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J. Tolosa et al. / Tetrahedron Letters 47 (2006) 4647–4651
synthesis was carried out by using only 1.2 equiv of
KButO in THF at 0 ꢁC. Although the presence of the
starting material 2a and the corresponding di-coupled
products were also observed in the crude mixture, all
the monophosphonates 3 could be easily separated by
column chromatography (silica gel, EtAcO) in good
yields (68–86%). A second HWE reaction with a different
benzaldehyde derivative (1 equiv) gave the desired asym-
metrical compounds 4a,b. Ultimately, two different
functional groups are located at the periphery or the
chain end of the final branched molecules. Similarly,
using 2b as a starting reagent, compounds 3c,d and
4c,d could be synthesized. Compounds 3 and 4 were ob-
(361 nm) and trans-4-dibutylaminostilbene (432 nm),
the emission spectrum is not due to the addition of both
fluorophore responses. The maximum of the low-energy
band has experienced a bathochromic shift with respect
to those of the individual fluorophores. This red shift
may be attributed to a charge separation effect because
of the presence of both electron-withdrawing and elec-
tron-donating groups, in spite of the meta-arrangement,
as indicated by the bathochromically shifted and lower
bands observed upon increasing solvent polarity (kmax
toluene: 458 nm; dichloromethane: 548 nm; acetonitrile:
615 nm). The smaller bands at 407 and 430 nm should
be the residual fluorescence from a second excited state
for each individual chromophore.
1
tained as single all-trans isomers, as determined by H
NMR spectroscopy. Finally, compound 1 was prepared
by a double Pd-catalyzed alkynylation involving reac-
tion of 4b with trimethylsilylacetylene, removal of the
SiMe3 protecting group and subsequent coupling with
4a. This approach allows accurate control over the
placement of different functionalities (electron-donating
and electron-withdrawing groups) in two segregated and
opposed parts of the molecule, while avoiding the need
for the introduction of additional functional group pro-
tection/deprotection steps.
The study of 4d, the other half of the molecule, indicates
a quasi-complete identity (453 nm) of the fluorescent re-
sponse when compared with the trans-4-diphenylamino-
stilbene (448 nm). The response is independent of the
excitation wavelength. This fact suggests that the emis-
sion occurs only from the trans-4-diphenylaminostilbene
unit (the emission maximum for trans-4-hexyloxystil-
bene is 379 nm), indicating an efficient energy transfer
from the trans-4-hexyloxystilbene moiety (Fig. 1). Here,
the charge separation does not take place, as expected,
between the two electron-releasing NPh2 and OC6H13
groups.
First, we recorded the absorption and emission spectra
of each separate half of the target molecule, that is, 4c
and 4d. The absorption spectrum of 4c—the half bearing
the electron-withdrawing group (CF3)—consists of the
superposition of each chromophores showing two max-
ima at 317 and 364 nm. The emission spectrum in
CH2Cl2, exciting at 317 nm, is characterized by two
bands (inset Fig. 1). One is centered at approximately
407 nm, while the maximum of the more intense band
appears around 548 nm. When exciting at 364 nm, the
band at 407 nm disappears, showing a shoulder at ca.
