Azaisoquinolinones: N positions tell you different stories in their optical properties

Article abstract of DOI:10.1021/jo402338n

Since isoquinolinones and their derivatives have been demonstrated to be powerful building blocks in constructing larger acenes and twistacenes, azaisoquinolinones and their analogues could also be important intermediates to approach larger N-heteroacenes. In this paper, we are interested in developing a concise method to synthesize novel azaisoquinolinones building blocks and studying their physical properties. Our results showed that the different N positions have a large effect on the optical and electrochemical properties of azaisoquinolinones. For example, protonation of 6- and 7-azaisoquinolinones shows different shifts of UV-vis and FL spectra. More interestingly, 6- and 7-azaisoquinolinones exhibited different interactions with metal ions in CH ₃CN solution. Upon the addition of 2 equiv of Fe³⁺, 6-azaisoquinolinone displayed an absorption wavelength red-shifted from 470 to 540 nm (Δλ = 70 nm) with a color change from yellow to red, while the interaction between Fe³⁺ and 7-azaisoquinolinone was very weak and there was no obvious color change (Δλ = 18 nm). Moreover, theoretical calculations confirmed the different optical properties with 6- and 7-azaisoquinolinones.

Full text of DOI:10.1021/jo402338n

Article
pubs.acs.org/joc
Azaisoquinolinones: N Positions Tell You Different Stories in Their
Optical Properties
Junbo Li, Junkuo Gao, Gang Li, Weiwei Xiong, and Qichun Zhang*
School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
S
* Supporting Information
ABSTRACT: Since isoquinolinones and their derivatives have
been demonstrated to be powerful building blocks in
constructing larger acenes and twistacenes, azaisoquinolinones
and their analogues could also be important intermediates to
approach larger N-heteroacenes. In this paper, we are
interested in developing a concise method to synthesize
novel azaisoquinolinones building blocks and studying their
physical properties. Our results showed that the different N
positions have a large effect on the optical and electrochemical
properties of azaisoquinolinones. For example, protonation of
6- and 7-azaisoquinolinones shows different shifts of UV−vis
and FL spectra. More interestingly, 6- and 7-azaisoquinolinones exhibited different interactions with metal ions in CH3CN
solution. Upon the addition of 2 equiv of Fe3+, 6-azaisoquinolinone displayed an absorption wavelength red-shifted from 470 to
540 nm (Δλ = 70 nm) with a color change from yellow to red, while the interaction between Fe3+ and 7-azaisoquinolinone was
very weak and there was no obvious color change (Δλ = 18 nm). Moreover, theoretical calculations confirmed the different
optical properties with 6- and 7-azaisoquinolinones.
solar cells,2c the same dilemma has been encountered in
challenging larger N-heteroacenes (more than six linearly fused
INTRODUCTION
Isoquinolinones (Scheme 1) are very attractive not only
■
benzene rings) due to their poor stability, especially the
because they are an important family of biologically relevant
existence of water during the reaction. Thus, a “clean reaction”
might be a feasible method to address larger N-heteroacenes.
However, in order to realize this strategy, a precursor with
build-in heteroatoms is required. Since our previous research
has already demonstrated that isoquinolinones and their
derivatives are powerful intermediates to prepare larger
oligoacenes and twistacenes,3 we believe that heteroatom (N,
P, B)-substituted isoquinolinones and their derivatives should
also be promising building blocks to construct larger
heteroacenes.
