Enhanced intersystem crossing in gable-type copper(II) porphyrin-free base porphyrin dimers: evidence of through-bond exchange interaction

Article abstract of DOI:10.1021/jp9941864

Spacer dependence of intersystem crossing (ISC) affected by a remote metal unpaired electron was studied in a series of copper(II) porphyrin-free base porphyrin dimers. In these dimers, fluorescence of the free base moiety is remarkably quenched by the copper counterpart which is linked via a phenanthrene, a naphthalene, or a benzene. Fluorescence lifetimes and quantum yields of the free base moiety in the heterodimers are 1/10-1/35 of the free base monomer depending upon the spacer, whereas no change was observed in the free base homodimers. The changes in the copper(II)-free base dimers are attributed to an enhancement of ISC in the free base half, due to interaction with an unpaired electron in Cu(II) ion of the other half. Estimated ISC rates from the experimental results exhibit a strong correlation with the number of bonds of the linkage but not with the center-to-center distance of the two halves. An expression for the ISC rate in the presence of an unpaired electron is derived. Exchange interaction between the copper unpaired electron and free base π-electrons gives rise to mixing between the excited states of the dimer when the free base moiety is in singlet and triplet excited states, and enhances ISC in the free part. The observed spacer dependence leads to the suggestion that the ISC rate is governed by through-bond exchange interaction.

Full text of DOI:10.1021/jp9941864

J. Phys. Chem. A 2000, 104, 4857-4865
4857
Enhanced Intersystem Crossing in Gable-Type Copper(II) Porphyrin-Free Base Porphyrin
Dimers: Evidence of Through-Bond Exchange Interaction
Namiki Toyama, Motoko Asano-Someda,* Takatoshi Ichino,† and Youkoh Kaizu
Department of Chemistry, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo, 152-8551, Japan
ReceiVed: NoVember 29, 1999; In Final Form: March 9, 2000
Spacer dependence of intersystem crossing (ISC) affected by a remote metal unpaired electron was studied
in a series of copper(II) porphyrin-free base porphyrin dimers. In these dimers, fluorescence of the free base
moiety is remarkably quenched by the copper counterpart which is linked via a phenanthrene, a naphthalene,
or a benzene. Fluorescence lifetimes and quantum yields of the free base moiety in the heterodimers are
1/10-1/35 of the free base monomer depending upon the spacer, whereas no change was observed in the
free base homodimers. The changes in the copper(II)-free base dimers are attributed to an enhancement of
ISC in the free base half, due to interaction with an unpaired electron in Cu(II) ion of the other half. Estimated
ISC rates from the experimental results exhibit a strong correlation with the number of bonds of the linkage
but not with the center-to-center distance of the two halves. An expression for the ISC rate in the presence
of an unpaired electron is derived. Exchange interaction between the copper unpaired electron and free base
π-electrons gives rise to mixing between the excited states of the dimer when the free base moiety is in
singlet and triplet excited states, and enhances ISC in the free base part. The observed spacer dependence
leads to the suggestion that the ISC rate is governed by through-bond exchange interaction.
1. Introduction
can be controlled by the spacer units.7-13,20-27 Particularly in
porphyrin heterodimers having two different metal ions, two
moieties are not equivalent due to their different excitation
energies and redox potentials. Therefore such features of these
porphyrin heterodimers can be utilized not only for designing
excellent donor-acceptor systems but also for studying the
effects of a remote unpaired electron in one moiety on the other
half of the dimer.
Recently, we have shown that efficient intramolecular energy
transfer takes place via the triplet manifolds in two rigidly linked
copper(II) porphyrin-free base porphyrin dimers, but also that
intersystem crossing (ISC) of the free base porphyrin moiety is
remarkably enhanced by the copper counterpart by means of
fluorescence and transient absorption spectroscopy.7 Since the
observed ISC rates are different between the two dimers and
such effect has not been observed in the corresponding
diamagnetic dimers, it is clear that the presence of an unpaired
electron in the copper half affects the ISC rate of the free base
half. However, a mechanism which accounts for the enhanced
ISC by a remote unpaired electron has not been fully substanti-
ated.
In the present work, we investigate spacer dependence of ISC
rates of the free base porphyrin moiety in gable-type copper-
(II)-free base porphyrin heterodimers (Figure 1) involving a
new dimer with a phenanthrene bridge. Particularly in the
phenanthrene-bridged dimer, the center-to-center distance be-
tween the two porphyrins is smaller than those of the other two
dimers, i.e., the benzene- and naphthalene-bridged dimers, but
the path length to combine the two moieties is the longest among
the three. From the fluorescence lifetimes and quantum yields
of both series of the free base homodimers and copper(II)-
free base heterodimers, we evaluate ISC rates of the free base
part in the dimers and show their spacer dependence. Separately,
we derive an expression of the ISC rate in the molecular system
involving a remote unpaired electron in terms of interaction
Long-range electronic interaction in molecular ensembles is
crucial to understanding intramolecular processes such as
electron and energy transfer in biological milieus and artificial
molecular systems.1-4 Besides the fact that electron and energy
transfer occurs through long-range interaction in such systems,
natural and artificial large molecular systems often involve a
paramagnetic species which is not located in the center of the
reaction but may affect photophysical processes through long-
range electronic and/or spin-spin interaction.
