Photophysical and Photochemical Properties of Viologen-Linked N-Alkylporphyrin Diads and Their Metal Complexes

Article abstract of DOI:10.1021/jp984221r

A new type of viologen-linked or -bridged N-alkylporphyrin and metalloporphyrin diads was synthesized and characterized by ¹H NMR spectroscopy. Although the viologen moiety of free base diads linked with a methylene chain is flexible in solutio

Full text of DOI:10.1021/jp984221r

J. Phys. Chem. B 1999, 103, 2867-2877
2867
Photophysical and Photochemical Properties of Viologen-Linked N-Alkylporphyrin Diads
and Their Metal Complexes
Keiichi Tsukahara,*,† Naoko Sawai,† Satoko Hamada,† Kayoko Koji,† Yoshiaki Onoue,‡
Takashi Nakazawa,† Ryoichi Nakagaki,§ Koichi Nozaki,# and Takeshi Ohno#
Department of Chemistry, Faculty of Science, Nara Women’s UniVersity, Nara 630, Japan,
Nara Ikuei High School, Nara 630, Japan, Faculty of Pharmaceutical Sciences, Kanazawa UniVersity,
Kanazawa, Ishikawa 920, Japan, and Department of Chemistry, Graduate School of Science,
Osaka UniVersity, Toyonaka, Osaka 560, Japan
ReceiVed: October 27, 1998; In Final Form: January 13, 1999
A new type of viologen-linked or -bridged N-alkylporphyrin and metalloporphyrin diads was synthesized
1
and characterized by H NMR spectroscopy. Although the viologen moiety of free base diads linked with a
methylene chain is flexible in solution, the insertion of metal ions results in the extended conformation,
where the viologen moiety is located nearly perpendicular to the N-alkylporphyrin plane. The diad, having
two porphyrin units bridged by a viologen moiety, is more rigid in conformation than the diad containing one
porphyrin unit. The fluorescence quantum yields in the viologen-linked and -bridged N-alkylporphyrins become
larger by insertion of H+, Mg2+, Al3+, and Si4+ into the N-alkylporphyrins. This may arise because the extended
and orthogonal conformer due to the electrostatic repulsion between the viologen and the central metal ion
decelerates the ET quenching of N-alkylporphyrin by the bound viologen unit.
Introduction
An electronic coupling between the donor and acceptor
orbitals with a π character must be weak in an orthogonal
orientation for porphyrin, and the ET rate is expected to be
slower than that for face-to-face interaction or the edge-to-edge
cofacial one. We have designed two types of viologen-linked
N-alkylporphyrins and their metal complexes. The first examples
are the diads (10 and 11, Chart 1) whose viologen moiety is
located nearly perpendicular to the porphyrin plane by not only
the conformational characteristic N-alkyl bond but also the steric
hindrance between the N-alkyl group and the relatively bulky
four phenyl groups at meso positions. In the diad systems, the
viologen unit is still able to wag and rotate around the bridging
methylene group. Therefore, the excited singlet state after
irradiation by light is expected to have two conformers
corresponding to an extended and perpendicular form and a
closed one, in the latter of which the viologen moiety is close
to the porphyrin plane. The second model is a new type of
viologen-bridged diad (14, Chart 2) to construct the orthogonal
orientation between porphyrin and viologen by restricting the
free rotation of the viologen unit. By inserting metal ions into
the N-alkylporphyrin diads, the extended conformer must exist
predominantly in solution due to charge repulsion between a
central metal ion and the viologen moiety. In this work, we
report the photophysical and photochemical properties of such
models of electron donor and acceptor systems. The preliminary
communications have been reported elsewhere.14,15
Porphyrins linked by viologen, which is a 4,4′-bipyridinium
ion, have been synthesized to achieve an efficient electron-
transfer (ET) quenching of the excited state of porphyrin or
metalloporphyrin by viologen.1 Since the first example of the
covalently linked viologen-tetraphenylporphyrin was reported
by Milgrom,2 several types of viologen-linked porphyrin have
been synthesized.3-8 The orientation between electron donor
and acceptor is one of the important factors determining ET
reaction rates.9,10 In some systems the fluorescence decay
profiles are fitted to the sum of two exponentials, indicating
the existence of two different conformers: the closed one, where
viologen is close to the porphyrin ring, and the extended one,
the viologen moiety of which is far away from the porphyrin
plane.6b,7b The closed conformer permits facile intramolecular
ET reaction from the excited singlet-state of porphyrin to the
bound viologen. All the porphyrins linked by electron donors
or acceptors are non-N-substituted ones.
Porphyrins substituted at a pyrrolenic nitrogen atom, N-
alkylporphyrins, are produced from the reactions of a number
of xenobiotics with the detoxifying cytochrome P450 enzymes
found in the liver and of hydrazines with hemoglobin and
myoglobin in vivo, resulting in the ferrochelatase inhibition.11
In addition to the biological significance of N-alkylporphyrins,
their structural features are worthy of remark. The N-alkylation
of porphyrins results in distortion of the porphyrin plane by
the sp3 hybridization of one of the pyrrolenic nitrogens; the alkyl
group is out-of-plane with respect to those unsubstituted
pyrrolenic nitrogens.11-13
Results and Discussion
Absorption Spectral Properties. Absorption spectra of
viologen-linked diads and their metal complexes in MeCN are
shown in Figures S1 and S2, and their numerical data are shown
in Tables S1 and S2. In the viologen- and bipyridine-linked
systems, the absorption due to the viologen unit appeared around
260 nm, where the absorption of the porphyrin moiety also
exists. The ratios of the molar absorption coefficient (ꢀ) of the
* To whom correspondence should be addressed. E-mail: tsukahara@
cc.nara-wu.ac.jp.
† Nara Women’s University.
‡ Nara Ikuei High School.
§ Kanazawa University.
# Osaka University.
10.1021/jp984221r CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/26/1999

2868 J. Phys. Chem. B, Vol. 103, No. 15, 1999
Tsukahara et al.
CHART 1
CHART 2
Soret band to that of the band around 260 nm are (6-7):1 in
diads and (8-9):1 in the viologen-bridged diads. The Soret
maxima of the diads did not shift from that for the unlinked
porphyrin (1) and the values of ꢀ for the diads 10 and 14 are
similar to and about twice that for the unlinked porphyrin (1),
respectively. It is, therefore, suggested that the intramolecular
π-π interaction between porphyrin and viologen is very weak.
The ground-state complexation between porphyrin and methyl
viologen in solution caused a red shift in the Soret band.17 Zinc-
(II) N-alkylporphyrins have partially resolved split Soret bands,
arising from the lower symmetry of the porphyrin.11 Lavallee
et al. have reported that the absorption spectra and the structures
of the N-alkylmetalloporphyrins of Mn(II), Fe(II), Co(II), Ni-
(II), and Cu(II) are similar to those for the Zn(II) complexes.11-13
In the case of the other N-alkylmetalloporphyrins, such as Na-
(I), Mg(II), Al(III), and Si(IV) complexes, Soret bands are not
split but broad. The Q-bands are completely different from those
for Zn(II) complexes; the first Q-band (around 680 nm) is the
largest in intensity, although the second Q-band (around 610
nm) is the largest for the Zn(II) porphyrins. The largest in the
first Q-band appears also in the monoprotonated species of the
free base N-alkylporphyrin, although its Soret band shifts
slightly. It is suggested that these metal ions, except Zn(II), are
only partially coordinated to three pyrrole nitrogens. The
remaining pyrrole nitrogen is protonated. This is confirmed by
1H NMR spectra of Al(III) and Si(IV) N-alkylporphyrins, where
the N-H proton signal still appeared (see below). This is just
as a “sitting-atop” complex that has been described for non-N-
substituted porphyrins.18
1
Structural Features of Diads. Figure 1 shows a H-1H
phase-sensitive NOESY NMR spectrum of the diad 10 at 34
°C in CD3CN. The N-CH2 proton signals in the bridged
methylene group appeared at a high field (-4.64 to -4.55 ppm)
due to the ring-current effect of the porphyrins and did not
exhibit a triplet signal, but a multiplet one, suggesting the
restriction of the free rotation of N-CH2-CH2 bonds. On the
other hand, a triplet signal of -CH2-N+ protons shows possible
free rotation around the -CH2-N+ bond. The signal for the
two peripheral protons on pyrrole ring A bearing the N-alkyl
group is shifted up field to 7.46 ppm in contrast to that for the
opposite pyrrole ring C that has a similar chemical shift to that
in the non-N-substituted H2tpp, suggesting that this ring stays

Viologen-Linked N-Alkylporphyrin Diads
J. Phys. Chem. B, Vol. 103, No. 15, 1999 2869
field doublets, respectively, because the former protons have
NOESY interactions with the o-phenyl I protons, for which
NOESY cross-peaks were observed with pyrrole A protons.