Scheme 1. Structures of Isoquinolinones (1-1), 2-Methyl-
1,4-diphenylisoquinolin-3(2H)-one (1), 7-Aza-2-methyl-1,4-
diphenylisoquinolin-3(2H)-one (2), and 6-Aza-2-methyl-1,4-
diphenylisoquinolin-3(2H)-one (3)
Here, we report the synthesis, characterization, physical
properties, and theoretical study of two novel azaisoquinoli-
alkaloids with key biological activities in the central nervous
system1 but also because they are one type of heteroacene with
tunable electronic properties2 and can act as promising building
blocks to construct larger oligoacenes through “clean”
reactions.3 The “clean reaction” strategy is based on thermally
eliminating a lactam bridge from precursors to give pure
polycyclic compounds through a retro Diels−Alder reaction at
high temperature.3 The attractive factor to employ a “clean”
reaction to approach larger acenes is that this method could
avoid the tedious purification problem due to the instability and
poor solubility of the larger acenes.4,5 Although N-hetero-
acenes6−9 have been demonstrated to be promising materials
for ion sensing8d,f or as active elements in organic semi-
conducting devices such as field effect transistors,7,9 light-
emitting devices,8i phototransistors,8h memory devices,8a,b and
nones: 7-azaisoquinolinone (2) and 6-azaisoquinolinone (3)
(Scheme 1). We found that compounds 2 and 3 have different
responses to protonation and metal ions.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. 6- and 7-azaisoquinoli-
nones were synthesized in two steps (Scheme 2). The [4 + 2]
reaction between 3,4-pyridyne (formed in situ by 3-amino-
pyridine-4-carboxylic acid and isoamyl nitrite) and mesoionic
pyrimidine10 produced a mixture of isomers 2-1 and 3-1.
Fortunately, the isomers can be separated through chromatog-
Received: October 19, 2013
Published: December 3, 2013
© 2013 American Chemical Society
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Calculations. Theoretical calculations gave the highest
occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) energy levels of
isoquinolinone 1 and azaisoquinolinones 2 and 3 using density
functional theory (DFT) at the B3LYP/6-31G* level.11 When
the 6- or 7-position CH group was substituted by an sp2 N
atom, both the HOMO and LUMO orbital energy levels were
decreased, but the tendencies were different. As shown in
Figure 3, the decrease of HOMO orbital energy for
Scheme 2. Synthetic Route to Azaisoquinolinones 2 and 3
raphy. Single crystals of 2-1 were obtained through slowly
evaporating a mixed solvent (VCH Cl /VCH OH = 4/1), and the
2
2
3
single-crystal structure has been solved for the configuration of
2-1 (Figure 1). Although the precursors 2-1 and 3-1 have
Figure 3. HOMO and LUMO orbital energies calculated by density
functional theory (DFT) at the B3LYP/6-31G* level.
azaisoquinolinone 2 (ΔHOMO1‑HOMO2 = 0.35 eV) is larger than
that for azaisoquinolinone 3 (ΔHOMO1‑HOMO3 = 0.26 eV), but
the decrease of LUMO orbital energy for azaisoquinolinone 2
(ΔLUMO1‑LUMO2 = 0.26 eV) is smaller than that for 6-
azaisoquinolinone 3 (ΔLUMO1‑LUMO3 = 0.36 eV), which induced
the band gap of 6-azaisoquinolinone 3 to be smaller than that
for 7-azaisoquinolinone (2). Moreover, the HOMO and
LUMO orbitals of compounds 1−3 (Figure 4) showed that
Figure 1. Crystal structure of compound 2-1.
similar structures, the temperatures required to remove the
lactam bridge are different (195 °C for compound 2-1 and 215
°C for compound 3-1; Figure S2, Supporting Information).
After the removal of the lactam bridge, the target compounds 2
and 3 were obtained. Figure 2 shows the 1H NMR spectra of 6-
1
Figure 2. H NMR spectra of azaisoquinolinones 2 and 3 in CDCl3.
Figure 4. Wave functions for the HOMO and LUMO of compounds
1−3.
azaisoquinolinone (3) and 7-azaisoquinolinone (2). The most
characterized shifts are protons (Ha, Hb, and Hc) on the
pyridine ring. When the position of the N atom was changed
from 7 to 6, the proton Ha was shifted downfield from 8.34 to
8.88 ppm. Meanwhile, the protons Hb and Hc were shifted
upfield from 7.94 and 7.96 ppm to 7.79 and 6.68 ppm,
respectively. Obviously, Ha, Hb, and Hc have different chemical
environments in compounds 2 and 3. When the position of
protons changed from the place near the electron-donating
group N−CH3 to the position close to the electron-with-
drawing group CO on the pyridinone ring, the chemical
shifts of protons moved downfield accordingly. All new
the 6-position substitution gives more contribution to the
LUMO orbital but the 7-position gives more contribution to
the HOMO orbital, which means that 6- and 7-position
substitution will strongly affect the bandgap of the related
compounds. These interesting theoretical data strongly
encouraged us to investigate the differences of 6- and 7-
azaisoquinolinones before using them to construct larger N-
containing heteroacenes. In addition, since azaisoquinolinones
contain N and O atoms, which are well-known atoms for the
coordination with metal ions,12 we believe that azaisoquinoli-
nones should have some responses to metal ions.
