Light-harvesting and photoisomerization in benzophenone and norbornadiene-labeled poly(aryl ether) dendrimers via intramolecular triplet energy transfer

Article abstract of DOI:10.1021/ja044800b

A series of benzophenone (BP) and norbornadiene (NBD)-labeled poly(aryl ether) dendrimers (Gn-NBD), generations 1-4, were synthesized, and their photophysical and photochemical properties were examined. The phosphorescence of the peripheral BP (donor) chr

Full text of DOI:10.1021/ja044800b

Published on Web 01/28/2005
Light-Harvesting and Photoisomerization in Benzophenone
and Norbornadiene-Labeled Poly(aryl ether) Dendrimers via
Intramolecular Triplet Energy Transfer
Jinping Chen,† Shayu Li,‡ Lu Zhang,† Baining Liu,† Yongbin Han,†
Guoqiang Yang,*,‡ and Yi Li*,†
Contribution from the Technical Institute of Physics and Chemistry, Chinese Academy of
Sciences, Beijing 100101, China, and Key Laboratory of Photochemistry, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100080, China
Received August 28, 2004; E-mail: yili@mail.ipc.ac.cn (Y.L.); gqyang@iccas.ac.cn (G.Y.)
Abstract: A series of benzophenone (BP) and norbornadiene (NBD)-labeled poly(aryl ether) dendrimers
(Gn-NBD), generations 1-4, were synthesized, and their photophysical and photochemical properties were
examined. The phosphorescence of the peripheral BP (donor) chromophore was efficiently quenched by
the NBD (acceptor) group attached to the focal point. Time-resolved spectroscopic measurements indicated
that the lifetime of the triplet state of the BP chromophore was shortened due to the proximity of the NBD
group. Selective excitation of the BP chromophore resulted in isomerization of the NBD group to
quadricyclane (QC). All of these observations suggest that an intramolecular triplet energy transfer occurs
in Gn-NBD molecules. The light-harvesting ability of these molecules increases with generation due to an
increase in the number of peripheral chromophores. The energy transfer efficiencies are ca. 0.97, 0.54,
0.45, and 0.37 for generations 1-4, respectively, and the rate constant of the triplet-triplet energy transfer
is ca. 106-107 s-1, which decreases inconspicuously with increasing generation. The intramolecular triplet
energy transfer is proposed to proceed mainly via a through-space mechanism involving the closest donor
(folding back conformation) and acceptor groups.
Introduction
workers initially reported a series of multichromophoric den-
drimers with different metal polypyridine complexes as the
Studies on the natural photosynthetic systems revealed that
the structure of the photosynthetic unit is a central reaction center
surrounded by light-harvesting complexes.1 The remarkable
character of the photosynthetic system is that the energy of any
photon absorbed by antenna complexes is transferred to the
reaction center with unit efficiency.2 Dendrimers are regularly
and hierarchically branched macromolecules with numerous
chain ends all emanating from a single core. The chromophores
can be accurately located at the core, focal point, periphery, or
even at each branching point of the dendritic structure. The
specific structure of the dendrimer makes it a mimic light-
harvesting system, where the antenna chromophores surround
the central reaction center.3
building block undergoing intramolecular energy transfer.4 Xu
and Moore developed a system based on phenylacetylene
dendrimers and showed an energy cascade from the dendrimer
to a single perylene, which is the first efficient, unidirectional
energy transfer example.5 Fre´chet and co-workers used the poly-
(aryl ether) as an antenna to harvest the energy then transfer
the energy to the ion at the focal point.6 Jiang and Aida, using
a system based on poly(aryl ether) dendrimers, have shown that
multiple low-energy photons could be harvested by the den-
drimer and transferred to an azobenzene core, accelerating its
isomerization from cis- to trans-azobenzene.7 The dendrimer-
independent energy transfer systems, in which the energy
transfer occurs between the peripheral chromophores and the
core and the dendrimer backbone acts as a spacer, were also
built and studied. A series of laser dye-labeled poly(aryl ether)
dendrimers were built by Fre´chet and co-workers. The energy
transfer efficiency from the periphery to the core in this system
was found to be efficient for generations 1-3, with only a slight
Recently, the studies of energy transfer in dendrimers have
been extensively examined by several groups. Balzani and co-
* Authors to whom correspondence should be addressed. Y.L.: 86-010-
64865872, phone; 86-010-64865872, fax. G.Y.: 86-010-82617263, phone;
86-010-82617315, fax.
