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J. Phys. Chem. B 2008, 112, 2842-2847
Quadratic Nonlinearity of One- and Two-Electron Oxidized Metalloporphyrins and Their Switching in Solution Abdul Wahab, Mily Bhattacharya, Sampa Ghosh, A. G. Samuelson, and Puspendu K. Das* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India ReceiVed: August 29, 2007; In Final Form: December 13, 2007
We report the quadratic nonlinearity of one- and two-electron oxidation products of the first series of transition metal complexes of meso-tetraphenylporphyrin (TPP). Among many MTPP complexes, only CuTPP and ZnTPP show reversible oxidation/reduction cycles as seen from cyclic voltammetry experiments. While centrosymmetric neutral metalloporphyrins have zero first hyperpolarizability, β, as expected, the cation radicals and dications of CuTPP and ZnTPP have very high β values. The one- and two-electron oxidation of the MTPPs leads to symmetry-breaking of the metal-porphyrin core, resulting in a large β value that is perhaps aided in part by contributions from the two-photon resonance enhancement. The calculated static first hyperpolarizabilities, β0, which are evaluated in the framework of density functional theory by a coupled perturbed Hartree-Fock method, support the experimental trend. The switching of optical nonlinearity has been achieved between the neutral and the one-electron oxidation products but not between the one- and the two-electron oxidation products since dications that are electrochemically reversible are unstable due to the formation of stable isoporphyrins in the presence of nucleophiles such as halides.
Introduction Switching bulk or molecular quadratic nonlinearity by an external stimuli such as a light beam, a potential gradient, etc. has been put forward as a part of a strategy to generate molecules or materials with large optical nonlinearities that are useful in optoelectronic and all-optical technologies. Quadratic nonlinearity or second-order molecular polarizability of molecules has been altered by oxidation, reduction, or chemical modifications.1 In a prototypical donor-acceptor compound of the type D-spacer-A, where the donor and the acceptor are separated by a backbone, this was achieved by reducing the donating power of the donor by oxidizing the donor, reduction of the accepting power of the acceptor by reducing the acceptor, or by structural or other chemical modifications of the linker. If the two forms, the native form and the one modified by any of the means described previously, are stable, then the system D-spacer-A can serve as a second-order NLO switch based on one of the forms being the on form and the other being the off form. The switching of quadratic nonlinearity was first demonstrated by Sakaguchi et al.2 in a mixed Langmuir-Blodgett film of (N,N′dioctadecyl-4,4′-dicarboxamide-2,2′-bipyridine)-bis(2,2′-bipyridine)ruthenium(II) perchlorate and dioctadecyldimethylammonium bromide in a two-laser experiment. When the film was exposed to pulsed laser irradiation (photochemical stimulation) at either 355 or 460 nm, the intensity of the second harmonic light at 532 nm, which was very strong when excited by a lone 1064 nm pulse, was remarkably suppressed. It was also shown that upon excitation at 378 nm, the photoinduced switching time of SHG was less than 2 ps and that the SHG intensity decreased to 70% of its initial value within that time.3 The recovery time for the SHG signal to its original value was several hundred picoseconds. The time taken by the Ru complex to relax was attributed to the time taken for recovery of the SHG signal. * Corresponding author. E-mail:
[email protected]; fax: 91-8023601552.