1
compounds have been fully characterized by H NMR, 13C
NMR, elemental analysis (EA), and HR-MS.
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Different Optical and Electrochemical Properties. The
UV−vis absorption and fluorescence spectra of compounds 1−
3 were recorded in CH3CN at room temperature. As shown in
Figure 5, isoquinolinone 1 (5 × 10−5 M) shows three
The calculated excitation wavelengths of compounds 1−3 are
3.13 eV/445 nm, 3.22 eV/437 nm, and 3.03 eV/458 nm,
respectively, which are slightly lower but in good agreement
with experiment. Figure S3 (Supporting Information) gives the
simulated absorption spectra of compound 1−3. The simulated
emission spectra show that the emission wavelengths of
compounds 1−3 are at 554, 562, and 580 nm, respectively.
The relatively larger red-shift tendency for 7-azaisoquinolinone
(3) is in accord with the experimental results (Figure S4,
Supporting Information).
The electrochemical properties of compounds 1−3 were
studied in a three-electrode electrochemical cell with
tetrabutylammonium hexafluorophosphate (Bu4NPF6, TBAF)
(0.1 M) as electrolyte and Ag/AgCl as reference electrode
(Figure 7 and Table 1). Compounds 1−3 all show one
Figure 5. UV−vis spectra of compounds 1−3 (5 × 10−5 M) in
CH3CN. The inset shows the color of compounds 1−3 (1 × 10−3 M)
in CH3CN.
absorption bands at 238, 304, and 448 nm. In comparison to
compound 1, the two azaisoquinolinones show different shifts.
For compound 2, the low-energy region was blue-shifted to 440
nm and the high-energy region was red-shifted to 312 and 251
nm, respectively. For compound 3, in contrast, the low-energy
region was red-shifted to 470 nm and the high-energy region
was blue-shifted to 272 and 231 nm. The solution containing
compound 3 became dark yellow. Moreover, the fluorescence
spectra also tell different stories. Compounds 1 and 2 have
nearly the same emission maximum wavelengths (λmax 531 nm)
and the same intensities, while compound 3 displays a longer
emission (544 nm) and a stronger fluorescent intensity in
comparison to 1 and 2 (Figure 6). The emission color for
Figure 7. Cyclic voltammetry curves of 1−3 in CH2Cl2 solution
containing 0.1 M TBAP electrolyte. Scanning rate: 100 mV/s.
irreversible oxidation peak. The onset oxidative potentials for
compounds 1−3 were measured as 1.00, 1.25, and 1.18 eV,
ox
respectively. On the basis of the equation HOMO = −Eonset
−
4.4 eV,13 the HOMO energy levels for 1−3 were calculated as
−5.40, −5.65, and −5.58 eV, respectively. It is worth noting
that the incorporation of an N atom at the 7-position (ΔEHOMO
= −0.25 eV) induced a larger decrease in HOMO than that at
the 6-position (ΔEHOMO = −0.18 eV). In addition, compound 1
shows an irreversible reduction peak, while both compounds 2
and 3 show a reversible reduction peak. Their onset reductive
potentials were measured as −1.67, −1.46, and −1.34 eV,
respectively. The relative LUMO energy levels were calculated
as −2.73, −2.94, and −3.06 eV, respectively. Moreover, the
decreased value of LUMO for 7-azaisoquinolinone (ΔELUMO
=
Figure 6. Fluorescence spectra (1 × 10−5 M) of compounds 1−3 in
CH3CN. The inset shows the fluorescence color of compounds 1−3
(1 × 10−3 M) in CH3CN.
−0.21 eV) is smaller than that for 6-azaisoquinolinone
(ΔELUMO = −0.33 eV), which might indicate that compound
3 (6-position) is more stable. The change in tendencies for the
HOMO and LUMO is in accord with the calculated results.
