Multiply reconfigurable 'plug and play' molecular logic via self-assembly

Article abstract of DOI:10.1039/b822181b

A substantial set of ion-driven molecular logic gates are implemented in turn by arranging the association between easily available lumophores and receptors in detergent micelles.

Full text of DOI:10.1039/b822181b

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COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Multiply reconfigurable ‘plug and play’ molecular logic via
self-assemblyw
A. Prasanna de Silva,*a Catherine M. Dobbin,a Thomas P. Vancea
and Boontana Wannalersebc
Received (in Cambridge, UK) 10th December 2008, Accepted 29th January 2009
First published as an Advance Article on the web 11th February 2009
DOI: 10.1039/b822181b
A substantial set of ion-driven molecular logic gates are
implemented in turn by arranging the association between easily
available lumophores and receptors in detergent micelles.
The concept of ‘plug and play’ adds greatly to the convenience
of semiconductor electronic devices, where the addition of new
modules directly enables new functions.1 Now we demonstrate
this concept for molecular logic devices2,3 so that different
logic configurations arise upon the straightforward addition of
new modules under self-assembly conditions.4 PASS 0,5 PASS 1,5
YES, NOT, OR, AND gates are implemented with the minimum
of organic synthesis effort.
Molecular logic continues to show significant conceptual
progress (reconfiguring,6 small-scale integration,7 numeracy,3o,8
gaming9) and applications of wide scope.3h,5,10 It is important to
demonstrate simpler ways of achieving molecular logic so that
more opportunities arise for conceptual development.
Fig. 1 Schematic representation of the micellar solutions containing
It has been clear for some time that self-assembled
‘lumophore–receptor’ systems could be profitable for sensing
and logic.11 Sensing ensembles12 have been some of the best
realizations. Detergent micelles also allow self-assembly of
lumophores and potential receptors.13 Pallavicini’s14 lumines-
cent ‘off–on–off’ system15 is a fine example which exploits
photoinduced electron transfer (PET).16 Since PET-based
luminescent systems are necessarily modular,11 now we can
develop a set of molecular logic gates serially.
various combinations of lumophores and receptors. These receive
inputs of various combinations of H+ and Ca2+. Emission at
625 nm, when excited at 450 nm, is the observed output. Physical
electronic symbols of the logic gates that each of these assemblies
produce are also given. The solutions are undeaerated in order to
maximize the convenience of the experiments.
the lumophore to reside within the detergent micelles (Fig. 1).
Interrogation as before produces a ‘high’ emission signal whether
H+ input is ‘low’ or ‘high’ (Table 1), since there are no basic
centres to engage H+ under these conditions. This emulates
PASS 1 logic. Any fluorescent or luminescent dye would satisfy
this requirement, but the relatively long emission lifetime of 1
(several hydrophobic tris(2,20-bipyridyl)Ru(II) complexes give an
average value of 200 ns)19 enables it to perform the subsequent
logic operations better than shorter-lived counterparts.
Though the physical electronic symbol may not reflect it at first
sight, PASS 0 is perhaps the simplest of Boolean logic operations17
(Fig. 1). To some, it would even appear trivial. Nevertheless, it has
been employed as a tag in molecular computational identification.5
An aqueous solution of the neutral detergent Triton X-100 above
its critical micellar concentration, when interrogated with 450 nm
excitation and 625 nm observation, shows no emission whether the
H
+ input is ‘low’ or ‘high’ (Table 1) since it is devoid of a suitable
chromophore. This truth table corresponds to a PASS 0 logic
device, which is driven by H+ input.
Addition of the hydrophobic tris(2,20-bipyridyl)Ru(II) complex
118 at 6.7 ꢀ 10ꢁ6 M to the Triton X-100 solution enables
a School of Chemistry and Chemical Engineering, Queen’s University,
Belfast, Northern Ireland BT9 5AG. E-mail: a.desilva@qub.ac.uk;
Fax: +44 2890 974890; Tel: +44 2890 974422
b Department of Chemistry, Faculty of Science, Kasetsart University,
Bangkok 10900, Thailand
Further addition of receptor 2 allows most of the micelles
occupied by 1 to co-include 2.20 Receptor 2 is conveniently
prepared from 2-nitrophenyl-n-octyl ether (a common plasti-
cizer for ion-selective electrode membranes)21 by hydrogena-
tion (Pd/C), N,N-dialkylation (BrCH2CO2Me–K2CO3) and
hydrolysis (KOH).z Interrogation as before at pH = 12 gives
c Department of Chemistry, Faculty of Science, Chulalongkorn
University, Bangkok 10330, Thailand
w Dedicated to Professor Seiji Shinkai on the occasion of his 65th
birthday.
ꢂc
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1386 | Chem. Commun., 2009, 1386–1388

