Optical switching and antenna effect of dendrimers with an anthracene core

Article abstract of DOI:10.1002/chem.200700718

Dendrimers 6Gⁱ (i=1-4) consisting of an anthracene core and Frechet dendrons which are attached via a CH₂OCH₂ chain in the 9-position undergo quantitative and completely reversible intramolecular [4π+4π] cycloaddition. The

Full text of DOI:10.1002/chem.200700718

FULL PAPER
DOI: 10.1002/chem.200700718
Optical Switching and Antenna Effect of Dendrimers with an Anthracene
Core
Derong Cao,*[a, b] Silvia Dobis,[c] Chunmei Gao,[b] Sabine Hillmann,[c] and
Herbert Meier*[a, c]
Abstract: Dendrimers 6Gi (i=1–4)
consisting of an anthracene core and
FrØchet dendrons which are attached
via a CH2OCH2 chain in the 9-position
undergo quantitative and completely
reversible intramolecular [4p+4p] cy-
cloaddition. The process can be moni-
tored by absorption and fluorescence
measurements. The FrØchet dendrons
act as an energy funnel that collects
and focuses the photon energy but
does not change the photostationary
states, which for both directions are
completely on the product side when
the separate chromophores are selec-
tively irradiated. The quantum yields
of anthracene fluorescence and of sin-
glet energy transfer from the dendrons
to the core were studied as a function
of dendrimer generation.
Keywords: cycloaddition
·
den-
drimers · energy transfer · fluores-
cence · photochemistry
Introduction
tations are possible. When the core chromophore ensures ef-
ficient and reversible photoreaction, the optical switch
(OSW) (Q) can be directly operated by irradiation with n
and n’ (Scheme 1). In addition, the large cross section of the
multidendron chromophores can be exploited, provided that
energy transfer (ET) takes place from the excited dendron
chromophores to the core (!). Depending on the excitation
energy (n’’>n’), singlet energy transfer is also possible on
the photoproduct (!). The population of triplet states can
be excluded by suitable choice of core and dendron chromo-
phores.
Molecular switches, which consist of mutually transformable
compounds with different absorption and fluorescence, are a
highly topical research field.[1–3] The concept presented here
concerns optical switching of systems which additionally
show light harvesting by means of an antenna effect.[4] The
molecular basis is provided by dendrimers whose dendrons
have a large cross section for absorption of light and effi-
ciently transfer energy to the photoreactive core chromo-
phore. In the arrangement core-chromophore–saturated
spacer–dendron-chromophore (Scheme 1) separate chromo-
phores are present that can be chosen so that selective exci-
The experimental realization discussed below is based on
dendrimers with an anthracene core, a CH2OCH2 spacer,
and poly(benzyl ether) dendrons (FrØchet dendrons).
[a] Prof. Dr. D. Cao, Prof. Dr. H. Meier
College of Chemistry
South China University of Technology
Guangzhou 510640 (China)
Fax : (+86)20-8711-0245
E-mail: drcao@scut.edu.cn
hmeier@mail.uni-mainz.de
[b] Prof. Dr. D. Cao, C. Gao
Guangzhou Institute of Chemistry
Chinese Academy of Sciences
Guangzhou 510650 (China)
[c] Dipl.-Chem. S. Dobis, S. Hillmann, Prof. Dr. H. Meier
Institute of Organic Chemistry
University of Mainz
Duesbergweg 10–14, 55099 Mainz (Germany)
Fax : (+49)6131-39-25396
Scheme 1. Optical switching (OSW) and its amplification by energy trans-
fer (ET) in dendrimer systems with a photoreactive core unit and light-
harvesting dendrons. (Representation of the case n<n’<n’’.)
Chem. Eur. J. 2007, 13, 9317 – 9323
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9317

1 þ 4 Gi ! 2 Giþ1 ! 3 Giþ1 ! 4 Giþ1 ði ¼ 0 À 3Þ
ð1Þ
Results and Discussion
Scheme 2 summarizes the structure and the preparation of
the dendrimers 6Gi (generation i=1–4) and the model com-
pound 9-[(benzyloxy)methyl]anthracene (6G0). We used a
combination of known reaction steps[5–9] for the preparation
of benzyl ether dendrons. 3,5-Dihydroxybenzoic acid methyl
ester (1) was treated with benzyl bromide (4G0) to give
ester 2G1, which was reduced with LiAlH4 to alcohol 3G1.
Reaction with PBr3 yielded the next higher bromo com-
pound 4G1 which was then used instead of 4G0 for the sub-
sequent analogous reaction sequence. The general sequence
is given in Equation (1).
Finally the dendron components, that is, the alcohols 3Gi,
were treated with 9-chloromethylanthracene (5a) to afford
target dendrimers 6Gi. The yields of this step ranged be-
tween 21 and 98% when phase-transfer conditions (KOH,
H2O/
(nC4H9)4N+BrÀ, chlorobenzene) were applied. The low
A
yield of 21% was typically for the fourth-generation 6G4,
whereby the bulky alcohol 3G4 had to attack the 9-chloro-
methyl group of 5a. Model compound 6G0 could be pre-
pared from 5a and benzyl alcohol (3G0) by using the same
type of Williamson synthesis.
