Intramolecular excitation energy transfer from visible-light absorbing chlorophyll derivatives to a near-infrared-light emitting boron dipyrromethene moiety

Article abstract of DOI:10.1246/cl.2010.953

Chlorophyll-a derivatives were covalently linked with a πextended boron aza-dipyrromethene moiety. In the synthetic dyads, photoinduced energy transfer from the former visiblelight absorbing chlorin chromophore to the latter highly fluorescent unit in the

Full text of DOI:10.1246/cl.2010.953

doi:10.1246/cl.2010.953
Published on the web July 31, 2010
953
Intramolecular Excitation Energy Transfer from Visible-light Absorbing Chlorophyll Derivatives
to a Near-infrared-light Emitting Boron Dipyrromethene Moiety
Yumiko Kataoka,1 Yutaka Shibata,2 and Hitoshi Tamiaki*1
1Department of Bioscience and Biotechnology, Faculty of Science and Engineering,
Ritsumeikan University, Kusatsu, Shiga 525-8577
2Division of Material Science-Physics, Graduate School of Science,
Nagoya University, Chikusa-ku, Nagoya, Aichi 464-8602
(Received June 3, 2010; CL-100527; E-mail: tamiaki@ph.ritsumei.ac.jp)
(i)
Chlorophyll-a derivatives were covalently linked with a ³-
extended boron aza-dipyrromethene moiety. In the synthetic
dyads, photoinduced energy transfer from the former visible-
light absorbing chlorin chromophore to the latter highly
fluorescent unit in the near-infrared region was observed in a
solution. The singlet-excited states of both the units were
partially quenched by the charge-separated state, which was
pronounced in polar organic solvents.
O
X = Me (3)
H (5)
(ii)
CH2COOCMe3 (6)
CH2COOH (7)
N
N
F
F
(iii)
(iv)
B
N
CH2COOCH2R
R = R1 (1)
X
R = R2 (2)
O
R1−Y
R2−Z
Understanding of excitation energy transfer in a (supra)-
molecule is important for construction of artificial photoactive
devices as well as elucidation of natural photosynthetic events.
Photoinduced intramolecular energy transfer from a visible-light
absorbing porphyrin ³-system to near-infrared-light absorbing
species including fullerenes and carbon nanotubes has been
investigated.1 Moreover, irradiation of chlorophyllous pigments
with visible light induced the migration of the excitation energy
to nearby chromophores such as bacteriochlorophyll derivatives,
cyanine dyes, and lanthanoid complexes, which emitted light in
the near-infrared region.2
Z
N
NH
N
NH
N
N
HN
HN
O
O
O
O
O
O
Y
Boron dipyrromethene (BODIPY) dyes are highly fluores-
cent and have been used as an efficient excitation energy donor
after absorbing visible light. Various reports are available on
intramolecular energy transfer from photoexcited BODIPY to
porphyrinoid moieties.3,4 Here we report synthesis of visible-
light absorbing chlorophyll-a derivatives covalently linked with
a near-infrared-light emitting aza-BODIPY moiety and singlet
energy transfer from photoexcited chlorin (Chl*) to BODIPY
units in synthetic dyads, where such a BODIPY derivative
functioned as an energy acceptor from porphyrinoids.
As a chlorophyll derivative that absorbs visible light, methyl
pyropheophorbide-a (4, Scheme 1) was used because of its easy
availability, and was prepared by modification of natural
occurring chlorophyll-a.5 The synthetic chlorophyllous moiety
gave a redmost (Qy) absorption maximum at 667 nm in
dichloromethane. For a singlet-excitation energy acceptor from
the chlorin chromophore, a ³-extended aza-BODIPY derivative
3 possessing -max = 739 nm6 was selected from various reported
BODIPYs.3,7 The dimethoxy compound 3 was treated with
boron tribromide and the reaction mixture containing dime-
thoxy-, monomethoxy-monohydroxy-, and dihydroxy com-
pounds was purified by silica gel flash column chromatography
to give the desired mono-ol 5 (55% yield, see experimental
details in Supporting Information).8 The phenol 5 was etherified
(54%) and the resulting tert-butyl ester 6 was quantitatively
cleaved by the action of trifluoroacetic acid. The reactive
carboxylic acid 7 was esterified with 2-hydroxyethylpyropheo-
Y = H (4), CH2OH (8)
Z = CH2OH (9)
Scheme 1. Synthesis of chlorin-BODIPY dyads 1/2: (i) BBr3,
CH2Cl2; (ii) BrCH2COOCMe3, K2CO3, Me2CO; (iii) CF3-
COOH, CH2Cl2; (iv) RCH2OH 8/9, EDC-DMAP, CH2Cl2.