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Table 1 Truth tables for a series of molecular logic gates self-
assembled in detergent micellesa
(Table 1) with a Ca2+-induced enhancement factor of 4.5 and
a Ca2+-binding constant (log b) of 1.6.
It is then logical to optically interrogate the aqueous Triton
X-100, 1 and 2 system with both H+ and Ca2+ inputs since the
receptor 2 responds to both. Such non-selective receptors have
led to OR logic in intramolecular cases.24 As Table 1 shows, a
‘high’ luminescence output is seen in all conditions except when
both H+ and Ca2+ inputs are ‘low’, provided that reasonable
input and output thresholding is applied. H+, Ca2+-driven OR
logic is the result. The Ca2+-binding constant of 1.6 and pKa of
5.8 determined above are applicable here.
H+-receptors other than 2 can be profitably examined.
t-Butylphenol (3) in its deprotonated form fits the bill. Phenolates,
as opposed to phenols, serve as electron donors to excited
tris(2,20-bipyridyl)Ru(II) complexes.25 Optical interrogation of
the aqueous solution of Triton X-100, 1 and 3 at pH = 12 gives
a ‘low’ emission output. In contrast, pH = 8 gives a luminescence
enhancement factor of 6.0. Corresponding intramolecular cases
are known.26 The pKa value of 3 under these conditions is 9.9.
The substantially different pKa values of 2 and 3 allows us to
choose a pH range (8–12) so that 2 loses its pH sensitivity and
only responds to Ca2+. We now have a suitably selective pair
of receptors, which, in non-micellar PET systems without self-
assembly, produces AND logic.2,3g,27 An aqueous micellar
solution of 1, 2 and 3 allows a substantial fraction of the
micelles containing 1 to also contain 2 and 3.20 Excitation at
450 nm and observation at 625 nm gives a ‘high’ emission
output only when both H+ and Ca2+ inputs are ‘high’, subject
to reasonable thresholds being used (Table 1), corresponding
to AND logic. A Ca2+-binding constant of 1.5 and pKa of 10.3
are found. These are reasonably close to the values obtained
for the log bCa2+ for 2 and the pKa of 3 separately.
Finally, we revisit frame (b) in Fig. 1. Instead of electrically
neutral Triton X-100, we now turn to anionic dodecylsulfate
micelles (SDS) with Na+ counter-ions. Complex 1 associates
with SDS micelles electrostatically as well as hydrophobically.
Now we evolve frame (b) to frame (f) in Fig. 1 by adding
4,40-bipyridyl (4) to act as the receptor for H+. Though 4
itself may not associate with SDS micelles, diprotonated 4
would associate electrostatically. Intramicellar PET from
tris(2,20-bipyridyl)Ru(II) to N,N-dimethylated 4 in SDS has
been known for a long time.28 A similar PET process from 1
to diprotonated 4 can be assigned as the cause of signifi-
cant luminescence quenching by a factor of 1.8 at pH = 2
(as compared to the emission at pH = 8). The operational pKa
value of 4 is 4.8 (Table 1). Since ‘high’ H+ input produces ‘low’
emission output and vice versa, we have NOT logic here.
In conclusion, all the single-input, single-output Boolean
logic operators and two important cases of the double-input,
single-output versions can be produced by simple addition of
lumophores and receptors into aqueous micellar solutions.
The micellar self-assembly approach is very convenient,
though the intensity ratios of the ‘high’ and ‘low’ output
states deserve improvement in future studies. This approach
