Sensors 2012, 12, 4421-4430; doi:10.3390/s120404421
2:1 Multiplexing Function in a Simple Molecular System
Sha Xu 1, Yu-Xin Hao 1, Wei Sun 2, Chen-Jie Fang 1,*, Xing Lu 1, Min-Na Li 1, Ming Zhao 1,*,
Shi-Qi Peng 1,* and Chun-Hua Yan 2,*
1 School of Chemical Biology and Pharmaceutical Sciences, Capital Medical University,
Beijing 100069, China; E-Mails: firstname.lastname@example.org (S.X.); email@example.com (Y.-X.H.);
firstname.lastname@example.org (X.L.); email@example.com (M.-N.L.)
2 Beijing National Laboratory for Molecular Sciences, State Key Lab of Rare Earth Materials
Chemistry and Applications & PKU-HKU Joint Lab in Rare Earth Materials and Bioinorganic
Chemistry, Peking University, Beijing 100871, China; E-Mail: firstname.lastname@example.org
* Authors to whom correspondence should be addressed; E-Mails: email@example.com (C.-J.F.);
firstname.lastname@example.org (M.Z.); email@example.com (S.-Q.P.); firstname.lastname@example.org (C.-H.Y.);
Tel.: +86-10-8391-1523 (C.-J.F.); Fax: +86-10-8391-1533 (C.-J.F.).
Received: 31 December 2011; in revised form: 18 February 2012 / Accepted: 12 March 2012 /
Published: 30 March 2012
Abstract: 1-[(Anthracen-9-yl)methylene] thiosemicarbazide shows weak fluorescence due
to a photo-induced electron transfer (PET) process from the thiosemicarbazide moiety to
the excited anthracene. The anthracene emission can be recovered via protonation of the
amine as the protonated aminomethylene as an electron-withdrawing group that suppresses
the PET process. Similarly, chelation between the ligand and the metal ions can also
suppress the PET process and results in a fluorescence enhancement (CHEF). When
solvents are introduced as the third control, a molecular 2:1 multiplexer is constructed to
report selectively the inputs. Therefore, a molecular 2:1 multiplexer is realized in a simple
Keywords: fluorescence; anthracene; molecular 2:1 multiplexer
There is increasing interesting in exploring single molecule species that could be potentially applied
in the construction of binary logic devices and future computers at the molecular scale [1–8]. Since the
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first molecular AND gate was reported , all common essential logic gates including AND, NOT,
OR, YES, INHIBIT, XOR, NAND and NOR, which are used in conventional silicon circuitry, have
been mimicked at the molecular level with chemical or optical signals [10–21]. With all these logic
gates in hand, the next step is to construct molecular logic networks taking advantage of functional
integration within a single molecule via rational chemical design. This is prior to relying on extensive
physical connection of elementary gates. Recently, molecular-scale arithmetic has also been
reported [21–30]. Those single molecule-based combinatorial circuits are more important because they
are fundamental to a complex information processing system. Tian reported a fluorophore capable of
logic memory . We have systematically explored the combination of logic functions and realized
a safe computing platform with user-identity-directed arithmetic functions to defend information
risk [32,33]. An important function in information technology, e.g. signal multiplexing, has
been realized based on the molecules [34–36]. The construction of molecule-based 1:2 digital
demultiplexer was also reported [37,38]. The signal multiplexing/demultiplexing was demonstrated in
8-methoxyquinoline and enzymes [39,40]. In spite of various logic functions mimicked at molecular
level, however, the combination and integration of advanced functions is still in the infant stage and
reports on this subject are rare [41–45].
In our previous work, up to seven binary logic gates were realized within a single molecule, in
which redox-active tetrathiafulvalene (TTF) was utilized as a switch to control the fluorescence .
Herein, we report a molecular system which is capable of performing multiplexing function in response
to chemical stimuli.
In the present work, the fluorescence of anthracene in the simple molecule 1-[(anthracen-9-yl)
methylene] thiosemicarbazide (L, Figure 1) [47,48] is tuned to realize a molecular 2:1 multiplexer with
anthracene as a signal unit via tuning the PET process. Protonation of amine and chelation with metal
ions can suppress the photo-induced electron transfer (PET) process. Combined with the solvents as
control inputs to switch the fluorescent output, the ligand L can report the binary state of either one of
these inputs or the other.
Figure 1. The molecular structure of the ligand L.
2. Experimental Section
The UV-vis absorption spectra were recorded on a Shimadzu 2500 UV-VIS spectrophotometer. The
fluorescence spectra were recorded on a Shimadzu RF-5301 spectrofluorophotometer using 5 nm input
and 5 nm output width. 1H- and 13C-NMR (TMS) are recorded on a Bruker Avance II 500MHz
spectrometer. The mass spectra were measured on a Waters Quattro micro TM API mass spectrometer.
Elemental analyses were performed on a Vario EI Elementar system.
