J. Phys. Chem. 1996, 100, 13609-13614
13609
One-Electron Oxidation of Metalloporphycenes As Studied by Radiolytic Methods Dirk M. Guldi,*,1a Jason Field,1b Jan Grodkowski,1c P. Neta,1d and Emanuel Vogel1e Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556, Physical and Chemical Properties DiVision, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, and Institut fu¨ r Organische Chemie, UniVersita¨ t zu Ko¨ ln, Greinstrasse 4, 50939 Ko¨ ln, Germany ReceiVed: March 29, 1996; In Final Form: May 29, 1996X
One-electron and two-electron oxidations of 2,7,12,17-tetrapropylporphycene (H2TPrPc) and its Fe, Co, Ni, Cu, and Sn complexes in CH2Cl2, CCl4, and 2-PrOH solutions have been studied by radiolytic techniques. Formation and decay of intermediates formed upon one-electron oxidation have been followed by kinetic spectrophotometric pulse radiolysis, and the absorption spectra of stable oxidation products have been recorded following γ-radiolysis. H2TPrPc is oxidized to the π-radical cation and then to the dication, which is stable in aprotic solvents but is transformed to a different product in 2-PrOH. Similar oxidation to the π-radical cation and then to the dication was observed for the CuII, SnIV, CoIII, and FeIII porphycenes. CoII and NiII porphycenes underwent radiolytic oxidation to form stable CoIII and NiIII products. The stability of the latter is in contrast with previous electrochemical observations, and the difference is ascribed to the effect of the axial ligand.
Introduction Porphycenes, one of several classes of porphyrin isomers,2 have evoked considerable interest in their electron transfer and catalytic properties and their potential application in biological processes.3 Metalloporphycenes, due to their rich absorption features in the visible spectrum, where human tissues are relatively transparent, have been successfully employed in the production of singlet oxygen as a product of excited state quenching. This led to their use in photodynamic therapy, where treatment of murine tumors with a tetra-n-propylporphycene demonstrated cellular uptake and antitumor activity.4 The lower symmetry of porphycenes (D2h) as compared to that of porphyrins (D4h) and the smaller size of the porphycene cavity (the four nitrogens comprise a rectangle with N-N distances of 2.83 and 2.63 Å) relative to that of porphyrin (the four nitrogens form a square with N-N distances of 2.89 Å) restrict the accommodation of reduced metal ions of large radii.2a This has been demonstrated for the radical-induced5 and electrochemical reduction6 of Ni(II) and Co(II) tetrapropylporphycene, which leads to the formation of π-radical anions, whereas the analogous metalloporphyrins exhibited reduction of the transition metals. Conversely, the smaller size of the porphycene cavity may have a stabilizing effect on high oxidation state transition metal porphycenes. Stable complexes of tervalent nickel with certain tetraazamacrocyclic ligands were prepared by electrochemical oxidation in aprotic solvents.7 Other complexes of tervalent nickel (with amines, amino acids, peptides, different tetraazamacrocycles,8 and certain porphyrins9), however, have been reported as short-lived transients in protic and aqueous media. Little information is available on the reactivity of metalloporphycenes toward oxidizing radicals. In the present study, we utilize radiolytic techniques, both time-resolved and steadystate, to study the oxidation of metalloporphycenes under various conditions, mainly by reaction with peroxyl radicals in organic solvents. Experimental Section The free base porphycene, 2,7,12,17-tetrapropylporphycene (H2TPrPc), and its Fe(III), Co(II), Ni(II), Cu(II), and Sn(IV) X
Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00942-2 CCC: $12.00
Figure 1. Porphycenes considered in this study.
