Intra- and intermolecular photochemical properties of poly(amidoamine) dendrons with an anthracene at focal point

Article abstract of DOI:10.1246/bcsj.76.743

Photophysical and photochemical properties of poly(amidoamine) dendrons containing an anthracene at the focal point have been studied by the steady-state and transient spectroscopic methods in solution. The fluorescence lifetime of the anthracene moiety in the higher generation dendron (G3.5) is slightly shorter than that of the lower generation homologue (G0.5). On the other hand, the triplet lifetime of G3.5 is longer than that of G0.5. The bimolecular rate constants for energy transfer from the triplet states of the anthracene moiety to O₂ (k_O2) and for T-T annihilation (k_TT) were almost the same for the anthryl dendrons, G3.5 and G0.5, suggesting that the dendron groups do not hinder the approach of O₂ or the anthracene moiety. Photodimerization of G3.5 took place at a similar rate to G0.5, while the photodissociation rates of these anthracene dimers increase in the order of anthracene > G0.5 > G3.5, suggesting the presence of some attractive forces between dendron wedges.

Full text of DOI:10.1246/bcsj.76.743

ꢀ
2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 743–747 (2003)
743
Intra- and Intermolecular Photochemical Properties of Poly(amidoamine)
Dendrons with an Anthracene at Focal Point
ꢀ
y
y
Mamoru Fujitsuka, Osamu Ito, Yutaka Takaguchi, Tomoyuki Tajima,
y
y
y
Kazuchika Ohta, Jiro Motoyoshiya, and Hiromu Aoyama
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, CREST (JST),
Katahira, Aoba-ku, Sendai, Miyagi 980-8577
yFaculty of Textile Science & Technology, Shinshu University, Ueda, Nagano 386-8567
(
Received October 1, 2002)
Photophysical and photochemical properties of poly(amidoamine) dendrons containing an anthracene at the focal point
have been studied by the steady-state and transient spectroscopic methods in solution. The fluorescence lifetime of the
anthracene moiety in the higher generation dendron (G3.5) is slightly shorter than that of the lower generation homologue
(
G0.5). On the other hand, the triplet lifetime of G3.5 is longer than that of G0.5. The bimolecular rate constants for energy
transfer from the triplet states of the anthracene moiety to O (kO ) and for T–T annihilation (kTT) were almost the same for the
anthryl dendrons, G3.5 and G0.5, suggesting that the dendron groups do not hinder the approach of O
2
2
2
or the anthracene
moiety. Photodimerization of G3.5 took place at a similar rate to G0.5, while the photodissociation rates of these anthracene
dimers increase in the order of anthracene > G0.5 > G3.5, suggesting the presence of some attractive forces between dendron
wedges.
New kinds of well-defined regularly branched macromole-
cules, dendrimers, have attracted much scientific attention in the
1
{7
past two decades.
In particular, photoreactive moieties
incorporated in dendritic architecture are of current interest from
the viewpoint of photoresponsive dendrimers whose structures
and properties can be switched under photoirradiation. In this
regard, several groups have reported the covalent fixation of a
highly photoreactive unit with a dendritic macromolecule as an
8
{17
intramolecular photo-switch.
For example, Archut et al.
reported the synthesesof dendrimers containing azobenzene units
8
in the periphery toward photoswitchable dendritic hosts.
Mizutani et al. have reported the photochemical trans-cis
isomerization of stilbene dendrimers. Tominaga et al. have
reported that the polyuracil dendrimers can be intramolecularly
locked via photoinduced dimerization of the uracil units.
Recently, we have reported the dechalcogenation reaction of
dendrimer dichalcogenides upon light irradiation. Although
photoreactions of dendrimers via intramolecular processes have
been extensively reported, much less is known about the
1
4
15
16
Scheme 1. Molecular structures employed in the present study.
1
8{22
intermolecular photoreaction of dendrimer or dendron.
Our
intermolecular ones is gathered using steady- and transient
fluorescence and absorption spectra.
group and Cao and Meier have recently reported on photo-
switching by intermolecular [4+4] photocycloaddition of anthra-
18;19
cene moieties at focal points of the dendrons.
