Imine formation as a simple reaction to construct copper-reactive cruciform fluorophores

Article abstract of DOI:10.1021/jo302456n

We developed a series of new conjugated cruciform fluorophores (XF) featuring imine groups. The condensation of an XF containing aldehyde functionalities and selected primary amines leads to several XF-imine derivatives. Upon addition of Cu²⁺ or Zn²⁺ ions to solutions of the imine XFs in different solvents, a red-shifted emission is detected, resulting in an altered emission color. The imine acts as a simple modular metallo-reactive fluorophore.

Full text of DOI:10.1021/jo302456n

Article
pubs.acs.org/joc
Imine Formation as a Simple Reaction to Construct Copper-Reactive
Cruciform Fluorophores
Thimon Schwaebel, Vera Schafer, Jan Wenz, Benjamin A. Coombs, Juan Tolosa, and Uwe H. F. Bunz*
̈
Organisch Chemisches Institut, Ruprecht Karls-Universitat Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
̈
S
* Supporting Information
ABSTRACT: We developed a series of new conjugated
cruciform fluorophores (XF) featuring imine groups. The
condensation of an XF containing aldehyde functionalities and
selected primary amines leads to several XF-imine derivatives.
Upon addition of Cu2+ or Zn2+ ions to solutions of the imine
XFs in different solvents, a red-shifted emission is detected,
resulting in an altered emission color. The imine acts as a
simple modular metallo-reactive fluorophore.
INTRODUCTION
RESULTS AND DISCUSSION
■
■
Cross-shaped aromatic species1−5 can be competent, operation-
ally functional, reactive fluorophores.6−11 We have specifically
investigated 1,4-distyryl-2,5-bis(arylethynyl)benzenes (XF) as
attractive targets. Having a unified and modularized synthesis,
XFs can be deployed in amine sensing12 and differentiation of
organic acids13 but are also metallo reactive, particularly if they
carry dialkylamino or pyridine functions on their traverses.
Consequently, consanguine groups of XFs with smoothly
tunable properties to detect and/or discern one or a group of
analytes should be easily created. Here, we introduce a
surprisingly simple way to postfunctionalize aldehyde-substi-
tuted XFs into metallo-responsive fluorophores by simple
addition of different primary amines and formation of imines.
The difference in response of an analyte class to one
fluorophore often leads to ambiguities in the identification of a
specific analyte; however, the usage of several consanguine
fluorophores allows easier identification of an analyte, allowing
discrimination. On the other hand, to develop a unique response
to a specific fluorophore it is also advantageous to have fluorophore
libraries with similar properties (same functional group) but with
differential responses to closely related analytes.14
Scheme 1 displays the synthesis of the platform 5. Starting from
1,17,18 a Horner reaction with 2 gives rise to the formation of 3 in
Scheme 1. Synthesis of Seven Different Imines 6a−g Starting
from the Bisaldehyde 5
To sense amines, we have employed an aldehyde-function-
alized donor-acceptor XF or an aldehyde-substituted distyr-
ylbenzene experiencing fluorescence turn-on upon exposure.15,16
Fluorescence turn-on was due to the rapid formation of the
imine. Consequently, aldehydes such as 5 are transformed into
functional imines when treated with suitable primary amines.
Here we present a series of seven consanguine XF-based imines
that display a ratiometric signal when exposed to Cu2+ or Zn2+
salts. Their color change (and concomitant spectroscopic
changes) is dependent upon the used amine, i.e., the formed
imine. The metallo response of the XF is tailored in the last, post-
functionalization step. In principle, this approach is attractive for
the generation of functional fluorophores.
