A unimolecular half-subtractor based on effective photoinduced electron transfer and intramolecular charge transfer processes of 1,8-naphthalimide derivative

Article abstract of DOI:10.1071/CH09455

We report a unimolecular system as a combinatorial logic half-subtractor based on a naphthalimide derivative. Chemical inputs (OH^- and H ⁺) can significantly change the absorption and fluorescence characteristics through modulating i

Full text of DOI:10.1071/CH09455

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2010, 63, 169–172
A Unimolecular Half-Subtractor Based on Effective
Photoinduced Electron Transfer and Intramolecular
Charge Transfer Processes of 1,8-Naphthalimide
Derivative
Bing LengA and He TianA,B
AKey Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University
of Science & Technology, Shanghai 200237, China.
BCorresponding author. Email: tianhe@ecust.edu.cn
We report a unimolecular system as a combinatorial logic half-subtractor based on a naphthalimide derivative. Chemi-
cal inputs (OH− and H+) can significantly change the absorption and fluorescence characteristics through modulating
intramolecular charge transfer and photoinduced electron transfer process upon protonation or deprotonation of the
molecule. An XOR gate is obtained following the absorbance variations at 485 nm, and an INHIBIT gate is induced when
the output signal is monitored at 535 nm in the fluorescence spectra. Large differences in output signals allow unequivocal
assignment of logic-0 and logic-1 of both logic gates, which may function in parallel to implement the half-subtractor.
Manuscript received: 31 August 2009.
Manuscript accepted: 21 October 2009.
OHϪ
Introduction
Input 1
Molecular logic gates that can implement the digital action of
Boolean logic are receiving more and more attention.[1] Many
examples of molecular logic gates such as OR, NOT, AND,
XOR, INHIBIT, and NOR, have been reported by employ-
ing a detectable spectroscopic change as output in response to
ionic, electronic, or photonic inputs.[2–7] Clearly, input–output
homogeneity is not available here, unlike bulk semiconductors
that employ all-electronic input and output signals. Fluorescent
molecular switches display emission ‘off–on’type changes, as a
result of the input-induced modulation of the thermodynamics
of the photoinduced electron transfer (PET)[8] process.The clear
intensity variations are in accordance with the digital action of
binary logic, so PET switches are suitable for molecular logic
functions. In addition, besides intensity modification of an exist-
ing signal, significant input-induced spectral shifts often occur,
providing a convenient method to change the logic expression
by monitoring the gate at a different wavelength.[1,9] Thus, more
than one logic gate can be defined based from the same molecule.
Combinations of molecular logic gates can perform arith-
metic operations on binary numbers.The fundamental principles
of molecular arithmetic have been carefully demonstrated in
previous work, where several examples of addition[10] and
subtraction[10b,10f,11] using molecules have been reported. For
example, theimplementationofahalf-subtractorthatcouldcarry
out elementary subtraction, can be achieved through parallel
operation of an XOR gate and an INHIBIT gate, whose out-
put variables produce both the ‘difference’ digit (D) and the
‘borrow’ digit (B), respectively.[12] It is worthwhile to note that
the logic function of the half-adder and half-subtractor can be
implemented using two molecular switches that respond to the
same two inputs. In original work by de Silva and coworkers, to
ICT
O
N
O
ICT
NH
N
C
N
Hϩ
H
PET
n-C4H9
Input 2
NAP
Fig. 1. Acid and base inputs-induced ICT and PET modifications.
demonstrate the proof-of-principle of Boolean algebra, a half-
adder was designed relying on two molecules that executed the
XOR operation and the AND operation, respectively.[13] How-
ever, in the progress of molecular logic gates, it is generally
preferable to develop smarter systems that integrate several logic
functions into a single molecule.[1,14]
Withtheseconsiderationsinmind, wetargetedasinglemolec-
ular entity of naphthalimide-based chromogenic and fluorogenic
compound (NAP, Fig. 1), which could be used to optically
carry out the operation of a half-subtractor in solution using
acid and base as two inputs and absorption and fluorescence
as two outputs. Naphthalimide derivatives are quite useful flu-
orescent dyes and have been used to design molecular switches
and chemosensors.[15] Typically, strong intramolecular charge
transfer (ICT) effect exists within the naphthalimide molecule,
leading to considerable dipole character, which permits the
significant input-induced spectral shifts upon perturbation of
π electron system.[9] Also, we introduced an additional PET-
sensitive aniline group to modulate the fluorescence through
protonation or deprotonation processes. From the molecular
© CSIRO 2010
10.1071/CH09455
0004-9425/10/020169