complexes (Figure 1), were synthesized as described before.2 The solvents used were analytical grade reagents from Mallinckrodt.1f Solutions containing 5 × 10-6 to 1 × 10-4 mol L-1 porphycene or metalloporphycene in the desired medium were freshly prepared and were irradiated under air. Steadystate irradiations were done in a Gammacell 220 60Co source with a dose rate of ≈1 Gy s-1. Irradiation times were up to several minutes. Optical absorption spectra were recorded within several minutes before and after irradiation. Pulse radiolysis experiments were performed by utilizing 50 ns pulses of 8 MeV electrons from a Model TB-8 / 16-1S electron linear accelerator (at Notre Dame) or 50 ns pulses of 2 MeV electrons from a Febetron Model 705 pulser (at NIST). Basic details of the equipment and the data analysis have been described elsewhere.10 Dosimetry was based on the oxidation of SCNto (SCN)2•-. The dose per pulse was varied between 14 and 27 Gy, which in aqueous solutions gives between 8 × 10-6 and 16 × 10-6 mol L-1 of radicals. Kinetic traces were recorded at various wavelengths, showing bleaching of the starting porphycene or formation of the porphycene product. The differential absorption spectra were measured after the completion of the kinetic process observed and were not corrected for the bleaching of the parent compound to obtain absolute spectra of the products. All experiments were carried out at room temperature (22 ( 2) °C. Results and Discussion Radiolytic Oxidation Reactions. Radiolytic oxidations of metalloporphycenes (MPc) were carried out in several solvents © 1996 American Chemical Society
13610 J. Phys. Chem., Vol. 100, No. 32, 1996
Guldi et al.
and solvent mixtures, and thus the oxidation reactions involve different radiolytically generated radicals. In aerated CH2Cl2, the porphycenes are oxidized mainly by the two peroxyl radicals, CH2ClO2• and CHCl2O2•, formed in the radiolysis of this solvent.11 Direct oxidation by the solvent radical cation ([CH2Cl2]•+) and by Cl atoms is unimportant since these species are very short-lived and the porphycene concentration is relatively low.
CH2Cl2 -Df [CH2Cl2]•+ + e- and •CH2Cl + Cl (1) Cl + CH2Cl2 f •CHCl2 + HCl
(2)
e-+ CH2Cl2 f •CH2Cl + Cl-
(3)
[CH2Cl2]•+ f •CHCl2 + H+
(4)
•
CH2Cl + O2 f •O2CH2Cl
(5)
•
CHCl2 + O2 f •O2CHCl2
(6)
•
O2CH2Cl/•O2CHCl2 + MPc f CH2ClO2-/CHCl2O2- + (MPc)+ (7)
(MPc)+ denotes an oxidized metalloporphycene, without specifying whether the site of oxidation is at the ligand or the metal center. Oxidation in neat CCl4 is mainly by the peroxyl radical CCl3O2•, although some reaction with Cl atoms may also take place in this system.12 Oxidation in aerated 2-PrOH/CCl4 solutions also is by the CCl3O2• radicals.13
(CH3)2CHOH -Df (CH3)2C˙ OH + e- + H+
(8)
e- + CCl4 f •CCl3 + Cl-
(9)
•
(CH3)2COH + CCl4 f •CCl3 + (CH3)2CO + H+ + Cl(10) CCl3 + O2 f CCl3O2•
(11)
CCl3O2• + MPc f CCl3O2- + (MPc)+
(12)
•
Free Base Porphycene. Pulse radiolysis of the free base porphycene in aerated CH2Cl2 or 2-propanol/CCl4 (9:1 by volume) resulted in strong bleaching of the starting compound at 372, 563, 602, and 635 nm and formation of broad absorptions in the 600-800 and 400-500 nm regions (Figure 2a). These spectral changes indicate one-electron oxidation of the porphycene to its π-radical cation, by comparison with results on porphyrins14 and from the close resemblance of the spectral changes in Figure 2a with those observed upon controlled oxidative electrolysis.6c
H2TPrPc - e- f H2TPrPc•+
(13)
To examine the stable products of the radical-induced oxidation, we carried out γ-radiolysis experiments with the free base porphycene in oxygen-saturated CH2Cl2 and recorded the optical absorption spectrum before and after several irradiation intervals (Figure 2b). The peaks of the starting material decayed gradually upon irradiation, while peaks at 325 and 384 nm and a broad absorption at 660-720 nm were formed. The spectral features, however, are different from those observed at short
Figure 2. Radiolytic oxidation of H2TPrPc: (a) differential spectrum monitored by pulse radiolysis in air-saturated 2-propanol/CCl4 (9:1), recorded 0.2 ms after the pulse, after completion of the oxidation reaction, showing bleaching of the starting material and formation of the broad absorption of the π-radical cation; (b) steady state radiolytic oxidation of H2TPrPc to the dication in oxygenated CH2Cl2 (the arrows indicate the direction of absorption changes during the course of irradiation, total irradiation dose 80 Gy).
times after the pulse (Figure 2a) and indicate that the product probably results from disproportionation of the π-radical cation. This product is probably the dication, H2TPrPc2+.
