Molecular logic gates and switches based on 1,3,4-oxadiazoles triggered by metal ions

Article abstract of DOI:10.1002/chem.200903265

Organic molecular devices for information processing applications are highly useful building blocks for constructing molecular-level machines. The development of "intelligent" molecules capable of performing logic operations would enable molecular-level devices and machines to be created. We designed a series of 2,5-diaryl-1,3,4oxadiazoles bearing a 2-(para-substituted) phenyl and a 5-(o-pyridyl) group (substituent X = NMe₂, OEt, Me, H, and Cl; 1a-e) that form a bidentate chelating environment for metal ions. These compounds showed fluorescence response profiles varying in both emission intensity and wavelength toward the tested metal ions Ni²⁺, Cu ²⁺, Zn²⁺, Cd²⁺, Hg²⁺, and Pb ²⁺ and the respons-es were dependent on the substituent X, with those of 1d being the most substantial. The 1,3,4-oxadiazole O or N atom and pyridine N atom were identified as metal-chelating sites. The fluorescence responses of 1d upon metal chelation were employed for developing truth tables for OR, NOR, INHIBIT, and EnNOR logic gates as well as "ON-OFF-ON" and "OFF-ONOFF" fluorescent switches in a single 1,3,4-oxadiazole molecular system.

Full text of DOI:10.1002/chem.200903265

DOI: 10.1002/chem.200903265
Molecular Logic Gates and Switches Based on 1,3,4-Oxadiazoles
Triggered by Metal Ions
Ai-Fang Li, Yi-Bin Ruan, Qian-Qian Jiang, Wen-Bin He, and Yun-Bao Jiang*[a]
Abstract: Organic molecular devices
for information processing applications
are highly useful building blocks for
constructing molecular-level machines.
The development of “intelligent” mole-
cules capable of performing logic oper-
ations would enable molecular-level
devices and machines to be created.
We designed a series of 2,5-diaryl-1,3,4-
oxadiazoles bearing a 2-(para-substitu-
ted)phenyl and a 5-(o-pyridyl) group
(substituent X=NMe2, OEt, Me, H,
and Cl; 1a–e) that form a bidentate
chelating environment for metal ions.
These compounds showed fluorescence
response profiles varying in both emis-
sion intensity and wavelength toward
es were dependent on the substi-
tuent X, with those of 1d being the
most substantial. The 1,3,4-oxadiazole
O or N atom and pyridine N atom
were identified as metal-chelating sites.
The fluorescence responses of 1d upon
metal chelation were employed for de-
veloping truth tables for OR, NOR,
INHIBIT, and EnNOR logic gates as
well as “ON-OFF-ON” and “OFF-ON-
OFF” fluorescent switches in a single
1,3,4-oxadiazole molecular system.
the tested metal ions Ni2+, Cu2+, Zn2+
,
Cd2+, Hg2+, and Pb2+ and the respons-
Keywords: bidentate ligands · che-
lates · fluorescence · metal ions ·
molecular devices
Introduction
vent polarity.[6] The tailored design of molecules exhibiting
multiple logic functions therefore remains to be explored. It
appears that “intelligent” molecules with distinct coordina-
tion behavior could be potential candidates for the construc-
tion of novel logic operations,[7] which would allow the func-
tions to be activated by the same kinds of metal-ion inputs.
We envisaged that a fluorescent ligand with varying metal-
chelating ability and chelating mode, with its emissive state
capable of modulation as well, would allow multiple logic
gates to be created from a single fluorescent ligand by using
individual metal ions or a combination of them as the input,
with fluorescence output in terms of both intensity and
wavelength.
1,3,4-Oxadiazoles are organic molecules known for their
excellent optical properties, and are thus extensively exploit-
ed as signaling components in molecular sensory systems.[8]
We proposed to employ 1,3,4-oxadiazole as the molecular
framework for constructing multiple fluorescent logic gates
and switches. This was based on the conjugated structure of
2,5-diaryl-1,3,4-oxadiazole, such that its electronic distribu-
tion and emissive state can be modulated by varying the
substituent at the 2-phenyl ring. Meanwhile, this structural
framework allows a bidentate metal-chelating environment
to be established when a metal binding site is incorporated
into the 5-aryl ring. As the O and N atoms in the 1,3,4-oxa-
diazole moiety can be respectively combined with this addi-
tional binding site, two options of the bidentate chelation
Molecular systems capable of performing logic operations
are attractive for the construction of molecular-level devices
and machines that are in high demand in information tech-
nology.[1,2] Since the first report of photonic molecular AND
logic gates,[3] logic systems consisting of chemically encoded
information as input and a fluorescence signal as output
have received considerable attention and molecular systems
showing functions such as AND, NAND, OR, NOR, XOR,
XNOR, and INHIBIT have been widely explored.[4,5] How-
ever, relatively few molecular systems capable of performing
multiple logic functions of superior information processing
capability have been reported.[6] In general, the reported
systems were based on the modulation of the emission prop-
erties of the designed molecules by different combinations
of external inputs, such as metal ions, solution pH, and sol-
[a] Dr. A.-F. Li, Y.-B. Ruan, Q.-Q. Jiang, W.-B. He, Prof. Y.-B. Jiang
Department of Chemistry, College of Chemistry
and Chemical Engineering
and the MOE Key Laboratory of Analytical Sciences
Xiamen University, Xiamen 361005 (China)
Fax : (+86)592-2185662
E-mail: ybjiang@xmu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903265.