It is quite
Experimental
curious that the intermolecular photoreactions smoothly proceed
in spite of the steric hindrance of bulky dendritic substituents. In
order tounderstand fully the photoreaction process, weperformed
Materials. Syntheses and purifications of poly(amidoamine)
dendrons, G0.5and G3.5, werecarried out according to our published
method. Highest grade methanol was used as a solvent for
spectroscopic measurements.
18
1
8
laser experiments employing anthryl dendrons.
In the present study, we investigated photophysical and
photochemical properties of poly(amidoamine) dendrons, G0.5
and G3.5, containing an anthracene at the focal point as shown in
Scheme 1. After getting the information about intramolecular
photophysical and photochemical properties, information about
Apparatus. Steady-state UV-visible absorption spectra were
measured with a JASCO/V-570 spectrophotometer. Steady-state
fluorescence spectra were measured using a Shimadzu RF-5300 PC
spectrofluorophotometer. The picosecond time-resolved fluores-
cence spectra and time profiles were measured using a THG (273 nm)

7
44 Bull. Chem. Soc. Jpn., 76, No. 4 (2003)
Photochemistry of Anthracene Dendrons
Fig. 2. Logarithum plots of fluorescence decays of (a) G3.5
and (b) G0.5 in methanol.
270–300 nm region may be attributed to the absorption of the
dendron wedge, poly(amidoamine) moiety.
The fluorescence spectra of G3.5 and G0.5 are also shown in
Fig. 1, in which the fluorescence intensities are normalized to the
absorption peaks at the longest wavelength. The fluorescence
peak positions of the dendron G3.5 were almost the same as those
of G0.5. The observed fluorescence intensity of G3.5 is lower
than that of G0.5, suggesting the low fluorescence quantum yield
(ꢁ ). Using unsubstituted anthracene as a reference (ꢁ ¼
Fig. 1. Steady-state absorption and fluorescence spectra of
(
a) G3.5 and (b) G0.5 in methanol.
of a Ti:sapphire laser (Spectra Physics, Tsunami) and a streak scope
Hamamatsu Photonics) as an excitation source and a detector,
f
f
25
(
0:30), the ꢁ value of G3.5 was evaluated to be 0.07, which is
f
respectively. Nanosecond transient absorption spectra in the UV and
visible regions were measured by means of laser flash photolysis. A
THG (355 nm) light from a Nd:YAG laser was used as an excitation
source. A Si-PIN photodiode was used for detection of the moni-
toring light from a pulsed Xe-lamp. Long time-scale measurements
smaller than that of G0.5 (ꢁ ¼ 0:13). This suggests that the
f
radiationless relaxation of the dendron G3.5 is more efficient than
that of G0.5.
Time-Resolved Measurements. Time profiles of fluores-
cence intensity after the picosecond-laser excitation of the
anthracene moiety are shown in Fig. 2. The decay time profiles
are fitted with a single exponential function, giving fluorescence
lifetimes (ꢂf) as listed in Table 1. The ꢂf value for the dendron
G3.5 (1.1 ns) is shorter than that of G0.5 (1.8 ns) and anthracene
(>2 ms) were performed using a photomultiplier tube and a
continuous Xe-lamp as a detector and a probe light, respectively.
Details are described in our previous report.
22
Results and Discussion
(5.5 ns). Although the observed difference between G0.5 and
Steady-State Absorption and Fluorescence Spectra.
Absorption and fluorescence spectra of higher generation anthryl
dendron G3.5 and its lower generation homologue G0.5 are
shown in Fig. 1. Both G3.5 and G0.5 show the absorption band
with vibronic peaks in the 320–400 nm region. These vibronic
anthracene is attributed to a substituent effect, the difference
between G3.5 and G0.5 is to the dendron effect, probably because
the dendron surrounding the anthracene moiety enhances the
radiationless relaxation process of the singlet excited state of the
anthracene moiety to the ground state.
1
bands are assigned to the La (p)-band of the anthracene moi-
3{25
Amorequantitativetreatmentwaspossibleusingarelationship
between ꢁ and relaxation rate constants such as k (radiative
2
ety.