2H2TPrPc•+ f H2TPrPc + H2TPrPc2+
(14)
Although this product was stable at least for several minutes in CH2Cl2, it was unstable in 2-PrOH/CCl4. In this solvent, no isosbestic points were observed upon γ-radiolytic oxidation and only broad absorptions were formed, indicating that the dication probably undergoes nucleophilic attack by the alcohol to form different products. The absorption peaks of the porphycene and metalloporphycene and of the oxidation products obtained under different conditions are summarized in Table 1. SnIV-Porphycene. Electrochemical experiments6c with SnIVTPrPc in CH2Cl2 indicated that this porphycene is reduced much more readily than the free base or other metalloporphycenes, whereas its oxidation was not observed within the range scanned by cyclic voltammetry in this solvent. Therefore, SnIVTPrPc is not expected to be oxidized rapidly by peroxyl radicals, but it may be oxidized by stronger oxidizing species such as Cl atoms or solvent radical cations, e.g. [CH2Cl2]•+. However, since these reactive species are very short-lived, their reaction with the SnIVTPrPc cannot be very effective at the low concentrations used. In fact, the radiolytic yields for oxidation of SnIVTPrPc were found to be very low and no stable radical cation or dication was observed. CuII-Porphycene. The differential absorption spectra observed in the pulse radiolysis of CuIITPrPc in aerated CH2Cl2 or 2-PrOH/CCl4 (Figure 3a) exhibit bleaching of the porphycene peaks and formation of broad absorption in the red. By similarity to the observations with the free base porphycene,
One-Electron Oxidation of Metalloporphycenes
J. Phys. Chem., Vol. 100, No. 32, 1996 13611
TABLE 1: Absorption Bands of the Porphycenes and Their Radiolytic Oxidation Products compound H2TPrPc
conditions CH2Cl2
Cu(II)TPrPc
CH2Cl2
Fe(III)TPrPc
CH2Cl2
Co(II)TPrPc
CH2Cl2
Ni(II)TPrPc
CH2Br2 CH2Cl2
BrCH2CH2Br
absorption bands, nm 372, 383 (sh), 563, 602, 635 700-840 ∼680 366 (sh), 386, 572, 614 700-840 (740, 820) 630, 650 362, 575 (sh), 615 645, 740 632, 655 322, 382, 550 (sh), 592 385, 570 (sh), 609 ∼760, 820 630, 650 392, 619 370 (sh), 397, 565 (sh), 601, 613 (sh) 399, 610 669, 740, 827 632, 655 397, 614
species H2Pc H2Pc•+ H2Pc2+ Cu(II)Pc Cu(II)Pc•+ Cu(II)Pc2+ Fe(III)Pc Fe(III)Pc•+ Fe(III)Pc2+ Co(II)Pc ClCo(III)Pc Co(III)Pc•+ Co(III)Pc2+ BrCo(III)Pc Ni(II)Pc ClNi(III)Pc Ni(III)Pc•+ Ni(III)Pc2+ BrNi(III)Pc
the observed spectra indicate that CuIITPrPc is oxidized to its π-radical cation, CuIITPrPc•+. This ligand oxidation is in line with previous results on copper porphyrins.15
CuIITPrPc - e- f CuIITPrPc•+
(15)
Upon γ-radiolysis of CuIITPrPc in oxygenated CH2Cl2 (Figure 3b) the typical Soret and Q-band decayed gradually while weak absorptions at 650-850 nm, with a shoulder at 740 and a weak maximum at 820 nm, were formed. The close resemblance of the results from the steady-state radiolysis with those from the time-resolved experiments suggests formation of CuIITPrPc•+ that is very stable under our experimental conditions. Comparison of our spectra with those obtained by thin-layer spectroelectrochemistry for H2TPrPc•+, NiIITPrPc•+, and CoIIITPrPc•+ 6c corroborates our assignment. Prolonged radiolysis of CuIITPrPc•+ in air-saturated CH2Cl2 led to formation of a product absorbing at 600-700 nm (small maxima at 630 and 650 nm). By comparison with the results obtained with the free base porphycene, this product is ascribed to the dication. Changing the solvent from neat CH2Cl2 or CCl4 to 2-PrOH/CCl4 (9:1) (Figure 3c) decreased the stability of the π-radical cation and the dication as discussed above. FeIII-Porphycene. The differential absorption spectrum recorded upon pulse radiolysis of FeIIITPrPc in CCl4 (Figure 4a) shows ground-state bleaching and parallel formation of broad absorption in the red, with no indication of a red shift of the main peaks. These findings indicate formation of a π-radical cation (FeIIITPrPc•+) rather than FeIVTPrPc.