Table 1 summarizes the 1H NMR chemical shifts for the bipyri-
dinium ring protons of the diads. In the trimethylene bridged
diads (10 and 10Zn), the proton signals for the bipyridinium
ring (2,6-, 3,5-, 3′,5′-, and 2′,6′-bpy) appeared in this order from
upper field, suggesting that the viologen moiety tends to locate
out-of-plane with respect to the porphyrin plane. Insertion of
Zn(II) into the free base diad 10 caused the changes in the proton
signals, upfield for pyrrole C ring and downfield for 2,6- and
3,5-bpy, suggesting that the viologen moiety locates more apart
from the central metal ion due to charge repulsion and/or steric
requirement. On the other hand, the signals of the bipyridinium
ring protons in the hexamethylene bridged diads (11 and 11Zn)
appeared differently from those for 10 and 10Zn; the order of
the signals from upper field is 3,5-, 3′,5′-, 2,6-, and 2′,6′-bpy,
being similar to that for free bipyridinium ions. Therefore, the
viologen moiety in 11 and 11Zn is free from the ring current
of porphyrin.
The calculated structure of the diad 10 by MM2 (Figure 2)
demonstrates that the closed form is more stable (a total energy,
1.4 kcal mol-1) than the extended form (6.0 kcal mol-1). On
the other hand, the extended conformer is slightly more stable
than the closed one for 10Zn by 3 kcal mol-1 (Figure 3). In the
case of 11 and 11Zn, the closed conformer of the viologen
moiety apart from the porphyrin ring was obtained: 3.8 kcal
mol-1 for 11 (Figure 3) and 3.3 kcal mol-1 for 11Zn. These
results are not inconsistent with the NMR data.
Figure 1. Phase sensitive 1H-1H NOESY spectrum of the diad 10 in
CD3CN at 34 °C. Only the negative peaks are drawn. Open and closed
signals indicate COSY and NOESY interactions, respectively.
TABLE 1: 1H NMR Chemical Shifts for Bipyridinium Ring
Protons of Diads in CD3CN at 27 °C
chemical shifta
compd
2,6-bpy
6.90
7.58
8.35
8.44
3,5-bpy
7.41
7.83
8.07
8.16
3′,5′-bpy
2′,6′-bpy
1
Figure 4 shows temperature-dependent H NMR spectra of
10
10Zn
11
11Zn
14
7.99
8.08
8.16
8.24
8.76
8.77
8.73
8.80
the viologen-bridged Zn(II) diad (14Zn). A considerably dif-
ferent 1H NMR spectrum, compared with that for the diad 10Zn,
was observed for the protons of -CH2-N+ (δ 2.38-2.50),
N-CH2 (δ -4.70 to -4.60 and -4.60 to -4.50), the 2,6-
bipyridinium ring (δ 7.29 and 7.58), and the pyrrole rings (δ
8.40 and 8.86), whose signals were separated. These results
suggest that the rotation of both viologen and trimethylene
groups is significantly restricted. The calculated structure of
14Zn based on MM2 (a total energy, 37 kcal mol-1) is
reasonable on the basis of the NMR data (Figure 5). The
6.60
7.20
14Zn
7.29, 7.58
7.29, 7.62
a The internal standard of chemical shifts is TMS.
approximately in the plane of the macrocycle, whereas ring A
is appreciably twisted from the plane.13,19 The proton signals
of the pyrrole B,Da and B,Db are assigned to upfield and down-
Figure 2. Structural models of the diad 10 obtained by MM2 calculation: (left) an extended form; (right) a closed form.

2870 J. Phys. Chem. B, Vol. 103, No. 15, 1999
Tsukahara et al.
Figure 3. Structural models of the diads 11 (left) and 10Zn (right) by MM2 calculation.
Figure 5. Structural model of the viologen-bridged diad 14Zn obtained
by MM2 calculation.
Figure 4. Temperature dependence of 1H NMR spectra of the viologen-
bridged diad 14Zn in CD3CN.
protons, respectively. The N-H proton signal still appeared and
was shifted downfield by ca. 1 ppm. These results strongly
suggest that the extended conformer predominantly exists in
the Si(IV) porphyrin due to the charge repulsion and that the
pyrrole C ring is twisted from the plane. A molecular model
based on MM2 demonstrates that the extended conformer is
more stable than the closed one more than 5 kcal mol-1 even
in the hexamethylene-bridged 11Si and that the N-H proton is
twisted to the same side as the N-alkyl group, whereas the
pyrrole C ring is at the opposite side. Insertion of Al(III) into
the diad 10 also changed the 1H NMR spectrum, but the changes
in chemical shifts were within 0.2 ppm, suggesting that the
conformational change of 10 on metalation by Al(III) is smaller
than in the case of Si(IV).
orientation of viologen in the diads 10Zn and 14Zn is quite
different from each other. The viologen unit of 10Zn is capable
of rotating around the methylene group (-CH2-N+), because
the proton signals of the methylene groups are not separated as
those for 14Zn. Intermediate properties in the conformation were
observed for the free-base triad 14. It is demonstrated from 1H
NMR measurements that the viologen moiety in 14 is more rigid
than that in 10.
Insertion of Si(IV) into the diad 10 changes drastically the
1H NMR spectrum. The signal of the pyrrole C protons was
shifted upfield by 0.35 ppm, whereas the other proton signals
were shifted downfield; the extent of the shift was 0.25, 0.20-
0.23, 0.3-0.5, 0.54, 0.74, and 0.10 ppm for the pyrrole B,D
ring, phenyl, bipyridinium, -CH2-N+, -CH2-, and N-CH2-
Fluorescence Properties. Fluorescence data for N-alkylpor-
phyrins and metalloporphyrins are summarized in Table 2.
N-Alkylporphyrins have unique fluorescence properties com-

Viologen-Linked N-Alkylporphyrin Diads
J. Phys. Chem. B, Vol. 103, No. 15, 1999 2871
TABLE 2: Fluorescence Data for Viologen-Linked
N-Alkylporphyrins and Their Metal Complexes in MeCN
under Argon at 25 °C
compd λex/nm
λ
em/nm
φf
τf/ns (yield/%)a
1
429 687, 745 (sh) 0.014
1.0 (100)
9.0 (100)b
1.0 (100)
0.9 (100)
1.6 (100)
0.9 (100)
1.0 (100)
1.0 (100)
1H+
1Na
1Zn
434 690, 745 (sh) 0.014
447 710
436 668, 732
0.014
0.011
0.014
0.014
0.015
1Mg 448 710
1Al
1Si
2
2Zn
3
3Zn
6
7
450 715
450 713
429 687, 745 (sh) 0.020 <0.6 (25), 3.0 (75)
436 669, 730 0.0058 1.1 (38), 1.9 (62)
429 687, 745 (sh) 0.016 <0.7 (20), 5.1 (80)
436 668, 730 0.012 1.7 (100)
429 687, 745 (sh) 0.0043 <0.3 (87), 3.0 (13)
430 689, 745 (sh) 0.0049 <0.3 (72), 3.9 (28)
7Zn
9
10
436 668, 730
0.0081 1.4 (100)
430 687, 745 (sh) 0.0049 <0.7 (90), 3.1 (10)
429 687, 745 (sh) 0.0003 <0.2 (80), 1.2 (20)c
2.0 (70), 9.0 (30)b
10H+
10Na
10Zn
434 690, 745 (sh) 0.0028 <0.7 (100)
447 710
0.0005 <0.7 (100)
436 670, 740 (sh) 0.0006 <0.2 (85), 2.2 (15)c
10Mg 448 710
0.0024 <0.7 (100)
0.0074 <0.7 (100)
0.012
10Al
10Si
11
450 715
450 713
Figure 6. Stern-Volmer plots for the fluorescence quenching of 1,
1Zn, and 1Si by MV2+ in degassed MeCN at 25 °C.