phorbide-a (8)9 to afford the desired chlorin-BODIPY dyad 1
(71%). As another hydroxylated chlorin, methyl 3-devinyl-3-
hydroxymethylpyropheophorbide-a (9) was used10 and the other
dyad 2 possessing a shorter covalent linkage was similarly
prepared (66%). Full characterization of the present synthetic
compounds is described in Supporting Information.8
In a diluted dichloromethane solution (1.8 ¯M), dyad 1 gave
the electronic absorption spectrum which was reproduced by the
sum of those of chlorin 4 and BODIPY 3 (Figure 1). The
superposition indicated no intramolecular interaction between
the chlorin and BODIPY units in the ground state as well as no
intermolecular interaction among the chromophores due to the
low concentration. The observed proton and carbon-13 signals
of 1, 3, and 4 in their NMR spectra (see the data in Supporting
Information8) also showed the limited intra- and intermolecular
interaction in the composite units of 1 in chloroform-d (ca.
1 mM). No interaction in 2 could be detected, although the
spacer was shorter than that of 1.11
Photoexcitation of 1 in dichloromethane (1.8 ¯M) at 410 nm
gave a weak emission band at 678 nm from the chlorin
chromophore and a prominent band at 752 nm from the
Chem. Lett. 2010, 39, 953-955
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

954
Table 1. Fluorescence lifetimes (¸flu/ns) of 1-3 monitored at
760 nm with excitation at 410 nm
0.30
0.25
0.20
0.15
0.10
0.05
0.00
¹1 b
(¸flurel)a [keT/108 s
]
1
2
3
4
¸
flu
Compound
Benzene
Dichloromethane Acetonitrile
1
2
3
1.96 (0.55) [2.3] 1.42 (0.43) [4.0] 0.88 (0.27) [8.3]
1.68 (0.47) [3.2] 1.03 (0.31) [6.7] 0.59 (0.18) [13.9]
3.59 (1.00)
3.31 (1.00)
3.27 (1.00)
a
b
¸
flu
rel = ¸flu/¸flu(3). keT = 1/¸flu ¹ 1/¸flu(3).
Chl*−BODIPY (1.85 eV)
Energy Transfer
400
500
600
700
800
Chl−BODIPY* (1.63 eV)
λ / nm
Electron
transfer
Electron
transfer
Figure 1. Electronic absorption spectra of Chl-BODIPY 1
with a long spacer (black solid), Chl-BODIPY 2 with a short
spacer (black dotted), BODIPY 3 (red broken), and Chl 4 (blue
broken) in CH2Cl2 (1.8 ¯M).
Chl+ −BODIPY− (1.17 eV)
Emission
of NIR
Absorption
of VIS
No emission
4000
1
2
3
3000
4
Scheme 2. Energy diagram of photoexcited Chl-BODIPY
dyad: the values in parenthesis indicate estimated energy levels
for dyad 1 in CH2Cl2.
2000
1000
0
tron transfer. Calculated free energy of photoinduced electron
transfer (¦GeT) from the Rehm-Weller equation12 showed that
the electron transfer from photoexcited chlorin to BODIPY
units was preferable: ¦GeT (in CH2Cl2) = ¹0.68 (for
600
650
700
750
800
850
900
¹•
Chl*-BODIPY ¼ Chl+•-BODIPY ) < ¹0.18 eV (for Chl*-
λ / nm
¹•
BODIPY ¼ Chl -BODIPY+•).11
Figure 2. Fluorescence emission spectra of Chl-BODIPY 1
with a long spacer (black solid), Chl-BODIPY 2 with a short
spacer (black dotted), BODIPY 3 (red broken), and Chl 4 (blue
broken) in CH2Cl2 (1.8 ¯M) with excitation at 410 nm.
Fluorescence lifetimes (¸flu) of the BODIPY unit in dyad 1
were dependent upon the solvents (Table 1). The values of ¸flu in
1 (¸flu(1)) decreased with an increase of solvent polarity; the
decrease would also be due to the electron-transfer quenching.
