A fluorophore capable of crossword puzzles and logic memory

Article abstract of DOI:10.1002/anie.200700526

(Chemical Equation Presented) Puzzlingly logical: The characteristic fluorescence of Hg²⁺-selective OFF-ON and Cu²⁺-selective ON-OFF operations can be monitored and controlled reversibly by the sequence and ratio of Hg²⁺ and Cu²⁺ inputs. These inputs have been used to construct a molecular keyboard that is capable of crossword puzzles and logic memory (see picture).

Full text of DOI:10.1002/anie.200700526

Angewandte
Chemie
DOI: 10.1002/anie.200700526
Molecular Devices
A Fluorophore Capable of Crossword Puzzles and Logic Memory**
Zhiqian Guo, Weihong Zhu,* Liangjun Shen, and He Tian*
Dedicated to Professor Vincenzo Balzani on the occasion of his 70th birthday
Computer memories have been reaching higher bit densities
for decades, although it is becoming ever more difficult to
keep up with this long-term trend. Many researchers have
therefore begun to think seriously about building memory
various molecular switches and logic gates have been
proposed that convert a change produced by external stimuli
such as chemical species and light into detectable outputs.[7]
Chemically triggered logic gates operating on solid supports
have stimulated chemists to develop a new class of program-
mable logic gates that are capable of executing logic
functions.[8] However, it is still a great challenge to implement
combinatorial logic circuits in a single molecular entity. To
date, only a few such examples can selectively and reversibly
identify HTM ions and emit distinct signals,[9] thereby limiting
the application of these systems as molecular logic gates.
DCPP, which can competitively coordinate Hg2+ and Cu2+
ions, is derived from the well known laser dye 4-dicyano-
methylene-6-[4-(dimethylamino)styryl]-2-methyl-4H-pyran
(DCM).[10] The synthesis of DCPP is straightforward and is
depicted in Scheme 1. The intermediate DCMP was synthe-
devices from the bottom up using individual molecules.[1]
A
very recent proof-of-concept study along these lines showed
that molecular memory cells can be made with a bit density of
1011 per square centimeter.[2] In addition, by harnessing the
principles of molecular Boolean logic, the first example of a
molecular system that mimics a keypad and is capable of
processing password entries was described. Such molecular
devices that are capable of distinguishing between different
chemical sequences might have considerable advantages over
simple molecular logic gates.[3]
Herein we present the novel fluorescent compound DCPP
(see Scheme 1), which possesses both 2-{2-isopropyl-6-[4-
(piperazin-1-yl)styryl]-4H-pyran-4-ylidene}malononitrile
(DCMP) and 2,6-bis(aminomethyl)pyridine moieties.
Remarkably, DCCP shows a reversible and controllable
fluorescent behavior as a result of competitive coordination
of Hg2+ and Cu2+ ions. The characteristic fluorescence of
Hg2+-selective OFF-ON and Cu2+-selective ON-OFF opera-
tions can be controlled by varying the sequence of Hg2+ and
Cu2+ inputs, and this has been used to construct sequential
logic circuits capable of memory function.[4] The output
signals in this molecular logic system are dependent not only
upon the appropriate combination of chemical inputs but also
upon the exact sequence of these inputs. An easy-to-handle
molecular keyboard capable of crossword puzzles can be
established based on the above logic operation.
The design and synthesis of chemosensors with high
selectivity and sensitivity for heavy and/or transition metal
(HTM) ions continues to be an active area of supramolecular
chemistry.[5] Fluorescent sensors that change their fluores-
cence properties upon binding HTM ions (such as Hg2+ and
Cu2+)[6] are one of the most widely used analytical tools, and
Scheme 1. Synthetic route to sensor DCPP.
[*] Z. Guo, Prof. Dr. W. Zhu, L. Shen, Prof. Dr. H. Tian
Laboratory for Advanced Materials and Institute of Fine Chemicals
East China University of Science & Technology
Shanghai 200237 (China)
sized in 68% yield by treating 4-(piperazin-1-yl)benzaldehyde
with
2-isopropyl-6-methyl-4-dicyanomethylene-4H-pyran.
DCPP was finally obtained from the reaction between
DCMP and 2,6-bis(bromomethyl)pyridine[11] in acetonitrile
with a yield of about 26%. The characteristic coupling
between protons e and f in the 1H NMR spectrum of DCMP
and DCPP (J = 16.0Hz) is indicative of the predominant trans
isomer.
