Modular logic gates: Cascading independent logic gates via metal ion signals

Article abstract of DOI:10.1039/c3dt52375f

Systematic cascading of molecular logic gates is an important issue to be addressed for advancing research in this field. We have demonstrated that photochemically triggered metal ion signals can be utilized towards that goal. Thus, independent logic gates were shown to work together while keeping their identity in more complex logic designs. Communication through the intermediacy of ion signals is clearly inspired from biological processes modulated by such signals, and implemented here with ion responsive molecules.

Full text of DOI:10.1039/c3dt52375f

Volume 43 Number 1 7 January 2014 Pages 1–362
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Akkaya et al.
Modular logic gates: cascading independent logic gates via metal ion
signals

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Modular logic gates: cascading independent logic
gates via metal ion signals†
Cite this: Dalton Trans., 2014, 43, 67
Esra Tanriverdi Ecik,a,b Ahmet Atilgan,a Ruslan Guliyev,c T. Bilal Uyar,a
Aysegul Gumusa,d and Engin U. Akkaya*a,c
Systematic cascading of molecular logic gates is an important issue to be addressed for advancing
research in this field. We have demonstrated that photochemically triggered metal ion signals can be uti-
lized towards that goal. Thus, independent logic gates were shown to work together while keeping their
identity in more complex logic designs. Communication through the intermediacy of ion signals is clearly
inspired from biological processes modulated by such signals, and implemented here with ion responsive
molecules.
Received 28th August 2013,
Accepted 27th September 2013
DOI: 10.1039/c3dt52375f
www.rsc.org/dalton
photonic or chemical signals to be produced as outputs. Such
compounds are typically metal ion complexes, which, on
Introduction
The field of molecular logic gates continues to flourish since irradiation with short wavelength light, undergo a photo-
the original conception by de Silva.1 Basic Boolean operators chemical cleavage reaction, releasing metal ions. The release is
now have a large number of molecular equivalents2 with due to reduced affinity of the cleaved pieces of the ligand for
various kinds of inputs and outputs. In addition to combina- the metal ions in question. Thus, depending on the factors
torial logic, a few examples of sequential logic appeared as such as the fluence rate of the irradiation, quantum yield of
well.3 However, some troubling questions remain, and shadow the photochemical reaction, relative affinities of the ligand
the work done in this field. Many in the field are convinced of and the degradation products for the metal ion, a reproducible
potential niche applications, most likely in the medical (thera- ion signal can be generated. Molecular logic gates, even in
peutic) context.4 Even so, advanced functions require an their earliest conception, were mostly chosen among ion
advanced level of logic gate cascading. In the digital electronic responsive molecules, and their response was typically a
elements, cascading of gates can be easily handled as the change in emission intensity or wavelength.
inputs and outputs are both electrical. In chemical logic gates,
the input/output heterogeneity is a major problem towards
cascading gates. On the other hand, there are many examples
of biological signal cascades,5 with messenger molecules, ion
signals and metabolic pathways. Thus, it makes most sense to
Results and discussion
make use of similar intermediary species to link or cascade
independent molecular logic gates. Photochemical and reversi-
ble H+ generation has already been applied in a cascading
scheme.6
Metal ion signaling seems especially enticing, considering
a multitude of literature examples7 for “caged” metal ions, and
the large variety of possible interactions and a diverse set of
In recent work, we reported cascading of logic gates by a
chemical reaction.8 However, for a broadly applicable cascad-
ing scheme, the use of metal ion signals looks more promis-
ing. Thus, our first cascaded logic gates are comprised of the
caged zinc(II) compound 1 and the dipicolylamine (DPA) sub-