Coe et al.4 first demonstrated completely reversible chemical switching in ruthenium amine complexes, although because of a weak signal, they could not measure the β value of the oxidized products. Loucif-Saibi et al.5 showed that photoisomerization of the dye, disperse red one (DR-1), dissolved in a PMMA matrix induced by 488 nm light acted as an external stimuli and led to the realization of bulk second harmonic switching. The first electrochemical reversible redox switching of β in a series of mono-, bi-, and trinuclear organometallic Fe(II) and/or Fe(III) complexes was reported by Weyland et al.6 They investigated the effect of oxidation of the metal center, which was integrated in the donor part of the complex on the β value. This is known as type I switching, where the donor capacity of the donor has been lowered by oxidation. In their case, the stepwise one- or two-electron oxidation of metal centers led to a change in the β value of the complex. In complexes where an increase in β upon one-electron oxidation was observed, the β value of the oxidized product could be as high as twice that of the neutral complex. However, in those examples, subsequent oxidation by withdrawing another electron from the complex invariably led to a decrease in the β value. In another example, type II switching where the acceptor capacity of the acceptor fragment based on the metal center has been lowered upon reduction has been reported.7 All examples studied so far for redox switching of quadratic nonlinearity have exploited the redox properties of the Fe(II)/ Fe(III) and Ru(II)/Ru(III) metal centers in the so-called onedimensional systems. However, much scope exists in realizing switchable optical nonlinearities in multidimensional systems.8 Here, we have chosen metalloporphyrins (Figure 1), which are two-dimensional NLO chromophores where the porphyrin core acts as the donor and the central metal ion as the acceptor. They undergo facile oxidation in one-electron transfer steps to yield π cation radicals (hereafter, π cation radicals are referred to as cation radicals) and dications.9,10 The X-ray crystal structure of the radical perchloratotetraphenylporphynatozinc(II) was
10.1021/jp076909m CCC: $40.75 © 2008 American Chemical Society Published on Web 02/14/2008
One- and Two-Electron Oxidized Metalloporphyrins
Figure 1. Structure of porphyrins used in this work.
determined, and it was found that the cation radical crystallizes in the monoclinic space group P21/c. The perchlorate group is covalently coordinated to the zinc, and the structure is asymmetrically distorted from the planar neutral state.11 A large number of asymmetrically substituted neutral metalloporphyrins has been investigated for their nonlinear optical properties, and a large molecular hyperpolarizability has been reported in them.12-16 However, their oxidized or reduced products have not been investigated. Resonance Raman spectra of metalloporphyrin cation radicals in CH2Cl2 show that the removal of one electron causes distortion (ruffling) of the porphyrin core and weakens the bonding in the macrocycle.17,18 It is expected that core distortions such as S4 ruffling will affect the electronic states of the cation radicals. Since porphyrins are highly symmetric core molecules and do not show much second-order nonlinearity in their planar form, any distortion should be conducive to induce large second-order nonlinearity in them. In this study, the quadratic nonlinearity of electrochemical oxidation products of TPP and its first-row transition metal derivatives were investigated, in solution, by the hyper-Rayleigh scattering (HRS) technique. We carried out DFT calculations of static hyperpolarizability, β0, of the oxidized metalloporphyrins using the Jaguar 5.5 program in the Schrodinger suite.19 Facile electrochemical switching of a quadratic NLO response in copper- and zinc-metalloporphyrins between their neutral and their one-electron oxidation states also was demonstrated. Experimental Procedures Chemicals. TPP and its metallo complexes (MnClTPP, FeClTPP, CoTPP, NiTPP, CuTPP, and ZnTPP) except for CrClTPP were obtained from Sigma-Aldrich (99+%) and used without further purification. CrClTPP was synthesized and purified using a literature procedure.20 p-Nitroaniline was obtained from Aldrich Chemical Co., recrystallized twice from methanol, and dried in vacuum over P2O5. Anhydrous 1,4dioxane was prepared from AR grade 1,4-dioxane obtained locally by distilling over Na metal, and AR grade CH2Cl2 was distilled over P2O5. Tetra-n-butylammonium perchlorate (TBAP) was synthesized from tetra-n-butylammonium bromide and perchloric acid by dissolving them in the minimum volume of water at molar equivalent. The white crystalline precipitate of TBAP was filtered off, washed with water, recrystallized twice from ethyl acetate, and dried in vacuum over P2O5. Electrochemical Measurements. Cyclic voltammetry (CV), HRS measurements, and UV-vis spectral measurements were carried out using a conventional three-electrode cell on a potentiostat (CH Instruments Electrochemical Analyzer, 617A). Platinum plates (1 cm × 2 cm) were used as both working and counter electrodes, while an Ag/AgCl electrode was used as a reference electrode. All solutions were purged by N2 gas prior to the electrochemical measurements. CV data were obtained at different scan rates varied from 30 to 800 mV s-1. All the
