Dihedral-angle modulation of meso-meso-linked ZnII diporphyrin through diamine coordination and its application to reversible switching of excitation energy transfer

Article abstract of DOI:10.1002/anie.200351177

The dihedral angle between the two porphyrins in meso-meso-linked Zn^II diporphyrin can be modulated effectively by adding 1,7-diaminoheptane, which coordinates at the both Zn^II centers (see picture). This mechanical torsion gives rise to a decrease in the energy of the S₁ state in the meso-meso-linked Zn^II-diporphyrin subunit, and results in the change in direction of the energy transfer in the excited state. This process can be reversed by adding acetic acid.

Full text of DOI:10.1002/anie.200351177

Communications
meso–meso-Linked ZnII Diporphyrin
Dihedral-Angle Modulation of meso–meso-
Linked ZnII Diporphyrin through Diamine
Coordination and Its Application to Reversible
Switching of Excitation Energy Transfer**
Hideyuki Shinmori, Tae Kyu Ahn, Hyun Sun Cho,
Dongho Kim,* Naoya Yoshida, and Atsuhiro Osuka*
Control of intramolecular excitation-energy transfer (EET)
and electron transfer (ET) has been a subject of considerable
attention in light of its importance in molecular electronics
including artificial photosynthesis,[1] molecular sensing sys-
tems,[2] molecular devices,[3] and so forth. When the direction
of an intramolecular EET reaction can be switched at will by
external physical and/or chemical input, it provides a switch-
ing EET module as a component of molecular devices.[4]
Recently, we reported the synthesis of meso–meso-linked
diporphyrins, whose Soret band is split into two bands by an
exciton coupling but the conjugative electronic interaction
between the porphyrin chromophores in the diporphyrin is
weak because of the nearly perpendicular conformation,
despite the direct meso–meso linkage.[5] Thus, a decrease in
the dihedral angle of diporphyrin, from an average of 908 to
an oblique geometry, is expected to lead to an enhancement in
the electronic coupling within the diporphyrin, which has
been confirmed by permanently distorted meso–meso-linked
diporphyrins bridged by a strap of variable length.[6] Namely,
the decrease in the dihedral angle between the porphyrin
rings in the diporphyrin causes progressive lowering of the S1-
state energy and an increase in energy of the HOMO orbital
in addition to the spectral changes from two Soret bands to
four Soret bands, which is a consequence of symmetry change
from D2d to D2.
Herein we report reversible switching of intramolecular
EET direction on the basis of the dihedral angle control of
meso–meso-linked ZnII diporphyrin through 1,7-diaminohep-
[*] Prof. D. Kim, Dr. T. K. Ahn, Dr. H. S. Cho
Center for Ultrafast Optical Characteristics Control and
Department of Chemistry, Yonsei University
Seoul 120–749 (Korea)
Fax: (+82)2-2123-2434
E-mail: dongho@yonsei.ac.kr
Prof. A. Osuka, Dr. H. Shinmori, Dr. N. Yoshida
Department of Chemistry, Graduate School of Science
Kyoto University, and
Core Research for Evolutional Science and Technology (CREST)
Japan Science and Technology Corporation
Sakyo-ku, Kyoto 606–8502 (Japan)
Fax: (+81)75-753-3970
E-mail: osuka@kuchem.kyoto-u.ac.jp
[**] The work at Yonsei University was financially supported by the
Creative Research Initiatives Program of the Ministry of Science and
Technology of Korea. N.Y. thanks the JSPS Research Fellowship for
Young Scientists.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200351177
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tane coordination. First, coordination behaviors of a,w-
diaminoalkanes (H2N(CH2)nNH2, nDA) to meso–meso-
linked ZnII diporphyrin 1 have been examined in terms of
association ability and association mode, by changing the
molecular length of nDA.[7] In the absence of an amine, the
absorption spectrum of 1 exhibits split Soret bands at 416 and
451 nm in toluene.[5] Following the changes in the absorption
spectra upon the addition of nDA to the corresponding
solutions, we determined the association constants (Ka) on the
basis of 1:1 complex formation (Table 1). A jump in Ka was
(5DA) and 7DA towards 1. The addition of 5DA to a toluene
solution of 1 caused the red shift of Soret bands to 424 and
459 nm with several isosbestic points (Figure 1a). These
spectral changes, which are common for nDA shorter than
5DA and for monoaminoalkanes, can be interpreted in terms
of the coordination of two diamine molecules to 1 without
affecting an averaged perpendicular conformation. Con-
versely, a similar addition of 7DA to a toluene solution of 1
gave rise to markedly different spectral changes, from two
Soret bands to four Soret bands (lmax = 412(shoulder), 427,
462, and 472 (shoulder) nm) as well as red shifts of Q bands
from 540 (shoulder) and 553 nm to 554, 575, respectively, and
the appearance of a band at 629 nm (Figure 1b). The resultant
absorption spectral features of complex 7DA–1 are, both in
the Soret band and Q band regions, quite similar to those of
Table 1: Association constants of 1 with nDA in toluene at 258C.