0.9 (100)
429 687, 745 (sh) 0.0009 <0.4 (90), 3.5 (10)
at 310 nm in the presence of excess MV2+ whose absorption
maximum is 258 nm. The fluorescence of the Zn(II) and Si-
(IV) complexes of 1 was also quenched by free MV2+ and the
Stern-Volmer plots were linear for both Zn(II) and Si(IV)
complexes (Figure 6). The intermolecular quenching rate
constants were obtained to be 7.3 × 109 M-1 s-1 for Zn(II)
and 6.1 × 109 M-1 s-1 for Si(IV): τ0 ) 1.6 ns for 1Zn and 1.0
ns for 1Si. There is no appreciable formation of exciplex for
the Zn(II) and Si(IV) complexes, arising from the charge
repulsion between viologen and central metal ions.
2.3 (75), 5.9 (25)b
11H+
11Na
11Zn
434 690, 745 (sh) 0.0094 <0.7 (100)
447 710
0.0017 <0.7 (100)
436 668, 740 (sh) 0.0023 <0.2 (30), 0.79 (60), 3.5 (10)c
11Mg 448 710
0.0046 <0.7 (100)
0.012
0.014
11Al
11Si
12
450 715
450 713
430 717
0.8 (100)
1.0 (100)
0.0001 <0.5 (90), 5.4 (10)
14
430 687, 730 (sh) 0.0018 <0.2 (50), 1.1 (50)c
2.9 (50), 9.0 (50)b
14H+
14Na
14Zn
14Mg 448 710
14Al
14Si
434 690, 740 (sh) 0.0087 <0.7 (100)
447 710
0.0027 <0.7 (100)
Intramolecular Quenching by Linked Viologen. Fluores-
cence spectra of the diad 10 and its metal complexes in MeCN
are shown in Figure 7 compared with those of the reference
compounds 1 and 1Zn. The fluorescence-quantum yield (φf)
and lifetime (τf) of the viologen-linked systems are both smaller
than those of 1 and 1Zn, suggesting that the excited singlet
state of the N-alkylporphyrin unit is quenched by the linked
viologen through an intramolecular ET. The free energy change
of the reaction can be estimated to be -0.55 and -0.42 eV for
10 and 10Zn, respectively, based on the redox potentials and
the exciting energies: E°(MV2+/MV•+) ) -0.44 V, E°(10•+/
1(10)*) ) -0.99 V, and E°(10Zn•+/1(10Zn)*) ) -0.86 V.
Transient picosecond-absorption spectroscopy for the Zn(II)
complexes, 1Zn, 10Zn, and 14Zn, are shown in Figures 8-10.
The formation of a porphyrin radical cation (λmax 470 nm (sharp)
and 680 nm (broad)) after irradiation of light within 10-30 ps
was confirmed (see Figures 8 and 9), although the absorption
of a viologen radical cation (λmax 600 nm) interfered with that
of the porphyrin radical cation. The lifetimes of the singlet state
were τs ) 2.0 ns for 1Zn, τs < 14 ps for 10Zn, and τs1 ≈ 20
ps (the fast phase) and τs2 ) 0.8 ns (the slow phase) for 14Zn.
An exciplex between zinc porphyrin and a viologen moiety may
be formed. Then the porphyrin radical cation decayed rapidly,
whose lifetime was ca. 50 ps for both 10Zn and 14Zn, and the
amount of the radical cation intermediate for 14Zn was smaller
than that for 10Zn. This is responsible for the fluorescence
behavior that the lifetimes of these systems have two compo-
nents, fast and slow (vide infra). The excited triplet state was
gradually formed over several microseconds: T-T absorption
maximum at 475 nm (broad).
436 668, 730 (sh) 0.0024 <0.2 (15), 1.3 (85)
0.0055 <0.7 (100)
0.013
0.014
450 715
450 713
0.9 (100)
1.0 (100)
a By using a nanosecond-fluorometer. b In propylene glycol at 0 °C.
c By using a picosecond-fluorometer.
pared with non-N-substituted porphyrins: red shifts in the
emission maxima, a considerable decrease in the quantum yield,
and larger Stokes’ shifts between the Qx(0,0) absorption and
Qx(0,0) fluorescence peaks.11,16 This fact can be explained by
the noncoplanarity of the porphyrin plane. Conformationally
distorted non-N-substituted porphyrins, such as dodecasubsti-
tuted porphyrins, have shown considerable decrease in the
fluorescence quantum yield.20 The low fluorescence quantum
yields may arise from increased rates of radiationless internal
conversion and intersystem crossing processes. The internal
conversion process is dependent on the vibrational wave function
overlap associated with a structural reorganization in the excited
state, and the increased intersystem crossing rate results
primarily from enhanced spin-orbit coupling in the nonplanar
complexes. We have shown that the low quantum yield of the
fluorescence of 1Zn or 2Zn arises mainly from the increased
intersystem crossing rate.16
The fluorescence from the excited singlet state of 1 (1(1)*)
and its Zn(II) and Si(IV) porphyrins was quenched by free MV2+
in MeCN similar to non-N-substituted porphyrins.7b A Stern-
1
Volmer plot for the quenching of (1)* by MV2+ showed a
saturation behavior, suggesting an exciplex formation between
1(1)* and MV2+. A new exciting spectral maximum appeared

2872 J. Phys. Chem. B, Vol. 103, No. 15, 1999
Tsukahara et al.
Figure 7. Fluorescence spectra of the diad 10, its metal complexes,
and the reference compounds 1 and 1Zn in degassed MeCN at 25 °C.
Exciting wavelengths are Soret maxima.
Figure 9. Transient absorption spectra after excitation of 10Zn (3.0
× 10-6 M) by picosecond-laser flash photolysis in Ar-saturated MeCN
at 25 °C. The exciting wavelength is 532 nm.
ceptor orbital is weak. There are two possibilities in the ET
pathway from donor and acceptor: a through-space mechanism
and a through-bond one.21 If only the through-bond mechanism
operated in the present system, the lifetimes for both closed
and extended forms might not be different. Although the ET
pathway is not clear at present, the coupling between the
π-donor and π-acceptor orbitals in orthogonal orientation might
be weaker than that in the closed orientation even by the
through-bond mechanism. In the case of the hexamethylene-
bridged diad 11, the lifetimes for the fast and slow components
are similar to and shorter than that for 10, respectively. The
short lifetime for the slow component in 11 might arise because
the viologen moiety of 11 is possible to locate close to the
porphyrin plane due to flexibility of the hexamethylene chain.
In the viologen-bridged zinc(II) diad 14Zn, the slow component
becomes predominant (85%) in MeCN, reflecting the rigid
ground-state conformation. In the case of the free-base diad 14,
the closed form contributes to the fluorescence-decay process
to greater extent than the extended one. The contribution of the
slow-fluorescence lifetime for the triad is greater than that for
the diad, which is confirmed by the transient absorption spec-
troscopy described above. The contribution of the slow com-
ponent is in the order 10 ≈ 10Zn < 14 < 14Zn, reflecting the
rigidity of the bound viologen as predicted from NMR measure-
ments.
Figure 8. Transient absorption spectra after excitation of 1Zn (3.0 ×
10-6 M) by picosecond-laser flash photolysis in Ar-saturated MeCN
at 25 °C. The exciting wavelength is 532 nm.