J. Phys. Chem. B, Vol. 112, No. 10, 2008 2843 porphyrin solutions were of 10-3 M concentrations containing 0.1 M TBAP. For quantitative oxidation for in situ HRS measurements, controlled-potential electrolysis was employed in a cylindrical glass cell with an Ag wire as the reference electrode. The total volume of the cell was 25 cm3. During measurements, solutions were stirred continuously for uniform mixing. The UV-vis absorption spectra of the oxidized porphyrins were recorded in situ by employing a miniature SD2000 fiber optic spectrometer system (Ocean Optics). For a 2-3 × 10-5 M CuTPP or ZnTPP solution, oxidation from the neutral form to the cation radical typically took ∼30 min and from the cation radical to the dication ∼50 min in our cell. Reduction of dications to neutral metalloporphyrins via cation radicals took ∼90 min at -0.3 V. Only cation radicals took ∼35 min for complete reduction at -0.3 V. By reducing the concentration and changing the dimensions of the electrodes, one can change the oxidation/reduction time. HRS Measurements. The detailed experimental setup and measurement procedures of the HRS technique have been described elsewhere.21 Briefly, a 1064 nm fundamental of a Q-switched Nd:YAG laser was focused using a plano-convex lens of 20 cm focal length on the electrochemical cell. The laser energy was kept within 18-21 mJ/pulse. The scattered second harmonic (SH) photons (I2ω) were collected at a 90° angle using a monochromator and a PMT. The signal from the PMT was amplified 5 times and averaged over 512 laser shots in a storage oscilloscope. The first hyperpolarizability, β, of a molecule dissolved in a solvent is related to the scattered SH light intensity (I2ω) by
I2ω ) G(Nsolventβ2solvent + Nsoluteβ2solute)I2ω
(1)
where Nsolvent and Nsolute are the number densities of the solvent and solute molecules, respectively, G is an instrument factor, and Iω is the intensity of the incident beam. Since all our solute molecules that are porphyrins absorb significantly at the SH wavelength (532 nm), eq 1 has to be corrected for selfabsorption as22
I2ω ) G(Nsolventβ2solvent + Nsoluteβ2solute)I2ω × 10-Nsolutel
(2)
where is the extinction coefficient of the solute at 532 nm, and l is the effective path length or half the cell diameter (1.25 cm). p-Nitroaniline (pNA) was used as an external reference compound (βpNA ) 16.9 × 10-30 esu in 1,4-dioxane, and we determined that βpNA ) 19.3 × 10-30 esu in CH2Cl2). Dilute solutions (10-5 to 10-6 M) were used to ensure linear dependence of I2ω/Iω2 on solute concentration. The quadratic dependence of the SH signal on Iω was checked (see Supporting Information). All the measurements were performed in CH2Cl2 at ∼20 °C. Contribution from two-photon fluorescence to the SH scattering signal was checked by dispersing the signal through the SH wavelength. The HRS signal was fitted to a Lorentzian function and analyzed.21 No background fluorescence was seen (see data in the Supporting Information). Computational Details. DFT calculations were performed using the Jaguar 5.5 program in the Schrodinger suite. Structures of neutral metalloporphyrins and their oxidized products were fully optimized in the ground state using the B3LYP functional with a LACVP** basis set (which uses the effective core potential (ECP) basis set for the transition metal atoms and 6-31G** for other atoms). Known X-ray crystal structures11,23 were used as input geometries for optimization. Molecular static first hyperpolarizabilities, β0, were calculated by the coupled
2844 J. Phys. Chem. B, Vol. 112, No. 10, 2008
Wahab et al. TABLE 2: Linear and Nonlinear Optical Properties of MTPP and Their Oxidized Species in CH2Cl2 λmax (nm) compda
B band
Q band (Qβ and QR)
TPP TPP+ TPP++ CuTPP CuTPP+ CuTPP++ ZnTPP ZnTPP+ ZnTPP++
413 435 438 414 416 417 418 411 441
515, 550, 591, 647 555, 605, 656 510, 609, 657 539, 617 592, 656 662 548, 587 603, 656 655
at 532 nm β × 1030 βo × 1030 (M-1 cm-1) (esu)b (esu)c 6021 2423 15228 15177 10369 10919 11329 11928 6014
0 234 393 0 351 371 0 407 606
0.1 10.4 28.7 0.1 16.8 43.2 0.1 18.1 50.1
a MTPP+: cation radicals and MTPP++: dications. b Measured β values with an estimated error of (13%. c Calculated β0 values.
