Fe-terpyridyl complex based multiple switches for application in molecular logic gates and circuits

Article abstract of DOI:10.1039/c4nj00121d

Molecular logic gates and circuits are constructed using the optical and electrochemical addressable-reversible-multiple switching event of the Fe(ii)-4′-pyridyl terpyridyl complex (1) using multiple analytes. The process involves oxidation-reduction of Fe²⁺ as well as successive quaternization-dequaternization of the free pendant pyridyl group monitored optically. The whole switching process could also be visualized by the naked eye as colour changes upon switching are quite apparent and instant. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4nj00121d

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Fe-terpyridyl complex based multiple switches for
application in molecular logic gates and circuits†‡
Prakash Chandra Mondal,*ab Vikram Singh*a and Bhaskaran Shankara
Cite this: New J. Chem., 2014,
38, 2679
Received (in Montpellier, France)
23rd January 2014,
Accepted 24th March 2014
Molecular logic gates and circuits are constructed using the optical and electrochemical addressable-
reversible-multiple switching event of the Fe(II)-40-pyridyl terpyridyl complex (1) using multiple analytes.
The process involves oxidation–reduction of Fe2+ as well as successive quaternization–dequaternization
of the free pendant pyridyl group monitored optically. The whole switching process could also be visua-
lized by the naked eye as colour changes upon switching are quite apparent and instant.
DOI: 10.1039/c4nj00121d
www.rsc.org/njc
interest from the scientific community for the development of
molecular logic gates in solution as well as on surfaces.13–19
1 Introduction
Molecular logic has seen exponential growth in recent times.1–3
Our group has been engaged in exploring the tremendous
A chemical approach towards molecular logic is inherently potential of terpyridyl-transition metal complexes both in
viable, since molecules are the smallest entities which have solution as well as on solid surfaces.20–22 Functionalized terpyridyl
discrete shape and properties.4–6 Molecular manipulation by and its derivatives offer several synthetic and structural advan-
chemists through a bottom-up approach could probably result tages due to its excellent structure–property correlations. For
in more efficient nanoscale devices, unlike the top-down example, (i) it has a strong ‘‘chelate effect’’ towards the transi-
approach, which has its own intrinsic limitations.7,8 Generally, tion metal ions, (ii) a facile synthetic route, (iii) versatile
molecules which could switch between at least two stable coordination modes and (iv) the strong p-electron accepting
states, upon specific external input(s), have found practical nature of the ligand stabilizes the metal ions in lower oxidation
applications in construction of molecular logic gates and states. In the present work, an optically rich and redox-active
circuits. Such intelligent use of inorganic/organic molecules Fe(II)-terpyridylcomplex (1) (Fig. 1) has been utilized for suc-
and their interplay with varying input(s) have resulted in the cessful demonstration of multiple switching events with
development of information technology at the molecular level.9 chemical(s) and electric potential as input and the process is
In this context, chemists have utilized and fine-tuned the redox, monitored optically using an off-the-shelf UV-Vis spectro-
photophysical and photochemical properties of polypyridyl photometer under transmission mode. These triggered, reversible
complexes of transition metal ions such as Fe(II), Ru(II), Ir(III), events prompted us to construct related molecular-integrated
Os(II) and Re(I) in the overall development of materials molecular logic gates and circuits. To the best of our knowledge,
science.10,11 Interestingly, these properties/states could easily this is the first report on reversible multiple state-switching of 1
be set/reset through external triggers/inputs, viz. light, electric both chemically and electrochemically. The optical read-out is fast
field, magnetic field or chemicals, while keeping the molecule and non-destructive, while electrochemical aspects demonstrate
stable in different states and often leading to visible colour redox stability and electro-chromic reversibility of 1. This study
changes or similar easy to recognize properties as outputs.12
This fine relationship between molecular properties in differ-
ent yet stable and identifiable states leads to molecular multi-
ple switches. Such molecule based switches have gained a lot of
a Department of Chemistry, University of Delhi, Delhi-110007, India.
