Unidirectional switching between two flavylium reaction networks by the action of alternate stimuli of acid and base.

Article abstract of DOI:10.1021/jp209913f

The introduction of an ester group in the flavylium core allowed the reversible conversion between two different flavylium compounds each one exhibiting its own reaction network. An unidirectional switching cycle between 7-diethylamino-2-(4-(methoxycarbon

Full text of DOI:10.1021/jp209913f

ARTICLE
pubs.acs.org/JPCA
Unidirectional Switching between Two Flavylium Reaction Networks
by the Action of Alternate Stimuli of Acid and Base.
Sandra Gago,† Vesselin Petrov,† Ana M. Diniz,† A. Jorge Parola,*,† Luís Cunha-Silva,‡ and Fernando Pina*,†
†REQUIMTE, Departamento de Química, Faculdade de Ci^encias e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica,
Portugal
‡REQUIMTE & Departamento de Química e Bioquímica, Faculdade de Ci^encias, Universidade do Porto, 4169-007 Porto, Portugal
ABSTRACT: The introduction of an ester group in the flavylium core
allowed the reversible conversion between two different flavylium
compounds each one exhibiting its own reaction network. An unidi-
rectional switching cycle between 7-diethylamino-2-(4-(methoxy-
carbonyl)phenyl)-1-benzopyrylium and 2-(4-carboxyphenyl)-7-
diethylamino-1-benzopyrylium was achieved by means of alternate
acid and base stimuli. Addition of base to a methanolic solution of the
ester derivative gives rise to the trans-chalcone of the parent carboxylic
acid, which upon acidification of the solution forms the respective
flavylium cation. This species esterifies under very acidic conditions to
restore the original methyl ester derivative. The chemical reaction networks of both compounds were fully characterized from their
thermodynamic and kinetic aspects, by a series of pH jumps followed by UVꢀvis absorption and emission spectroscopy, stopped
1
flow and H NMR. The crystal structure of the trans-chalcone of the ester derivative was unveiled showing a supramolecular
structure involving hydrogen bonding.
It is easy to show4 that eq 1ꢀ4 behave as a single acid base
’ INTRODUCTION
equilibrium, eq 5
The pH-dependent network of flavylium compounds is well
AHþ þ H2O h CB þ H3Oþ
established in particular for those lacking hydroxyl substituents
where formation of B4 has been observed. In the case of the
K0a ¼ Kh2 þ Kh4 þ Kh2Kt þ Kh2KtKi
ð5Þ
flavylium cation (lacking of substituents),1ꢀ3 in moderately
acidic to neutral aqueous solutions, a sequence of reactions takes
place, starting from the hydration in position 2 to give B2, eq 1,
being [CB] = [B2] + [B4] + [Cc] + [Ct].
When the cisꢀtrans isomerization is much slower than the
other reactions a pseudo stationary state can be defined by means
of eq 6, and behaving also as a single acidꢀbase equilibrium
AHþ þ H2O h CB^ þ H3Oþ
K^ ¼ Kh2 þ Kh4 þ Kh2Kt
ð6Þ
a
being [CB^] = [B2] + [B4] + [Cc].
One useful way to the comprehension of the pH dependent
thermodynamic and kinetics of the flavylium network of chemi-
cal reactions is the construction of an energy level diagram as the
one shown in Scheme 1.5
At acidic medium the thermodynamic stable species is the
flavylium cation. As the pH raises B2, B4 and Cc are formed as
transient species, but the final equilibrium involves essentially the
trans-chalcone.
The kinetics of the (not substituted) flavylium system were
also reported, Scheme 2.1ꢀ3
After the work of McClelland and Gedge,2 it was established
that the terms in gray are due to the catalysis, and thus after a pH
Received: October 14, 2011
Revised:
December 1, 2011
Published: December 28, 2011
r
2011 American Chemical Society
372
dx.doi.org/10.1021/jp209913f J. Phys. Chem. A 2012, 116, 372–380
|

The Journal of Physical Chemistry A
ARTICLE
Scheme 1
Scheme 2
jump from 1 (flavylium cation) to for example pH = 5, B2 and B4
are formed with rates approximately 5 sꢀ1 and 15 sꢀ1 (parallel
reactions) respectively and Cc from B2 in ca. 2 sꢀ1. The cisꢀtrans
isomerization is much slower and thus a pseudo equilibrium
involving AH+, B2, B4, and Cc is established in the time scale of
one second. In this pseudo equilibrium the major species are
B2 and Cc, as shown in Scheme 1. From this stage the iso-
merization takes place and its rate is given by eq 7 where the
term that multiplies ki accounts for the mole fraction dis-
tribution of Cc at the pseudo equilibrium. Representation of
the rate constant of the cisꢀtrans isomerization rate con-
stant as a function of pH should give a sigmoid curve with a
plateau at higher pH values given by eq 8.1
sigmoid curve according to eq 7 and 8 is obtained, as observed
for the flavylium cation itself.
When comparing the absorptionspectra of thedifferent species
constituting the network, those of the flavylium cation and in less
extent of the chalcones are red-shifted, while both B2 and B4 have
absorption bands rather blue-shifted, due to the disruption of the
πꢀπ* conjugation in position 2 and 4 respectively.