Effect of typical metal ions of Na+, Mg2+, Al3+, and Si4+ on
the fluorescence quantum yield was examined in MeCN (see
Table 1 and Figure 7). The fluorescence maximum for these
complexes appeared around 715 nm and the fluorescence
quantum yields were in the order Na+ < Mg2+ < Al3+ < Si4+
for 10, 11, and 14. Decrease in the fluorescence quantum yield,
compared with the reference compound 1, arises from the ET
quenching by the bound viologen. Figure 11 shows the plots of
relative fluorescence quantum yield for these metalloporphyrin
diads (10, 11, and 14) and that for the unlinked porphyrin 1
against the effective charge on the metal ions (z/r). The Zn(II)
porphyrins were not shown in Figure 11, because the structures
of the Zn(II) porphyrins are different from those of the
The fluorescence lifetimes of the viologen-linked systems
have two components (see Table 2). The lifetime for the fast
component in MeCN was less than 0.2 ns, which is beyond the
instrumental response. Therefore, the fluorescence lifetimes for
1, 10, 11, and 14 were also measured in propylene glycol. The
lifetimes for the fast and slow component for 10 and 14 are
shorter than and similar to that for the reference compound 1,
respectively. The fast component might arise from a closed form
(see Figure 2), where the viologen moiety is close to the por-
phyrin plane. The slow-fluorescence decay must occur in an
extended and perpendicular form, where the electronic coupling
between the porphyrin π-donor orbital and the viologen π-ac-

Viologen-Linked N-Alkylporphyrin Diads
J. Phys. Chem. B, Vol. 103, No. 15, 1999 2873
Figure 10. Transient absorption spectra after excitation of 14Zn (3.0
× 10-6 M) by picosecond-laser flash photolysis in Ar-saturated MeCN
at 25 °C. The exciting wavelength is 532 nm.
Figure 12. Absorption spectral changes after excitation of 10Zn (3.0
× 10-6 M) by a nanosecond-laser flash photolysis in degassed MeOH.
The spectra were recorded at 0.03, 2, 4, 10, and 16 µs after irradiation.
Figure 13. Decay of MV•+ after irradiation with a Xe flash lamp (a
pulse width of 30 µs) of the degassed MeCN solution of 10Zn (3.0 ×
10-6 M) in the presence of free MV2+ (1.0 × 10-3 M). The data are
fitted to the second-order decay kinetics.
We have shown that the excited triplet state of 1Zn or 2Zn
was intermolecularly quenched by free methyl viologen,16
similar to non-N-substituted Zn(II) porphyrins. In the case of
the Zn(II) diad 10Zn, transient nanosecond-laser spectroscopy
showed that no radical-ion intermediate was detected (Figure
12) and the lifetime of the excited triplet state of 10Zn
(3(10Zn)*) became shorter than that of 1Zn: τt ) 3.0 µs (10Zn)
Figure 11. Plots of the relative fluorescence quantum yield vs zr-1
for the fluorescence of the diads 10, 11, and 14 and their metal
complexes in degassed MeCN at 25 °C. The compound 1 and its metal
complexes were used as references.
t
porphyrins of these typical metal ions (vide supra). This result
indicates that the insertion of positive charged metal ions into
N-alkylporphyrin decelerated the ET reaction and that in the
case of 11Si and 14Si almost no quenching occurred. The ET
rate might decrease in the extended and perpendicular confor-
mation, being induced by electrostatic repulsion between the
viologen moiety and a central metal ion. The redox potential
of Al(III) and Si(IV) porphyrins (E°(MP•+/MP)) becomes only
0.2 V higher than that of the free base porphyrin, which cannot
explain the great deceleration of the ET reaction.
and τ0 ) 7.9 µs (1Zn) in MeCN and τt ) 3.5 µs (10Zn) and
t
τ0 ) 4.4 µs (1Zn) in MeOH. The estimated free energy change
for the back ET reaction of 10Zn•+ with MV•+ is exothermic:
∆G° ) E°(MV2+/MV•+) - E°(10Zn•+/10ZnP) ) -0.44 - 1.05
) -1.49 eV. Therefore, these results strongly suggest that the
back ET reaction between the Zn(II) porphyrin radical cation
and the linked viologen radical cation is much faster than the
3
ET quenching of (10Zn)* by the linked viologen. The rate
3
constants of the intramolecular quenching of (10Zn)* by the
linked viologen are estimated to be 2.1 × 105 s-1 in MeCN

2874 J. Phys. Chem. B, Vol. 103, No. 15, 1999
Tsukahara et al.
and 5.9 × 104 s-1 in MeOH. These are smaller than those for
the corresponding non-N-substituted Zn(II) porphyrins.3,6 This
arises from the low driving force of the reaction: ∆G° ) E°-
(MV2+/MV•+) - {E°(10Zn•+/10ZnP) - E00(3(10Zn)*)} )
-0.44 - (1.05 - 1.48) ) -0.01 eV. When free methyl viologen
(1.0 × 10-3 M) was added into the solution of 10Zn (3.0 ×
10-6 M) followed by irradiation of light with a Xe flash lamp
(a 30 µs pulse width), both methyl viologen and Zn(II)porphyrin
radical cations (λmax 400 and 600 nm for the former and λmax
470 and 680 nm for the latter) were detected and decayed over
15 ms with a second-order kinetics (Figure 13); the second-
order rate constants are 3.2 × 109 M-1 s-1 in MeCN and 5.8 ×
109 M-1 s-1 in MeOH. This strongly suggests that the
intramolecular ET quenching of 3(10Zn)* by the linked viologen
competes with the intermolecular process in the presence of
and [Zn(BrPrtpp)Cl]‚0.5H2O (2Zn)) were prepared by the
previously reported method.16
N-(3-Iodopropyl)-5,10,15,20-Tetraphenylporphyrin Hydro-
gen Tetrafluoroborate ([H2IPrtpp]BF4, 4) and N-(6-Iodo-
hexyl)-5,10,15,20-tetraphenylporphyrin Hydrogen Tetraflu-
oroborate ([H2IHxtpp]BF4‚CH3C6H5, 5). Syntheses were car-
ried out by the method of Setsune and Takeda.26 The mixture
of H2tpp (5.13 g, 8.34 mmol) and [Ph2SprI]BF4 (10.5 g, 0.0238
mol) or [Ph2ShxI]BF4 (11.5 g, 0.0238 mol) was refluxed in 330
mL of 1,2-dichloroethane for 72 h under argon. After the solvent
was removed, the residue was dissolved in dichloromethane
(CH2Cl2) and the solution was loaded on a silica gel column
(o.d. 3.8 cm × 20 cm). A purple band was eluted with CH2Cl2
to recover H2tpp (4.00 g, 78%). The second green band was
eluted with CH2Cl2-acetone (100:1 v/v), followed by a rechro-
matography on a silica gel column to yield 1.86 g (25%) of 4
and 2.14 g (28%) of 5. The excess amounts of diphenylsulfo-
nium salts were eluted with acetone. A green band remained
on the top of the column. 4: 1H NMR (270 MHz, CDCl3, 27
°C, TMS) δ 8.89 (d, J ) 4.1 Hz, 2H, pyrrole B,Da), 8.77 (s,
2H, pyrrole C), 8.70 (d, J ) 4.1 Hz, 2H, pyrrole B,Db), 8.39
(br, 4H, o-phenyl I), 8.21 (br, 4H, o-phenyl II), 7.70-7.92 (m,
12H, m-, p-phenyl I, II), 7.67 (s, 2H, pyrrole A), 1.30 (t, J )
5.4 Hz, 2H, -CH2I), -2.54 (br, 1H, NH), -1.38 to -1.19 (m,
2H, -CH2-), -4.91 (br, 2H, N-CH2-). Anal. Calcd for
C47H36N4IBF4: C, 64.85; H, 4.17; N, 6.44. Found: C, 65.05;
H, 4.16; N, 6.34. 5: 1H NMR (270 MHz, CDCl3, 27 °C, TMS)
δ 8.89 (d, J ) 6.6 Hz, 2H, pyrrole B,Da), 8.64 (s, 2H, pyrrole
C), 8.43 (d, J ) 6.6 Hz, 2H, pyrrole B,Db), 8.33 (br, 4H,
o-phenyl I), 8.25 (br, 4H, o-phenyl II), 7.67-8.02 (m, 12H,
m-, p-phenyl I, II), 7.79 (s, 2H, pyrrole A), 2.67 (t, J ) 7.0 Hz,
2H, R-CH2I), 1.02-1.13 (m, 2H, â-CH2-), 0.06-0.17 (m, 2H,
γ-CH2-), -0.60 to -0.49 (m, 2H, δ-CH2-), -1.69 to -1.59
(m, 2H, ꢀ-CH2-), -5.08 (br, 2H, N-CH2-). Anal. Calcd for
C50H42N4IBF4‚CH3C6H5: C, 68.14; H, 5.02; N, 5.58. Found:
C, 68.56; H, 4.93; N, 5.96.
free MV2+
.
