Multistate/Multifunctional Behaviour of 4′-Hydroxy-6-nitroflavylium: A Write-Lock/Read/Unlock/Enable-Erase/Erase Cycle Driven by Light and pH Stimulation

Article abstract of DOI:10.1002/chem.200305348

We have investigated the network of reactions observed for the photochromic 4′-hydroxy-6-nitroflavylium compound in aqueous solutions upon pH changes (including pH jump and stopped flow experiments) and light excitation. The changes observed in the NMR and UV/Vis spectra allowed identification of ten different forms in which this compound can be transformed depending on the experimental conditions. Equilibrium and kinetic constants have been determined. Compared with other members of the flavylium family, 4′-hydroxy-6-nitroflavylium is characterized by a large cis→trans isomerization barrier, and a very efficient hydration reaction. These peculiar features allow writing, reading, storing and erasing photonic information on 4′-hydroxy-6-nitroflavylium by a novel cyclic process that involves the following steps: write-lock/read/unlock/enable-erase/erase.

Full text of DOI:10.1002/chem.200305348

FULL PAPER
Multistate/Multifunctional Behaviour of 4’-Hydroxy-6-nitroflavylium:
A Write-Lock/Read/Unlock/Enable-Erase/Erase Cycle Driven by
Light and pH Stimulation
Margarida C. Moncada,[a, b] A. Jorge Parola,[a] Carlos Lodeiro,[a] Fernando Pina,*[a]
Mauro Maestri,[c] and Vincenzo Balzani[c]
Abstract: We have investigated the
network of reactions observed for the
photochromic 4’-hydroxy-6-nitroflavyli-
um compound in aqueous solutions
upon pH changes (including pH jump
and stopped flowexperiments) and
light excitation. The changes observed
in the NMR and UV/Vis spectra al-
lowed identification of ten different
forms in which this compound can be
transformed depending on the experi-
mental conditions. Equilibrium and ki-
netic constants have been determined.
Compared with other members of the
flavylium family, 4’-hydroxy-6-nitrofla-
vylium is characterized by a large cis!
trans isomerization barrier, and a very
efficient hydration reaction. These pe-
culiar features allowwriting, reading,
storing and erasing photonic informa-
tion on 4’-hydroxy-6-nitroflavylium by
a novel cyclic process that involves the
following steps: write-lock/read/unlock/
enable-erase/erase.
Keywords: flavylium
salts
¥
heterocycles ¥ molecular devices ¥
optical memories ¥ supramolecular
chemistry
Introduction
This compound can perform a write lock read unlock
erase process, that is, a cycle (Figure 1a) in which i) a bit of
information is photochemically ™written∫ on a molecule,
ii) ™locked∫ by a chemical input (+I), iii) ™read∫ by light ab-
sorption, iv) ™unlocked∫ by another chemical input (ꢀI),
and v) ™erased∫ by a photon or thermal energy. This occurs
because, i) the Ct form of the 4’-hydroxyflavylium com-
pound can be photochemically converted into its Cc isomer,
which ii) can be transformed by addition of acid into the
stable AH+ species, and iii) is photostable and can thus be
spectroscopically determined by UV/Vis spectroscopy (Fig-
ure 1b). When necessary, the reaction can be reverted:
iv) addition of a base leads AH+ back to Cc, which v) can
be reconverted to Ct by heating or light excitation.
Great attention is currently devoted to the development of
9]
molecular-level devices.[1 Photochromic compounds and
other systems exhibiting multistate/multifunctional behav-
iour have been proposed as switches and memory ele-
13]
ments.[1,10
In the last fewyears we have carried out a systematic in-
vestigation on the multistate/multifunctional properties of
photochromic flavylium compounds capable of performing
as molecular-level optical memories.[14,15] The complex net-
work of reactions displayed in acidic medium by these com-
pounds is illustrated in Scheme 1 for the case of the 4’-hy-
droxyflavylium species.[16]
A necessary requirement for performing the above-men-
tioned cycle is the presence of an energy barrier in order to
prevent a fast cis!trans thermal isomerization. The lack of
this barrier would in fact result in the immediate erasing of
the information written by the trans!cis photochemical re-
action.