The networks of flavylium compounds have been exploit-
ed as models for optical memories permitting to define
writeꢀreadꢀerase cycles at the molecular level operated by light
and pH inputs.4 In particular when light is used as input the trans-
chalcone is transformed into the cis form that spontaneously
leads to the colorful flavylium cation. One interesting improve-
ment is the possibility of enlarging the number of species in the
network by designing a flavylium able to be reversibly converted
in another flavylium, joining together two different reaction net-
works. This was partially achieved with 40-acetamidoflavylium,
which could be converted into the corresponding 40-aminoflavy-
lium upon hydrolysis under acidic conditions.6 However, the
system is irreversible. Here we report on a flavylium ester deriv-
ative that hydrolyzes to the corresponding acid under basic
conditions and re-esterificates to the original compound in
acidic conditions.
KhKt
kcisꢀtrans
¼
ki þ k
ð7Þ
ꢀi
½Hþꢁ þ Kh þ KhKt
Kt
kcisꢀtransðplateauÞ ¼
ki þ k
ð8Þ
ꢀi
1 þ Kt
In eq 8, the term that multiplies the rate constant ki, represents
the mole fraction of Cc when the pH is sufficiently high to
convert all the flavylium cation into B2 and Cc at the pseudo
equilibrium, as occurs at the plateau.
It is also important to retain the two limiting cases for the
(slower) kinetics of the trans-chalcone appearance from flavy-
lium cation upon pH jumps from very acid to the basic region: (i)
in the case of hydration control a U shaped curve is obtained un-
less for higher pH values the cisꢀtrans isomerization becomes
slower (change of mechanism) leading to a plateau at higher
pH values ; (ii) when the isomerization is the controlling step a
’ EXPERIMENTAL SECTION
General. All reagents and solvents were of analytical grade. 1H and
NMR spectra were recorded at 400 MHz respectively with a Bruker
AMX400 in CD3OD referenced to the solvent. Elemental analyses (EA)
were performed in a Thermofinnigan Flash EA 112 series. Mass spectra
373
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The Journal of Physical Chemistry A
ARTICLE
were run on an Applied Biosystems Voyager-DE PRO. Spectroscopic
experiments were carried out in buffered7 water/ethanol 80:20 (v/v)
solvent mixture. pH values were adjusted by the addition of a 0.1 M
NaOH aqueous solution to a 0.1 M HCl aqueous solution, and measured
with a MeterLab pHM240 pH meter from Radiometer Copenhagen.
For solutions with HCl concentration above 0.1 M, pH was calculated
from ꢀlog [HCl]. UV/vis absorption spectra were recorded with a
Varian-Cary 100 Bio spectrophotometer or in a Shimadzu VC2501-PC.
The stopped flow experiments were conducted in an Applied Photo-
physics SX20 stopped-flow spectrometer provided with a PDA.1/UV
photodiode array detector.
Table 1. Crystal and Structure Refinement Data for Com-
pound trans-Chalcone, Ct
formula
C21H23NO4
Synthesis of 7-Diethylamino-2-(4-(methoxycarbonyl)-
phenyl)-1-benzopyrylium Hydrogen Sulfate. The compound
was prepared from condensation of 4-diethylamino-2-hydroxybenzalde-
hyde (0.31 g; 1.6 mmol) with 4-acetylbenzoic acid (0.26 g; 3 mmol).8
The reagents were dissolved in 6 mL of acetic acid, 2 mL of H2SO4, and
2 mL of CH3OH. The reaction mixture was stirred overnight. By the
following day ethyl acetate was added and a purple solid precipitated,
was filtered off and carefully washed with diethyl ether and dried yielding
0.58 g of 7-diethylamino-2-(4-(methoxycarbonyl)phenyl)-1-benzopyry-
Mr
353.40
crystal description
crystal size/mm
temperature/K
crystal system
space group
a /Å
orange plates
0.20 ꢂ 0.09 ꢂ 0.01
150(2)
triclinic
Pꢀ1
12.039(2)
13.110(2)
13.346(2)
85.202(8)
67.787(7)
68.725(8)
1813.7(5)
4
b /Å
1
lium hydrogensulfate (1.4 mmol, 85%): H NMR (CD3OD, 300 K,
400.13 MHz), δ (ppm): 8.69 (1H, H4, d, 3J = 7.8 Hz), 8.40 (2H, d, H3 ,
c /Å
0
3
3
α /°
β /°
γ /°
volume /Å3
Z
0
0
0
H5 , J = 8.8 Hz); 8.27 (2H, d, H2 , H6 , J = 8.7 Hz); 8.01 (2H, dd, H3,
H5, 3J = 8.7 Hz, 4J = 5.9 Hz); 7.57 (1H, dd, H6, 3J = 9.5 Hz, 4J = 2.4 Hz);
7.34 (1H, d, H8, 4J = 1.7 Hz); 3.98 (3H, s, COOꢀCH3); 3.83
(4H, q, NꢀCH2), 1.39 (6H, t, CH3). MALDIꢀTOF/MS: m/z (%):
calcd for C21H22NO3, 336.16; found, 336.16 [M+] (100%). Anal. Calcd
for C21H22NO3HSO4 (Mr = 433.47): C, 58.19; H, 5.35; N, 3.23. Found:
C, 57.88; H, 5.60; N, 3.05.
