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Light-Induced Electron Spin Polarization in the Ground State of Water-Soluble
Copper Porphyrins
VLADIMIR ROZENSHTEIN,a ALEXANDER BERG,a HAIM LEVANON,a,* UWE KRUEGER,b DIETMAR STEHLIK,b
YURI KANDRASHKIN,c AND ART VAN DER ESTc
a
Department of Physical Chemistry and the Farkas Center for Light-Induced Processes, The Hebrew University of Jerusalem,
Jerusalem 91904, Israel
b
Fachbereich Physik, Freie Universität Berlin, D-14195 Berlin, Germany
c
Department of Chemistry, Brock University, St. Catharines, Ontario, Canada L2S 3A1
(Received 29 June 2003 and in revised form 9 October 2003)
Abstract. Light-induced spin-polarized transient EPR spectra are reported for
several water-soluble copper porphyrins. The spectra are assigned to the doublet
ground state, with emissive spin polarization resulting from photoexcitation and
subsequent electronic relaxation. In contrast to other systems for which polarization
of a doublet ground state has been observed, the exchange interactions in the copper
porphyrins are strong and the geometry is fixed. It is proposed that intersystem
crossing from the photoexcited trip-doublet to the trip-quartet state can lead to net
polarization of the spin system and that this polarization is maintained during
electronic decay, possibly via charge-transfer and exciplex states. The intensity of
the observed spin polarization is essentially independent of the molecular orientation
in the external field, but is strongly dependent on the nature of the charged peripheral
groups. Possible reasons for this behavior are discussed.
INTRODUCTION
Many light-induced processes generate electron spin
polarization (ESP) due to spin selectivity associated
with electronic transitions and/or electron transfer (ET).
Therefore, ESP spectra measured by time-resolved EPR
(TREPR) spectroscopy can provide information on the
excited-state dynamics and reaction pathways even if
some of the states involved are very short-lived and
cannot be observed directly. Most ESP studies deal with
photoexcited systems that are diamagnetic in their
ground states. In such cases, light excitation produces a
two-spin system that can be described in a singlet and
triplet basis set. The spin polarization of such systems
and the mechanisms leading to the observed polarization are known in detail.1 A number of three-spin systems have also been studied. For example, triplet
quenching by stable radicals in solution leads to radical
polarization, due to quartet–doublet mixing during radical–triplet encounters combined with spin-sorting and
spin-selective relaxation. In these systems, the radical–
Israel Journal of Chemistry
triplet pair mechanism (RTPM)2–6 and the ESP transfer
(ESPT) mechanism7 require diffusion of the triplets and
radicals. In the latter case, the net polarization of the
radical reflects the initial polarization of the triplet. Because of the rapid molecular motion and short lifetimes,
the excited quartet and doublet states usually cannot be
observed.
Recently, a number of complexes have been studied
in which a nitroxide is tethered or ligated to porphyrins,8–10 phthalocyanine,10–12 anthracene,13,14 thioxanthonedioxide,15 quinones,16 or fullerenes and their derivatives.17–19 ESP in electrostatically bound porphyrin
heterodimers containing paramagnetic copper has also
been reported.20 In all of these linked systems, the
RTPM and ESPT mechanisms do not account for the
observed spin-polarized EPR spectra of the excited and
ground states. Several mechanisms have been invoked
to explain the observed ESP but, as in solution, they all
*Author to whom correspondence should be addressed. E-mail:
[email protected]
Vol. 43
2003
pp. 373–381
374
involve spin-selective relaxation and doublet–quartet
mixing.10,12,14-19 It was shown that these processes depend strongly on the exchange interaction (J) and are
most effective in weakly coupled three-spin systems,
i.e., when J is less than or comparable to the Zeeman
energy.21
We report here on spin-polarized TREPR spectra of
three water-soluble copper porphyrin (CuP) monomers.
In terms of their spin states, the paramagnetic metalloporphyrins are similar to the tethered radical–chromophore complexes and porphyrin dimers. However,
the exchange interaction in these CuPs is much larger
than the Zeeman energy. Because of their fixed geometry, level crossings in the weak coupling regime, which
are important in solution, are not accessible here. Thus,
CuPs complement the weakly coupled systems, allowing the effect of strong exchange coupling on the generation of net spin polarization to be studied.