E-mail: mondalpc@gmail.com, vikram.singh19@yahoo.co.in
b Department of Chemical Physics, Weizmann Institute of Science, Rehovot-7610001,
Israel
† Dedicated to Late Dr. Tarkeshwar Gupta.
‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/
Fig. 1 Structure of the Fe-terpyridyl complex (1).
c4nj00121d
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could possibly be an addition to the further development of 0.316 mmol) was dissolved in 10 mL of hot methanol and then
solution based molecular logic gates.
FeCl2 (21 mg; 0.158 mmol) in 10 mL methanol was added drop-
wise. The reaction mixture was refluxed with stirring for 4 h
under a N2 atmosphere. Subsequently, the reaction mixture was
slowly cooled to room temperature and 1 was precipitated out by
addition of an excess of a saturated methanolic solution of
NH4PF6, which was then filtered off. The residue was washed
with an excess amount of water followed by diethyl ether and
recrystallized using a mixture of acetonitrile and acetone (1 : 1, v/v)
resulting in a purple colour microcrystalline solid. 1H NMR
(400 MHz, CD3CN); d/ppm (Fig. S2, ESI‡): 9.20 (s, 4H, Ar H),
9.02 (d, J = 8.0 Hz, 4H), 8.61 (d, J = 11 Hz, 4H), 8.23 (d, J = 10.2 Hz,
4H), 7.90 (t, J = 8.2 Hz, 4H), 7.15 (d, J = 5.8 Hz, 4H), 7.10 (t, J = 7 Hz,
4H). 13C NMR (400 MHz, CD3CN, d): 118.29, 122.58, 122.98,
125.04, 128.36, 139.82, 148.75, 152.08, 154.22, 158.77, 161.84.
IR, (KBr): n = 838 (vs), 1408 (m) 1598 cmꢀ1 (m). UV-Vis (CH3CN):
lmax (e) = 569 nm (23 000); ESI-TOF m/z (%): 338 (90) [M-2PF6]2+,
339 (46) [(M-2PF6) + H+]2+, 821(30) [(M-PF6)]+, anal. calcd. for
C40H28N8FeP2F12: C, 45.47; H, 3.63; N, 10.61. Found: C, 45.16;
H, 3.34; N, 9.96%.
2 Experimental
2.1 Materials
2-Acetyl pyridine, pyridine-4-carboxaldehyde, FeCl2, NH4PF6,
NOBF4, CD3CN, CDCl3 were purchased from Sigma-Aldrich
and used as received. Hydrochloric acid (HCl), triethyl amine
(Et3N) were purchased from s. d. fine chemicals (Mumbai,
India). Tetra-n-butyl ammonium hexafluorophosphate (TBAPF6)
was purchased from Alfa-Aesar and it was further recrystallized
and dried under vacuum before use. All other chemicals were
used as received without any further purification. All solvents
(AR grade) were purchased from Merck (Mumbai, India) and
s. d. fine chemicals (Mumbai, India) and were purified using
literature procedures so as to make them moisture free, and
were further degassed using N2 and kept in a N2 filled glove box
(O2 o 2 ppm).
2.2 Physical measurements
2.4 Preparation of solutions of 1 and other chemical reagents
UV-Vis spectra were recorded at room temperature on a double
beam JASCO (Model V-670) spectrophotometer. A one cm path
length quartz cuvette with a Teflon stopper was used for data
collection. Suitable baselines were recorded for preliminary
adjustments. 1H NMR spectra were recorded on a JEOL 400
NMR (JNMECX 400P) using a suitable deuterated solvent. ESI-
TOF (electrospray-ionization, time-of-flight) mass spectra were
recorded on a microTOF QII from BrukerDaltonik (Germany).
Electrochemical experiments were performed using a CH
Instrument (Model 660D) electrochemical Workstation equipped
with a BASi bulk electrolysis three electrode cell. Cyclic voltammo-
grams (CV) of complex 1 was recorded in dry acetonitrile with
tetra-n-butyl ammonium hexafluorophosphate (TBAPF6) as the
supporting electrolyte using glassy carbon as the working
electrode (WE, B0.2 cm2), Pt wire as the counter electrode
and Ag/AgCl (in 1 M KCl) as the reference electrode (RE). For
bulk electrolysis with coulometry experiments a similar proce-
dure was followed except that porous glassy carbon (coil) was
used as the working electrode (50 mm high, 5 mm thick, 40 mm
diameter; surface area = 10.5 cm2 cmꢀ2). The TBAPF6 was dried
at 100 1C for 30 min before use. The solution was degassed
by bubbling N2 for at least 15 min before the experiment. All
the experiments were performed at room temperature unless
mentioned otherwise.
