Construction of a rigid Zn porphyrin-C60 dyad within dendritic structure: Dendrimer effect on singlet energy transfer

Article abstract of DOI:10.1021/ol071296h

A snowflake-shaped Zn porphyrin dendrimer with a C₆₀ terminal was prepared. Covalent linkage of the C₆₀ unit results in significant quenching of the Zn porphyrin fluorescence mainly due to energy transfer from the Zn porphyrin core t

Full text of DOI:10.1021/ol071296h

ORGANIC
LETTERS
2007
Vol. 9, No. 17
3315-3318
Construction of a Rigid Zn
Porphyrin C60 Dyad within Dendritic
−
Structure: Dendrimer Effect on Singlet
Energy Transfer
Masatoshi Kozaki,* Kogen Akita, Shuichi Suzuki, and Keiji Okada*
Graduate School of Science, Osaka City UniVersity, 3-3-138 Sugimoto, Sumiyoshi-ku,
Osaka 558-8585, Japan
kozaki@sci.osaka-cu.ac.jp; okadak@sci.osaka-cu.ac.jp
Received June 1, 2007
ABSTRACT
A snowflake-shaped Zn porphyrin dendrimer with a C60 terminal was prepared. Covalent linkage of the C60 unit results in significant quenching
of the Zn porphyrin fluorescence mainly due to energy transfer from the Zn porphyrin core to the C60 terminal. Comparison of the energy-
transfer efficiency with similar dendrons indicates that the dendritic structure considerably delays the energy-transfer rate.
Dendrimers with multiple functional groups have been
extensively studied in the past decade because of a variety
of promising applications.1 We recently prepared a snowflake-
shaped dendrimer comprising rigid, conjugated chains cov-
ered with flexible dendritic branching chains.2 This arrange-
ment allowed us to assemble various functional groups within
the dendritic structure. It should be noted that the conjugated
chains not only provide a scaffold for the construction of a
well-designed and highly organized assembly but also serve
as a mediator in both the electron- and energy-transfer
processes.3 However, it is not clear how the branching side
chains and dendritic structure influence the conducting
properties of the conjugated chain.4 We have recently shown
that the electron-transfer rate is enhanced by shielding of
the conjugated chain with branching chains and also by
assembly of the dendrons within the dendrimer.2b This unique
shielding and dendrimer effect prompted us to construct a
similar Zn porphyrin-C60 dyad within the dendritic structure
(Figure 1). The C60 skeleton is a well-known energy and
electron acceptor from the singlet excited state of Zn
porphyrin.5 Therefore, the study of Zn porphyrin-C60 dyads
should provide information about the effect of the dendritic
architecture on the energy- and/or electron-transfer pro-
cesses.6 Although multiple porphyrin-C60 dyads have been
(1) (a) Newcome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendrimers and
Dendrons: Concepts, Syntheses, Applications; VCH: Weinheim, Germany,
2001. (b) Fre´chet, J. M. J.; Tomalia, D. A. Dendrimers and other Dendritic
Polymers; John Wiley & Sons: New York, 2002.
(2) (a) Kozaki, M.; Okada, K. Org. Lett. 2004, 6, 485-488. (b) Kozaki,
M.; Akita, K.; Okada, K. Org. Lett. 2007, 9, 1509-1512.
(3) (a) Osuka, A.; Tanabe, N.; Kawabata, S.; Yamazaki, I.; Nishimura,
Y. J. Org. Chem. 1995, 60, 7177-7185. (b) Davis, W. B.; Svec, W. A.;
Ratner, M. A.; Wasielewski, M. R. Nature 1998, 396, 60-63. (c) Eng, M.
P.; Albinsson, B. Angew. Chem., Int. Ed. 2006, 45, 5626-5629.
(4) (a) Karakaya, B.; Claussen, W.; Gessler, K.; Saenger, W.; Schlu¨ter,
A. D. J. Am. Chem. Soc. 1997, 119, 3296-3301. (b) Sato, T.; Jiang, D.-L.;
Aida, T. J. Am. Chem. Soc. 1999, 121, 10658-10659. (c) Apperloo, J. J.;
Janssen, R. A. J.; Malenfant, P. R. L.; Groenendaal, L.; Fre´chet, J. M. J. J.
Am. Chem. Soc. 2000, 122, 7042-7051. (d) Schenning, A. P. H. J.; Arndt,
J.-D.; Ito, M.; Stoddart, A.; Schreiber, M.; Siemsen, P.; Martin, R. E.;
Boudon, C.; Gisselbrecht, J.-P.; Gross, M.; Gramlich, V.; Diederich, F. HelV.
Chim. Acta 2001, 84, 296-334. (e) Li, W.-S.; Jiang, D.-L.; Aida, T. Angew.
Chem., Int. Ed. 2004, 43, 2943-2947.
10.1021/ol071296h CCC: $37.00
© 2007 American Chemical Society
Published on Web 07/19/2007