1
The intense single band at 250 nm was attributed to the
23{25
f
f
Bb (ꢀ)-band of the anthracene moiety.
No shift of the
nr
relaxation rate constant) and k (non-radiative relaxation rate
26
absorption peaks of the anthracene moiety was evident in the
comparison of G3.5 with G0.5. The absorption band of G3.5 in
constant) (Eq. 1):
Table 1. Rate Parameters of the Excited States of Anthryl Dendrons and Anthracene in Methanol
Compounds
ꢂ
f/ns
fluo/nm)
ꢁf
ꢁisc
ꢂ
T/ms
kO2
kTT
ꢁ1
3
ꢁ1
ꢁ1
3
ꢁ1
(
ꢃ
(ꢃT/nm)
mol dm s
9
mol dm s
9a)
G3.5
G0.5
Anthracene
1.1 (393)
1.8 (391)
5.5 (380)
0.070
0.13
0.30
0.23
0.70
0.71
170 (430)
77 (430)
77 (430)
1:8 ꢂ 10
3:7 ꢂ 10
9
9a)
2:1 ꢂ 10
4:2 ꢂ 10
b)
9
9a)
2:4 ꢂ 10
4:1 ꢂ 10
ꢁ
1
3
ꢁ1
a) Molar extinction coefficients at 430 nm; 43000 mol dm cm for methylanthracene in alcohol was employed as a model of
ꢁ1
2
8
ꢁ1
3
28
dendrons, 52000 mol dm cm for anthracene in alcohol. b) From Ref. 25.

M. Fujitsuka et al.
Bull. Chem. Soc. Jpn., 76, No. 4 (2003)
745
Fig. 4. Laser power dependence of the decay at 430 nm of
G3.5 in long time scale in deaerated methanol. Inset: Plots
of Eq. 4.
Fig. 3. Nanosecond transient absorption spectra observed by
the laser light (355 nm) excitation of G3.5 in deaerated
methanol. Inset: Time profiles at 430 nm in the presence and
2
absence of O .
dependence as shown in Fig. 4. At higher laser power, the decay
can be fitted with mixed order kinetics of first- and second-order.
The second-order kinetics can be attributed to the T–T annihila-
tion process (Eq. 3), which is prominent at the high concentration
of the triplet state under the high laser power.
ꢁ
f ¼ kf=ðkf þ knrÞ
ð1Þ
Using the ꢁf and ꢂf (¼ 1=ðkf þ knrÞ) values, the kf and knr values
8
8
ꢁ1
were evaluated to be 0:73 ꢂ 10 and 4:9 ꢂ 10 s for G0.5 and
8
8
ꢁ1
ð3Þ
0
:65 ꢂ 10 and8:7 ꢂ 10 s forG3.5, respectively. Aprominent
difference was found in the knr values rather than in the kf values.
Nanosecond transient absorption spectra observed by the laser
excitation of the anthracene moiety of the G3.5 dendron are
shown in Fig. 3. An absorption peak that appeared at 430 nm was
attributed to the triplet-triplet absorption band of the anthracene
where kTT isa rateconstant of the T–T annihilationprocess. Inthe
first-order plots of the decay curve, the slope of the initial part,
which is referred to ꢀk1st, increases with the initial absorbance
2
5;27;28
moiety. Similar transient spectra with a peak at 430 nm
were observed for G0.5 and anthracene.