F
calculated /g cmꢀ3
1.294
F(000)
752
Results for 7-Diethylamino-2-(4-(methoxycarbonyl)-
phenyl)-1-benzopyrylium Chloride Obtained from Recrys-
tallization of 2-(4-Carboxyphenyl)-7-diethylamino-1-ben-
zopyrylium Hydrogen Sulfate in Acidic (HCl) Methanol.
MALDIꢀTOF/MS: m/z (%): calcd for C21H22NO3+, 336.16; found,
μ /mmꢀ1
θ range /°
index ranges
0.089
3.61ꢀ23.11
ꢀ12 e h e 13
ꢀ14 e k e 14
ꢀ14 e l e 14
19 454
336.2 [M+] (100%). Anal. Calcd for C21H22NO3Cl 3H2O (Mr =
3
418.70): C, 59.22; H, 6.53; N, 3.29. Found: C, 59.91; H, 6.46; N, 3.35.
Synthesis of 2-(4-Carboxyphenyl)-7-diethylamino-1-benzo-
pyrylium Hydrogen Sulfate. This compound was prepared from
condensation of 4-diethylamino-2-hydroxybenzaldehyde (0.58 g; 3 mmol)
and 4-acetylbenzoic acid (0.49 g; 3 mmol). The reagents were dissolved
in 9 mL of acetic acid and 3 mL of H2SO4. The reaction mixture was
stirred overnight. By the following day, diethyl ether was added and a
purple solid precipitated, was filtered off and carefully washed with
reflections collected
independent reflections
final R indices [I > 2σ(I)]
4692 (Rint = 0.0829)
R1 = 0.0506
wR2 = 0.0978
R1 = 0.1261
wR2 = 0.1307
0.206 and ꢀ0.212
final R indices (all data)
largest diff peak and hole/e Å3
Scheme 3
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ARTICLE
Figure 1. Spectral variations of the compound 2-(4-carboxyphenyl)-7-diethylamino-1-benzopyrylium, 3.43 ꢂ 10ꢀ5 M, in ethanolic (20%) aqueous
solutions upon equilibration in the dark (3 h) as a function of pH: A, 2 < pH < 7; B, 7 < pH < 11.5; C, spectra at extremely acidic solutions; D,
simultaneous fitting of the absorption at 531 nm (b) and emission at λexc = 500 nm, λem = 630 nm (O).
Figure 2. (A) Spectral variations of the compound 2-(4-carboxyphenyl)-7-diethylamino-1-benzopyrylium 3.4 ꢂ 10ꢀ5 M, upon a pH jump from
1.1 to 7.9. (B) The same for pH = 12.2. (C) The same for pH = 8.9.
diethyl ether and dried yielding 1.22 g of 2-(4-carboxyphenyl)-7-diethyl-
amino-1-benzopyrylium hydrogen sulfate (2.9 mmol, 96%). H NMR
Isolation of the trans-Chalcone of 7-Diethylamino-2-(4-
(methoxycarbonyl)phenyl)-1-benzopyrylium: (E)-1-(4-Acetyl-
phenyl)-3-(4-diethylamino)-2-hydroxyphenyl)prop-2-en-
1-one. The pH of 10 mL of ethanol/water (1:1) solution of
7-diethylamino-2-(4-(methoxycarbonyl)phenyl)-1-benzopyrylium hydro-
gensulfate was adjusted to 7.0 with addition of 1 M NaOH dropwise. This
mixture was warmed to reflux for 1 h and allowed to cool overnight. The
solvent was decanted and the precipitated solid washed with ether several
times and vacuum-dried to give a brown solid (25 mg, 18%).
1
(CD3OD/DCl, pD ≈ 1.0, 400.13 MHz), δ (ppm): 8.7 (1H, H4, d,
3
3J
= 7.8 Hz), 8.39 (2H, d, H3 , H5 , J
= 8.4 Hz);
0
0
0
0
0
0
H4ꢀH3
H3 ,H5 ‑H2 ,H6
3
0
0
0
0
0
0
8.22 (2H, d, H2 , H6 , J H2 ,H6 ‑H3 ,H5 = 8.4 Hz); 8.02 (2H, t, H3, H5,
3J
= 7.8 Hz, J
= 9.5 Hz); 7.55 (1H, dd, H6, JH6ꢀH5
=
3
3
H3ꢀH4
H5ꢀH6
4
4
9.5 Hz, JH6ꢀH8 = 1.9 Hz); 7.33 (1H, d, H8, JH8ꢀH6 = 1.5 Hz); 3.81
(4H, m, NꢀCH2), 1.37 (6H, s, CH3). MALDIꢀTOF/MS: m/z (%)
calcd for C20H20NO3 , 322.14; found, 322.2 [M+] (100%). Anal. Calcd
+
for 0.9(C20H20NHO3 2HSO4) + 0.1(C20H20NO3 HSO4)9: C, 47.31;
1H NMR (400.13 MHz, CD3OD, room temperature, 400.13 MHz):
δ (ppm) 8.13 (2H, d, H20 + H60 or H30 + H50, 3J = 8.1 Hz), 8.08 (d, 1H,
3
3
H, 4.53; N, 2.76. Found: C, 47.54; H, 4.43; N, 2.88.