Conclusions
The fluorescence quantum yields in the viologen-linked and
-bridged N-alkylporphyrins become larger by insertion of H+,
Mg2+, Al3+, and Si4+ into the N-alkylporphyrins. This may arise
because the extended and orthogonal conformer due to the
electrostatic repulsion between the viologen and the central metal
ion decelerates the ET quenching of N-alkylporphyrin by the
bound viologen unit.
Experimental Section
Materials. 5,10,15,20-Tetraphenylporphyrin (H2tpp), 1,3-
dibromopropane, 1,6-dibromohexane, 1-iodopropane, nitro-
methane, 4,4′-bipyridine, and 3,4-dihydro-2H-pyrido[1,2-a]-
pyrimidin-2-one were purchased from Tokyo Kasei Kogyo Co.,
Ltd. and were used without further purification. Silver hexafluo-
rophosphate was purchased from Aldrich Chemical Co., Inc.
Tetrahydrofuran (THF), N,N-dimethylformamide (DMF), metha-
nol (MeOH), acetonitrile (MeCN), and propylene glycol used
were of Spectrosol (Dojindo Laboratory) for syntheses and
spectroscopic measurements and were of Luminasol (Dojindo
Laboratory) for luminescence and kinetic measurements. Ac-
tivated alumina (Wako Chemical Industries, Ltd., 200 mesh),
silica gel (Wakogel C-200), and Sephadex LH-20 (Pharmacia)
were used for a column chromatography. Other chemicals were
of guaranteed reagent grade from Wako Chemical Industries,
Ltd. All of the solvents were dried by standard methods. 1,1′-
Dimethyl-4,4′-bipyridinium perchlorate monohydrate ([MV]-
(ClO4)2‚H2O) and its hexafluorophosphate ([MV](PF6)2) were
obtained from the corresponding chloride salt (Tokyo Kasei)
by an anion exchange with sodium perchlorate and sodium
hexafluorophosphate in water, respectively. 1-Methyl-4-(4-
pyridyl)pyridinium hexafluorophosphate ([mbpy]PF6) were pre-
pared by the reported method with slight modifications.22 1,3-
Diiodopropane and 1,6-diiodohexane were prepared by reacting
the corresponding dibromoalkane with sodium iodide in acetone
at room temperature.23 Diphenyl(3-iodopropyl)sulfonium tet-
rafluoroborate ([Ph2SprI]BF4) and diphenyl(6-iodohexyl)sulfo-
nium tetrafluoroborate ([Ph2ShxI]BF4) were prepared by reacting
of diiodoalkane with diphenyl sulfide in nitromethane by the
same method as reported in the literature.24 1,1′′-Trimethyl-
enebis(4-(4-pyridyl)pyridinium) hexafluorophosphate ([pdb]-
(PF6)2) was prepared by treating the corresponding dibromide25
with sodium hexafluorophosphate in water. N-(3-Propyl)-5,10,-
15,20-tetraphenylporphyrin (HPrtpp‚1.5H2O, 1), N-(3-bromopro-
pyl)-5,10,15,20-tetraphenylporphyrin (HBrPrtpp‚2.5H2O, 2),
N-(3-hydroxypropyl)-5,10,15,20-tetraphenylporphyrin (HHOPrtpp‚
H2O, 3), and zinc(II) porphyrins ([Zn(Prtpp)Cl]‚0.5H2O (1Zn)
N-(3-Iodopropyl)-5,10,15,20-tetraphenylporphyrin (HIPrt-
pp, 6) and N-(6-Iodohexyl)-5,10,15,20-tetraphenylporphyrin
(HIHxtpp, 7). Compound 4 (0.983 g, 1.13 mmol) or compound
5 (1.14 g, 1.13 mmol) was treated with 3,4-dihydro-2H-pyrido-
[1,2-a]pyrimidin-2-one (0.184 g, 1.24 mmol) in 20 mL of CH2-
Cl2 at room temperature for 1 h. The solution was filtrated and
evaporated to dryness. The residue was dissolved in toluene,
and the solution was loaded on an alumina column (o.d. 2.0
cm × 15 cm). The purple band was eluted with CH2Cl2. After
the solvent was evaporated, the residue was recrystallized from
CH2Cl2-MeOH to give a purple powder: yield, 0.816 g (92%)
for 6 and 0.876 g (94%) for 7. 6: 1H NMR (270 MHz, CDCl3,
34 °C, TMS) δ 8.81 (s, 2H, pyrrole C), 8.65 (d, J ) 4.8 Hz,
2H, pyrrole B,Da), 8.47 (d, J ) 4.8 Hz, 2H, pyrrole B,Db), 8.26-
8.45 (br, 4H, o-phenyl I), 8.13-8.20 (br, 4H, o-phenyl II), 7.75-
7.85 (m, 12H, m-, p-phenyl I, II), 7.52 (s, 2H, pyrrole A), 1.44
(t, J ) 7.1 Hz, 2H, -CH2I), -2.2 (br, NH), -1.09 (quintet,
2H, J ) 6.4 and 7.1 Hz, -CH2-), -4.40 (t, J ) 6.4 Hz, 2H,
N-CH2-). Anal. Calcd for C47H35N4I: C, 72.12; H, 4.51; N,
7.16. Found: C, 72.47; H, 4.56; N, 7.11. FAB-MS (3-
nitrobenzyl alcohol (3-NBA)): m/z (rel intensity, %) 783 {-
1
[HIPrtpp]+, 100}. 7: H NMR (270 MHz, CDCl3, 34 °C, TMS)
δ 8.79 (s, 2H, pyrrole C), 8.64 (d, J ) 4.9 Hz, 2H, pyrrole
B,Da), 8.45 (d, J ) 4.9 Hz, 2H, pyrrole B,Db), 8.26-8.40 (br,
4H, o-phenyl I), 8.10-8.17 (br, 4H, o-phenyl II), 7.73-7.85
(m, 12H, m-, p-phenyl I, II), 7.45 (s, 2H, pyrrole A), 2.67 (t, J
) 6.8 Hz, 2H, R-CH2I), 1.02-1.13 (m, 2H, â-CH2-), 0.08-
0.16 (m, 2H, γ-CH2-), -0.47 to -0.59 (m, 2H, δ-CH2-),

Viologen-Linked N-Alkylporphyrin Diads
J. Phys. Chem. B, Vol. 103, No. 15, 1999 2875
-1.49 to -1.57 (m, 2H, ꢀ-CH2-), -4.48 (br, 2H, N-CH2-).
Anal. Calcd for C50H41N4I: C, 72.81; H, 5.01; N, 6.79. Found:
C, 73.44; H, 5.15; N, 6.74. FAB-MS (3-NBA): m/z (rel
intensity, %) 825 {[HIHxtpp]+ 100}.
8.60 (m, 4H, o-phenyl I), 8.14-8.22 (m, 4H, o-phenyl II), 8.16
(d, J ) 6.8 Hz, 2H, 3′,5′-bpy), 8.07 (d, J ) 6.8 Hz, 2H, 3,5-
bpy), 7.79-7.88 (m, 6H, m-, p-phenyl I), 7.72-7.83 (m, 6H,
m-, p-phenyl II), 7.40 (s, 2H, pyrrole A), 4.32 (s, 3H, N+-
CH3), 3.90 (t, J ) 7.7 Hz, 2H, R-CH2-N+), 1.05-1.14 (m,
2H, â-CH2-), -0.07 to +0.09 (m, 2H, γ-CH2-), -0.66 to
-0.53 (m, 2H, δ-CH2-), -1.62 to -1.54 (m, 2H, ꢀ-CH2-),
-2.0 (br, NH), -4.57 to -4.49 (m, 2H, N-CH2-). Anal. Calcd
for C61H52N6P2F12‚0.5CH3C6H5: C, 64.28; H, 4.68; N, 6.97.