The electronic states of isoquinolinones are strongly
governed by its pyridone end unit.2 There are two possible
major contributing resonance structures: one is the neutral
polyene state (left), and the other is the charge-separated state
(mesoionic state, right) (Scheme 3). Moreover, the electronic
state can be readily favored through the choice of solvents. For
isoquinolinone 1, in aprotic solvents (CH2Cl2, THF, DMF,
compounds 1 and 2 was green, while it was yellow for
compound 3 (Figure 6, inset). Clearly, the UV−vis and FL
results suggested that the N positions in azaisoquinolinones
have a large effect on their optical responses. Detailed
information about the absorption and fluorescence changes
for the different azaisoquinolinones can also be obtained from
TDDFT (time-dependent DFT) at the B3LYP/6-31G* level.11
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Table 1. Experimental and Calculated HOMO−LUMO Gaps of Compounds 1−3
a
a
b
c
d
e
f
f
f
red
ox
compound Eonset (V)
Eonset (V)
Egap (eV)
LUMO (eV)
HOMO (eV)
Egap (eV )/λmax (nm) LUMO (eV) HOMO (eV) Egap (eV)
1
2
3
−1.67
−1.46
−1.34
1.00
1.25
1.18
2.67
2.71
2.52
−2.73
−2.94
−3.06
−5.40
−5.65
−5.58
2.77/448
2.82/440
2.64/470
−1.76
−2.02
−2.12
−4.89
−5.24
−5.15
3.13
3.22
3.03
a
d
b
c
Obtained from cyclic voltammograms in CH2Cl2. Reference electrode: Ag/AgCl. Egap = Eonsetox − Eonsetred. Calculated from cyclic voltammograms.
Calculated according to the formula EHOMO = ELUMO − Egap. Optical band gap, Egap = 1240/λmax. Obtained from theoretical calculations.
e
f
showed little change within a wide pH span of 1−10. However,
for azaisoquinolinones, the protonation of N atoms has a large
effect on their optical properties. For 7-azaisoquinolinone 2,
when the pH value of the solution changed from 10 to 1, the
maximum absorption wavelength was red-shifted from 425 to
448 nm (Δλ = 23 nm) and, accordingly, the emission
wavelength was also red-shifted from 531 to 551 nm with
increased intensity. Interestingly, when the N position was
switched from the 7-position to the 6-position, a larger UV−vis
absorption shift from 457 to 515 nm (Δλ = 58 nm) was
observed and the emission wavelength was red-shifted from 544
to 604 nm with some fluorescence quenching. The HOMO and
LUMO orbital energies of protonated 6- and 7-azaisoquinoli-
nones (3H, 2H) were calculated using density functional theory
(DFT) at the B3LYP/6-31G* level (Figure S5, Supporting
Information). The decrease of LUMO orbital energy with the
protonation of 7-azaisoquinolinone 2 is smaller than that of the
6-position N-heteroisoquinolinone 3 (ΔLUMO2‑LUMO2H = 4.05
eV; ΔLUMO3‑LUMO3H = 4.32 eV), but the decreases in values of
the HOMO levels are nearly equal (ΔHOMO2‑HOMO2H = 3.58 eV;
ΔHOMO3‑HOMO3H = 3.62 eV), which inferred that the band gap
of protonated 3 is smaller than that of protonated 2; this could
explain why compound 3 has a greater red-shift absorption than
2 during the protonation.
Optical Responses with Different Metal Ions. The
optical responses of compounds 1−3 with different metal ions
were also investigated. Figure 10 shows the UV−vis spectra of
6-azaisoquinolinone 3 upon the addition of 2 equiv of different
common metal ions in CH3CN. As shown in Figure 10, the
original color of the solution of 3 is yellow and the maximum
absorption wavelength is 470 nm. Upon the addition of 2 equiv
of Fe3+, the maximum absorption wavelength was largely red-
shifted from 470 to 540 nm and the color of the resulting
solution changed from yellow to red (Figure 10b). Meanwhile,
the addition of other common metal ions such as Cu2+, Zn2+,
Ni2+, Co2+, Mn2+, Co2+, Pb2+, Ag+, Ca2+, Mg2+, Na+, K+, and Li+
has little effect on the UV−vis absorption and there is almost
no change in the color of the solutions. These results showed
that compound 3 has high selectivity for Fe3+. The titration
curve showed that, upon the addition of Fe3+ to a CH3CN
solution of 3, the maximum absorption peak at 470 nm
Scheme 3. Resonance Structure of Isoquinolinone 1
DMSO), the maximum absorption wavelength is around 448
nm. In protic solvents such as methanol, the maximum
absorption peak was blue-shifted to 434 nm, whereas in
strongly acidic solvents such as trifluoroacetic acid (TFA), the
absorption was blue-shifted to 372 nm (Figure 8a). In relatively
weaker acids such as acetic acid, the maximum absorption was
at 421 nm, between those for dichloromethane and trifluoro-
acetic acid. The phenomenon is similar to what we have
observed in benzo-fused isoquinolinones.2
For azaisoquinolinones 2 and 3, the solvent-induced
absorption changes are different. In protic solvent, especially
in acidic solvent, there are two equilibriums in the system. One
is the equilibrium between protonation and deprotonation of
the N atom, and another is the resonance between the polyene
state and mesoionic state. For 7-azaisoquinolinone (2), the
absorption intensity at 440 nm was decreased and blue-shifted
to 438 and 434 nm in the CH3COOH and CF3COOH solvent
systems, respectively. Moreover, a new peak at 334 nm
emerged (Figure 8b). Interestingly, the 6-azaisoquinolinone 3
showed different shifts in comparison to 7-azaisoquinolinone 2.