FeIIITPrPc - e- f FeIIITPrPc•+
(16)
Previous results16 indicated that (EtioPc)FeIII(C6H5) (EtioPc ) 2,7,12,17-tetraethyl-3,6,13,16-tetramethylporphycene), in which C6H5 is σ-bonded to the metal center, undergoes three electrochemical one-electron oxidation steps, leading to an Fe(IV)porphycene and then to the π-radical cation and the dication. The σ-bonded C6H5 presumably increases the electron density at the metal site relative to FeIIITPrPc and makes the metalcentered oxidation more favorable. To examine the effect of axial ligands on the site of oxidation, we added pyridine to the FeIIITPrPc solutions. This strong ligand replaces the weaker axial ligands present (Cl- and solvent molecules) as indicated by a blue shift of the absorption bands (617 f 612 nm with a
Figure 3. Radiolytic oxidation of CuIITPrPc: (a) differential spectrum monitored by pulse radiolysis in air-saturated 2-propanol/CCl4 (9:1), recorded 0.4 ms after the pulse, after completion of the oxidation reaction, showing bleaching of the starting material and formation of the broad absorption of the π-radical cation; (b) steady state radiolytic oxidation of CuIITPrPc to the π-radical cation in oxygenated CH2Cl2 (the arrows indicate the direction of absorption changes during the course of irradiation, total irradiation dose 120 Gray); (c) prolonged steady state radiolytic oxidation of CuIITPrPc•+ to the dication in oxygenated 2-propanol/CCl4 (1:1) (the arrows indicate the direction of absorption changes during the course of irradiation).
volume fraction of 5% pyridine). The high electron donating ability of the pyridine ligand increases the electron density at the metal and thus should facilitate its oxidation. However, pulse radiolysis of FeIIITPrPc in CCl4 or CH2Cl2 containing 5% pyridine showed differential absorption spectra similar to those found without pyridine, indicating a ligand centered oxidation. Steady-state radiolysis of FeIIITPrPc in CH2Cl2 also showed little effect of pyridine on the outcome of the oxidation. The spectral changes observed with solutions containing 5% pyridine (Figure 4b) are similar to those found in the absence of pyridine; both resemble the above findings with the free base porphycene. No clean isosbestic points were found. An intermediate was formed at low doses, displaying maxima at 382 nm and around 645 and 740 nm, which indicate that it is the π-radical cation. This FeIIITPrPc•+, however, decays within minutes, and the final product absorbs at 632 and 655 nm, indicating formation of the dication. Radiolysis in the solvent mixture 2-PrOH/CCl4 could not be studied with this porphycene because of a slow thermal reaction
13612 J. Phys. Chem., Vol. 100, No. 32, 1996
Figure 4. Radiolytic oxidation of FeIIITPrPc: (a) differential spectrum monitored by pulse radiolysis in air-saturated CCl4 containing 5 vol % pyridine, recorded 0.2 ms after the pulse, after completion of the oxidation reaction, showing bleaching of the starting material and formation of the broad absorption of the π-radical cation; (b) steady state radiolytic oxidation of FeIIITPrPc to the π-radical cation in oxygenated CH2Cl2/pyridine (5 vol %) (the arrows indicate the direction of absorption changes during the course of irradiation, total irradiation dose 24 Gy).