stituted Bodipy dye 2. The ligand used in compound 1 has
minor substitution differences with the previously reported
compound7b (and was synthesized in 6 steps from simpler pre-
cursors essentially following the literature procedure7b). The
Bodipy derivative 2 was also synthesized following established
protocols9 for Bodipy synthesis (ESI†).
The Bodipy derivative 2 is weakly fluorescent in acetonitrile
solutions. The quenching is widely ascribed to the PeT process
from the electron rich meso substituent;10 however in polar sol-
vents, a dark ICT state may play a role as well.11 Ion binding
enhances the emission intensity at 510 nm. On the other
aUNAM-National Nanotechnology Research Center, Bilkent University, 06800 Ankara,
Turkey. E-mail: eua@fen.bilkent.edu.tr; Fax: +90 312-266-4068;
Tel: +90 312-290-2450
bDepartment of Chemistry, Gebze Institute of Technology, Kocaeli, 41400, Turkey
cDepartment of Chemistry, Bilkent University, Ankara, 06800, Turkey
dDepartment of Chemistry, Yuzuncu Yil University, Van, 65080, Turkey
†Electronic supplementary information (ESI) available: Experimental details;
Synthetic routes; Characterization data; Photophysical parameters. See DOI:
10.1039/c3dt52375f
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hand, EDTA is a non-selective chelator for many metal ions in
aqueous and organic solutions.
Using EDTA as an input, we can devise an INH logic gate in
a straightforward way (Fig. 1). Here is how that gate works:
irradiation at 360 (input 1 = 1) results in a Zn(II) signal only
when EDTA is not present (input 2 = 0). That defines an inde-
pendently functioning INH gate. Bodipy dye 2 produces photo-
nic output (emission at 510 nm) only if there are sufficient free
Zn(II) ions in solution and if the compound is excited at
480 nm. When the two modular molecular logic gates
designed this way are placed in the same solution, the two
gates are cascaded, i.e., the output Zn(II) is taken up by the
dipicolyl-Bodipy which in turn generates green emission if it is
also excited separately at 480 nm. Actual implementation is
more successful if instead of 1 : 1 equivalency, more (2 equi-
valents) caged Zn(II) compound is added (Fig. 2). When both 1
and 2 were added at 5.0 μM concentrations, even at full degra-
dation of the cage, the emission due to 2 is low; this is due to
the lower amount of Zn(II) release on degradation. At 2 equi-
valents of the cage compound under the same conditions
almost 70% of the fully complexed 2 emission was obtained.
Thus, in the optimal implementation of cascaded INH–
AND logic modules, the solution initially contains 10.0 μM
cage-compound 1 (INH module) and 5.0 μM compound 2
(AND module). The cascaded gate response is strong, with a
very large increase in the emission intensity at 510 nm (Fig. 3).
Encouraged by the success of the logic gate implemen-
tation, we wanted to demonstrate that higher order cascading
is also possible (Fig. 5). We previously reported a through
space energy transfer for coupled AND logic gates.8 The energy
transfer at the concentrations used in the study becomes possi-
ble only if the two AND logic gate modules are chemically
tethered. This is to say that compound 3 can be viewed as two
Fig. 2 Fluorescence response of compound 2 upon the uncaging of
(a) 1 equivalent and (b) 2 equivalents of cage compound 1 (5.0 µM each)
recorded in acetonitrile. Initially compound 2 exhibits no fluorescence
(quenched due to the active PET process), irradiation of the solution at
360 nm resulted in the complete photolysis of 1 which can be followed
using the enhanced emission spectrum of compound 2. Highest intensity
curves (a-purple, b-orange) represent the maximum emission intensities
of 2 which were obtained by the addition of 1.0 equivalent of zinc(II)
cations in the form of triflate salt. (λex = 480 nm, slit width = 5–2.5.)
Fig. 3 Cascading of the two independent gates, INH and AND logic
gates. (a) The maximum emission intensity of 2 is obtained by the
addition of 1 equivalent of zinc(II) triflate salt; (b) irradiation of the
10.0 μM solutions of compound 1 in acetonitrile solutions with 360 nm
light releases Zn(II) ions, which are chelated by 2 (5.0 μM) to generate
a very strong emission at 510 nm when excited at 480 nm; (c) 1 + 2;