DCPP is an A-p-D type laser dye with a broad absorption
band resulting from an ultra-fast internal charge-transfer
(ICT) process. As shown in Figure 1, free DCPP has a
characteristic emission at around 621 nm with moderate
Fax: (+86)21-6425-2758
E-mail: whzhu@ecust.edu.cn
tianhe@ecust.edu.cn
[**] This work was supported by the NSFC (China) (National Basic
Research 973 Program, grant no. 2006CB806200), the Education
Committee of Shanghai, and the Scientific Committee of Shanghai.
W.H.Z. thanks the Program for New Century Excellent Talents in
University (NCET).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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upon titration with Cu2+, with the fluorescence being almost
completely quenched to the baseline upon addition of two
equivalents of Cu2+ (Figure 1b). This quenching is not
unexpected because this ion can, and in fact does, quench
most excited states by energy and/or electron transfer since it
is easily reduced. Notably, the fluorescence peak of DCPP is
not so strongly affected by the titration with other metal ions.
None of the above phenomena were observed for DCMP,
which suggests that the configuration of DCPP is critical for
the selectivity and sensitivity to metal ions.
1H NMR studies of the interaction of Hg2+ with DCPP
give useful information consistent with the above analysis.
The peaks at around d = 7.75 ppm (Figure 2), which were
assigned to pyridine proton d (Scheme 2) in free DCPP, shift
significantly downfield to around d = 8.06 ppm upon metal
coordination. The signals of the protons b of the piperazine
moiety also undergo a downfield shift from d = 2.62 to
Figure 1. Emission spectra of DCPP (5.0 mm) in a mixture of H2O and
CH3CN (4/6, v/v) with a buffer solution of Na2HPO4/KH2PO4 (0.01m,
pH 7.0) upon titrating different molar ratios of Hg2+ (a) and Cu2+ ions
(b) with excitation at 440 nm. Irel =100 corresponds to I=1000 in the
case of (a) and to I=390 in the case of (b). Insets: Job’s plots; the
total concentration of DCPP and Hg2+ or Cu2+ ion is 10.0 mm.
efficiency (FDCPP = 0.09)[12] resulting from this ICT process.
There are three kinds of nitrogen atoms in DCPP that can
coordinate to metal ions (Scheme 1), namely the aniline
nitrogen (N1), the piperazine nitrogen (N2), and the pyridine
nitrogen (N3) atom, which play different roles in the
fluorescence. Thus, metal ions will increase the fluorescence
intensity (fluorescence ON) when coordinated to N2 and N3 as
this coordination will stop the photo-induced electron trans-
fer (PET) from N2 to N1.[5f] On the other hand, metal ions
bonded to N1 will tend to reduce the fluorescence (fluores-
cence OFF) due to a decrease in the electron-donating ability
of this nitrogen atom.[13] Such a design is likely to yield distinct
fluorescence signals depending on the target species coordi-
nated to specific nitrogen atoms.
Figure 2. Parts of the 1H NMR spectra of: a) free DCPP and b) DCPP-
Hg2+ in CD3CN.
Indeed, a distinct enhancement of the fluorescence
intensity for DCPP was observed upon interaction with
Hg2+ (Figure 1a). An enhancement factor (EF, I/I0) of 2.2 at
621 nm was found in the presence of about five equivalents of
Hg2+, thus indicating that the PET quenching pathway is
efficiently blocked by the likely coordination of the Hg2+ ion
to N2 and N3 in DCPP. In contrast, a quenching was observed
Scheme 2. Proposed sensing process of DCPP by characteristic
square-planar coordination with Cu2+ and tetrahedral coordination
with Hg2+, both of which can be reset by addition of edta.
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2.76 ppm upon metal coordination and they clearly broaden.
These deshielding effects confirm the coordination of Hg2+ to
N2 and N3 of DCPP (Scheme 2). The protons a behave
differently upon addition of Hg2+, with one of the two peaks
shifting downfield from d = 3.30to 3.41 ppm and the other
shifting slightly upfield from d = 3.30to 3.26 ppm. This
behavior is attributed to a shielding effect induced by the
coordination of Hg2+ to N2, which might indicate that the
Hg2+ ion coordinates to only one of the two N1 atoms in
DCPP (Scheme 2). This observation is consistent with a
previous report concerning the regular tetrahedral coordina-
tion with Hg2+.[6d]
Unfortunately, the 1H NMR spectrum of DCPP-Cu2+ does
not provide useful information due to the paramagnetic
nature of the metal ion. An intense peak for DCPP-Cu2+ at m/
z 910.4 (100%), with the correct isotope distribution, in the
electrospray ionization (ESI) HPLC mass spectrum, however,
provides strong evidence for the formation of this complex
(see Figure S14 in the Supporting Information). It is likely
that DCPP is coordinated to the Cu2+ ion in a characteristic
square-planar coordination, as shown in Scheme 2. The
energy-minimized conformations of DCPP-Hg2+ and DCPP-
Cu2+ were also simulated with the Hyperchem software to
further support the above-mentioned square-planar coordi-
nation with Cu2+ and the characteristic tetrahedral coordina-
tion with Hg2+ (see Figures S15–S17 in the Supporting
Information).