Scheme 2. Structures of the target dendrimers 6Gi (i=1–4), their precursors 1, 2Gi, 3Gi, 4Gi, 5a, and model compounds 6G0 and 5b. Preparation mode
of 6Gi. a) K2CO3, Me2CO; b) LiAlH4, THF; c) PBr3, C6H5CH3; d) (nBu)4N+BrÀ, C6H5Cl/KOH, H2O.
9318
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 9317 – 9323

Dendrimers with an Anthracene Core
FULL PAPER
In reactions of 9-chloromethyl- or 9-bromomethylanthra-
cene, bis(9-anthrylmethyl) ether (7), lepidopterene (8), and
9,10-anthraquinone (9) were often observed as products or
byproducts.[10–14] They can be easily detected by their charac-
teristic intense NMR signals (Scheme 3). We found traces of
7–9 in all raw materials 6Gi, particularly in 6G1.
Scheme 3. By-products in the reaction of 5a and 3Gi and their most in-
1
tense H/13C NMR signals.
Since even small amounts of 7–9 disturb fluorescence
measurements and/or photochemical reactions, we purified
the dendrimers 6Gi extremely carefully (see Experimental
Section).
The spectroscopic characterization of dendrimers 6Gi is
based on homo-and heteronuclear 2D NMR measurements
(COSY, HMQC, HMBC). Scheme 4 shows as an example
the complete assignment of all H and 13C NMR signals of
1
Scheme 4. 1H and 13C NMR data of 6G2 (d values in CDCl3 relative to
TMS as internal standard).
1
6G2 to certain nuclei. Table 1 summarizes the H NMR data
of 6Gi (i=0–4), which demonstrate the uniformity of the
compounds.
The absorption spectra of dendrimers 6Gi show two dis-
tinct and well-separated regions, namely, the anthracene
(Chr-1) absorption with the typical vibrational structure be-
tween 300 and 410 nm (p band) and the absorption of the
benzenoid pp* transitions below 300 nm (measurement in
CH2Cl2). The range between 250 and 300 nm contains not
only the long-wavelength bands of the 1,3,5-trisubstituted
benzene rings (Chr-2) and the terminal benzene rings (Chr-
3), but also the b and the hidden a band of the anthracene
moiety (S0!S2, S3). Due to the saturated linkers, the three
aromatic moieties of the dendrimer molecules can be re-
garded as independent chromophores. However, the ratio of
the three chromophores Chr-1,2,3 present in 6Gi depends
on the generation i: anthracene (Chr-1):1,3,5-trisubstituted
benzene rings (Chr-2):terminal benzene rings (Chr-3)=1:-
regioselective head-to-tail process.[16,17] A monomolecular
photoreaction could not be found, even at high dilution. The
dendrimers 6Gi (i=1–3) behave completely differently.[18]
Highly purified samples in dilute solution in benzene under-
go intramolecular [4p+4p] cycloaddition of the central ben-
zene ring of the anthracene unit and the inner benzene ring
of the dendron (Scheme 5). This quantitative process leads
to complete disappearance of the anthracene absorption.
Anthracene fluorescence, which competes with the photore-
action, disappears simultaneously. The difference between
the photoreactivity of 6G0 and 6Gi (iꢀ1) is due to the elec-
tron density of the inner benzene ring. Only a high electron
density in the 4-position of this ring guarantees a completely
selective intramolecular process. We assume either an “in-
tramolecular exciplex” E, which is stabilized by charge
transfer, or a dipolar, covalently fixed intermediate I, ob-
tained as a consequence of a photo-electron-transfer (PET)
mechanism. These assumptions are consistent with an al-
A
of 6G2 and its fluorescence.
Irradiation (lꢀ300 nm) of 6G0 leads to the well-known
[4p+4p] cyclodimerization.[15] In this case we found a highly
Table 1. 1H NMR data of the compounds 6G0À4 (d values in CDCl3, TMS as internal standard).
Compound
CH2
Aromat. CH
4-H
m (2H)
a
b
g
d
e
z
1-H
m (2H)
2-H, 3-H, C6H5
m
10-H
s (1H)
C6H3
m
s (2H)
s (2H)
s (4H)
s (8H)
s (16H)
s (32H)
6G0
6G1
6G2
6G3
6G4
5.49
5.49
5.48
5.48
5.48
4.72
4.65
4.64
4.63
4.56
8.31
8.33
8.31
8.32
8.36
7.31–7.54 (9H)
7.28–7.52 (14H)
7.30–7.50 (24H)
7.25–7.50 (44H)
7.20–7.45 (84H)
8.01
8.01
7.99
7.96
8.00
8.46
8.46
8.45
8.43
8.45
4.98
4.91
4.91
4.92
6.55–6.67 (3H)
6.53–6.65 (9H)
6.50–6.70 (21H)
6.40–6.70 (45H)
5.00
4.93
4.94
5.00
4.97
4.99
Chem. Eur. J. 2007, 13, 9317 – 9323
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9319

H. Meier, D. Cao et al.
In addition to direct excitation of Chr-4, efficient energy
transfer
S1
A
(Chr-4)!S1
G
G
seems to be plausible, since the overlap criterion of donor
deactivation energy (fluorescence or radiationless decay)
and acceptor excitation energy is ideally fulfilled.[20] Direct
proof of an energy transfer S
1)+S0(Chr-2,3) is possible by monitoring the anthracene
fluorescence [S (Chr-1)]. Scheme 6 demonstrates
1(Chr-1)!S0
A
(Chr-1)!S1
A
AHCTREUNG
A
ACHTREUNG
Figure 1. Long-wavelength absorption and normalized fluorescence band
(lexc =260 nm) of 6G2 in CH2Cl2.