Among various photophysical processes which might be
affected by a remote unpaired electron, one of the simplest
processes is intersystem crossing (ISC). Although it is well-
known that the ISC rate is governed by spin-orbit (SO)
coupling in most molecules,5 it is also true that the ISC rate is
sometimes much faster than that expected from SO coupling
when the system contains a paramagnetic species.6,7 To elucidate
the role of a “remote” metal unpaired electron in intersystem
crossing, we have been investigating porphyrin heterodimers
in which only one half has a paramagnetic central metal ion,
i.e., copper(II), and the other half is a free base porphyrin. Spacer
dependence study of ISC occurring in the diamagnetic moiety
of the dimer is expected to lead to a clear understanding of
how the remote paramagnetic metal plays a role in terms of
ISC rates through long-range interaction.
A great many kinds of porphyrin dimers and oligomers have
been studied extensively over the past two decades with
relevance to the primary process of photosynthesis and photo-
harvesting systems.4,7-37 Porphyrin dimers with a rigid bridge
are advantageous for the systematic investigation of intra-
molecular processes because the mutual distance and orientation
* Corresponding author.
† Present address: Radiation Laboratory, University of Notre Dame,
Notre Dame, IN 46556.
10.1021/jp9941864 CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/29/2000

4858 J. Phys. Chem. A, Vol. 104, No. 21, 2000
Toyama et al.
Figure 1. Schematic molecular structures of the copper(II) porphyrin-
free base porphyrin dimers bridged via a phenanthrene, a naphthalene,
and a benzene.
between the unpaired electron and the π-electrons. On the basis
of the derived equation for the ISC rate, we will present a
mechanism leading to enhanced ISC through long-range ex-
change interaction, which is consistent with the observed spacer
dependence.
2. Experimental Section
2.1. Measurements. Time-resolved fluorescence measure-
ments were performed on a Hamamatsu C4790 fluorescence
lifetime measurement system.7 Free base porphyrins were
selectively excited at 590 nm by use of a Laser Photonics
LN120C Nitro Dye Laser. Emission was dispersed by a
Chromex 250IS imaging spectrograph and then detected by a
streak scope Hamamatsu C4334 under the single photon
counting condition. Both the time- and wavelength-resolved data
were transferred to a personal computer through a GPIB
interface. Fluorescence lifetimes were determined by fitting the
decay profiles produced from the data set summed over an
appropriate wavelength window.
Steady-state absorption and emission spectra were taken on
a Hitachi 330 spectrophotometer and a Hitachi 850 spectrof-
luorometer, respectively. For the emission measurements, por-
phyrins were dissolved in toluene to give a final concentration
of ∼1 × 10-5 M (O.D. at 590 nm ) ∼0.1 with a 10 mm path
length). Sample solutions were purged with argon gas and sealed
in a quartz cell. Toluene used for solvent was shaken with
sulfuric acid following simple distillation, and then neutralized
with dilute NaHCO3 a.q. After washing with water and drying
over CaCl2, redistillation was carried out on CaH2.
Figure 2. Absorption (s) and emission (---) spectra of (a) Cu-Pn-
H2, (b) Cu-Np-H2, (c) Cu-Bz-H2, and (d) a 1:1 linear combination
of two monomers, TPPCu and TPPH2, in toluene at room temperature.
In (d), absorption spectra of TPPCu (‚‚‚) and TPPH2 (- ‚ -) are also
shown. Emission intensities in (a), (b), and (c) are scaled by (a) 10, (b)
13, and (c) 29 times relative to that of (d).
1
m/z 1251 (M+ + 1). H NMR(CDCl3) δ -2.89 (s, 4H), 7.55
(m, 6H), 7.68 (m, 12H), 7.95 (d, J ) 7 Hz, 4H), 8.09-8.14 (m,
8H), 8.34 (s, 2H), 8.41 (d, J ) 8 Hz, 2H), 8.56 (d, J ) 8 Hz,
2H), 8.71-8.73 (m, 12H), 8.85 (d, J ) 5 Hz, 4H), 9.58 (s,2H).
Monometalation of H2-Pn-H2 into Cu-Pn-H2 (Cu-Pn-
H2 denotes [5-[3′-[6′-(10,15,20-triphenyl-21H,23H-porphin-5-
yl)]phenanthyl]-10,15,20-triphenyl-porphyrinato]copper(II)-
N21,N22,N23,N24 ) was done by reacting with copper(II) acetate,
in a manner similar to that for the corresponding benzene- and
naphthalene-bridged dimers.7 This procedure gives a mixture
of bismetal, monometal, and free base porphyrins (i.e., Cu-
Pn-Cu, Cu-Pn-H2, H2-Pn-H2). Cu-Pn-H2 was easily
separated from the other two by column chromatography
(alumina; CH2Cl2, silica gel; CHCl3/hexane) and further puri-
fication was carried out on column chromatography and
recrystallization (CHCl3/MeOH). The final product was identi-
fied by FAB-MS spectroscopy; m/z 1313 (M+ + 1). The
absorption and emission data are presented under Results.
2.2. Materials. Porphyrin monomers were prepared and
purified according to the reported method.38 The preparation
of the benzene- and naphthalene-linked porphyrin dimers was
described previously.7
Phenanthrene-bridged free base homodimer (H2-Pn-H2) was
synthesized essentially in the same method as that of the
naphthalene-bridged dimer (H2-Np-H2). The key material for
the spacer unit, 3,6-dimethylphenanthrene, was prepared ac-
cording to the method of Davy et al.39 The dimer was prepared
via a stepwise ring closure, and the first condensation of the
porphyrin ring was done by the method of Adler et al.40 The
second ring closure was carried out by the method of Lindsey
et al.41 by using trifuluoroacetic acid (TFA) as catalyst. The
yield obtained for H2-Pn-H2 with TFA catalysis (25.9%) was
higher than that with BF3 catalysis (12.4%). The free base
porphyrin dimer was purified by column chromatography
(alumina; CH2Cl2, silica gel; CHCl3/hexane) and recrystallization
(CHCl3/MeOH) repeatedly, to be sufficient for the lifetime
measurements.