† Technical Institute of Physics and Chemistry, Chinese Academy of
Sciences.
(4) (a) Denti, G.; Campagna, S.; Serroni, S.; Ciano, M.; Balzani, V. J. Am.
Chem. Soc. 1992, 114, 2944. (b) Balzani, V.; Campagna, S.; Denti, G.;
Juris, A.; Serroni, S.; Venturi, M. Acc. Chem. Res. 1998, 31, 26.
(5) (a) Devadoss, C.; Bharati, P.; Moore, J. S. J. Am. Chem. Soc. 1996, 118,
9635. (b) Shortreed, M. R.; Swallen, S. F.; Shi, Z.-Y.; Tan, W.; Xu, Z.;
Devadoss, C.; Moore, J. S.; Kopelman, R. J. Phys. Chem. B 1997, 101,
6318.
(6) (a) Kawa, M.; Fre´chet, J. M. J. Chem. Mater. 1998, 10, 286. (b) Kawa,
M.; Fre´chet, J. M. J. Thin Solid Films 1998, 331, 259.
(7) (a) Jiang, D.-L.; Aida, T. Nature 1997, 388, 454. (b) Jiang, D.-L.; Aida,
T.; Yashima, E.; Okamoto, Y. Thin Solid Films 1998, 331, 254.
‡ Institute of Chemistry, Chinese Academy of Sciences.
(1) McDermott, G.; Prince, S. M.; Freer, A. A.; Hawthornthwaite-Lawless, A.
M.; Papiz, M. Z.; Cogdell, R. J.; Isaace, N. W. Nature 1995, 374, 517.
(2) Hu, X.; Damjanovic, A.; Ritz, T.; Schulten, K. Proc. Natl. Acad. Sci. U.S.A.
1998, 95, 5935.
(3) (a) Tomalia, D. A.; Naylor, A.; Goddard, W. A. Angew. Chem., Int. Ed.
1990, 29, 138. (b) Fre´chet, J. M. J. Science 1994, 263, 1710. (c) Newkome,
G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendrimers and Dendrons: Concept,
Synthesis, Application; VCH: Weinheim, Germany, 2001.
9
10.1021/ja044800b CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 2165-2171
2165

A R T I C L E S
Chen et al.
decreasing for generation 4 (∼93%).8 Mu¨llen, De Schryver, and
their co-workers developed a system in which the donor (D)
and the acceptor (A) were separated by rigid polyphenylene
dendrimers and found that intramolecular directional Fo¨rster
resonance energy transfer took place very efficiently.9
play an antenna role in the energy transfer. The dendrimers are
synthesized up to the fourth generation, as shown in Figure 1.
In these molecules, the BP chromophore can be selectively
excited. After intersystem crossing with 100% efficiency, the
triplet energy of BP is transferred to NBD, resulting in the
isomerization of the latter to the quadricyclane (QC) group. The
efficiency and the absolute rate constant of intramolecular triplet
energy transfer were examined by steady-state and time-resolved
spectroscopy. The rate constant decreases inconspicuously as
the generation increases. These findings provide a new example
of the use of the dendrimer for light harvesting; the periphery
“antenna” chromophores “harvest” photon energy which is then
utilized to activate the core group via the Dexter energy transfer
mechanism.
Up to now, most of the studies and other experiments10 on
intramolecular energy transfer between the periphery chro-
mophores and the core in dendrimers are related to the singlet
states only, and the energy transfer proceeds via Fo¨rster
mechanism. The Fo¨rster mechanism is a through-space dipole-
dipole interaction, and the D-A orbital overlap is not necessary.