In the weakly acidic solvent CH3COOH, the absorption was
blue-shifted from 470 to 458 nm and the onset of the peak
reached 600 nm, but in the strongly acidic trifluoroacetic acid,
the absorption peak at 470 nm was divided into two peaks at
440 and 500 nm, respectively. The more complicated shifts in
azaisoquinolinones are attributed to its greater number of
possible resonance structures.
Optical Responses with Different pHs. The N atom in
azaisoquinolinones should have different responses to the
variation of pH values. As shown in Figure 9, the UV−vis and
FL spectra changes of compounds 1−3 were investigated in a
mixed-solvent system (CH3CN/PBS 1/1, v/v). Figure 9a,b are
the UV−vis and FL spectra of isoquinolinone 1 in different pH
solutions. The maximum absorption and emission wavelengths
Figure 8. UV−vis absorption spectra of compound 1−3 (5 × 10−5 M) in different solvents: (a) isoquinolinone 1; (b) 7-azaisoquinolinone 2; (c) 6-
azaisoquinolinone 3.
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Figure 9. UV−vis absorption (50 μM) and FL spectra (10 μM) of compounds 1−3 at different pH values (CH3CN/PBS, 1/1 v/v): (a, b)
isoquinolinone 1 (c, d) 7-azaisoquinolinone; (e, f) 6-azaisoquinolinone.
Figure 11. UV−vis absorption spectra of 3 (50 μM) in CH3CN in the
presence of increasing concentrations of Fe3+. The inset plots the
absorbance at 470 and 540 nm versus the Fe3+ concentration.
Figure 10. (a) UV−vis absorption spectra of 3 (50 μM) upon addition
of different metal ions (2 equiv) in CH3CN. (b) Photograph of 3 (50
amine. First, 2 equiv of Fe3+ was added to 3, and to this
μM) with different metal ions.
solution was added 2 equiv of diethylenetriamine or
decreased while the peak at 540 nm increased gradually. The
point for the titration to reach saturation is ∼2 equiv of Fe3+
(Figure 11). Furthermore, an isosbestic point at 500 nm was
observed, which indicated the existence of an equilibrium
between 3 and the 3-Fe3+ complex.14 The reversible binding of
Fe3+ was checked using diethylenetriamine or triethylenetetr-
triethylenetetramine, which completely restored the original
UV−vis and FL spectra of 3 (Figures S6 and S7, Supporting
Information).
Job plot data give a 3:2 ratio between 3 and Fe3+ (Figure 12),
which indicated that both the N atom and CO group might
be coordinated with Fe3+ and a polymer-like complex might be
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Figure 12. Job plots according to the method of continuous variations.