Received: November 8, 2012
Published: November 29, 2012
© 2012 American Chemical Society
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80% yield. Sonogashira reaction of 3 with 4 and deprotection
furnishes the light yellow dialdehyde 5 in a total yield of 75%; 5
can be prepared in gram quantities but is reactive. It gives the
dimethyl acetal after 1 day, when dissolved in methanol, even
without the addition of acid. When 5 is dissolved in the amines
a−g, the imines 6a−g form smoothly. While chromatography
leads to hydrolysis, the imines are purified by precipitation from
methanol and/or crystallization; 6a−g give the correct NMR
spectroscopic data. The imines 6 are only weakly solvatochromic,
as expected for acceptor-substituted distyrylbenzene derivatives
(Figure 1, top row, small strips).
From Figure 1 some trends can be gleaned. If an excess of
Cu(OTf)2 is used, imines 6a−g give a color reaction in all
solvents with exception of methanol, DMSO, and DMF. Hexanes
are problematic, but for reasons of lack of solubility of the metal
salt and the coordinated fluorophore. The imines 6a−e change
color from blue to yellow or orange, when Cu(OTf)2 is
coordinated. The imines 6f and 6g, however, only display a slight
shift in color even when exposed to a large excess of Cu(OTf)2.
Both imines share a formal 1,2-diamine as common structural
element. A possible reason for the decreased emission wave-
length shift is the combination of electronegative (−I) and
coordination effects of the nitrogen substituents, which simul-
taneously effective, lead to a lowered positive partial charge on
the imine nitrogen, when coordinating to Cu2+ ions. This leads to
a counterintuitive situation that strong binding is accompanied
by a small ratiometric signal.
How do substituents attached to the imine nitrogen affect the
binding to the metal cations? We investigated 6b−g in the
presence of Cu(OTf)2 acetonitrile. Figure 2 displays the
absorption and emission spectra of the XF-imines 6b−g in
acetonitrile before (thick black line) and after (red line) the
addition of Cu(OTf)2. The XFs themselves without the metal
salts have roughly all the same absorption and emission
characteristics. To observe metal binding, we added 100 equiv
of Cu(OTf)2 to a 1 μM solution of XF in acetonitrile observing
the unbound and the bound species with an additional band in
the absorption spectra for 6b, 6c, and 6e. The absorption spectra
of 6d, 6f, and 6g did not change for reasons that are not entirely
clear.6−11 Excitation at an isosbestic point shows for 6b, 6c, and
6e a new band appearing at around 600 nm in the emission for 6d
and 6g quenching and for 6f increased fluorescence intensity. If
the excitation wavelength is chosen to be in the new absorption
region, emission spectra of the low intensity band at 600 nm were
recorded for 6b−f.
Table 1 displays the spectroscopic data of the imines 6b−g and
their metal adducts from Figure 2. The major component of the
fluorescent lifetimes is around 0.8−2.5 ns for all of the imine
fluorophores 6b−g, quite typical for distyrylbenzene deriva-
tives.11,21 The emission lifetime changes differently for the
fluorophores upon coordination of copper; 6b, 6c, and 6e display
similar τfl = 1.8−1.9 ns for their Cu-bound forms. The emission
decreases and is red-shifted when going from the uncoordinated
to the coordinated form. Along, the lifetimes of both signals
shorten. The addition of Cu(OTf)2 to the solution of derivatives
6d, 6f, and 6g changes their fluorescent intensity but increases
their fluorescence lifetime.
Figure 1. Addition of an excess of Cu(OTf)2 to solutions of the
modularly synthesized imines 6a−g in different solvents. The top row
displays solutions of the imines 6a−g without the added metal cations.
All of the solutions are blue fluorescent and therefore only shown as
small segments. The concentration of the imines 6a−g is 10 μM in the
different solvents. The fluorescence intensity of the blue solutions is
stronger than the emission of the orange or yellow solutions by a factor
of 10−20. For clarity, the vials have been photographed such that the
color hues are most realistically depicted, not according to the intensity
of emission.