Estimated ¦GeT supported the preferable intramolecular electron
transfer from photoexcited BODIPY to chlorin units: ¦GeT (in
BODIPY unit (Figure 2). Considering that the light at 410 nm
was predominantly absorbed by the chlorin moiety, the energy
transfer from the photoexcited chlorin to BODIPY units was
observed in the dyad 1. Compared with the fluorescence
emission spectrum of an equimolar mixture of 3 and 4,
fluorescence from the chlorin chromophore (-abs = 667 and
¹•
CH2Cl2) = ¹0.46 (for Chl-BODIPY* ¼ Chl+•-BODIPY ) <
¹•
+0.05 eV (for Chl-BODIPY* ¼ Chl -BODIPY+•).13 From the
electron transfer rate (keT) = 1/¸flu(1) ¹ 1/¸flu(3), the keT values
¹1
were calculated to be 2.3, 4.0, and 8.3 © 108 s for benzene,
dichloromethane, and acetonitrile, respectively. An energy
diagram for photoexcited chlorin-BODIPY dyads is drawn in
Scheme 2. Unfortunately, the analysis of fluorescence profiles at
760 nm gave no clear rise constant in the present measurements
(ps time scale), so the rate of excitation energy transfer from
Chl* to BODIPY in dyad 1 was not determined.
Quantum yields (Φflu) of fluorescence emission from
BODIPY* by photoexcitation of Chl in dyad 1 were dependent
on the solvents and decreased with an increase of their polarities
as expected (Table 2). This decrease is explained by 1) less
formation of BODIPY* through intramolecular energy transfer
from primarily produced and partially electron-transfer
quenched Chl* and/or 2) more quenching of BODIPY* through
intramolecular electron transfer in a polar solvent.
-
em = 678 nm) was largely (>99%) quenched by the BODIPY
unit (-abs = 737 and -em = 752 nm) in a molecule. Based on the
analysis of fluorescence excitation spectrum at 810-nm emission
and visible absorption spectrum of dyad 1 (Figure S1).8 the
efficiency of intramolecular energy transfer was estimated to be
about 80%. The inconsistency of the quenching value with
energy-transfer efficiency indicated the presence (ca. 20%) of
other quenching processes in 1.
In a benzene solution of 1, the intramolecular fluorescence
quenching value of photoexcited chlorin chromophore and
singlet-excitation energy-transfer efficiency were determined
to be >99 and >95%, respectively. The nearly consistent
percentage data indicated that the other quenching pathway was
inhibited in a less polar solvent, suggesting that the other
fluorescence quenching was ascribable to intramolecular elec-
In dyad 2 which possessed a shorter spacer between Chl
and BODIPY units, similar behavior in fluorescence data was
Chem. Lett. 2010, 39, 953-955
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

955
Table 2. Fluorescence quantum yields (¯flu) of 1-3 monitored
at an emission band of the BODIPY moiety (720-850 nm) with
excitation at 410 nm
Schaffner, Angew. Chem., Int. Ed. Engl. 1996, 35, 772; A.
Osuka, Y. Wada, K. Maruyama, H. Tamiaki, Heterocycles
1997, 44, 165; V. I. Prokhorenko, A. R. Holzwarth, M. G.
Müller, K. Schaffner, T. Miyatake, H. Tamiaki, J. Phys.
Chem. B 2002, 106, 5761; Y. Chen, K. Ohkubo, M. Zhang,
E. Wenbo, W. Liu, S. K. Pandey, M. Ciesielski, H. Baumann,
T. Erin, S. Fukuzumi, K. M. Kadish, R. Fenstermaker, A.
Oseroff, R. K. Pandey, Photochem. Photobiol. Sci. 2007, 6,
1257; H. L. Kee, J. R. Diers, M. Ptaszek, C. Muthiah, D.
Fan, J. S. Lindsey, D. F. Bocian, D. Holten, Photochem.
Photobiol. 2009, 85, 909; Q.-M. Wang, S. Sasaki, H.
Tamiaki, Chem. Lett. 2009, 38, 648.
A. Loudet, K. Burgess, Chem. Rev. 2007, 107, 4891.
a) R. W. Wagner, J. S. Lindsey, J. Am. Chem. Soc. 1994, 116,
9759. b) E. A. Ermilov, J.-Y. Liu, D. K. P. Ng, B. Röder,
Phys. Chem. Chem. Phys. 2009, 11, 6430. c) C. Y. Lee, J. T.