A 10 mM solution of 1 was prepared in CH3CN (purple coloured
solution) and kept inside a N2 glove box and is pipetted out as
and when required. The complex solution was sufficiently
stable in the presence of air/light within the experimental
time-frame. 100 ppm of a fresh NOBF4 (source of NO+) solution
in dry CH3CN was prepared, as per requirement, for further
dilution to one and two eq. solutions with respect to 1. Conc.
HCl and Et3N were diluted to one and two eq. with respect to 1,
using deionized water.
2.5 Chemical induced optical switching
Optical changes in 1 induced by NO+ and H+. At first, the
UV-Vis spectrum of 2 mL of 10 mM solution of 1 was recorded in
transmission mode in the 200–800 nm range with CH3CN for
baseline correction. (Note: any external chemical added after-
wards was added to both the sample and reference so as to measure
optical changes occurring for 1 only). One eq. of freshly prepared
NO+ solution was then added using a 0 to 200 mL pipette and
the corresponding UV-Vis spectrum was recorded. Then
another equivalent of NO+ was added and the UV-Vis spectrum
was recorded thereafter. Furthermore, in a separate experi-
ment, an excess of NO+ (42 eq.) was added and corresponding
changes in the UV-Vis spectrum were recorded. A similar
procedure was followed for addition of HCl solution. All addi-
tions and recording optical absorption were performed under
standard conditions.
2.3 Methods
Synthesis of 40-pyridyl-2,20:60,200-terpyridine (pytpy) (Scheme S1,
ESI‡). pytpy was synthesized following a published procedure23
resulting in needle shaped white crystals. 1H NMR (400 MHz,
CDCl3): d/ppm (Fig. S1, ESI‡) 8.76 (s, 2H), 8.78 (d, 2H), 8.68 (d, 2H),
7.8 (d, 2H), 7.89 (t, 2H), 8.75 (d, 2H), 7.39 (t, 2H); ESI-MS: m/z 310
[M+]; UV-Vis in CHCl3 (lmax): 316, 278 and 245 nm.
For switching back optical changes in 1. To the 1 + NO+
(excess) solution, 5 mL of deionized water and two equivalents
of Et3N were added in a stepwise manner with recording of the
UV-Vis spectrum at each step. To the 1 + H+ solution two
equivalents of Et3N was added and its UV-Vis spectrum was
recorded.
Synthesis of 1 (Scheme S2, ESI‡). 1 was prepared according to
a reported procedure.16 In brief, 40-pyridyl-terpyridine (98 mg;
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2.6 Spectro-electrochemical switching
is obviously dependent on the ligand environment, while Fe(III)
analogues generally lack this characteristic.21 Moreover, inter-
conversion of the two oxidation states could be performed
using suitable oxidizing/reducing agents. Particularly, addition
of water could easily convert Fe(III) to Fe(II). In the case of 1, the
MLCT band appeared at lmax = 569 nm in dry CH3CN. Addi-
tionally, the free pendant pyridyl groups of 1 could potentially
interact or form adducts with several cationic species, includ-
ing NO+ and H+, due to the presence of non-bonding electrons
on the N-atom.28
At first, the cyclic voltammogram of 10 mM of 1 in dry CH3CN
was recorded with 20 mM TBAPF6 as the supporting electrolyte.