We propose that the net polarization is generated
during the transition from the trip-doublet to the tripquartet and is subsequently transferred to the ground
state, possibly via intermediate quenching state(s). Several authors have suggested that either a charge transfer
(CT) state, in which a π-electron of the porphyrin ring is
transferred to the metal,22–26 or (d–d*) excitation of the
Cu2+ 24–27 affects the excited-state lifetimes of CuPs. The
fact that the ESP amplitude depends strongly on the
nature of the charged side groups of the porphyrin implies that the relaxation pathway of the excited states is
sensitive to electrostatic effects, as might be expected
for states with CT character. However, the involvement
of such states remains an open question.
MATERIALS AND METHODS
Solutions of copper meso-tetrakis[4-trimethyl-anilinium]porphyrin (CuTTAP), copper meso-tetrakis(p-carboxylphenyl)porphyrin (CuTCPP), and copper meso-tetrakis(psulfonatophenyl) porphyrin (CuTSPP) were prepared
by dissolving CuTTAPCl4, Na4CuTCPP, and Na4CuTSPP
in DMSO/H2O, MeOH, and DMSO/Glycerol to give a
final concentration of 10–4–10–3 M. The samples were
then degassed by repeated freeze-pump-thaw cycles and
sealed under vacuum. TREPR measurements and data
acquisition are described elsewhere.28 The experiments
were carried out in the solid matrices at T = 10–30 K using
photoexcitation at 542 nm. Spin-polarized spectra were
extracted from the complete time/field data sets by integrating the signal intensity in a given time window. Background signals were removed by subtracting the spectrum
in a time window immediately before the laser flash.
Continuous wave (CW) steady-state EPR spectra of the
ground state were obtained using the same set-up but with
field modulation detection and without light excitation.
Israel Journal of Chemistry
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Fig. 1. Structures of the three ionic copper porphyrins: copper
meso-tetrakis[4-trimethylanilinium]porphyrin (CuTTAP),
copper meso-tetrakis(p-carboxylphenyl) porphyrin (CuTCPP),
and copper meso-tetrakis(p-sulfonatophenyl)porphyrin
(CuTSPP).
EXPERIMENTAL RESULTS
The structures of the water-soluble CuPs, CuTTAP, CuTCPP,
and CuTSPP are shown in Fig. 1. Because the charge of the
side groups is localized on the peripheral substituents, the
CuPs can be thought of as consisting of outer ion-pairs made
up of the peripheral groups and their counterions Na+4(CuP)4or (CuP)4+Cl–4 and an inner ion-pair comprised of the central
metal and the porphyrin ring (Cu2+P2–) ≡ (CuP).
Figure 2 shows the spin-polarized TREPR spectra of the
three CuPs in frozen solutions at cryogenic temperatures. All
three porphyrins exhibit purely emissive spectra with different
amplitudes, but with the same spectral width and line shape.
The absolute magnitude of the polarization is difficult to quantify from transient EPR experiments. With the direct detection
technique, the Boltzmann spectrum cannot be detected. On the
other hand, the TREPR spectra of the copper porphyrins are
considerably weaker than the corresponding spectra of the
triplet state of diamagnetic porphyrins measured under similar
conditions.
In Fig. 3 (top), the steady-state EPR spectrum of the
CuTCPP ground state measured using the standard field
modulation technique (100 kHz) is compared with a simulation. In Fig. 3 (bottom), the dashed line is the experimental
spin-polarized TREPR spectrum measured using direct detection, and the solid line has been obtained by integrating and
inverting the experimental ground-state spectrum measured
using field modulation, shown at the top of Fig. 3. As can be
375
Fig. 2. X-Band spin-polarized TREPR spectra of photoexcited CuTTAP (25 K, DMSO), CuTCPP (25 K, MeOH), and CuTSPP
(10 K, DMSO/H2O). The spectra are the integrated signal intensity in a time window: 0–6 µs (CuTTAP), 0–4 µs (CuTCPP) and
0–2 µs (CuTSPP). Excitation wavelength: 542 nm; pulse energy: 2.5 mJ; sample concentration: 1 mM. Note that the spin
polarization for CuTSPP is very weak and no signal was observable at temperatures above 10 K.
