Toward volatile and nonvolatile molecular memories: Fluorescence switching based on fluoride-triggered interconversion of simple porphyrin derivatives

Article abstract of DOI:10.1002/chem.200802469

A study was conducted to demonstrate the development of volatile- and nonvolatile-type memory elements due to fluorescence switching and based on structural modifications of dye molecules. These memory elements were operated by using fluoride anions as writing components. The study used simple porphyrin derivatives 1 and 2 that possessed four and two redox-active 3,5-di-tert-butyl-4-hydroxyphenyl groups. These derivatives exhibited optical properties, including intense fluorescence of their solutions. Porphyrin derivative 1 was prepared by the condensation of pyrrole with 3,5-di-tert-butyl-4- hydroxybenzaldehyde in propionic acid at reflux, while derivative 2 was prepared by a similar method using a mixture of the appropriate benzaldehydes. Memory cycling of the two derivatives was carried out by a 10^-5M solution of 1 in dichloromethane, prepared from a 5×10^-4M stock solution and an addition of tetra-n-butylammonium fluoride to the solution.

Full text of DOI:10.1002/chem.200802469

DOI: 10.1002/chem.200802469
Toward Volatile and Nonvolatile Molecular Memories: Fluorescence
Switching Based on Fluoride-Triggered Interconversion of Simple Porphyrin
Derivatives
Atsuomi Shundo, Jonathan P. Hill,* and Katsuhiko Ariga*[a]
Molecular species are promising candidates for incorpora-
tion into electronic or optical devices because of their syn-
thetic flexibility, processability and small size.[1] During pro-
totyping of suitable molecules, it is likely that new methods
for information storage and processing will be discovered.
Previously, molecular solid-state and solution-based ana-
logues of traditional solid-state electronic processes have
been presented,[2,3] some involving tetrapyrrolic molecules,[4]
and those works have illustrated the scope of using discrete
molecular species in a variety of states. Solution-based
switching presents a novel concept for information process-
ing, and it could be combined with other solution-state tech-
nologies, such as nanofluidics or ink-jet fabrication. Solu-
tion-based technologies should also facilitate interface with
biochemical systems.
erated primarily by using fluoride anions as specific writing
components. The use of fluoride anions is notable because
complementary anion-related molecular devices, including
logic gates,[5] shuttles[6] and allosteric switches,[7] are already
available, so that combination with these systems might be
possible in the pursuit of increasing complexity.
Our study uses the simple porphyrin derivatives 1 and 2
shown in Scheme 1. Derivatives 1 and 2 possess four and
two redox-active 3,5-di-tert-butyl-4-hydroxyphenyl groups,
The initial challenge in preparing an information process-
ing system is to develop molecular memory and logic func-
tions.[2] Data might be stored in binary form, based on
changes in optical or electronic properties, and toggled
using an external stimulus, such as light, temperature, chem-
ical concentration, voltage or a magnetic field. Changes in
optical absorbance and/or fluorescence can be detected as
photonic output(s) and these may contain more information
than simple electronic outputs. Thus, the switching of pho-
tonic output from dye molecules is an ideal prototype for
solution-based memory. In this work, we have developed
both volatile- and nonvolatile-type memory elements due to
fluorescence switching and based on subtle structural modi-
fications of dye molecules. These memory elements are op-
Scheme 1. Structures of porphyrin derivatives 1 and 2 for volatile and
nonvolatile memory systems.
respectively, substituted at their meso positions and each
compound is stable in neutral solution. They exhibit optical
properties typical of porphyrin derivatives, including intense
fluorescence of their solutions.[8] Structural variation at the
meso substituents of these porphyrin derivatives permits
access to quite different memory cycles for 1 and 2, which
could be repeated as shown in Figure 1. Addition of a large
excess of tetra-n-butylammonium fluoride (FÀ), which is re-
quired for fast reaction (see below), to a solution of 2 in
CH2Cl2 (10À5 m; Figure 1B, a) results in a nonfluorescent so-
lution (Figure 1B, b). Subsequent removal of FÀ by washing
with water reinstates its fluorescence (Figure 1B, c). This
volatile behaviour is typical of anion-responsive molecular
switches,[9] since toggling from OFF to ON states is usually
due to hydrogen bonding of anions, thus requiring their con-
stant presence at ON. In the case of 1, emission quenching
by the addition of FÀ is also observed under similar condi-
tions (Figure 1A, a and b), but the non-emissive state is re-
[a] Dr. A. Shundo, Dr. J. P. Hill, Dr. K. Ariga
World Premier International (WPI) Research Center for
Materials Nanoarchitectonics (MANA)
National Institute for Materials Science (NIMS) 1-1 Namiki
Tsukuba 305-0044 (Japan)
Fax : (+81)29-860-4832
E-mail: Jonathan.Hill@nims.go.jp
ariga.katsuhiko@nims.go.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802469.