The Fe(II)/Fe(III) couple was observed at DEpa = 1.24 V and DEpc
=
1.17 V with E1/2 = 1.20 V at a scan rate of 0.3 V sꢀ1. Now, with a
similar concentration of 1 bulk electrolysis was performed by
coulometry at an initial potential of 1.4 V, so as to oxidise 1. At an
interval of 500 s, 2 mL of solution was drawn from the electro-
lytic cell and its optical absorption was recorded and the process
was continued till there was no further alteration in the optical
absorption or when the increase in charge saturates with respect
to time. For Fe(III) to Fe(II) conversion, a potential of 1.0 V was
applied and a similar procedure was followed thereafter. The
UV-Vis spectrum was recorded after considering TBAPF6 solution
in CH3CN as baseline for preliminary adjustments.
Chemical inputs induced optical changes. The optical and
structural attributes of state-switching of 1 were obtained by
using carefully chosen chemical input(s) applied at different
stages of the switching experiment, viz. NO+, H+, H2O and Et3N.
1
Addition of one eq. of NO+ to 1 shifted the MLCT band peak
position to 586 nm (+17 nm shift) along with a visible, instant
colour change to light purple/blue (Fig. 2(23+)). Another eq. of
2.7 Molecular computational studies
1
NO+ further shifted the MLCT band to 590 nm (+4 nm shift)
The ground state geometry optimizations of 12+, 23+, 34+ and 45+
were carried out in the gas phase using the B3LYP method and
the LANL2DZ24,25 basis set with an effective core potential was
used for the iron atom. The 6-31G*26 basis set was used for all
other atoms except iron. The initial geometry was obtained
from the standard geometrical parameters. All calculations
were performed using the Gaussian 03 program package.27
and the colour of the solution changed to blue (Fig. 2(34+)).
Moreover, optical absorption of the 1MLCT band increased with
each addition step as signified by 13% and 34% increases in
molar absorptivity (‘e’) for each addition, respectively. Addition
of excess of NO+ (42 eq.) to 1 produced remarkable deviation
from the above behaviour. With each further aliquot (1 eq. each),
the 1MLCT band centred at lmax = 590 nm started to diminish in
intensity, finally resulting in a broad band in the range B600–
800 nm and a concomitant blue to green colour change
(Fig. 2(45+)).
3 Results and discussions
3.1 Optical monitoring of switching events
Fe(II)-terpyridiyl complexes characteristically exhibit a 1MLCT
band in the visible region of the spectrum, whose peak position
Explanation and verification of observed changes. The most
plausible explanation for observed bathochromic and hyper-
chromic shifts in optical absorption spectra of 1 upon addition
Fig. 2 (i) Native colour of 1 in CH3CN and (below) the corresponding UV-Vis spectrum (1MLCT) with lmax = 569 nm; (ii) addition of 1 eq. of NO+ to 1 with
a colour change from purple to blue and (below) ‘a’ represents bathochromic shift of 17 nm to 586 nm and an increase in the molar absorptivity in 1MLCT
of 1; (iii) addition of another eq. of NO+ to 1 changed the colour to light blue and (below) ‘b’ represents changes in the UV-Vis spectrum; (iv) excess of
NO+ results in green colour and (below) ‘c’ represents vanishing of 1MLCT leading to Fe3+ formation. Successive use of H2O and Et3N leads to the original
state of 1. The black line in the UV-Vis spectra represents baseline with dry CH3CN.
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of NO+ could potentially be the sequential quaternization of pyridyl N-atoms as proposed earlier (dicationic species). More-
pendant pyridyl N-atoms. Induced electron deficiency in the over, the dicationic species (adduct) could be considered as a
terpyridyl unit forces it to withdraw electronic charge from the weak Lewis acid and it was presumed that action of a mild
metal centre. This perhaps resulted in lowering of the band gap Lewis base such as Et3N would neutralize as well as dequater-
between metal and ligand centred molecular orbitals and hence nize the pyridyl N-atoms. This was indeed what we observed
bathochromism is observed.21 Hyperchromic effects at fixed after addition of two eq. of Et3N solution to the solution of
1
concentrations are generally related to the increase in ‘e’. dicationic species. The MLCT band reverted to lmax = 569 nm
Formation of different species from 1 could only bring about along with an instant change in colour from blue to purple
changes in ‘e’. The concept of quaternization is based on the (Fig. S4(2), ESI‡). Addition of water to the 1 + H+ solution
fact that the ionization energy of the appended pyridine unit is produced no effect, while addition of 2 eq. of Et3N restored the
slightly greater than that of NO and consequently the charge 1MLCT peak position. To check the effect of direct addition of
transfer process (pyridyl N-atom to NO+ or Fe(II) to NO+) is either H2O or Et3N to 1, control experiments were performed.
