Cation-triggered 'switching on' of the red/near infra-red (NIR) fluorescence of rigid fluorophore-spacer-receptor ionophores

Article abstract of DOI:10.1039/b006430k

Fluorophore-spacer-receptor ionophores 1 and 2 show a strong cation-induced enhancement of the fluorescence at λ > 650 nm in the presence of Hg(II) and Ag¹ (1) or Pb(II), alkaline-earth and alkali metal ions (2).

Full text of DOI:10.1039/b006430k

Cation-triggered ‘switching on’ of the red/near infra-red (NIR) fluorescence of
rigid fluorophore–spacer–receptor ionophores
Knut Rurack,*a Ute Resch-Genger,a Julia L. Bricksb and Monika Spielesa
a Dept. I.3902, Federal Institute for Materials Research and Testing (BAM), Richard-Willstaetter Str. 11, D-12489
Berlin, Germany. E-mail: knut.rurack@bam.de
b Institute of Organic Chemistry, National Academy of Sciences of the Ukraine, Murmanskaya 5, 253660, Kiev-94,
Ukraine
Received (in Liverpool, UK) 3rd August 2000, Accepted 22nd September 2000
First published as an Advance Article on the web 9th October 2000
Fluorophore–spacer–receptor ionophores 1 and 2 show a
strong cation-induced enhancement of the fluorescence at l
> 650 nm in the presence of HgII and AgI (1) or PbII,
alkaline-earth and alkali metal ions (2).
Molecular signaling systems relying on drastic changes in
fluorescence intensity and/or band position upon binding to an
inorganic/organic substance have lately received much atten-
tion as photonic molecular devices.1 As functionalized hosts for
inorganic guests such as main group, heavy, and transition
metal ions or anions, ionophores that consist of an ion-
responsive receptor and a (potentially, i.e. in the ‘switched on’
the absorption and emission bands of 1–4 in acetonitrile, the
state) highly emissive chromophore, separated by a short alkyl
solvent we used for the complexation studies (Table 1). The
spacer thus preventing pronounced electronic interaction in the
broad, structureless and largely Stokes-shifted absorption and
ground state, are of particular interest.2 This molecular
emission bands underline the CT character of the optical
constitution allows to combine an electron-donating receptor
transitions in the 1,3-chromophore. In accordance with results
with a reducable chromophore to invoke fast signaling proc-
obtained for their 3-benzothiazol-2-yl-substituted analogues,4b
esses such as intramolecular electron transfer (ET).3 Among
both, the intensive low energy absorption band (e ≈ 2 3
composite ET fluoroionophores, dyes containing a small and
104 M21 cm21 for all the dyes) and the considerably high
rigid spacer are especially advantageous in terms of minimally
fluorescence quantum yield of the reference compound 3 reveal
separated molecular subunits with efficient transduction capa-
that allowed CT transitions determine the spectroscopic proper-
2
bility. Recently, such probe molecules based on the D -
2
ties of the 3-PhNI-D -pyrazolines. When comparing the
pyrazoline chromophore were introduced, showing a cation-
fluorescence quantum yield and lifetime data of 3 with those of
induced ‘switching on’ of the fluorescence upon binding to
1, 2 and 4, the quenching electron transfer interaction (virtually
main group4 and/or heavy metal ions.4b In these substituted
