A R T I C L E S
Gomes et al.
guests.12 Flavylium salts, with their positive 1-benzopyrylium
moieties, seem excellent candidates to be hosted by these
molecular clips.
Table 1. ESI-MS Data Found for the Precipitates of the 1:1
Mixtures of Molecular Clip C1 with the Flavylium Salts 1-3 from
Water (Dissolved in Methanol)
ion
(m/z)exptl
(m/z)calcd
Figure 2 shows the structures of the flavylium salts 1-4, the
water-soluble molecular clips C1 and C2 substituted with
hydrogen phosphate or sulfate groups in the central bridging
unit, and the phosphate-substituted bridge C3 to be used as guest
and host molecules, respectively, in this study. Flavylium salts
[C1 - 2Li+]2-
298.0347
862.1909
619.0641
298.0360
890.2230
444.6075
294.1495
298.0367
597.0837
849.1598
253.0848
298.0400
862.1965
619.0699
298.0400
890.2289
444.6108
294.1498
298.0400
597.0874
849.1660
253.0859
-
[1 + C1 - PF6 - 2Li+]-
-
+
[1 - PF6
]
[C1 - 2Li+]2-
-
[2 + C1 - BF4- - 2Li+]-
13 and 2,14 with amino substituents, were chosen on the basis
[2 + C1 - BF4 - 2Li+ - H+]2-
1
-
+
[2 - BF4
]
of their stability until pH 4-5 where they start to be hydrated;
they are in principle suitable to study the interaction of these
flavylium salts with the hydrogen phosphate-substituted clip C1
in a pH range where C1 remains unprotonated, whereas the
phosphate groups (OPO32-) of bridge C3 are certainly proto-
nated to the corresponding hydrogen phosphate groups (OP(O-
H)O2-) under these slightly acidic conditions. Flavylium salt
315 is hardly hydrated in aqueous solutions due to the methyl
group in position 4 (that reduces positive charge on position
2); compound 3 was chosen to allow NMR studies on both states
AH+ and A, with no worries with the hydration reaction.
Flavylium salt 4,16 a very well studied compound, is hydrated
at pH 2-3, and the resulting chalcones are photochromic. It
was chosen to test the effect of host-guest complex formation
on the network of reactions shown in Figure 1. Particularly,
we were interested in the following two questions: (1) Does
the host molecule in the host-guest complex protect the
flavylium core of the guest molecule from water attack? (2) Is
the trans-cis isomerization of the chalcones affected upon
complexation?
[C1 - 2Li+]2-
[C1 - 2Li+ + H+]-
[3 + C1 - Cl- - 2Li+]-
[3 - Cl-]+
Fluorescence quantum yields were determined using rhodamine
6G in ethanol as standard for flavylium salts 1 and 2 (AH+ form)
and 3 (A form); perylene in toluene was used as standard for 3
(AH+ form). Standard fluorescence quantum yields and refraction
indexes were taken from literature.17,18
Time-resolved fluorescence decays with picosecond resolution
were obtained by the single-photon time technique using laser
excitation at 390 nm and recording the emission at 510 nm. The
setup consisted of a Ti:Sapphire laser Tsunami (Spectra Physics)
pumped with a solid-state laser Millennia Xs (Spectra Physics),
delivering 70 fs pulses at a repetition rate of 80 MHz. The laser
repetition rate was reduced to 4 MHz using a pulse-picker (APE),
and the output was frequency doubled to 390 nm (∼1 nJ per pulse)
and vertically polarized. The fluorescence passed through a polarizer
set at the magic angle and was selected by a Jobin-Yvon HR320
monochromator with a grating of 100 lines/mm and detected by a
Hamamatsu 2809U-01 microchannel plate photomultiplier. The
experimental excitation pulse (fwhm ) 35 ps) was measured using
a scattering solution (Ludox AM30, Aldrich) in water. The decays
were stored in a multichannel analyzer working with 1024 channels.
The fluorescence emission was observed at 510 nm using a cutoff
filter to effectively eliminate the scattered light from the sample.
The experimental decay curves were fitted to simulated curves using
a nonlinear least-squares reconvolution method.
Experimental Section
Synthesis. Synthesis of water-soluble clip C1 and C2 and the
bridge C3 was recently described.11,12 Flavylium salts 1-4 were
available from previous studies.13–16
Mass Spectrometry. Electrospray ionization (ESI) mass spectra
were recorded on a Bruker BioTOF II mass spectrometer.
UV-Vis Absorption and Emission Studies. All flavylium
solutions were freshly prepared using acidified (HCl) water or
methanol. The association constants were determined by adding
increasing amounts of a stock solution of the clip to 2 mL of the
flavylium. After each addition the absorption and emission spectra
were taken. Absorption spectra were run on a Shimadzu UV-
2501PC or a CARY 100Bio, and fluorescence spectra were run on
a Jobin-Yvon Spex, Fluorolog FL3-22.
1H NMR, 13C NMR, DEPT H,H-COSY, C,H-COSY, NOE-
SY, HMQC, HMBC, 1H NMR Titration Experiments. A Bruker
DRX 500 was used. The undeuterated amount of the solvent was
1
used as an internal standard. The H and 13C NMR signals were
assigned by the 2D experiments mentioned above. In the titration
experiments, the total guest concentration [S]0 was kept constant,
whereas the total host concentration [R]0 was varied. This was
achieved by dissolving a defined amount of receptor R in 0.6 mL
of the solution containing the guest concentration [S]0. The
association constants K and the maximum complexation-induced
1H NMR shifts, ∆δmax, were determined from the dependence of
the guest 1H NMR shifts, ∆δ, on the host concentrations by
nonlinear regression analysis using the computer program Table-
Curve 2D, version 5.01.
Kinetic Studies. pH jumps were carried out by mixing 100 µL
of universal buffer at desired pH,19 400 µL of NaOH 0.01 M, and
500 µL of a stock solution of the flavylium 4 at pH 2.0 (1.4 ×
10-5 M) or 200 µL of NaOH 0.01 M, 200 µL of a stock solution
of the flavylium cation at pH 2.0 (3.4 × 10-5 M), and 500 µL of
water. In the cases where the clip is present, 30 µL of clip C2 2.9
× 10-3 M or 28.5 µL of clip C2 3.1 × 10-3 M were previously
mixed with the used amount of the flavylium ion. The absorption
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Ceroni, P.; Balzani, V.; Cartagena, M. C.; Kla¨rner, F.-G.; Schrader,
T.; Vo¨gtle, F. New J. Chem. 2009, 33, 397–407.
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(15) Moncada, M. C.; Moura, S.; Melo, M. J.; Roque, A.; Lodeiro, C.;
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R.; Mac¸anita, A. L.; Pina, F. J. Am. Chem. Soc. 1994, 116, 1249–
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8924 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009