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isotropic thermal displacements parameters (Uiso) fixed at 1.2 or
1.5 ꢂ Ueq of the carbon atom to which they are attached.
Information concerning the crystallographic data collection and struc-
ture refinement are summarized in Table 1. Crystallographic data
(excluding structure factors) for the structure in this paper have been de-
posited with the Cambridge Crystallographic Data Centre as supplementary
publication No. CCDC-838069. Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
U.K. (fax, +44-(0)1223ꢀ336033, or e-mail, deposit@ccdc.cam.ac.uk).
’ RESULTS AND DISCUSSION
2-(4-Carboxyphenyl)-7-diethylamino-1-benzopyrylium Hy-
drogen Sulfate. The pH dependent spectral variations of
equilibrated solutions of the compound 2-(4-carboxyphenyl)-
7-diethylamino-1-benzopyrylium, Scheme 3, are represented in
Figure 1.
Figure 3. Representation of the rate constants of the direct pH jumps
versus pH. Fitting was achieved according to eq 10 (hydration) and
eq 11 (isomerization).
The data are compatible with a sequence of reactions as shown
in Scheme 3. The small spectral variations of the absorption regard-
ing the first inflection (pK = 3.4) are attributed to the deprotona-
tion of the carboxylic moiety, parts A and D of Figure 1. The
deprotonation of the carboxylic substituent is better detected by
fluorescence, Figure 1D (open circles), as previously reported for
the analogous compound 4-(2-carboxyphenyl)-7-diethylamino-
40-dimethylamino-1-benzopyrylium.18 The higher acidity of the
carboxylic substituent, pKa = 3.4, in comparison with benzoic acid
pKa = 4.2, is expected from the influence of the positive charge of
the flavylium cation.
H4, 3J = 15.0 Hz), 8.03 (2H, d, H20 + H60 or H30 + H50, 3J = 8.3 Hz), 7.49
(1H, d, H5, 3J = 9.6 Hz), 7.48 (1H, d, H3, 3J = 15.0 Hz), 6.31
(1H, dd, H6, 3J = 8.9 Hz, 4J = 1.5 Hz), 6.14 (1H, d, H8, 4J = 2.0 Hz),
3.93 (3H, s, COOꢀCH3), 3.41 (4H, q, NꢀCH2), 1.19 ppm (6H, t,
CH3). MS (EI): m/z (%) calcd for MS (EI): m/z (%) calcd for
C21H23NO4 , 353.16; found, 353.2 [M+•] (56%); 338.14 (100%)
+
[M ꢀ CH3+•]. Anal. Calcd for C21H23NO4 1.5H2O (Mr = 380.43):
3
C, 66.30; H, 6.89; N, 3.68. Found: C, 66.23; H, 6.95; N, 3.68.
Ionized trans-Chalcone of 2-(4-Carboxyphenyl)-7-diethy-
lamino-1-benzopyrylium Hydrogen Sulfate (Ct2ꢀ). H NMR
1
The network of chemical reactions characteristic of the
flavylium system, eq 5, takes place at higher pH values, pK0a = 5.2,
and by consequence all the species involved are deprotonated at
the carboxylic moiety, in particular the flavylium cation in this pH
range can be considered as having a zwitterionic nature. The
major species of CB in eq 5 is the ionized trans-chalcone, Ctꢀ,
based on the shape and position of the absorption spectrum and
(400.13 MHz, CD3OD/NaOD, room temperature, 400.13 MHz):
3
δ = 8.33 (1H, d, H4, J = 14.9 Hz), 8.09 (2H, d, H20, H60 or H30,
H50, 3J = 8.4 Hz), 7.99 (2H, d, H20, H60 or H30, H50, 3J = 8.4 Hz), 7.38
(1H, d, H5, 3J = 9.0 Hz), 7.29 (1H, d, H3, 3J = 14.9 Hz), 6.07 (1H, dd,
3
4
4
H6, J = 9.0 Hz, J = 2.5 Hz), 5.94 (1H, d, H8, J = 2.5 Hz), 3.54
(4H, q, NꢀCH2), 3.35 (3H, s, CH3OH, product of the ester hydrolysis),
1.18 ppm (6H, t, CH3).
1
on H NMR data, see below. Finally in Figure 1B the ionized
trans-chalcone (Ct2ꢀ) is formed from the trans-chalcone (Ctꢀ).
At extremely acidic pH values the amine protonates with pKa ≈
ꢀ0.95 (Figure 1C).
Single-Crystal X-ray Diffraction. Single crystals of the trans-
chalcone, C21H23NO4, of 7-diethylamino-2-(4-(methoxycarbonyl)-
phenyl)-1-benzopyrylium were harvested from the crystallization vial
and immersed in highly viscous FOMBLIN Y perfluoropolyether
vacuum oil (LVAC 140/13). A suitable crystal was selected and mount-
ed on a HamptonResearch CryoLoop with the assistance of a Stemi 2000
stereomicroscope.10 Data were collected on a Bruker X8 Kappa APEX II
charge-coupled device (CCD) area-detector diffractometer (Mo
Kα graphite-monochromated radiation, λ = 0.71073 Å) controlled by the
APEX2 software package,11 and equipped with an Oxford Cryosystems
Series 700 cryostream monitored remotely using the software interface
Cryopad.12 Images were processed using the software package
SAINT+,13 and data were corrected for absorption by the multiscan
semiempirical method implemented in SADABS.14 The structure was
solved by direct methods implemented in SHELXS-97,15,16 and refined
from successive full-matrix least-squares cycles on F2 using SHELXL-
97.15,17 All non-hydrogen atoms were successfully refined using aniso-
tropic displacement parameters.