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Chem. Eur. J. 2009, 15, 2486 – 2490

COMMUNICATION
Figure 2. 1H NMR spectra (in CD2Cl2) of A) 1 , B) 1 in the presence of
FÀ (9.2 equiv), C) 3 obtained by removal of FÀ from the solution contain-
ing 1 and FÀ and D) 1 in the presence of ClÀ (13.2 equiv). The peaks
À
À
marked with * and † represent those due to H2O and CH2 in tetra-n-
butylammonium salts, respectively.
Figure 1. Repeated memory cycles using 1 (A) and 2 (B), in which each
state was detected by its emission intensity (659 nm for 1; 655 nm for 2)
obtained from 10À6 m solutions of 1 and 2 in CH2Cl2 excited at the wave-
length of the Soret band (426 nm for 1; 423 nm for 2). Photographs
showed each 10À5 m solution of 1 and 2 under a UV lamp at 365 nm.
cating that no serious changes to the tetrapyrrole framework
occur. During the reduction of 3 to 1 in CH2Cl2/methanol
(c.a. 30:1 v/v)[13] at room temperature there is a gradual in-
crease in the Soret band intensity (426 nm) with a concur-
rent decrease in intensity of the absorbance bands due to 3
(351 and 518 nm), showing isosbestic points (see the Sup-
porting Information). Regression analysis of the plots of
time dependency of the absorbance at 426 nm provide a cor-
relation coefficient of r>0.999, and the conversion rate and
half-life were calculated to be 5.6ꢁ10À2 minÀ1 and 12.3 min,
respectively. To summarise, the nonvolatile memory system
based on 1 is achieved as follows (Scheme 2): 1) fluoride
anions induce the oxidative conversion from 1 to 3 with sub-
sequent complexation of 3 with FÀ (writing), 2) then, remov-
al of FÀ does not cause any change in the structure of 3, al-
though a dissociation of the complex between 3 and FÀ
Fluorescence emission spectral changes upon addition of FÀ into 10À6
m
solutions of 1 (C) and 2 (D) in CH2Cl2.
tained even after removal of FÀ (Figure 1A, c). In our initial
experiments, the subsequent dispersion of an excess of as-
corbic acid in that solution followed by filtration caused
return to the initial fluorescent state (Figure 1A, d). This
nonvolatile behaviour of 1 cannot be explained by a simple
noncovalent interaction with FÀ.
To investigate the origin of the nonvolatile behaviour of
1
1, H NMR, FTIR, UV/Vis spectroscopies were carried out
at each step. Addition of 9.2 equiv of FÀ to a CD2Cl2 solu-
tion of 1 (~1.4 mm) resulted in new resonances at 7.60, 6.82
and 1.36 ppm with disappearance of the resonances due to 1
in the 1H NMR spectrum (Figure 2B). Monitoring of this
spectral change (238C) revealed a gradual increase in inten-
sity of these new peaks with a concurrent decrease of those
due to 1. Upon removal of FÀ, these peaks shifted upfield
and a new peak appeared at d=8.83 ppm (Figure 2C). The
resulting spectrum is identical to that of 3,[10] obtained ac-
cording to another method,[11] which indicates that porphy-
rin 1 has been oxidised to the oxoporphyrinogen 3. The FÀ-
induced aerobic oxidative conversion from 1 to 3 was con-
firmed by FTIR and UV/Vis spectroscopies (the C=O vibra-
tional absorption is replaced by one OH in the FTIR spec-
trum, the UV/Vis spectrum is identical to that of 3; see the
Supporting Information). 1H NMR spectral changes ob-
served by removal of FÀ are due to dissociation of the 3·2FÀ
complex[12] because complementary spectral changes were
observed by the addition of FÀ to a solution of 3 in CD2Cl2
(see the Supporting Information). Subsequently, a reducing
reagent, such as ascorbic acid or stannous chloride, reduced
3 to 1. Addition of ascorbic acid (10 equiv) to a 10À5 m solu-
tion of 3 in CH2Cl2 restores the UV/Vis spectrum of 1, indi-
Scheme 2. The writing–retention–erasing cycle of 1.