Fig. 3. Comparison of the light-induced and steady-state EPR spectra of CuTCPP. Top: Steady-state spectrum measured using
100 kHz field modulation (upper trace) and a simulation (lower trace) using the parameters: g(Cu) = 2.187, g⊥ ((Cu)) = 2.045,
A (Cu) = 188.8 × 10–4 cm–1, A⊥(Cu) = 30.8 × 10–4 cm–1, A (N) = 14.4 × 10–4 cm–1, A⊥(N) = 13.6 × 10–4 cm–1; Euler angles
between hyperfine and g tensors: (A (Cu) and g (Cu)) = 0º, 7º, 0º; (A (N) and g (Cu)) = 0º, 0º, 0º ; Gaussian line width of the
individual hyperfine lines: 12 G. Bottom: Spin-polarized TREPR spectrum (dashed curve) and numerically integrated and
inverted steady-state spectrum (solid curve) of CuTCPP. The TREPR spectrum is the same as shown in Fig. 2. The steady-state
spectrum was measured under the same conditions except that the sample concentration was 2 mM.
Rozenshtein et al. / Electron Spin Polarization in Copper Porphyrins
376
seen, the two spectra in the lower part of Fig. 3 are virtually
identical. For the spectrum of the ground state, the EPR spectral width is governed primarily by the hyperfine splitting. For
the trip-doublet and trip-quartet states these couplings are
expected to be three times less than that of the ground state.29
Thus, the good agreement between the steady-state and lightinduced spectra indicates that the ground state becomes spin
polarized as a result of photoexcitation and subsequent decay
of the excited states. Moreover, the spin polarization is emissive and nearly independent of the orientation of the magnetic
field with respect to the magnetic tensor axes. The close
similarity of all spectra in Fig. 2 suggests that any mechanistic
conclusions will be common to all three CuPs studied here.
Figure 4 shows the time dependence of the TREPR signals
of CuTTAP (30 K, DMSO/glycerol), CuTCPP (16 K, MeOH),
and CuTSPP (10 K, DMSO/H2O). The upper two traces
(CuTTAP and CuTCPP) are clearly biexponential and a fit of
Fig. 4. Experimental TREPR kinetic traces (dashed curves) of
CuTTAP (30 K, DMSO/glycerol), CuTCPP (16 K, MeOH)
and CuTSPP (10 K, DMSO/water) and fits of the decays
(solid curves) with the function S(t) = A[a exp(–t/τ1) + (1-a)
exp(–t/τ2)] and the following parameters: τ1 = 5.8 µs; τ2 =
48.0 µs, a = 0.76 (CuTTAP, top trace); τ1 = 890 ns and a = 1.0
(CuTSPP, bottom trace); τ1 = 2.4 µs; τ2, = 47.0 µs, a = 0.63
(CuTCPP, middle trace).
Israel Journal of Chemistry
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the transients (solid lines) gives lifetimes of τ1 = 2.4 µs and
τ2 = 47.0 µs for CuTCPP, and τ1 = 5.8 µs and τ2 = 48.0 µs for
CuTTAP with roughly a 2:1 amplitude ratio for both cases.
For CuTSPP, on the other hand, the signal intensity is weak,
and only a single lifetime of 1 µs can be estimated from the
data. The kinetics were found to be independent of microwave
power (0.08–2.8 mW) and did not exhibit any transient
nutations. Thus, the experiments are carried out at underdamping conditions.30,31
ENERGETICS AND POSSIBLE RELAXATION
PATHWAYS
It is convenient to consider the copper porphyrins as
semi-ionic metal–ligand complexes consisting of Cu2+
(3d9, S = 1/2) ligated to the four pyrrole nitrogens, and
P2– (porphyrin rings with S = 0 or S = 1). In the complex,
the spin-orbital states of the copper and the porphyrin
ring are perturbed by the interaction between them.