causing red shift of the absorption peaks and then decay of all absorptions due to precipitation. We speculate that a chain reaction develops with 2-PrOH/CCl413 that produces HCl and may be initiated by traces of FeII in the material, but this system was not investigated further. Co-Porphycene. The differential absorption spectra monitored by pulse radiolysis of CoIITPrPc in aerated CH2Cl2 or 2-PrOH/CCl4 (9:1) (Figure 5a) show bleaching at 325 and 600 nm and formation of peaks at 400 and 630 nm, with no broad absorption in the 650-800 nm region. This red shift of the major peaks is in agreement with the spectroelectrochemical results6c and is similarly ascribed to oxidation of CoIITPrPc to CoIIITPrPc. γ-Radiolysis of a similar solution (Figure 5b) resulted in a gradual disappearance of the 322, 382, and 592 peaks and simultaneous formation of new peaks at 385 and 609 nm. These changes are identical with those observed by pulse radiolysis, and the good isosbestic points (at 385, 540, and 605 nm) confirm the clean transformation of the divalent to the tervalent cobalt complex. Oxidation of CoIITPrPc by Br2 also resulted in the formation of CoIIITPrPc, although the peaks are slightly red shifted from those observed by γ-radiolysis in CH2Cl2 because of the axial ligation of Br- vs Cl- (similar to the shifts observed below with the Ni complex). Bathochromically shifted absorption peaks (385 f 392 nm; 609 f 619 nm) were also observed upon radiolytic oxidation in CH2Br2 where the product is ligated with the Br- released in the radiolysis. As we have indicated in a recent study,5 CoIITPrPc is slowly oxidized by air in solution. Pulse radiolysis of CoIIITPrPc showed differential absorption changes that were dominated by ground-state bleaching at 380 and 615 nm and formation of new bands at ∼750 and 820 nm (Figure 6a). γ-Radiolysis also indicated gradual decay of the peaks of CoIIITPrPc while distinct broad bands at ∼760 and
Guldi et al.
Figure 5. Radiolytic oxidation of CoIITPrPc: (a) differential spectrum monitored by pulse radiolysis in air-saturated CH2Cl2, recorded 0.5 ms after the pulse, after completion of the oxidation reaction, showing oxidation of the metal center from CoII to CoIII; (b) steady state radiolytic oxidation of CoIITPrPc to the CoIII complex in oxygenated CH2Cl2 (the arrows indicate the direction of absorption changes during the course of irradiation, total irradiation dose 44 Gy).
820 nm were formed (Figure 6b). The absorptions bands at 760 and 820 nm closely resemble those found in an electrochemical investigation, suggesting generation of a stable π-radical cation. Further irradiation of the π-radical cation resulted in spectral transformations that indicate formation of the dication, CoIIITPrPc2+, showing broad absorption between 600 and 700 nm with small maxima around 630 and 650 nm (not shown). This assignment is supported by the above results on the two-electron oxidation of H2TPrPc and CuIITPrPc. NiII-Porphycene. The differential absorption spectra observed upon pulse radiolysis of NiIITPrPc in aerated CH2Cl2 or 2-PrOH/CCl4 (9:1) (Figure 7a) exhibit bleaching of the metalloporphycene peaks at 395 and 605 nm, formation of peaks at slightly higher wavelengths, and no absorptions in the 700800 nm region. These spectra clearly indicate oxidation of the nickel center rather than the ligand. Previous findings with metalloporphyrins also indicate that changes in the redox state of the metal result in only minor shifts of the peaks.17
NiIITPrPc - e- f NiIIITPrPc
(17)
Steady-state radiolysis of NiIITPrPc in aerated CH2Cl2 or 2 PrOH/CCl4 (1:1) also led to red shifts of the peaks, from 397 and 601 nm to 399 and 610 nm, with general broadening of the absorption bands (Figure 7b). The observation of isosbestic points at 350, 395, 545, and ∼625 nm, which agree very well with the zero-crossing points of the differential spectrum, indicates a clean radiolytic transformation of the starting NiIITPrPc to the product NiIIITPrPc in both experiments and underscores the remarkable stability of NiIIITPrPc under these conditions. Metal-centered oxidation of NiIITPrPc changes the squareplanar configuration of the divalent complex to an octahedral configuration, where the nickel is located in the plane deter-
One-Electron Oxidation of Metalloporphycenes
J. Phys. Chem., Vol. 100, No. 32, 1996 13613
Figure 6. Radiolytic oxidation of CoIIITPrPc: (a) differential spectrum monitored by pulse radiolysis in air-saturated 2-propanol/CCl4 (9:1), recorded 0.2 ms after the pulse, after completion of the oxidation reaction, showing bleaching of the starting material and formation of the broad absorption of the π-radical cation; (b) steady state radiolytic oxidation of CoIIITPrPc to the π-radical cation in oxygenated CH2Cl2 (the arrows indicate the direction of absorption changes during the course of irradiation, total irradiation dose 100 Gy).