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FULL PAPER
may be created. We therefore designed a series of 2,5-
diaryl-1,3,4-oxadiazoles bearing a 2-(para-substituted)phenyl
and a 5-(o-pyridyl) group (substituent X=NMe2, OEt, Me,
H, and Cl; 1a–e, respectively; Scheme 1). The o-pyridyl
1,3,4-oxadiazole fluorophore. This provides support to the
molecular design strategy that the electronic distribution
may be tuned by substitution at the 2-phenyl ring and by
metal chelation to the rest of the molecule as well.
Absorption and fluorescence spectra of 1: The absorption
spectra of 1a–e were recorded in solvents of increasing po-
larity, namely, cyclohexane (CHX), diethyl ether (DEE),
ethyl acetate (EtOAc), dichloromethane (CH2Cl2), and ace-
tonitrile (CH3CN). Some of the spectral parameters are
summarized in Table S1 in the Supporting Information. In
general, the absorption of 1a and 1b–e exhibits a major
band at 340 and 287–309 nm, respectively (Figure 2; Fig-
Scheme 1. Molecular structure of 2,5-diaryl-1,3,4-oxaACTHUNRTGNEUNG(thio)diazoles 1–3.
group at the C5 position of the oxadiazole ring was expected
to couple with the 1,3,4-oxadiazole ring to form an (O, N) or
(N, N) bidentate chelation environment for metal ions,
whereas the 2-phenyl group was introduced to test if the
electronic distribution and the emissive state could be tuned
by way of substitution. We found that the emission of 1a–e
was indeed highly sensitive to the substitution, and the re-
sponse profiles in terms of emission intensity and wave-
Figure 2. Absorption spectra of 1a–e (10 mm) in CH3CN.
length toward the tested metal ions Ni2+, Cu2+, Zn2+, Cd2+
,
Hg2+, and Pb2+ varied quite a lot, depending on the substi-
tuent X. On the basis of these observations, we developed a
unique molecular system capable of performing multiple
logic functions (OR, NOR, INHIBIT, and EnNOR) as well
as “ON-OFF-ON” and “OFF-ON-OFF” fluorescent
switches by simply varying the combination and level of
metal-ion inputs in a single fluorescent ligand 1d. To the
best of our knowledge, this is the first molecular system ca-
pable of performing multiple logic functions based on the
1,3,4-oxadiazole fluorophore.
AHCTUNGTREGuNNUN res S1–S4 and Table S1 in the Supporting Information).
The significantly redshifted absorption of 1a compared to
that of 1b–e (Figure 2) is presumably due to the strong elec-
tron-donating ability of the NMe2 substituent. Molar absorp-
tion coefficients at the 104 mꢀ1 cmꢀ1 order of magnitude are
indicative of the p!p* transition character. It was found,
from the data compiled in Table S1 (Supporting Informa-
tion), that the absorption of 1 underwent a redshift with in-
creasing electron-donating ability of the substituent X in the
same solvent, whereas the absorption of 1 was insensitive to
the solvent polarity.
Results and Discussion
The fluorescence of 1a–e experiences a similar redshift
with increasing electron-donating ability of X in the same
solvent (Table S1 in the Supporting Information). However,
the emission of 1a–e was found to be influenced by the sol-
vent polarity to an extent depending on X (Figure 3). The
emission of 1a shows a significant redshift with increasing
solvent polarity from 388 nm in CHX to 525 nm in CH3CN,
which is in contrast to the solvent-polarity-insensitive nature
of the absorption spectrum. This observation reveals that
the emissive state of 1a is of a charge-transfer (CT) type.
Upon the same variation of solvent, the emission of 1b and
1c shifts to the red by 39 and 13 nm, respectively, whereas
that of 1d and 1e hardly changes its position (Figure 3). The
observation that the solvatochromic effect becomes less and
less with decreasing electron-donating ability of X in 1 indi-
cates that the CT in 1, if any, occurs from the X-substituted
Crystal structure of 1b: Figure 1 shows the crystal structure
of 1b, which confirms the planar structure of the 2,5-diaryl-
Figure 1. Crystal structure of 1b.
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Y.-B. Jiang et al.
Table 1. Absorption and fluorescence spectral parameters of 1 and 3 in the
presence of 25 equivalents of metal ions in CH3CN.
