1623
the DDQ-adduct 7a was treated with 50 equivalents of £-
terpinene at 100 °C in toluene to obtain the pentacene dendrimer
8a, the product of the retro-Diels-Alder reaction, in 90% yield.
In order to investigate the effect of the dendritic wedges of 8a,
we synthesized the pentacene derivative 8b having four 3,5-
dimethoxybenzyl groups at the 2,3,9,10-positions in the same
way. In marked contrast to pristine pentacene, the pentacene
dendrimers 8a and 8b were very soluble in various organic
solvents (toluene, benzene, chloroform, dichloromethane, ace-
tone, and ethyl acetate), although pentacene derivatives, which
are soluble in acetone and/or ethyl acetate, are quite rare. A
solid-state sample of the dendrimer 8a was stable for several
months at 4 °C in the dark.
(310 nm) of 8a is stronger than that of 5a (Figure S2).19 Although
the reason for the increasing fluorescence intensity of 8a is not
clear at present, the fluorescing property of 8a might be attributed
to the dendritic architecture.
We investigated the stability of the pentacene dendrimers 8a
and 8b in toluene by monitoring the change in their UV-vis
absorption with time upon exposure to ambient white light
(- = 400-650 nm) and air (Figure S3).19 The initial purple color
of the solution changed smoothly to an orangish tint, and then to
pale yellow within 30 min. The half-life (t1/2) of the pentacene
dendrimer 8a was estimated to be 22.4 min,17 in marked contrast
to the t1/2 value of pristine pentacene (t1/2 µ 3 min).11d The t1/2
value of 8b was estimated to be 7.4 min, which is shorter than
that of 8a but longer than that of pristine pentacene. Hence, the
site-isolation effect of the dendritic wedges of the dendrimer 8a
might be important for the stabilization of the central ring of
pentacene.
Despite the difference in reactivity in the central-, middle-,
and outer-ring of pentacene, there has been no clear-cut
experimental report on the regioselective reaction employing
pentacene derivatives of which the C1, C4-8, and C11-14
positions, which are potentially reactive in [4 + 2] cycloaddi-
tion, are unsubstituted, with the exception of pristine pentacene
and 2,3,9,10-tetrakis(trimethylsilyl)pentacene.12a After the pho-
tolysis of 8a, the 6,13-endoperoxide 9a was obtained quantita-
tively and isolated by column chromatography (SiO2, eluent:
chloroform) (Scheme 3). The structure of 9a was confirmed by
1H and 13C NMR spectroscopy and MALDI-TOF MS. The
1H NMR spectrum of 9a showed a peak of the bridgehead
protons at 6.34 ppm. The aromatic ring protons of the naph-
thalene moiety appeared at 7.99 and 8.27 ppm. The 13C NMR
results showed two bridgehead carbons at 67.5 ppm.15,18 The
MALDI-TOF MS of 9a showed a molecular ion peak at m/z
2174.01 (9a, C126H118O34 requires m/z 2174.75) using negative-
ion mode. On the other hand, photolysis of 8b gave the
endoperoxide 9b (89%) together with the pentacene quinone 10
(11%) as judged by the 1H NMR spectra. The difference in
photoreactivity between 8a and 8b can probably be interpreted
in terms of the instability of 9b. In marked contrast to the
chemoselective photolysis of dendrimer 8a, other end-substi-
tuted pentacenes transformed to many products such as photo-
dimers, endoperoxides, and their decomposition products.15
Hence, the dendritic wedges of 8a and 8b might prevent
The structures of the pentacene dendrimers 8a and 8b were
1
confirmed by H and 13C NMR spectroscopy, UV-vis and IR
spectrometry, and MALDI-TOF mass spectrometry (MS). The
1H NMR spectrum of 8a in CDCl3 showed three singlet signals
at 8.42, 8.80, and 9.06 ppm, assigned to protons attached to the
C1 (C4, C8, and C11), C5 (C7, C12, and C14), and C6 (C13)
positions of the pentacene core, respectively. In the 13C NMR
spectrum of 8a, six sp2 carbons of the pentacene ring were
observed in the range 127.7 to 139.1 ppm. The MALDI-TOF
mass spectrum of 8a in the positive-ion mode showed a
molecular ion peak at m/z 2143.71 [M + H]+ (C126H119O32
requires m/z 2143.76).
The UV-vis spectra of 8a and 8b in CHCl3 exhibited
absorptions at 510, 547, and 593 nm (Figure S1).19 Fluorescence
maxima at 612 (for 8a under excitation at 540 nm) and 609 nm
(for 8b under excitation at 540 nm) were observed in CHCl3
(Figure 1). The electron-withdrawing ester connections of the
dendritic substituents of 8a and 8b cause bathochromic shifts in
the absorption and fluorescence spectra compared with pristine
pentacene, for which absorption bands appear at 495, 530, and
578 nm15 and an emission band is observed at 580 and 631 nm.16
It is notable that the fluorescence intensity (-ex = 580 nm) of 8a
was stronger than that of 8b. The excitation spectrum of 8a in
CHCl3 showed a band assignable to the dendron unit at
283 nm,13b which was not observed in 8b. The emission spectrum
of 8a showed a band at 310 nm due to the dendritic subunits
(Figure S2).19 It is notable that the intensity of the emission band
Figure 1. Excitation and emission spectra of 8a (black line:
1.14 © 10¹5 M in CHCl3, -Em = 640 nm, -Ex = 580 nm) and 8b
(dotted line: 1.14 © 10¹5 M in CHCl3, -Em = 640 nm, -Ex
580 nm).
=
Scheme 3. Photooxygenation of dendrimers 8a and 8b.
Chem. Lett. 2012, 41, 1622-1624
© 2012 The Chemical Society of Japan
Tajima, Tomoyuki