mined by the four ligating nitrogen atoms and binds two axial ligands. Thus, stabilization of the tervalent nickel complex requires that the additional positive charge of the oxidized metal be balanced by an anionic ligand. Radiolysis of CH2Cl2 or 2-PrOH/CCl4 produces Cl-, which can serve as such a ligand.
NiIITPrPc + ROO• f ROO-NiIIITPrPc
(18)
ROO-NiIIITPrPc + HCl f Cl-NiIIITPrPc + ROOH (19) To confirm this assignment and the stability of NiIIITPrPc, we also oxidized NiIITPrPc radiolytically in oxygenated 1,2dibromoethane and chemically with Br2 in CH2Cl2. In these cases, essentially the same products are formed, but with a Bras the axial ligand. The peaks of the oxidation product were at 397 and 614 nm, significantly red shifted relative to the peaks of NiIITPrPc, with no absorption above 650 nm.18 The stability of the metal-oxidized nickel porphycene is remarkable, since oxidation of nickel(II)-porphyrins by pulse radiolysis and γ-radiolysis resulted in the formation of transient Ni(III)-porphyrins or π-radical cations of nickel(II)-porphyrins. In all cases, however, the one-electron-oxidation products, whether Ni(II) π-radical cation or Ni(III), decayed to yield twoelectron ring oxidation products.9a The above results on the oxidation of NiIITPrPc to NiIIITPrPc are in contrast with those obtained by cyclic voltammetry and thin-layer spectroelectrochemisty in CH2Cl2, which indicated oxidation at the porphycene ligand.6c This difference arises most probably from the availability of the anion to serve as an axial ligand that stabilized the Ni(III) state; in the radiolytic or chemical oxidation, Cl- or Br- was available, but in the
Figure 7. Radiolytic oxidation of NiIITPrPc: (a) differential spectrum monitored by pulse radiolysis in air-saturated CCl4, recorded 0.5 ms after the pulse, after completion of the oxidation reaction, showing oxidation of the metal center from NiII to NiIII; (b) steady state radiolytic oxidation of NiIITPrPc to the NiIII complex in oxygenated CH2Cl2 (the arrows indicate the direction of absorption changes during the course of irradiation, total irradiation dose 38 Gy); (c) prolonged steady state radiolytic oxidation of NiIIITPrPc to the π-radical cation in oxygenated CH2Cl2 (the arrows indicate the direction of absorption changes during the course of irradiation, total irradiation dose 95 Gy).
electrochemical oxidation the electrolyte was tetrabutylammonium perchlorate, which provides a very weak complexing anion. Upon further radiolysis in aerated CH2Cl2 or 2-PrOH/CCl4 (1:1), the generated NiIIITPrPc undergoes further oxidation resulting in the formation of broad absorption in the red with bands at 669, 740, and 827 nm. These peaks are very similar to those described above for the products of oxidation of CuII and CoIII-porphycene and are similarly ascribed to NiIIITPrPc•+.