Scheme 6. Downhill focal energy-transfer (ET) route in the dendrimers
6Gi and competing peripheral energy migration (EM).
the “downhill” process of singlet energy transfer. Apart
from step-by-step transfer, direct transfer from the periph-
ery to the core seems to be possible, since the distances are
small enough in comparison to the Fçrster radius of related
systems.[20] One must consider the overlap of the long-wave-
length transition S1!S0 of Chr-2,3 (donor) and a higher
electron transition S0!Sn of Chr-1 (acceptor).
Fluorescence (F) of dendrimers 6Gi (i=1–3) was studied
in comparison to that of 1:1 mixtures of 5b and 3Gi (i=1–
3). Dilute solutions (3”10À7 m in CH2Cl2) were prepared so
that the absorbance of dendrimer and mixture was exactly
the same at l=260 nm. It turned out that the absorption of
the anthracene chromophore and the dendron chromo-
phores are strictly additive. Since intermolecular singlet
energy transfer (Fçrster transfer) can be excluded under the
conditions used, the fluorescence of the mixture is only
based on the absorption A (l=260 nm) of the anthracene
chromophore of 5b: A higher electronically excited state Sn
is generated that undergoes internal conversion (IC) to S1,
which fluoresces. The fluorescence quantum yields FF’=
23.5% of 6G1 and 6G2 are virtually the same within the
error limit (Table 2) as that of 5b, but somewhat lower than
the quantum yield of unsubstituted anthracene (27%). The
third-generation dendrimer has a quantum yield FF’ of
17%, which is again somewhat lower. The side chains in the
9-position of the anthracene moiety enhance the role of ra-
diationless decay, because they increase the number of vi-
brational modes. Table 2, however, demonstrates that the
overall fluorescence intensity F’ of all three dendrimers 6Gi
(i=1,2,3) is higher than the fluorescence intensity F of
model compound 5b on irradiation at lexc =260 nm. This
Scheme 5. Intramolecular [4p+4p] cycloaddition and cycloreversion
6GiQ7GiÀ1
.
lowed concerted route 6Gi!E!7GiÀ1 and a stepwise pro-
cess 6Gi!E!I!7GiÀ1
.
Irradiation of the cyclomer 7GiÀ1 at 254 nm or heating
above 608C leads back to 6Gi. Simultaneously, the anthra-
cene fluorescence returns. Thus, the molecular switch can be
monitored by absorption and fluorescence.
Irradiation of 7GiÀ1 at 254 nm provokes excitation S0!S1
of the terminal and trisubstituted benzene rings (Chr-3 and
Chr-2) and, moreover, excitation of the dialkoxy 1,4-cyclo-
hexadiene chromophore (Chr-4). Like in 1,4-cyclohexadiene
or norbornadiene, interaction of the homoconjugated olefin-
ic p bonds can be assumed, which leads to splitting into an
higher and lower energy transitions.[19] However, all these
electron transitions of 7GiÀ1 are superimposed in the range
between 240 and 290 nm. The electronic transition energies
DE are very similar [Eq. (2)].
DEðChr-4Þ ꢁ DEðChr-2Þ ꢂ DEðChr-3Þ
ð2Þ
9320
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 9317 – 9323

Dendrimers with an Anthracene Core
FULL PAPER
Table 2. Fluorescence data of dendrimers 6Gi (i=1–3) (lexc =260 nm, 3”
10À7 m solution in CH2Cl2).
608C)!6Gi, we did not find any fatigue. The photochemical
reversion 7GiÀ1-(lꢀ254 nm)!6Gi also shows high fatigue
resistance. However, long-term irradiation at 254 nm leads
to decomposition.[23]
Generation Emission Relative
Fluorescence Quantum yield
i
maxima
fluorescence
intensity
quantum
yield
of energy
transfer
FT [%]
The multiple benzenoid chromophores in the dendrons
permit singlet energy transfer to the anthracene core with a
quantum yield FT of 9, 10, and 12% for 6G1, 6G2, and 6G3
(lexc =260 nm). Accordingly the fluorescence intensity is en-
hanced (14–41%), but the maximum increase is reached for
the second-generation dendrimer 6G2 because the fluores-
cence quantum yield FF’ for third-generation 6G3 decreases
from 23.5% for 6G1 and 6G2 to 17% for 6G3. We ascribe
this result to more efficient peripheral energy migration,
which the 31 benzenoid chromophores present in 6G3
render more favorable. Moreover, we assume energy trans-
l [nm]
F’
G
E
1
2
3
393Æ1
414Æ1
441Æ1
393Æ1
415Æ1
440Æ1
393Æ1
415Æ1
441Æ1
1.14Æ0.1
1.41Æ0.1
1.33Æ0.1
23.5Æ2
23.5Æ2
17.0Æ2
9Æ1
10Æ1
12Æ1
effect must be due to singlet energy transfer (ET) from the
benzene rings to the anthracene core [Eq. (3)].
fer from the dendrons to the center of photoproducts 7GiÀ1
,
but direct proof of this by fluorescence measurements is not
possible. The photostationary states for irradiation at l1 ꢀ
300 nm are totally on the side of 7GiÀ1, and those for irradi-
ation at l2 =254 or 260 nm are totally on the side of 6Gi.