Hydrogen atoms associated with the hydroxyl (OꢀH) groups were
markedly visible in difference Fourier maps and were included in the
structure with the OꢀH distances restrained to 0.90(2) Å, and Uiso fixed
at 1.5 ꢂ Ueq of the parent oxygen atom. H atoms bound to carbon were
located at their idealized positions using appropriate HFIX instructions
in SHELXL: 43 for the aromatic, 23 for the ꢀCH2 carbons and 137 for
the terminal ꢀCH3 methyl groups. All these atoms were included in
subsequent refinement cycles in riding-motion approximation with
From the data of Figure 1, eq 9 can be written
K0a ¼ Kh4 þ Kh2 þ Kh2Kt þ Kh2KtKi ¼ 10ꢀ5:2
ð9Þ
The formation of (ionized) trans-chalcone (Ct2ꢀ) at pH = 12
1
was confirmed by H NMR on the basis of the large coupling
constant between H3 and H4, Figure 7B. Titration of this species
followed by UVꢀvis back to acid up to pH = 7 reproduces the
data from Figure 1B.
In order to characterize the kinetics of the network of chemical
reactions, a series of pH jumps from equilibrated solutions at
pH = 1.1 to higher pH values was performed, Figure 2. If the pH
jump is carried out to pH = 7.9, Figure 2A, the flavylium cation
(ionized on the carboxylic group) is the species observed im-
mediately after the jump and the system evolves to the trans-
chalcone (also ionized in the carboxylic group). At pH = 8.9,
Figure 2C, the first absorption spectrum after the pH jump
indicates the presence of the flavylium cation that evolves over
time to ionized trans-chalcone through three kinetic processes,
the first one much faster (see below stopped flow experiments).
In the case of the same experiment to pH = 12.2, Figure 2B, the
species immediately formed after the jump is not the flavylium
cation, but instead a species with an absorption band centered at
509 nm appears. This behavior is an indication that hydration of
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Figure 4. (A) Spectral variations of the compound 2-(4-carboxyphenyl)-7-diethylamino-1-benzopyrylium 4.3 ꢂ 10ꢀ5 M, after a pH jump from 1.1 to
12.2 followed by stopped flow: (B) stopped flow traces after a pH jump from 1.1 to the basic region; (C) rate constants as a function of the hydroxyl
concentration.
pseudo equilibrium.
khydration ðsꢀ1Þ ¼ 1:4 ꢂ 10ꢀ4 þ 35½Hþꢁ þ 1:2 ꢂ 103½OHꢀꢁ
Kh2 ¼ 4 ꢂ 10ꢀ6 Mꢀ1
ð10Þ
10ꢀ9:4
½Hþꢁ þ 10ꢀ9:4
kisomerizationðsꢀ1Þ ¼
ꢂ 0:0014 þ 1:4 ꢂ 10ꢀ4
ð11Þ
The concept of pseudoequilibrium presented in eq 6 can be
extended to the basic region, by adding eq 12, considering that in
eq 6 the species are ionized at the carboxylic substituent.
Figure 5. (A) Spectral modifications of the compound 2-(4-carboxyphenyl)-
7-diethylamino-1-benzopyrylium 4.3 ꢂ 10ꢀ5 M in waterꢀethanol (20%)
upon a reverse pH jump from solutions at pH = 13 (nonequilibrated)
followed by stopped flow. (B) The same upon equilibration followed by a
normal spectrophotometer.
AHþ, ꢀ þ H2O h CB^ þ H3Oþ K^
a
½CB^ꢁ ¼ ½B4ꢀꢁ þ ½B2ꢀꢁ þ ½Ccꢀꢁ
ð12Þ
Ccꢀ h Cc2ꢀ þ Hþ
KCc
ꢀ=Cc2ꢀ
The mole fraction of Cc2ꢀ being given by eq 13
Kh2KtKCcꢀ=Cc2ꢀ
Scheme 4
χCc2ꢀ
¼
ð13Þ
2
½Hþꢁ þ Ka^½Hþꢁ þ Kh2KtKCcꢀ=Cc2ꢀ
According to the fitting of eq 11, ((Kh2KtKCcꢀ/Cc2ꢀ)/(Ka^)) =
10ꢀ9.4 and the intrinsic value of the isomerization rate constant is
ki0 = 0.0014 sꢀ1. Considering that the experimental evidence
points out to [CB] ≈ [Ccꢀ] and K^a ≈ Kh2Kt it can be concluded
^
that KCcꢀ/Cc2ꢀ ≈10ꢀ9.4. This value is close to KCtꢀ/Ct2ꢀ =10ꢀ9.2
,
the flavylium cation takes place at this pH value before the time
needed to record an absorption spectrum in our spectrophot-
ometer (ca. 1 min). The nature of the species formed as the result
of this hydration will be discussed below on the basis of the
stopped flow measurements.
Figure 1D, as expected from the usually observed similarity
between the acidity constants of the cis- and trans-chalcones.19
In order to characterize the pseudoequilibrium, a series of pH
jumps from pH = 1.0 monitored by stopped flow was carried out,
Figure 4.
