Modulate photoinduced electron transfer efficiency of bipolar dendritic systems

Article abstract of DOI:10.1021/ol801096c

(Chemical Equation Presented) Covalent linkage of dendritic carbazole-based donors and 1,3,4-oxdiazole-based acceptors renders novel bipolar dendrimers that can efficiently facilitate the photoinduced electron transfer (PET) process. Photodynamic studies

Full text of DOI:10.1021/ol801096c

ORGANIC
LETTERS
2008
Vol. 10, No. 15
3211-3214
Modulate Photoinduced Electron
Transfer Efficiency of Bipolar Dendritic
Systems
Yu-Hsien Lin, Hsu-Hsuan Wu, Ken-Tsung Wong,* Cheng-Chih Hsieh, Yi-Chih Lin,
and Pi-Tai Chou*
Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan
kenwong@ntu.edu.tw; chop@ntu.edu.tw
Received May 12, 2008
ABSTRACT
Covalent linkage of dendritic carbazole-based donors and 1,3,4-oxdiazole-based acceptors renders novel bipolar dendrimers that can efficiently
facilitate the photoinduced electron transfer (PET) process. Photodynamic studies indicated that the PET rate of bipolar dendrimers DA1 and
DA2 can be modulated by the number of acceptors presented in the molecule.
Photoinduced electron transfer (PET) is an essential process
for solar energy conversion both in biological photosynthesis
and artificial photovoltaic devices. PET occurring in molec-
ular systems can be achieved by a covalently linked donor
(D) and acceptor (A) with judicious choice of a bridging
system. The rate and efficiency of electron-transfer processes
can be controlled over parameters such as distance and spatial
orientation between donor and acceptor. In the biological
light-harvesting systems, the light-capturing chromophores
are assembled into well-organized arrays which then funnel
the excitation energy to the photosynthetic center in a very
efficient way, triggering the charge separation for subsequent
chemical reactions.1 Inspired by this natural preorganization
process, scientists have made significant endeavors for
assembling molecules into an integrated system to mimic
the natural photosynthesis center. In this regard, the unique
molecular architectures of dendrimers provide the essential
structural features for the preparation of artificial light-
harvesting systems.2 Most dendrimers enabled for efficient
electron transfer are usually featured with electron-donating
dendrons, which serve as antenna located at the peripheral
positions of a well-defined electron-accepting core or focal
point. The rate and efficiency of PET can thus be manipulated
by independent modifications on the chemical structures of
donor and acceptor as well as the donor’s dendritic genera-
tions.3 More recently, efficient PET processes can also be
achieved in supramolecular arrays formed by noncovalent
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10.1021/ol801096c CCC: $40.75
Published on Web 06/27/2008
2008 American Chemical Society