Figure 1. Chemical structures of dendrimers 1 and 2 and dendron 3.
successfully constructed at a dendrimer surface,7 dendrimers
containing a porphyrin core linked with a single C60 unit
are unprecedented.8 Here we report synthesis of a Zn
porphyrin-C60 dyad within the snowflake-shaped dendrimer
and a significant dendrimer effect, mainly on the singlet
energy transfer.
presence of sodium hydroxide to give Zn porphyrin 8 in 93%
yield. Dendron 10 was obtained in 85% yield by the
Sonogashira coupling reaction of Zn porphyrin 8 with TBS
(tert-butyldimethylsilyl)-terminated dendron 9 that was pre-
pared according to our reported procedure.2 Removal of the
TMS groups followed by the Sonogashira coupling reaction
of dendron 11 with phenyl-terminated dendron 1411 gave
dendrimer 12 in 40% yield (two steps). The TBS group in
dendrimer 12 was removed by the treatment of TBAF in
THF to afford dendrimer 13 in 84% yield. Finally, dendrimer
1 was obtained by the lithiation of the dendrimer 13 using
LHMDS (lithium hexamethyldisilazane) in the presence of
Dendrimer 1 was synthesized according to Scheme 1. First,
pyrrole and aldehydes 49 and 5 were condensed under acidic
conditions to give A3B type porphyrin 6 in 10% yield.
Porphyrin 6 was separated without any difficulty due to a
polar 2-hydroxy-2-propyl group. After the incorporation of
Zn metal ion, the selective removal of the 2-hydroxy-2-propyl
protecting group in the presence of TMS groups10 was
achieved by refluxing Zn porphyrin 7 in toluene in the
C
60 in THF followed by protonation with TFA.12 During the
treatment of TFA, the Zn porphyrin unit was partially
demetalated. The crude product was then heated with zinc
acetate in chloroform to afford dendrimer 1 as a purple
powder in 38% yield (two steps). Dendron 3 was obtained
by a similar method (Scheme S3). Dendrimer 1 was soluble
in various organic solvents such as chloroform, dichlo-
romethane, THF, and toluene and unambiguously character-
ized by means of NMR, elemental analysis, and MALDI-
TOF mass spectroscopy. Interestingly, the 1H NMR spectra
of 1 showed broad signals at room temperature probably
because of the slow conformational change of the dendritic
chain. Sharp well-resolved signals were obtained at 80 °C
in tetrachloroethane-d2.
(5) (a) Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi,
S.; Okada, T.; Shirakawa, M.; Sakata, Y. J. Am. Chem. Soc. 1996, 118,
11771-11782. (b) Kuciauskas, D.; Liddell, P. A.; Lin, S.; Stone, S. G.;
Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem. B 2000, 104, 4307-
4321. (c) Vail, S. A.; Krawczuk, P. J.; Guldi, D. M.; Palkar, A.; Echegoyen,
L.; Tome´, J. P. C.; Fazio, M. A.; Schuster, D. I. Chem.sEur. J. 2005, 11,
3375-3388.
(6) For some recent papers containing porphyrin-conjugated oligomer-
60 triad without dendritic side chains, see: (a) Ikemoto, J.; Takimiya, K.;
C
Aso, Y.; Otsubo, T.; Fujisuka, M.; Ito, O. Org. Lett. 2002, 4, 309-311. (b)
de la Torre, G.; Giacalone, F.; Segura, J. L.; Mart´ın, N.; Guldi, D. M.
Chem.sEur. J. 2005, 11, 1267-1280.
(7) (a) Hasobe, T.; Kamat, P. V.; Absalom, M. A.; Kashiwagi, Y.; Sly,
J.; Crossely, M. J.; Hosomizu, K.; Imahori, H.; Fukuzumi, S. J. Phys. Chem.
B 2004, 108, 12865-12872. (b) Li, W.-S.; Kim, K. S.; Jiang, D.-L.; Tanaka,
H.; Kawai, T.; Kwon, J. H.; Kim, D.; Aida, T. J. Am. Chem. Soc. 2006,
128, 10527-10532.
(8) Dendron containing multiple porphyrin units and a C60: (a) Choi,
M.-S.; Aida, T.; Luo, H.; Araki, Y.; Ito, O. Angew. Chem., Int. Ed. 2003,
42, 4060-4063. Dendrimer involving a C60 core and a porphyrin unit: (b)
Camps, X.; Dietel, E.; Hirsch, A.; Pyo, S.; Echegoyen, L.; Hackbarth, S.;
Ro¨der, B. Chem.sEur. J. 1999, 5, 2362-2373.
(9) Gryko, D.; Li, J.; Diers, J. R.; Roth, K. M.; Bocian, D. F.; Kuhr, W.
G.; Lindsey, J. S. J. Mater. Chem. 2001, 11, 1162-1180.
(10) Rodr´ıgues, J. G.; Esquivias, J.; Lafuente, A.; D´ıaz, C. J. Org. Chem.
2003, 68, 8120-8128.
(11) Dendron 14 was prepared according to Scheme S1 in the Supporting
Information.
(12) (a) Komatsu, K.; Murata, Y.; Takimoto, N.; Mori, S.; Sugita, N.;
Wane, T. S. M. J. Org. Chem. 1994, 59, 6101-6102. (b) Shirai, Y.; Zhao,
Y.; Cheng, L.; Tour, J. M. Org. Lett. 2004, 6, 2129-2132.
3316
Org. Lett., Vol. 9, No. 17, 2007

