Molecular logics: A mixed bodipy-bipyridine dye behaving as a concealable molecular switch

Article abstract of DOI:10.1039/c0nj00770f

A species based on a bodipy chromophore and containing bipyridine chelating sites behaves as a concealable molecular switch, featuring part of the properties of D-latch circuits by integrating two logic gates, a NOR and an INHIBIT gate, with both gates sh

Full text of DOI:10.1039/c0nj00770f

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PAPER
Molecular logics: a mixed bodipy–bipyridine dye behaving as a
concealable molecular switchw
Fausto Puntoriero,a Francesco Nastasi,a Thomas Bura,b Raymond Ziessel,*b
Sebastiano Campagna*a and Antonino Giannetto*a
Received (in Montpellier, France) 8th October 2010, Accepted 7th February 2011
DOI: 10.1039/c0nj00770f
A species based on a bodipy chromophore and containing bipyridine chelating sites behaves as a
concealable molecular switch, featuring part of the properties of D-latch circuits by integrating
two logic gates, a NOR and an INHIBIT gate, with both gates sharing the same inputs.
basis of complex biological processes.6 However, as recently
Introduction
mentioned by many researchers,1,4d,5,7 probably the strongest
aspect of molecular logic research is to introduce new
viewpoints to look at chemical systems, so contributing to
develop new ideas.
Molecular logics, initially introduced in the late eighties1 and
fostered by de Silva’s suggestion2 that luminescence signals
controlled by cations, protons and other chemical species
could be seen to be analogous to digital responses in electronic
logic gates, is a very active research field.
In this latter regard, molecular logics has another advantage
over traditional, silicon-based electronics, as novel logic
functions can be designed/performed, inspired by similar,
but not identical, existing processes operating in the
electronic realm.
Molecular logics is indeed
a powerful approach to
process information at the molecular level. For example,
molecular properties such as absorption, redox behavior,
and luminescence can be affected by the presence of other
molecules, ions, and/or specific light and/or redox inputs. So,
by using an electronic analogy, the addition of ions, protons,
etc. represents the inputs which are decoded by a specific
molecule (or molecular assembly), giving rise to specific
outputs (most commonly, luminescence at a given wavelength).
The specific molecule behaves as a molecular logic gate,
depending on the truth table generated by the combination
of inputs and outputs.3
Recently, research on molecular logics has allowed us to
design systems capable to perform quite complex and valuable
logic functions, based on the combination of multiple gates.
Examples are the molecular half-adders,8 full-adders,4f keypad
lock,9 multiplexers,4a,10 and encoder–decoder4d,11 systems
recently reported.
Most of the molecular logic gates reported up to now are
based on combinational logic. A quite intriguing perspective in
the molecular logic research field is the design of systems
capable to work by a sequential logic that is a type of logic
circuit whose output depends not only on the present input but
also on the history of the input. This is in contrast to
combinational logic, whose output is a function of, and only
of, the present input. In other words, sequential logic has a
sort of data storage (memory) that combinational logic does
not. An example of circuits operating by a sequential logic is
D-latch circuits.
In gated D-type latches,12 two external inputs are available:
one (the exchanging input or data input, named IN in
Scheme 1) has the function of exchanging the output between
exit-1 and exit-2; it can be thought of as a switch. The second
input (the output enabler, named E in Scheme 1) has the
function of blocking the output as it is at the time the second
input is activated, disabling any successive effect of the first
input IN until the second input E is removed.