In a simplistic way, imines are vinylogous to pyridines. As a
consequence, we imagined that coordination of metal cations to
derivatives of 6 should lead to red-shifted (and weakened)
emission. Addition of Cu(OTf)2 to 6b in different solvents
(Figure 1) resulted in emission color changes. Red shift and
decline of intensity of the emission are visible in acetonitrile,
DCM, THF, and ethanol. For Zn(OTf)2 a similar red shift of the
emission and decreased intensity are observed (see the
Supporting Information, Figure S1). When compared to each
other, the imines show fewer color changes with Zn(OTf)2 than
upon addition of Cu(OTf)2. The imine 6d, featuring an
additional coordinating dimethylamino group, gives brightly
colored responses when Cu2+ or Zn2+ salts are added. The
addition of Cu(OTf)2 to 6d in THF leads to yellow fluorescence.
The addition of additional pyridine units is a successful strategy
to increase the binding affinity of fluorophores to zinc ions19,20
resulting in a stronger color change imine in 6g. The color of
solutions of 6g changes from blue to green-yellow with Zn2+ and
with Cu2+ to green-turquoise.
To obtain more precise information about the binding abilities
of our imines, we performed titrations of 6b−g with copper-
(II)triflate (XF concentration 1.0 μM). We selected wavelengths
of increasing or decreasing signals in both absorption and
emission spectra to obtain the binding constant (Figure 3). As we
have not been able to extract a good model for the stoichiometry
of metal cation and XF, the log β obtained by the fit eq 1 is an
approximate but appropriate substitute for binding constant
values with one type of imine nitrogen for coordination:22,23
Iend − Istart
I(x) = Istart
+
1 + 10(log β−x)n
(1)
I(x) is the relative intensity (absorption or fluorescence) at a
given copper concentration [Cu2+], x is the negative logarithm
of the copper concentration −log [Cu2+] and Istart and Iend are the
relative intensities at the beginning and the end of the titration.
Log β is at the inflection point, where the difference of intensity
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Figure 2. Addition of 100 equiv of Cu(OTf)2 to solutions of 6b−g in acetonitrile. (Left) Absorption and emission spectra with their measured
intensities. (Right) Normalized absorption and emission spectra. Emission spectra were recorded with the excitation at the isosbestic point.
Table 1. Photophysical Properties of Imines 6b−g in Acetonitrile and in the Presence of Cu(OTf)2
a
compd λmax abs (nm) λmax abs (+ 100 equiv of Cu2+) (nm) λmax em (nm) λmax em (+ 100 equiv of Cu2+) (nm) λem (nm)
τ (ns) τ (+ 1000 equiv of Cu2+) (ns)
6b
339, 383
341, 410
437, 458
459, 590sh
460
600
460
600
470
460
600
460
460
1.82
1.57, 2.76
0.43
1.83
6c
339, 384
350, 410
435, 457
457, 598
1.77
0.72, 2.59
0.52
1.81
6d
6e
332
331
435, 458
435, 458
461
1.03
1.97, 3.26
1.21, 2.70
0.54
338, 382
342, 403
461, 600sh
1.92
1.88
6f
338, 379
339, 386
338, 382sh
340, 390
461
460
461
0.79, 2.53
1.80
2.33, 3.23
2.19, 3.17
6g
437, 459
a
Excitation wavelength 376 nm.
values of the derivatives are between 5.1 and 6.0. Having only the
imine nitrogen, 6b has the smallest log β (average 5.35). Having
additional heteroatoms for 6c−g, the average log β values
increases to 5.77 (6f) and 5.96 (6d, 6g). Both values indicate that
the presence of a chelating nitrogen increases the binding
significantly. Consequently, the imine 6g formed from picolyl-
amine is particularly efficient in the detection of copper cations as
suggested by the work of Urano et al. for other pyridine
containing ligands.21
of Istart and Iend is half. The value n describes the slope of the curve
and does not necessarily correlate with the stoichiometry of the
formed complex.
Table 2 displays the parameter log β for 6b−g resulting from
the nonlinear least-squares fit of selected experimental data.
Some of the titration plots of 6d and 6f display a different shape.