More insight on the system was obtained by representing the
rate constants of the direct pH jump experiments like those
reported in Figure 2, as a function of pH, Figure 3. At lower pH
values the fitting follows eq 10, and we can attribute this process
to an hydration control (see below in the stopped flow sec-
tion the details for the calculation of the [OHꢀ] coefficient).
At higher pH values the hydroxyl attacks the flavylium cation and
hydration becomes faster than the isomerization rendering
this last process as the rate-determining step. The plateau at
higher pH values is reached when the ionized- Cc2‑ attains the
pseudo equilibrium, i.e., an equilibrium of all the basic species
before significant formation of the Ct2‑. The fitting achieved
by means of eq 11 reflects the fraction of Cc2‑ existent at the
The stopped flow data shows that, besides the slow process
shown in Figure 2B attributed to the cisꢀtrans isomerization
(time scale of hours), two other previous kinetic processes are
detected, Figure 4. At basic medium (pH = 12.2) a very fast
process with rate constant 21.9 sꢀ1, corresponds to the
disappearance of the flavylium cation, i.e., its hydration. This is
followed by a second process leading to a raise of the absorption
in the visible. These results can be explained if during the first
step B4ꢀ, B2ꢀ, and Cc2‑ are formed and in a second step the
kinetic product B4ꢀ gives rise to the final pseudoequilibrium
involving essentially Cc2‑. This corresponds to the pseudo-
equilibrium above-reported, the state from which the final
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The Journal of Physical Chemistry A
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Figure 6. (a) Asymmetric unit cell of the crystalline structure of the trans-chalcone of 7-diethylamino-2-(4-(methoxycarbonyl)phenyl)-1-benzopyr-
ylium showing the labeling scheme for the oxygen and nitrogen atoms; All atoms are represented as thermal ellipsoids drawn at the 50% probability level,
excluding the hydrogen atoms which are drawn as spheres with arbitrary radii. (b) Strong OꢀH O (orange dashed lines) and weak CꢀH
O
3 3 3
3 3 3
(green dashed lines) hydrogen bounds between adjacent molecules.
closure.1 The faster process is proportional to the hydroxyl ion
concentration, Figure 4C, with an observed rate constant 1.2 ꢂ
103 sꢀ1 and can be attributed to the sum of the rate constants
OH
B2
OH
B4
k
+ k , responsible for the hydroxyl ion attack to the
flavylium cation in positions 2 and 4, respectively.
A sequence of pH jumps from equilibrated solutions at pH = 1 to
13 and immediately back to pH = 1 have been performed, the
absorption being monitored in the last step, Figure 5A. Protonation
is faster than the mixing time of the stopped flow and by con-
sequence all the species present at pH = 13 are immediately frozen
in the respective protonated species. In other words, all B2ꢀ, B4ꢀ
and Cc2‑ existent at pH = 13 upon the initial pH jump become B2,
B4, and Cc, the observed kinetic processes regard these species.
The biexponential trace in the inset of Figure 5A is in ac-
cordance to a faster kinetics that converts B2 and B4 into
flavylium cation (2.4 sꢀ1), followed by a slower one leading to
more flavylium cation from Cc via B2. On the other hand, the
conversion of Ct to flavylium cation is much slower as shown in
Figure 5B. The ratio of the amplitudes in Figure 5A is 0.38
meaning that the pseudo equilibrium at pH = 13 was constituted
by 38% of B2ꢀ and B4ꢀ and 62% of Cc2‑.
7-Diethylamino-2-(4-(methoxycarbonyl)phenyl)-1-ben-
zopyrylium Hydrogen Sulfate. The compound 7-diethylamino-
2-(4-(methoxycarbonyl)phenyl)-1-benzopyrylium, Scheme 4, can
be obtained according to the synthetic procedure described in the
experimental part or from the previously studied 2-(4-carbo-
xyphenyl)-7-diethylamino-1-benzopyrylium in acidic methanol,
see below. For this reason the following experiments have been
carried out in water containing 20% of ethanol. In this case the
solutions are stable, unless differently is mentioned.
Crystal Structure of the trans-Chalcone. A crystalline ma-
terial of trans-chalcone of 7-diethylamino-2-(4-(methoxycarbonyl)
phenyl)-1-benzopyrylium suitable for single-crystal X-ray diffraction
analysis was obtained by recrystallization in ethanol of the
precipitate obtained in a mixture water:ethanol, and the unveiled
structure confirms undoubtedly the formation of the chalcone
species in the configuration trans (Figure 6a). Systematic searches
in the literature and in the Cambridge Structure Database (CSD,
Version 5.32 of 2011)20,21 confirm that this compound was not
reported previously. The crystal structure was determined in the
triclinic system and in the centrosymmetric space group Pꢀ1
Figure 7. 1H NMR spectra at room temperature in CD3OD: (A)
7-diethylamino-2-(4-(methoxycarbonyl)phenyl)-1-benzopyrylium; (B) the
same compound after addition of NaOD (Ct2‑ form); (C and D) evolution
of the system after addition of DCl to the previous solution (+ denotes the
presence of the flavylium cation and / the presence of the Ct form).
step occurs. The existence of a very fast tautomeric equilibrium
(between B2ꢀ and Ccꢀ) is expected from the previously
described catalytic effect of the hydroxyl ion in the ring-opening
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The Journal of Physical Chemistry A
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Figure 8. Absorption spectra of the compound 7-diethylamino-2-(4-(methoxycarbonyl)phenyl)-1-benzopyrylium, 2.7 ꢂ 10ꢀ5 M in waterꢀethanol
(80:20) upon equilibration: (A) ꢀ3.9 < pH < 7.9, fitting was achieved for pK0a = 4.95; (B) at extremely acidic medium (1.1 ꢂ 10ꢀ5 M, in water),
inflection point occurs for [HCl] ≈ 9 M.