In principle, molecular logic gates are by themselves more
powerful than silicon-based analogues, as the same molecule
can behave as different, distinct logic gates, depending on the
inputs and outputs considered.4 Leaving aside speculation
related to the construction of chemical computers, design of
novel molecular logic gates is also attracting a large interest
because these species can be connected to applications in the
fields of sensing and labeling5 and since the behavior of
molecular logic gates could assist to understand the chemical
a Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica
`
Fisica, Universita di Messina, via Sperone 31, 98166 Vill. S. Agata,
Messina, Italy. E-mail: campagna@unime.it
Laboratoire de Chimie Organique et Spectroscopies Avancees
b
´
(LCOSA), Centre National de la Recherche Scientifique (CNRS),
Ecole de Chimie, Polyme`res Mate´riaux (ECPM), Universite´ de
Strasbourg (UdS), 25 rue Becquerel, 67087 Strasbourg Cedex 02,
France. E-mail: ziessel@unistra.fr
w Electronic supplementary information (ESI) available: Absorption
and emission spectral changes of 1 upon Cu2+ complexation; inter-
active figure illustrating the behavior of 1. See DOI: 10.1039/
c0nj00770f
Here we report on a hybrid bodipy–bipyridine dye, 1 (see
Fig. 1), which is able to process two inputs, protons and Cu2+
ions, giving rise to luminescence outputs which resemble in
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Scheme 1 The logic scheme of a possible D-latch system. E represents
the enabler/disabler input, and IN is the data input.
Fig. 2 Aromatic region of the proton NMR of compound 1 in
CD2Cl2 labeled S1 in the spectra. Inset: expanded aliphatic region,
S2 accounts for residual water.
Fig. 1 Molecular formula of 1.
some aspects the behavior of a gated D-latch system, although
some essential features of an effective D-latch system behavior
are not matched (so a molecular-level counterpart of D-latch
circuits still remains elusive). In essence, 1 can behave as a
disguised molecular switch, by integrating the behavior of two
logic gates, a NOR and an INHIBIT gate, which share the
same inputs.
Fig. 3 Emission spectral changes of 1 upon protonation (acetonitrile
solution). Concentration of 1 is 9.4 ꢀ 10ꢁ6 M; in the panel, molar
ratios between added triflic acid and 1 are reported.
Results and discussion
originated from a singlet excited state with partial charge
transfer (CT) character, where the tertiary amine plays the
role of the donor and the bodipy core is the acceptor. Addition
of protons (triflic acid; 1 : 8 molar ratio between 1 and H+)
leads to disappearance of the 695 nm emission, with the
appearance of a structured emission peaking at 570 nm
(Fig. 3), as a consequence of protonation of the tertiary amine
group and destabilization of the CT state, so yielding the
quite intense and higher-energy emission of the bodipy core
(F = 0.63, t = 5 ns).z
Compound 1 comprises an extended fluorescent core made of
a
4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (bodipy) dye,
and a complexation pocket made of a tertiary amine tethered
with two bipyridine subunits. The general route for the
synthesis of 1 has been already reported as a preliminary
communication and full synthetic details are given in the
Experimental part.13 Compound 1 has been unambiguously
characterized by modern NMR techniques. In particular the
splitting of the b-pyrrolic protons into two singlets at 5.97
(assigned to proton f) and at 6.58 ppm (proton e) is diagnostic
for the level of substitution (Fig. 2). This is in agreement with
the observation of three methyl signals at 2.47 (protons g),
1.44 (protons c) and 1.40 ppm (protons d). The presence of
two bipyridine side-arms is confirmed by the integration of
the methylene groups at 4.99 ppm accounting for 4H and the
deshielded doublet at 8.64 ppm assigned to the protons in the
ortho position of the pyridine rings (protons o). The 16.2 Hz
proton–proton coupling constant for the vinyl protons at
7.18 ppm confirms the E-conformation of the double bond.
In acetonitrile solution, 1 exhibits a relatively strong emission
(F = 0.26, t = 2 ns), peaking at 695 nm (see Fig. 3),
Protonation of the tertiary amine can be reversed by addi-
tion of a strong base, so (referring to Scheme 1) the presence of
an acid is the exchanging input capable of switching the
emission output between exit-1 (emission at 695 nm) and
exit-2 (emission at 570 nm). The other input is the presence
of Cu2+ ions: indeed Cu2+ is promptly coordinated by the
bipyridine subunits of 1, quantitatively, as indicated by
z Please note that a large excess of acid is needed to protonate the
tertiary amine group of 1, since the initial acid addition leads to
protonation of the bipyridine units, with only small changes in the
emission spectrum.
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Fig. 5 Emission spectral changes of 1 upon Cu2+ complexation.