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J. P. Hill, K. Ariga, and A. Shundo
occurs (retention) and 3) 3 is reduced to 1 by using ascorbic
acid (erasing).
dicates deprotonation of phenolic groups in 2 by FÀ: Addi-
tion of FÀ (11 equiv) to a solution of 2 in CD2Cl2 resulted in
upfield shifts of the b-pyrrolic and phenol group meta-
proton peaks, and to the disappearance of the phenolic OH
peak (see the Supporting Information). Cyclic voltammo-
grams of 1 and 2 recorded in o-dichlorobenzene (with 0.1m
Bu4NClO4) contained two irreversible oxidation peaks.[15]
Differential pulse voltammetry revealed that oxidation
peaks of 2 are located at more positive potentials than those
of 1 (DE1/2 =E1/2(2)ÀE1/2(1) =0.04 and 0.12 V for the first and
second oxidations, respectively; see Supporting Informa-
tion), indicating that 2 is relatively electron deficient and
consequently more difficult to oxidise. Therefore, the differ-
ing memory behaviours of 1 and 2 can be explained as fol-
lows: the phenolic meso substituents of both 1 and 2 are de-
protonated by fluoride anions, but only deprotonated 1 can
be transformed to stable porphyrinogen 3 because the de-
protonated form of 1 is susceptible to oxidation by ambient
dioxygen, whereas that of 2 is not.
An initial step towards fabricating a memory device in-
volved using an organogelator, l-glutamide-derived lipid,[16]
as an addressable matrix. Figure 4 shows a photograph of 1-
containing organic gel, in which “F” is written by using a so-
lution of fluoride anions. OFF and ON states can be clearly
detected because of their differing emission intensities. This
illustrates the possibility that combination with an appropri-
ate medium leads to a multi-bit memory device.
It should be noted that oxidation from 1 to 3 does not
occur in the presence of less basic anions. Additions of ClÀ,
À
À
À
BrÀ, IÀ, NO3 , BF4 or PF6 to solutions of 1 in CH2Cl2
result in no significant changes in optical absorbance or
fluorescence (see the Supporting Information). 1H NMR
spectra of a solution containing 1 and excess ClÀ in CD2Cl2
also showed no variation in chemical shifts of peaks as-
signed to 1 (Figure 2D). These results indicate that Lewis
basicity of anions is important for the oxidation of 1.
1H NMR spectroscopic monitoring of the oxidation of 1 by
using different quantities of FÀ clearly indicates that the rate
of conversion from 1 to 3·2FÀ depends on FÀ concentration
and we observed that oxidation only reaches completion fol-
lowing addition of at least four equivalents of FÀ, that is,
only if phenolic groups of 1 are fully deprotonated. Impor-
tantly, the phenolic OH peak disappears rapidly during the
initial stages of reaction (see the Supporting Information),
suggesting that deprotonation of the phenolic groups of 1 by
FÀ triggers its oxidation. Thus, we routinely used quantities
of fluoride anions in excess of four equivalents to ensure
eventual oxidation of 1. Also, oxidation of 1 in the presence
of five equivalents of FÀ was complete within 20 min under
a dioxygen atmosphere, whereas complete oxidation took
more than 3 h under a lower dioxygen concentration (see
the Supporting Information). This strongly suggests that am-
bient dioxygen is responsible for oxidation of deprotonated
1, and this aspect of 1 has been investigated previously.[14] In
fact, oxidation of 1 in the presence of strong bases, such as
hydroxide anions, has been observed. However, fluoride
anions might be more useful in combination with comple-
mentary anion-related molecular devices.
When excess FÀ was added to a solution of 2 in CH2Cl2,
the solution colour changed from pale yellow to pale red
(Figure 3B). This differs from the case for 1 in which pale
yellow turns to deep blue[12] (Figure 3A). The UV/Vis spec-
trum for the solution of 2 in the presence of excess FÀ con-
tains a split Soret band (413 and 503 nm) of decreased inten-
sity straddling the original Soret band position (419 nm),
and a broad absorption band centred around 777 nm, which
is characteristic of porphyrins with deprotonated phenolic
Figure 4. Photograph (under UV lamp at 365 nm) of 1-containing organic
gel, in which “F” was written by the solution of FÀ. The organogelator
and 1 were dissolved in a mixture of C6H6 and CH2Cl2 (4:1) at 408C and
then cooled to room temperature to form a gel in the bottom of a vial
with a diameter of 2 cm. “F” was written by using a microsyringe.