This allows the chromophores to be separated by a relatively
large distance (10-100 Å). Within dendritic structures, the
distance between D and A is less than several tens of angstroms,
even D and A are located at the periphery and the core,
respectively, in high generations. It is expected that the energy
transfer via the Fo¨rster mechanism can occur efficiently from
the periphery to the core in the dendritic structure, and the
experimental results confirm this hypothesis.8,9 However, den-
drimers capable of light harvesting through the triplet-triplet
energy transfer mechanism have rarely been demonstrated. As
we know, the triplet-triplet energy transfer is the most common
and most important type of energy transfer involved in chemical
and biochemical processes.11 The mechanism for triplet energy
transfer is usually described by Dexter electron-exchange
interaction12 and may be visualized in terms of two electron
transfer processes or one electron transfer and one hole transfer
processes.11 Generally, this electron exchange requires strong
D-A orbital overlap, and therefore the energy transfer rate
constant in this case decreases exponentially with increasing
D-A distance. Thus, one might expect that the rate constant
of triplet energy transfer will become negligibly small as the
D-A distance increases beyond 10 Å,11 except the energy
transfer occurs via a “through-bond mechanism” in a conjugated
or rigid system. Furthermore, most of the studies of intramo-
lecular energy transfer of dendrimer concern photophysical
processes. There are only a few examples of application of
intramolecular energy transfer dealing with photochemical
process.7
Experimental Section
Materials. Reagents were purchased from Aldrich or Acros and were
used without further purification, unless otherwise noted. Tetrahydro-
furan (THF) was distilled over Na/benzophenone under an argon
atmosphere. Acetone was dried with anhydrous K2CO3 and distilled.
Spectral-grade 2-methyltetrahydrofuran and dichloromethane (CH2Cl2)
were used for absorption and emission spectra, flash photolysis, and
steady-state photoirradiation measurements.
Instrumentation. 1H NMR spectra were recorded on either a Varian
Gemini 300 MHz or a Bruker 400 MHz spectrometer. IR spectra were
run on a Bio-Rad Win IR spectrometer. MALDI-TOF mass spectrom-
etry was performed on a Bruker BIFLEX spectrometer. Elemental
analyses were carried out on a Flash EA1112 spectrometer. Melting
points were determined on a XT4A apparatus and were uncorrected.
HPLC was recorded at a Hitachi system with an Alltima LC-Si 5 µm
column (4.6 mm ID, 25 cm) and a UV-vis detector. Steady-state
absorption spectra and phosphorescence spectra were measured by a
Shimadzu UV-1601PC spectrometer and a Hitachi F-4500 spectrometer,
respectively.
Phosphorescence Measurements. Phosphorescence studies were
performed in 2-methyltetrahydrofuran at 77 K, and the sample solutions
were degassed by at least three freeze-pump-thaw cycles at a pressure
of 5 × 10-5 Torr. The excitation wavelength was 343 nm. For
comparison of the emission efficiency of Gn-NBD with Gn-QC, the
spectra were run using solutions with identical optical density at the
excitation wavelength. The relative emission efficiencies were measured
from the peak areas of the emission spectra.
In the present work, we create a dendritic system in which
the energy transfer between the peripheral chromophores and
the core is suitable for Dexter triplet state energy transfer, and
a photochemical reaction of the core is used as the probe to
detect the energy transfer occurrence. Benzophenone (BP)
chromophores and norbornadiene (NBD) group are attached to
the periphery and the core of the poly(aryl ether) dendrimer,13
respectively, and the dendritic backbone acts as a scaffold that
holds D and A in a desired spatial arrangement and does not
Redox Potentials of BP and MNBD. The redox potentials of BP
and MNBD were determined by cyclic voltammetry in dichloromethane,
using a 10 µm platinum microelectrode and a Ag/Ag+ (the concentration
of Ag+ is 0.01 M) reference electrode in the presence of 0.1 M
tetrabutylammonium perchlorate as the supporting electrolyte.
Laser Flash Photolysis. Nanosecond transient absorption spectra
were performed on a LP-920 pump-probe spectroscopic setup (Ed-
inburgh). The excited source was the unfocused third harmonic (355
nm, 7 ns fwhm) output of a Nd:YAG laser (Continuum surelite II);
the probe light source was a pulse-xenon lamp. The signals were
detected by Edinburgh analytical instruments (LP900) and recorded
on a Tektronix TDS 3012B oscilloscope and computer.
(8) (a) Adronov, A.; Gilat, S. L.; Fre´chet, J. M. J.; Ohta, K.; Neuwahl, F. V.
R.; Fleming, G. R. J. Am. Chem. Soc. 2000, 122, 1175. (b) Adronov, A.;
Fre´chet, J. M. J. Chem. Commun. 2000, 1701.
(9) (a) Gronheid, R.; Hofkens, J.; Ko¨hn, F.; Weil, T.; Reuther, E.; Mu¨llen, K.;
De Schryver, F. C. J. Am. Chem. Soc. 2002, 124, 2418. (b) Cotlet, M.;
Gronheid, R.; Habuchi, S.; Stefan, A.; Barbafina, A.; Mu¨llen, K.; Hofkens,
J.; De Schryver, F. C. J. Am. Chem. Soc. 2003, 125, 13609.