3. Results
3.1. Absorption Spectra. Figure 2 shows absorption spectra
of the three copper(II)-free base porphyrin heterodimers and
monomers TPPCu and TPPH2, along with the 1:1 sum spectrum
of the monomers. In all three dimers, the absorption Q-band is
almost identical to that of the superposition of the monomers,
and thus the interaction between the two moieties should be
weak in the Q-band. In contrast, the absorption B-band exhibits
an energy splitting depending upon the spacer molecule, while
such splitting could not be observed in either the monomers or
the sum spectrum.
3,6-Bis[5′-(10′,15′,20′-triphenylporphinyl)]phenanthrene.
Anal. Calcd. for C90H58N8 (mol wt 1251.51): C, 86.38; H, 4.67;
N, 8.95. Found: C, 86.15; H, 4.96; N, 8.75. FAB-MS,
In the corresponding free base homodimers, similar charac-
teristic features of the absorption Q- and B-bands are observed
as shown in Figure 3. The absorption Q-bands of the ho-

ISC in Copper(II) Porphyrin-Free Base Porphyrin Dimers
J. Phys. Chem. A, Vol. 104, No. 21, 2000 4859
TABLE 1: Absorption and Emission Properties of the Monomers and Dimers in Toluene
λB/nm (ꢀ/105 mol-1 cm-1
)
λQ/nm (ꢀ/103 mol-1 cm-1
)
λf/nm
TPPH2
TPPCu
419(4.63)
417(4.57)
514(19.7)
540(20.6)
516(38.9)
516(39.5)
515(39.8)
516(22.0)
515(22.1)
515(22.5)
549(8.1)
591(5.5)
647(3.7)
652
717
H2-Pn-H2
H2-Np-H2
H2-Bz-H2
Cu-Pn-H2
Cu-Np-H2
Cu-Bz-H2
418(7.26)
419(5.43)
417(4.94)
416(6.95)
418(4.98)
415(4.86)
429(sh)
551(18.1)
551(19.4)
550(18.2)
543(25.5)
543(26.1)
543(26.4)
592(11.3)
592(11.2)
592(11.2)
590(6.3)
590(6.4)
590(6.3)
648(8.0)
648(8.2)
648(7.9)
648(3.8)
648(3.8)
648(3.7)
653
653
652
653
652
652
717
717
717
717
717
717
428(5.50)
428(4.69)
428(sh)
427(4.75)
428(4.38)
Figure 4. Fluorescence and its excitation spectra of Cu-Pn-H2
together with the absorption spectrum of the Q-band in toluene.
of the hetero- and homo-dimers are also presented in Figures 2
and 3, respectively. Note that the fluorescence intensities of the
copper(II)-free base dimers (Figure 2) are scaled by a factor
of 10-30 (see figure caption) relative to the monomer. The
fluorescence intensities of the copper(II)-free base dimers are
much weaker than the monomer, whereas the free base
homodimers do not show any decrease of the fluorescence
quantum yield.
Figure 4 compares the fluorescence excitation and absorption
spectra of Cu-Pn-H2. Evidently, the excitation spectrum is
coincident with the absorption spectrum of the free base
monomer but not with that of the copper(II)-free base dimer
(Figures 2 and 3). The other two dimers, Cu-Bz-H2 and Cu-
Np-H2, also exhibit the same excitation spectrum (not shown).
This indicates that energy transfer via porphyrin singlet mani-
folds does not take place from the copper moiety to the free
base in the heterodimers and only the excitation of the free base
part yields the free base fluorescence. This situation of copper-
(II)-free base dimers is quite different from that of zinc(II)-
free base dimers which undergo efficient singlet energy
transfer.8-12, 14,15
Since the excitation to the copper moiety does not yield S1
fluorescence of the free base, we compare the relative fluores-
cence intensity for the selective excitation of the free base
moiety. The relative fluorescence intensities of the heterodimers
to the monomer are 0.097, 0.080, and 0.035, for Cu-Pn-H2,
Cu-Np-H2, and Cu-Bz-H2, respectively. It should be noted
that the fluorescence intensity of Cu-Pn-H2 is the largest while
this dimer has the shortest center-to-center distance between
the two halves.52
Figure 5 shows transient traces of fluorescence signals of H2-
Pn-H2 and Cu-Pn-H2 dimers. In both dimers, fluorescence
decays obeying a first-order kinetics. The fluorescence lifetime
of Cu-Pn-H2 was determined as 1.1 ns whereas that of H2-
Pn-H2 is 12.4 ns. Table 2 presents fluorescence lifetimes and
relative fluorescence yields of the copper(II)-free base dimers
as well as those of the free base homodimers. Here, the relative
fluorescence yield is defined as the fluorescence intensity per
the unit concentration of the free base moiety. As listed in Table
2, the variation of the lifetimes is very similar to that of the
relative quantum yield: In the heterodimers, fluorescence
Figure 3. Absorption (s) and emission (---) spectra of (a) H2-Pn-
H2, (b) H2-Np-H2, (c) H2-Bz-H2, and (d) the monomer free base
TPPH2 in toluene at room temperature. Note that the intensities in (d)
are magnified twice.
modimers are approximately twice as large as that of the free
base monomer. It is noted that, in some of reported gable-type
free base porphyrin dimers, the absorption Q-bands are red-
shifted by 3-5 nm25,26 relative to the corresponding monomer.