Found: C, 64.42; H, 4.37; N, 7.15. FAB-MS (3-NBA): m/z
(rel intensity, %) 869 {[HMVhxtpp]+, 100}, 435 {[HMVhx-
tpp]2+, 48}.
5,10,15,20-Tetraphenyl-N-[3-(4-(4-pyridyl)pyridinio)pro-
pyl]porphyrin Bromide ([HBpyPrtpp]Br‚3H2O, 8). Com-
pound 2 (0.080 g, 0.10 mmol) and 4,4′-bipyridine (0.319 g, 2.04
mmol) were dissolved in 3 mL of DMF, and the solution was
heated at 75-80 °C for 46 h under argon. DMF was evaporated,
and the residue was washed with cold toluene. The desired
compound was further purified by a Sephadex LH-20 column
chromatography (o.d. 2.0 cm × 5.0 cm) with MeCN as an
eluent. Recrystallization from CH2Cl2-toluene gave a purple
powder 8 (0.057 g, 63%). 8: 1H NMR (270 MHz, CDCl3, 34
°C, TMS) δ 8.84 (s, 2H, pyrrole C), 8.69 (d, J ) 5.9 Hz, 2H,
2′,6′-bpy), 8.48 (d, J ) 3.7 Hz, 2H, pyrrole B,Da), 8.32 (d, J )
3.7 Hz, 2H, pyrrole B,Db), 8.17-8.27 (br, 4H, o-phenyl I),
8.01-8.08 (br, 4H, o-phenyl II), 7.69-7.82 (m, 12H, m-,
p-phenyl I, II), 7.52 (s, 2H, pyrrole A), 7.10 (d, J ) 5.9 Hz,
2H, 3′,5′-bpy), 6.93 (d, J ) 5.1 Hz, 2H, 3,5-bpy), 6.82 (d, J )
5.1 Hz, 2H, 2,6-bpy), 2.53 (t, J ) 5.5 Hz, 2H, -CH2-N+),
-0.71 to.62 (m, 2H, -CH2-), -4.77 to -4.71 (m, 2H,
N-CH2-). Anal. Calcd for C57H43N6Br‚3H2O: C, 72.37; H,
5.22; N, 8.88. Found: C, 72.43; H, 4.80; N, 8.73. Application
of the same method to HIPrtpp gave an iodide ([HBpyPrtpp]I‚
CH3C6H5, 9). Yield: 0.081 g (79%). Anal. Calcd for C57H43N6I‚
CH3C6H5: C, 74.56; H, 4.99; N, 8.15. Found: C, 74.31; H,
4.90; N, 8.49.
5,10,15,20-Tetraphenyl-N-[3-(1′-(3-(4-(4-pyridyl)pyridin-
io)propyl)-4,4′-bipyridinio)propyl]porphyrin Hexafluorophos-
phate ([HPdbPrtpp](PF6)3‚0.5CH2Cl2, 12). A DMF solution
(15 mL) of compound 2 (0.145 g, 0.186 mmol) or compound 6
(0.153 g, 0.186 mmol) with [pdb](PF6)2 (2.39 g, 3.71 mmol)
was heated at 75-80 °C for 48 h under argon. After DMF was
evaporated, the residue was washed with cold water to remove
excess [pdb](PF6)2 and then with cold toluene to remove
compound 2 or 6. The resulting purple solid was dissolved in
MeCN, and the solution was purified by Sephadex LH-20
column chromatography (o.d. 2.0 cm × 4.0 cm). Recrystalli-
zation from CH3CN-CH2Cl2 gave a purple powder 12 (0.183
g, 66%). 12: 1H NMR (270 MHz, CD3CN, 34 °C, TMS) δ
9.13 (d, J ) 6.6 Hz, 2H, 2′′,6′′-bpy), 9.07 (d, J ) 6.6 Hz, 2H,
2′,6′-bpy), 9.03 (d, J ) 6.6 Hz, 2H, 2′′′,6′′′-bpy), 8.90 (d, J )
6.6 Hz, 2H, 3′′,5′′-bpy), 8.85 (d, J ) 4.8 Hz, 2H pyrrole B,Da),
8.75 (s, 2H, pyrrole C), 8.55-8.60 (m, 4H, o-phenyl I), 8.39
(d, J ) 6.6 Hz, 2H, 3′′′,5′′′-bpy), 8.25-8.34 (m, 4H, o-phenyl
II), 8.17 (d, J ) 6.6 Hz, 2H, 3′,5′-bpy), 8.00-8.13 (m, 6H, m-,
p-phenyl I), 8.04 (s, 2H, pyrrole A), 7.86-7.98 (m, 6H, m-,
p-phenyl II), 7.82 (d, J ) 4.8 Hz, 2H pyrrole B,Db), 7.75 (d, J
) 6.6 Hz, 2H, 3,5-bpy), 7.36 (d, J ) 6.6 Hz, 2H, 2,6-bpy),
4.86 (t, J ) 8.8 Hz, 2H, R′′-CH2-N+), 4.83 (t, J ) 8.1 Hz,
2H, N+-CH2-(R′)), 2.72-2.86 (m, 2H, -CH2-(â′)), 2.13 (t, J
) 7.3 Hz, 2H, R-CH2-N+), -0.69 to -0.58 (m, 2H, â-CH2-
), -5.13 to -5.08 (m, 2H, N-CH2-). Anal. Calcd for
C70H57N8P3F18‚0.5CH2Cl2: C, 56.92; H, 3.93; N, 7.53. Found:
C, 56.76; H, 3.51; N, 7.77.
N-[3-(1-Methyl-4,4′-bipyridinio)propyl]-5,10,15,20-tetra-
phenylporphyrin Hexafluorophosphate ([HMVprtpp](PF6)2‚
3H2O, 10) and N-[6-(1-Methyl-4,4′-bipyridinio)hexyl]-5,10,-
15,20-tetraphenylporphyrin Hexafluorophosphate ([HMV-
hxtpp](PF6)2‚0.5CH3C6H5, 11). A DMF solution (6 mL) of
compound 2 (0.188 g, 0.241 mmol) or compound 6 (0.189 g,
0.241 mmol) with [mbpy]PF6 (1.14 g, 3.61 mmol) was heated
at 75-80 °C for 48 h under argon. DMF was evaporated, and
the residue was washed with cold water and toluene to remove
unreacted [mbpy]PF6 and compound 2 or 6, respectively. The
crude product was purified by Sephadex LH-20 column chro-
matography (o.d. 2.0 cm × 10 cm). Only one purple band was
eluted with MeCN. Removal of MeCN and washing with cold
MeOH gave a purple powder. Yield: 0.124 g (0.106 mmol,
44%) for 10 and 0.117 g (0.116 mmol, 48%) for 11. The
protonated green species of 10 and 11 were sometimes obtained
after washing with cold water; then they were further depro-
tonated by using 3,4-dihydro-2H-pyrido[1,2-a]pyrimidin-2-one,
followed by the purification with Sephadex LH-20 column
1,1′-Bis[3-(5,10,15,20-tetraphenylporphyrinyl)propyl]-4,4′-
bipyridinium Dibromide ([(HP)2MV]Br2‚3CH2Cl2‚(CH3)2-
NCOH, 13) and Hexafluorophosphate ([(HP)2MV](PF6)2‚
5H2O, 14). Method A. To the DMF solution (3 mL) of
compound 2 (0.120 g, 0.154 mmol) was added 4,4′-bipyridine
(9.6 mg, 0.061 mmol) in DMF (0.26 mL) over 9 h with stirring
at 75-80 °C under argon. After 39 h, DMF was evaporated
and the residue was washed with cold toluene to remove
unreacted 2 and 4,4′-bipyridine. Further purification by Sepha-
dex LH-20 column chromatography (o.d. 2.0 × 6.0 cm) with
MeCN and recrystallization from CH2Cl2-toluene yielded a
purple powder (0.025 g, 10%).