Results and Discussion
(10) (a) Stewart, G. M.; Fox, M. A. J. Am. Chem. Soc. 1996, 118, 4354. (b)
Aida, T.; Jiang, D.-L. J. Am. Chem. Soc. 1998, 120, 10895. (c) Gilat, S.
L.; Adronov, A.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 1999, 38, 1422.
(d) Hahn, U.; Gorka, M.; Vogtle, F.; Vicinelli, V.; Ceroni, P.; Maestri, M.;
Balazani, V. Angew. Chem., Int. Ed. 2002, 41, 3595.
Synthesis and Stability of the Dendrimers. The synthesis
and characterization of the compounds are described in Sup-
porting Information. All of the compounds have been purified
by column chromatography. The target compounds, Gn-NBD
(n ) 1-4), were characterized by 1H NMR, elemental analysis,
IR, mass spectrometry (MALDI-TOF or EI), and HPLC (see
(11) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cumming:
Menlo Park, CA, 1978; Chapter 9.
(12) (a) Dexter, D. L. J. Chem. Phys. 1953, 21, 836. (b) Katz, J. L.; Jortner, J.;
Chol, S. I.; Rice, S. A. J. Chem. Phys. 1963, 39, 1897.
(13) (a) Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638. (b)
Hawker, C. J.; Fre´chet, J. M. J. J. Chem. Soc., Chem. Commun. 1990, 1010.
9
2166 J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005

BP and NBD-Labeled Poly(aryl ether) Dendrimers
A R T I C L E S
Figure 1. Structure of MNBD and Gn-NBD compounds.
Supporting Information). For Gn-QC (n ) 1-4), the valence
isomers with structure similar to that of Gn-NBD were used as
1
the model compounds. They were also characterized by H
NMR and mass spectrometry (MALDI-TOF or EI). Gn-NBD
(n ) 1-4) must be stored in the dark because they are very
sensitive to the light, which can induce the isomerization of
Gn-NBD to Gn-QC in solution even with room light, especially
for the higher generation.
Steady-State Absorption and Phosphorescence Spectro-
scopy. The absorption spectra of dendrimers (Gn-NBD), the
models for the donor (Gn-QC) (see the structure in Supporting
Information), and the model for the acceptor (dimethyl bicyclo-
[2.2.1]hepta-2,5-diene-2,3-dicarboxylate (MNBD)) were mea-
sured in dichloromethane. Figure 2 illustrates the absorption
spectra of G2-NBD, G2-QC, and MNBD in dichloromethane.
There is no measurable interaction between the NBD, and BP
chromophores of Gn-NBD in the ground state can be observed
from the absorption spectra. Significantly, the absorption of the
Figure 2. Absorption spectra for acceptor model compound MNBD, donor
model compound G2-QC, and target compound G2-NBD.
BP group extends to a wavelength longer than that of the NBD
group, which suggests that singlet-singlet energy transfer from
the excited BP chromophore to the NBD group is endothermic
and, consequently, unlikely. Furthermore, this fact permits
selective excitation of the BP moiety in the Gn-NBD system.
9
J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005 2167

A R T I C L E S
Chen et al.
Figure 3. Absorption spectra of Gn-NBD in CH2Cl2.
Figure 5. Transient absorption spectra of the benzophenone triplet state
formed upon laser photolysis of G3-NBD and G3-QC in CH2Cl2.
energy change involved in an electron-transfer process can be
calculated by the Rehm-Weller equation:14
∆G (kcal/mol) ) 23.06[E(D•+/D) - E(A/A•-) - e2/rꢀ] -
E
00 (kcal/mol) (1)
E00 is the excited-state energy and in this study represents
the triplet state energy of the 4-substituent benzophenone group,
which is 69 kcal/mol.15 The e2/rꢀ represents the Columbic
energy associated with bringing separated radical ions at a
distance r in a solvent of dielectric constant ꢀ (r ) 9.2 Å in
G1-NBD, 11.7 Å in G2-NBD, 14.4 Å in G3-NBD, and 16.5 Å
in G4-NBD; see below). Calculation according to eq 1 shows
that ∆G ) 20.1-20.5 kcal/mol from G1-NBD to G4-NBD,
suggesting that electron transfer from NBD to the triplet state
BP group would be very inefficient if any did occur. Further-
more, in the flash photolysis study, we could not detect any
transient absorption attributable to a BP anion radical (see
below). On the other hand, the triplet energy of NBD (53 kcal/
mol)16 is much lower than that of the BP group (69 kcal/mol).