The decay time-profiles of the triplet state of the anthracene
0
(ꢀA ) according to Eq. 4:
T
ꢁ
d½lnðꢀAinitialÞꢃ=dt ¼ ꢀk1st ¼ k
0
þ ð2kTT="ÞꢀA
0
ð4Þ
2
moiety are shown in the inset of Fig. 3. In the absence of O , after
T
an initial fast decay, the absorbance shows a slow decay. The
initial fast decay may be attributed to the decay tail of the singlet
excited state of the anthracene moiety. When the solution was
where ꢀAinitial, k
part, the intrinsic rate constant for the triplet state, and the molar
0
, and "are the absorbance change in the initial
extinction coefficient of the triplet absorption band of the
T
2
ꢁ3
25
saturated by O
the decay rate of the triplet state increased. This finding can be
2
(½O
2
ꢃ ¼ 1:02 ꢂ 10ꢁ mol dm in methanol),
anthracene moiety, respectively. The k
from the intercept of the plot of Eq. 4, as shown in the inset of
0
value can be evaluated
attributed to the triplet energy transfer from anthracene to O
(Eq. 2):
2
Fig. 4. The triplet lifetimes (ꢂT) were evaluated from the relation,
T
ꢂ
T ¼ 1=k
0
. The kTT values are evaluated from the slope of the
plots (2kTT="), employing the " values of 9-methylanthracene
(43000 mol dm cm ), which is slightly smaller than the
k
O2
3
ꢀ
1
ð2Þ
ꢁ1 ꢁ1 28
3
Anth þ O
2
ꢁꢁꢁꢁꢁꢁꢁꢁ! Anth þ O
2
ꢁ
1
3
ꢁ1 28
unsubstituted anthracene (52000 mol dm cm ).
These
Acceleration of the decay of the triplet state was also observed
estimated parameters are listed in Table 1. The ꢂT value of
G3.5 is longer than that of G0.5 by a factor of 2, which is the
opposite tendencyoftheꢂf values. Inthe caseofthe tripletstate, a
solvent effect may control the lifetimes; thus, the dendron of
higher generation abrogates methanol solvent from the proximity
of anthracene, which causes deceleration of relaxation of the
anthracene triplet state in G3.5 compared with relaxations of
triplet states of G0.5 and anthracene.
in air-saturated solution (½O
ꢃ ¼ 2:1 ꢂ 10ꢁ mol dm ). Each
3
ꢁ3
2
decay obeys first-order kinetics giving the first-order rate constant
(
k1st). From the slope of the plots of the k1st values against O
2
concentrations, thesecond-orderrateconstant(kO )wasevaluated
9
2
ꢁ1
3
ꢁ1
9
to be 1:8 ꢂ 10 mol dm s for G3.5, which is almost the same
ꢁ1
3
ꢁ1
as those of G0.5 (2:1 ꢂ 10 mol dm s ) and anthracene
9
ꢁ1
3
ꢁ1
(
2:2 ꢂ 10 mol dm s ). Since small O
2
molecule can
approach to the focal anthracene by sneaking through the space
between the dendron branches, no appreciable slow-down was
observed in the second-order rate constant, kO , of the higher
The kTT value of G3.5 is almost the same as those of G0.5 and
anthracene, suggesting that the rates of the bimolecular reactions
are not so much affected by the steric crowding of the dendron
groups. These kTT values are almost 1/3 of the diffusion-
2
generation of the dendron.
In the absence of molecular oxygen, the decay time-profiles in
the longer time-scale measurements show the laser power
10
ꢁ1
3
ꢁ1
controlled limit (kdiff ¼ 1:2 ꢂ 10 mol dm s in metha-
29
nol).

7
46 Bull. Chem. Soc. Jpn., 76, No. 4 (2003)
Photochemistry of Anthracene Dendrons
Fig. 5. Dimerization by the 355 nm laser light irradiation of
G3.5 in deaerated methanol. Inset: Plots of decreases of the
absorbance at longest wavelength for G3.5, G0.5, and
anthracene in deaerated methanol.
Fig. 6. Dissociation by the 266 nm laser light irradiation of
G3.5-dimer in deaerated methanol. Inset: Plots of increase
in the absorbance at longest wavelength for anthracene
dimers in deaerated methanol.
Under the optical match condition, the initial absorbance
laser light in the deaerated solution, the absorption of the
anthracene decreasesasshown inFig. 5. From the absorptionand
observed under the low laser power is the relative measure of the
quantum yields for the intersystem crossing (ꢁisc). The initial
absorbance of G3.5 is 1/3 of those of G0.5 and anthracene. Thus,
the ꢁisc value of G3.5 is 1/3 of those of G0.5 and anthracene.