The total concentration of 3 and Fe3+ is 10−4 M.
formed. However, attempts to grow the complex crystal
between compound 3 and Fe3+ were not successful. Meanwhile,
upon the addition of Fe3+, the UV−vis absorption wavelength
of isoquinolinone 1 was blue-shifted (Figure S8, Supporting
Information) and the fluorescence was quenched (Figure S9,
Supporting Information), which indicated that the CO group
might have some coordination with Fe3+. The calculated
binding constant of 3 with Fe3+ is 1.78 × 106 M−3/2 (Figure
S10, Supporting Information). Moreover, the absorbance value
at 540 nm was linear with Fe3+ concentration in the range (1−
9) × 10−5 M. The linear regression equation was determined to
be A = (4.86 × 103)[Fe3+] − 0.04 (n = 9, R = 0.99805) (Figure
13). The detection limit was calculated to be 2.5 × 10−7 M with
Figure 14. (a) Fluorescence response of 3 (10 μM) upon addition of
different metal ions (2 equiv) in CH3CN. (b) Photograph of 3 (10
μM) with different metal ions.
fluorescence without a change in the emission wavelength and
Zn2+ led to fluorescence intensity enhancement with the
emission wavelength red-shifted to 560 nm. Other metal ions
showed little change in emission wavelength and emission
intensity. Figure 15 gives the fluorescence response of 3 (50
μM) upon addition of different concentrations of Fe3+.
Figure 15. Fluorescence response of 3 (50 μM) upon addition of
different concentrations of Fe3+ (2 equiv) in CH3CN. The inset plots
the fluorescence intensity at 554 nm versus Fe3+ concentration.
Figure 13. Plot of absorption value at 540 nm versus the
concentration of Fe3+.
Unlike 6-azaisoquinolinone, which could be used as a highly
selective colorimetric probe for Fe3+, 7-azaisoquinolinone
shows a very different optical response to Fe3+. Figure 16a
shows the UV−vis absorption spectra upon the addition of
different metal ions. Obviously, the addition of Fe3+ induced a
smaller red shift (Δλ = 18 nm) in comparison to that for 6-
azaisoquinolinone (Δλ = 70 nm), while other metal ions only
induce a slight shift of the absorption wavelength. A
photograph (Figure 16b) shows that the originally light yellow
the equation detection limit = 3Sd/ρ, where Sd is the standard
deviation of blank measurement and ρ is the slope of the
absorbance intensity versus Fe3+ concentration.15
Figure 14 shows the fluorescence spectra of compound 3 (10
μM) in CH3CN upon the addition of different metal ions. On
the addition of 2 equiv of Fe3+ to the solution, the fluorescence
of the solution was quenched partially and red-shifted from 554
to 618 nm, while Cu2+ and Ni2+ partially quenched the
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CONCLUSIONS
■
In summary, we have synthesized and fully characterized two
novel azaisoquinolinones: 6- and 7-azaisoquinolinone. The-
oretical calculations and electrochemical results showed that the
N atom at the 6-position of the isoquinolinone structure had a
greater contribution to the LUMO, while 7-position sub-
stitution contributes more to the HOMO. Our research showed
that the 6-azaisoquinolinone exhibited a larger red shift in UV
absorption and fluoroscence in comparison to 7-azaisoquino-
linone during protonation. More interestingly, 6-azaisoquino-
linone can be used as a colorimetric probe for Fe3+, while the 7-
azaisoquinolinone did not show any color change. All the above
results clearly demonstrated that the N-substituted positions
have a great effect on the optical properties. Future work in our
group will focus on using the 6- and 7-azaisoquinolinones as
building blocks to construct larger N-containing heteroacenes
and trying to understand how N-positions affect the properties
of targeted N-heteroacenes.
EXPERIMENTAL SECTION
■
General Procedure. The stock solutions (10 mM) of chloride
salts of Fe3+, Cu2+, Zn2+, Ni2+, Co2+, Hg2+, Ca2+, Mg2+, Na+, K+, and Li+
and nitrate salts of Pb2+, Mn2+, and Ag+ were prepared in deionized
water. Stock solutions (1 mM) of compounds 1−3 were prepared in
acetonitrile.
Figure 16. (a) UV−vis absorption spectra of 2 (50 μM) upon addition
of different metal ions (2 equiv) in CH3CN. (b) Photograph of 2 (50
μM) with different metal ions.