Thus, triplet-triplet energy transfer from the triplet excited BP
chromophore to the NBD group is thermodynamically possible.
Therefore, we infer that the quenching of the BP phosphores-
cence in Gn-NBD is due to the intramolecular triplet-triplet
energy transfer to the NBD group.
Flash Photolysis. The evidence for the intramolecular triplet-
triplet energy transfer in Gn-NBD based on phosphorescence
efficiency is further strengthened by flash photolysis study.
Pulsed-laser photolysis of the model compounds, Gn-QC, and
the target compounds, Gn-NBD, was performed in degassed
CH2Cl2 by using 355 nm excitation light, which gives rise to a
strong transient absorption spectra with a maximum at 536 nm.
The typical transient absorption spectra for G3-NBD and G3-
QC are given in Figure 5.
Figure 4. Phosphorescence spectra of G2-NBD and G2-QC in 2-meth-
yltetrahydrofuran at 77 K (λex ) 343 nm). Concentrations computed with
benzophenone are 2 × 10-5 M.
Figure 3 shows the absorption spectra of the Gn-NBD series
indichloromethane at the same concentration. Within experi-
mental error, the absorption due to BP is proportional to the
number of peripheral chromophores, which means that the light-
harvesting capability of the molecules can be enhanced by
increasing the generation number.
The emission spectra of Gn-NBD and Gn-QC were studied
in glassy 2-methyltetrahydrofuran at 77 K. An example of the
emission spectra of G2-NBD and G2-QC is presented in Figure
4. No fluorescence from these two compounds was observed
with excitation at the maximal absorption of BP (343 nm), while
a phosphorescence characteristic of the benzophenone with
maxima at 416, 445, 480, and 519 nm was detected for both
G2-NBD and G2-QC. The shape and peak positions of these
two phosphorescence spectra are essentially identical, but the
overall intensities are different. The phosphorescence efficiency
of G2-NBD is ca. 52% less than that of model compound G2-
QC. The other generations show similar results but various
degrees of decreasing in phosphorescence efficiency, G1 with
95%, G3 with 38%, and G4 with 36%. This finding indicates
that the phosphorescence of BP is quenched by the NBD group
in Gn-NBD. Measurements at different concentrations reveal
that the quenching is intramolecular.
This absorption is assigned to the lowest triplet state of the
BP chromophore on the basis of the following observations.
First, this absorption is essentially identical to that of the alkyl
benzophenone-4-carboxylate triplet state independently gener-
To clarify the reason for the intramolecular quenching of BP
phosphorescence by the NBD group in Gn-NBD, we measured
the redox potentials of BP and the model compound, MNBD.
The reduction potentials of MNBD, E(NBD•+/NBD), and BP,
E(BP/BP•-), were determined in dichloromethane to be +1.50
and -2.40 V, respectively, with respect to Ag/Ag+. The free
(14) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259.
(15) The triplet energy of 4-substituted benzophenone was estimated from the
0-0 band of the phosphorescence spectrum of Gn-NBD in 2-methyltet-
rahydrofuran.
(16) Wu, Q. H.; Zhang, B. W.; Ming, Y. F.; Cao, Y. J. Photochem. Photobiol.,
A: Chem. 1991, 61, 53.
9
2168 J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005

BP and NBD-Labeled Poly(aryl ether) Dendrimers
A R T I C L E S
Table 2. Energy Transfer Efficiencies and Rate Constants for
Compounds Gn-NBD
1
P
compound
k
ET (s-
)
ΦET
ΦET
G1-NBD
G2-NBD
G3-NBD
G4-NBD
2.49 × 107
7.32 × 106
8.16 × 106
6.11 × 106
0.98
0.56
0.51
0.37
0.95
0.52
0.38
0.36
intended to signify the exact number of distinct processes being
observed.
For the donor model compounds Gn-QC, the average lifetime
decreases as the generation increases. The generation-dependent
trend for the target compounds Gn-NBD is the same as that of
the donor model compounds. These should be mainly induced
by the triplet-triplet annihilation. Meanwhile, the shorter
average lifetime of the triplet state for Gn-NBD in comparison
with that of corresponding Gn-QC is consistent with the result
of the phosphorescence experiments and indicates that an
intramolecular triplet-triplet energy transfer from BP to NBD
chromophores in Gn-NBD indeed occurs. The rate constant (kET)
and efficiency (ΦET) for the energy transfer can be calculated
Figure 6. Decay traces for G3-NBD and G3-QC.