Absorption
Fluorescence
lflu [nm] I/I0
[a]
[b]
labs [nm] e [104 mꢀ1 cmꢀ1
]
Ks [105 mꢀ1
]
F[c]
1a
235/284/ 1.27/1.11/3.35
343
–
525
528
526
–
0.477
1a+Ni2+ 293/320/ 1.52/1.74/2.35
>101
>101
0.01 0.005
0.002 0.024
0.08 0.022
0.21 0.074
0.002 0.002
0.21 0.059
382
1a+Cu2+ 321/376/ 1.95/1.34/0.22
630
1a+Zn2+ 292/320/ 1.50/1.79/2.22 1.27ꢁ0.043 529
378
1a+Cd2+ 292/325/ 1.40/1.88/1.99 0.21ꢁ0.005 526
366
1a+Hg2+ 296/325/ 1.62/2.11/1.85
>101
517
392
1a+Pb2+ 292/325/ 1.53/2.13/2.01 0.66ꢁ0.012 526
Figure 3. Normalized fluorescence spectra of 1a–e (spectra a–e, respec-
tively) in CHX (1), DEE (2), EtOAc (3), CH2Cl2 (4), and CH3CN (5).
371
1b
300
2.81
2.23
1.91
2.18
2.19
1.85
2.01
2.62
2.27
2.02
2.27
2.25
1.90
2.05
2.57
2.30
2.08
2.33
2.34
2.02
2.07
2.94
2.60
2.33
2.63
2.64
2.27
2.46
2.51
2.25
2.02
2.26
2.33
2.02
2.23
–
388
390
390
–
0.781
1b+Ni2+ 322
1b+Cu2+ 332
1b+Zn2+ 315
1b+Cd2+ 309
1b+Hg2+ 329
1b+Pb2+ 314
4.33ꢁ0.70
0.02 0.041
0.005 0.007
0.65 0.511
0.94 0.721
0.02 0.022
0.19 0.385
>101
2-phenyl group to the electron-deficient oxadiazole moiety.
Fluorescence quantum yields of 1 (Table S1 in the Support-
ing Information) were sensitive to the electronic character
of X, too. For example, quantum yields in CHX decreased
dramatically from 0.71 (X=OEt) to 0.11 (Me), 0.015 (H),
and 0.047 (Cl).
0.38ꢁ0.007 508
0.06ꢁ0.004 491
>101
557
0.28ꢁ0.006 389
1c
290
–
350
352
350
–
0.255
1c+Ni2+ 308
1c+Cu2+ 314
1c+Zn2+ 301
1c+Cd2+ 298
1c+Hg2+ 312
1c+Pb2+ 301
3.42ꢁ0.41
0.03 0.006
0.01 0.004
2.8
1.6
0.30 0.031
0.26 0.094
–
0.08 0.002
0.05 0.002
32.5
11.5
0.42 0.009
0.56 0.009
–
0.09 0.002
0.04 0.002
10.3
4.0
0.15 0.011
0.39 0.023
–
0.04 0.003
0.01 0.003
4.4
2.3
0.41 0.013
0.77 0.026
>101
All these spectral properties confirm that the fluorescence
of 1 is highly sensitive to the substituent X or the electronic
distribution in the whole molecule, and the emissive state of
1 also undergoes variation with X. It was therefore expected
that metal chelation to the bidentate structure formed by
1,3,4-oxadiazole and the 5-(o-pyridyl) moiety would act as
an alternative way of tuning the electronic distribution and
emission of the fluorescent ligands. In accordance with this
assumption, we observed that the fluorescence quantum
yield of 1d in CH3CN (0.015) was much lower than that
(0.83) of its model compound 2, in which the 5-(o-pyridyl)
moiety in 1d was replaced by a 5-phenyl group (Scheme 1).
This suggests that when a metal ion binds to the pyridine
N atom in 1d, for example, chelation-induced fluorescence
enhancement would be observed. It was hence made clear
that the emission of a sophisticatedly chosen member of 1
could be employed to construct metal-ion-triggered molecu-
lar logic gates or fluorescent switches.
0.28ꢁ0.005 414
0.721
0.386
0.08ꢁ0.008 403
>101
452
0.22ꢁ0.006 350
1d
287
–
334
0.015
1d+Ni2+ 301
1d+Cu2+ 307
1d+Zn2+ 296
1d+Cd2+ 293
1d+Hg2+ 306
1d+Pb2+ 296
1.42ꢁ0.049 338
13.93ꢁ3.12 334
0.22ꢁ0.006 380
0.08ꢁ0.007 377
0.624
0.239
>101
416
0.18ꢁ0.004 334
1e
289
–
337
0.044
1e+Ni2+ 303
1e+Cu2+ 310
1e+Zn2+ 299
1e+Cd2+ 293
1e+Hg2+ 308
1e+Pb2+ 298
1.24ꢁ0.055 341
11.66ꢁ2.75 344
0.17ꢁ0.004 390
0.03ꢁ0.003 387
0.537
0.194
>101
429
0.13ꢁ0.005 350
3
308
321
329
319
311
325
311
–
375
375
375
0.029
3+Ni2+
3+Cu2+
3+Zn2+
3+Cd2+
3+Hg2+
3+Pb2+
4.92ꢁ0.74
>101
0.32ꢁ0.008 403
0.121
0.059
0.035ꢁ0.003 395
Absorption and fluorescence spectra of 1 in the presence of
metal ions: Interaction of 1 with metal ions was probed by
the absorption and fluorescence responses of 1 in CH3CN
>101
424
0.064ꢁ0.005 375
[a] Binding constant of metal ion. [b] Fluorescence enhancement factor, the
ratio of fluorescence intensity in the presence of 25 equivalents of metal ion
(I) to that in the absence of the metal ion (I0). [c] Fluorescence quantum
yields of 1a–e and 3 in the presence of 25 equivalents of metal ion, measured
by referring to quinine sulfate standard.[10]
toward the tested metal ions Ni2+, Cu2+, Zn2+, Cd2+, Hg2+
,
and Pb2+ (Table 1). It was found that 1a–e responded simi-
larly in their absorption spectra toward these metal ions
(Figures S5–S9 in the Supporting Information). The appear-
ance of isosbestic points in the titration traces suggested the
formation of well-defined binding complexes between 1a–e
and the tested metal ions, which thereby allows their binding
constants (Ks) to be fitted by a nonlinear fitting procedure