Optical Study. A 3.0 mL portion of acetonitrile solvent was placed
in a quartz cell (10.0 mm width), and 150 μL (for UV−vis) or 30 μL
(for FL) of compounds 1−3 was added. To these solutions was added
30 μL (for UV−vis) or 6 μL (for FL) of metal ions. For all
measurements, the excitation wavelength was at 450 nm for 1 and 2,
and 470 nm for 3. The slit width for both excitation and emission was
5.0 nm.
solution (5 × 10−5 M) was nearly unchanged after the addition
of different metal ions, including Fe3+. The fluorescence change
seems more interesting. Unlike the case for compound 3, which
shows partial fluorescence quenching and a red shift upon the
addition of Fe3+, the fluorescence intensity of compound 2 was
enhanced while the emission wavelength was also red-shifted
from 531 to 555 nm. Figure 17b shows that the emission color
changes from blue to orange after the addition of 2 equiv of
Fe3+.
Synthesis. Compound 2-1. Under an atmosphere of dry argon,
mesoionic pyrimidine 3 (2 g, 6.83 mmol) and excess isoamyl nitrite (6
mL) were added to 150 mL of 1,2-dichloroethane. The mixture was
heated to reflux, and then 3-aminopyridine-4-carboxylic acid (1.14 g,
8.26 mmol) dispersed in 50 mL of 1,2-dichloroethane was added to
the mixture dropwise. After addition, the mixture was refluxed for 12 h.
Then, the mixture was cooled to room temperature and the solvent
was removed under reduced pressure. The resultant residue was
purified by silica gel column chromatography using CH2Cl2/CH3OH
(100/1, v/v) as eluent to afford compound 2-1 as a white solid (630
mg, 25%). Mp: decomposition at 195 °C. IR (KBr, cm−1): ν 3035,
1
2958, 1716, 1675, 1581, 1477, 1414, 1375, 1206, 1010, 756, 704. H
NMR (CDCl3, 300 MHz): δ 2.72 (s, 6H), 6.87 (d, 1H, J = 6 Hz),
7.47−7.53 (m, 3H), 7.66−7.70 (m, 3H), 7.86 (d, 2H, J = 6 Hz), 8.13
(d, 2H, J = 6 Hz), 8.46 (d, 1H, J = 6 Hz), 8.75 (s, 1H). 13C NMR
(CDCl3, 75 MHz): δ 34.2, 66.5, 80.1, 120.8, 127.8, 128.3, 129.4, 130.1,
130.5, 130.6, 131.4, 135.9, 141.6, 148.1, 149.7, 171.0. HR-MS: [M +
H]+ calcd 370.1556, found 370.1554. Anal. Calcd for C23H19N3O2: C,
74.78; H, 5.18; N, 11.37. Found: C, 74.56; H, 5.02; N, 11.56.
Compound 3-1. The same procedure was used as for the synthesis
of 2-1: white solid, yield 20%. Mp: decomposition at 215 °C. IR (KBr,
cm−1): ν 3053, 2943, 1716, 1677, 1583, 1452, 1414, 1379, 1196, 1087,
1
1006, 833, 753, 698. H NMR (CDCl3, 300 MHz): δ 2.69 (s, 6H),
7.46−7.56 (m, 4H), 7.64−7.72 (m, 3H), 7.80−7.83 (m, 2H), 8.12 (s,
1H), 8.17−8.19 (m, 2H), 8.59 (d, 1H, J = 6 Hz). 13C NMR (CDCl3,
75 MHz): δ 34.1, 64.6, 80.3, 115.9, 127.8, 128.3, 129.4, 130.1, 130.3,
130.5, 130.6, 131.3, 133.9, 147.0, 148.4, 148.9, 171.4. HR-MS: [M +
H]+ calcd 370.1556, found 370.1554. Anal. Calcd for C23H19N3O2: C,
74.78; H, 5.18; N, 11.37. Found: C, 74.62; H, 5.12; N, 11.46.
7-Aza-2-methyl-1,4-diphenylisoquinolin-3(2H)-one (2). A neat
sample of compound 2-1 (300 mg, 0.81 mmol) was purged three
times using nitrogen before heating at 195 °C under vacuum for 6 h to
give a yellow solid (250 mg, nearly 100%). Mp: 251−253 °C. IR (KBr,
cm−1): ν 3056, 1629, 1609, 1532, 1440, 1294, 1024, 817, 758, 706. 1H
NMR (CDCl3, 300 MHz): δ 3.60 (s, 3H), 7.07 (d, 1H, J = 6 Hz),
Figure 17. (a) Fluorescence response of 2 (10 μM) upon addition of
different metal ions (2 equiv) in CH3CN. (b) Photograph of 2 (10
μM) with different metal ions.