NiIIITPrPc - e- f NiIIITPrPc•+
(20)
Prolonged radiolysis of NiIIITPrPc•+ resulted in formation of the dication, as can be deduced from the characteristic absorption peaks at 632 and 655 nm (not shown).
13614 J. Phys. Chem., Vol. 100, No. 32, 1996 Conclusion NiII-porphycenes
are oxidized first at the metal In summary, center to form stable NiIII complexes if a proper anion is available to serve as the axial ligand to stabilize the oxidized species. In the absence of such ligands, they are oxidized to the π-radical cations. The stability of NiIII-porphycenes appears to be higher than that of comparable porphyrins, probably due to the smaller size of the porphycene cavity which forms a tighter complex with the small NiIII ion. NiIIIporphycenes are further oxidized at the ligand to yield π-radical cations and then dications. Similar two-step ligand oxidation was found also with FeIII, CoIII, CuII, and the free base porphycenes. The π-radical cations in all of these cases exhibit broad absorptions over the 700-840 nm range and beyond, while the dications absorb mainly below 700 nm, with small maxima around 630 and 650 nm. These species are somewhat stable in CH2Cl2 and CCl4, in which they could be observed, at least partially, following γ-radiolysis (i.e. within minutes after formation), but they disappear rapidly in 2-PrOH, probably by nucleophilic attack of the solvent. Most of the oxidized species observed with porphycenes were more stable than those reported for porphyrins. This is generally in line with the oxidation potentials of the porphycene being slightly lower than those of the corresponding porphyrins. Acknowledgment. This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy. This is Contribution NDRL-3909 from the Notre Dame Radiation Laboratory. References and Notes (1) (a) University of Notre Dame. (b) University of Notre Dame; on leave from University of Waterloo, ON. (c) Nationial Insitute of Standards and Technology; on leave from the Institute of Nuclear Chemistry and Technology, Warsaw, Poland. (d) National Institute of Standards and Technology. (e) Universita¨t zu Ko¨ln. (f) The identification of commercial equipment or material does not imply recognition or endorsement by the National Institute of Standards and Technology, nor does it imply that the material or equipment identified is necessarily the best available for the purpose. (2) (a) Vogel, E.; Ko¨cher, M.; Schmickler, H.; Lex, J. Angew. Chem., Int. Ed. Engl. 1986, 25, 257. (b) Vogel, E.; Balci, M.; Pramond, K.; Koch, P.; Lex, J.; Ermer, O. Angew. Chem., Int. Ed. Engl. 1987, 26, 928. (c) Sessler, J. L. Angew. Chem., Int. Ed. Engl. 1994, 33, 1348. (3) (a) Bormann, S. Chem. Eng. News 1995, June 26, 30. (b) Ofir, H.; Regev, A.; Levanon, H.; Vogel, E.; Ko¨cher, M.; Balci, M. J. Phys. Chem.