Figure 3. Emission spectra of DCPP (5.0 mm) in a mixture of H2O and
CH3CN (4/6, v/v) with a buffer solution of Na2HPO4/KH2PO4 (0.01m,
pH 7.0) upon titrating different molar ratios of Cu2+ and Hg2+ when
excited at a wavelength of 440 nm: Cu2+-Hg2+-Cu2+ OFF-ON-OFF.
mation) of this system, a sequential logic circuit capable of
memory function was built. The appropriate choice of
addition or subtraction chemical inputs induced by Cu2+ or
Hg2+ ions was used to control the algebraic operation. The
metal ions Cu2+ (3 equiv) and Hg2+ (5 equiv) are defined as
In 1 and In 2, respectively, in Figure 4. The value of I/I0 at
621 nm is designated as either output “0” (I/I0 < 1.20) or “1”
(I/I0 > 1.20). The sequence of adding Cu2+ and Hg2+ ions,
Fluorescence titrations of DCPP with various metal ions
(see Figures S4 and S5 in the Supporting Information) clearly
showed that the fluorescence change due to the addition of
metal ions other than Cu2+ and Hg2+ is negligible. Both the
linear decrease of fluorescence intensity within the equivalent
range of Cu2+ ion and the Jobꢀs plot (Figure 1b) indicate that
DCPP forms a 1:1 complex with Cu2+. The association
constant (Kass) of the DCPP-Cu2+ complex, as calculated
from the titration curve, is 4.36 ” 105 mÀ1 [14a]
Similarly, the
.
appropriate Jobꢀs plot also suggests that DCPP forms a 1:1
complex with Hg2+, with a Kass of 1.85 ” 105 mÀ1, as calculated
from a non-linear least-squares equation (see the Supporting
Information). The detection limits for Cu2+ and Hg2+ can be
estimated from the titration results to be 1.2 ” 10À7 and 3.0”
10À7 m, respectively.[14b]
Figure 4. Output (I/I0) resulting from different input sequences:
a) In 1 (Cu2+ (3 equiv) first) and In 2 (Hg2+ (5 equiv) second); b) In 1
(Hg2+ (5 equiv) first) and In 2 (Cu2+ (3 equiv) second); the dashed line
shows the threshold (1.20) determined after 5 min.
Since the Kass values of DCPP-Cu2+ and DCPP-Hg2+ are
approximately equal Cu2+ and Hg2+ can compete against each
other to coordinate with DCPP. As shown in Figure 3, a
significant quenching of the fluorescence of DCPP (fluores-
cence OFF) is obtained upon adding Cu2+ due to the
formation of DCPP-Cu2+. Sequential titration with 50equiv-
alents of Hg2+ and 30equivalents of Cu 2+ caused the
fluorescence to vary from its initial maximum intensity
(ON) to quenched fluorescence (OFF). The competing
coordination of Cu2+ and Hg2+ with DCPP therefore results
in a transformation between DCPP-Cu2+ and DCPP-Hg2+.
Moreover, a specific signal pattern was also observed upon
changing the addition sequence of Hg2+ (5 equiv) and Cu2+
(3 equiv) (see Figures S6 and S7 in the Supporting Informa-
tion).
which leads to the input string 11, determines the values of the
output. Thus, when the first input signal is Cu2+ (3 equiv) as
“1” and the second input signal is Hg2+ (5 equiv) as “1”, the
output digit is “0” (Figure 4a). However, changing the
sequence of adding Cu2+ and Hg2+ ions gives the output
digit “1” (Figure 4b). Accordingly, the output value in the
previous string is memorized and maintained when both
inputs become “1”.
To visualize these sequence-dependent phenomena
directly, this sequential logic operation with memory function
can be constructed as a crossword puzzle. As illustrated in
Figure 5, the input of Hg2+ (5 equiv) is designated as the
character “U” and the input of Cu2+ (3 equiv) as the character
“S”. When the input signal “U” is added first, emission as the
character “E” (strong fluorescence signal, ON) is observed as
a result of memorizing the input sequence. This combination
of inputs gives the word “USE” in the crossword. Inverting
To take advantage of the reversibility, selectivity, and fast
response (see Figures S10and S11 in the Supporting Infor-
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controlling the input sequence. This concept can be expected
to greatly enhance combinatorial logic operations in sequen-
tial logic circuits with a memory function.