Figure 2. Cyclic voltammograms of TPP and its transition metal complexes in CH2Cl2 containing 0.1 M TBAP at a scan rate of 0.1 V s-1.
TABLE 1: Half-Wave Redox Potentials (V vs Ag/AgCl) and Peak Separation Potentials (∆E, mV) of TPP and Its Metallo Complexes in CH2Cl2 Containing 0.1 M TBAP compd
Oxd1
∆E
Oxd2
∆E
TPP CrClTPP MnClTPP FeClTPP CoTPP NiTPP CuTPP ZnTPP
1.07 1.15 1.16 1.19 0.81 1.10 1.05 0.82
55 110 24 64 62 51 54 55
1.30 1.39 1.52 1.45 1.03 1.18 1.29 1.12
50 63 79 54 52 53 54
Oxd3
∆E
1.18
42
perturbed Hartree-Fock (CPHF) method. For comparison with experimental values, the calculated β0 values were multiplied by a factor of 2 (B convention) as suggested by Willetts et al.24 Results and Discussion Figure 2 displays the cyclic voltammograms of all the metalloporphyrins in CH2Cl2 investigated by us at a 0.1 V s-1 scan rate. The half-wave redox potentials and redox peak separation potentials (∆E) versus Ag/AgCl are summarized in Table 1. The oxidation processes are labeled as Oxdn (n ) 1, 2, or 3). The measured half-wave potentials for one- and twoelectron oxidations agree well with literature values.9,25,26 From Figure 2, Table 1, and other peak current and peak separation characteristics,27 we infer that most of the metalloporphyrins including TPP show irreversible or quasi-reversible oxidation steps except CuTPP and ZnTPP. Both CuTPP and ZnTPP show two facile, well-resolved, and reversible stepwise one-electron oxidation processes as reported earlier.26 On the basis of a series of measurements such as EPR, magnetic susceptibility, magnetic circular dichroism, and linear spectroscopy, it has been established that the oxidations in CuTPP and ZnTPP are centered on the porphyrin ligands.9,10,28 In most of the metalloporphyrins, the electrons are removed from the porphyrin ring in succession upon oxidation except in CoTPP, where the first oxidation occurs at the metal center and the other two one-electron oxidations take place in the porphyrin ring. Although the oxidation steps in CoTPP were suggested to be reversible29 in CH2Cl2 containing 0.1 M TBAP in earlier studies, we find that the peak position corresponding to Oxd1 varies with the voltage scan rate and that ∆E is more than the expected 59 mV. The
ratio of the redox peak currents of Oxd1 and Oxd3 deviate from unity, and peak currents are also not proportional to the square root of the scan rate. These led us to infer that oxidation steps in CoTPP are quasi-reversible except for the second step (Oxd2), which is reversible. For in situ electrochemical oxidation of the metalloporphyrins, we employed controlled-potential electrolysis in the electrochemical cell. Potentials slightly higher than the oxidation potentials obtained from the CV measurements were applied for complete conversion of the metalloporphyrins to the corresponding cation radicals and dications. First hyperpolarizabilities of metalloporphyrins and their cation radicals and dications were measured in CH2Cl2 by HRS and are shown in Table 2. TBAP was used as a supporting electrolyte, which was checked to have a very low HRS response. In the irreversible (TPP, CrClTPP, MnClTPP, and FeClTPP) and quasi-reversible (CoTPP and NiTPP) porphyrin systems, complete redox conversion was not possible within a few hours as monitored by the change in absorption spectrum of the porphyrin compounds. The overlapping nature of the redox processes, such as in NiTPP, complicated the electrolysis process further. Since the oxidation process was not completed, the concentration of the oxidized product could not be determined with accuracy, and hence, β for the oxidized species of these complexes could not be measured. However, for the reversible