However, such a significant red shift is not observed in the
present dimers, and the properties of the absorption Q-band can
be described approximately as a superposition of the constitu-
ents.7 On the other hand, the absorption B-bands of the free
base homodimers are not the superposition of those of the
monomers but very are similar to the corresponding copper-
(II)-free base heterodimers. The peak positions of the absorp-
tion bands are summarized in Table 1.
It is noted that the lowest excited singlet state of the free
base is lower than the corresponding excited state of the copper
porphyrin (see Figure 2d). At around 590 nm, copper porphyrin
exhibits almost no absorption whereas the free base has
appreciable intensity. Thus, we can use this wavelength to excite
the free base part in the dimer selectively.
3.2. Fluorescence Spectra, Quantum Yields, and Lifetimes.
The free base monomer exhibits intense fluorescence from the
S1 state, whereas the copper porphyrin does not fluoresce due
to rapid intersystem crossing.38,42-51 In the copper(II)-free base
dimers, fluorescence only from the free base moiety is observed
and the spectral shape is the same as that of the monomer
independent of the excitation wavelength. Fluorescence spectra

4860 J. Phys. Chem. A, Vol. 104, No. 21, 2000
Toyama et al.
nbb
TABLE 2: Relative Fluorescence Intensities, Lifetimes, and Evaluated ISC Rates in Toluene
If
τf/ns
kISC/107 s-1
rc-ca/10-10
m
TPPH2
“1”c
12.5((0.5)
12.4((0.5)
12.4((0.5)
12.4((0.5)
1.1((0.05)
0.89((0.04)
0.43((0.04)
10.3((0.5)
6.8((0.6)
H2-Pn-H2
H2-Np-H2
H2-Bz-H2
Cu-Pn-H2
Cu-Np-H2
Cu-Bz-H2
Pd-Bz-H2
0.96 ((0.05)
1.00 ((0.05)
1.02((0.05)
0.097((0.01)
0.080((0.01)
0.035((0.005)
0.85((0.05)
7.1((0.6)
6.8 ((0.6)
6.6 ((0.6)
0.88((0.04) × 102
1.1((0.05) × 102
2.3((0.2) × 102
8.5((0.8)
10.9
13.2
11.0
5
4
2
a The center-to-center distance between the two moieties of the dimer, estimated by MM2 calculation.52 b Number of bonds of the linkage.
c Fluorescence quantum yield of TPPH2 is 0.12.55,56
lifetimes is inconsistent with pathways involving back energy
transfer from the free base S1 to the copper excited states via
thermal processes: τ(S1,Cu-Pn-H2), 1.1 ns at 300 K, 2.3 ns
at 77 K; τ(S1,TPPH2), 12.5 ns at 300 K; 15.5 ns at 77 K. On
the basis of the above observations, we conclude that the
decreases in fluorescence yields and lifetimes are ascribed to
enhancement of ISC in the free base half. In the following, we
evaluate ISC rates of the free base moiety and discuss the origin
of the spacer dependence.
4.1.1. Estimation of ISC Rates in the Copper(II)-Free Base
Dimers. In the dimers, the fluorescence lifetime (τf) and yield
(φf) are given by
1/τf ) kr + kic + kisc
(1)
(2)
φf ) kr/(kr + kic + kisc)
where kr, kic, and kisc are the rate constants for radiative decay,
internal conversion, and intersystem crossing of the free base
moiety, respectively. As has been described in section 3.2, we
do not take into account absorption processes of the copper
moiety in the dimer. The radiative decay rate constants in the
free base moiety can be derived from the ratio of the relative
quantum yield to the lifetime by using eqs 1 and 2, and are
found to be identical to that of the monomer (kr ) 9.5 × 106
s-1) within experimental errors. On the other hand, it is difficult
to determine the two nonradiative rate constants, kic and kisc, at
the same time from the fluorescence measurements. Here, we
assume that the internal conversion rate, kic, in the dimers is
the same as that of the monomer. This is because neither the
formation of the dimer nor the insertion of the copper(II) into
the other half is expected to affect the internal conversion rate
as discussed previously.7 In the case of the monomer we can
evaluate kic since the ISC rate in the monomer has been known.
In the monomer TPPH2, the intersystem crossing yield, φisc, was
reported as ∼0.85 (0.82-0.88)53,54 whereas the fluorescence
quantum yield in toluene was determined as 0.12.55,56 By using
these values and the monomer lifetime of 12.5 ns, a value of
2.3 × 106 s-1 is obtained for kic in the monomer.
Figure 5. Fluorescence decay profiles of (a) H2-Pn-H2 and (b) Cu-
Pn-H2 in toluene at room temperature. Emission data are given by
dots, together with the best fitting decay curves. The response for the
excitation laser is shown by the solid line.
lifetimes decrease in the same order as that of the relative
quantum yield, i.e., Cu-Pn-H2 > Cu-Np-H2 > Cu-Bz-
H2, whereas those of the free base homodimers are identical to
that of the monomer. Again, the lifetime of the Cu-Pn-H2 is
the longest while the center-to-center distance is the shortest.
For a comparison, the fluorescence quantum yield and lifetime
of Pd(II)-Bz-H2 are also presented in Table 2.