interactions between electron-donating dendrimers and elec-
tron-accepting chromophores.4
Carbazole and 1,3,4-oxdiazole were selected as donor and
acceptor, respectively, because both of them were ubiqui-
tously exploited in organic electronics for charge transport
functions.5 Covalent linkage of dendritic donors and accep-
tors creates a novel bipolar system equipped with multiple
donors and acceptors that may facilitate the PET process.
Photodynamic studies indicate that the PET rate of bipolar
dendrimers DA1 and DA2 can be modulated by the number
of acceptors presented in the D-A dendrimers.
The synthesis of D-A dendrimers (DA1 and DA2) is
depicted in Scheme 1. The 9-phenylcarbazole-based dendron
(D) was synthesized by our group previously through
efficient C-N bond formation reactions.6 The acceptor
counterpart (A-Br) bearing two 1,3,4-oxadiazole moieties was
synthesized according to the reported procedures.7 The
covalent hybridization of the donor and the acceptor was
successfully achieved by a Pd-catalyzed C-N bond coupling
reaction of 9-phenylcarbazole-based dendron D with 1,3,4-
oxdiazole-containing bromideA in the presence of Pd2(dba)3
as a catalyst and 2-dicyclohexylphospine biphenyl as the
ligand, yielding DA1 and DA2 in 30% and 67%, respec-
tively.
Instead of making a funnel-like dendritic system, we have
long anticipated that dendrimers incorporated with preorga-
nized donors (donor dendron) for capturing excitation energy
and with multiple electron acceptors (acceptor dendron) for
trapping the photoexcited electrons could give rise to a new
type of bipolar dendrimer with a more efficient PET process.
In this paper, we report the synthesis, electrochemical, and
phophysical properties of two novel D-A dendrimers (DA1
and DA2, Scheme 1), in which the electron-donating
Scheme 1. Synthesis of Bipolar Dendrimers DA1 and DA2
Both of DA1 and DA2 are amorphous materials, exhibiting
Tg at 237 and 250 °C, respectively, analyzed by differential
scanning calorimetry (DSC), and show high thermal tolerance
(Td > 448 °C), analyzed by thermogravimetric analysis
(TGA). The bipolar characters of DA1 and DA2 were first
probed by cyclic voltammetry (CV). Both compounds exhibit
two reversible one-electron oxidation potentials (DA1: 0.25,
0.44; DA2: 0.28, 0.46 V vs ferrocene/ferrocenium ion, Fc/
Fc+) followed by a multiple-electron oxidation peak centered
at 0.80 V (DA2 shown in Figure 1, DA1 shown in Figure
S-1, Supporting Information). We assigned the first two peaks
to the successive oxidations of the donor’s central part: 3,6-
diaminocarbazole. Upon attaching to the acceptor(s), these
two oxidations are slightly shifted to higher potentials as
compared to those of the donor-only compound D, which
displays the first two oxidation peaks at 0.22 and 0.42 V.
The third oxidation potential (0.80 V) then was ascribed to
the oxidations of peripheral carbazoles. DA1 and DA2
showed reversible reduction peaks at -2.47 and -2.23 V,
respectively. The model compound A (see the Supporting
Information) also displayed a reversible reduction potential
at -2.42 V. Thus, the cathodic reduction couples can be
unambiguously attributed to the reduction process at the
phenylene ring containing 1,3,4-oxdiazole moieties. Interest-
ingly, the DA2 has a lower reduction potential compared to
9-phenylcarbazole-based dendron was coValently hybridized
with an electron-accepting 1,3,4-oxdiazole-based dendron.
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Org. Lett., Vol. 10, No. 15, 2008