no changes in spectral band position, see above) of a 5-p-anilino
2
1,3,5-triaryl-D -pyrazolines, not only does the spacer-separated
donor is evident. Since the radiative rate constants kf = ff/tf of
receptor at the 5-position acts as an electron donor but the main
chromophore itself (including the ring fragment C(3)NN(2)–
N(1)) consists of a donor (D: N(1) and 1-substituent) and an
1–4 are very similar (kf ≈ 0.9 3 108 s21), the rate constant of
the ET quenching process in 1, 2 and 4 can be calculated from
acceptor (A: C(3) = N(2) and 3-substituent) subunit, and thus
the intramolecular ET reaction from the 5-donor does not
Table 1 Spectroscopic data of 1 and its HgII and AgI complexes, 2 and its
LiI, NaI, KI, MgII, CaII, SrII, BaII and PbII complexes, 3 and 4 in acetonitrile
quench a chromophore-localized state (as, for instance in
at room temperaturea
donor–alkyl-substituted anthracenes or pyrenes)5 but rather a
highly emissive charge transfer (CT) excited state.4b This
labs/nm lem/nm ff
tf/ns
log KS
constitution harbors two important advantages for rational
probe design: (i) rigidized D–A-fluorophores with allowed CT
1
488
483
484
493
489
487
484
484
484
485
483
490
484
680
667
673
679
675
673
669
668
668
670
667
682
670
0.007b
20c
0.076
1.57
1.22
0.029
0.35
0.65
1.14
1.08
1.23
0.99
1.53
1.32
0.017
—
> 5.2d
4.95
—
2.92
2.47
2.99
4.40
3.74
3.68
> 5.2d
—
2
17HgII
17AgI
2
transtitions (e.g. the basic 1,3-diaryl-D -pyrazoline chromo-
phore)4b usually show broad and largely Stokes-shifted absorp-
tion and emission bands often accompanied by high fluores-
cence quantum yields and (ii) the tuning of composite
D–A-chromophores by combining specific D and A units is
synthetically well feasible. Here, we extended this design
concept to the red/near infra-red (NIR) region by the introduc-
tion of the strong N-phenyl-1,8-naphthalimide (PhNI) acceptor
15c
0.003b
11c
27LiI
27NaI
27MgII
27CaII
27SrII
27BaII
27PbII
3
17c
40c
43c
39c
37c
2
to the 3-position of the D -pyrazoline ring yielding 1–4. 1 and
47c
2 are equipped with cation-sensitive anilino crown units
showing strongly different ion binding preferences and 3 and 4
act as model compounds.
0.18b
0.002b
4
—
a Experimental conditions: c (dye) = 5 3 1026 M, lexc ≈ 480 nm (at the
respective isosbestic points) for steady-state, 480 nm for time-resolved
fluorescence measurements. The cation selectivity is identical to that of
related pairs of aza-oxa and aza-thia crowns.4b,9 b Fluorescence quantum
yield determined relative to fluorescein 27 in 0.1 M NaOH (ff = 0.90 ±
0.03).12 c Relative fluorescence enhancement with respect to ff of the
corresponding free dye. d Too high to be determined with acceptable
accuracy with the method employed.
The efficient separation of the electron donating 5-p-receptor
2
unit and the main 1,3-diaryl-D -pyrazoline chromophore by the
rigid spacer is suggested from the nearly identical positions of
† Electronic supplementary information (ESI) available: experimental
details including synthesis of 1–4 and optical spectroscopy. See http://
www.rsc.org/suppdata/cc/b0/b006430k/
DOI: 10.1039/b006430k
Chem. Commun., 2000, 2103–2104
This journal is © The Royal Society of Chemistry 2000
2103