Using these ꢁisc values, the knr value was further divided into
the rate constants of internal conversion process (kic) and
18
MALDI-TOF MS spectra of the products, the dimerization
reaction of the anthracene moieties in G3.5 was confirmed
(Scheme 2). Almost complete consumption of the monomeric
anthracene absorption under repeated irradiation of the laser light
(Fig. 5) indicates that the reaction occurs quantitatively.
26
intersystem crossing process (kisc) according to Eq. 5:
In the inset of Fig. 5, the absorption intensities due to the
anthracene moieties were plotted against the number of the laser
shots. Although anthracene shows the fastest decrease, the
anthryl dendrons G3.5 and G0.5 show the almost the same
dimerization rate. The difference between the anthracene and
G0.5can be attributed to the retardation effect of the substituent at
the 9-position of anthracene, which will be the bridgehead
position after the photodimerization. The similarity of the
dimerization rates between the dendrons G3.5 and G0.5 indicates
thatthedendrongroupdoesnotretardthedimerizationrate, which
is in good agreement with the similarity of the observed kTT
values, because dimerization of anthracene derivatives occurs via
ꢁ
isc ¼ kisc=ðkf þ kic þ kiscÞ
ð5Þ
8
The kic and kisc values were evaluated to be 0:96 ꢂ 10 and 4:0 ꢂ
8
ꢁ1
8
8
ꢁ1
1
0 s for G0.5 and 6:5 ꢂ 10 and 2:2 ꢂ 10 s for G3.5,
respectively. A prominent difference was found in the kic values
rather than in the kisc values. This indicates that the radiationless
relaxation process of the singlet excited state of G3.5 is mainly
attributedtothe internalconversion tothe groundsinglet state, but
nottothetripletstateoftheanthracenemoiety. Abovefindingthat
the kic valueofG3.5islargerthanthatof G0.5maybeattributedto
the higher generation dendron wedge, which probably stimulates
the relaxation of the excited singlet state of the anthracene moiety
via the vibroinc interaction.
2
6
the triplet excited states. This suggests that the fixation reaction
of the anthracene moiety at focal points in the dendron takes place
at a rate similar to that in anthracene in solution. The locking
reaction of the dendrons seems to occur efficiently even in higher
Photodimerization of Anthracenes. Upon the selective
photoirradiation of anthracene chromophores with the 355 nm
Scheme 2. Photodimerization of G3.5.

M. Fujitsuka et al.
Bull. Chem. Soc. Jpn., 76, No. 4 (2003)
747
generation.
Photodissociation of Anthracene Dimers.
212, Springer, Berlin (2001).
F. V o¨ gtle, ‘‘Dendrimers d,’’ Topics in Current Chemistry
217, Springer, Berlin (2001).
G. R. Newkome, C. N. Moorfield, and F. V o¨ gtle, ‘‘Dendritic
Molecules Concepts, Syntheses, Perspectives,’’Wiley-VCH, Wein-
heim (1996).
Under the
6
photoirradiation of the anthracene dimers with the 266 nm laser
light, the recovery of the absorption band of monomeric
anthracene was observed as shown in Fig. 6. Since the 266 nm
laser light excites predominantly the anthracene dimers under the
low anthracene monomer concentrations, dissociation of the
dimers occurs at the initial stage. But when the monomer was
accumulated, the excitation of the monomer regenerates the
dimer. Thus, the saturation of the reaction occurs as shown in the
inset of Fig. 6. The dissociation of unsubsituted anthracenedimer
is faster and produces a higher yield than the dimers with 9-
substituents. Dendron G3.5 shows a further slower dissociation
rate and a lower yield than G0.5, suggesting that the unlocking
reaction is slightly dependent on the dendron generation. This
implies that dendron wedges in the G3.5-dimer have attractive
forces, probably due to mutual intertangles.
7
8
A. Archut, G. C. Azzellini, V. Balzani, L. De Cola, and F.
V o¨ gtle, J. Am. Chem. Soc., 120, 12187 (1998).
C. M. Junge and D. V. McGrath, J. Am. Chem. Soc., 121,
912 (1999).
9
4
8
10
57.
D. M. Junge and D. V. McGrath, Chem. Commun., 1997,
1
1
1
2
D.-L. Jiang and T. Aida, Nature, 388, 454 (1997).