Concentration of 1 is 2.84 ꢀ 10ꢁ6 M. Solid line, emission spectra of 1;
dashed line (mostly overlapped with baseline), emission spectra of 1
after adding CuCl2 in the molar ratio 1/1; dotted line, emission spectra
of 1 after adding CuCl2 and (successively) Na2EDTA in the molar
ratio 1/1/3. Excitation is performed at 375 nm, an isosbestic point.
Solvent: acetonitrile.
Fig. 4 Absorption spectral changes of 1 upon CuII complexation.
Concentration of 1 is 8.6 ꢀ 10ꢁ6 M; in the panel, molar ratios between
added CuCl2 and 1 are reported. Solvent: acetonitrile.
absorption spectral changes of 1 in the presence of CuCl2
(see Fig. 4).y The coordination of Cu2+ fully quenches the
emission of 1 at any wavelengths (see Fig. 5): most likely the
formation of the Cu2+ complex introduces a low-lying
metal-centered excited state, to which all the other excited
states of 1 deactivate, and which decays to the ground state
by radiationless processes.z In the presence of Cu2+, any
addition/removal of protons has no effect on the emission
properties, and both exits are deactivated. However, removal
of Cu2+ ions, for example by EDTA, restores the emission
outputs (Fig. 5). The presence of Cu2+ ions plays therefore the
role of output enabler/disabler input.
The truth table of the above described system is shown in
Fig. 6, which also shows the logic scheme of 1 (in this scheme,
the E and IN inputs of Scheme 1 are called IN-1 and IN-2,
respectively) and the corresponding switch circuit schematization,
based on two NOR logic gates, with the four possible
combinations of input digits (00, 01, 10, 11). When input-1
(represented by Cu2+, the enabler/disabler input) is absent,
input-2 (the exchanging or switching input, i.e. acidification)
switches the system output from the output digit 01 to the
output digit 10. In the presence of input-1, no exit is active
(output digit is always 00), both in the presence and absence of
protons.
We wish once more to clearly point out that our system is
not a D-latch system, whose design remains an elusive task at
the molecular level; however, our system is inspired by
D-latches as far as the effect of the enabler/disabler input on
the functioning of the ‘‘exchanging’’ input is concerned: both
in D-type latches and in our system, the exchanging input can
be made inefficient. From an operational viewpoint, the
difference is that in fully-operative D latches the enabler/
Fig.
6 Truth table, combined logic gate diagrams and circuit
y The quantitative complexation is shown by the fact that any
successive addition of CuCl2 does not modify the spectrum corres-
ponding to the 1 : 1 molar ratio shown in Fig. 4. Also, the several
isosbestic points which are maintained during all the coordination
process indicate that a single process (i.e., copper coordination) takes
place.
-
containing subunit via oxidative electron transfer. The mechanism of
the quenching process is not investigated here.
schematizations of 1. All the four possible combinations of inputs
are illustrated. Interactive diagrams, also showing effects on the
emission spectra, are given as ESI.w
disabler input blocks the function of the exchanging input
but leaves one output active, which now becomes independent
of the state of the system as far as the exchanging input is
z The bodipy luminescence could also be quenched by the Cu2+
c
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concerned, whereas in 1 the enabler/disabler input deactivates
both outputs (in this sense, a specific memory function of
D-latches is missed by 1). However the information linked
to the exchanging input of the molecular system is still
maintained, as removing the enabler/disabler input re-activates
the circuit and its specific output, conforming to the situation
determined by input-2. The enabler/disabler input in 1 can be
seen as a ‘‘hide’’ signal, as the output (and therefore, the
information embedded in the system by input-2) is temporarily
hidden, but it is stored and can be made visible again at will
when the enabler/disabler input is cancelled. In other words, 1
behaves as a concealable molecular switch, where one input
(the addition of Cu2+ ions) can disguise the state of the proton
switchable chromophore. In essence, 1 features the integration
of two logic gates: a NOR and an INHIBIT gate, with both
gates sharing the same inputs.
TLC was performed on silica gel plates coated with a
fluorescent indicator.
UV-Vis absorption spectra were recorded with a Jasco
560 spectrophotometer. Steady-state luminescence spectra
were recorded with a Horiba Jobin-Yvon Fluoromax P
spectrofluorimeter equipped with
a Hamamatsu R3896
photomultiplier, and were corrected for photomultiplier
response using a program purchased with the fluorimeter.