complements the use of luminescent, but covalently bound,
switching systems for the examination of micellar environments.29
We thank the Department of Employment and Learning of
Northern Ireland, the Royal Government of Thailand, the Allen
McClay Trust and Prof. Boosayarat Tomapatanaget for support.
System
TX
Input1
Input2
—
Output
Logic
0
0
(low, 0.0)
0
(low, 0.0)
1
(high, 57)
1
(high, 63)
0
(low, 6)
1
(high, 44)
0
PASS 0
(10ꢁ12 M H+
1
)
)
)
TX
—
—
—
—
—
0
PASS 0
PASS 1
PASS 1
YES
(10ꢁ2 M H+
0
)
TX + 1
(10ꢁ12 M H+
1
TX + 1
(10ꢁ2 M H+
0
)
TX + 1 + 2
TX + 1 + 2
TX + 1 + 2b
TX + 1 + 2b
TX + 1 + 2
TX + 1 + 2
TX + 1 + 2
TX + 1 + 2
TX + 1 + 3
TX + 1 + 3
(10ꢁ12 M H+
1
YES
(10ꢁ2 M H+
—
)
YES
(0.0 M Ca2+) (low, 7)
—
1
1
YES
(0.2 M Ca2+) (high, 32)
0
0
0
OR
(10ꢁ8 M H+
)
)
)
)
(0.0 M Ca2+) (low, 9)
0
1
1
OR
(10ꢁ8 M H+
(0.2 M Ca2+) (high, 30)
1
0
1
OR
(10ꢁ4 M H+
(0.0 M Ca2+) (high, 36)
1
1
1
OR
(10ꢁ4 M H+
0
(0.2 M Ca2+) (high, 34)
—
—
0
0
(low, 10)
1
(high, 62)
0
YES
(10ꢁ12 M H+
1
)
YES
(10ꢁ8 M H+
)
TX + 1 + 2 + 3 0
AND
AND
AND
AND
NOT
NOT
(10ꢁ12 M H+) (0.0 M Ca2+) (low, 4)
TX + 1 + 2 + 3 0
(10ꢁ12 M H+) (0.2 M Ca2+) (low, 11)
TX + 1 + 2 + 3 1
(10ꢁ8 M H+
TX + 1 + 2 + 3 1
(10ꢁ8 M H+
1
0
0
0
)
)
)
)
(0.0 M Ca2+) (low, 15)
1
1
(0.2 M Ca2+) (high, 36)
SDS + 1 + 4
0
—
1
(10ꢁ8 M H+
1
( high, 52)
0
(low, 28)
SDS + 1 + 4
—
(10ꢁ2 M H+
a
2 ꢀ 10ꢁ3 M Triton X-100 (TX) or 4 ꢀ 10ꢁ2 M SDS aqueous solution
with a combination of 6.7 ꢀ 10ꢁ6 M 1, 10ꢁ3 M 2, 10ꢁ3 M 3 and 10ꢁ3
M 4 as indicated above. The outputs are luminescence quantum yields
(fLum) given as 103fLum values. The ‘low’ and ‘high’ levels of inputs
are determined by the ion-binding strengths of the various receptors
2–4 in the micellar media. For instance, a ‘high’ level of H+ input
requires a pH value at least 1 unit lower than the operational pKa value
of the receptor component of the logic gate. Cases without receptors
employ the levels used in related systems containing receptors. The
‘low’ and ‘high’ levels of outputs are determined by setting thresholds
so that the predicted truth table is reasonably attained. A 103fLum
value of 20 is chosen as the threshold for the OR gate. A 103fLum value
b
of 25 is chosen as the threshold for the AND gate. pH = 8.
a ‘low’ luminescence output due to intramicellar PET from the
aromatic amine to 1.13 In contrast, the emission output is
enhanced by a factor of 7.0 at pH = 2 since the quenching
effect of the amine is negated by its protonation.22 So the
emission output follows the H+ input14 (Table 1), corres-
ponding to H+-driven YES logic. The analysis of the output–
input profile23 gives the H+-binding constant (pKa) of 2 as 5.8.
The micellar solution containing lumophore 1 and receptor
2 can also be interrogated at ‘low’ (0 M) and ‘high’ (0.2 M)
inputs of Ca2+ at pH = 8. Ca2+-driven YES logic is seen
ꢂc
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Notes and references
z Methyl ester of 2; 1H-NMR (500 MHz, CDCl3): d = 6.75–6.92
(m, ArH, 4H), 4.13 (s, NCH2, 4H), 3.92 (t, OCH2, 2H), 3.68
(s, OCH3, 6H), 1.88 (m, OCH2CH2, 2H), 1.53 (m, OCH2CH2CH2,
2H), 1.39 (m, CH2(C4H8)CH3, 8H), 0.98 (t, O(C7H14)CH3, 3H).
13C-NMR (125 MHz, CDCl3): 172.5, 151.2, 139.3, 122.7, 120.5, 119.3,
112.9, 70.1, 69.0, 55.0, 53.7, 32.2, 30.5, 29.7, 26.5, 23.1, 14.6. MS(EI):
m/z = 365.2228 (M+), 306 (M+ ꢁ CO2CH3). Calculated m/z for
C20H31O5N = 365.2202.
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1388 | Chem. Commun., 2009, 1386–1388