inhibited to a large extent and adduct formation prevails.28–30 2 eq. each of H2O and Et3N were added either individually or
The probable absence of quaternization sites for further added together to a solution of 1, but no noticeable change was
aliquots of NO+ makes the charge transfer from the metal observed even after 30 min of reaction.
centre Fe(II) to NO+ the most reasonable conclusion, ably
Thus, 1 could exist in four stable states formed stepwise,
supported by the diminishing 1MLCT band, appearance of a viz. monocationic, dicationic, dicationic-oxidized and 1 itself,
broad band in the range B600–800 nm and visible colour upon careful sequential addition of NO+. Additionally, monopro-
change. This potentially signifies the oxidation of Fe(II) to tonated and bisprotonated states were observed in the case of
Fe(III).31 Moreover, absence of H2O and O2 in CH3CN rules addition of H+. Moreover, facile switching between different states
out the probable formation of acid (HBF4) and protonation of could be achieved by addition of selected chemical reagents as
pyridyl N-atoms upon reaction with HBF4. To further verify that input. This behaviour of 1 prompted its use as a molecular switch,
adduct formation governs the process, dil. HCl was added to a as the different molecular states were inter-convertible as well as
solution of 1 and the UV-Vis spectrum was recorded (Fig. S3, stable within the experimental time-frame. Moreover, the whole
ESI‡). Upon addition of one eq. of dil. HCl, the colour changed exercise was repeated at least B4 times to confirm the switching
1
to light purple and the MLCT band peak position shifted to behaviour using the same solution of 1. Fig. 3 (left panel)
582 nm (+13 nm shift) and ‘e’ was enhanced by 10% (Fig. S3(2), demonstrates the effect of set–reset events on optical absorption
ESI‡). Addition of two equivalents of dil. HCl produced blue at lmax = 590 nm for addition of NO+ to 1. It does not include the
colour and the 1MLCT band peak position shifted to lmax
=
action of Et3N. With each cycle, there was only B5–8% signal loss
590 nm with a net 26% increase in ‘e’ (Fig. S3(3), ESI‡). Further (change in optical absorption) at each step of switching from 1 to
addition of equivalents of dil. HCl did not induce any further the dicationic-oxidized state and back, which could be termed as
spectral or colour changes in 1. These spectral changes could reasonable performance in terms of efficiency. Similar signal
well be ascribed to the protonation of pendant pyridyl N-atoms.32 losses were observed for addition of H+. Moreover, signal devia-
We observed a significant difference in addition of NO+ and H+ to tion from three independent switching experiments with different
1: changes in molar absorptivity, ‘De’, in the case of addition of solutions of 1 (10 mM each) was only B3–6%, (Fig. 3, right panel)
NO+ are greater than that with addition of H+. ‘e’ is the intrinsic which potentially indicates the sufficient accuracy of the multiple
property of a species. If there had been no adduct formation switching events.
between 1 and NO+ (A; see below), the species generated in both
cases would probably have been the same (–C5H4N+) (B; see
below) and De values similar, which is not the case. Thus, it is
tentatively stated that NO+ leads to adduct formation, while H+
promotes charge transfer.
NOþ þ ꢀC5H4N ! NOþꢁ þ ꢀC5H4N þ ꢀC5H4Nþ þ NO
A
B
Reverting to the original optical signal. In continuation,
addition of only 5 mL of deionized H2O to a NO+ saturated
solution of 1 resulted in quick reappearance of the 1MLCT band
at lmax = 590 nm and change of the colour to blue from green
(Fig. S4(1), ESI‡). Further addition of H2O neither resulted in
Fig. 3 Black spheres represent addition of NO+ and grey squares addition
of H+. (left) Optical switching of the same solution of 1 across four cycles.