Emission lifetimes were measured with an Edinburgh
OB-900 single-photon counting spectrometer equipped with
a Hamamatsu PLP-2 laser diode (pulse width at 408 nm, 59 ps)
and/or with a PicoQuant PDL 800-D pulsed laser diode (pulse
width at 308 nm, 50 ps). The emission decay traces (emission
lifetimes measured at the emission maximum wavelengths)
were analyzed by the Marquadt algorithm. Experimental
uncertainty for absorption spectra maxima is 2 nm, for molar
absorption is 10%, for luminescence emission maxima is 4 nm,
for luminescence lifetime is 10%.
Finally, with reference to the connections between
molecular logics and complex biological systems as far as the
information processing is concerned, we would like to note
that the behavior of 1 could basically recall the way in which
some multifunctional enzymes (ME) work. An ME is an
enzyme which can perform different functions (outputs),
depending on changes of its environment (the ‘‘exchanging’’
input);14 however, when a suitable inhibitor (the ‘‘enabler/
disabler’’ input) is present, all the enzyme functions (or most
of them) can be suppressed.
Preparation and characterization of compound 1. In a round-
bottomed flask equipped with a Dean stark apparatus,
4-(bis((6-(pyridin-2-yl)pyridin-2-yl)methyl)amino)benzaldehyde
(85 mg, 0.185 mmol) and piperidine (2 mL) were added to a
stirred solution of bodipy (5.5 mg, 0.123 mmol) in toluene
(20 mL). The solution was heated at reflux for 12 h. After
cooling to rt, the mixture was washed with water and brine.
The organic phase was filtered over hydroscopic cotton wool
and rotary evaporated. The residue was purified by column
chromatography on silica gel eluting with a gradient of ethyl
acetate/petroleum ether (20/80 to 40/60) to give compound 1
(72 mg, 65%). 1H NMR (CD2Cl2, 300 MHz): d = 1.40 (s, 3H),
1.44 (s, 3H), 2.47 (s, 3H), 4.99 (s, 4H), 5.97 (s, 1H), 6.58 (s,
Conclusions
In short, we showed that 1 can exhibit a behavior which recalls
in part that of a D-type latch circuit, when protons and Cu2+
ions are used as the chemical inputs. The complete behavior of
a D-latch system (including the quite interesting memory
functions typical of D-latch systems) is not matched, however
a behavior allowing to include a hiding function on the output
of a stored information is obtained.15 Our results further
indicate that electronic logics can inspire the design of systems
towards alternative ways of processing the information at the
molecular level.
3
3
1H), 6.85 (d, J = 9 Hz, 2H), 7.07 (d, J = 8.3 Hz, 2H), 7.18
(d, 3J = 16.15 Hz, 1H), 7.27–7.31 (m, 5H), 7.41 (d, 3J =
8.6 Hz, 2H), 7.73–7.84 (m, 6H), 8.29 (d, 3J = 7.9 Hz, 2H), 8.41
(d, 3J = 7.9 Hz, 2H), 8.64 (d, 3J = 4.1 Hz, 2H); 13C{1H}
NMR (CDCl3, 50 MHz): d = 14.2, 15.0, 46.3, 57.6, 78.6, 83.1,
112.9, 115.1, 118.0, 119.7, 120.8, 121.1, 121.4, 122.9, 123.9,
125.8, 128.7, 129.5, 132.8, 136.1, 137.1, 137.6, 137.9, 138.0,
141.0, 142.7, 149.3, 149.7, 153.7, 154.8, 156.2 ppm; ESI-MS:
m/z: 889.2 (100) [M]+; elemental analysis calcd for
This work is supported by MIUR (Prin 2008), the
University of Messina, and the Centre National de la
Recherche Scientifique (CNRS).
C48H39BF2IN7 (Mr
= 889.58): C, 64.81; H, 4.42; N,
11.02%. Found: C, 64.52; H, 3.98; N, 10.84%.
Notes and references
Experimental part
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using standard Schlenk tube techniques. All chemicals were
used as received from commercial sources without further
purification unless otherwise stated. Toluene was distilled
from P2O5 under an argon atmosphere. The 200, 300, 400
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c
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c
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