Therefore, eq 1 could not be used for fitting those data. For these
curves a sigmoidal fitting function with two possible inflection
points was applied (see the Supporting Information), or these
curves were not used at all. From these nonlinear least-squares
fits only the first log β was used. This value is in good accordance
with the other log β values resulting from fits using eq 1. The log
β value describes the binding of the imine XF to copper. All log β
CONCLUSIONS
■
We efficiently prepared seven consanguine, fluorescent imines
6a−g in one step by reaction of amines a−g with the dialdehyde
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Figure 3. (Top) Absorption spectra and emission spectra recorded with different excitation wavelengths for the titration of 6b with Cu(OTf)2.
(Bottom) Relative titration plots from the absorption and emission spectra for selected wavelengths (solid squares) and the nonlinear least-squares fit
(red line) of the experimental data to eq 1.
Table 2. Binding Data Obtained from the Titration for
Selected Wavelengths of Absorption and Emission Data
escape our notice that this modular system should be useful for
the synthesis of any type of reactive fluorophore that can be
constructed by a sensory appendage and a fluorescent nucleus
using imine formation.24,25
6b
log β
6c
log β
6e
log β
Abs (nm) 300
Abs (nm) 450
5.53
5.35
5.47
5.30
5.08
log β
292
450
5.60
5.63
5.80
5.37
5.18
log β
290
450
5.43
5.54
5.68
5.39
5.30
log β
EXPERIMENTAL SECTION
■
a
c
d
Em (nm) 458
Em (nm) 650
457
625
458
650
600
6g
1H NMR spectra were recorded on 250 and 300 MHz spectrometers.
a
c
d
b
13C NMR spectra were recorded on a 75 MHz spectrometer. The
b
b
Em(nm) 606
600
HRMS measurements were performed using systems with quadruple
ionic trap or time-of-flight mass spectrometers. Time-correlated single
photon counting lifetime measurements were made using a pulsed laser
diode (wavelength 376 nm).
6d
6f
Abs (nm) 292
Abs (nm) 450
6.04
5.97
6.09
5.74
290
5.80
5.87
5.64
300
5.82
6.02
6.05
450
440
e
b
g
1,4-Diiodo-(E)-2,5-bis(4-(1,3-dioxolan-2-yl)styryl)benzene
(3): Compound 1 (3.40 g, 19.0 mmol) was dissolved in water-free THF
(150 mL) under N2 atmosphere. Then 2 (6.00 g, 9.5 mmol) and KOtBu
(2.00 g, 19.0 mmol) were added at 0 °C. The reaction mixture was
stirred for 2 h at 22 °C. Hydrolysis and centrifugation, washing with
methanol, gave 3 (5.10 g, 80%): mp 265 °C dec; 1H NMR (250 MHz,
CD2Cl2) δ 8.14 (s, 2H), 7.59 (d, J = 8.18 Hz, 4H), 7.48 (d, J = 7.80 Hz,
4H), 7.25 (d, J = 16.31 Hz, 2H), 7.04 (d, J = 16.01 Hz, 2H), 5.79 (s, 2H),
4.15−3.99 (m, 8H); 13C NMR: the solubility of 3 was too low to obtain a
spectrum; IR (cm−1) 2946, 2868, 1606, 1455, 1426, 1382, 1361, 1332,
1298, 1284, 1240, 1219, 1175, 1112, 1086, 1039, 1014, 984, 958, 943,
925, 888, 872, 856, 834, 810, 730, 721, 655, 621, 609, 535, 502, 483, 460,
437, 420, 401; HRMS (EI) m/z calcd for C28H24I2O4 [M]+ 677.9764,
found 677.9744.