Table 1. Triplet State Lifetime for Dendrimers Gn-NBD and
Gn-QC, All Measured in CH2Cl2 by Flash Photolysis
compound
τ1 (ns)
a1
τ
2 (ns)
a2
τ
(av) (ns)
G1-NBD
G2-NBD
G3-NBD
G4-NBD
G1-QC
G2-QC
G3-QC
G4-QC
14
21
25
23
633
139
57
0.90
0.70
0.72
0.80
0.82
0.82
0.72
0.76
269
206
160
214
9348
333
314
290
0.10
0.30
0.28
0.20
0.18
0.18
0.28
0.24
40
77
63
61
2202
174
129
98
from the average lifetime of Gn-NBD (τNBD) and Gn-QC (τQC
)
according to eqs 2 and 3, respectively. The results are shown
in Table 2. Along with the generation increase, the distance
between donor and acceptor becomes larger, and we expected
that kET should decrease dramatically. However, the experi-
mental results show that kET for different generations is at the
same magnitude. The reason for this unexpected result is
discussed in the Mechanism of Energy Transfer section below.
37
ated.17 Second, the 536 nm species is rapidly quenchable by
O2. The transient absorption of the model compounds Gn-QC
is identical with that of Gn-NBD. Taking the transient spectra
of Gn-NBD and Gn-QC at 536 nm as a function of time gives
the decay curves of the compounds. Figure 6 shows the decay
traces of G3-NBD and G3-QC, and the entire set of acquired
data is summarized in Table 1.
kET ) 1/τNBD - 1/τQC
ΦET ) 1 - τNBD/τQC
(2)
(3)
None of the compounds investigated exhibit monoexponential
decay profiles. This result is quite compatible with the data
reported for laser-dye-labeled poly(aryl ether) dendrimers by
Fre´chet and co-workers.8 They propose that the flexible nature
of the dendrimers is responsible for the nonmonoexponential
behavior. The conformational freedom of the dendritic backbone
creates a variety of local microenvironments for the individual
dyes. In our system, the flexible poly(aryl ether) dendritic
backbone is also used. Since the photophysical properties of
BP are not very sensitive to the environments, the changes of
the local microenvironment induced by the dendrimer confor-
mations would not affect the triplet state lifetimes of the
individual BP dramatically. The molecule modeling (calculated
by HyperChem 6.0) shows that the BP groups are close to each
other, especially in higher generations (G2-G4). Molecular
modeling suggests that 4 Å is the shortest distance between the
BP chromophores for generations 2-4, and triplet-triplet
annihilation could easily occur within that space.11,18 So, we
propose that nonmonoexponential behavior is mainly due to the
intramolecular triplet-triplet annihilation. Phosphorescence
measurement and the laser power dependence of the transient
data support this supposition. All acquired data could be well
fitted by double-exponentials with acceptable ø2 value. However,
it should be noted that the number of exponentials used is not
The energy transfer efficiency, ΦET, decreases with the
generation increase, and this result agrees with the trend obtained
by the phosphorescence measurements (ΦETP). A little difference
of the energy transfer efficiencies from the steady-state and time-
resolved data for the same generation can be rationalized to the
accuracy and the different experimental conditions. Usually, the
increase in donor-acceptor separation that occurs as dendrimer
generation increases is thought to be the reason for the decrease
in energy transfer efficiency. In our work, the triplet-triplet
annihilation of BP chromophores could be the main cause of
the decrease of energy transfer efficiency because of the small
change of the energy transfer rate constant with the generation.
(See more in the Energy Transfer Mechanism part.)
Photosensitized Isomerization of the Norbornadiene to the
Quadricyclane Group in Gn-NBD. The photosensitized va-
lence isomerization of NBD to QC has been the subject of
intense experimental and theoretical investigations19 in view of
its significance in solar energy storage20 and mechanism
interests.21 Benzophenone and some other aromatic ketones are
known to sensitize NBD f QC isomerization through triplet-
(19) Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401.
(20) (a) Yashida, Z. I. J. Photochem. 1985, 29, 27. (b) Harel, Y.; Adamson, A.
W.; Kutal, C.; Gotsch, P. A.; Yasufuku, K. J. Phys. Chem. 1987, 91, 901.