for a 1:1 binding complex[9] (Table 1). In general, the Ks
values of 1a–e in CH3CN became lower when substituent X
was more electron-withdrawing. The fluorescence response
profiles of 1a–e toward these metal ions, however, were
found to differ very markedly. The CT fluorescence of 1a in
CH3CN was quenched by all the tested metal ions (Fig-
ure S10 in the Supporting Information). In contrast, the
fluorescence of 1c–e was quenched by Ni2+, Cu2+, Hg2+
,
and Pb2+, whereas it was enhanced by Zn2+ and Cd2+
(Figure 4 and Figures S11–S13 in the Supporting Informa-
tion). With the latter two metal ions a redshifted new emis-
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Molecular Logic Gates and Switches
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Figure 4. Fluorescence spectra of 1d (10 mm) in CH3CN in the presence of metal ions of increasing concentration. The concentration of metal ion in-
creased as indicated by the arrows in the same manner from 0 to 250 mm. The excitation wavelengths for acquiring the fluorescence spectra of the tested
metal ions Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, and Pb2+ were the isosbestic wavelengths of 296, 300, 296, 297, 300, and 299 nm, respectively.
sion band appeared together with an isoemissive point. It
was also found that the redshifted band of 1b–e in the pres-
ence of Zn2+ also depended on the substituent X (Fig-
ure S14 in the Supporting Information). The fluorescence re-
sponse factors (IꢀI0)/I0 of 1a–e in the presence of 25 equiva-
lents of a given metal ion are plotted in Figure 5.
The results presented in Figures 4 and 5 reveal rich pro-
files of fluorescence response of 1 toward metal ions, in
both intensity and wavelength. This again supports our mo-
lecular design of 1 and builds up the basis for constructing
metal-ion-triggered logic gates and molecular switches using
1. Our systematic investigation thus leads to the observation
that 1d among 1a–e shows the most pronounced fluores-
cence responses toward metal ions (Figure 5). This fact indi-
cates that a subtle balance of the electronic characters of
the 2- and 5-aryl groups is critical. Compound 1d was there-
fore chosen for examining the logic functions. It should be
emphasized that other metal ions, such as Na+, Mg2+, and
Ag+, were also tested and were found to exert practically
no influence on the absorption and fluorescence of 1d in
CH3CN (Figures S15–S17 in the Supporting Information).
Related experiments were thereafter not continued on these
three metal ions.
Coordination modes of 1 with metal ions: To probe the co-
ordination modes of 1 toward metal ions, the absorption and
fluorescence spectra of the tested molecule 1d and control
molecules 2 and 3 (Scheme 1) in CH3CN in the presence of
the tested metal ions were examined. The absorption and
fluorescence spectra of 2, which bears a 5-phenyl group
rather than the 5-(o-pyridyl) group in 1d, hardly showed
any response toward the metal ions (Figures S18 and S19 in
the Supporting Information), which indicates that the pyri-
dine N atom in 1d coordinates to the metal ions. With the
control compound 3 that bears a 1,3,4-thiodiazole instead of
the 1,3,4-oxadiazole in 1d, the fluorescence response to
Zn2+ in CH3CN was much weaker than that of 1d, whereas
its responses to Cu2+ and Hg2+ were similar to those of 1d
(Figure 6 and Figure S20 in the Supporting Information).
This finding suggests that the 1,3,4-oxadiazole O atom in 1d
dictates the fluorescence enhancement response of 1d
Figure 5. a–e) (IꢀI0)/I0 of 1a–e, respectively, in CH3CN in the presence of
25 equivalents of a given metal ion (250 mm). [1a–e]=10 mm.
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Y.-B. Jiang et al.
Figure 6. Plots of fluorescence enhancement factor (I/I0) in CH3CN
versus concentration ratio of Zn2+ (a), Hg2+ (b), and Cu2+ (c) to 1d and
3. [1d]=[3]=10 mm.
toward Zn2+ but not Cu2+ and Hg2+. The slight difference
in molecular structure of 1d and 3 may also contribute to
the observed difference in their fluorescence responses
toward Zn2+. Calculations at the B3LYP/6-31G* level led to
the optimized ground-state structures of 1d and 3 (Fig-
ure S21 in the Supporting Information). The structure shows
that 1d is planar, whereas the 5-(o-pyridyl) ring in 3 is
slightly twisted from the 2-phenyl-1,3,4-thiodiazole plane.