(d) 1 + 2 + EDTA; (e) the presence of EDTA (5.0 μM) reduces the available
free Zn(II) significantly, with an expected result of negligible emission
from the Bodipy dye.
AND logic gates cascaded by chemical reaction. As the primary
AND gate module in compound 3 is a dipicolylamine-deriva-
tive, we wanted to couple this AND–AND cascade which was
shown to function independently previously, with photochemi-
cally released Zn(II) signal. The energy transfer between the
AND–AND module is only possible when Hg(II) ions are added
as well; this causes a blue shift in the absorbance spectrum
increasing the spectral overlap and hence the efficiency of
through space energy transfer (Fig. 4). The absorbance band
of the distyryl-Bodipy compound shows a hypsochromic shift
of 40 nm on binding of the mercuric ions. Strong red emission
from the AND–AND cascade is also contingent upon the
release of Zn(II), which blocks the PeT quenching operational
in the meso-dipicolylaminophenyl substituted Bodipy unit.
Fig. 1 Independent INH and AND logic gates. Irradiation of the 10.0 μM
solutions of compound 1 in acetonitrile solutions releases Zn(II) ions,
which are chelated by 2 to generate emission signals.
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Fig. 6 Spectral response of the cascaded INH–AND–AND logic modules:
(a) emission spectra of 3 (3.0 µM) in acetonitrile in the presence of 1
(+hv@360 nm) and Hg(II) ions (3.0 µM, 18.0 µM, respectively); (b) 3 + [1–Zn(II)] +
Hg(II) + hv@360 nm; (c) 3 + 1 + hv@360 nm; (d) 3 + 1; (e) 3 + [1–Zn(II)] +
hv@360 nm; (f) 3 + [1–Zn(II)]. Also note that [1–Zn(II)] refers to the caging
ligand alone, i.e., without the Zn(II) ions. All solutions were excited at 560 nm.
Fig. 4 Absorbance spectra of compound
3 (3.0 µM) recorded in
acetonitrile in the presence of compound 1 and Hg(II) cations (3.0 µM,
18.0 µM, respectively). Also note that [1-Zn(II)] refers to the caging ligand
alone, i.e., without the Zn(II) ions.
Conclusion
The dissociation constants of the dipicolylamine (2.1 ×
10−8 M, in acetonitrile2b) and for the 1/Zn(II) complexes (2.3 ×
10−13 M in aqueous medium7b) are supportive of the photo-
induced ion migration. The use of selective ligands minimizes
any chances for crosstalk between the gate inputs in solution.
Thus, 3.0 μM solutions of compound 1 and compound 3,
when excited at 360 nm light, set off a sequence of events,
which are in accordance with cascaded INH–AND–AND gates
(Fig. 6).
While we are cognizant of the fact that chemical analogues of
the electronic logic gates do not need to follow the same
developmental stages, it is clear that advanced information
processing at the molecular level will require a set of modular
logic gates, which can talk to each other by exchanging inputs
and outputs. Some homogeneity in the choice of inputs and
outputs will certainly help in establishing the modularity of
the logic gates. Metal ion signals may be a promising choice.
In biological systems, ion signals, together with small
Fig. 5 Cascaded logic modules: the Bodipy dye on the left is weakly emissive due to PeT. On Zn(II) ion complexation, one or more non-radiative
pathways become inoperative. Excited state energy is then transferred to the longer wavelength distyryl-Bodipy. This dye becomes strongly emissive
only if energy transfer is possible and Hg(II) in addition to Zn(II) is made available in the solution. The result is the AND–AND cascade. Photochemical
generation of zinc ions, in turn, incorporates the first gate as well, resulting in the INH–AND–AND cascade.
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molecule fluxes, are to a great extent responsible for initiating
and maintaining many important processes essential to life
itself. Thus, we feel cautiously optimistic that controlled ion
fluxes may indeed play an important role in modular assembly
of molecular information processors designed and imple-
mented for a particular task in mind.
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Acknowledgements
The authors gratefully acknowledge support from TUBITAK in
the form of a postdoctoral scholarship to E. T. Ecik and
R. Guliyev and a doctoral scholarship to T. B. Uyar.
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