Association constants have been calculated assuming a 1:1 complex.
n
Ka [”104 mÀ1
]
n
Ka [”104 mÀ1
]
2
4
5
6
7
9.0
11
16
10
160
8
9
10
12
710
690
2600
910
permanently distorted meso–meso-linked diporphyirn
bridged by a 1,4-dioxyteteramethylene strap (Scheme 1). It
2
observed between 1,6-diaminohexane (6DA) and 1,7-diami-
noheptane (7DA), and between 1,9-diaminononane (9DA)
and 1,10-diaminodecane (10DA). Figure 1 shows typical
contrasting coordination behaviors of 1,5-diaminopentane
Figure 1. Absorption spectra of 1 (3.5”10À6 m) with 5DA (a) and
7DA (b) in toluene at 258C: [5DA]=0–1.6”10À4 m; [7DA]=0–
3.5”10À5 m. A=absorption.
Scheme 1. Structure of the porphyrins referred to.
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is thus conceivable that the association of 7DA with 1 takes
nearly 908 to 60–708.[6b] The absorption and fluorescence
spectra of 7DA–2 complex are quite similar to those of more
distorted diporphyrin 3 bridged by a dioxymethylene strap, in
which the dihedral angle is estimated to be less than 508.[6b]
Therefore, it is likely that 7DA is ideally suited for changing
the dihedral angle of the meso–meso-linked ZnII diporphyrin
and the additional torsional strain imposed by the coordina-
tion of 7DA is effective for the modulation of the interpor-
phyrin electronic interaction. Furthermore it is noteworthy
that the addition of acetic acid to a solution of complexes
7DA–1 or 7DA–2 restores the original absorption and
fluorescence spectra of 1 or 2, plausibly through a simple
acid–base reaction without affecting the ZnII metallation.
Thus these torsional changes can be effected in a reversible
manner.
With these reversible modulations of the optical proper-
ties of 1 at our disposal, we have explored a molecular system
that enables reversible switch in direction of the EET. We
designed triporphyrin 4, in which a meso–meso-linked ZnII-
diporphyrin unit is covalently linked to a 5,15-diaryl free-base
porphyrin through a 1,4-phenylene bridge. Model 4 was
prepared by the Suzuki coupling reaction of bromide 5 with
boronate 6 ([Pd(PPh3)4], K2CO3, toluene) in 81% yield.[8] The
absorption spectrum of 4 is almost identical to the sum of
those of 1 and the monomer of 5,15-diaryl free-base
porphyrin, thus indicating negligible electronic interaction
in the ground state. The steady-state fluorescence spectrum of
4 arises only from the 5,15-diaryl free-base porphyrin, thus
indicating the nearly quantitative EET from the meso–meso-
linked ZnII-diporphyrin subunit to the free-base porphyrin
subunit. The addition of 7DA to a toluene solution of 4
caused distinct spectral changes in the absorption spectra,
which are essentially the same as those observed for 1, thus
suggesting a similar decrease in the dihedral angle of the
meso–meso-linked diporphyrin. Interestingly, the fluores-
cence spectrum of 4 exhibits a clear change of the emitting
chromophore, from the 5,15-diaryl free-base porphyrin in free
4 to the meso–meso-linked ZnII-diporphyrin in 7DA–4
complex (Figure 3a). These results indicate that the intra-
molecular EET direction can be switched upon the addition
of 7DA. Furthermore, the addition of acetic acid to complex
7DA–4 recovers the original absorption and fluorescence
spectra of the free 4 (Figure 3b), hence restoring the intra-
molecular EET from the ZnII diporphyrin to the free base
porphyrin, probably through liberation of 7DA from the ZnII-
diporphyrin moiety.[9]
place through two amino groups coordinated to the two zinc
centers, hence forcing a tilt of the two porphyrins from a
dihedral angle of about 908 to a decreased dihedral angle.