Presumably this could result from a larger repulsion be-
tween the S atom and pyridine N atom in 3 than that be-
tween the O atom and pyridine N atom in 1d. The X-ray
crystal structure of 1b (Figure 1) agrees well with the calcu-
lated planar structure of 1. These findings indicate that the
O atom in the 1,3,4-oxadiazole group of 1d possibly acts as
Figure 7. 1H NMR spectra of 1d (20 mm) in CD3CN in the presence of in-
creasing equivalents of ZnACHTUNRGTNEUNG(ClO4)2. For proton numbering, see Scheme 2.
A possible 2:1 1d–Zn2+ complex was probed by 1H NMR spectroscopy,
presumably due to the much higher ligand concentration employed here
than in the absorption and fluorescence spectral measurements at the
ACTHNUTRGNEUNG
10 mm level. Job plots for the 1d–Zn2+(Cu2+) complex in CH3CN ob-
tained by monitoring the fluorescence intensity data indicated a 1:1 bind-
ing stoichiometry (Figure S22 in the Supporting Information).
CH1 proton was shielded by the aryl ring current of 1d; at
higher Zn2+ concentrations, this 2:1 complex decomposed
into a 1:1 complex in which no shielding effect existed any
more (Scheme 2a). It was therefore made clear that the
CH1 proton behaved differently from others in the 5-pyridyl
ring.[11] This observation helps to probe the coordination of
Zn2+ with 1d at its pyridine N atom and oxadiazole O atom.
In the case of Hg2+ titration, similar downfield shifts in the
signals of the CH2, CH3, and CH4 protons were observed.
The signal of the CH1 proton, however, behaved oppositely,
in that it shifted first to the downfield and then slightly to
the upfield (Figure S23 in the Supporting Information).
Based on these observations, it was reasonable to conclude
that, although the pyridine N atom in 1d was a binding site
the other binding site with Zn2+
.
1H NMR titrations of 1d by Zn2+ and Hg2+ in CD3CN
were carried out to better understand the coordination of
1d with metal ions. The 5-pyridine proton signals of 1d
were affected by the introduction of Zn2+ in that the signals
of protons CH1, CH2, CH3, and CH4 shifted significantly
(Figure 7). In contrast, all the 2-phenyl protons were only
slightly influenced by Zn2+ coordination. This titration
result clearly confirmed the essential role of the pyridine
N atom in its coordination with
Zn2+
. More importantly, the
signals of protons CH2, CH3,
and CH4 of the 5-pyridyl group
shifted significantly downfield
upon
Zn2+
coordination,
whereas that of the CH1 proton
at the ortho position of the pyr-
idine N atom shifted upfield
upon the addition of 0.5 equiv-
alent of Zn2+ and thereafter
downfield (Figure 7). The shift
profile of the CH1 signal was
assumed to result from the ini-
tial formation of a 2:1 ligand–
metal complex in which the Scheme 2. Proposed coordination modes of 1d with a) Zn2+ and b) Hg2+ and Cu2+
.
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Molecular Logic Gates and Switches
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for both Zn2+ and Hg2+, the other coordination sites of 1d
with Zn2+ and Hg2+ differed from each other. It was thus
proposed that Hg2+ chelated to the pyridine N atom and the
4-N atom in the 1,3,4-oxadiazole moiety of 1d, as shown in
pathway b in Scheme 2. The formation of a 2:1 1d–Hg2+
complex at a lower equivalent of Hg2+ was assigned to be
responsible for the downfield shift of the CH1 proton signal,
presumably due to an intermolecular hydrogen-bonding in-
teraction between this proton and the 3-N atom of the other
oxadiazole moiety. The proposed coordination of 1d with
Cu2+ (Scheme 2b) agrees with that of a previously reported
relevant complex.[12]
rescence intensity of 1d (10 mm) as a function of 5 equiva-
lents of Cu2+ (50 mm) and 5 equivalents of Hg2+ (50 mm) as
inputs is read as a NOR logic response. The truth table pre-
sented in Figure 9 indicates that the fluorescence of 1d ap-
Logic gating and switching behavior of 1d with metal ions
as inputs: Different fluorescence outputs of 1b–e, in particu-
lar 1d, upon the addition of the tested metal ions Ni2+
,
Cu2+, Zn2+, Cd2+, Hg2+, and Pb2+ provide entries for devel-
oping molecular logic gates by using metal ions as inputs.
The fluorescence properties of 1d in the presence and ab-
sence of metal inputs were therefore employed to construct
molecular logic gates with the fluorescence enhancement
factor (I/I0) as the threshold level. The OR gate is one of
the basic logic gates that implement logical disjunction,
which results in a high output (1) if one or both inputs to
the gate are high (1).[13] The fluorescence of 1d (10 mm),
which can be enhanced by Zn2+ or Cd2+ or both, enables
the OR logic function when 1 equivalent of Zn2+ (10 mm)
and 5 equivalents of Cd2+ (50 mm) are taken as inputs
(Figure 8).
Figure 9. Fluorescence spectra of 1d (10 mm) in CH3CN in the presence
of chemical inputs. [Cu2+ ]=[Hg2+ ]=50 mm. The excitation wavelength
was 300 nm. The inset shows the logic table and the respective symbolic
representation of the NOR function.
pears at 334 nm (output “1”) when neither Cu2+ nor Hg2+ is
added, whereas it is quenched by applying one or both
inputs, which affords output “0”.