Table 1. Photophysical Properties of D-A Dendrimers DA1
and DA2 in Various Solvents
solvent
λmax (nm)
relaxation dynamicsa (ns)
Φ
DA1
toluene
450
450 nm: τ1 ) 0.290 (0.62)
τ2 )10.79 (0.38)
0.147
580 nm: τ1 ) 0.285 (-0.33)
τ2 )11.36 (0.67)
THF
435
560
450 nm: τ1 ) 0.068 (0.98)
τ2 ) 5.65 (0.02)
0.047
0.012
0.121
600 nm: τ1 ) 0.065(-0.67)
τ2 ) 8.95 (0.33)
Figure 1. Cyclic voltammogram of compound DA2 (oxidation in
CH2Cl2 and reduction in THF).
CH2Cl2
435
620
450 nm: τ ) 0.0594b
600 nm: τ1 ) 0.0603 (-0.36)b
τ2 ) 4.78 (0.64)
DA2
toluene
that of DA1; this result points out that the LUMO energy of
a D-A system can be subtly modulated by the number of
acceptors attached.8
495
450 nm: τ1 ) 0.263 (0.64)
τ2 ) 12.75 (0.36)
580 nm: τ1 ) 0.250 (-0.22)
τ2 ) 14.18 (0.78)
THF
590
630
450 nm: τ1 ) 0.0021 (0.99)b
τ2 ) 15.79 (0.01)
0.026
0.012
610 nm: τ ) 17.70
CH2Cl2
480 nm: τ1 ) 0.0014 (0.80)b
τ2 ) 4.02 (0.20)
600 nm: τ1 ) 0.0016 (-0.28)b
τ2 ) 4.92 (0.72)
a Data in parentheses is the fitted pre-exponential factor normalized to
1. b The ultrafast relaxation dynamics were measured by a femtosecond
up-conversion system.
electronic coupling) between D and A, so that D and A can be
treated as separated entities in the ground state. Furthermore,
the absorption spectra exhibit nearly solvent polarity indepen-
dence in the two D-A systems. The similar solvation effect
indicates a rather small dipolar change between ground and
Franck-Condon excited states. Conversely, the luminescence
spectra of these D-A dendrimers show remarkable solvent-
polarity dependence and reveal multiple emission in polar
solvents.
As a prototypical example, DA1 revealed a broad emission
band maximized at ∼450 nm in toluene (excitation at 380 nm).
Upon increasing solvent polarity, the broad emission band (in
toluene) gradually separated to dual emission with peak
wavelength at 435 nm (F1 band) and 560 nm (F2 band) in e.g.
THF. While the F1 band revealed nearly solvent independence,
the peak wavelenght of the F2 band at 560 nm further red-
shifted to 620 nm in CH2Cl2 (see Table 1). The quantum yield
of the entire emission decreased from 0.15 in toluene to 0.01
in CH2Cl2. The entire emission originating from a common
ground-state species is ascertained by the same fluorescence
excitation spectra throughout the monitored emission wave-
length of 450-700 nm, which are also effectively identical with
the absorption spectrum. In yet another approach, the control
D and A moieties exhibit solvent independent, normal Stokes
shifted emission maxima at 430 and 344 nm, respectively (see
Figure S-2, Supporting Information).
Figure 2. (a) Absorption and emission spectra of DA1 in toluene
(black), THF (red), and CH2Cl2 (green) where τex ) 380 nm. Inset:
emission spectrum of DA1 excited at 300 nm in CH2Cl2. (b)
Absorption and emission spectra of DA2 in toluene (black), THF
(red), and CH2Cl2 (green). All samples are prepared to be ∼10-5
M in various solvents.
Figure 2 depicts the steady-state absorption and emission
spectra of DA1 and DA2 in various solvents. Pertinent
spectroscopic and dynamic parameters are listed in Table 1.
For both DA1 and DA2, the absorption spectral features could
be well convoluted by a linear combination of D and A
chromophores, indicating a negligibly small interaction (weak
Similar solvent-polarity dependent emission was obtained for
DA2. In toluene, DA2 revealed a broad emission band with
(8) Ku, S.-Y.; Cheng, Y.-M.; Lin, X.-Y.; Hung, Y.-Y.; Pu, S.-C.; Wong,
K.-T.; Chou, P.-T.; Lee, G.-H. J. Org. Chem. 2006, 71, 456.
Org. Lett., Vol. 10, No. 15, 2008
3213