Singlet energy transfer does not change that. Scheme 7 sum-
marizes the complex photophysical (absorption A (l1) or A
(l2), fluorescence F, internal conversion IC, singlet energy
transfer ET) and photochemical processes (PhR) of differ-
ent chromophores (anthracene Chr-1, benzene Chr-2,3, ho-
modiene Chr-4).
A
ET
F
S ðChr-2=Chr-3Þ
0
S ðChr-2=Chr-3Þ
1
S ðChr-1Þ S ðChr-1Þ
ƒ!
ƒ!
!
1
0
ð3Þ
Going from the first to the third generation of 6Gi, the
fluorescence quantum yield FF’ decreases, but the quantum
yield for energy transfer FT increases; 12% of the excited
benzene rings in 6G3 undergo singlet energy transfer to the
anthracene core (Table 2).
The opposite trends of FF’ and FT (i=1–3) lead to a max-
imum fluorescence enhancement of 41% for the second-
generation 6G2. The amount of absorption S0!Sn of the an-
thracene moiety at 260 nm is taken into account in this eval-
uation (see Experimental Section).
This situation is comparable with the attachment of FrØ-
chet dendrons to porphyrins.[21,22] It is remarkable that
energy transfer leads to enhanced fluorescence of the den-
Experimental Section
General: Melting points were determined on a Büchi 510 melting point
apparatus and are uncorrected. The UV/Vis spectra were obtained with a
Zeiss MCS 320/340, and the fluorescence spectra with a Perkin-Elmer LS
50B spectrometer. The 1H and 13C NMR spectra were recorded with
Bruker AMX 400 and ARX 400 spectrometers. The mass spectra were
obtained on a Finnigan MAT 95 (FD and EI) and on a Shimadzu
AXIMA-CFR (MALDI-TOF) spectrometer. The elemental analyses
were determined in the Microanalytical Laboratory of the Chemistry De-
partment of the University of Mainz.
Preparation of dendrons and model compounds: Compounds 1, 4G0, and
5a,b are commercially available. Dendrons 2Gi (i=1–4), 3Gi (i=1–4),
and 4Gi (i=1–3) were prepared with a combination of known proce-
dures: Step a in Scheme 2 was performed according to referen-
ces [5,6,10], step b according to references,[6,7,9] and step c in analogy
to reference [8]. 9-[(Benzyloxy)methyl]anthracene (6G0) was obtained as
described earlier.[17]
drimers 6Gi but not to a visible photoreaction 6Gi!7GiÀ1
.
Of course, the photoprocess competes with fluorescence of
S1, but the reverse process 7GiÀ1!6Gi is obviously more ef-
ficient on irradiation at l=260 nm. We attribute this to a
high quantum yield of the cycloreversion and to additional
S1(Chr-2/Chr-3)+S0
G
(Chr-4)+S0(Chr-2/Chr-3)
energy transfer (cf. Scheme 1). The photostationary state for
both irradiation wavelengths (lꢀ300 nm and l=254 or
260 nm) is completely on the product side. Energy transfer
does not change that.
General procedure for preparation of dendrimers 6Gi
(i=1–4): A mix-
G
ture of 9-chloromethylanthracene (5a, 227 mg, 1.0 mmol), alcohol 3Gi
(1.05–1.20 mmol), tetrabutylammonium bromide (81–106 mg, 0.25–
0.33 mmol) in chlorobenzene (30–50 mL), and KOH (224 mg, 4.0 mmol)
in H2O (1–2 mL) was stirred at 608C in an Ar atmosphere. The reaction
time was three days for 6Gi (i=1,2,3) and six days for 6G4. CH2Cl2
(100 mL) and H2O (100 mL) were added at room temperature. The aque-
ous layer was separated and extracted with CHCl3 (3”50 mL). The uni-
fied organic phases were dried with Na2SO4 and evaporated. The crude
product was purified by column chromatography (SiO2 (50”3 cm), cyclo-
hexane/CH2Cl2/ethyl acetate 1/1/0.03). Intense daylight should be avoided
during all operations.
Conclusion
In contrast to 9-[(benzyloxy)methyl]anthracene (6G0) as
model compound, irradiation (lꢀ300 nm) of the corre-
sponding dendrimers 6Gi results in intramolecular [4p+4p]
cycloaddition to form the cyclomers 7GiÀ1. The quantitative
process is thermally (T>608C) and photochemically (l=
254 nm) reversible and therefore an ideal example of a mo-
lecular switch. For the photochemical process 6Gi-(lꢀ
300 nm)!7GiÀ1 and its thermal reversion 7GiÀ1-(Tꢀ
9-{[3,5-Bis(benzyloxy)benzyloxy]methyl}anthracene (6G1): Yield 393 mg
(77%), m.p. 1088C, yellowish solid; 13C NMR (CDCl3): d=64.0, 1C (a-
CH2), 70.0, 2C (g-CH2), 72.2, 1C (b-CH2), 101.6, 1CH/106.7, 2CH (tri-
subst. benzene), 124.4, 2CH/124.9, 2CH/126.1, 2CH/129.0, 2CH (C-1, C-
Chem. Eur. J. 2007, 13, 9317 – 9323
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9321

H. Meier, D. Cao et al.
300 nm). After about 30 min, TLC
(SiO2, toluene) showed total consump-
tion of the starting material. The Ar
stream was stopped and the solution
evaporated. The obtained residue was
pure product 7GiÀ1. Heating or traces
of acids must be strictly avoided.