RESEARCH FRONT
170
B. Leng and H. Tian
(a)
(b)
XOR
0.10
INHIBIT
Input 1
OHϪ
Input 2
Hϩ
Output 2
A (485 nm)
OHϪ
0.08
0.06
0.04
0.02
0.00
0
0
0
1
0 (high)
1 (low)
Hϩ
No ions and
1
1
0
1
1 (low)
Hϩ ϩ OHϪ
0 (high)
(c)
Input 1
Input 2
Output 2
400
450
500
550
600
650
700
XOR
Wavelength [nm]
Fig. 2. (a) Absorption spectra of NAP (4 × 10−6 M) with or without the addition of 500 mM H+ and/or 500 mM OH− in acetonitrile/
water (4/1 v/v); (b) truth table for the XOR logic gate; (c) logic circuit.
(a)
(b)
INHIBIT
Hϩ
600
500
400
300
200
100
0
Input 1
OHϪ
Input 2
Hϩ
Output 1
F (535 nm)
0
0
1
1
0
1
0
1
0 (low)
1 (high)
0 (low)
0 (low)
(c)
No ions and
Hϩ ϩ OHϪ
OHϪ
600
Input 1
Input 2
Output 1
500
550
650
700
INHIBIT
Wavelength [nm]
Fig. 3. (a)Fluorescencespectraof NAP (4 × 10−6 M)withorwithouttheadditionof500 mM H+ and/or500 mM OH− inacetonitrile/water
(4/1 v/v) (λex = 450 nm); (b) truth table for the INHIBIT logic gate; (c) logic circuit.
structure of NAP, the naphthalimide moiety and the aniline
moiety were linked by a CH=N group, providing conjugation
between these two parts, although the conjugation was not ideal
due to lower molecular rigidity.[16] With the same two inputs to
switch the absorption and fluorescence, the XOR and INHIBIT
gateswereconstructedandtheirparalleloperationcouldproduce
a unimolecular half-subtractor.
the electron-donating ability of the substitute in the para position
of the benzene ring.[16] It is found that in the presence of a strong
base (NaOH), deprotonation of amino moiety on the 4-position
of naphthalimide ring could cause a significant increase in the
charge density on the amido nitrogen, with associated enhance-
ment in the push–pull effect of the ICT transition (Fig. 1). As
a consequence, this enhancement in ICT afforded a red-shifted
absorption peak at 587 nm. However, when in the presence of a
strong acid (HCl), the protonation of the aniline nitrogen reduced
its electron-donating character, weakening the ICT effect within
thewholemolecule, resultinginablue-shiftedmaximumabsorp-
tion peak at 455 nm. The spectral changes induced by addition
of acid and base were investigated and shown in Figs 2 and 3.
Since the naphthalimide portion is not at the same molecular
panel with the aniline portion in the NAP molecule, the two parts
would not be the same conjugated system.[16] As a result, the
fluorescence of NAP in an acetonitrile/water solution remained
yellow in colour with a maxima centred around 535 nm. The
colour and position of the emission band were similar to those
of 4-amino-1,8-naphthalimide. It should be notable that aniline
moiety is an efficient electron donor in the PET process, several
Results and Discussion
Changes of Optical Properties of NAP upon Addition
of Acid and Base
The absorption and emission bands of naphthalimide deriva-
tives are largely dependent on the electron donating potential
of 4-substituents.[9] In the case of an amino group with strong
electron-donating ability, the characteristic absorption peaks
generally appear at wavelength range from 400 to 450 nm. How-
ever, a solution of NAP (4 × 10−6 M) in acetonitrile/water (4/1
v/v) showed a red-shifted absorption band centred at 485 nm.
The large bathochromic shift was attributed to the lengthening
of the conjugated system within the molecule and an increase in