The INHIBIT gate can be interpreted as a particular inte-
gration of AND and NOT logic gates such that the output
signal is inhibited by one of the active inputs.[15] A basic
two-input INHIBIT action can be obtained for 1d (10 mm)
with Zn2+ (10 mm) and Cu2+ (50 mm) as inputs. Enhancement
of 1d fluorescence is observed only in the presence of
1 equivalent of Zn2+ and the absence of Cu2+, so that the
output is read as “1”. Under other circumstances the fluo-
rescence of 1d is quenched, thus leading to output “0”. The
corresponding truth table for the INHIBIT function is illus-
trated in Figure 10. It was therefore made clear that the IN-
Figure 8. Fluorescence spectra of 1d (10 mm) in CH3CN in the presence
of chemical inputs. [Zn2+ ]=10 mm, [Cd2+ ]=50 mm. The excitation wave-
length was 300 nm. The inset shows the logic table and the respective
symbolic representation of the OR function. I/I0 is the ratio of the fluo-
rescence spectrum area (310–520 nm) in the presence of metal ion to that
in the absence of the metal ion.
The NOR gate, as a universal gate that allows the combi-
natorial creation of all other Boolean operations, is of po-
tential interest.[14] A NOR gate, which integrates NOT and
OR logic gates, is performed only when neither of two
Figure 10. Fluorescence spectra of 1d (10 mm) in CH3CN in the presence
of chemical inputs. [Zn2+ ]=10 mm, [Cu2+ ]=50 mm. The excitation wave-
length was 300 nm. The inset shows the logic table and the respective
symbolic representation of the INHIBIT function.
inputs is present. The combination of metal ions Ni2+, Cu2+
,
Hg2+, and Pb2+ as inputs leads to fluorescence quenching of
1d, and expresses the NOR logic function. The level of fluo-
Chem. Eur. J. 2010, 16, 5794 – 5802
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5799

Y.-B. Jiang et al.
HIBIT function could be expanded to an enabled NOR
(EnNOR) logic gate by combining a NOR operator.[6j,16]
Here, Zn2+ is a controlling input, which means that whenev-
er it is “1” the NOR gate works whereas this gate closes if it
is “0”. The corresponding logic circuit is depicted in Fig-
ure 11b.
Figure 12. Outputs (I/I0) of 1d (10 mm) in CH3CN upon alternate addition
of Zn2+ and Cu2+: a) 5.0 mm Zn2+, 25.0 mm Cu2+, 50.0 mm Zn2+, 150.0 mm
Cu2+, 300.0 mm Zn2+, 900.0 mm Cu2+ and b) 10.0 mm Cu2+, 10.0 mm Zn2+
,
30.0 mm Cu2+, 60.0 mm Zn2+, 180.0 mm Cu2+, 360.0 mm Zn2+. The excita-
tion wavelength was 300 nm.
logic gates and switches. By modifying the substituent X at
the 2-phenyl group, we demonstrated that the electronic dis-
tribution and the emissive state of the molecule can be
modulated and that the emission depended sensitively on
the electronic distribution. The 5-(o-pyridyl) group was in-
corporated to form a bidentate coordination environment
with a 1,3,4-oxadiazole core, which creates two distinct coor-
dination modes for binding with different metal ions. 2,5-
Diaryl-1,3,4-oxadiazoles were shown to exhibit fluorescence
response profiles varying in both intensity and wavelength
toward metal ions such as Ni2+, Cu2+, Zn2+, Cd2+, Hg2+
,
and Pb2+. This provides access to establish multiple logic
functions in a single molecular system. OR, NOR, INHIBIT,
and EnNOR logic gates as well as “ON-OFF-ON” and
“OFF-ON-OFF” fluorescent switches were accordingly con-
structed from one derivative (1d) by varying the combina-
tions and levels of metal-ion inputs. It is expected that struc-
tural modifications on 2- and/or 5-aryl moieties would
create more structural motifs for extended design and appli-
cations in molecular “intelligence”, which is now under way
in this laboratory.
Figure 11. a) Fluorescence spectra of 1d (10 mm) in CH3CN in the pres-
ence of chemical inputs. [Zn2+ ]=10 mm, [Cu2+ ]=[Hg2+ ]=50 mm.
b) The logic table and respective symbolic representation of the EnNOR
function. The excitation wavelength was 300 nm.
The rich fluorescence response profiles of 1d toward Zn2+
and Cu2+ also make it feasible to build up fluorescent
switches.[17] The fluorescence of 1d in CH3CN was enhanced
upon addition of Zn2+
, whereas it was significantly
quenched by Cu2+. Therefore, the fluorescent “ON” and
“OFF” states can be modulated by the relative concentra-
tion of Zn2+ and Cu2+. By setting I/I0 >2.0 as the threshold
for the fluorescent “ON” state and I/I0 <0.5 as that for the
fluorescent “OFF” state, the fluorescent “ON” and “OFF”
states can be established by alternate addition of Zn2+ and
Cu2+. Sequential titration of 1d by Zn2+ and Cu2+ caused
the fluorescence to be enhanced (ON) and quenched (OFF)
(Figure 12). The fluorescent switches resulted from the com-
petitive coordination of Zn2+ and Cu2+ with 1d that formed
Experimental Section
Materials: Chemicals used for synthesis were commercially available.