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7.40−7.51 (m, 7H), 7.65−7.67 (m, 3H), 7.95 (d, 1H, J = 6 Hz), 8.34
(s, 1H). 13C NMR (CDCl3, 75 MHz): δ 36.2, 113.5, 115.2, 118.8,
127.7, 128.5, 128.9, 129.5, 130.6, 130.8, 131.3, 134.4, 139.2, 144.8,
155.2, 155.3, 160.2. MALDI-TOF: [M + H]+ calcd 313.1341, found
313.1340. Anal. Calcd for C21H16N2O: C, 80.75; H, 5.16; N, 8.97.
Found: C, 80.70; H, 5.02; N, 8.79.
6-Aza-2-methyl-1,4-diphenylisoquinolin-3(2H)-one (3). A neat
sample of compound 3-1 (300 mg, 0.81 mmol) was purged three
times using nitrogen before heating at 215 °C under vacuum for 6 h to
give a red solid (240 mg, nearly 100%). Mp: 202−204 °C. IR (KBr,
cm−1): ν 3056, 1627, 1604, 1520, 1436, 1371, 1292, 1024, 750, 700.
1H NMR (CDCl3, 300 MHz): δ 3.63 (s, 3H), 6.58 (d, 1H, J = 6 Hz),
7.38−7.46 (m, 3H), 7.52−7.55 (m, 4H), 7.63−7.65 (m, 3H), 7.79 (d,
1H, J = 6 Hz), 8.88 (s, 1H). 13C NMR (CDCl3, 75 MHz): δ 36.7,
116.4, 118.1, 122.9, 128.2, 128.5, 129.0, 129.5, 130.3, 131.1, 132.2,
133.5, 132.5, 137.1, 150.4, 151.8, 160.5. HR-MS: [M + H]+ calcd
313.1341, found 313.1340. Anal. Calcd for C21H16N2O: C, 80.75; H,
5.16; N, 8.97. Found: C, 80.67; H, 5.10; N, 8.86.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, a table, and CIF files giving crystal data of 2-1,
simulated UV−vis and FL spectra, and original FTIR, 1H NMR,
13C NMR, and HR-MS spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for Q.Z.: qczhang@ntu.edu.sg.
■
Notes
The authors declare no competing financial interest.
(8) (a) Gu, P. Y.; Zhou, F.; Gao, J.; Li, G.; Wang, C.; Xu, Q. F.;
Zhang, Q.; Lu, J.-M. J. Am. Chem. Soc. 2013, 135, 14086. (b) Li, G.;
Zheng, K.; Wang, C.; Leck, K. S.; Hu, F.; Sun, X. W.; Zhang, Q. ACS
Appl. Mater. Interfaces 2013, 5, 6458. (c) Li, G.; Long, G.; Chen, W.;
Hu, F.; Chen, Y.; Zhang, Q. Asian J. Org. Chem. 2013, 2, 852. (d) Li,
G.; Wu, Y. C.; Gao, J. K.; Li, J. B.; Zhao, Y.; Zhang, Q. C. Chem. Asian
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(f) Zhao, J.-F.; Li, G.; Wang, C.-Y.; Chen, W.-Q.; Chye, S.; Loo, J.;
Zhang, Q.-C. RSC Adv. 2013, 3, 9653. (g) Li, G.; Wu, Y. C.; Gao, J. K.;
Wang, C. Y.; Li, J. B.; Zhang, H. C.; Zhao, Y.; Zhao, Y. L.; Zhang, Q.
C. J. Am. Chem. Soc. 2012, 134, 20298. (h) Wu, Y. C.; Yin, Z. Y.; Xiao,
J. C.; Liu, Y.; Wei, F. X.; Tan, K. J.; Kloc, C.; Huang, L.; Yan, Q. Y.;
Hu, F. Z.; Zhang, H.; Zhang, Q. C. ACS Appl. Mater. Interfaces 2012, 4,
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ACKNOWLEDGMENTS
■
Q.Z. acknowledges financial support from AcRF Tier 1 (RG
16/12) and Tier 2 (ARC 20/12 and ARC 2/13) from the
MOE and the CREATE program (Nanomaterials for Energy
and Water Management) from the NRF.
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