem. 1999, 38, 2722-2725.
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