Scheme 1. Synthesis of Dendrimer 1
The cyclic voltammogram of 1 in 1,2-dichloroethane
to dendron 3, the dendrimer 2 has absorption identical to
that of dendrimer 1. Neither a significant shift nor a change
in the spectral shape was observed. A weak absorption of
the C60 unit throughout the visible region was not clearly
observed because of the spectral overlapping. Overall, these
spectral features suggest that there is an insignificant
electronic interaction between the Zn porphyrin core and the
C60 terminal in the ground state for 1 and 3.
showed two reversible reduction waves at -1.12 and -1.49
V vs Fc/Fc+, assigned to the successive reduction of the C60
terminal (Table 1). The first reduction potential slightly shifts
Table 1. Redox Potentials (V vs Fc/Fc+) of Dendrimer 1 and
Dendron 3a
Excitation of the Zn porphyrin core in dendrimer 1 in DMF
resulted in a typical Zn porphyrin fluorescence with emission
maxima at 615 and 660 nm, as in the case of dendrimer 2
Ox2
Ox1
Red1
Red2
compd
E1/2
E1/2
E1/2
E1/2
1
3
b
b
-1.12
-1.06
-1.49
-1.44
0.59
0.25
a In 1,2-dichloroethane, 0.1 M (C4H9)4NClO4, 100 mV s-1, Pt working
electrode, SCE pseudo-reference electrode (Fc/Fc+ ) +0.48 V vs SCE).
b Broad waves.
in the negative direction by 0.06 and 0.08 V compared with
that of dendron 3 and 1-ethynyl-2-methyl[60]fullerene,
respectively.5c Although broad oxidation waves were ob-
served for dendrimers 1 and 2, probably because of the slow
electron-transfer rate between the encapsulated Zn porphyrin
core and an electrode, dendron 3 showed sharp reversible
Zn porphyrin oxidation waves at 0.25 and 0.59 V vs Fc/Fc+
that are comparable to those of tetraphenyl Zn porphyrin
(0.30, 0.60 V vs Fc/Fc+). These results indicate that the
linkage of the C60 unit and the Zn porphyrin core causes
negligible electronic perturbation at each chromophore.
The UV-vis absorption spectrum of dendrimer 1 in DMF
showed characteristic bands for a Zn porphyrin unit (Soret
band: λmax ) 435 nm, Q-bands: λmax ) 564, 606 nm) along
with absorption bands due to the branched benzyl ether
(around 285 nm) and conjugated (λmax ) 365 nm) chains
(Figure 2). Although the Zn porphyrin absorption of den-
drimer 1 slightly shifts to a longer wavelength region relative
Figure 2. Absorption spectra of 1, 2, and 3 in DMF.
Org. Lett., Vol. 9, No. 17, 2007
3317