430 nm instead. In comparison to those of models taken
for a single arm, such as trans-4-trifluoromethyl-
The absorption spectrum of the four-armed tolane 1 is a
simple superposition of the absorptions due to the five
independent chromophores (stilbene and tolane moie-
ties) and shows a continuous absorption from 250 to
440 nm. The absorption maxima are observed at
308 nm (e = 97,500) and 373 nm (e = 65,500). The emis-
sion spectra obtained by irradiation at different wave-
lengths are the same regardless of the excitation
frequency, with a maximum at 536 nm and a shoulder
at ca. 470 nm (Figs. 1 and 2). In comparison to those
of models taken for a single arm, such as tolane
(318 nm), trans-4-trifluoromethyl- (361 nm), trans-4-
hexyloxy- (379 nm), trans-4-dibutylamino- (432 nm),
and trans-4-diphenylaminostilbene (448 nm), the emis-
sion maximum of the longest-wavelength transition of
1 is again red-shifted. This behavior can be interpreted
in terms of significant electronic interaction between
the fluorophores in the excited state. The absorbing
and emitting states are not the same, something that
has been previously demonstrated in phenylacetylene
derivatives with meta-arrangements.13
Selected data for compound 1: 1H NMR (CDCl3, 500 MHz): d 0.91
(pseudo t, 3H, J = 7.0 Hz, CH3), 0.97 (t, 6H, J = 7.5 Hz, CH3), 1.32–
1.42 (m, 8H, 4 · CH2), 1.43–1.53 (m, 2H, CH2), 1.56–1.62 (m, 4H,
2 · CH2), 1.76–1.83 (m, 2H, CH2), 3,30 (t, 4H, J = 7.5 Hz,
2 · NCH2), 3.99 (t, 2H, J = 7.0 Hz, OCH2), 6.65 (A of ABq, 2H,
J = 9.0 Hz, ArH), 6.87 (A of ABq, 1H, J = 16.0 Hz, CH@), 6.91 (A
of ABq, 2H, J = 8.5 Hz, ArH), 6.97 (A of ABq, 1H, J = 16.0 Hz,
CH@), 7.00 (A of ABq, 1H, J = 16.5 Hz, CH@), 7.03–7.10 (m, 4H),
7.10–7.17 (m, 7H), 7.20 (s, 2H, 2 · CH@), 7.25–7.30 (m, 4H), 7.39–
7.43 (m, 4H), 7.47 (B of ABq, 2H, J = 8.5 Hz, ArH), 7.53–7.59 (m,
5H), 7.61–7.64 (m, 5H). 13C NMR and DEPT (CDCl3, 125 MHz): d
159.1 (C), 148.1 (C), 147.6 (C), 147.5 (C), 140.6 (C), 139.1 (C), 138.3
(C), 138.2 (C), 137.1 (C), 131.1 (C), 130.5 (CH), 130.3 (CH), 129.6
(C), 129.4 (q, J = 32 Hz, C), 129.4 (CH), 129.3 (CH), 129.2 (CH),
129.2 (CH), 128.6 (CH), 128.0 (CH), 127.9 (CH), 127.8 (CH), 127.6
(CH), 127.5 (CH), 126.7 (CH), 126.0 (CH), 125.7 (q, J = 4 Hz, CH),
125.4 (CH), 124.6 (CH), 124.5 (CH), 124.5 (CH), 124.2 (q,
J = 270 Hz, CF3), 124.0 (C), 123.9 (C), 123.8 (C), 123.4 (CH),
123.1 (CH), 123.0 (CH), 122.2 (CH), 114.8 (CH), 111.6 (CH), 89.4
(C„), 89.2 (C„), 68.1 (OCH2), 50.8 (NCH2), 31.6 (CH2), 29.5
(CH2), 29.2 (CH2), 25.7 (CH2), 22.6 (CH2), 20.4 (CH2), 14.0 (CH3),
14.0 (CH3). MALDI-TOF, m/e 1049.5 (M++1). Anal. Calcd for
In any case, the four-armed tolane shows a single char-
acteristic response upon excitation over a wide spectral
range. The obtained profiles indicate that some energy
transfer must occur from one half of the molecule to
the other. However, in this case the energy transfer is
not as efficient as in compound 4d. Emission occurs
mainly at 536 nm, a band, which resembles that of com-
pound 4c, while the shoulder at ca. 470 nm is the resid-
ual fluorescence of the half related with 4d.
Excitation spectra of 1 observed at different emission
wavelengths reveal more evidence for energy transfer.
Thus, for example, observing fluorescence at 610 nm,
C
73H71F3N2O: C, 83.55; H, 6.82; N, 2.67. Found: C, 83.80; H, 6.62;
N, 2.71.