Figure 9. (A) Spectral variations after a pH jump from equilibrated solutions of the compound 7-diethylamino-2-(4-(methoxycarbonyl)phenyl)-1-
benzopyrylium in waterꢀethanol (80:20) at pH = 1 to pH = 7.5 (1.8 ꢂ 10ꢀ5 M); (B) Observed rate constants as a function of pH upon direct pH jumps
from pH = 1.0.
Scheme 5
c.a. 35.16°, while the correspondent angle in the molecule 2 (that
contains the N2 atom) is c.a. 31.87°. Adjacent Ct molecules
interact through strong OꢀH O intermolecular hydrogen
3 3 3
bonds (orange dashed lines in Figure 6b) involving one hydroxyl
group and oxygen atom of the carbonyl group (CdO) in the
neighboring molecule leading to the formation of one-dimen-
sional supramolecular arrangement: O4ꢀH1 O7 with
3 3 3
d(H1 O7) = 1.858(19) Å and angle(O4ꢀH1ꢀO7) = 171(4)°, and
3 3 3
O2ꢀH2 O3 with d(H2
=
1.814(19)
Å
and
O3)
3 3 3
3 3 3
angle(O8ꢀH2ꢀO3) = 177(3)°; symmetry transformations used to
generate equivalent atoms, (i) x, yꢀ1, z. This supramolecular
arrangement is further reinforced by the cooperative effect
of various intermolecular CꢀH
O weak interactions
3 3 3
(green dashed lines in Figure 6b): C14ꢀH14
O6,
O2,
3 3 3
3 3 3
C16ꢀH16
C37ꢀH37
O7, C18ꢀH18A
O3, C39ꢀH39B
O6, C35ꢀH35
(more details related with the crystallographic data collection and
structure refinement can be found in the Experimental Section
and Table 1). The asymmetric unit cell (asu) consists of two
crystallographic independent Ct molecules, each one rotated
180° in the horizontal plane relatively to the other. Furthermore,
the two molecules display distortion conformational considerably
distinct. In molecule 1 (that contains the N1 atom) the dihedral
angle between the average planes of the two aromatic rings is
3 3 3
3 3 3
3 3 3
3 3 3
O2, with H
O dis-
3 3 3
tances in the range between 2.558(3) and 2.716(3) Å and
the CꢀHꢀO angles ranging from 131.3(2) and 177,6(2)°.
De-Esterification of 7-Diethylamino-2-(4-(methoxycarbonyl)-
phenyl)-1-benzopyrylium. When aqueous solutions of the com-
pound7-diethylamino-2-(4-(methoxycarbonyl)phenyl)-1-benzopyr-
1
ylium in d-methanol are basified the H NMR spectrum taken
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The Journal of Physical Chemistry A
ARTICLE
immediately after shows unequivocally the disappearance of the
singlet of the methoxy group and the appearance at 3.34 ppm
of a singlet that is characteristic of (nondeuterated) methanol.
Acidification of this solution shows the appearance of the proto-
nated trans-chalcone that evolves to 2-(4-carboxyphenyl)-7-diethyl-
amino-1-benzopyrylium, Figure 7.
The spectral variations of equilibrated solutions of the com-
pound 7-diethylamino-2-(4-(methoxycarbonyl)phenyl)-1-benzo-
pyrylium, are shown in Figure 8. The study was carried out
in acidic-neutral medium in order to avoid the formation of
2-(4-carboxyphenyl)-7-diethylamino-1-benzopyrylium.
The absorption spectra presented in Figure 8A shows a single
set of isosbestic points corresponding to the equilibrium between
flavylium cation and trans-chalcone. This behavior is very simi-
lar to the parent compound 2-(4-carboxyphenyl)-7-diethyl-
amino-1-benzopyrylium and a comparable global acidity
constant K0a is obtained. The spectra at lower pH values are
also similar to those of the free carboxylic acid, showing a
pKa ≈ ꢀ0.95.
(3) Pina, F.; Melo, M. J.; Maestri, M.; Passaniti, P.; Camaioni, N.;
Balzani, V. Eur. J. Org. Chem. 1999, 3199–3207.
(4) Pina, F.; Parola, A. J.; Gomes, R.; Maestri, M.; Balzani, V.
Multistate/Multifunctional Molecular-Level Systems: Photochromic
Flavylium Compounds. In Molecular Switches, 2nd ed.; Feringa, B.,
Browne, W. R., Eds.; Wiley-VCH: Weinheim, Germany, 2011; Vol. 1,
pp 181ꢀ226.
(5) Pina, F.; Melo, M. J.; Ballardini, R.; Maestri, M. New. Chem. 1997,
21, 696–976.