Experimental Section
The synthesis of DCPP was performed as follows. A mixture of
DCMP (135 mg, 0.363 mmol), 2,6-di(bromomethyl)pyridine (44 mg,
0.165 mmol), potassium carbonate (5 mg) as catalyst, and acetonitrile
(30mL) was stirred and refluxed for 15 h under nitrogen. After
cooling to room temperature the mixture was concentrated under
vacuum to afford a dark red solid. The product was purified by
column chromatography with dichloromethane/methanol (15/1, v/v)
as eluent to give DCPP (85 mg, yield: 28%); m.p. 188–1908C;
1H NMR (400 MHz, CDCl3): d = 1.33 (d, J = 6.8 Hz, 12H; CH-
(CH3)2), 2.69 (t, J = 4.8 Hz; 8H; CH2), 2.88 (m, 2H; CH(CH3)2),
3.37 (t, J = 4.8 Hz; 8H; CH2), 3.75 (s, 4H; CH2-pyridine), 6.46 (s, 2H;
Figure 5. Crossword puzzles that allow direct visualization of the
sequence dependence. For details see text.
=
pyran-H), 6.51 (d, J = 16.0Hz, 2H; CH ), 6.58 (s, 2H; pyran-H), 6.89
=
(d, J = 8.4 Hz, 4H; phenyl-H), 7.33 (d, J = 16.0Hz, 2H; CH ), 7.38 (d,
the addition sequence of Cu2+ and Hg2+ gives an obvious
fluorescence quenching (OFF, designated as the character
“N”). This input sequence gives the word “SUN” in the
crossword, therefore different character strings are produced
by changing the input sequences (Figure 5). Furthermore, the
cycle can be repeated by adding a strong chelate such as
ethylenediamine tetraacetic acid (edta; see Scheme 2 and
Figure S8 in the Supporting Information) due to the large
difference in association constants (Kass) between edta and
J = 7.6 Hz, 2H; pyridine-H), 7.43 (d, J = 8.4 Hz, 4H; phenyl-H),
7.69 ppm (2d, J = 7.6 Hz, 1H; pyridine-H); 13C NMR (400 MHz,
CDCl3): d = 20.29, 32.96, 47.60, 52.93, 58.00, 64.33, 103.51, 106.06,
114.20, 114.79, 115.57, 121.65, 124.74, 129.44, 136.98, 138.02, 152.51,
156.66, 157.68, 159.94, 169.89 ppm; MS(ESI positive ion mode for
M+H): calcd. 848.4400; found 848.4416.
Received: February 6, 2007
Revised: April 23, 2007
Published online: June 20, 2007
DCPP with these metals (logKass of edta with Cu2+ and Hg2+
[15]
is 18.70and 21.80,
respectively, which indicates that edta
Keywords: copper · fluorescent probes · logic memory ·
.
binds much more strongly with Cu2+ and Hg2+ ions than
DCPP). This means that the logic operation reported here can
be reset.
mercury · molecular devices
A similar sequence-dependent phenomenon has been
reported in the literature,[1c] although the short timescale
limited its application. Our system overcomes this problem as
it remains stable for a reasonably long time (> 12 h; see
Figures S12 and S13 in the Supporting Information) over the
threshold of 1.2. It should be pointed out, however, that the
Hg2+:Cu2+ ratio is critical to the sequence dependence.[16]
Thus, changing the ratio of Hg2+ and Cu2+ to (50and
5 equiv, respectively (Figure 3 and Figure S9 in the Support-
ing Information), gave only one fluorescence signal (ON),
even when changing the addition sequence. Thus, the
coordination of DCPP with Hg2+ and Cu2+ is a competitive
process that occurs under kinetic control.
In summary, a novel fluorescent probe (DCPP) that
possesses both DCMP and 2,6-bis(aminomethyl)pyridine
moieties has been designed for the detection of Hg2+ and
Cu2+ ions. The characteristic fluorescence of Hg2+-selective
OFF-ON and Cu2+-selective ON-OFF operations can be
monitored and controlled reversibly by the addition sequence
and ratio of Hg2+ and Cu2+ inputs, and this has been used to
construct a crossword puzzle and a logic memory at the
molecular level. This molecular logic system has the following
advantages over other reported combinatorial logic circuits:
1) a characteristic signal pattern that performs distinct
algebraic operations solely in the fluorescence mode; 2) a
single fluorescence intensity measurement setup for direct
“reading” of the arithmetic results at the same excitation
wavelength; 3) a reset function; and 4) is easy to handle by
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[16] We thank Prof. A. P. de Silva and the referees who suggested a
kinetic process for this sequence-dependent phenomenon.
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