T. Nagasaki, S. Tamagaki, and K. Ogino, Chem. Lett., 1997,
7
17.
1
3
S. Yokoyama, T. Nakahama, A. Otomo, and S. Mashiko,
Chem. Lett., 1997, 1137.
T. Mizutani, M. Ikegami, R. Nagahata, and T. Arai, Chem.
Lett., 2001, 1014.
M. Tominaga, K. Konishi, and T. Aida, Chem. Lett., 2000,
1
4
Summary
1
5
Intramolecular properties such as fluorescence lifetime, triplet
state lifetime and quantum yield of the intersystem crossing of the
anthracene moiety in the higher generation dendron are con-
siderably different from those of lower generation dendron and
pristineanthracene. Onthe otherhand, bimolecular rateconstants
374.
16 Y. Takaguchi, S. Suzuki, T. Mori, J. Motoyoshiya, and H.
Aoyama, Bull. Chem. Soc. Jpn., 73, 1857 (2000).
17 M. Smet, L.-X. Liao, W. Dehaen, and D. V. McGrath, Org.
Lett., 2, 511 (2000).
for the energy transfer with O
2
and T–T annihilation processes
18 Y. Takaguchi, T. Tajima, K. Ohta, J. Motoyoshiya, and H.
Aoyama, Chem. Lett., 2000, 1388.
were similar between the G3.5 and G0.5. Similarity was also
observed in the dimerization rates. On the other hand, the
unlocking reaction of the anthracene dimers for higher dendron
generation is less efficient than that for lower dendron generation.
1
2
9
0
D. Cao and H. Meier, Angew. Chem., Int. Ed., 40, 186 (2001).
Y. Takaguchi, T. Tajima, K. Ohta, J. Motoyoshiya, H.
Aoyama, T. Wakahara, T. Akasaka, M. Fujitsuka, and O. Ito,
Angew. Chem., Int. Ed., 41, 817 (2002).
2
1
Y. Takaguchi, S. Suzuki, K. Ohta, J. Motoyoshiya, and H.
Aoyama, Phosphorus, Sulfur Silicon, 176, 61 (2001).
R. Kunieda, M. Fujitsuka, O. Ito, M. Itoh, Y. Murata, and K.
Komatsu, J. Phys. Chem. B, 106, 7193 (2002).
This study was partially supported by the Ministry of
Education, Culture, Sports, Science and Technology, the Mitsu-
bishi Foundation, and the Kao Foundation For Arts and Sciences.
2
2
2
2
3
4
J. R. Platt, J. Chem. Phys., 17, 484 (1949).
P. Clar, ‘‘Polycyclic Hydrocarbons,’’ Vol.a&b, Academic
References
Press, London (1964).
1
J. M. J. Fr e´ chet and D. A. Tomalia, ‘‘Dendrimers and Other
Dendritic Polymers,’’ John Wiley & Sons, Ltd., Chichester (2001).
G. R. Newkome, C. N. Moorfield, and F. V o¨ gtle, ‘‘Den-
drimers and Dendrons,’’ Wiley-VCH, Weinheim (2001).
F. V o¨ gtle, ‘‘Dendrimers,’’ Topics in Current Chemistry 197,
Springer, Berlin (1998).
25 S. L. Murov, I. Carmichael, and G. L. Hug, ‘‘Handbook of
Photochemistry,’’ 2nd ed, Marcel Dekker, New York (1993).
26 N. J. Turro, ‘‘Modern Molecular Photochemistry,’’ 2nd ed,
University Science Book, Sausalito, California (1991).
27 I. Carmichael and G. L. Hug, J. Phys. Chem. Ref. Data, 15, 1
(1986).
2
3
4
10, Springer, Berlin (2000).
F. V o¨ gtle, ‘‘Dendrimers b,’’ Topics in Current Chemistry
28 K. Kikuchi, ‘‘Triplet-Triplet Absorption Spectra,’’ Bunshin
Publishing, Tokyo (1989).
29 O. Ito and M. Matsuda, Can. J. Chem., 56, 1080 (1978).
2
5
F. V o¨ gtle, ‘‘Dendrimers c,’’ Topics in Current Chemistry