(6) Giestas, L.; Folgosa, F.; Lima, J. C.; Parola, A. J.; Pina, F. Eur. J.
Org. Chem. 2005, 4187–4200.
(7) K€uster, F. W.; Thiel, A. Tabelle per le Analisi Chimiche e Chimico-
Fisiche, 12nd ed.; Hoepli: Milano, Italy, 1982; pp 157ꢀ160. The
universal buffer used was prepared in the following way: 2.3 cm3 of
85% (w/w) phosphoric acid, 7.00 g of monohydrated citric acid, and
3.54 g of boric acid are dissolved in water; 343 mL of 1 M NaOH was
then added and the solution diluted to 1 dm3 with water.
(8) Gia, O.; Anselmo, A.; Conconi, M. T.; Antonello, C.; Uriarte, E.;
Caffieri, S. J. Med. Chem. 1996, 39, 4489–4496.
(9) Apparently the compound precipitated as a mixture of 90%
dication (protonation of the diethylamino group) and 10% monocation.
(10) Kottke, T.; Stalke, D. J. App. Cryst. 1993, 26, 615–619.
(11) APEX2 Data Collection Software Version, v. 2.1-RC13; Bruker
AXS: Delft, The Netherlands, 2006.
Solutions of the compound 7-diethylamino-2-(4-(methoxy-
carbonyl)phenyl)-1-benzopyrylium at pH = 1.0 were submitted
to pH jumps to higher pH values and the UVꢀvis absorption
modifications monitored, Figure 9.
The initial spectrum in Figure 9A is the flavylium cation that
evolves to the trans-chalcone, an identical pattern being observed
in the pH range between 3 and 8. This behavior indicates a
process controlled by the hydration reaction similarly to the
carboxyl derivative. Fitting was achieved for kh2 = 0.00026 sꢀ1
and kꢀh2 = 15 Mꢀ1 sꢀ1, pKh = 4.8, that compares with 5.4 for the
carboxylic derivative showing that the ester compound is more
easily hydrated.
Solutions of the compound 2-(4-carboxyphenyl)-7-diethyla-
mino-1-benzopyryliumin methanol are converted into the methyl
ester derivative 7-diethylamino-2-(4-(methoxycarbonyl)phenyl)-
1-benzopyrylium upon addition of hydrochloric acid. This fact
allows to conceive an unidirectional cycle (Scheme 5) where the
flavylium ester derivative is converted into the free carboxylic acid
through the respective trans-chalcone upon alternating acidꢀbase
inputs. While the base catalyzed hydrolysis of esters is a well-
known reaction, the esterification of a carboxylic cid at room
temperature is less usual. In the case of the present compound the
positive charge of the benzopyrylium unit facilitates the attack of
methanol to the carboxylic acid group on the para position.
(12) Cryopad Remote monitoring and control, v. 1.451; Oxford
Cryosystems: Oxford, U.K., 2006.
(13) SAINT+ Data Integration Engine, v. 7.23a; Bruker AXS: Madison,
WI, 1997ꢀ2005.
(14) Sheldrick, G. M. SADABS Bruker/Siemens Area Detector Absorp-
tion Correction Program, v. 2.01; Bruker AXS: Madison, WI, 1998.
(15) Sheldrick, G. M. Acta Crystallogr. A 2008, 64, 112–122.
(16) Sheldrick, G. M. SHELXS-97, Program for Crystal Structure
Solution; University of G€ottingen: G€ottingen, Germany, 1997.
(17) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
Refinement, University of G€ottingen: G€ottingen, Germany, 1997.
(18) Gavara, R.; Laia, C. A.T.; Parola, A. J.; Pina, F. Chem.—Eur. J.
2010, 16, 7760–7766.
(19) Petrov, V.; Gomes, R.; Parola, A. J.; Jesus, A.; Laia, C. A. T.;
Pina, F. Tetrahedron 2008, 64, 714–720.
(20) Allen, F. H. Acta Crystallogr., Sect. B 2002, 58, 380–388.
(21) Allen, F. H.; Motherwell, W. D. S. Acta Crystallogr., Sect. B 2002,
58, 407–422.
’ NOTE ADDED AFTER ASAP PUBLICATION
This article posted ASAP on December 28, 2011. Equation 6 has
been revised. The correct version posted on January 3, 2012.
’ AUTHOR INFORMATION
Corresponding Author
*Fax: +351212948550. Telephone: +351212948355. E-mail:
(A.J.P.) ajp@dq.fct.unl.pt; (F.P.) fjp@dq.fct.unl.pt.
’ ACKNOWLEDGMENT
This work was supported by Fundac-~ao para a Ci^encia e
Tecnologia through Projects PTDC/QUI-QUI/104129/2008
and Pest-C/EQB/LA0006/2011 and Grants SFRH/BD/48226/
2008 (A.D.) and SFRH/BPD/18214/2004 (V.P.).
’ REFERENCES
(1) Pina, F.; Melo, M. J.; Laia, C. A. T.; Parola, A. J.; Lima, J. C. Chem.
Soc. Rev. [Online early access] 2011, DOI: 10.1039/c1cs15126f.
(2) McClelland, R. A.; Gedge, S. J. Am. Chem. Soc. 1980, 102,
5838–5848.
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