(c) Basu, A.; Saple, A. R.; Sapre, N. Y. J. Chem. Soc., Dalton Trans. 1987,
1797.
(21) (a) Schwarz, W.; Dangel, K. M.; Jones, G., II; Bargan, J. J. Am. Chem.
Soc. 1982, 104, 5686. (b) Arai, T.; Oguchi, T.; Wakabayashi, T.; Tsuchiya,
M.; Nishimura, Y.; Oishi, S.; Sakuragi, H.; Tokumarce, K. Bull. Chem.
Soc. Jpn. 1987, 60, 2937.
(17) Tung, C. H.; Zhang, L. P.; Li, Y. J. Phys. Chem. 1996, 100, 4480.
(18) Saltiel, J.; Atwater, B. W. AdVance in Photochemistry; Wiley-Inter-
science: New York, 1987; Vol. 14, p 1.
9
J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005 2169

A R T I C L E S
Scheme 1
Chen et al.
Scheme 2
phosphorescence and flash photolysis experiments mentioned
above. Φiso(NBD) represents the quantum yield of the isomer-
ization reaction of the NBD triplet state and is 0.185.17 These
in turn give Φiso(Gn-NBD) as 0.18, 0.10, 0.082, and 0.068 for
generations 1-4, respectively. The quantum yield of the
isomerization of Gn-NBD decreases with increasing generation
due to the decrease of the energy transfer efficiency.
Further intramolecular photosensitized isomerization experi-
ments of Gn-NBD in degassed CH2Cl2 were done to determine
the light-harvesting ability of Gn-NBD. The same concentration
(2.0 × 10-5 M) was used in the experiments for Gn-NBD. After
irradiation for 5 min with a 450 W high-pressure mercury lamp
in a merry-go-round apparatus, the conversions of Gn-NBD to
Gn-QC were 9, 15, 22, and 29% (the experiment accuracy is
(3%) for G1-G4, respectively. The observed rates of the
isomerization for G1-NBD-G4-NBD were 3.6 × 10-7, 6.0 ×
10-7, 8.8 × 10-7, and 1.2 × 10-6 M min-1, respectively.
Obviously, the higher isomerization rate for higher generation
is attributed to the light-harvesting ability of Gn-NBD, referring
to the larger molar absorption coefficient of higher generation.
triplet energy transfer.22 Thus, study of the intramolecular
photosensitized isomerization of the NBD group in Gn-NBD
may provide evidence of triplet-triplet energy transfer in the
dendritic system.
Irradiation with λ > 350 nm of a 2.0 × 10-5 M (concentration
is calibrated by BP chromophore) solution of Gn-NBD in
degassed CH2Cl2 at room temperature results in valence
isomerization of the norbornadiene group to the quadricyclane
group (Gn-QC), as shown in Scheme 1. Under this condition,
only the BP chromophores absorb the light. Thus, the isomer-
ization of NBD to QC must be attributed to the energy transfer.
The yield of the isomerization product is almost 100% on the
basis of the consumption of the starting material. The assignment
of the product as the quadricyclane derivative relies mainly on
Mechanism of BP to NBD Group Intramolecular Triplet-
Triplet Energy Transfer. The flash photolysis and photosen-
sitization reaction experiments reveal that the excitation of the
BP chromophore in Gn-NBD results in an intramolecular
triplet-triplet energy transfer to the NBD group and subse-
quently leads to the isomerization of the latter group to QC
(Scheme 2). The efficiencies of such triplet-triplet energy
transfer are ca. 97, 54, 45, and 37% (average of steady-state
and transient data), and the rate constants are ca. 2.5 × 107, 7.3
× 106, 8.2 × 106, and 6.1 × 106 s-1 for generations 1-4,
respectively.
1
its H NMR spectrum, which is in close agreement with that
reported in the literature.16 The mass spectrometry shows that
the product and the starting material have the same molecular
weight. Measurements of the product formation at different
concentrations revealed that the isomerization is induced by
intramolecular photosensitization. On the basis of the experi-
mental results mentioned above, the primary photophysical and
photochemical processes in Gn-NBD can be expressed by
Scheme 2.