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achieved even with advantageous emission features such as
broad and largely Stokes shifted bands and considerably high
fluorescence quantum yields in the red/NIR spectral region.
We gratefully acknowledge the financial support by the
Deutsche Forschungsgemeinschaft and the Bundesministerium
für Bildung und Forschung.
Notes and references
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3045; (j) G. McSkimming, J. H. R. Tucker, H. Bouas-Laurent and J.-P.
Desvergne, Angew. Chem., Int. Ed., 2000, 39, 2167; (k) A. P. de Silva,
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2 For examples on anions: D. H. Vance and A. W. Czarnik, J. Am. Chem.
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Bleasdale and P. A. Gale, Chem. Commun., 1999, 1723; for examples on
heavy and transition metal cations: P. Ghosh, P. K. Bharadwaj, S.
Mandal and S. Ghosh, J. Am. Chem. Soc., 1996, 118, 1553; B.
Ramachandram and A. Samanta, Chem. Phys. Lett., 1998, 290, 9; for
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1213; K. Yoshida, T. Mori, S. Watanabe, H. Kawai and T. Nagamura,
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refs. 1b, k.
3 R. A. Bissell, A. P. de Silva, H. Q. N. Gunaratne, P. L. M. Lynch, G. E.
M. Maguire, C. P. McCoy and K. R. A. S. Sandanayake, Top. Curr.
Chem., 1993, 168, 223.
4 (a) A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson and M.
Nieuwenhuizen, Chem. Commun., 1996, 1967; (b) K. Rurack, J. L.
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5 A. P. de Silva and S. A. de Silva, J. Chem. Soc., Chem. Commun., 1986,
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6 M. A. Kahlow, T. J. Kang and P. F. Barbara, J. Phys. Chem., 1987, 91,
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Fig. 1 Fluorescence titration spectra (lexc = 478 nm) of 1 (5 3 1026 M)
with Hg(ClO4)2 (concentration range: 5 3 1027 to 2 3 1023 M) in
acetonitrile. Upper inset: selected absorption titration spectra (isosbestic
point: 478 nm; arrow indicates the direction of change upon HgII addition).
Lower inset: fit (–––) of the integrated intensity (0) of the fluoroescence
titration.
the measured fluorescence lifetime according to kET = tf(X)21
2 tf(3)21 with X = 1, 2, 4 by using that of 3 as a reference
(Table 1). This yields kET of 12, 34 and 58 ns21 for 1, 2 and 4,
respectively. These rate constants are rather small and are thus
not related to the solvent relaxation time (the longitudinal
solvent relaxation time in acetonitrile was determined to
0.2 ps)6 suggesting a nonadiabatic electron transfer process.
Upon cation addition to 1 and 2, the changes in spectral band
position are comparatively small (Table 1 and, as an example,
the titration of 1 with Hg(ClO4)2 shown in Fig. 1) as is expected
for an ET signaling mechanism. Accordingly, binding of a
cation to the electron donating 5-p-receptor strongly alters its
redox potential and weakens its donor strength thus decelerting
the ET quenching process. As is evident from Table 1, the
cation-induced changes in fluorescence quantum yield and
lifetime are drastic and are directly related to the charge density
of the metal ion, i.e. the inhibition of the ET reaction being
stronger for HgII than for AgI (for 1) and for alkaline-earth metal
ions relative to alkali metal ions (for 2). For the complexes with
divalent HgII, CaII and PbII, all of them tightly binding to the
fluorescent sensor molecules, even a complete ‘switching off’
of the ET process is manifested by rate constants of radiative
and non-radiative deactivation which are nearly identical to
those of the reference compound 3, lacking a 5-p-anilino donor.
In all the cases, the fit of the spectrofluorometric titration
data4b,7 yielded a 1+1 complex stoichiometry.
Besides the favorable fluorescence enhancement character-
istics, the cation selectivity can easily be directed by tuning of
the 5-p-receptor. Whereas 1, containing four ‘soft’8 sulfur donor
atoms in the 15-crown-5 receptor, only binds to the thiophilic
heavy and transition metal ions HgII and AgI (for a detailed
discussion of ion selectivities and preferences, see refs 4(b) and
9), 2 with a monoaza-tetraoxa-15-crown-5 unit shows changes
in its spectroscopic properties in the presence of the ‘hard’8
group I and II metal ions as well as PbII. The power of the
present design concept, especially for HgII (1) and PbII (2)
commonly known as fluorescence quenchers10 and for sensing
applications in the red/NIR, is apparent.11
8 R. G. Pearson, J. Am. Chem. Soc., 1963, 85, 3533.
9 K. Rurack, J. L. Bricks, G. Reck, R. Radeglia and U. Resch-Genger,
J. Phys. Chem. A, 2000, 104, 3087.
10 A. W. Varnes, R. B. Dodson and E. L. Wehry, J. Am. Chem. Soc., 1972,
94, 946; H. Masuhara, H. Shioyama, T. Saito, K. Hamada, S. Yasoshima
and N. Mataga, J. Phys. Chem., 1984, 88, 5868.
11 It is interesting that the corresponding substituted chalcone-type dyes
(PhNI–CO–CHNCH–C6H4R with R = H, NMe2 or crowns), which
2
were obtained as intermediates within the D -pyrazoline synthesis (see
ESI†), do not fluoresce (ff < 1 3 1025) and thus cannot be exploited
for sensing purposes as, for instance their benzothiazole derivatives.9
This is most probably due to the close neighborhood of the carbonyl and
the PhNI group, since electron acceptors at the 4-position of the
naphthalimide chromophore are known to quench its fluorescence.13
12 J. Olmsted, III, J. Phys. Chem., 1979, 83, 2581.
In summary, we have shown that upon combining intra-
molecular charge and electron transfer processes in a simple
fluorophore–spacer–receptor ionophore with a small but rigid
spacer, an efficient cation-triggered ‘switching on’ of the
intramolecular charge transfer fluorescence can selectively be
13 A. Pardo, J. M. L. Poyato and E. Martin, J. Photochem., 1987, 36,
323.
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Chem. Commun., 2000, 2103–2104