‘a’ indicates changes in absorbance of the dicationic state for a single
solution in four cycles. Similarly ‘b’ and ‘c’ indicate changes in absorbance
of the oxidized and original state after regeneration respectively. ‘1’ and ‘2’
indicate changes in absorbance of the bisprotonated and original state
after regeneration respectively. Overall, B5–8% signal loss was observed
after four cycles. (right) B3–6% deviation in absorbance of 1 at different
wavelengths under study, after addition of either NO+ or H+ in three
separate experiments. Dotted lines are guide to eyes.
reposition of the 1MLCT band to the original value of lmax
=
569 nm nor produced any colour change or change in ‘e’ value.
This suggested that addition of H2O induced possible
reduction of Fe(III) to Fe(II), which is actually a well-known
phenomenon, but failed to dequaternize the adduct forming
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Fig. 5 (1) Bulk electrolysis with coulometry of 1 in CH3CN. ‘a’ represents
oxidation of 1 at an applied potential of 1.4 V while ‘b’ signifies reduction of
1 from the oxidized state at an applied potential of 1.0 V; (2) corresponding
changes in the UV-Vis spectrum of 1 during electrochemical oxidation and
(3) visual colour changes upon application of potential.
Fig. 4 Molecular orbital pictures (HOMO and LUMO) of 12+, 23+, 34+ and
45+ in the gas phase.
3.2 Properties of molecular orbitals
DFT studies revealed that the highest occupied molecular
orbital (HOMO) of 12+ and 23+ is dominated by the pyridyl
moiety of pytpy (see Fig. S5a–d for optimized structures and
Tables S1 and S2 for selected bond lengths and angles, ESI‡).
On the other hand, in the HOMO of 34+ and of 45+ the electronic
charge density is mainly distributed over metal d orbitals and
the pyridyl unit. It is observed that for all different states of 1,
lowest unoccupied molecular orbitals (LUMO’s), electron den-
sities are predominantly residing on pytpy moiety. For the
corroboration of experimentally observed results, calculation
of HOMO–LUMO energy gaps was performed (12+ for 3.39 eV,
23+ for 0.28 eV, 34+ for 2.08 eV and 45+ for 3.63 eV), which is in
general agreement with the changes observed in the 1MLCT
band of 1 upon addition of NO+ (Fig. 4).
Fig. 6 (left) Changes in absorbance of 1 in CH3CN upon application of
potential across five cycles for a single sample resulting in B5% signal
loss. Top sphere to bottom sphere: Eapplied = 1.4 V; bottom to top sphere:
Eapplied = 1.0 V. (right) Signal deviation of B4–6% (in terms of absorbance)
was observed in three separate spectro-electrochemical switching
events. Fe2+ represents the reduced state (presence of 1MLCT) and Fe3+
the oxidized state (absence of 1MLCT). Dotted lines are guide to eyes.
3.3 Spectro-electrochemical switching events
During bulk electrolysis with coulometry, an applied potential signal deviation from three independent reversibility experi-
of 1.4 V produced a significant visible change in the colour of ments each performed for at least five cycles (Fig. 6, right
the solution which was confirmed by reduction in the optical panel). In addition, cyclic voltammograms of 1 were recorded
absorption at 569 nm. The purple colour of the solution for 300 cycles at a scan rate of 0.3 V sꢀ1 and no degradation of 1
became colourless within B3000 s and changes in the optical was noticed under experimental conditions (Fig. S7, ESI‡).
absorption became stagnant at B6000 s. This observation was These experiments suggest that electrochemical switching
in sync with the saturation of charge with respect to time and is events are sufficiently efficient and accurate along with satis-
presented in Fig. 5. At this point, the applied potential was factory stability of different oxidation states of 1.
changed to 1.0 V and the experiment was re-run. Within B2500 s,
3.4 Construction of logic gates and circuits
the colour changed to purple from colourless and optical absorp-
tion was restored almost to the original level (Fig. S6, ESI‡).