Hupp, Langmuir 2010, 26, 3760.
rel a
Φflu (Φflu
)
Compound
Benzene
Dichloromethane Acetonitrile
1
2
3
0.10 (0.53) 0.08 (0.38)
0.09 (0.47) 0.05 (0.24)
0.19 (1.00) 0.21 (1.00)
0.02 (0.11)
0.01 (0.06)
0.18 (1.00)
aΦflurel = Φflu /Φflu(3).
observed. Estimated keT values for Chl-BODIPY* ¼ Chl+•-
3
4
¹•
BODIPY in 2 were larger than those in 1 (Table 1), because
the average distance between the two units in 2 was shorter than
that of 1 in a solution. Smaller Φflus of 2 are also interpreted by
more efficient intramolecular electron transfer in 2.
In conclusion, intramolecular singlet energy transfer from
photoexcited chlorin (porphyrinoid) chromophore to BODIPY
unit was first shown in synthetic dyads. In the photoexcited
molecule, electron-transfer quenching processes were also
observed, which were regulated by the solvent polarity as well
as the length of the covalently linked spacer between Chl and
BODIPY moieties.
5
K. M. Smith, D. A. Goff, D. J. Simpson, J. Am. Chem. Soc.
1985, 107, 4946; H. Tamiaki, S. Takeuchi, S. Tsudzuki, T.
Miyatake, R. Tanikaga, Tetrahedron 1998, 54, 6699.
W. Zhao, E. M. Carreira, Chem.®Eur. J. 2006, 12, 7254.
J. Killoran, S. O. McDonnell, J. F. Gallagher, D. F. O’Shea,
New J. Chem. 2008, 32, 483; K. Umezawa, A. Matsui, Y.
Nakamura, D. Citterio, K. Suzuki, Chem.®Eur. J. 2009, 15,
1096.
6
7
We thank Dr. Michio Kunieda of Ritsumeikan University for
useful discussion. This work was partially supported by a Grant-
in-Aid for Scientific Research (A) (No. 22245030) from the
Japan Society for the Promotion of Science (JSPS) (to HT) and a
Sasakawa Scientific Research Grant from The Japan Science
Society (to YK).
8
9
Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
T. Miyatake, H. Tamiaki, A. R. Holzwarth, K. Schaffner,
Photochem. Photobiol. 1999, 69, 448.
10 H. Tamiaki, M. Amakawa, Y. Shimono, R. Tanikaga, A. R.
Holzwarth, K. Schaffner, Photochem. Photobiol. 1996, 63,
92.
References and Notes
1
V. Chukharev, N. V. Tkachenko, A. Efimov, H.
Lemmetyinen, Chem. Phys. Lett. 2005, 411, 501; Z. Guo,
F. Du, D. Ren, Y. Chen, J. Zheng, Z. Liu, J. Tian, J. Mater.
Chem. 2006, 16, 3021; H. He, A. G. Sykes, P. S. May, G. He,
Dalton Trans. 2009, 7454; B. Grimm, A. Hausmann, A.
Kahnt, W. Seitz, F. Spänig, D. M. Guldi, in Handbook of
Porphyrin Science, ed. by K. M. Kadish, K. M. Smith, R.
Guilard, World Scientific Publishing, Singapore, 2010,
Vol. 1, Chap. 2, pp. 133-219.
11 Similarly, the electronic absorption spectrum of 1 in a
diluted acetonitrile solution was consistent with the sum of
those of 4 and 3.
12 G. J. Kavarnos, N. J. Turro, Chem. Rev. 1986, 86, 401.
13 Based on the following data: Eox (3) = 0.34, Ered (3) =
¹1.07, Eox (4) = 0.37, and Eox (4) = ¹1.60 V (vs.
(C5H5)2Fe/(C5H5)2Fe+, in 0.1 M (C4H9)4NClO4/CH2Cl2);
E
0,0 (3) = 1.63 and E0,0 (4) =1.85 eV; factors for Coulombic
stabilization and ion-pair solvation = ¹0.27 eV reported in
ref. 4b.
2
H. Tamiaki, T. Miyatake, R. Tanikaga, A. R. Holzwarth, K.
Chem. Lett. 2010, 39, 953-955
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/