4. Discussion
4.1. Analysis of Fluorescence Yields and Lifetimes. In the
phenanthrene-linked copper(II)-free base dimer, Cu-Pn-H2,
the relative fluorescence yield and lifetime of the free base half
are remarkably decreased compared to the monomer, as has been
observed in Cu-Bz-H2 and Cu-Np-H2. It should be noted
that the extent of decrease in Cu-Pn-H2 is smaller than that
of the other two copper(II)-free base dimers, despite the fact
that the center-to-center distance is the shortest. On the other
hand, the quantum yields and lifetimes of the three free base
homodimers are essentially the same as those of the monomer.
As discussed in ref 7, the decreases in the copper(II)-free base
heterodimers can be ascribed to enhanced ISC from the S1 to
T1 state in the free base moiety but not to other intramolecular
processes.
From the fluorescence lifetime and quantum yields in the
dimers, kisc can be estimated by using the values of kr and kic
and eq 1. Results are summarized in Table 2, along with the
center-to-center distance between the two halves and number
of bonds on the pathway to link the two porphyrins. The ISC
rates in the hybrid dimers are increased by 10-35 times
compared to the monomer depending upon the spacer molecule.
4.1.2. Spacer Dependence of the ISC Rates. As has been
discussed, it is clear that the presence of the copper(II) ion in
the other half affects the ISC rates of the free base half. In
addition, this effect is not due to the so-called “heavy atom
effect”, since such significant enhancement has not been
observed in various zinc(II)-free base dimers.8,9b,25 This argu-
ment is supported by the fact that Pd-Bz-H2 shows only a
Support for this conclusion came from the transient absorption
measurements, which did not show any trace of other intermedi-
ates.7 In addition, temperature dependence of fluorescence

ISC in Copper(II) Porphyrin-Free Base Porphyrin Dimers
J. Phys. Chem. A, Vol. 104, No. 21, 2000 4861
section, we derive wave functions of the excited states of the
copper(II) porphyrin-free base porphyrin dimers where the free
base part is excited and the copper half is in the ground state.
Here we only consider excitations in the free base π-system
but do not involve (π,π*) excitations in the copper moiety, since
we can conclude that the ISC does not compete against any
other intramolecular process and is independent of (π,π*)
transitions in the copper half.
In free base monomer and typical diamagnetic metallopor-
phyrins, the characteristic nature of the two lowest excited
singlet and triplet states can be well explained by four transitions
between two HOMOs and two LUMOs in the porphyrin
π-system.57,58 In the free base monomer with D2h symmetry,
two HOMOs are b1u and au orbital, whereas two LUMOs are
b2g and b3g.42,59 When the dimer system has an unpaired
d-electron in the other half, the system can be described as linear
combinations of the products of the d-orbital and free base
π-orbitals. This is analogous to the treatment of the copper
porphyrin monomer38,45 although the copper d-electron in our
case is located rather away from the free base π-electron system.
The ground state of the system consisting of the four free base
π-orbitals and a copper d-orbital is doublet, and the wave
functions for Sz ) 1/2 and -1/2 substates are given by
Figure 6. Semilogarithmic plots of kisc versus (a) the center-to-center
distance, rc-c, which is calculated on MM2 and (b) number of bonds.
The diameter of the circles in the figure indicates the size of
experimental errors.
slightly shorter lifetime of the free base moiety (see Table 2)
although Pd has a much larger atomic number (Z ) 46)
compared to zinc (Z ) 30) and copper (Z ) 29). As proposed
previously, the exchange interaction between the π-electrons
in the free base half and an unpaired electron in the copper(II)
is very likely to gain the ISC rate in the free base moiety. Before
discussing the mechanism in detail (see next section), we first
consider what property of the dimers determines the spacer
dependence of the ISC rates.
Figure 6 shows semilogarithmic plots of kisc (a) vs center-
to-center distance of the dimer, rc-c, and (b) vs number of bonds
combining the two porphyrins with the shortest pathway. In
Figure 6a, there is no clear relation between kisc and rc-c, whereas
Figure 6b shows a good linear relation between ln kisc and
number of bonds. This implies that the interaction through the
linkage is likely to be dominant in determining the ISC rate
rather than the spatial interaction between the two moieties.
Figure 6b leads to the suggestion that the ISC rate can be written
as
|GR ) |dauaub1ub1u|
|Gâ ) |dauaub1ub1u|
(4)
where d stands for the copper unpaired electron and the overbar
(s) stands for â spin.
In monomer porphyrin π-system, the two lowest excited
singlet states, i.e., Q and B states, are approximately described
as a 1:1 mixture of two configurations due to the characteristic
nature of the porphyrin π-system.42,55 For the excited states of
the dimer where the free base half is in the excited singlet states
and the copper(II) is the ground state, the following four
configurations can be given as the Sz ) 1/2 states:
kisc exp {-â(n - 1)}
(3)
where n - 1 is the number of bonds and â corresponds to the
1
1
slope of the line in the Figure 6b, which is found to be 0.23
x
|QxR ) {|d‚ (b1ub2g) - |d‚ (aub3g) }/ 2
Å-1
.