7,22-Bis(benzyloxy)-3-oxahexacyclo-
[7.6.6.25,8.01,5.010,15.016,21]tricosa-
6,10,12,14,16,18,20,22-octaene (7G0):
Yield: quantitative, m.p. 708C
(decomp); 1H NMR (CDCl3): d=3.24
(m, 1H; 8-H), 3.59/4.25 (AB, 2J=
À10.8 Hz, 4H; g-CH2), 3.90 (s, 2H; b-
CH2), 4.28 (d, 3J=11.4 Hz, 1H; 9-H),
4.48 (d, 4J=2.0 Hz, 2H; 6-H, 23-H),
4.75 (s, 2H; a-CH2), 7.07–7.36 ppm
(m, 18H; aromat. H); 13C NMR
(CDCl3): d=50.7, 1 CH (C-9), 52.8, 1
CH (C-8), 56.6, 1 CH (C-5), 65.4, 1
CH (C-1), 70.2, 2 CH2 (g-CH2), 71.1,
1CH2 (a-CH2), 80.9, 1CH2 (b-CH2),
106.4, 2CH (C-6, C-23), 122.5, 2CH/
125.4, 2CH/126.0, 2 CH/127.7, 2 CH/
128.0, 2 CH/128.1, 4CH/128.4, 4CH
(aromat. CH), 136.6, 2Cq/144.5, 2Cq/
146.0, 2Cq (aromat. Cq), 162.8 ppm,
2Cq (CqO, C-7, C-22); FD MS: m/z
(%): 511 (15) [M+], 421 (100); ele-
mental analysis calcd (%) for
Scheme 7. Photophysical and photochemical process of the dendrimers 6Gi (i=1–3) with chromophores Chr-
1,2,3 and their cyclomers 7GiÀ1 with chromophores Chr-2,3,4: A: Absorption, l1 ꢀ300 nm, l2 =254 or 260 nm;
F: Fluorescence; IC: Internal conversion; ET: Singlet energy transfer; PhR: Photoreaction.
C36H30O3 (510.6):
found: C 84.74, H 6.25.
7,22-Bis[3,5-bis(benzyloxy)benzyloxy-
C 84.68, H 5.92;
AHCTREUNG
2, C-3, C-4), 127.5, 4CH/127.9, 2CH/128.5, 4CH (C6H5), 128.4, 1CH (C-
3-oxahexacyclo
[7.6.6.25,8.01,5.010,15.016,21]tricosa-6,10,12,14,16,18,20,22-oc-
A
10), 128.6, 1Cq (C-9), 131.1, 2Cq/131.4, 2Cq (C-4a, C-8a), 138.6, 2Cq
(C6H5), 140.9, 1Cq/160.0 ppm, 2Cq (trisubst. benzene); FD MS: m/z (%):
510 (100) [M+]; elemental analysis calcd (%) for C36H30O3 (510.6): C
84.68, H 5.92; found: C 84.61, H 6.04.
1
taene (7G1): Yield: quantitative, m.p. 748C (decomp); H NMR (CDCl3):
d=3.22 (m, 1H; 8-H), 3.48/4.14 (AB, 2J=À11.0 Hz, 4H; g-CH2), 3.85 (s,
2H; b-CH2), 4.23 (d, 3J=10.6 Hz, 1H; 9-H), 4.41 (d, 4J=1.9 Hz, 2H; 6-
4
H, 23-H), 4.71 (s, 2H; a-CH2), 5.01 (“s”, 8H; d-CH2), 6.44 (d, J=2.3 Hz,
9-({3,5-Bis[3,5-bis(benzyloxy)benzyloxy]benzyloxy}methyl)anthracene
G
4H; trisubst. benzene), 6.53 (t, 4J=2.3 Hz, 2H; trisubst. benzene), 6.97–
7.35 ppm (m, 28H; aromat. H); 13C NMR (CDCl3): d=50.7, 1 CH (C-9),
52.7, 1 CH (C-8), 50.5, 1Cq (C-5), 65.4, 1Cq (C-1), 70.1, 6CH2 (g-CH2, d-
CH2), 71.1, 1CH2 (a-CH2), 80.8, 1CH2 (b-CH2), 101.6, 2CH (trisubst.
benzene), 106.5, 2CH (C-6, C-23), 107.0, 4CH (trisubst. benzene) 122.4,
2CH/125.4, 2CH/126.0, 2CH/127.5, 8CH/127.6, 2CH/128.0, 4CH/128.6,
8CH (aromat. CH), 136.8, 4Cq/138.9, 2Cq/144.4, 2Cq/145.9, 2Cq (aromat.