CuTPP and ZnTPP systems, the β values of cation radicals and dications were easily determined (Table 2) since the quantitative transformation to the oxidized species in a short time span (30 min) could be monitored accurately by a change in the UV-vis spectra and coulometric current variation. Figure 3a,b presents the absorption spectra of CuTPP and ZnTPP, respectively, in CH2Cl2 during the progress of the electrochemical oxidation. Important absorption parameters are listed in Table 2. Spectral data of the oneand two-electron oxidized products show good agreement with those reported in the literature. For comparison, β of the TPP ligand and its oxidation products were also measured. Since TPP oxidation steps are irreversible, we waited for a long time for the oxidation to reach >80% completion (as judged from the decrease in intensity of the absorption spectrum), and the concentration of the oxidized products was assumed to be 100% of that of the initial neutral compound. Therefore, the β values for the radical cation and dication of TPP shown in Table 2 have large errors (∼20%) and should only be considered to be approximate. In fact, the incompletely oxidized metalloporphyrins also showed a large SH signal upon successive oxidation, but due to the lack of knowledge regarding the correct concentrations of the oxidized products, their β values were not determined. The first one-electron oxidized product of CoTPP exhibited a very weak SH signal in comparison to other MTPPs,
One- and Two-Electron Oxidized Metalloporphyrins
Figure 3. Absorption spectra of (a) CuTPP (2.546 × 10-5 M) and (b) ZnTPP (2.002 × 10-5 M) during oxidation from neutral to cation radicals, MTPP+ in CH2Cl2. Insets show corresponding absorption spectra during oxidation from MTPP+ to MTPP++. Spectra 1, neutral metalloporphyrin; spectra 2, cation radical; and spectra 3, dication.
suggesting that oxidation of the metal center in CoTPP is not conducive for the enhancement of β. However, the controlledpotential electrolysis in CoTPP appeared to be slow. According to Gouterman’s model,30 the electronic transition from the π HOMO (a1u and a2u) to the π* LUMO (eg) orbitals is primarily responsible for the highly intense B or Soret band (assigned as a1u f eg) around 400 nm and a moderately intense Q (QR and Qβ) band (assigned as a2u f eg) around 500-660 nm in free base porphyrins. The HOMO and HOMO - 1 states (a1u and a2u) of free base porphyrins are nearly degenerate, while the LUMO and LUMO + 1 states (eg) are rigorously degenerate.31 The energies of these porphyrin MOs are altered by the particular metal ion that binds to the core. Absorption spectra of CuTPP and ZnTPP show a weak QR band at 580-625 nm, a strong Qβ band at 530-560 nm, and a much stronger B band at 380-440 nm. After one-electron oxidation, the well-resolved absorption bands of the neutral metalloporphyrins appear to be red-shifted with some additional broad features appearing in the spectrum. The calculated32 and previously reported10,18,28 spectra of ZnTPP+ and CuTPP+ are in agreement with our spectrum. However, dication spectra of metalloporphyrins are
J. Phys. Chem. B, Vol. 112, No. 10, 2008 2845 not known in the literature except for CoTPP and MgTPP. Further oxidation of both CuTPP+ and ZnTPP+ makes the Q band vanish and the B bands less intense. In addition, ZnTPP++ shows another broad absorption band at 725-900 nm. For the neutral porphyrins, a scattered SH signal could not be detected. This is not surprising since β is expected to be zero in perfect D4h (MTPP) or D2h (TPP) symmetries. Upon successive oxidation, large β values were obtained for the cation radicals and dications of CuTPP and ZnTPP. The