1,4-Bis(4-tert-butylphenylethynyl)-(E)-2,5-bis(4-formylstyryl)-
benzene (5): In a dried Schlenk flask 3 (2.43 g, 3.60 mmol) was added
to a degassed solution of NEt3 (2.00 mL) in water free THF (250 mL)
under N2 atmosphere. The mixture was dissolved by heating. After
cooling, 4 (1.70 g, 10.7 mmol), Pd(PPh3)2Cl2 (50.0 mg, 71.2 μmol), and
CuI (14.0 mg, 73.5 μmol) were added, and the mixture was stirred for
72 h under N2 atmosphere. The solvent was removed, and the crude
product washed with methanol twice to give a yellow solid (2.22 g,
Em (nm) 675
Em (nm) 650
635
460
f
a
b
Excitation wavelength 359 nm. Excitation wavelength 470 nm.
c
d
Excitation wavelength 355 nm. Excitation wavelength 358 nm.
Excitation wavelength 390 nm. Excitation wavelength 455 nm.
e
f
g
Excitation wavelength 360 nm.
5. The imines form readily in good to excellent yields and display
attractive and tunable binding to Cu2+ and Zn2+ salts in THF or
in acetonitrile. Coordination leads to a significant red shift in
emission, suggesting that imines 6 are potentially attractive
ratiometric fluorophores. The absorption and emission red shift
and weakening of emission in the imines upon metalation is
easily understood when looking at the imines as vinylogous
pyridines. Metal binding leads to the stabilization of the LUMO,
but influences the position of the HOMO much less. Using
different aromatic mono- or dialdehydes in combination with
suitable amines should allow to generate a whole family of
modularly constructed metal-reactive fluorophores. It did not
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84%); 1.66 g (2.3 mmol) of this material was dissolved in a THF (350
mL)/water (250 mL) mixture and concd. HCl (50.0 mL) was added.
After 24 h, the extraction with dichloromethane furnished 5 (1.30 g,
89%): mp >305 °C dec; 1H NMR (300 MHz, CDCl3) δ 10.02 (s, 2H),
7.94 (s, 2H), 7.91 (d, J = 8.3 Hz, 4H), 7.83 (d, J = 16.4 Hz, 2H), 7.72
(d, J = 8.1 Hz, 4H), 7.55 (d, J = 8.5 Hz, 4H), 7.45 (d, J = 8.5 Hz, 4H),
7.33 (d, J = 16.5 Hz, 2H), 1.37 (s, 18H); 13C NMR (75 MHz, CDCl3) δ
191.6, 152.5, 143.3, 137.2, 135.7, 131.5, 130.4, 129.5, 129.2, 129.1, 127.3,
125.8, 123.1, 120.0, 96.6, 86.9, 35.1, 31.3; IR (cm−1) 2958, 2904, 2867,
1693, 1597, 1565, 1513, 1474, 1463, 1389, 1362, 1304, 1267, 1213,
1165, 1103, 1015, 958, 860, 828, 807, 788, 735.71, 652, 626, 559, 514;
HRMS (EI) m/z calcd for C48H42O2 [M]+ 650.3184, found 650.3144.
General Procedure for Imine Formation of 5 with Amines.
A reaction mixture of 5 (300 mg, 460.9 μmol) and the amine (1.8 mmol)
was stirred in THF (20 mL) for 24 h at room temperature. Removal of
solvent followed by suspension in methanol and centrifugation followed
by washing with methanol furnished 6 as a yellow solid.
1,4-Bis(4-tert-butylphenylethynyl)-(E)-2,5-bis(4-((cyclopro-
pylimino)methyl)styryl)benzene (6a): 93% yield (314 mg); mp
>236 °C dec; 1H NMR (300 MHz, CDCl3) δ 8.44 (s, 2H), 7.88 (s, 2H),
7.71 (d, J = 16.4 Hz, 2H), 7.68 (d, J = 8.2 Hz, 4H), 7.57 (d, J = 8.3 Hz,
4H), 7.54 (d, J = 8.6 Hz, 4H), 7.43 (d, J = 8.5 Hz, 4H), 7.25 (d, J =
16.4 Hz, 2H), 3.09−3.00 (m, 2H), 1.37 (s, 18H), 1.01−0.94 (m, 8H);
13C NMR (75 MHz, CDCl3) δ 158.1, 152.2, 139.2, 137.4, 136.2, 131.5,
130.2, 128.9, 128.1, 127.1, 126.8, 125.7, 122.7, 120.2, 96.2, 87.3, 42.3,
35.0, 31.3, 9.1; IR (cm−1) 2959, 2865, 1599, 1507, 1301, 1170, 1103,
1012, 955, 884, 809, 564, 535; HRMS (ESI) m/z calcd for C54H53N2
[M + H]+ 729.4203, found 729.4213.