Cq), 159.4, 4Cq/162.6 ppm, 2Cq (aromat. CqO); FD MS: m/z (%): 935
(100) [M+]; elemental analysis calcd (%) for C64H54O7 (935.1): C 82.20,
H 5.82; found: C 82.19, H 5.34.
(6G2): Yield 885 mg (95%), m.p. 1328C, yellowish powder; FD MS: m/z
(%): 936 (100) [M+H+]; elemental analysis calcd (%) for C64H54O7
(935.1): C 82.20, H 5.82; found: C 82.44, H 6.06.
9-[(3,5-Bis
N
N
thyl]anthracene (6G3): Yield 1.74 g (98%), yellowish glassy compound;
13C NMR (CDCl3): d=64.0, 1C (a-CH2), 70.0, 2C (g-CH2), 70.1, 12C (d-
CH2, e-CH2), 72.2, 1C (b-CH2), 101.6, 7CH/106.4, 12 CH/106.8, 2CH (tri-
subst. benzene), 124.4, 2CH/124.9, 2CH/126.1, 2CH/129.0, 2CH (C-1, C-
2, C-3, C-4), 127.5, 16CH/128.0, 8CH/128.5, 16CH (C6H5), 128.4, 1CH
(C-10), 128.5, 1Cq (C-9), 131.1, 2Cq/131.4, 2Cq (C-4a, C-8a), 136.7, 8Cq
(C6H5), 139.2, 4Cq/139.3, 2Cq/140.9, 1Cq (trisubst. benzene), 159.9, 2Cq/
160.0, 4Cq/160.1 ppm, 8Cq (CqO, trisubst. benzene); FD MS: m/z (%):
1785 (100) [M+H+]; elemental analysis calcd (%) for C120H102O15
(1784.1): C 80.79, H 5.76; found: C 80.64, H 6.01.
7,22-Bis
N
G
clo[7.6.6.25,8.01,5.010,15.016,21]tricosa-6,10,12,14,16,18,20,22-octaene
(7G2):
Yield: quantitative, m.p. 808C (decomp); 1H NMR (CDCl3): d=3.29 (m,
2
1H; 8-H), 3.52/4.19 (AB, J=À11.2 Hz, 4H; g-CH2), 3.91 (s, 2H; b-CH2),
4.28 (d, 3J=10.7 Hz, 1H; 9-H), 4.47 (d, 4J=1.5 Hz, 2H; 6-H, 23-H), 4.76
(s, 2H; a-CH2), 4.99 (“s”, 8H; d-CH2), 5.03 (s, 16H; e-CH2), 6.48–6.71
(m, 18H; trisubst. benzene), 6.98–7.50 ppm (m, 40H; aromat. H); FD
MS: m/z (%): 1785 (100) [M+H+].
9-{[3,5-Bis
A
{3,5-bis
A
oxy)benz
ACHTREUNG
preparation which used 0.14 mmol 5, yellowish glassy compound.
13C NMR (CDCl3): d=65.2, 1C (a-CH2), 70.0, 6C (g-CH2, d-CH2), 70.1,
24C (e-CH2, z-CH2), 72.6, 1C (b-CH2), 101.6, 15CH/106.4, 30CH (tri-
subst. benzene), 125.0, 2CH/127.2, 4CH/129.0, 2CH (C-1, C-2, C-3, C-4),
124.6, 32CH/128.0, 16 CH/128.6, 32CH (C6H5), 128.7, 1CH (C-10), 133.5,
134.1, 136.8, 139.2, (aromat. Cq, partly superimposed), 160.0, 160.1 ppm
(aromat. Cq, partly superimposed). The MALDI-TOF mass spectrum
showed the dendron mass, but not the mass of the dendrimer 6G4 (m/z
3482).
Reverse reaction: A short heat shock to neat 7GiÀ1 (i=1–3) (Tꢀ608C)
or short irradiation (tꢂ1 min) at l=254 nm (10À4 m solution in CH2Cl2)
in the absence of traces of acids yields quantitatively the dendrimers 6Gi
(i=1–3).
Fluorescence measurements: 3”10À7
m
solutions of 6Gi (i=1–3) in
CH2Cl2 were compared with 1:1 mixtures of 3Gi (i=1–3) and 5b which
had the same absorbance. The excitation wavelength was in both cases
l=260 nm. The fluorescence intensities F and F’, respectively, were ob-
tained by integration using the software of the fluorescence spectrometer.
The data compiled in Table 2 were obtained according to Equations (4)
and (5),
General procedure for the irradiation of 6Gi
(i=1–3): A slow Ar stream
U
was passed for 30 min through a solution of 0.1 mmol of 6Gi in dry ben-
zene (165–170 mL), after which irradiation was started with a 450-W
Hanovia medium-pressure lamp equipped with a Duran glass filter (lꢀ
9322
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 9317 – 9323

Dendrimers with an Anthracene Core
FULL PAPER
e1
e1
0
0
ꢀTꢀF
1984; Angew. Chem. Int. Ed. 2002, 41, 1900–1904; n) M.-S. Choi, T.
Aida, T. Yamazaki, Angew. Chem. 2001, 113, 3294–3298; Angew.