However, the strength of the perturbation is small
compared to the exchange interaction between the
valence electrons of the porphyrin, but large compared
to the Zeeman and zero-field interactions. Because of
this, the overall states are often referred to as “singdoublet” (2S), “trip-doublet” (2T), and “trip-quartet” (4T)
with “sing” and “trip” indicating either a singlet or
triplet configuration of the π-electrons of P.32 Doublet or
quartet refers to the overall spin multiplicity. Figure 5
shows a general energy scheme for such systems and the
possible excitation decay pathways. J is the exchange
interaction between the triplet state of the porphyrin ring
and the copper doublet. This exchange interaction removes the spin degeneracy and forms two states, i.e.,
doublet and quartet, with energy separation of 3 J (a few
hundred cm–1 for systems such as CuTPP).29,33,34 Upon
photoexcitation at 542 nm, the sing-doublet state (2S1) is
populated. As shown by Gouterman32 and discussed
recently by Toyama et al.,35 the difference in the
exchange integrals J(dx2–y2,π) – J(dx2–y2,π*) mixes the
trip-doublet states (2T1, 2T2) with the corresponding
sing-doublet. Thus, decay of the sing-doublet to the tripdoublet is both orbitally and spin allowed. A number of
low-lying electronic trip-doublet states exist that could
be populated by this process.24,26,36 In Fig. 5, two of these
states, labeled 2T1 and 2T2, are shown. These states correspond to promotion of an electron from a1u (π) or
a2u(π) orbital, respectively, to the doubly degenerate
eg(π) orbital. They will be further split if the degeneracy
of eg orbital is lifted, e.g., by a molecular distortion or
solvent effects.32 The dotted lines with labels 3T1 and 3T2
show where the triplet states of the porphyrin would lie
in the absence of exchange coupling between the triplet
and doublet spins. In the presence of spin-orbit coupling, intersystem crossing from the trip-doublet states
377
Fig. 5. Energy scheme for the copper porphyrins.
2
S0, 2S1 are the ground- and excited sing-doublet
states; 2T1, 4T1, 2T2, 4T2 are the lowest trip-doublet
and trip-quartet states, which are doubly degenerated and of Eu symmetry; 2(d–d*) is the lowest
excited state of the copper; 2CT1, 2CT2 are low-lying
ligand-to-metal charge transfer states.
to the trip-quartet states (4T1 and 4T2) can occur and may
compete with internal conversion from 2T1 to 2T2.
Quenching of the trip-doublet/trip-quartet states of
CuP may proceed via the d–d* and CT states (Fig. 5). In
terms of orbital energies,37 the copper dx2–y2 orbital is
expected to lie between the porphyrin ring HOMOs
a1u(π) and a2u(π) and LUMO eg(π*) (Fig. 6). Several
electronic transitions are possible: (a) metal-to-metal
(d* ← d), e.g., (dx2–y2 ← dz2); (b) metal-to-ligand
(eg(π*) ← dx2–y2) or (a2u ← dx2–y2); and (c) ligand-to-metal
(dx2–y2 ← eg(π*)). The first two transitions generate
excited states (d–d* and CT states), while the ligand-tometal transition results in the CT ground state. All of
these states are doublets. We will show that two quenching routes with participation of both d–d* and ligand-tometal intermediates are possible.
An important consideration for such quenching is the
possible formation of aggregates either between the
porphyrin and the solvent or between the porphyrin
molecules themselves. It is well known that nitrogenand oxygen-containing compounds, like methanol,
water, and DMSO used here, can act as axial ligands to
the metal porphyrins.26,38 Axial ligation can also occur
Fig. 6. Molecular orbitals of quasi-independent copper and
porphyrin ring moieties. LMCT is the ligand-to-metal charge
transfer.
Rozenshtein et al. / Electron Spin Polarization in Copper Porphyrins
378
via dimerization of CuPs, which is strongly affected by
the nature of the porphyrin and solvent involved.39–44
Self-aggregation has been reported for tetraphenyl porphyrins, including water-soluble derivatives.39,40,44 Nevertheless, because very similar polarization patterns are
obtained for different porphyrins in different solvents,
an influence from aggregation on the electron spin polarization of the copper porphyrins studied here is unlikely, even though such dimers may be formed.45 Moreover, an important consequence of the axial ligation of
copper is the strong effect on the excited-state lifetimes.