tigated the solvent effect on the fluorescence intensity. The
free energy change in the electron-transfer process was
estimated by applying the Weller approximation.13 Obvi-
ously, the free energy values are much more negative in polar
solvents: ∆G ) -0.13 in 1,4-dioxane (ꢀ ) 2.2), -0.91 in
THF (ꢀ ) 7.6), and -1.13 eV in benzonitrile (ꢀ ) 26).
However, the efficiency of fluorescence quenching for
dendrimer 1 was almost constant for these solvents: 39%
quenching (ΦF ) 2.3 × 10-2) in 1,4-dioxane; 33% quenching
(ΦF ) 2.3 × 10-2) in THF; 39% quenching (ΦF ) 3.5 ×
10-2) in benzonitrile; and 38% (ΦF ) 2.8 × 10-2) quenching
in DMF (ꢀ ) 37). These results undoubtedly indicate that
the major quenching mechanism in dendrimer 1 is not the
electron transfer but the energy transfer from the excited Zn
porphyrin core to the C60 terminal. The poor fluorescence
quantum yield of the C60 skeleton5c and the possible electron-
transfer process from the excited C60 unit are consistent with
no emission from the C60 terminal.15
Table 2. Photophysical Data of 1, 2, and 3 in DMF
λmax
compd
Soret band
Q-bands
λem
ΦF
max
1
2
3
435
436
430
564, 606
563, 604
562, 602
615, 660
614, 659
609, 661
0.028
0.074
0.015
and dendron 3 (Table 2). The fluorescence from the Zn
porphyrin core was effectively quenched for dendrimer 1
(ΦF ) 0.028) relative to that of dendrimer 2 (ΦF ) 0.074)
as shown in Figure 3, where all the spectra were normalized
Interestingly, the intensity of the fluorescence of dendron
3 is several times smaller than that of dendrimer 1 for all
solvents (ΦF ) 6.5 × 10-3 in 1,4-dioxane; 1.1 × 10-2 in
THF; 2.3 × 10-2 in benzonitrile; and 1.5 × 10-2 in DMF
for 3). This result suggests that energy transfer from the
singlet excited state of the Zn porphyrin core to the C60
terminal is somewhat retarded by the dendritic structure.
We have previously clarified that the electron-transfer rate
is enhanced by the dendritic structure for the similar Zn
porphyrin dendrimer with four anthraquinolyl terminals.2b
Therefore, the dendrimer architecture effect on energy
transfer is in sharp contrast to that of electron transfer. Further
photophysical studies involving kinetics using laser flash
photolysis of these dendrimers may disclose the detailed
mechanism of the dendrimer effect.
In summary, we successfully incorporated a Zn porphy-
rin-C60 dyad within the dendritic assembly by applying the
snowflake-shaped dendrimer. The Zn porphyrin fluorescence
is quenched in consequence of the covalent linkage of the
C60 terminal. The intramolecular energy transfer from the
excited singlet state of the Zn porphyrin core to the C60
terminal was shown to be a major contributing mechanism
for the quenching. Comparison of the fluorescence quenching
between dendrimer 1 and the model dendron 3 revealed an
interesting dendrimer effect, namely, that dendrimer 1 has a
lower quenching efficiency in the energy-transfer process
than dendron 3 in all the solvents examined.
Figure 3. Florescence of 1, 2, and 3 in degassed DMF (431 nm
excitation). All the spectra were normalized to a constant absorbance
at the excitation wavelength.
by the absorbance at the excitation wavelength. No apparent
C60 emission was detected around 700 nm. In dyads 1 and
3, both the electron and the singlet energy transfers from
the excited Zn porphyrin core to the C60 terminal are
exothermic processes in DMF (∆G ) -1.16 and -0.33 eV
for 1, respectively).13 The observed weaker fluorescence for
1 and 3 may reflect both electron- and energy-transfer
processes in comparable weight or in one of the processes
predominantly. It has been well-established that solvent
polarity has strong effects selectively on the electron-transfer
process.14 To differentiate the electron- and energy-transfer
processes revealed in the fluorescence intensity, we inves-
Acknowledgment. This work was partially supported by
a Grant-in-Aid for Science Research (Nos. 17550044,
19655016, and 18039033 “Super-Hierarchical Structures”)
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
(13) The ∆G values were estimated by applying the Weller equation
(see Supporting Information). (a) Weller, A. Z. Phys. Chem. Neu. Folg.
1982, 133, 93-98. (b) Lor, M.; Viaene, L.; Pilot, R.; Fron, E.; Jordens, S.;
Schweitzer, G.; Weil, T.; Mu¨llen, K.; Verhoeven, J. W.; Van der Auweraer,
M.; De Schryver, F. C. J. Phys. Chem. B 2004, 108, 10721-10731.
(14) Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi, S.;
Okada, T.; Shirakawa, M.; Sakata, Y. J. Am. Chem. Soc. 1996, 118, 11771-
11782.
Supporting Information Available: Detailed synthetic
procedure and NMR, UV-vis-NIR, and emission spectral
data. This material is available free of charge via the Internet
at http://pubs.acs.org.
(15) Kuciauskas, D.; Gilbert, S. L.; Seely, G. R.; Moore, A. L.; Moore,
T. A.; Gust, D.; Drovetskaya, T.; Reed, C. A.; Boyd, P. L. J. Phys. Chem.
1996, 100, 15926-15932.
OL071296H
3318
Org. Lett., Vol. 9, No. 17, 2007