Chem. Int. Ed. 2001, 40, 3194–3198; o) V. R. Neuwahl, R. Righini,
A. Adronov, P. R. L. Malenfant, J. M. J. FrØchet, J. Phys. Chem. B
2001, 105, 1307–1312; p) M. Plevoets, F. Vçgtle, L. De Cola, U. Bal-
zani, New J. Chem. 1999, 23, 63–69; q) S. L. Gilat, A. Adronov,
J. M. J. FrØchet, Angew. Chem. 1999, 111, 1519–1524; Angew. Chem.
Int. Ed. 1999, 38, 1422–1427; and S. L. Gilat, A. Adronov, J. M. J.
FrØchet, J. Org. Chem. 1999, 64, 7474–7484; r) K. Kimura, T. Shiba,
T. Muto, K. Hanabusa, H. Shirai, Macromolecules 1999, 32, 8237–
8239; s) M. Kawa, J. M. J. FrØchet, Chem. Mater. 1998, 10, 286–296;
t) D.-L. Jiang, T. Aida, J. Am. Chem. Soc. 1998, 120, 10895–10901;
u) V. Balzani, S. Campagna, G. Denti, A. Juria, S. Serroni, M. Ven-
turi, Acc. Chem. Res. 1998, 31, 26–34; v) A. Bar-Haim, J. Klafter, J.
Phys. Chem. B 1998, 102, 1662–1664; w) M. R. Shortreed, S. F.
Swallen, Z.-Y. Shi, W. Tan, Z. Xu, C. Devadoss, J. S. Moore, J. Phys.
Chem. B 1997, 101, 6318–6322; x) C. Devadoss, P. Bharathi, J. S.
Moore, J. Am. Chem. Soc. 1996, 118, 9635–9644; y) G. M. Stewart,
M. A. Fox, J. Am. Chem. Soc. 1996, 118, 4354–4360.
ꢀF
F 0
F
e1 þ e2
e1 þ e2
ꢀF
¼
¼
ð4Þ
ð5Þ
e1
e1 þ e2
ꢀ
ꢁ
e1 F 0 ꢀF
ꢀT
ꢃ
0 À1
ꢀF
e2
F
where F’/F is the ratio of fluorescence intensities of 6Gi and the mixture
3Gi/5b for excitation of both at l=260 nm (equal absorbance); e1 the
molar absorption coefficient of the anthracene chromophore (Chr-1) at
l=260 nm (measurement on 5b); e2 the molar absorption coefficient of
the benzene-ring chromophores (Chr-2, Chr-3) at l=260 nm (measure-
ment on 3Gi); fF’ the quantum yield of the S1!S0 fluorescence of the
dendrimers 6Gi with (lexc =260 nm); fF the quantum yield of the fluores-
cence of 5b (lexc =260 nm); and fT the quantum yield of singlet energy
transfer from the benzene rings to the anthracene core.
The fluorescence quantum yields of 5b and 6Gi (i=1,2,3) were obtained
by comparison with unsubstituted anthracene as standard (FF =27%).
[5] C. J. Hawker, J. M. J. FrØchet, J. Am. Chem. Soc. 1990, 112, 7638–
7647.
[6] D. Seebach, J.-M. Lapierre, G. Greiveldinger, K. Skobridis, Helv.
Chim. Acta 1994, 77, 1673–1688.
[7] G. L’abbØ, B. Forier, W. Dehaen, Chem. Commun. 1996, 2143–2144.
[8] B. Forier, W. Dehaen, Tetrahedron 1999, 55, 9829–9846.
[9] W. Freeman, L. A. J. Chrisstoffels, J. M. J. FrØchet, J. Org. Chem.
2000, 65, 7612–7617.
[10] M. Hartmann, M. Raethe, Z. Chem. 1979, 19, 373–374.
[11] Castellan, J.-M. Lacoste, H. Bouas-Laurent, J. Chem. Soc. Perkin
Trans. 2 1979, 411–419.
Acknowledgements
We are grateful to the Deutsche Forschungsgemeinschaft, the Fonds der
Chemischen Industrie, the National Natural Science Foundation of China
(20272059, 20572111), and to Specialized Chinese Research Fund for the
Doctoral Program of Higher Education (20060561024) for financial sup-
port. We thank Prof. Henri Bouas-Laurent (University of Bordeaux) for
many valuable discussions.
[12] H. Bouas-Laurent, J.-P. Desvergne, A. Castellan, J. Parrot, M.
Webb, G. Bourgeois, Bull. Soc. Chim. Fr. 1989, 449–454.
[13] G. Felix, R. Lapouyade, A. Castellan, H. Bouas-Laurent, J. Gaultier,
C. Hauw, Tetrahedron Lett. 1975, 16, 409–412.
[14] H.-D. Becker, K. Andersson, K. Sandros, J. Org. Chem. 1980, 45,
4549–4555.
[15] a) H. Bouas-Laurent, A. Castellan, J.-P. Desvergne, R. Lapouyade,
Chem. Soc. Rev. 2001, 30, 248–263; b) H. Bouas-Laurent, A. Castel-
lan, J.-P. Desvergne, R. Lapouyade, Chem. Soc. Rev. 2000, 29, 43–
55; c) H. D. Becker, Chem. Rev. 1993, 93, 145–172; d) H. D. Becker
in Advances in Photochemistry, Vol. 15 (Eds.: D. H. Volman, G. S.