It has been well established that triplet-triplet energy transfer
proceeds via the Dexter electron-exchange mechanism, and its
rate constant decreases exponentially with an increasing of the
distance between D and A.11 The energy transfer is normally
expected to become very inefficient as the D-A distance
increases beyond 10 Å. In this study, the BP and NBD
chromophores are located at the periphery and the core of the
poly(aryl ether) dendrimer, Gn-NBD (n ) 1-4). Since the poly-
(aryl ether) dendrimer is not rigid, it is difficult to determine
the interchromophoric distances within each generation. We used
the HyperChem 6.0 program to calculate the lowest-energy
conformation for each generation. While optimizing the con-
formation, the solvent effect could not be taken into account
due to the limitation of the HyperChem program. The distance
The quantum yield of this intramolecular photosensitization
isomerization, Φiso(Gn-NBD), can be calculated according to
eq 4:
Φiso(Gn-NBD) ) ΦiscΦETΦiso(NBD)
(4)
where Φisc represents the quantum yield of the intersystem
crossing from the singlet to the triplet excited state of the BP
group and is assumed to be unity.11 ΦET was obtained by
(22) (a) Murov, S.; Hammond, G. S. J. Phys. Chem. 1968, 72, 3797. (b)
Hammond, G. S.; Wyatt, P.; Deboer, C. D.; Turro, N. J. J. Am. Chem.
Soc. 1964, 86, 2532.
9
2170 J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005

BP and NBD-Labeled Poly(aryl ether) Dendrimers
A R T I C L E S
Table 3. Distance between the BP and NBD
Conclusions
D
−
A distance (Å)
G1
G2
G3
G4
Flash photolysis and photochemical reaction studies demon-
strate that triplet energy in Gn-NBD can be transferred from
the peripheral BP groups to the core NBD group with 97, 54,
45, and 37% efficiency for generations 1-4, respectively,
leading to the valence isomerization of the NBD group to QC.
The higher isomerization rate at higher generation indicates that
the light-harvesting ability of Gn-NBD increases with generation
due to the number of peripheral chromophores that doubles from
one dendrimer generation to the next, although the energy
transfer efficiency decreases with increasing generation. The
triplet energy transfer rate constant is ca. 106-107 s-1, almost
independent of the generation. The intramolecular triplet energy
transfer proceeds mainly via a through-space mechanism
involving the closest donor (folding back conformation) and
acceptor groups. These findings reveal that one can use the
peripheral antenna chromophore to harvest photon energy which
is then utilized to initiate chemical reaction of a functional group
at the core of the dendritic system.
average
shortest
9.2
7.2
11.7
7.0
14.4
6.2
16.5
6.3
between the BP and NBD groups is defined as the distance
between the C of the carbonyl in BP and the C of the vinyl
closely connected to the dendrimer backbone. The average
distances from our calculation are shown in Table 3, which are
consistent with the data obtained by Fre´chet et al.8 The average
distance between D and A increases with the generation and is
9.2 Å for generation 1. At such separation between the
chromophores, triplet-triplet energy transfer via a through-space
exchange process would be very inefficient and this is contrary
to the experimental results. From the molecular modeling
(shown in Supporting Information), we noticed that there is
always a BP group folding back which is close to the acceptor
at the core. Calculation of the shortest distance between the BP
and NBD groups gave a similar number for generations 1-4,
ca. 6-7 Å. In these distances, the triplet-triplet energy transfer
could occur, which is accordant with the time-resolved results.
One may argue that the energy transfer could proceed via a
through-bond mechanism. However, if this energy transfer is
via a through-bond mechanism, the rate constant will decrease
with the generation increase. This is not consistent with the
observation for the generation independence of the rate con-
stants. On the basis of the calculation and experimental results
mentioned above, we propose that the energy transfer mainly
proceeds via a through-space mechanism. After the BP group
is selectively excited, the intersystem crossing occurs with 100%
efficiency. The triplet energy of BP migrates from one BP group
to another rapidly and is transferred to the NBD group mainly
via the closest BP group, which results in the isomerization of
NBD to the QC group.
Acknowledgment. Financial support for this work by Na-
tional Science Foundation of China (NO. 20273081), the
Ministry of Science and Technology of China (G2000078100),
and Chinese Academy of Sciences is gratefully acknowledged.
We also acknowledge help with the manuscript by Coracle
Optics Co.
Supporting Information Available: Experimental procedures
for the synthesis of all dendritic compounds, including analytical
data. Structures of model compounds, phosphorescence spectra
of Gn-NBD and Gn-QC, and molecular modeling for Gn-NBD.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JA044800B
9
J. AM. CHEM. SOC. VOL. 127, NO. 7, 2005 2171