[a] Dr. M. C. Moncada, Dr. A. J. Parola, Dr. C. Lodeiro, Prof. F. Pina
Departamento de QuÌmica
Centro de QuÌmica-Fina e Biotecnologia - REQUIMTE
Faculdade de CiÜncias e Tecnologia
Universidade Nova de Lisboa
Previous studies[14,16] have shown that a hydroxyl substitu-
ent in position 7 decreases the isomerization barrier, where-
as a hydroxyl substituent in position 4’ has the opposite
effect. In order to get more insight concerning the role
played by substituents on the performance of this family of
compounds, we have synthesized a flavylium species bearing
an electron withdrawing nitro group in position 6 and a hy-
droxyl unit in position 4’. We have studied the interconver-
2829-516 Monte de Caparica (Portugal)
E-mail: fjp@dq.fct.unl.pt
[b] Dr. M. C. Moncada
Instituto Superior de CiÜncias da SaÇde
Monte de Caparica (Portugal)
[c] Prof. M. Maestri, Prof. V. Balzani
Dipartimento di Chimica ™G. Ciamician∫
Universit‡ di Bologna, via Selmi 2
40126 Bologna (Italy)
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DOI: 10.1002/chem.200305348
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FULL PAPER
Scheme 1.
Figure 1. A write/lock/read/unlock/erase process in a generic photochromic compound (a) and in the 4’-hydroxyflavylium compound (b).
AHþ Ð B þ Hþ Kh
B Ð Cc Kt
ð2Þ
ð3Þ
ð4Þ
sion reactions among the various forms of the 4’-hydroxy-6-
nitroflavylium compound in acid and basic media upon light
excitation and changes in the pH of the solution (including
pH jumps and stopped flowexperiments) and we have
found that it exhibits some peculiar properties. The com-
plete pattern of processes occurring for this compound is
represented in Scheme 2.
Cc Ð Ct Ki
This set of equilibria can be simplified in one single acid
base equilibrium, Equation (5), if the A, B, Cc, and Ct spe-
cies are represented together as a generic conjugate base
CB, which is in equilibrium with the flavylium cation
AH+ :[14,15,21,22]
Results and Discussion
Acid media: In acid media, flavylium compounds bearing a
hydroxyl in position 4’ can be present in five fundamental
forms (Schemes 1 and 2), namely the flavylium cation AH+,
the quinoidal base A formed upon deprotonation of the fla-
vylium cation, the hemiketal species B obtained by hydra-
tion of the flavylium cation, the cis-chalcone form Cc ob-
tained by tautomerisation of the hemiketal, and the trans-
chalcone Ct form resulting from isomerization of the Cc.[16,2]
AHþ Ð CB þ Hþ Ka0 ¼ KaþKhþKhKtþKhKtKi
ð5Þ
According to Equation (5), AH+ is the stable form for pH
< pK’a and the set of species CB are stable for pH > pK’a,
(however, as we shall see below, in basic medium other
forms can be obtained, see Scheme 2). A useful way to
obtain information on the network of reactions is to perform
pH jump experiments from pH < pK’a to pH > pK’a, and
to monitor the spectral variations that take place as a func-
tion of time.
AHþ Ð A þ Hþ Ka
ð1Þ
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Light and pH Stimulation
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Scheme 2.
With a few exceptions, and we will see that the 4’-hy-
droxy-6-nitroflavylium compound is indeed one of them, the
kinetic processes that occur upon such pH jumps can be de-
scribed as follows (Scheme 3): 1) a very fast deprotonation
reaction leads to the complete or partial disappearance of
the flavylium cation AH+ with formation of the quinoidal
base A as a transient product; 2) the hemiketal B and cis-
chalcone Cc species (which are in very fast equilibrium) are
formed through the hydration of the flavylium cation as the
rate determining step; when there is a kinetic barrier to the
cis!trans isomerization, a pseudo equilibrium is reached at
this stage; 3) on a longer time scale, the cis!trans isomeri-
zation reaction leads to the thermodynamic equilibrium,
with formation of Ct.
The pH dependent absorption spectra of dark equilibrat-
ed solutions (two months in the dark at room temperature)
in the case of 4’-hydroxy-6-nitroflavylium are displayed in
Figure 2a. Inspection of this Figure shows that on increasing
pH the absorption band of the flavylium cation (lmax
=
450 nm) decreases; additionally an absorption band arises
with shape and energy which are characteristic of the chal-
cone.[14,15] A further investigation has shown that at this
stage about 97% of the trans-chalcone form is present.