1
chromatography described above. 10: H NMR (270 MHz, CD3-
CN, 35 °C, TMS) δ 8.83 (s, 2H, pyrrole C), 8.76 (d, J ) 6.8
Hz, 2H, 2′,6′-bpy), 8.59 (d, J ) 4.8 Hz, 2H, pyrrole B,Da), 8.59
(d, J ) 4.8 Hz, 2H, pyrrole B,Db), 8.32-8.40 (m, 4H, o-phenyl
I), 8.12-8.22 (m, 4H, o-phenyl II), 7.99 (d, J ) 6.8 Hz, 2H,
3′,5′-bpy), 7.88-7.94 (m, 6H, m-, p-phenyl I), 7.76-7.84 (m,
6H, m-, p-phenyl II), 7.55 (s, 2H, pyrrole A), 7.41 (d, J ) 6.8
Hz, 2H, 3,5-bpy), 6.90 (d, J ) 6.8 Hz, 2H, 2,6-bpy), 4.40 (s,
3H, N+-CH3), 2.32 (t, J ) 6.6 Hz, 2H, -CH2-N+), -0.66 to
-0.57 (m, 2H, -CH2-), -2.3 (br, NH), -4.64 to -4.55 (m,
2H, N-CH2-). Anal. Calcd for C58H46N6P2F12‚3H2O: C, 59.49;
H, 4.48; N, 7.18. Found: C, 59.51; H, 3.99; N, 7.07. FAB-MS
(3-NBA): m/z (rel intensity, %) 827 {[HMVprtpp]+, 100}, 414
{[HMVprtpp]2+, 30}. 11: 1H NMR (270 MHz, CD3CN, 34 °C,
TMS) δ 8.77 (s, 2H, pyrrole C), 8.73 (d, J ) 6.8 Hz, 2H, 2′,6′-
bpy), 8.59 (d, J ) 4.8 Hz, 2H, pyrrole B,Da), 8.35 (d, J ) 6.8
Hz, 2H, 2,6-bpy), 8.38 (d, J ) 4.8 Hz, 2H, pyrrole B,Db), 8.25-
Method B. The mixture of compound 8 (0.070 g, 0.078
mmol) with compound 2 (0.159 g, 0.204 mmol) in DMF (6
mL) was heated at 75-80 °C for 48 h under argon with stirring.
After DMF was evaporated, the residue was washed with cold
toluene and further purified by Sephadex LH-20 column
chromatography with MeCN. Recrystallization from CH2Cl2-
toluene gave a purple powder 13 (0.062 g, 49%). Using
compound 6 instead of 2 gives the corresponding iodide of 13.
The bromide or iodide salt was converted to a hexafluorophos-
phate with silver hexafluorophosphate in MeCN to give a purple
powder 14 (0.066 g, 95%). 13: Anal. Calcd for C104H78N10-
Br2‚3CH2Cl2‚(CH3)2NCOH: C, 67.56; H, 4.69; N, 7.88.

2876 J. Phys. Chem. B, Vol. 103, No. 15, 1999
Tsukahara et al.
Found: C, 67.30; H, 4.88; N, 8.44. 14: 1H NMR (270 MHz,
CD3CN, 34 °C, TMS) δ 8.77 (d, J ) 6.6 Hz, 4H, pyrrole B,Da),
8.67 (s, 4H, pyrrole C), 8.42-8.55 (m, 8H, o-phenyl I), 8.12-
8.18 (m, 8H, o-phenyl II), 7.88-8.07 (m, 12H, m-, p-phenyl
I), 7.96 (s, 4H, pyrrole A), 7.65-7.92 (m, 12H, m-, p-phenyl
II), 7.40 (d, J ) 6.6 Hz, 4H, pyrrole B,Db), 7.20 (d, J ) 6.0
Hz, 4H, 3,5-bpy), 6.60 (d, J ) 6.0 Hz, 4H, 2,6-bpy), 2.15 (t, J
) 8.1 Hz, 4H, -CH2-N+), -0.70 to -0.60 (m, 4H, -CH2-),
-4.65 to -4.45 (m, 4H, N-CH2-). Anal. Calcd for C104H78N10-
P2F12‚5H2O: C, 68.27; H, 4.85; N, 7.65. Found: C, 68.34; H,
4.61; N, 7.76. FAB-MS (3-NBA): m/z (rel intensity, %) 812
{[[(HP)2MV]2+ - [Htpp(CH2)3]+]+, 100}, 1468 {[(HP)2MV]+,
9; 17 in 1-thioglycerol}.
1H NMR (270 MHz, CDCl3, 34 °C, TMS) δ 8.95 (d, J ) 5.1
Hz, 2H, pyrrole B,Da), 8.86 (d, J ) 5.1 Hz, 2H, pyrrole B,Db),
8.85 (s, 2H, pyrrole C), 8.50-8.60 (br, 4H, o-phenyl I), 8.20-
8.31 (br, 4H, o-phenyl II), 8.19 (s, 2H, pyrrole A), 7.76-7.89
(m, 12H, m-, p-phenyl I, II), 2.64 (t, J ) 6.8 Hz, 2H, R-CH2I),
0.99-1.10 (m, 2H, â-CH2-), 0.15-0.24 (m, 2H, γ-CH2-),
-0.75 to -0.67 (m, 2H, δ-CH2-), -0.87 to -0.81 (m, 2H,
ꢀ-CH2-), -4.58 (t, J ) 7.3 Hz, 2H, N-CH2-). Anal. Calcd
for C50H40N4IZnCl‚H2O: C, 63.71; H, 4.49; N, 5.94. Found:
C, 63.41; H, 4.22; N, 5.86. 14Zn ([(ZnClP)2MV](PF6)2‚3H2O):
1H NMR (270 MHz, CD3CN, 34 °C, TMS) δ 8.80-8.95 (m,
8H, pyrrole B,D), 8.68 (br, 4H, o-phenyl I), 8.40 (d, J ) 8.8
Hz, 4H, pyrrole C), 8.28 (br, 4H, o-phenyl I), 8.10 (br, 8H,
o-phenyl II), 7.97 (br, 4H, pyrrole A), 7.80-8.00 (m, 24H, m-,
p-phenyl I,II), 7.62 (d, J ) 6.0 Hz, 4H, 3,5-bpy), 7.58 (d, J )
6.0 Hz, 2H, 2- or 6-bpy), 7.29 (d, J ) 6.0 Hz, 2H, 2- or 6-bpy),
2.38-2.50 (m, 4H, -CH2-N+), -0.18 to -0.13 (m, 4H,
-CH2-), -4.60 to -4.50 (m, 2H, N-CH2-), -4.70 to -4.60
(m, 2H, N-CH2-). Anal. Calcd for C104H76N10Zn2Cl2P2F12‚
3H2O: C, 62.10; H, 4.11; N, 6.96. Found: C, 62.14; H, 3.88;
N, 7.08. FAB-MS (3-NBA): m/z (rel intensity, %) 911
{[[(ZnClP)2MV]2+ - [ZnCl(tpp(CH2)3)]+ - H]+, 100}, 1667
{[(ZnClP)2MV]+, 30}.
Insertion of Zinc(II) into Porphyrins. A typical method is
described for chloro{[3-(1-methyl-4,4′-bipyridinio)propyl]-5,-
10,15,20-tetraphenylporphyrin}zinc(II) ([ZnCl(MVprtpp)](PF6)2‚
0.5CH3C6H5, 10Zn). Compound 10 (0.078 g, 0.067 mmol) and
zinc(II) chloride (0.091 g, 0.67 mmol) were dissolved in 5 mL
of THF including 2,6-lutidine (7.4 mg, 0.069 mmol), and the
mixture was heated at 40 °C for 40 min under argon with
stirring. After THF was evaporated, the residue was washed
with cold water and then with cold toluene. The resulting purple
powder was dissolved in MeCN and purified by Sephadex LH-
20 column chromatography with MeCN. Recrystallization from
CH2Cl2-toluene gave a purple powder (0.054 g, 64%). The
same method as previously described in the literature16 was used
Insertion of Na(I), Mg(II), Al(III), and Si(IV) into Por-
phyrins. Metalloporphyrins were prepared in situ by adding
NaPF6, MgCl2‚6H2O, AlCl3‚6H2O, and SiCl4 in slight excess
in MeCN. The 1:1 complex formation was confirmed by a molar
ratio method at Soret band maxima for Na+, Mg2+, Al3+, and
Si4+ ions.27 An attempt to isolate these metalloporphyrins as a
solid was unsuccessful; removal of excess metal ions gave a
mixture of metalloporphyrin with free-base porphyrin. A small
amount of water was added into MeCN due to the lower
solubility of the metal salts except for NaPF6 and SiCl4. The
water content of the sample solution was less than 0.055 M.
