of actinides vs lanthanides; however, no selectivity was
shown toward any particular lanthanide.8a In contrast, a
calix[6]areneꢀnapthoic acid derivative shows fluorescence
enhancement toward La3þ and Y3þ by ∼2- and ∼3-fold
respectively, without much selectivity.8b The binding of
fluoride is expedited in the presence of Csþ due to ion pair
formation by a crown-calix-pyrrole conjugate.9 To our
knowledge, we have not found any calix[6]arene conjugate
doublets at 3.23 and 4.23 ppm suggesting that L is in a
flattened cone conformation.12a
The ion recognition of L has been studied by fluores-
cence spectroscopy in ethanol by exciting the solutions at
330 nm and by varying the mole ratio of the added metal
ion, viz., La3þ, Pr3þ, Nd3þ, Sm3þ, Eu3þ, Gd3þ, Dy3þ, and
Ho3þ (S10ꢀS11 in SI). L is a weak emitter, and its fluores-
cence intensity increases as a function of added La3þ
concentration until 8 equiv, beyond which it saturates. Thus
it exhibits an overall enhancement of ∼70 ( 10-fold due to
the binding of La3þ to L yielding a Ka of 36 694 ( 760 Mꢀ1
based on the BenesiꢀHildebrand equation (Figure 1)
(S12 in SI). Indeed a value of ∼33 800 Mꢀ1 is obtained
based on isothermal titration calorimetry (S13 in SI). The
quantum yields of L and its La3þ complex were 0.001 and
0.075 respectively with reference to anthracene (S14 in SI).
The minimum detectable concentration of La3þ by L is
65 ( 5 ppb (490 nM) (S15 in SI). Similar titrations were
carried out with the control molecules, viz., L4, L5, and L6,
and found that these do not exhibit any considerable
change in the fluorescence intensity upon addition of
La3þ. L7 shows selectivity toward La3þ with an ∼16-fold
increase in fluorescence intensity, but its sensitivity is much
less than L.
h
that is suitable for sensing La3þ followed by F. Therefore,
the present communication deals with the selective sensing
property by a triazole linked picolylimine appended 1,3,5-
tris-conjugate of calix[6]arene (L) toward La3þ and the
{L La3þ} complex toward Fꢀ.
3
The precursors, L, and the control (L4, L5, L6, and L7)
molecules were synthesized (Scheme 1) and character-
ized (S01ꢀS09 in the Supporting Information (SI)).10,11
The bridged diastereotopic protons were observed as two
a
Scheme 1. Synthesis of L, L4, L5, and L6
Figure 1. Fluorescence titration of L {25 μL of 5 μM solution}
with M3þ {5 μM, variable volume} ethanol: (a) Fluorescence
spectra obtained during the titration of Lwith La3þ (λex =330 nm).
(b) Histogram showing the number of folds of fluorescence
enhancement (I/I0) in the titration of L with M3þ (450 nm band)
(Inset: Histogram of relative intensity plot for control molecules).
a (a) CH3I, K2CO3, Dry Acetone, 70 °C, 5.5 bar, 24 h. (b) Propargyl
bromide, Cs2CO3, dry dimethylformamide, 90 °C, 4 h. (c) 5-tert-Butyl-
3-(azidomethyl)-2-hydroxybenzaldehyde, CuSO4 5H2O, sodium ascor-
3
bate, (tert-BuOHþdichloromethane (9:1))/Water as 1:1 ratio, rt, 12 h.
(d) Picolyl amine (L), and benzyl amine (L5), CH3OH, rt, 12 h. (i) SnCl4,
Bu3N, (CH2O)n, toluene, reflux, 8 h, N2 atm. (ii) HCHO, HCl, rt, 48 h.
(iii) NaN3, dimethylformamide, rt, 12 h.
This result suggests that the imino-phenolic-binding
core isrequired, while additional binding througha pyridyl
moiety results in further enhancement in the emission.
Fluorescence titrations carried out with the other seven
lanthanides showed no appreciable change in the intensity
of L, suggesting that L is selective toward La3þ among
these eight ions. When L binds to La3þ, the CdN bond
isomerization and excited-state proton transfer (ESPT) are
inhibited, leading to a fluorescence increase.13
These results were further supported by spectrophoto-
metric titrations. The absorption spectra exhibited isosbes-
tic points at 270, 290, and 340 nm upon the titration,
indicating the transition between the unbound and the
La3þ bound species. At higher concentrations of La3þ, the
spectra exhibited an increase in the absorbance at 370 and
(8) (a) Macerata, E.; Sansone, F.; Baldini, L.; Ugozzoli, F.; Brisach,
F.; Haddaoui, J.; Hubscher-Bruder, V.; Arnaud-Neu, F.; Mariani, M.;
Ungaro, R.; Casnati, A. Eur. J. Org. Chem. 2010, 2675–2686. (b) Liu,
J.-M.; Chen, C.-F.; Zheng, Q.-Y.; Huang, Z.-T. Tetrahedron Lett. 2004,
45, 6071–6074.
(9) Sessler, J. L.; Kim, S. K.; Gross, D. E.; Lee, C. H.; Kim, J. S.;
Lynch, V. M. J. Am. Chem. Soc. 2008, 130, 13162–13166.
(10) (a) Gutsche, C. D.; Dhawan, B.; No, K. H.; Muthukrishnana, R.
J. Am. Chem. Soc. 1981, 103, 3782–3792. (b) Janseen, R. G.; Verboon,
W.; Reinhoudt, D. N.; Casnati, A.; Feriks, M.; Pochini, A.; Ugozzoli,
F.; Ungaro, R.; Nieto, P. M.; Carramolino, M.; Cuevas, F.; Prados, P.;
de Mendoza, J. Synthesis 1993, 380–386. (c) Duynhoven, J. P. M. v.;
Janssen, R. G.; Verboom, N.; Franken, S. M.; Casnati, A.; Pochini, A.;
Ungaro, R.; Merdoza, J. d.; Nieto, P. M.; Pradas, P.; Reinboudt, D. N.
J. Am. Chem. Soc. 1994, 116, 5814–5822.
(11) (a) Pathak, R. K.; Dikundwar, A. G.; Guru Row, T. N.; Rao,
C. P. Chem. Commun. 2010, 4345–4347. (b) Joseph, R.; Rao, C. P. Chem.
Rev. 2011, 111, 4658–4702.
ꢁ ꢀ
(12) (a) Darbost, U.; Seneque, O.; Li, Y; Bertho, G.; Marrot, J.;
Rajer, M.-N.; Reinaud, O.; Jabin, I. Chem.;Eur. J. 2007, 13, 2078–
2088. (b) Izzet, G.; Zeng, X.; Akdas, H.; Marrot, J.; Reinaud, O. Chem.
Commun. 2007, 810–812.
(13) Pathak, R. K.; Hinge, V. K.; Rai, A.; Panda, D.; Rao, C. P.
Inorg. Chem. 2012, 51, 4994–5005.
Org. Lett., Vol. 14, No. 12, 2012
2969