1
1
4.2. Mechanism of Enhanced Intersystem Crossing. In
copper(II) porphyrin monomer, exchange interaction between
an unpaired electron and porphyrin π-electrons splits porphyrin
triplets into so-called “trip-doublets” (2T) and “trip-quartets”
x
|QyR ) {|d‚ (b1ub3g) + |d‚ (aub2g) }/ 2
1
1
x
|BxR ) {|d‚ (b1ub2g) + |d‚ (aub3g) }/ 2
1
1
(4T) with an energy gap of several hundred cm-1 38,45-49
whereas
,
x
|ByR ) {|d‚ (b1ub3g) - |d‚ (aub2g) }/ 2
(5)
porphyrin singlet states become “sing-doublet” (2S) states as
the whole system.45 Nonzero exchange interaction gives rise to
where
2
2
mixing between the S1 and T1 states, thus resulting in very
1
2
2
fast ISC from the S1 to T1 within a picosecond time scale
(<8 ps).43,44 This value is more than 3 orders of magnitude
quicker than that observed in diamagnetic zinc(II) porphyrins
(∼2 ns)55 wherein ISC rate is governed mainly by SO coupling.
Here it is noted that zinc and copper have much the same atomic
numbers (30 vs 29).
x
|d‚ (b1ub2g) ) {|dauaub1ub2g| - |dauaub1ub2g|}/ 2
1
x
|d‚ (aub3g) ) {|daub3gb1ub1u| - |dau b3gb1ub1u|}/ 2
1
x
|d‚ (b1ub3g) ) {|dauaub1ub3g| - |dauau b1ub3g|}/ 2
1
x
In the copper(II)-free base dimers, interaction between
π-electrons in the free base and an electron in the copper dσ
orbital is considered to be much weaker than that in the
monomer copper porphyrin. However, such weak interaction
is expected to interpret the increase of ISC rates. The aim of
this section is to derive an expression for the ISC rate in terms
of the interaction between the copper half and the free base
half and present a mechanism which is consistent with the
observed spacer dependence.
|d‚ (aub2g) ) {|daub2gb1ub1u| - |daub2gb1u b1u|}/ 2 (6)
The two states |QxR and |QyR correspond to the first excited
singlet states of the free base monomer, Qx and Qy states,
respectively, and they have almost equal energy and transition
moment but different symmetry. Similarly, |BxR and |ByR
correspond to the second singlet excited states. For the sym-
metrical reason, |QxR can mix with |BxR but not with |QyR
and |ByR . Since the wave functions with y-symmetry (B2u
representation) compose essentially the same energy matrix as
that of x-symmetry (B3u representation), hereafter we will show
4.2.1. WaVe Functions of the Excited States of the Free Base
Half in Copper(II)-Free Base Porphyrin Dimers. In the present

4862 J. Phys. Chem. A, Vol. 104, No. 21, 2000
Toyama et al.
only x-symmetry wave functions and matrix elements. The
corresponding Sz ) -1/2 states for x-symmetry are
Since the initial states of ISC are ψQ states, e.g., |QxR , and
i
the final states are ψT states, e.g., |Tbx(+1)â and |Tbx(0)R ,
j
the ISC rate of the free base moiety is written as
1
1
x
|Qxâ ) {|d‚ (b1ub2g) - |d‚ (aub3g) }/ 2
2
kisc
| ψ |V|ψ | Fj
(13)
∑∑
Tj
Qi
1
1
i
j
x
|Bxâ ) {|d‚ (b1ub2g) + |d‚ (aub3g) }/ 2
(7)
Nonzero exchange interaction between the π-electrons in the
free base and a copper unpaired electron gives rise to the
coupling matrix elements between ψQ and ψT states. For
example, the |QxR state has coupling with |Tbx(+1)â and
|Tbx(0)R while the |Qxâ state has coupling with |Tbx(0)â and
|Tbx(-1)R . The coupling matrix elements which lead to the
ISC transitions are given by
In contrast to the (monomer) porphyrin singlet states, the two
lowest triplet states can be described rather as a single
configuration.38,42 When the free base moiety is in a triplet, the
whole system of the dimer is a coupled triplet-doublet system
and Sz numbers are 3/2, 1/2, -1/2, and -3/2. Thus, in the dimer,
the wave functions for Sz ) 3/2 substates of the dimer can be
written as
i
j
QxR|V|Tbx(0)R ) - Qxâ|V|Tbx(0)â )
Sz ) 3/2
|Tax(+1)R ) |d‚3(aub3g)+1 ) |daub3gb1ub1u|
1
-
(Kb1u - Kb2g)
x
2 2
|Tbx(+1)R ) |d‚3(b1ub2g)+1 ) |dauaub1ub2g|
(8)
QxR|V|Tbx(+1)â ) - Qxâ|V|Tbx(-1)R )
where Ta and Tb stand for two electronic excited triplet states,
i.e., (T1, T2 ) states of the free base moiety and (+1) stands for
the spin-quantum number of the triplet state. Whether Ta or Tb
is the lower depends on the peripheral substituents of the
macrocycle.42,38,55 Here, we assume that the lowest electronic
1
2
(Kb1u - Kb2g) (14)
where Kb1u and Kb2g are the exchange interaction between the
copper unpaired electron and free base π -electron in b1u and
b2g orbitals, respectively.
3
excited state is Tb, i.e., the state with (b1ub2g) configuration,
3
since the corresponding monomer has (b1ub2g) configuration
as the lowest.42,55
Kb1u ) (b1ud|db1u)
Kb2g ) (b2gd|db2g)
The wave functions for Sz ) -3/2, 1/2, and -1/2 substates
of Tbx state are
(15)
The whole energy matrix is presented in Table 3. Substituting
eq 13 into eq 14, we obtain the following expression for the
ISC rate:
Sz ) -3/2
|Tbx(-1)â ) |d‚3(b1ub2g)-1 ) |dauau b1u b2g|
(9)
Sz ) 1/2
|Tbx(+1)â ) |d‚3(b1ub2g)+1 ) |dauau b1u b2g|
|Tbx(0)R ) |d‚3(b1ub2g)0 ) {|dauaub1ub2g| +
2
kisc c2(Kb1u - Kb2g
)
(16)
As a result, the ISC rate is proportional to the square of a
difference of the two exchange integrals between the unpaired
electron in the copper(II) and π-electrons in the free base HOMO
and LUMO.
x
|dauau b1ub2g|}/ 2 (10)
Sz ) -1/2
4.2.3. Spacer Dependence of Exchange Interaction. As has
been discussed in section 4.1.2, the spacer dependence of the
ISC rates shows a good correlation with the number of bonds
combining the two moieties but not with the center-to-center
distance of the dimer. According to the mechanism above, the
ISC rate is given as a function of the exchange integrals between
the electrons in the free base π-system and a copper d orbital.
