demonstrated perfect emission OFF-ON switch effect and
good selectivity.1,2,17,18 With DFT calculations we demon-
strated that DNBS induces a dark excited state to the probe
(S1, the transition oscillator strength for S0 f S1 is close to
zero; thus S0 f S1 is a forbidden transition and S1 f S0
relaxation is nonradiative),17 and thus the probe is nonfluo-
rescent.17,19 Cleavage of the electron sink DNBS with thiols
will re-establish the radiative S1 state of the fluorophore
(oscillator strength of S0 f S1 is close to unity, indicating
the transition is allowed and S1fS0 is probably a radiative
transition), and thus the emission is switched on.17
Previously we designed OFF-ON fluorescent thiol probes
with DNBS as intramolecular electron sink.17 However, our
probes suffer from drawbacks: (1) the pyrene moiety is not
an ideal electron donor; the nonefficient electron transfer (ET)
deteriorates the quenching efficiency or the contrast ratio,23b
and as a result, only 20- to 53-fold emission enhancement
was observed;17 (2) low emission quantum yield in aqueous
solutions, which is common for most of the fluorophores;11,17
and (3) short luminescent lifetimes (ns).17
To tackle the above limitations, herein we designed a thiol
probe by using Ru(II) poly(1,10-phenanthroline) complex as
the luminophore, which is known to show environment-non-
sensitive metal-to-ligand charge transfer (3MLCT) red emission
(at ca. 600 nm), large Stokes shift (ca. 150 nm, 5250 cm-1),
and long luminescent lifetimes (µs, 10-6 s).27,28 Thus we
envisaged the new phosphorescent thiol probe 2 (Scheme 1).
However, most of the probes are based on fluorescence,1
for which the singlet excited state is the emissive state
(usually S1, Kasha’s rule).11,17 These thiol probes show some
drawbacks: (1) small Stokes shift, e.g., probes based on
BODIPY or rhodamines;4,5 (2) short excitation-emission
wavelength, which induces undesired background fluores-
cence;20 (3) microenvironment-sensitive emission, e.g., being
effected by pH or polarity of the media;16 and (4) short
luminescent lifetimes (ns, 10-9 s, due to the spin manifold
of the S0 f S1 transition),11 and so it is difficult for these
probes to be used for lifetime-based analysis.11 Therefore,
new thiol probes with drastically different lumophores are
desired to tackle these problems.
Scheme 1. Syntheses of Complex 1 and Probe 2
Much investigation has been focused on binding sites and
the luminophores of molecular probes. However, fewer
efforts have been made to explore new sensing mecha-
nisms.16,21-23 Recently we have been interested in the study
of molecular probes with new binding sites, chromophores,
and new sensing mechanisms.17,24-26
(7) (a) Zhang, M.; Yu, M.; Li, F.; Zhu, M.; Li, M.; Gao, Y.; Li, L.; Liu,
Z.; Zhang, J.; Zhang, D.; Yi, T.; Huang, C. J. Am. Chem. Soc. 2007, 129,
10322. (b) Huang, K.; Yang, H.; Zhou, Z.; Chen, H.; Li, F.; Yi, T.; Huang,
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(8) Amoroso, A. J.; Arthur, R. J.; Coogan, M. P.; Court, J. B.; Fernandez-
Moreira, V.; Hayes, A. J.; Lloyd, D.; Milletb, C.; Pope, S. J. A. New
J. Chem. 2008, 32, 1097
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(9) Chen, Y.; Parr, T.; Holmes, A. E.; Nakanishi, K. Bioconjugate Chem.
2008, 19, 5
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(10) Lin, W.; Yuan, L.; Cao, Z.; Feng, Y.; Long, L. Chem.sEur. J.
2009, 15, 5096
(11) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.;
Kluwer Academic/Plenum Publishers: New York, 1999
(12) Zhang, D.; Zhang, M.; Liu, Z.; Yu, M.; Li, F.; Yi, T.; Huang, C.
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Tetrahedron Lett. 2006, 47, 7093
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(13) Rusin, O.; Luce, N. N. St.; Agbaria, R. A.; Escobedo, J. O.; Jiang,
S.; Warner, I. M.; Dawan, F. B.; Lian, K.; Strongin, R. M. J. Am. Chem.
Soc. 2004, 126, 438
(14) Lin, W.; Long, L.; Yuan, L.; Cao, Z.; Chen, B.; Tan, W. Org. Lett.
2008, 10, 5577
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(15) Wang, W.; Rusin, O.; Xu, X.; Kim, K. K.; Escobedo, J. O.;
Fakayode, S. O.; Fletcher, K. A.; Lowry, M.; Schowalter, C. M.; Lawrence,
C. M.; Fronczek, F. R.; Warner, I. M.; Strongin, R. M. J. Am. Chem. Soc.
2005, 127, 15949
(16) Lee, J. H.; Su Lim, C.; Tian, Y. S.; Han, J. H.; Cho, B. R. J. Am.
Chem. Soc. 2010, 132, 1216
(17) Ji, S.; Yang, J.; Yang, Q.; Liu, S.; Chen, M.; Zhao, J. J. Org. Chem.
2009, 74, 4855
(18) Fujikawa, Y.; Urano, Y.; Komatsu, T.; Hanaoka, K.; Kojima, H.;
Terai, T.; Inoue, H.; Nagano, T. J. Am. Chem. Soc. 2008, 130, 14533
(19) Zhao, G.; Liu, J.; Zhou, L.; Han, K. J. Phys. Chem. B 2007, 111, 8940
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(20) Lee, K. S.; Kim, T. K.; Lee, J. H.; Kim, H. J.; Hong, J. I. Chem.
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(21) James, T. D. Top. Curr. Chem. 2007, 277, 107
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(22) Nagaki, A.; Kim, H.; Yoshida, J. Angew. Chem., Int. Ed. 2008,
47, 7833
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Probe 2 is based on tuning the MLCT photophysics by
DNBS. The typical MLCT photophysics of the Ru(II)
complex, i.e., the photoinduced ET from the Ru(II) center
(23) (a) Hong, V.; Kislukhin, A. A.; Finn, M. G. J. Am. Chem. Soc.
2009, 131, 9986. (b) Cody, J.; Mandal, S.; Yang, L.; Fahrni, C. J. J. Am.
Chem. Soc. 2008, 130, 13023–13032
.
Org. Lett., Vol. 12, No. 12, 2010
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