RESEARCH FRONT
Unimolecular Half-Subtractor
171
(a)
(b)
XOR
Input 1 Input 2
Output 1
Output 2
Borrow (B) Difference (D)
OHϪ
Hϩ
Difference
A485 nm
OHϪ
Hϩ
F (535 nm)
A (485 nm)
0
0
1
1
0
1
0
1
0
1
0
0
0
1
1
0
INHIBIT
Borrow
F535 nm
Fig. 4. (a) Logic circuit of the half-subtractor, with ‘difference’ and ‘borrow’ outputs collected at 485 nm (absorption)
and 535 nm (fluorescence) respectively; (b) truth table for the operation of the molecular half-subtractor.
molecular switches and chemosensors have been designed to dis-
play fluorescence ‘off–on’ type response upon the inhibition of
PET through metal ions coordination to an aniline nitrogen.[17]
Therefore, the very weak fluorescence of NAP was of course
expected (Φ = 0.024, Rhodamine B in CH3CN was used as flu-
orescence quantum yield standard), considering it an effective
PET molecular switch. However, addition of acid protonated
the aniline nitrogen, effectively retarding PET, as a result, the
fluorescence of NAP was switched on at the same wavelength
(535 nm). The quantum yield was determined to be 0.39 after
acid addition, showing a bright yellow emission. Only a slight
increase and small red shift was observed in the case of base
addition.
0.10
0.08
0.06
0.04
Hϩ
OHϪ
Hϩ
0
OHϪ
1
2
3
4
5
Cycles
Implementation of Unimolecular Half-Subtractor
Fig. 5. Modulated absorbance at 485 nm of NAP (4 × 10−6 M) in
acetonitrile/water (4/1 v/v) during alternating additions of OH− and H+.
Because the absorption wavelength and fluorescence intensity of
NAP could be well tuned by the simple addition of acid or base,
we extended its application to XOR and INHIBIT logic gates
based on the switching optical signals at different wavelengths.
For this reconfigurable logic system, the strong base (OH−) and
acid (H+) wereused as chemical input 1 andinput 2, respectively.
The emission and absorption were followed at wavelengths that
held the sharp spectral changes, in other words, the emission
intensity at 535 nm was defined as output 1 and the absorbance
at 485 nm was used as output 2.
As illustrated in Fig. 2a, absorbance at 485 nm was reduced
when OH− or H+ was applied, whereas in the absence or simul-
taneous addition of both OH− and H+, the absorbance kept a
high value. In binary language, the output 2 is in a logic-1 state
only when input 1 and input 2 are both in the same 0 or 1 state,
while the output 2 is in a logic-0 state when input 1 and input 2 are
in different states. So the absorbance output at 485 nm coincided
with the XNOR logic gate. It is worthwhile to note that the XOR
logic operator can be realized by transformation of XNOR logic.
Due to the fact that the inputs and the outputs are of different
kind, the negative logic can be applied only to outputs that result
in a new XOR logic.[11b,11c] The truth table and logic circuit of
the XOR logic gate are shown in Figs 2b and 2c.
The protonation of the aniline nitrogen upon addition of acid
led to significant increase of fluorescence intensity at 535 nm. In
this case, the input 1 is in the 0 state and input 2 signal is in the 1
state, conferring a 1 state to output 1. All of the other input states
are shown to be weakly fluorescent at 535 nm (Fig. 3). The fluo-
rescence behaviour correlates very well with the INHIBIT logic
operation. Another INHIBIT logic gate can also be constructed
following the absorbance variations at 587 nm. Here, the two
inputs nullify each other, so the addition of one input after
another to a solution of NAP could revert to its original state.
As demonstrated above, the XOR and INHIBIT gates may
function in parallel to implement a half-subtractor. Fig. 4 shows
a schematic diagram and truth table of the single molecule-based
half-subtractor. In binary subtraction, the XOR gate output cor-
responds to the ‘difference’ digit derived from the absorption
change at 485 nm, whereas the INHIBIT gate output corresponds
to the ‘borrow’digit that is derived from the fluorescence change
at 535 nm.
Because of the full reversibility of the protonation and
deprotonation processes, the chemical input-induced modifica-
tion of absorption and fluorescence spectra can be repeated.
By alternating additions of OH− and H+, respectively, we ver-
ified that the photochemical processes are highly reproducible
over more than five cycles. As Fig. 5 demonstrates, the cur-
rent half-subtractor has excellent recovery properties and can be
used repeatedly, indicating that accumulation of salts formed
in the acid-base neutralization did not influence the system
performance over several repeated cycles.
Conclusions
A half-subtractor that could carry out elementary subtraction has
been implemented utilizing a single molecule with very large
differences in absorption wavelengths and emission intensity,
from simple protonation and deprotonation processes. This
molecular device has the ability to reset the output, such that
the half-subtractor can be operated repeatedly. Notably, the real-
ization of binary subtraction operations involving more than one
logic expression through a judicial choice of input and output
signals has provides a means to design complicated logic circuits