hyperpolarizability increased from neutral to cation radical to dication, and the stepwise increment was more pronounced in ZnTPP. Several factors can be ascribed to this: (a) the removal of one or two porphyrin π electrons causes ruffling of the porphyrin core, (b) the breaking away from the centrosymmetric geometry is further aided by the participation of the ClO4- ions as an axial ligand to the metal center, and (c) the metalloporphyrin cation radicals and dications show moderate absorption at 532 nm, perhaps leading to the two-photon resonance enhancement of β. The fact that β values of CuTPP and ZnTPP cation radicals and dications are not derived in part from the contribution of multiphoton fluorescence to the HRS signal is evidenced from the spectral pattern of the HRS signal dispersed through the monochromator before detection. The evidence for the deviation of the core structure from planarity comes from X-ray crystallographic studies, where a large ruffling of the porphyrin core in solid ZnTPP11 and CuTPP23 cation radicals containing the ClO4- counterion has been reported. Also, in going from the neutral to the oxidized form, the direction of charge transfer is altered. In the neutral form, porphyrin acts as the donor, while in the oxidized form, it acts as an acceptor. The acceptor strength of the porphinato cation is further increased by oxidation, which leads to the enhancement of β. Molecular static hyperpolarizabilities, β0, of all the oxidized metalloporphyrins were calculated using the coupled perturbed Hartree-Fock method under the framework of DFT as implemented in the Jaguar 5.5 program in the Schrodinger suite and are displayed in Table 2. Although the calculated gas phase β0 values cannot be compared directly with experimental β values since the dispersion and solvent effects in β are not accounted for in the calculation, it is helpful in understanding the trend in hyperpolarizabilities in transition metal porphyrins as a function of structural change. In fact, the ruffled geometry of the porphyrin core upon oxidation is clear from the optimized geometry displayed in Figure 4. The ground state structure of the oxidized metalloporphyrins belongs to C1 symmetry. It was found that all 10 isotropic components of β have nonzero values (data not shown) consistent with C1 symmetry. This also provides indirect evidence about the distortion of the core structure upon oxidation. The HRS β for unpolarized measurements in the laboratory frame was calculated from the components using a literature procedure.33 The β0 value for pNA was calculated and found to be 10.2 × 10-30 esu. The calculated hyperpolarizabilities of all the oxidized species are higher than that of pNA, which is consistent with the measured β values. Although dispersion corrected β0 values can be extracted from the measured hyperpolarizabilities using the two-state model34 for comparison with calculations, we chose not to do that since multiple electronic transitions in metalloporphyrins around the SH wavelength in our measurement make the choice of the excited state for the two-state model very difficult. Nevertheless, the trends of the measured β values in oxidized porphyrins match well with those from calculations. In CuTPP and ZnTPP, the reverse oxidation of the cation radicals is facile and leads to the formation of original symmetric
2846 J. Phys. Chem. B, Vol. 112, No. 10, 2008
Wahab et al.
Figure 4. Optimized structures of neutral, cation radical, and dication of ZnTPP.
Figure 5. Electrochemical switching of HRS responses between neutral porphyrins (MTPP) and their cation radicals (MTPP+) in CH2Cl2: (a) CuTPP (2.724 × 10-5 M) and (b) ZnTPP (2.536 × 10-5 M).