1
groups as meso substituents. H NMR spectroscopy also in-
In conclusion, we have demonstrated fluoride-writable
memory systems using simple porphyrin derivatives based
on the relative oxidisability of these porphyrins in the pres-
ence of fluoride anions. Subtle structural modification at the
porphyrin meso substituents enabled us to achieve both vol-
atile and nonvolatile modes. In both cases, OFF and ON
states exhibited large differences in fluorescence emission
intensity (Figure 1C and D), which has great potential as a
read-out signal for memory systems.
Figure 3. Changes in electronic absorption spectra upon addition of FÀ to
10À5 m solutions of 1 (A) and 2 (B) in CH2Cl2.
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Fluoride-Triggered Interconversion of Simple Porphyrin Derivatives
COMMUNICATION
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Experimental Section
Synthesis of 1 and 2: Porphyrin 1 was prepared by the condensation of
pyrrole with 3,5-di-tert-butyl-4-hydroxybenzaldehyde in propionic acid at
reflux as previously reported.[12] Porphyrin 2 was prepared by a similar
method by using a mixture of the appropriate benzaldehydes. Thus, pyr-
role (6.7 g, 0.1 mol) was added to a mixture of 3,5-di-tert-butyl-4-hydroxy-
benzaldehyde (11.7 g, 0.05 mol) and benzaldehyde (5.3 g, 0.05 mol) in
propionic acid (500 mL) at reflux. Heating at reflux was continued for
three hours. Subsequently, the reaction mixture was reduced to half its
volume by distillation and allowed to cool. Methanol (300 mL) was
added to the mixture and it was allowed to stand until a purple precipi-
tate had formed (~2 days). The precipitate was filtered, washed with
methanol and subjected to gel permeation chromatography (BioBeads
SX-1) eluting with dichloromethane. Since only the least soluble porphyr-
ins (i.e., meso-tetraphenylporphyrin, 5-(3,5-di-tert-butyl-4-hydroxyphen-
yl)-10,15,20-triphenylporphyrin, and 2) had precipitated from the reaction
mixture upon addition of methanol, isolation of 2 was simplified and it
was collected as the first band eluting from the GPC column. Fractions
containing only 2 were combined, the solvent was evaporated and the
solid was recrystallised from dichloromethane/methanol to give 2 as a
purple microcrystalline solid (245 mg, 1.1%). 1H NMR (CD2Cl2,
300 MHz, 208C): d=8.94 (4H, d; pyrrolic b-H), 8.84 (4H, d; pyrrolic b-
H), 8.24 (4H, m; phenyl-H), 8.06 (4H, s; hydroxyphenyl substituent Ar-
H), 7.78 (6H, m; phenyl-H), 5.61 ppm (2H, s; phenol-OH), 1.64 (36H, s;
tert-butyl-H); MALDI-TOF-MS (dithranol): m/z: 871.8 [M+H]+. The
isomeric identity of the compound was confirmed by observing the split-
ting pattern of the pyrrolic protons (see Figure S1 in the Supporting In-
formation).
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Memory cycling: Memory cycling of 1 and 2 was carried out as follows:
1) a 10À5 m solution of 1 in dichloromethane was prepared from a 5ꢁ
10À4 m stock solution. 2) A large excess of tetra-n-butylammonium fluo-
ride (>100 equiv) was added to that solution (Writing). 3) The solution
obtained was washed with water until its colour was constant. The solu-
tion was evaporated then diluted to its initial concentration (Retention).
4) l-Ascorbic acid was dispersed in the solution (20 mgmLÀ1) and stirred
at room temperature for several days. Residual l-ascorbic acid was re-
moved by syringe filtration (pore diameter: 0.2 mm) (Erasing). In the
case of 2 only procedures 1) to 3) were applied. UV/Vis spectra of the
solutions of 1 and 2 obtained after each procedure were measured in a
2.0 mm path length quartz cell. Fluorescence spectra for each solution
after dilution (to c=10À6 m) were also measured in a 10 mm path length
quartz cell. For all measurements excitation wavelengths were 426 nm for
1 and 423 nm for 2. The intensity changes of the emission (659 nm for 1;
655 nm for 2) were monitored during repeated memory cycles.
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Acknowledgements
We thank Professors H. Ihara and M. Takafuji (Kumamoto University)
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(NIMS) for their assistance during electrochemical measurements. This
work was supported by a Japan Society for the Promotion of Science
(JSPS) Fellowship (to A.S.), and was also supported by World Premier
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Keywords: fluoride
oxoporphyrinogen · porphyrinoids · switching
·
molecular
devices
·
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J. P. Hill, K. Ariga, and A. Shundo
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[13] Under these conditions, ascorbic acid is fully dissolved as indicated
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Received: November 26, 2008
Published online: February 2, 2009
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