Association constants of 1 with longer diamines such as 10DA
and 12DA are apparently larger than that with 7DA, but the
resulting complexes exhibit an absorption spectra similar to
that of the 5DA–1 complex, thus suggesting that these long
diamines have sufficient molecular length to form a 1:1
complex without affecting the average perpendicular con-
formation of the meso–meso-linked ZnII diporphyrin. Job
plots indicate a 1:1 molecular ratio for the formation of
complexes 7DA–1 and 10DA–1 (Supporting Information).
Different association modes between complexes 5DA–1 and
7DA–1 are also evident in the fluorescent spectra (Figure 2),
which feature modest spectral changes in the former, and a
distinct red shift and intensity enhancement in the latter. The
fluorescence spectrum of complex 7DA–1 (Figure 2b) is
reminiscent of that of the distorted diporphyrin 2 coordinated
with amine, which again indicates a change in the dihedral
angle of 1 upon the association of 7DA. Among the a,w-
diaminoalkanes examined, 7DA is the most effective in
inducing the distorted conformation as judged from the
optical properties of the complexes formed. Judging from the
X-ray crystal structure of the CuII analogue of 2, the
coordination of 7DA reduces the dihedral angle from
The intramolecular EET processes have been examined
by picosecond time-resolved transient absorption spectro-
scopy. Figure 4a shows the transient absorption spectra of
free 4 taken by excitation at 580 nm, mainly pumping the
meso–meso-linked ZnII-diporphyrin part of the molecule. The
spectrum at 1 ps delay time indicates strong bleaching bands
at 450 and 555 nm because of the formation of the singlet
excited state of the ZnII diporphyrin. These bleaching bands
disappear rapidly with t = 5.5 ps (Supporting Information)
followed by an increase of a new bleaching band at 500 nm
arising from the formation of the singlet excited state of the
free base porphyrin with t = 5.0 ps as can be seen in the
spectrum at 50 ps delay time and the temporal profile at
Figure 2. Fluorescence spectra of 1 (3.5”10À6 m) with 5DA (a) and
7DA (b) in toluene at 258C: lex =435 nm, [5DA]=0 (a) or
2.9”10À4 m (c); [7DA]=0 or 8.0”10À6 m. I=intensity (arbitrary
units).
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Figure 3. Fluorescence spectra of 4 (1.8”10À6 m) with 7DA in toluene
at 258C: lex =468 nm, [7DA]=0–1.7”10À5 m, (a). Fluorescence spectra
of 4 (1.8”10À6 m) and 7DA (6.7”10À6 m) plus acetic acid, [acetic
acid]=0–2.6”10À2 m, (b).
Figure 4. Transient absorption spectra of 4 in toluene by excitation at
580 nm (a) and of 7DA–4 complex by excitation at 400 nm (b).
O.D.=optical density.
500 nm (Supporting Information). These results unambigu-
ously indicate the efficient intramolecular EET from the ZnII-
diporphyrin to the free base porphyrin part with kEET = 2 ”
1011 sÀ1. The resultant singlet excited state of the free-base
porphyrin has a lifetime of about 9 ns, as determined by the
fluorescence lifetime measurement, and thus is not quenched
by the ZnII-diporphyrin moiety. This result provides firm
evidence for the occurrence of EET, not electron transfer.