Therefore, the sizes of exchange integrals are expected to depend
on the number of bonds of the spacer. Such an interaction may
be called a through-bond interaction,61-63 in analogy to what
has been found in long-range intramolecular electron and energy
transfer processes.64-66 Here, we further consider the ISC rate
in eq 16 in terms of the spacer dependence.
|Tbx(0)â ) |d‚3(b1ub2g)0 ) {|dauaub1ub2g| +
x
|dauau b1ub2g|}/ 2
|Tbx(-1)R ) |d‚3(b1ub2g)-1 ) |dauau b1u b2g| (11)
If the exchange coupling between the free base triplet and
the copper doublet electrons is strong, the wave functions in
the coupled triplet-doublet system would mix with each other
to a large extent and then split into quartet and doublet states.
However, if the coupling is weak, mixing is small and wave
functions in eqs 8-11 will remain as a good basis set.34 In the
next section we will consider matrix elements of this triplet-
doublet system and derive an expression for the ISC rate.
4.2.2. Matrix Elements Leading to Enhanced ISC in the
Copper(II)-Free Base Porphyrin Dimer. According to the well-
known theory of radiationless transitions,61 transition probability
per unit time is given by
Based on the through-bond model originally presented by
McConnell,61 exchange coupling between the two terminal
moieties which are linked with n pieces of bridges can be given
in a modified form62
(n-1)
t
∆
Ki ) Ci
(17)
4π2
h
( )
2
i
wfi )
| ψf|V|ψi | F
(12)
where ∆i is an energy difference between a virtual orbital of
the bridge and a porphyrin π-orbital i, and t is the coupling
term between the bridged orbitals. Ci is a function including
∆i, the coupling term between the edge of the bridge and the
where V is a perturbation term of the Hamiltonian responsible
for the transition from the initial state ψi, to the final state ψf,
F is the density of the final states, and h is Planck’s constant.

ISC in Copper(II) Porphyrin-Free Base Porphyrin Dimers
J. Phys. Chem. A, Vol. 104, No. 21, 2000 4863
TABLE 3: Energy Matrices of Excited States in a System Porphyrin Singlet/Triplet Coupled with Cooper Doublet
(x-symmetry)
Sz ) 3/2
|Tax(+1)R
J1 - K1 - (Kau + Kb3g
|Tbx(+1)R
)
0
J2 - K2 - (Kb1u + Kb2g
)
Sz ) 1/2
|Tbx(+1)â
(Kb1u - Kb2g)/2
|BxR
|QxR
|Tax(+1)â
|Tax(0)R
|Tbx(0)R
-(Kb1u - Kb2g)/2
J - K + 2K′ + 2K′′ -
(A + B)/2
δꢀ
(Kau - Kb3g)/2
x
x
x
-(Kau - Kb3g)/2
2
2
J - K + 2K′ - 2K′′ -
(A + B)/2
-(Kau - Kb3g)/2 (Kb1u - Kb2g)/2
J1 - K1
x
(Kau - Kb3g)/2
2
-(Kb1u - Kb2g)/2
2
0
0
x
-(Kau + Kb3g)/
2
J2 - K2
0
x
-(Kb1u + Kb2g)/
2
J1 - K1 - (Kau + Kb3g)/2
0
J2 - K2 - (Kb1u + Kb2g)/2
Sz ) -1/2
|Bxâ
|Qxâ
|Tax(0)â
|Tbx(0)â
|Tax(-1)R
|Tbx(-1)R
J - K + 2K′ + 2K′′ -
(A + B)/2
δꢀ
-(Kau - Kb3g)/2
-(Kb1u - Kb2g)/2
-(Kb1u - Kb2g)/2
0
x
x
(Kau - Kb3g)/2
2
(Kb1u - Kb2g)/2
2
2
J - K + 2K′ - 2K′′ -
(A + B)/2
(Kau - Kb3g)/2
x
x
-(Kau - Kb3g)/2
2
(Kb1u - Kb2g)/2
J1 - K1 - (Kau + Kb3g)/2
0
x
-(Kau + Kb3g)/
2
J2 - K2 - (Kb1u + Kb2g)/2
0
x
-(Kb1u + Kb2g)/
2
J1 - K1
0
J2 - K2
Sz ) -3/2
|Tax(-1)â
J1 - K1 - (Kau + Kb3g
|Tbx(-1)â
)
0
J2 - K2 - (Kb1u + Kb2g
)
In these matrices
J1 ) 2{(b1ub1u|auau) + (b1ub1u|b3gb3g) + (b1ub1u|dd)} + (b1ub1u|b1ub1u) + (auau|b3gb3g) + (auau|dd) + (b3gb3g|dd)
J2 ) 2{(b1ub1u|auau) + (auau|b2gb2g) + (auau|dd)} + (auau|auau) + (b1ub1u|b2gb2g) + (b1ub1u|dd) + (b2gb2g|dd)
J ) (J1 + J2)/2
Kb1u ) (b1ud|db1u), Kb2g ) (b2gd|db2g), Kau ) (aud|dau), Kb3g ) (b3gd|db3g)
K1 ) (b1uau|aub1u) + (b1ub3g|b3gb1u) + (aub3g|b3gau) + Kb1u
K2 ) (b1uau|aub1u) + (aub2g|b2gau) + (b1ub2g|b2gb1u) + Kau
K ) (K1 + K2)/2
K′ ) {(aub3g|b3gau) + (b1ub2g|b2gb1u)}/2
K′′ ) (aub3g|b2gb1u)
A ) (Kau + Kb1u)/2
B ) (Kb3g + Kb2g)/2
δꢀ ) (J1 - J2) - (K1 - K2) + 2{(aub3g|b3gau) - (b1ub2g|b2gb1u)} + {Kau - Kb1u + Kb3g - Kb2g}/2
where
(ab|cd) ) ∫φ/a(1) φc/(2)e2/r12φb(1) φd(2) dr1 dr2
where
porphyrin π-orbital i, and exchange interaction between dσ and
π-electrons in the copper porphyrin. Therefore the ISC rate in
eq 16 is rewritten as
â ) -2 ln(t/∆)
(21)
2
(n-1)
t
t
In the above, â depends on the interaction between the bridge
orbitals and the energy difference between the porphyrin orbital
and the virtual bridge orbital. It is noted that â does not depend
on the interaction between the copper electron and porphyrin