peak wavelength at 495 nm in toluene and dual emission bands
upon increasing the solvent polarity to THF and CH2Cl2, as
shown in Figure 2. However, prominent difference can also be
highlighted between DA1 and DA2. For comparison, in the
same solvents, the intensity ratio for F1 versus F2 band in DA2
was much lower than that of DA1, and F1 of DA2 even became
obscure in CH2Cl2. This intriguing observation can be rational-
ized via probing the corresponding relaxation dynamics for both
DA1 and DA2 (vide infra).
of the associated photophysical parameters are listed in Table 1.
The decay at 450 nm can be well fitted by two single exponential
components, 290 ps and 10.79 ns. Conversely, the red side at 580
nm consists of a rise component of 285 ps and a decay of 11.36
ns. The decay of 290 ps at 450 nm, within experimental error, is
consistent with the rise (285 ps) monitored at 580 nm, establishing
a precursor-successor type of PET process. Furthermore, the
identical decay component of ∼11 ns between the two wavelength
leads us to conclude the strong overlap of locally excited (F1) and
electron transfer (F2) emission of DA1 in toluene. Upon increasing
solvent polarity, a decrease of the F1 intensity implies a faster PET
process. This viewpoint was further supported by the relaxation
dynamics of F1 band of 68 and 59 ps in THF and CH2Cl2,
respectively (Table 1). Based on the same methodology, we then
focused on the relaxation dynamics of DA2 in toluene and also
resolved a fast decay (263 ps) and another exceedingly long lifetime
(12.75 ns) at short wavelength and a consistent risetime (250 ps)
at longer wavelength of DA2 (Table 1). Upon increasing the
solvent polariy, PET in DA2 is apparently more facile than that
in DA1, as supported by the resolution of faster decay 2.1 and
1.4 ps for DA2 (the F1 band) in THF and CH2Cl2, respectively.
The results qualitatively fit mechanism of PET, in which the
charge transfer species is further stabilized with increasing the
solvent polarity, resulting in the reduction of solvent-induced
barrier and hence the faster reaction rate.9 Finally, the relatively
long radiative lifetime (.10 ns) for DA1 and DA2 in various
solvents could be rationalized by a meta conjugation between
oxdiazole and 4-(9H-carbazol-9-yl)benzenamino moieties. In
the excited state, there exists a better communication mechanism
through the meta-connectivity than in the ground state.10 As a
result, a slow charge recombination rate is expected for the
charge transfer state (the F2 band).
As for the origin of the F2 band, we have prepared DA1 and
DA2 dendrimers in various solvents with different concentra-
tions of 2 × 10-6, 1 × 10-5, and 1 × 10-4 M to examine the
associated photophysical properties. As a result, the absorption
spectra, the intensity ratio for F1 versus F2 bands and the
emission profile all showed concentration independence. The
results eliminate the possibility that the F2 band originates from
the aggregation effect and/or the excimer emission. These, in
combination with the cascade type of HOMO/LUMO arrange-
ment between D and A (measured by CV, see the Supporting
Information), lead us to conclude the occurrence of PET
between D and A, resulting in the F2 emission (see Figure S-3,
Supporting Information). Accordingly, the F1 band can be
unambiguously assigned to the normal emission of the donor
moiety. It should be noted that when excitation at the shorter
wavelength of 300 nm, in which the transition, in part, is also
attributed to the acceptor moiety, DA1 exhibited distinct triple
emission, consisting of acceptor (∼360 nm), donor (450 nm)
and the charge transfer emission (620 nm) in CH2Cl2. The sum
of emission nearly covers the entire range of visible spectrum
(see inset of Figure 2a). The resulting multiple emission implies
finite rate of PET process, as supported by the following reaction
dynamics.
In the steady-state approach, the emission of DA1 in toluene
revealed a much broader bandwidth than that of the donor only
moiety (Figure S-2, Supporting Information) in the same solvent.
We thus suspect that PET is also operative on DA1 (or DA2) in
toluene. It is thus crucial to monitor the relaxation dynamics at
different wavelength of DA1 in toluene. As shown in Figure 3,
the decay traces were drastically different at the very blue side
and the red tail of the emission of DA1 (or DA2) in toluene. Details
In conclusion, two bipolar dendritic molecules DA1 and
DA2 composed of carbazole-based donors and a different
number of 1,3,4-oxadiazole-based acceptors have been
synthesized. These two dendritic systems exhibited efficient
charge transfer emission, allowing us to probe the detailed
electron transfer processes by spectroscopy and the associated
relaxation dynamics. The photodynamics indicate that the
dendrimer (DA2) equipped with more acceptors can ef-
ficiently facilitate the PET process. Our results may spark
the demanding for a new molecular design of bipolar
dendrimers use for highly efficient light-harvesting systems.
Acknowledgment. This study was supported financially
by the National Science Council and Ministry of Education
of Taiwan.
Supporting Information Available: Detailed experimen-
tal procedures of synthesis and photodynamics measure-
ments; spectroscopic characterization of new compounds;
cyclic voltammograms of compounds DA1, D, and A;
photophysics of D and A. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL801096C
Figure 3. Relaxation dynamics of DA1 in toluene monitored at
298 K at (A) 450 nm and (B) 580 nm. (C) The instrument response
function of 380 nm. Note that with best curve fitting, the fast decay
of 450 nm (290 ps) emission correlates very well with the rise of
580 nm emission (285 ps).
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