RESEARCH FRONT
172
B. Leng and H. Tian
based on one molecule, that could inturn effectively enhance the
power of the molecular processor.
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1H NMR and 13C NMR spectra in DMSO-d6 were recorded on
a Bruker AM-500 spectrometer with tetramethylsilane (TMS)
as the internal standard. Mass spectra (MS) were obtained with
a HITACHI-80 spectrometer. UV-Vis absorption spectra were
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N-Butyl-4-(p-N,N-dimethylaminobenzylimino)
amino-1,8-naphthalimide (NAP)
To a 100 mL round-bottomed flask was added N-butyl-4-
hydrazino-1,8-naphthalimide (1.0 g, 3.5 mmol), p-N,N-
dimethylaminobenzaldehyde (0.54 g, 3.6 mmol), and ethanol
(60 mL). The mixture was refluxed for 2.5 h. After cooling to
room temperature, the precipitated product was filtered and
dried. The obtained crude solid was purified by recrystallization
from ethanol/acetone (2/1) to afford the title compound NAP
as red-brown crystals (1.2 g, 83%), mp: 267–269◦C. δH 8.79
(1H, d, J 8.47, naphthalene-H), 8.47 (1H, d, J 7.26, naphthalene-
H), 8.36 (2H, ss, naphthalene-H, NH), 7.77 (1H, t, J 8.26,
7.47, naphthalene-H), 7.65 (3H, dd, J 8.54, 8.85, naphthalene-H,
benzene-H), 6.79 (2H, d, J 8.9, benzene-H), 4.03 (2H, t, J 7.3,
NCH2), 3.00 (6H, s, N(CH3)2), 1.60 (2H, m, CH2), 1.35 (2H,
m, CH2CH3), 0.93 (3H, t, J 7.3, CH3). δC 163.6, 162.8, 147.9,
146.6, 144.9, 133.5, 130.5, 129.2, 128.4, 128.1, 124.4, 122.1,
121.8, 118.4, 111.3, 109.8, 106.1, 40.3, 29.8, 19.8, 13.7 (one
signal obscured by overlapping). m/z (HR-MS EI); Anal. Calc.
for C25H26N4O2 [M+] 414.2056. Found 414.2057.
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Acknowledgements
This work was supported by NSFC/China, National Basic Research 973
Program (2006CB806200) and Scientific Committee of Shanghai.
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