From the absorbance variation at 450 nm, a value of ꢀ0.6
was obtained for the overall pK’a [Eq. (5)]. This value can
be compared with pK’a 1.9 for the parent 4’-hydroxyflavyli-
um cation,[16] showing that the 6-nitro substituent has a con-
siderable destabilization effect on the cationic form with re-
Scheme 3.
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F. Pina et al.
FULL PAPER
In conclusion, in freshly pre-
pared solutions at pH
< 4,
where A and Ct are not pres-
ent, the overall constant, usual-
ly called K^ , can be written as
a
K^a =Kh+KhKt =0.125. On the
contrary, in solutions at the
final equilibrium, where A is
not
present,
K’a
=
Kh+KhKt+KhKtKi
=
3.98.
From the difference between
the two equilibrium constants
(K’a and K^ ), the mole fraction
a
Figure 2. Absorption spectra of dark equilibrated (a) and immediately prepared (b) aqueous solutions of 4’-hy-
droxy-6-nitroflavylium, 2.0î10ꢀ5 m, as a function of proton concentration.
distribution of Ct at the final
equilibrium, given by the ratio
KhKtKi/K’a =0.97 can be calcu-
lated.[23]
spect to the CB species. This behaviour is due to the very ef-
ficient hydration reaction, see below.
Basic media: We have also studied the behaviour of 4’-hy-
droxy-6-nitroflavylium in basic media. A pH jump from
pH 0.7 to 12 causes the immediate formation of the quinoi-
dal base A (see stopped flowdata below). A is then trans-
formed (several days) into an anionic form of cis-chalcone.
Acid titration of the cis-chalcone form obtained immedi-
ately after a pH jump to 12 leads to the spectral changes re-
ported in Figure 3a and c. Identical titrations, Figure 3b and
d, can be carried out on the trans-chalcone species obtained
either several days after a pH jump to 12 or by irradiation
of the cis-chalcone at the same pH, see photochemical ex-
periments. The titration data showthat for both cis and
trans isomers, two novel species can be obtained on decreas-
ing pH, leading to the conclusion that at pH 12 the cis and
The pH dependence of the absorption spectra was also
examined immediately after the preparation of the solutions
(Figure 2b). The flavylium cation absorption decreases again
with increasing pH, but comparison with Figure 2a shows
that Ct, whose absorption is higher than that of Cc below
16]
400 nm,[14
is not formed; this indicates the existence of a
cis!trans isomerization barrier at room temperature. This
barrier was determined to be 90.4 kJmolꢀ1, on the basis of a
van×t Hoff plot. Immediately after the pH jump, the flavyli-
um cation is involved in a pseudo-equilibrium with Cc and
eventually B2, an interpretation that was confirmed by
1H NMR spectroscopy, see below.
An interesting feature of the present system (see stopped
flowmeasurements below) is that no quinoidal base A is
formed, unless the final pH of the jump is higher than 4. In
this case the formation of B2 through the hydration reaction
of AH+ takes place at a pH value lower than the pKa of the
AH+/A acid base reaction. In other words, the first step
upon a pH jump from very acidic solutions (pH<0) to pH
<4 is no longer the deprotonation, as usually observed for
other flavylium compounds, but the formation of B2 and Cc,
(Scheme 4). The quinoidal base, A, is formed only in less
acidic or basic media and disappears by a newchannel,
which corresponds to the attack of OHꢀ to position 2 (or 4)
leading to B2ꢀ (or B4ꢀ) and Cc, Ccꢀ or Cc2ꢀ depending on
pH, see below.
Figure 3. Absorption spectra of 4.0î10ꢀ5 m solutions of Cc (a) and Ct (b)
*
*
)
at different pH values; absorbance changes at 354 ( ) and 303 nm (
*
*
Scheme 4.
for Cc (c) and at 395 ( ) and 304 nm ( ) for Ct (d).
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Light and pH Stimulation
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trans isomers are present in their dianionic forms. From the
fitting of the plots of Figure 3c and d, the values of the equi-
librium constants for the two consecutive acid dissociation
processes of Cc and Ct can be obtained:
values of the rate constants for hydration/dehydration and
tautomerization equilibria, observed in stopped flowexperi-
ments.