1
for the synthesis of compound 7Zn. 10Zn: H NMR (270 MHz,
CD3CN, 35 °C, TMS) δ 8.93 (d, J ) 4.8 Hz, 2H, pyrrole B,Da),
8.87 (s, 2H, pyrrole C), 8.77 (d, J ) 4.8 Hz, 2H, pyrrole B,Db),
8.77 (d, J ) 6.8 Hz, 2H, 2′,6′-bpy), 8.38 (s, 2H, pyrrole A),
8.22-8.32 (m, 4H, o-phenyl I), 8.05-8.10 (m, 4H, o-phenyl
II), 8.08 (d, J ) 6.8 Hz, 2H, 3′,5′-bpy), 7.79-8.02 (m, 6H, m-,
p-phenyl I, II), 7.83 (d, J ) 6.8 Hz, 2H, 3,5-bpy), 7.58 (d, J )
6.8 Hz, 2H, 2,6-bpy), 4.37 (s, 3H, N+-CH3), 2.36 (t, J ) 7.3
Hz, 2H, -CH2-N+), -0.26 to -0.15 (m, 2H, -CH2-), -4.63
to -4.57 (m, 2H, N-CH2-). Anal. Calcd for C58H45N6-
ZnClP2F12‚0.5CH3C6H5: C, 58.49; H, 3.91; N, 6.65. Found:
C, 57.95; H, 3.97; N, 6.39. FAB-MS (3-NBA): m/z (rel
intensity, %) 926 {[[ZnCl(MVprtpp)]2+ - H+]+, 100}, 1072
{[ZnCl(MVprtpp)]2+‚PF6-, 40}. 11Zn ([ZnCl(MVhxtpp)](PF6)2‚
0.5CH3C6H5): 1H NMR (270 MHz, CD3CN, 34 °C, TMS) δ
8.97 (d, J ) 4.8 Hz, 2H, pyrrole B,Da), 8.89 (d, J ) 4.8 Hz,
2H, pyrrole B,Db), 8.84 (s, 2H, pyrrole C), 8.80 (d, J ) 6.8 Hz,
2H, 2′,6′-bpy), 8.25, 8.62 (br, 4H, o-phenyl I), 8.44 (d, J ) 6.8
Hz, 2H, 2,6-bpy), 8.28 (s, 2H, pyrrole A), 8.24 (d, J ) 6.8 Hz,
2H, 3′,5′-bpy), 8.16 (d, J ) 6.8 Hz, 2H, 3,5-bpy), 8.09 (br, 4H,
o-phenyl II), 7.83-7.91 (m, 6H, m-, p-phenyl I,II), 4.37 (s, 3H,
N+-CH3), 3.98 (t, J ) 7.7 Hz, 2H, R-CH2-N+), 1.08-1.20
(m, 2H, â-CH2-), 0.05-0.17 (m, 2H, γ-CH2-), -0.83 to -0.71
(m, 2H, δ-CH2-), -0.92 to -0.86 (m, 2H, ꢀ-CH2-), -4.69 to
-4.63 (m, 2H, N-CH2-). Anal. Calcd for C61H51N6ZnClP2F12‚
0.5CH3C6H5: C, 59.37; H, 4.25; N, 6.44. Found: C, 59.86; H,
3.96; N, 6.35. FAB-MS (3-NBA): m/z (rel intensity, %) 966
Measurements. Fluorescence spectra were measured in
degassed MeCN and MeOH solutions at 25 °C with a Hitachi
850 spectrofluorometer. The fluorescence quantum yield was
determined by using that of 1 as a standard compound.16 The
excitation wavelength was the Soret maximum. Fluorescence
lifetimes were measured at 25 °C using a Horiba NAES-500
nanosecond-fluorometer. Glass filters were used for cutting off
the exciting light (B390 (HOYA)) and the emission (U330
(HOYA), L42 (TOSHIBA), and Y51 (HOYA)). The fluores-
cence was detected by a single-photon counting system and
analyzed as the sum of two exponential components after
deconvolution of the instrument response function. A picosecond-
photon-counting streak scope system (Hamamatsu Photonics
C4780) was also used for fluorescence lifetime measurements
of compounds 14, 10Zn, 11Zn, and 14Zn, where an N2/dye
laser was used for excitation at 430 nm (a 300 ps pulse width
and 10 µJ/pulse). T-T absorption spectra and the lifetimes of
the excited triplet state were measured using nanosecond-laser
flash photolysis;28 a XeCl excimer laser was used as a light
source where the excitation wavelength was 308 nm, and a Xe
lamp was used as a spectrum flash lamp. Time-resolved
difference absorption spectra over a delay time ranging from
0-6000 ps were measured using a picosecond-laser spectros-
copy system,29 where a mode-locked Nd3+:YAG laser (Con-
tinuum PY61C-10, fwhm ) 17 ps, 10 Hz) was used for
excitation at 532 nm. Conventional pulse flash photolysis was
also carried out for a slow reaction using a Photal RA-412 pulse
flash apparatus with a Xe flash lamp (a 30 µs pulse width). 1H
NMR spectra were measured with a JEOL JNM-GX270 FT
NMR spectrometer. UV-vis and IR spectra were recorded with
{[[ZnCl(MVhxtpp)]2+ - H+ - 2H]+, 100}, 968 {[[ZnCl-
(MVhxtpp)]2+ - H+], 90}, 1114 {[ZnCl(MVhxtpp)]2+‚PF6
,
-
32}. 3Zn ([ZnCl(HOPrtpp)]‚0.5H2O): 1H NMR (270 MHz,
CDCl3, 34 °C, TMS) δ 8.94 (d, J ) 5.1 Hz, 2H, pyrrole B,Da),
8.86 (d, J ) 5.1 Hz, 2H, pyrrole B,Db), 8.84 (s, 2H, pyrrole C),
8.53-8.63 (br, 4H, o-phenyl I), 8.21 (s, 2H, pyrrole A), 8.10-
8.20 (br, 4H, o-phenyl II), 7.70-7.90 (m, 12H, m-, p-phenyl I,
II), 1.83 (t, J ) 6.0 Hz, 2H, -CH2OH), -0.67 to -0.57 (m,
2H, -CH2-), -4.46 (t, J ) 7.3 Hz, 2H, N-CH2-). Anal. Calcd
for C47H35N4OZnCl‚0.5H2O: C, 72.22; H, 4.64; N, 7.17.
Found: C, 72.22; H, 4.72; N, 6.96. 7Zn ([ZnCl(IHxtpp)]‚H2O):

Viologen-Linked N-Alkylporphyrin Diads
J. Phys. Chem. B, Vol. 103, No. 15, 1999 2877
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Acknowledgment. This research was partly supported by
Grants-in-Aid for Scientific Research Nos. 06640724 and
08454209 from the Ministry of Education, Science, Sports, and
Culture, Japan. The authors thank Professor K. Kasuga and Dr.
M. Handa, Shimane University, for elemental analyses, Professor
K. Morishige of Kinki University for FAB-MS spectra, and
Hamamatsu Photonics for the picoseond fluorescence measure-
ments.
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Supporting Information Available: Absorption spectral
data for N-alkylporphyrins and their zinc(II) complexes in
MeCN (Table S1), absorption spectral data for viologen-linked
N-alkylmetalloporphyrin diads in MeCN (Table S2), absorption
spectra of the viologen-linked diads 10 and 14 and their Zn(II)
complexes in MeCN (Figure S1), and absorption spectra of the
metal complexes of viologen-linked diad 10 in MeCN (Figure
S2). This information is available free of charge via the Internet
at http://pubs.acs.org.
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