kisc ) c′ Cb1u
(n-1) - Cb2g
(18)
(
∆
)
(
)
[
]
∆
b1u
b2g
Since the energy of the virtual bridge orbital is expected to
be much higher than the energies of the porphyrin LUMOs and
HOMOs, we can assume ∆1bu ≈ ∆2bg ≈ ∆. Thus the ISC rate
is given by
π-electrons. From Figure 6b, we have obtained â ) 0.23 Å-1
,
which agrees with values observed for electron and energy
transfer in similar series of aromatically bridged porphyrin
dimers.10a This agreement supports the contention that the
through-bond exchange interaction plays a dominant role in
determining the ISC rate.
4.2.4. Other Factors. Although the ISC rates show an
exponential dependence on the number of bonds, we might have
to consider effects of π-electrons in the arene spacers. Recently,
it was reported that the spacer arene mediates singlet-singlet
2(n-1)
t
2
kisc ) c′(Cb1u - Cb2g
)
(19)
(
)
∆
Accordingly we obtain the expression
kisc ) C exp{-â(n - 1)}
(20)

4864 J. Phys. Chem. A, Vol. 104, No. 21, 2000
Toyama et al.
(6) Kalyanasundram, K. In Photochemistry of Polypyridine and Por-
phyrin Conplexes; Academic Press: London, 1992; Chapters 1 and 2 and
references therein.
energy transfer in a series of zinc(II) porphyrin-free base
porphyrin dimers bridged by a benzene, a naphthalene, and an
anthracene.9 In this series of head-to-tail type dimers, not only
the distance but also the number of bonds between the donor
and acceptor is the same. The observed energy transfer rates in
the benzene- and naphthalene-bridged dimers are much the same
but that for the anthracene-bridged dimer is approximately twice
as large as the other two. They suggested mediation of energy
transfer by the bridging chromophore, possibly through (π,π*)
excited states of the aromatic spacer.
(7) Asano-Someda, M.; Kaizu, Y. Inorg. Chem. 1999, 38, 2303.
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D. F.; Donohoe, R. J. J. Am. Chem.Soc. 1996, 118, 11181. (b) Strachan,
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In our case, the lowest singlet excitation energies of benzene,
naphthalene, and phenanthrene are 38 400, 32 200, and 28 900
cm-1, respectively.67 Since phenanthrene has the lowest energy
among the three, the largest influence by (π,π*) excited states
of the spacer would be expected in the phenanthrene-linked
dimer. However, the observed ISC rate is the slowest in this
dimer and the semilogarithmic plot of ISC rates in Figure 6b
shows a quite good linear relation. This implies that the effect
from the (π,π*) excited states of the spacer molecule is not so
large as that which could be observed appreciably. This situation
is similar to what was reported in a series of the polyyne-bridged
dimers by Osuka et al.10b In their dimers, the energy transfer
rate decreases exponentially with an increase of the length of
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5. Conclusions
The observed spacer dependence not only leads to the
conclusion that the through-bond interaction is likely to be
dominant rather than that of through-space in governing the ISC
rate, but also provides support for the contention that exchange
interaction between a metal unpaired electron in one half and
π-electrons in the other half enhances ISC in the free base half.
These results show that a remote unpaired electron in the metal
ion can play an important role in relaxation processes of the
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Acknowledgment. The authors are grateful to Professors
K. Eguchi and K. Kakinuma for the measurements of FAB
1
MASS and 500 MHz H NMR spectra and thank Professor
Emeritus K. Yamamoto for the use of his photoreaction
apparatus in Tokyo Institute of Technology. This work was
partially supported by a Grant-in-Aid for Scientific Research
from the Ministry of Education, Science, Culture and Sports
Japan (No. 11740318, 11694061).
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Supporting Information Available: 1H NMR data for the
compounds obtained in the synthetic procedures of H2-Pn-
H2 and H2-Np-H2. This information is available free of charge
via the Internet at http://pubs.acs.org.
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