Dissolution of 4’-hydroxy-6-nitroflavylium tetrafluorobo-
rate in basic D2O solutions leads to a single set of peaks
that is assigned to the Cc2ꢀ species, since a jump to strongly
Cc Ð Ccꢀ þ Hþ pKCc1 ¼ 6:2
Ccꢀ Ð Cc2ꢀ þ Hþ pKCc2 ¼ 8:1
Ct Ð Ctꢀ þ Hþ pKCt1 ¼ 5:7
ð6Þ
ð7Þ
ð8Þ
ð9Þ
1
acidic solutions, gives the H NMR spectrum of AH+. The
peaks on the spectrum of Cc2ꢀ were assigned on the basis of
COSY spectra. Irradiation of a freshly prepared Cc2ꢀ solu-
tion at 366 nm, followed by UV/Vis spectroscopy until stabi-
1
lization, leads to a solution that shows a H NMR spectrum
Ctꢀ Ð Ct2ꢀ þ Hþ pKCt2 ¼ 8:0
whose peaks can be assigned to Ct2ꢀ on the basis of COSY
spectra and by comparison with the spectrum of similar ion-
ized chalcones.[16]
1
1H NMR studies: A H NMR study was carried out on the
system in order to characterize and confirm the assignment
of species in solution at different pH values. The data are
summarized in Table 1.
The studies in acid and basic media together with the as-
1
signment through H NMR spectroscopy are summarized in
Table 2 where the main species are characterized by their
absorption maxima and respec-
tive molar absorptivities.
Table 1. 1H NMR chemical shifts d [ppm] and scalar J couplings [Hz] of several forms of 4’-hydroxy-6-nitrofla-
vylium tetrafluoroborate in D2O solutions at T = 301.0ꢁ0.5 K.
Stopped flow experiments: The
stopped flowanalysis of a series
of pH jumps was carried out as
shown in Figure 4. Below pH 4,
the species observable immedi-
ately after the dead time of the
stopped flowapparatus is still
AH+[a]
Cc2ꢀ[b]
Ct2ꢀ[b,c]
Proton
d
J
d
J
d
J
H2’+H6’
H3’+H5’
H3
H4
H5
8.20
6.83
8.24
8.83
8.72
8.50
7.99
9.3
9.3
9.3
9.3
2.4
9.3, 2.4
9.3
7.55[d]
6.26[d]
6.36[d]
6.95[d]
7.7[e]
8.9
8.9
12.5
7.79
6.46
7.7[e]
7.7[e]
8.38
7.86
6.41
8.9
8.9
[e]
[e]
12.5
[e]
2.8
9.3, 2.8
9.3
[e]
[e]
H7
H8
7.7[e]
6.25[e]
the flavylium cation. In
a
second process, which follows a
first-order kinetics, with a rate
[a] pD=ꢀ0.80; [b] pD ~ 12; [c] after irradiatian of Cc2ꢀ at 366 nm; [d] tentative assignment; [e] overlapped
peak.
Table 2. Absorption maxima and molar absorptivities of the main species present in aqueous solutions of 4’-
hydroxy-6-nitroflavylium tetrafluoroborate.
Dissolution of 4’-hydroxy-6-
nitroflavylium tetrafluoroborate
in 20% (w/w) DCl (pD ꢀ0.80)
gave a single set of peaks that
must be due, under this acid
conditions, to the flavylium
cation, AH+. The two doublets
at 8.20 and 6.83 ppm, integrat-
AH+
Cc
Ccꢀ
Cc2ꢀ
Ct
Ctꢀ
Ct2ꢀ
lmax [nm]
450
28900
308
13400
407
13700
361
20700
314
23700
388
20100
397
28700
e [molꢀ1 dm3 cmꢀ1
]
constant of 9 sꢀ1, AH+ disappears to give Cc in equilibrium
with B2. On the contrary, a pH jump to 13.2 immediately
leads to the quinoidal base A (see the absorption band at
500 nm) that evolves to the Cc2ꢀ species according to a first-
order process with a rate constant of 38 sꢀ1.
ing for two protons each, are assigned to protons H2’ +H6’
and H3’ +H5’, respectively, by comparison with similar flavy-
lium compounds.[16,23,24] The doublet at 8.72 ppm is assigned
to proton H8 on the basis of the small meta scalar coupling
4
constant, JH5,H7 =2.4 Hz. This allows the assignment of the
The results obtained for the pH jumps to neutral or mod-
erately acidic media can be interpreted considering that the
process with rate constant 9 sꢀ1 should correspond
(Scheme 2) to the hydration reaction (global rate constant
equal to kh+kꢀh[H+]).[16,25] The pKa 5.5 of Equation (1) was
also determined from the plot of the pH dependent absorb-
ance (500 nm) of the species A immediately formed upon a
series of pH jumps from pH 0.7 to >4.