CH3CN was of HPLC quality. CHX, DEE, EtOAc, CH2Cl2, and CH3CN
for spectral studies were redistilled. Metal ions were used as their per-
chlorates. Solvents for NMR measurements had a deuteration grade of
>99 atom D%. Compounds 1d,[18] 1e,[19] 2,[18] and 3[20] have been report-
ed in the literature.
1d–Zn2+ and/or 1d–Cu2+
.
Measurements and methods: Absorption and fluorescence spectra were
recorded in a 1 cm quartz cell. All spectral titrations were carried out by
keeping the concentration of 1–3 constant (10 mm) while varying the
metal-ion concentrations (0–250 mm). The binding constants Ks were de-
termined from the absorbance in CH3CN at 258C. The absorbance data
were fitted to Equation (1),[9] in which A is the absorbance of the ligand
in the presence of a given amount of metal ion, A0 is the absorbance of
the initial solution of the free ligand, Alimit is the absorbance of full com-
plexation, and C0 and CM are the molar concentrations of ligand and
Conclusion
2,5-Diaryl-1,3,4-oxadiazoles bearing a 2-(para-substituted)-
phenyl and a 5-(o-pyridyl) group (substituent X=NMe2,
OEt, Me, H, and Cl; 1a–e) were designed for fluorescent
5800
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Chem. Eur. J. 2010, 16, 5794 – 5802

Molecular Logic Gates and Switches
FULL PAPER
metal ion, respectively. Fluorescence quantum yields were measured
2-p-Tolyl-5-(pyridin-2-yl)-1,3,4-oxadiazole (1c): 1H NMR (500 MHz,
CDCl3, TMS): d=2.44 (s, 3H), 7.33 (d, J=8.0 Hz, 2H), 7.46 (t, J=
6.0 Hz, 1H), 7.89 (t, J=8.0 Hz, 1H), 8.10 (d, J=7.0 Hz, 2H), 8.31 (d, J=
8.0 Hz, 1H), 8.81 ppm (d, J=4.5 Hz, 1H); 13C NMR (100 MHz, CDCl3,
TMS): d=21.6, 120.8, 123.1, 125.7, 127.2, 129.7, 137.1, 142.6, 143.7, 150.2,
163.6, 165.7 ppm; HRMS (ESI): m/z: calcd for C14H12N3O: 238.0980
[M+H+]; found: 238.0983 [M+H+].
2-Phenyl-5-(pyridin-2-yl)-1,3,4-oxadiazole (1d): 1H NMR (500 MHz,
CDCl3, TMS): d=7.47–7.49 (m, 1H), 7.52–7.57 (m, 3H), 7.89–7.93 (m,
1H), 8.22–8.24 (m, 2H), 8.32 (d, J=8.0 Hz, 1H), 8.82–8.83 ppm (m, 1H);
13C NMR (100 MHz, CDCl3, TMS): d=123.3, 123.7, 125.8, 127.3, 129.0,
132.0, 137.2, 143.7, 150.3, 163.9, 165.6 ppm; HRMS (ESI): m/z: calcd for
C13H10N3O: 224.0824 [M+H+]; found: 224.0817 [M+H+].
using quinine sulfate as a standard (0.546 in 0.5m H2SO4).[10]
ꢀ
ꢁ
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
AlimitꢀA0
2c0
2
A ¼ A0 þ
cMþ1=Ksþc0ꢀ ðcMþ1=Ksþc0Þ ꢀ4c0cM
ð1Þ
1H NMR (500 MHz) and 13C NMR (100 MHz) spectra were recorded in
CDCl3 with TMS as an internal standard. 1H NMR (400 MHz) titrations
of 1d by Zn2+ and Hg2+ were carried out in CD3CN. HRMS spectra
were obtained by using methanol as the solvent.
Single-crystal X-ray diffraction data were collected at 273 K. Absorption
corrections were applied by using the multiscan program SADBS. The
structure was solved by direct methods, and non-hydrogen atoms were
refined anisotropically by a least-squares method on F2 by using the
SHELXTL-97 program. The hydrogen atoms were generated geometri-
cally (C–H, 0.96 ꢁ). CCDC-745239 (1b) contains the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
2-(4-Chlorophenyl)-5-(pyridin-2-yl)-1,3,4-oxadiazole
(1e):
1H NMR
(500 MHz, CDCl3, TMS): d=7.47–7.50 (m, 1H), 7.52 (d, 2H, J=9.0 Hz,
2H), 7.89–7.93 (m, 1H), 8.16 (d, J=8.5 Hz, 2H), 8.32 (d, J=7.5 Hz, 1H),
8.82 ppm (d, J=4.5 Hz, 1H); 13C NMR (100 MHz, CDCl3, TMS): d=
122.1, 123.3, 125.9, 128.5, 129.4, 137.2, 138.3, 143.5, 150.3, 164.0,
164.8 ppm; HRMS (ESI): m/z: calcd for C13H9ClN3O: 258.0434 [M+H+];
found: 258.0434 [M+H+].