neutral complexes that have zero β. This forms the basis of on-off or off-on switching of quadratic nonlinearity in these metalloporphyrins. We followed the second harmonic intensity (I2ω) during oxidation of neutral CuTPP and ZnTPP to cation radicals and their conversion back to neutral porphyrins. The electrochemical on-off switching of β is apparent from the HRS signal plot in Figure 5a,b. The switching cycle is reproducible up to several cycles as evidenced from the in situ absorption spectra (not shown) of the components. We assign the neutral state of CuTPP or ZnTPP with a zero β value as the off state and the respective cation radical with a large β value as the on state. However, the second oxidation step from cation radical to dication was not quantitatively reversible, and on-off switching was not recorded. In fact, it is known that MTPP dications are powerful electrophiles and unstable in the presence of a variety of nucleophiles such as water, methanol, halides, etc. to produce isoporphyrins.35 These isoporphyrins are stable and have been characterized as the perchlorate salts.
Conclusion Quadratic nonlinearity of the first transition metal series metalloporphyrins, MTPPs, has been investigated under oneand two-electron oxidations in CH2Cl2 at 1064 nm. Among the transition metal porphyrins, only CuTPP and ZnTPP show reversible two one-electron oxidation steps. Neutral porphyrins exhibit zero quadratic nonlinearity, but oxidation leads to a substantial increase in their β values. The experimental trend in β is also supported by DFT calculations. Electrochemical switching of nonlinearity between the neutral and the cation radical states has been achieved. Although the second oxidation is electrochemically reversible, repeated NLO switching could only be demonstrated for the first oxidation step. The observation of a large HRS response in the oxidized metalloporphyrins paves the way of realizing very large quadratic nonlinearity in some selected metalloporphyrin systems under oxidation and on-off switching in those higher dimensional systems for device applications.
One- and Two-Electron Oxidized Metalloporphyrins Acknowledgment. We are grateful to N. Munichandriah and K. Sakthivel for helping us with the electrochemical experiments and to S. Ramakrishnan for lending us the Fiber Optic spectrometer for in situ absorption measurements. A.W. is thankful to the Department of Biotechnology for the award of a Research Associateship. M.B. and S.G. are thankful to the Council of Scientific and Industrial Research for Senior Research Fellowships. Generous funding from the Department of Science and Technology for our research is acknowledged. Supporting Information Available: Dependence of SH signal (I2ω) on the incident laser power (Iω) for cation radicals and dications of CuTPP and ZnTPP (Figures 1S-4S). Typical wavelength scans of HRS signal over spectral range of 510550 nm for cation radicals and dications of CuTPP and ZnTPP (Figures 5S-8S). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Asselberghs, I.; Clays, K.; Persoons, A.; Ward, M. D.; McCleverty, J. J. Mater. Chem. 2004, 14, 2831. (2) Sakaguchi, H.; Nagamura, T.; Matsuo, T. Jpn. J. Appl. Phys. 1991, 30, L377. (3) Sakaguchi, H.; Jahn, L. A. G.; Prichard, M.; Penner, T. L.; Whitten, D. G.; Nagamura, T. J. Phys. Chem. 1993, 97, 1474. (4) Coe, B. J.; Houbrechts, S.; Asselberghs, I.; Persoons, A. Angew. Chem., Int. Ed. 1999, 38, 366. (5) Loucif-Saibi, R.; Nakatani, K.; Delaire, J. A.; Dumont, M.; Sekkat, Z. Chem. Mater. 1993, 5, 229. (6) Weyland, T.; Ledoux, I.; Brasselet, S.; Zyss, J.; Lipinte, C. Organometallics 2000, 19, 5235. (7) Malaun, M.; Kowallick, R.; McDonagh, A. M.; Marcaccio, M.; Paul, R. L.; Asselberghs, I.; Clays, K.; Persoons, A.; Bildstein, B.; Fiorini, C.; Nunzi, J.-M.; Ward, M. D.; McCleverty, J. A. J. Chem. Soc., Dalton Trans. 2001, 3025. (8) Coe, B. J. Acc. Chem. Res. 2006, 39, 383. (9) Wolberg, A.; Manassen, J. J. Am. Chem. Soc. 1970, 92, 2982. (10) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J. Am. Chem. Soc. 1970, 92, 3451.
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