(E)-2,5-Bis(4-((benzylimino)methyl)styryl)-1,4-bis(4-tert-butyl-
phenylethynyl)benzene (6b): 86% yield (330 mg); mp >190 °C dec;
1H NMR (300 MHz, CDCl3) δ 8.41 (s, 2H), 7.92 (s, 2H), 7.81 (d, J =
8.2 Hz, 4H), 7.76 (d, J = 16.5 Hz, 2H), 7.63 (d, J = 8.3 Hz, 4H), 7.55 (d,
J = 8.4 Hz, 4H), 7.43 (d, J = 8.5 Hz, 4H), 7.38−7.26 (m, 12H), 4.85 (s,
4H), 1.36 (s, 18H); 13C NMR (75 MHz, CDCl3) δ 161.5, 152.0, 139.7,
139.3, 135.6, 131.3, 131.3, 129.9, 128.8, 128.7, 128.5, 128.0, 127.0, 126.9,
126.9, 125.6, 122.6, 120.0, 96.1, 87.1, 65.1, 34.9, 31.2; IR (cm−1) 3027,
2959, 2902, 2865, 2208, 1907, 1800, 1638, 1599, 1512, 1452, 1267,
1103, 1024, 957, 831, 730, 695, 559, 524; HRMS (EI) m/z calcd for
C62H56N2 [M]+ 828.4444, found 828.4444.
127.2, 126.4, 123.5, 121.1, 96.9, 87.8, 65.1, 62.6, 35.5, 31.5; IR (cm−1)
3241, 3030, 2954, 2867, 2328, 2210, 1642, 1599, 1506, 1062, 960, 831,
815, 559, 516; HRMS (ESI) m/z calcd for C52H53N2O2 [M + H]+
737.4102, found 737.4107.
1,4-Bis(4-tert-butylphenylethynyl)-(E)-2,5-bis(4-(((2-(dimethyl-
amino)ethyl)imino)methyl)styryl)benzene (6f): 80% yield (291 mg);
mp >140 °C dec; 1H NMR (300 MHz, CDCl3) δ 8.31 (s, 2H), 7.89 (s,
2H), 7.74 (d, J = 8.4 Hz, 4H), 7.73 (d, J = 16.2 Hz, 2H), 7.59 (d, J = 8.3
Hz, 4H), 7.54 (d, J = 8.4 Hz, 4H), 7.43 (d, J = 8.5 Hz, 4H), 7.27 (d, J =
16.3 Hz, 2H), 3.77 (t, J = 6.7 Hz, 4H), 2.66 (t, J = 7.0 Hz, 4H), 2.33 (s,
12H), 1.36 (s, 18H); 13C NMR (75 MHz, CDCl3) δ 161.5, 152.2, 139.7,
137.4, 135.8, 131.4, 130.1, 128.9, 128.7, 127.1, 125.7, 122.7, 120.1, 96.2,
87.2, 60.3, 60.2, 46.1, 35.0, 31.3. IR (cm−1) 2955, 2904, 2862, 2767,
2204, 1692, 1363, 1166, 959, 831, 812, 559; HRMS (ESI) m/z calcd for
C56H63N4 [M + H]+ 791.5047, found 791.5051.