Hammond, K. Gollnik), Wiley, New York, 1990, pp. 139–227.
[16] S. Dobis, D. Schollmeyer, C. Gao, D. Cao, H. Meier, Eur. J. Org.
Chem. 2007, 2964.
[1] Recent books and reviews on this topic: a) F. M. Raymo, M. Toma-
sulo, Chem. Eur. J. 2006, 12, 3186–3193; b) F. M. Raymo, M. Toma-
sulo, Chem. Soc. Rev. 2005, 34, 327–336; c) F. M. Raymo, M. Toma-
sulo, J. Phys. Chem. A 2005, 109, 7343–7352; d) V. I. Minkin, Chem.
Rev. 2004, 104, 2751–2776; e) Photochromism:Molecules and Sys-
tems (Eds.: H. Dürr, H. Bouas-Laurent) Elsevier, Amsterdam, 2003;
f) H. Bouas-Laurent, H. Dürr, Pure Appl. Chem. 2001, 73, 639–665;
g) D. D. Nolte, Mind at Light Speed:A New Kind of Intelligence ,
Free Press, New York, 2001; h) Molecular Switches (Ed.: B. L. Ferin-
ga, Wiley-VCH, Weinheim, 2001; i) S. O. Kasap, Optoelectronic and
Photonics:Principles and Practices , Prentice-Hall, Upper Saddle
River, 2001; j) M. Irie (Ed. ) Chem. Rev. 2000, 100, 1683–1890;
k) Organic Photochromic and Thermochromic Compounds (Eds.:
J. C. Crano, R. J. Guglielmetti), Plenum Press/Kluwer Academic,
New York, 1999; l) Photo-Reactive Materials for Ultrahigh Density
Optical Memory (Ed.: M. Irie) Elsevier, Amsterdam, 1994.
[17] See however J.-P. Desvergne, A. Castellan, H. Bouas-Laurent, Tetra-
hedron Lett. 1981, 22, 3529–3532.
[18] Dendrimer 6G4 behaves like the other 6Gi. We obtained its photo-
product 7G3 in quantitative yield but characterized it only by its
1H NMR spectrum, which is in accordance with 7Gi (i=0,1,2).
[19] M. Hesse, H. Meier, B. Zeeh, Spektroskopische Methoden in der or-
ganischen Chemie, 7th ed., Thieme, Stuttgart, 2005, p. 9.
[2] de Meijere, L. Zhao, V. N. Belov, M. Bossi, M. Noltemeyer, S. W.
Hell, Chem. Eur. J. 2007, 13, 2503–2516, and references therein.
[3] See also M. Z. Alan, T. Yoshioka, T. Ogata, T. Nonaka, S. Kurihara,
Chem. Eur. J. 2007, 13, 2641–2647.
ˇ ´
´
[20] J. Michel, V. Bonacic-Koutecky, Electronic Aspects of Organic Pho-
tochemistry, Wiley, New York, 1990, p. 81.
[4] Selected recent articles on antenna effects: a) S. Saha, J. F. Stoddart,
Chem. Soc. Rev. 2007, 36, 77–92; b) T.-Y. Luh, Y.-J. Cheng, Chem.
Commun. 2006, 4669–4678; c) K. Biradha, M. Sarkar, L. Rajput,
Chem. Commun. 2006, 4169–4179; d) T. Ishi-i, S. Shinkai, Top. Curr.
Chem. 2005, 258, 119–160; e) T. S. Balaban, Acc. Chem. Res. 2005,
38, 612–623; f) R. E. Bauer, A. C. Grimsdale, K. Müllen, Top. Curr.
Chem. 2005, 245, 253–286; g) E. A. Medlycott, G. S. Hanan, Chem.
Soc. Rev. 2005, 34, 133–142; h) X. Kim, A. Oswka, Acc. Chem. Res.
2004, 37, 735–745; i) M. Kawa, Top. Curr. Chem. 2003, 228, 193–
204; j) V. Balzani, P. Ceroni, M. Maestri, C. Saudan, Top. Curr.
Chem. 2003, 228, 159–191; k) J.-F. Nierengarten, Top. Curr. Chem.
2003, 228, 87–110; l) D. M. Guldi, Chem. Soc. Rev. 2002, 31, 22–36;
m) T. Weil, E. Reuther, K. Müllen, Angew. Chem. 2002, 114, 1980–
[21] D.-L. Jiang, T. Aida, J. Am. Chem. Soc. 1998, 120, 10895–10901.
[22] See also Y. Wakabayashi, M. Tokashi, A. Hibara, D.-L. Jiang, T.
Aida, T. Kitamori, J. Phys. Chem. B 2001, 105, 4441–4445.
[23] Practical optimization should combine a complete intramolecular
photocycloaddition with a partial cycloreversion with the most fa-
vorable wavelength between 254 and 400 nm. Thus, the disappear-
ance and appearance of the fluorescence should permit high-perfor-
mance monitoring of optical switching.
Received: May 10, 2007
Revised: June 27, 2007
Published online: August 31, 2007
Chem. Eur. J. 2007, 13, 9317 – 9323
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9323