Guldi et al. 1987, 91, 2686. (c) Levanon, H.; Toporowicz, M.; Ofir, H.; Fessenden, R. W.; Das, P. K.; Vogel, E.; Ko¨cher, M.; Pramond, K. J. Phys. Chem. 1988, 92, 2429. (d) Waluk, J.; Mu¨ller, M.; Swiderek, P.; Ko¨cher, M.; Vogel, E.; Hohlneicher, G.; Michl, J. J. Am. Chem. Soc. 1991, 113, 5511. (4) (a) Richert, C.; Wessels, J. M.; Mu¨ller, M.; Kisters, M.; Benninghaus, T.; Goetz, A. E. J. Med. Chem. 1994, 37, 2797. (b) Aramendia, P. F.; Redmond, R. W.; Nonell, S.; Schuster, W.; Braslavsky, S. E.; Schaffner, K. Photochem. Photobiol. 1986, 44, 555. (c) Guardino, M.; Biolo, R.; Jori, G.; Schaffner, K. Cancer Lett. 1989, 44, 1. (5) Guldi, D. M.; Neta, P.; Vogel, E. J. Phys. Chem. 1996, 100, 4097. (6) (a) Renner, M. W.; Forman, A.; Wu, W.; Chang, C. K.; Fajer, J. J. Am. Chem. Soc. 1989, 111, 8618. (b) Gisselbrecht, J. P.; Gross, M.; Ko¨cher, M.; Lausmann, M.; Vogel, E. J. Am. Chem. Soc. 1990, 112, 8618. (c) Bernard, C.; Gisselbrecht, J. P.; Gross, M.; Vogel, E.; Lausmann, M. Inorg. Chem. 1994, 33, 2393. (d) D’Souza, F.; Boulas, P.; Aukauloo, A. M.; Guilard, R.; Kisters, M.; Vogel, E.; Kadish, K. M. J. Phys. Chem. 1994, 98, 11885. (e) Kadish, K. M.; van Caemelbecke, E.; Boulas, P.; D’Souza, F.; Vogel, E.; Kisters, M.; Medforth, C. J.; Smith, K. M. Inorg. Chem. 1993, 32, 4177. (7) Busch, D. H. Acc. Chem. Res. 1978, 11, 392. (8) (a) Lati, J.; Koresh, J.; Meyerstein, D. Chem. Phys. Lett. 1975, 33, 386. (b) Jacobi, M.; Meyerstein, D.; Lilie, J. Inorg. Chem. 1979, 18, 429. (c) Maruthamuthu, P.; Patterson, L.; Ferraudi, G. Inorg. Chem. 1978, 17, 1630. (d) Zeigerson, E.; Bar, I.; Bernstein, J.; Kirschbaum, L. J.; Meyerstein, D. Inorg. Chem. 1982, 21, 73. (9) Nahor, G. S.; Neta, P.; Hambright, P.; Robinson, L. R. J. Phys. Chem. 1991, 95, 4415. (10) (a) Neta, P.; Huie, R. E. J. Phys. Chem. 1985, 89, 1783. (b) Schuler, R. H. J. Phys. Chem. 1996, submitted for publication. (11) (a) Emmi, S. S.; Beggiato, G.; Casalbore-Miceli, G. Radiat. Phys. Chem. 1989, 33, 29. (b) Alfassi, Z. B.; Mosseri, S.; Neta, P. J. Phys. Chem. 1989, 93, 1380. (12) Grodkowski, J.; Neta, P. J. Phys. Chem. 1984, 88, 1205. (13) Brault, D.; Neta, P. J. Phys. Chem. 1983, 87, 3320. (14) (a) Neta, P. J. Phys. Chem. 1981, 85, 3678. (b) Guldi, D. M.; Neta, P.; Hambright, P. J. Chem. Soc., Faraday Trans. 1992, 88, 2013. Harriman, A.; Richoux, M.-C.; Neta, P. J. Phys. Chem. 1983, 87, 4957. (15) Harriman, A.; Richoux, M.-C.; Neta, P. J. Phys. Chem. 1983, 87, 4957. (16) Kadish, K. M.; D’Souza, F.; Van Caemelbecke, E.; Boulas, P.; Vogel, E.; Aukauloo, A. M.; Guilard, R. Inorg. Chem. 1994, 33, 4474. (17) (a) Guldi, D. M.; Hambright, P.; Lexa, D.; Neta, P.; Saveant, J.M. J. Phys. Chem. 1992, 96, 4459. (b) Nahor, G. S.; Neta, P.; Hambright, P.; Robinson, L. R.; Harriman, A. J. Phys. Chem. 1990, 94, 6659. (c) Dolphin, D.; Niem, T.; Felton, R. H.; Fuita, I. J. Am. Chem. Soc. 1975, 97, 5288. (18) Since the square-planar nickel(II) complex does not coordinate any ligands at the two axial positions, the observed red shift cannot be ascribed to a simple complexation of a bromide ligand. This assignment has been substantiated by adding various concentrations of pyridine, which is a stronger ligand than chloride and bromide, and finding no shifts in the absorption band. Thus, the red-shift can be unambiguously ascribed to a bromide complexation of the hexacoordinated nickel(III)-porphycene.
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