1,4-Bis(4-tert-butylphenylethynyl)-(E)-2,5-bis(4-(((pyridin-2-
ylmethyl)imino)methyl)styryl)benzene (6g): 75% yield (162 mg);
mp >150 °C dec; 1H NMR (300 MHz, CDCl3) δ 8.59 (d, J = 4.2 Hz,
2H), 8.48 (s, 2H), 7.90 (s, 2H), 7.83 (d, J = 8.2 Hz, 4H), 7.75 (d, J = 16.5 Hz,
2H), 7.71−7.65 (m, 2H), 7.62 (d, J = 8.2 Hz, 4H), 7.55 (d, J = 8.4 Hz, 4H),
7.49−7.40 (m, 6H), 7.28 (d, J = 16.2 Hz, 2H), 7.22−7.16 (m, 2H), 4.98
(s, 4H), 1.36 (s, 18H); 13C NMR (75 MHz, CDCl3) 162.8, 159.5, 152.2,
149.4, 140.0, 136.8, 135.7, 134.3, 133.9, 131.5, 130.1, 129.0, 127.3, 127.1,
125.7, 122.8, 122.5, 122.2, 120.1, 96.3, 87.2, 67.1, 35.1, 31.4;. IR (cm−1)
2958, 2903, 2865, 1693, 1597, 1266, 1103, 957, 831, 812, 750, 560, 524;
HRMS (ESI) m/z calcd for C60H55N4 [M + H]+ 831.4421, found
831.4428.
ASSOCIATED CONTENT
* Supporting Information
■
S
Copies of NMR spectra, spectroscopic data and titration plots.
This material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: Uwe.bunz@oci.uni-heidelberg.de.
■
Notes
1,4-Bis(4-tert-butylphenylethynyl)-(E)-2,5-bis(4-(((1-(tert-butox-
The authors declare no competing financial interest.
ycarbonyl)piperidin-4-yl)imino)methyl)styryl)benzene (6c): 87%
1
yield (409 mg); mp >255 °C dec; H NMR (300 MHz, CDCl3) δ
ACKNOWLEDGMENTS
We thank the “Sturktur- und Innovationfonds des Landes Baden-
Wurttemberg” for generous support.
8.35 (s, 2H), 7.90 (s, 2H), 7.75 (d, J = 8.4 Hz, 4H), 7.74 (d, J = 16.2 Hz,
2H), 7.60 (d, J = 8.3 Hz, 4H), 7.54 (d, J = 8.5 Hz, 4H), 7.43 (d, J = 8.5 Hz,
4H), 7.28 (d, J = 16.3 Hz, 2H), 4.10 (d, J = 12.0 Hz, 4H), 3.40 (quin, J =
6.8 Hz, 2H), 3.01 (quin, J = 6.7 Hz, 4H), 1.75 (m, 8H), 1.49 (s, 18H),
1.36 (s, 18H); 13C NMR (75 MHz, CDCl3) δ 159.2, 155.1, 152.2, 139.8,
137.4, 135.9, 131.4, 130.1, 129.0, 128.8, 127.1, 127.1, 125.7, 122.7, 120.1,
96.2, 87.2, 79.6, 67.5, 35.0, 33.5, 33.4, 31.3, 28.4; IR (cm−1) 2964, 2864,
2202, 1685, 1426, 1364, 1234, 1167, 832, 812, 563, 532; HRMS (ESI)
m/z calcd for C68H79N4O4 [M + H]+ 1015.6096, found 1015.6101.
1,4-Bis(4-tert-butylphenylethynyl)-(E)-2,5-bis(4-(((3-(dime-
thylamino)propyl)imino)methyl)styryl)benzene (6d): 82% yield
■
̈
REFERENCES
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(207 mg); mp >181 °C dec; H NMR (300 MHz, CDCl3) δ 8.29 (s,
2H), 7.90 (s, 2H), 7.74 (d, J = 16.4 Hz, 2H), 7.74 (d, J = 8.3 Hz, 4H),
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