Palladium-Catalyzed Cycloisomerizations of Diarylacetylenes
FULL PAPER
bands. Figure 2 shows that the population of HOMO on po-
sition C6 is larger than that of LUMO on the same atom.
The electron-donating group at position C6 causes a larger
potential increase in the HOMO than in the LUMO, and
leads to a redshift in both the absorption and emission
bands. Unlike other electron-withdrawing groups, such as
fluoro and trifluoromethyl, the ester group causes a batho-
chromic shift in 3h and displays a tendency similar to that
observed in the methoxy-substituted product 3s. The exten-
sion of the p system generated by the ester group in the
LUMO, as shown in Figure 2, decreases its potential and
redshift in both the absorption and emission bands. Dialkyl-
substituted cycloadduct 7a-nBu, which lacks the aryl group
at position C1, causing shorter wavelengths in both absorp-
tion and emission bands relative to 3a. As noted above, the
dihedral angle A/Ar3 was small in compound 3p. Therefore,
Ar3 would significantly contribute to the p-system of the
backbone that causes the redshift in both the absorption
and emission bands. However, this tendency was not ob-
served when the photophysical properties of 3a, 3p, and 3q
are compared. Perhaps the effect of electron-withdrawing
substituents located at positions C5 and C7 was larger than
that of the coplanarity of A/Ar3. The absorption spectra of
compound 11j are very similar to 3a, but a solution of the
former shifts bathochromically. Like dibenzopentalenes
13,[28] compound 11j contains 4n p electrons, and it is practi-
cally nonfluorescent in either liquid or solid form. Accord-
ing to the computational results based on time-dependent
DFT (TD-DFT), its HOMO-to-LUMO transition is symme-
try forbidden (see the Supporting Information).
The aggregation-induced emission (AIE) of 3a was ob-
served in our previous study.[9,29] The solution containing 3a
in dichloromethane or THF is practically nonfluorescent,
but it becomes highly luminescent in crystal (or solid) form.
To understand this abnormal AIE, the quantum yield (FF)
of compounds 3, 6, and 7a-nBu (ca. 10À5 m) was examined in
a mixture of THF/water (Table 4). In compounds 3a–d, 3g,
3h, 3k, 3n, 3p, and 3q, fluorescence is weak to absent when
the water content is below 60% (W60). However, they
become fluorescent at W75 or higher because aggregations
of these molecules can efficiently inhibit the free vibration
and rotation of aryl groups, thus increasing quantum yield.
The photoluminescent behaviors of these compounds clearly
reveal their AIE properties. Dialkyl-substituted 7a-nBu has
a less crowded environment. Its AIE properties must be en-
hanced under more aggregated conditions. In contrast to the
aforementioned compounds, the THF solutions of 3r and 3s
are photoluminescent, and the bulky substituents of these
two molecules partly block the nonradiative pathway, even
under nonaggregated conditions. Their emission behaviors
are classified as aggregation-enhanced emission (AEE).[29]
However, as water is added to the solutions (exceeding
W75), the FF values dramatically decrease, possibly due to
the precipitation of molecules. Notably, the THF solution of
compound 6 is a bright fluorescent green (FF =49.2%),
even under ambient lighting because such bulky side arms
completely block the nonradiative pathway. Therefore, com-
pound 6 resembles a normal luminescent molecule, and its
FF value is inversely related to water content. Moreover,
the aggregation properties of compounds 3g and 3r in the
solution were analyzed by dynamic light scattering (DLS).
As expected, no significant particles are observed in their
W0 solutions, whereas 3g (W90) and 3r (W75) aggregate in
the aqueous solutions with the particle size in the 170–420
and 120–150 nm range, respectively.
Electrochemistry: Table 5 shows the redox properties of cy-
cloadducts and their analogues as characterized by cyclic
voltammetry. The first oxidation potential depends on sub-
stituent type and position and also on the extension of the
p-system in the backbone. An electron-donating substituent,
that is, the methyl and methoxy groups at the C6-position in
compounds 3 reduces the oxidation potential, whereas an
electron-withdrawing group, such as fluoro, trifluoromethyl,
and methyl ester, increases the value. However, an electron-
donating substituent at C5- and C7-positions would only
slightly reduce the oxidation potential as revealed by com-
paring 3b (0.49 V) vs. 3k (0.58 V) and 3g (0.27 V) vs. 3r
(0.69 V). These results are consistent with the computational
Table 5. Cyclic voltammetric analyses for cycloadducts.[a]
Compd.
Eox (V vs. Fe/Fe+)
Ered (V vs Fe/Fe+)
3a
3b
3c
3d
3g
3h
3k
3n
3r
3s
6
0.70
0.49, 0.87 (Ep), 0.95 (Ep)
1.12 (Ep)
0.79 (Ep), 1.04 (Ep), 1.19 (Ep)
0.27, 0.57 (Ep)
1.02 (Ep)
À2.05 (Ep)
À2.48 (Ep)
À2.00 (Ep)
0.58
0.69, 1.03 (Ep), 1.17 (Ep)
0.69 (Ep)
0.54 (Ep), 0.99 (Ep)
0.41
À0.75
À1.64
11j
0.46, 0.98 (ir)
[a]Experimental conditions: compounds (concentration ca. 10À3 m) in a
background electrolyte solution of 0.1m TBAPF6 (tetrabutylammonium
hexafluorophophate) in CH2Cl2. Working electrode: glass carbon (GC)
electrode, reference electrode: Ag/AgNO3 (0.1m AgNO3 and 0.1m N-
AHCTNUTGREGN(NNU nBu)4ClO4 in acetonitrile), counter electrode: platinum wire. Scan rate:
50 mVsÀ1. Internal standard: ferrocene.
Figure 3. The redox potentials determined by CV (open) and HOMO
and LUMO levels generated from the DFT computation (solid).
Chem. Eur. J. 2011, 17, 7220 – 7227
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