For copper porphyrins, the 2T and 4T states typically
decay to the ground state within tens of ps in coordinating solvents,26 while in non-coordinating solvents the
decay is three orders of magnitude slower.46,47 Because a
single axial ligand removes the center of symmetry in
the molecule, it makes the (d* ← d) transitions partially
allowed. Thus, the fast quenching of the copper porphyrin excited states in coordinating solvents is thought to
also involve exciplex states formed between the solvent
and the porphyrin (dx2–y2 ← dz2) excited state (cf.
Fig. 6).26 The energy of the exciplex strongly depends
on the interaction between the Cu2+ and the axial ligand,
which is governed by the ion–ligand distance, and, thus,
can vary over a wide range for different complexes. In
some cases, the energy of the exciplex state may be
close to the energy of the trip/doublet–trip/quartet levels
(cf. Fig. 5).23 However, it is unclear whether such ligation occurs in the water-soluble porphyrins studied here.
An alternative explanation for the very short exciplex
lifetimes is the possible involvement of a CT state (CT2
in Fig. 5).24,26,38,48–50 This proposal is based on the idea
that in coordinating solvents, axial ligation lowers the
energy of the CT level, resulting in accelerated quenching.23,34,51,52 Because of the bonding between the metal
and ligand, the (d–d*) and CT states are mixed53 so that
the quenching state(s) have both (d–d*) and CT character. In other words, the (d–d*) and CT2 states, shown in
Fig. 5, can be considered as an overall quenching state.
Although the quenching is more effective in coordinating solvents, it also occurs in non-coordinating solvents. Since decay via the (d–d*) states is unlikely in
such cases, an additional quenching route, involving
another charge transfer state (CT1 in Fig. 5)23 has been
proposed. The CT1 state is stabilized by interaction with
the highly polar solvent to be energetically below
2
T1/4T1 states.38,54,55 The interactions of the inner CuP
charges with the peripheral ones can also stabilize the
CT state as compared to CuTPP (without peripheral
charges).
To summarize this part, we propose two possible
quenching routes in our systems. The first takes place in
the non-coordinated complexes and involves the CT1
Israel Journal of Chemistry
43
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state, while the second one occurs via (d–d*) and CT2
states (Fig. 5) if singly coordinated complexes are present.
SPIN POLARIZATION MECHANISMS
We propose two mechanisms to account for the
observed ESP. In both cases, the net polarization of the
trip-doublet and trip-quartet states is transferred to the
ground state via the quenching states discussed above.
The polarization of the trip-doublet and trip-quartet is
generated either during decay of the sing-doublet or via
spin-selective intersystem crossing (ISC) between the
trip-doublet and trip-quartet states.
1. In terms of the energy scheme in Fig. 5, the initial
step following photoexcitation is decay of the singdoublet (2S1) to the trip-doublet (2T). The exchange interaction between the partially filled d orbital of the
copper and the π and π* orbitals of the porphyrin, mixes
the trip-doublet and sing-doublet states. Thus, decay to
the trip-doublet is both orbitally and spin allowed,
which accounts for the very short lifetime of the singdoublet.35 This process is not expected to be spin selective because the mixing due the exchange interaction
does not break the symmetry of either state with respect
to the external field. However, we nonetheless include
this as a possibility, based on experimental evidence for
spin polarization of the trip-doublet state in a number of
related systems17,56 and estimations of the relative magnitudes of the decay constants from optical data. If spin
polarization is generated in the 2Ti states, it can be
transferred to the ground sing-doublet state (2S0) via
either the 2CT2 ← d–d * ← 2T2 or 2CT1 ← 2T1 route (cf.
Fig. 5). Both CT states are expected to decay by back
electron transfer (BET) (Fig. 5).