The reverse EET in 7DA–4 complex was more difficult to
detect owing to larger absorbance of the ZnII-diporphyrin in
comparison with that of the free-base porphyrin unit, which
did not allow a selective excitation of the latter. Figure 4b
shows the transient absorption spectra of complex 7DA–4
taken by excitation at 400 nm that corresponds to the
pumping of the ZnII diporphyrin and the free base porphyrin
roughly in a 1:1.5 ratio. At 462nm, at which the bleaching
arises from the superposition of the singlet excited states of
the ZnII diporphyrin complexed with 7DA as well as from the
free base porphyrin, the temporal profile has revealed a rapid
recovery with t ꢀ 10 ps, which is assignable to the EET from
the free base porphyrin to the ZnII diporphyrin.
meso–meso-linked ZnII-diporphyrin subunit, which, in turn,
can be erased upon the addition of acetic acid in a reversible
manner. This sequence, when applied to the triporphyrin
model 4, nearly completes the switch in the direction of the
intramolecular EET. Further extension of this methodology is
currently being investigated in our laboratories.
Experimental Section
Triporphyrin 4: meso-Brominated meso–meso ZnII diporphyrin 5
(10 mg, 0.006 mmol), porphyrin boronate 6 (30 mg, 0.051 mmol),
K2CO3 (44 mg, 0.32mmol), and Pd(PPh 3)4 (1.2mg, 0.001 mmol) were
dissolved in dry toluene (7 mL). The solution was degassed by three
freeze–pump–thaw cyles and then heated to 808C for 17 h under
argon. The reaction mixture was cooled to room temperature, and the
solvent was removed by evaporation. The desired porphyrin was
separated by silica-gel column chromatography (eluent; CH2Cl2:hex-
ane = 1:1) and was further purified by recrystallization from CH2Cl2
1
and MeOH to give 4 as a violet solid. Yield; 10 mg, 81%. H NMR
(500 MHz, CDCl3, 2 58C, TMS): d = 10.45 (s, 2H, meso), 10.41 (s, 1H,
meso), 9.62(d, J = 5 Hz, 2H, Por-b), 9.60 (d, J = 5 Hz, 2H, Por-b), 9.53
(d, J = 5 Hz, 2H, Por-b), 9.51 (d, J = 5 Hz, 2H, Por-b), 9.48 (d, J =
5 Hz, 2H, Por-b), 9.26 (d, J = 5 Hz, 2H, Por-b), 9.21 (d, J = 5 Hz, 2H,
Por-b), 9.15 (d, J = 5 Hz, 2H, Por-b), 8.80 (d, J = 5 Hz, 2H, Por-b), 8.77
(d, J = 5 Hz, 2H, Por-b), 8.78 and 8.73 (d-d, J = 8 Hz, 4H, Phenylene),
8.35–8.32(m, 2H, Ph), 8.22(d, J = 5 Hz, 2H, Por-b), 8.19 (d, J = 2Hz,
4H, Ar), 8.16 (d, J = 5 Hz, 2H, Por-b), 8.14 (d, J = 2Hz, 4H, Ar),
7.86–7.84 (m, 3H, Ph), 7.76 (t, J = 2Hz, 2H, Ar), 7.73 (t, J = 2Hz, 2H,
As demonstrated above, the dihedral angle between the
two porphyrins in the meso–meso-linked ZnII diporphyrin can
be modulated effectively upon the addition of 7DA through
the 1:1 coordination at both ZnII centers. This mechanical
torsion gives rise to a decrease in the S1-state energy in the
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Ar), 1.50 ppm (s, 36H, tBu), 1.48 (s, 36H, tBu); MALDI-TOF MS m/z
1955, calcd for C128H122N12Zn2 1955; UV/Vis (toluene): lmax(loge) =
410(5.64), 458(5.49), 500(4.59), 541(4.65), 558(4.82), 601(4.05), and
633(3.52) nm; fluorescence (toluene): lmax = 634 and 698 nm.
Received: February 13, 2003
Revised: April 14, 2003 [Z51177]
Keywords: coordination modes · energy transfer ·
.
molecular devices · porphyrinoids · zinc
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[9] The observed fluorescence upon addition of acetic acid was the
same as that of nonprotonated free-base porphyrin, thus exclud-
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about 1000-fold excess of acetic acid was needed to recover the
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