Based on the experimental results, we constructed two and
The colour changes upon applied potential and as verified three input molecular logic gates and circuits using 1 and the
through UV-Vis spectroscopy could be easily attributed to chemical inputs at the molecular level. For designing logic
Fe(II)/Fe(III) conversion based on the reversible and oxidation gates, addition of NO+ or H+ was considered as Input = 1, else
state dependent electrochromic effect. Moreover, in a similar Input = 0. Similarly, the significant optical changes were
experiment performed at least five times with the same observed considered as Output = 1, else Output = 0. In the
solution, an B5% signal loss was observed (Fig. 6, left panel). presence of H+, the MLCT transition of 1 showed a red shift,
Furthermore, it was also observed that there was only B4–6% while addition of Et3N reconfigured the MLCT transition.
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Fig. 7 Representation of the INHIBIT logic gate constructed using two
inputs (H+ and Et3N) applied on (1). Given alongside is the associated truth
table.
To accomplish a logic gate, addition of H+ was considered as
Input1 = 1, while addition of Et3N as Input2 = 1, while the
shifting of the MLCT band to lmax = 590 nm was considered as
Output = 1. When both the chemical inputs (H+ and Et3N) were
added (Input1 and Input2 = 1) simultaneously, Output = 0 was
obtained. Interestingly, when Input1 was applied, Output was
observed but no Output was observed upon exclusive applica-
tion of Input2. This result is presented in Fig. 7 as the INHIBIT
molecular logic gate along with the truth table.
Molecular logic circuits using three chemical inputs and two
outputs have also been designed using 1. The three chemical
inputs used were NO+ (excess), Et3N and H2O as Input1, Input2
and Input3, respectively. The absorbance ("1" for above the
threshold value, "0" for below the threshold value) and the red
shift of the MLCT band to lmax = 590 nm were considered as
Output1 and Output2 respectively (Fig. 8). The truth table for
multiple chemical inputs and multiple outputs has been pre-
sented. The Output1 was obtained for all combinations except
when Input1 = 1, Input2 = 0 and Input3 = 0, while Output2 was
observed ("1") exclusively when Input1 and Input3 were applied
and Input2 was absent The observation afforded the three input
logic circuits as presented in Fig. 9.
In summary, it has been demonstrated that 1 possess
controllable multiple switching characteristic and is able to
generate logic gates and circuits. The operation mechanism has
also been proposed and justified. The complex showed a multi-
ple switching event and NO+ plays a dual role i.e., quaternization
at lower concentration and oxidation at higher concentration.
The molecular switch in response to the external chemical
Fig. 9 Representation of molecular logic circuits constructed using three
(NO+, Et3N, H2O) inputs and resulting in two outputs. Given alongside is
the associated truth table.
inputs is capable of executing multiple binary Boolean logics
at the molecular level. These attractive features make 1 a smart
candidate for the development of hybrid materials33,34 and it
could potentially be used for chemically addressable information
processing devices at the molecular level.35 Notably, the above
complex showed reversible color change upon applying potential
so, it could potentially find application as an electrochromic
material36,37 and in molecular memory devices.38,39
Acknowledgements
PCM and VS thank Council of Scientific and Industrial Research
and University Grants Commission (New Delhi, India), respec-
tively, for Senior Research Fellowship. The authors would like to
thank the reviewers for their fruitful suggestions.
Notes and references
1 G. de Ruiter and M. E. van der Boom, Acc. Chem. Res., 2011,
44, 563.
2 G. de Ruiter and M. E. van der Boom, Angew. Chem., Int. Ed.,
2010, 122, 173.
3 S. J. Langford and T. Yann, J. Am. Chem. Soc., 2003,
125, 11198.
4 U. Pischel, Angew. Chem., Int. Ed., 2007, 46, 4026.
5 P. Ball, Nature, 2000, 406, 118.
6 V. Balzani, A. Credi, F. M. Raymo and J. F. Stoddart, Angew.
Chem., Int. Ed., 2000, 112, 3484.
Fig. 8 Combined UV/Vis spectra of 1 undergoing switching between
different reversible stable states upon external chemical stimulus. (a) MLCT
band of 1 with lmax = 569 nm; (b) 1 eq. of NO+, (c) 2 eq. of NO+, (d) excess
of NO+; (e) 5 mL of DI H2O; (f) addition of Et3N. The black line represents
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