These quenching routes are supported by the available kinetic data for related copper porphyrins. For
CuTPP, the quenching rate constant via the CT states
(kET1 and kET2 ≈ 2 × 1010s–1) is higher by an order of
magnitude than that of 4Ti ← 2Ti ISC, i.e., kDQ ≈ 2 ×
109 s–1.34,57,58 Since recovery of the ground state occurs
within tens of ns,34 it is reasonable to assume that BET1
and BET2 are the rate-limiting processes, with a rate
constant kBET ≈ 108 s–1. At this stage, we cannot distinguish between the two possible quenching routes, i.e.,
via 2CT1 (BET1) or 2CT2 (BET2) (cf. Fig. 5).
The above discussion leads to the following conclusions: (1) kdd, kET1, kET2 >> kDQ; (2) kdd >> kET2; and (3)
kBET1 ≈ kBET2 = kBET. Here, kdd is the rate constant of
(d–d*) exciplex formation. Since ET and BET reactions
occur within a few ns or less (i.e., faster than the spin–
lattice relaxation (SLR) within 2Ti and both 2CT states)
spin alignment should be conserved.59 Retention of spin
orientation in the d–d* process is accompanied by intramolecular energy transfer from the triplet porphyrin
379
moiety to Cu2+, namely 2(2Cu2+,3P2p–*) → 2(2Cu2p+*,1P2–),
where superscript, *, and subscript, p, stand for electronically excited and spin-polarized species, respectively. Unlike its water-soluble analogues, CuTPP does
not display light-induced spin polarization. Thus, although the mechanism above is consistent with the observed kinetics, other possibilities must be considered.
2. A different mechanism, which may account for the
ESP generated in the ground state, is based on spinselective intersystem crossing (ISC) from the 2T to the
4
T state (cf. Fig. 5) and is similar to the reversed triplet
mechanism (RTM) proposed for systems with singlet
ground states.60,61 This mechanism is also conceptually
similar to that used by Corvaja et al. for the weak
coupling regime.62 In their systems, the energy separation between quartet and doublet states is close to the
Zeeman splitting. The 4T ← 2T ISC rates were found to
be selective, with initial quartet sublevel populations in
the order: [4T+3/2] > [4T+1/2] > [4T–1/2] > [4T–3/2]. Recently, a
treatment of ISC between doublet and quartet states,
stimulated by our experimental results, was developed
for systems with spin-orbit coupling as the dominant
interaction63 and it was shown that the powder spectrum
of the quartet state could also exhibit net polarization in
strongly coupled systems. Because the decay rates for
the two spin sublevels of trip-doublet are different, this
mechanism generates transient spin polarization also in
the trip-doublet state. Decay of these states via CT1 and
CT2 ← d–d* (cf. Fig. 5) can transfer this polarization to
the ground state.
As discussed previously,63 spin-orbit coupling mixes
the trip-doublet and trip-quartet states. In the metalloporphyrins this mixing is particularly effective because
the LUMO is doubly degenerate. Distortions of the
molecule, which lift this degeneracy, leave a small energy
gap between the two lowest orbital states. It is well known
that second-order perturbation terms of spin-orbit
coupling lead to a contribution to the zero-field splitting
(ZFS) parameters. The large values of these parameters
for CuPs (DQ = –(0.1 – 0.35) cm–1 and EQ = –(0.25 –
0.35) cm–1)64–68 suggest that the second-order spin-orbit
contributions are large. However, we do not have sufficient information about the electronic structure of the
molecules studied here to accurately estimate the relative magnitude of the first-order and second-order
terms. However, regardless of which terms dominate,
efficient ISC between the 2Ti and 4Ti states is expected.
Consistent with this expectation, the measured lifetime
of the 4Ti ← 2Ti transition is short (~450 ps).57,58
Both the first- and second-order spin-orbit terms
break the symmetry with respect to the external field;
thus, ISC from the trip-doublet to the trip-quartet is
expected to produce net spin polarization. Although
analytical expressions were derived recently,63 it is difficult to predict the sign and magnitude of the net spin
polarization without detailed knowledge of the electronic structure. This is because the first-order perturbation terms predict absorptive net polarization of the tripquartet, while the second-order terms generate polarization of either sign. Both terms depend on the relative
magnitude of energy separation of the trip-doublet and
trip-quartet compared to the energy separation between
the two lowest orbital configurations, as well as the
magnitude and sign of the spin-orbit contribution to the
ZFS.