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3. Results and Discussion
3.1. Synthesis of the Ligand L
When anthracene-10-carbaldehyde was treated with thiosemicarbazide in methanol, the ligand L
was readily formed as an orange microcrystalline solid appearing in the reaction medium, and it was
characterized with elemental analysis (EA), IR, MS, and NMR spectra data. 1H-NMR (DMSO-d6)
δ = 7.58 (m, 2H, anthryl), 7.65 (m, 2H, anthryl), 7.73 (s,1H, NH2), 8.15 (d, 2H, anthryl), 8.32 (s, 1H,
NH2), 8.58 (d, 2H, anthryl), 8.71 (s, 1H,CH=N), 9.34 (s, 1H, anthryl), 11.66 (s, 1H, NH). 13C-NMR
(DMSO- d6) δ = 125.2, 125.5, 126.0, 127.8, 129.4, 129.9, 130.1, 131.4, 142.7, 178.6. IR (KBr):
ν = 3436 (NH2), 3250 (NH), 1601 (C=N), 1285 (C=S) cm–1. ESI-MS, m/z (%): 279 [M+]. EA (%):
C16H13N3S, calcd.: N, 15.05; C, 68.82; H, 4.66; found: N, 15.25; C, 68.61; H, 4.80.
3.2. Photophysical Properties of the Ligand L
The photophysical properties of the ligand L were initially examined in tetrahydrofuran (THF) and
methanol. As shown in Figure 2, characteristic anthracene absorption bands appear in the range of
335–500 nm in UV-Vis spectra .
Figure 2. Absorption spectra of the ligand L in THF and methanol (2.0 × 10−5 M).
The fluorescence spectrum of the ligand in THF solution displays peaks at 412, 437 and 470 nm
(Figure 3). The emission intensity of the ligand L is significantly reduced through a photo-induced
electron transfer (PET) process, with the electron-transfer from the electron-donor thiosemicarbazide
moiety to the excited anthracene. When the amine group is proton-free, it could serve as a PET donor
(ΔGPET = −0.1 eV) [49–51]. If protons are present in sufficient concentration, the protonated
aminomethylene behaves as an electron-withdrawing group which disturbs and suppresses the PET
process from the thiosemicarbazide moiety, thus producing an enhancement of the fluorescent
emission. In addition, chelation enhanced fluorescence (CHEF) is popular for chemosenors base on
PET mechanism to sensing metal ions . Two above points have been useful to design
chemosensors based on the PET mechanism.
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Figure 3. Fluorescence emission spectra of the ligand L with addition of H+ and/or Cu2+ in
THF (2.0 × 10−5 M).
Solvents also impact the fluorescent properties of the ligand. The ligand L exhibits a weak emission
band centered at 470 nm in THF (Figure 3) whereas it exhibits a relatively stronger emission centered
at 481 nm in methanol (Figure 4), with the vibration fine structure missed and the fluorescence
maximum wavelength red-shift. This phenomenon induced by polar solvents has been reported, and it
has been attributed to the formation of exciplexes or specific solute-solvent complexes in the excited
Figure 4. Fluorescence emission spectra of the ligand L with addition of H+ and/or Cu2+ in
methanol (2.0 × 10−5 M).
The emission enhancement is significant under the addition of acid to the THF solution of the
ligand, while the emission is slightly enhanced in the methanol solution. The enhanced fluorescence
upon protonation at amine group is caused by the sufficient protons which disturb and suppress the
PET process and result in a recovered emission [12,32,33]. The similar enhancement due to CHEF
effect is observed upon addition of copper ions to the THF solution. The ESI-MS result of the mixture
of the ligand and copper ions show a m/z peak at 297.1 ([M++H2O]) and 659.2 ([CuL2(H2O)2]). To
understand the chelation ability of the ligand towards Cu2+, the HOMO (high occupied molecular orbit)
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and LUMO (lowest unoccupied molecular orbit) distribution of the L were determined by density
functional theory (DFT) calculations. As shown in Figure 5, the HOMO distribution is mainly located
diffusely over the thiourea group, thus the S and N atoms are electron-rich centers and exhibit high
affinity for the metal ion, which contributes to CHEF effect.
Figure 5. HOMO −1, HOMO, LUMO and LUMO +1 distributions of the ligand at the
B3LYP/6-31G (d,p) level of theory calculated.
HOMO −1 HOMO
LUMO +1 LUMO
In methanol solution, however, the fluorescence is quenched. This might be due to the intrinsic
fluorescence quenching by mechanisms inherent to paramagnetic species Cu2+ in high dielectric and
polar methanol. The communication between paramagnetic Cu2+ and fluorophore is predominant, and
thus the fluorescence is quenched.
3.3. Binary Logic Analysis for Molecular Logic Circuit
Fluorescence is one of the most widely employed signals owing to its high sensitivity, feasibility in
detection, and low cost in operation. Since systems containing fluorophores can be switched between
emissive state and quenched state via manipulation of the PET process, the chemical system described
above could be a simple functional model of logic gate with tuning the fluorescence as output signal.
It is interesting to note that the logic analysis of the present system is complex if the solvent is
considered as the control input. In this context, the logic system described here is a chemical model of
a 2:1 multiplexer. Physically, a multiplexer is a communication device that combines multiple inputs
into an aggregate signal to be transported via a single transmission channel. Simply, it is a data selector
which outputs any one of several possible inputs via a control switch.
In the present case, the solvent as the third input acts like a mechanical rotary switch to control the
logic functions of the molecule, enabling the molecule to output selectively the desirable data from all
the inputs. With the input of the proton and the metal ion Cu2+ while solvent as the control, changes of
the fluorescent emission of the ligand corresponding to two different inputs mimic the digital selection