After pH jumps at basic pH values, the rate constant of A
disappearance depends linearly on the hydroxyl concentra-
tion (Figure 5); this suggests that the quinoidal base A,
formed during the dead time of the stopped flowapparatus,
undergoes a hydroxyl attack, most probably leading to B2ꢀ
and then, depending on pH, to Ccꢀ or Cc2ꢀ through a fast
ring opening. The occurrence of the quinoidal base attack
double doublet peak at 8.50 Hz as proton H7. Irradiation of
this proton allows to assign the doublet at 7.99 ppm to
proton H8. The two remaining doublets at 8.83 and
8.24 ppm must be assigned to ring C protons H4 and H3,
the lower field doublet corresponding to proton H4 by com-
parison with similar flavylium salts.[16,23,24]
On increasing pD in the range ꢀ0.80<pD<2.2, a single
set of peaks is maintained with an overall shift of the spec-
trum to higher field values (further increase of pD to values
in the neutral region leads to strong precipitation of a slight-
ly orange solid that prevents NMR spectra to be obtained).
This shift is compatible with the presence of a fast equilibri-
1
um between AH+ and B2+Cc in the H NMR time scale
(ca. 10ꢀ2 10ꢀ4 sꢀ1), which is in accordance with the high
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F. Pina et al.
FULL PAPER
pH 3.2, Cc is the main species
and the faster process is the
tautomerization
reaction
(global rate=kt+kꢀt) that leads
to B2 (Scheme 2). The second
and slower process can thus be
identified as the dehydration
reaction that forms the flavyli-
um cation AH+ (global rate=
kh+kꢀh[H+]). From the data of
the direct and reverse pH
jumps, it can be obtained kh =
9 sꢀ1 and kꢀh =65 sꢀ1.
Figure 4. Spectral variations observed in stopped flowexperiments upon pH jumps from pH 0.7 to: a) pH 4,
b) pH 13.2; t [s].
Photochemistry: Irradiation of
the 4’-hydroxy-6-nitroflavylium
compound at pH 12 (Cc2ꢀ
form) was carried out at 366 nm
by the hydroxyl group opens a newreaction channel as illus-
trated in Scheme 4 (see also Scheme 2).
In order to get more insight on the system, a freshly pre-
pared solution at pH 0.7 (AH+ form) was submitted to a
pH jump to 3.2, attaining the pseudo equilibrium, and subse-
(see Figure 7). The final photoproduct is Ct2ꢀ, since its acidi-
fication leads to Ct.
Irradiation of the Ct isomer in very acidic (4m HCl)
medium leads spectral changes compatible with the forma-
tion of AH+, with a quantum yield of 0.02. At pH 2.99, a
photostationary state involving the Ct and Cc species is at-
tained with a similar quantum yield (F=0.03). Formation of
AH+ occurs only in very acid media (4m HCl), because of
the extremely lowp K^a of this compound (Figure 8).
Multistate/multifunctional processes: As mentioned in the
Introduction, the 4’-hydroxyflavylium species can perform as
a molecular-level optical memory by a write lock read
unlock erase cycle (Figure 1b) in which i) a photonic bit of
information is ™written∫ in the photochemical conversion
of Ct into Cc, ii) ™locked∫ into AH+ by an acid input,
iii) ™read∫ in the photostable AH+ species by light absorp-
tion, iv) ™unlocked∫ to Cc by a base input, and v) ™erased∫
by a photon that reconverts Cc into Ct.
Compared with other members of the flavylium family,
this compound is characterized by a large cis!trans isomeri-
zation barrier together with a narrow pH domain of stability
of the AH+ species. This compound performs like an optical
memory by means of a different cyclic process that involves
Figure 5. Dependence on the hydroxyl concentration of the first order
rate constant of the process that takes place upon a pH jump from 0.7 to
the basic region.
quently to a pH jump to pH 0. The last step, that leads
again to AH+ formation, was followed by stopped flow
technique, Figure 6. The de-
crease in absorbance (Fig-
ure 6b) at 320 nm shows two
consecutive
faster one
processes;
the
(k>7î102 sꢀ1)
cannot be observed with suffi-
cient accuracy because it is
masked by the dead time of the
apparatus; while the second,
monitored either at 320 nm (de-
crease of Cc absorption) or at
470 nm (increase of AH+ ab-
sorption), follows first-order ki-
netics with rate constant of
65 sꢀ1.