1
Geometry optimizations and vibrational analysis were performed by den-
sity functional theory with Beckeꢂs three-parameter hybrid exchange
functional and the Lee–Yang–Parr correlation functional (B3LYP) imple-
mented in the Gaussian 03 package. The 6-31G* basis set was used in all
calculations.
2,5-Diphenyl-1,3,4-oxadiazole (2): H NMR (500 MHz, CDCl3, TMS): d=
7.52–7.57 (m, 6H), 8.14–8.16 ppm (m, 4H); 13C NMR (100 MHz, CDCl3,
TMS): d=123.9, 126.9, 129.1, 131.7, 164.6 ppm; HRMS (ESI): m/z: calcd
for C14H11N2O: 223.0871 [M+H+]; found: 223.0875 [M+H+].
2-Phenyl-5-(pyridin-2-yl)-1,3,4-thiadiazole (3): 1H NMR (500 MHz,
CDCl3, TMS): d=7.39–7.41 (m, 1H), 7.51–7.52 (m, 3H), 7.87 (t, J=
8.0 Hz, 1H), 8.05 (t, J=3.5 Hz, 2H), 8.40 (d, J=8.0 Hz, 1H), 8.68 ppm
(d, J=5.0 Hz, 1H); 13C NMR (100 MHz, CDCl3, TMS): d=121.0, 125.3,
128.0, 129.2, 130.3, 131.2, 137.2, 149.2, 149.8, 169.9, 170.0 ppm; HRMS
(ESI): m/z: calcd for C13H10N3S: 240.0595 [M+H+]; found: 240.0590
[M+H+].
Preparation and characterization of 1–3: The substituted benzoyl chlo-
ride (2.2 mmol) in CH2Cl2 (10 mL) was added dropwise to a dried round
flask containing picolinohydrazine or benzoyl hydrazine (2.0 mmol), pyri-
dine (1.0 mL), and N,N-dimethyl-4-aminopyridine (DMAP; 60 mg) in di-
chloromethane (15 mL). The mixture was stirred at room temperature
for 6 h and then washed with dilute aqueous HCl (1m, 3ꢃ10 mL) and
water (3ꢃ10 mL), and dried over sodium sulfate. After removal of the
solvent at reduced pressure, N’-(4-substituted-benzoyl)picolinohydrazide
or N’-benzoylbenzohydrazide was obtained as a white solid in 85% yield,
and was directly used for the next step.
1 and 2 were respectively synthesized from N’-(4-substituted-benzoyl)pi-
colinohydrazide and N’-benzoylbenzohydrazide (1.5 mmol) by heating its
POCl3 (10 mL) solution at reflux for 6 h. After cooling, the solution was
poured into iced water and neutralized with saturated NaHCO3 solution.
The resulting solution was extracted with CHCl3 (3ꢃ15 mL) and the or-
ganic phase was washed with water (3ꢃ15 mL) and saturated NaHCO3
solution (3ꢃ15 mL), and dried over sodium sulfate. After evaporation of
the solvent under reduced pressure, the residue was subjected to column
chromatography on silica gel (ethyl acetate/petroleum ether, 1:3) to
afford 1 or 2 in 75–80% yield.
Acknowledgements
This work was supported by the NSFC of China through grant nos.
J0630429, 20675069, and 20835005.
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2-(4-Dimethylamino)-5-(pyridin-2-yl)-1,3,4-oxadiazole (1a): 1H NMR
(500 MHz, CDCl3, TMS): d=3.07 (s, 6H), 6.75 (d, J=9.0 Hz, 2H), 7.42–
7.45ACHTUNGTRENNUNG(m, 1H), 7.85–7.89 (m, 1H), 8.06 (d, J=9.0 Hz, 2H), 8.29 (d, J=
8.0 Hz, 1H), 8.79–8.81 ppm (m, 1H); 13C NMR (100 MHz, CDCl3, TMS):
d=40.0, 110.6, 111.5, 122.9, 125.3, 128.7, 137.0, 144.1, 150.1, 152.6, 162.8,
166.3 ppm; HRMS (ESI): m/z: calcd for C15H15N4O: 267.1246 [M+H+];
found: 267.1241 [M+H+].
2-(4-Ethoxyphenyl)-5-(pyridin-2-yl)-1,3,4-oxadiazole
(1b):
1H NMR
(500 MHz, CDCl3, TMS): d=1.46 (t, J=7.0 Hz, 3H), 4.12 (q, J=7.0 Hz,
2H), 7.01 (d, J=9.0 Hz, 2H), 7.45–7.47 (m, 1H), 7.88–7.91 (m, 1H), 8.14
(d, J=9.0 Hz, 2H), 8.30 (d, J=8.0 Hz, 1H), 8.80–8.82 ppm (m, 1H);
13C NMR (100 MHz, CDCl3, TMS): d=14.7, 63.8, 114.9, 115.9, 123.1,
125.6, 129.1, 137.1, 143.9, 150.2, 162.0, 163.4, 165.6 ppm; HRMS (ESI):
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