SIGNAL INTENSITY AND SPIN LATTICE
RELAXATION
Regardless of which mechanism accounts for the spin
polarization, the fact that it is observed in the ground
state leads to the conclusion that the ET rate should
exceed SLR rates within the doublet and quartet states.
The anisotropy of the ZFS generally leads to fast SLR
transitions between the sublevels of 4T. Thus, it is unlikely that the net polarization will be observed in the
trip-quartet state itself. However, if electronic transitions (e.g., via charge separation) are faster than the spin
relaxation, the net polarization will be transferred to a
long-lived ground state and, thus, can be observed.
The decay curve of the ground-state spin polarization
was found to be bi-exponential, with the characteristic
times of 3–5 and 50 ms. The origin of this behavior is
not clear. However, it is conceivable that these decays
are due to two types of CuP complexes, e.g., four- and
fivefold ligated CuPs or monomers in equilibrium with
dimers. In such a case, different magnetic interactions
within the different possible forms of the molecule
could induce different SLR rates. Another explanation
for the bi-exponential decay is possible with nearly
equal rates of SLR and 4Ti decay to the ground state.
Indeed, for CuTPP at 80 K, phosphorescence decay in
the range of 25–600 ms was measured.33,69
An interesting finding in the observed spin-polarized
spectra is the strong dependence of their intensity on the
nature of the peripheral groups (cf. Fig. 2). This effect is
attributed to different efficiencies of the ESP transfer to
the ground state. The energy of the CT state and/or the
position of its potential well along the reorganization
coordinate are expected to be sensitive to the charges on
the peripheral groups. This relates to the redistribution
of charge on the porphyrin, which occurs during the ET
process, where an electron is transferred to the Cu2+ at
the center of the molecule, leaving a net positive charge
on the porphyrin moiety. Thus, the positively-charged
anilinium groups in CuTTAP will tend to destabilize the
CT state, while the negatively-charged carboxylate and
Rozenshtein et al. / Electron Spin Polarization in Copper Porphyrins
380
sulfonate groups will stabilize it, with a smaller stabilization energy expected for the carboxylate. However,
since the degree of polarization depends on the relative
magnitude of many parameters, all of which may be
affected by the nature of the peripheral groups, straight
interpretation of the relationship between the charge on
the peripheral groups and the strength of the polarization is still an open question.
The excited states of the CuPs, which include the
triplet of the porphyrin ring, should exhibit anisotropy
with respect to molecular orientation relative to the
magnetic field. The theoretical treatment points out that
the low-field end of the powder spectrum in the excited
state should be slightly reduced.63 Our results indicate
that such an effect is not observed in the ground state,
possibly due to an averaging effect of several quenching
pathways.
CONCLUDING REMARKS
We have demonstrated in this work that upon photoexcitation, water-soluble CuPs with strong doublet–
triplet spin exchange (~200 cm–1) manifest a spin-polarized doublet ground state. The proposed mechanisms of
ESP are based on spin-selective populating or depopulating the trip-doublet state combined with additional
quenching via (d–d*) and CT states. It is of interest that
related CuTPP derivatives with uncharged side-groups
do not exhibit ESP.56 It is likely that the peripheral
charges cause a shift in the energy of the quenching
states. The ESP patterns and the mechanisms outlined
here show that weak spin–spin coupling and the accompanying doublet–quartet mixing are not prerequisites
for the generation of net polarization in a coupled
triplet–doublet spin pair.
Acknowledgments. We wish to thank Dr. Motoko AsanoSomeda for helpful discussions and sharing unpublished data
with us. The Farkas Center is supported by the Bundesministerium für Forschung und Technologie and the Minerva
Gesellschaft für die Forschung GmbH. The research described
here was partially supported by the United States-Israel Binational Science Foundation (BSF), the Deutsche Forschungsgemeinschaft (Sfb 337), the Natural Sciences and Engineering
Research Council, the Canada Foundation for Innovation, and
the Ontario Innovation Trust.
REFERENCES AND NOTES
(1) Salikhov, K.M.; Molin, Y.N.; Sagdeev, R.Z.;
Buchachenko, A.L. Spin Polarization and Magnetic
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