This behaviour can be inter-
Figure 6. pH Jump from a pseudo-equilibrated solution at pH 3.2 to pH 0, followed by stopped flow: a) spec-
&
*
*
preted by considering that, at tral changes î 0.001, & 0.01, 0.02, 0.1, 0.2; b) kinetic plots at 320 and 470 nm.
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Light and pH Stimulation
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Scheme 5.
Figure 7. Irradiation at 313 nm of the cis-chalcone (Cc2ꢀ) of 4’-hydroxy-6-
nitroflavylium 1.25î10ꢀ5 m at pH 12; from t = 0, 2.5, 5, 7.5, 10, 15, 20,
30, 40 min.
(0.26 g, 28.5%). It may be recrystallized from acetic acid. Elemental
analysis calcd (%) for C15H10BF4NO4: C 50.74, H 2.84, N 3.94; C 51.46, H
2.81, N 3.71; MS-FAB(+): m/z (%): 269 [M+H](; 1H NMR: see Table 1.
All other chemicals were of analytical grade. All experiments were car-
ried out in aqueous solutions. The pH was adjusted by addition of HCl
and NaOH, or buffer, and was measured in a Metrohm 713 pH meter.
UV/Vis absorption spectra were recorded in a Perkin Elmer lambda 6 or
Shimadzu UV2501-PC spectrophotometers.
Light excitation was carried out using a medium-pressure mercury arc
lamp, and the excitation bands were isolated with interference filters
(Oriel). The incident light intensity was measured by ferrioxalate actino-
metry.[27] The flash photolysis experiments were performed as previously
described.[28]
The 1H NMR experiments were recorded in a Bruker ARX-400 spec-
trometer operating at 400.13 MHz. The tetrafluoroborate salt of the fla-
vylium was dissolved in D2O, acidified with 20% (w/w) DCl, or basified
with 40% (w/w) NaOD. The reported pD values are direct readings of
the pH meter which can be corrected for the isotope effect through the
equation pD = pH+0.4.[29]
Reaction profiles were collected on a SX 18 MV stopped flow (Applied
Photophysics) spectrophotometer interfaced to a computer for data col-
lection and analysis. The standard flowtube has an observation path
length of 1 cm. The driving ram for the mixing system was operated to
the recommended pressure of 8.5 bar. Under these conditions, the time
required to fill the 1 cm cell was experimentally determined to be
1.35 ms (based on a test reaction).
Figure 8. Spectral changes observed on irradiation at 313 nm of Ct 1.25î
10ꢀ5 m in [HCl]=4m; t = 0, 1, 3, 5, 10, 15, 20, 25, 30, 45, 60, 75, 90, 105,
135, 165, and 195 min.
the following steps: write-lock/read/unlock/enable-erase/
erase (Scheme 5): i) starting with the Ct form at pH 0, light
excitation causes the isomerization to Cc (write) that spon-
taneously converts (auto-lock) to AH+ because of the acidi-
ty of the solution; ii) AH+ is photostable and can thus be
examined by UV/Vis absorption spectroscopy (read) with-
out being erased; iii) when the information has to be erased,
AH+ can be converted into Cc2ꢀ by a pH jump to pH 12
(unlock); iv) Cc2ꢀ can then be isomerized by light excitation
to Ct2ꢀ, a process (enable-erase) that finally allows forma-
tion of the original Ct species by a pH jump to pH 0 (erase).
Acknowledgement
Financial support from FundaÁào para a CiÜncia e Tecnologia POCTI
32442/99 and FEDER, Portugal, MIUR (Supramolecular Devices Proj-
ect), Italy, are gratefully acknowledged. C.L. acknowledges the financial
support provided through the European Community×s Human Potencial
Programme under contract HPRN-CT-2000-00029 [Molecular Level De-
vices and Machines]™.
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4’-Hydroxy-6-nitroflavylium tetrafluoroborate was prepared by condensa-
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