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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 43 2003 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 43 2003 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 2003 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 Effects in Radical Reactions; Elsevier: Amsterdam, 1984. (2) Goudsmit, G.-H.; Paul, H.; Shushin, A.I. 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