Photochemirtry and Phorobiology Vol. 55. No. 5 ,
pp. 677-680, 1992
Printed in Great Britain. All rights rcscrved
Copyright
003 1-8655/92 $05 .00+0.00 1992 Pergamon Press pic
0
TWO NEW STERICALLY HINDERED PHTHALOCYANINES: SYNTHETIC AND PHOTODYNAMIC ASPECTS BORISD. RIHTER',MARIAD. BOHORQUEZ~, MICHAEL A. J. RODGERS~* and MALCOLM E. KENNEY'* 'Department of Chemistry, Case Western Reserve University, Cleveland, O H 44106, USA and T e n t e r for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403. USA (Received 16 October 1991; accepted 4 November 1991) Abstract-The synthesis and the properties of 1,4-dibutoxy-2,3-dicyanotriptycene, of a metal-free tetradibenzobarrelenooctabutoxyphthalocyanine,and of the corresponding zinc phthalocyanine are described. The two phthalocyanines do not aggregate when dissolved in benzene at concentrations up to 450 pV. For the metal-free and the zinc compounds, the red band maxima are at 736 and 757 nm, the triplet maxima are at 590 and 605 nm, the triplet state lifetimes are 58 and 177ps, and the protoporphyrin-IX dimethyl ester-to-compound bimolecular rate constants for triplet energy transfer are 2.61 X 108 and 1.47 x lox M - ' s-l. Triplet energy transfer from the metal-free compound to O2 is endoergonic by 1.0 kcal mol-I. The potential of the zinc compound for photodynamic therapy is touched upon.
INTRODUCTION
Photodynamic therapy (PDT)t is a promising experimental treatment for neoplastic disease. An essential feature of this therapy is a photosensitizer that is activated by visible or near infrared radiation. In the studies done on the clinical level so far, two mixtures, hematoporphyrin derivative and Photofrin 11, have been used as the photosensitizer (Kessel, 1990). Currently, a search is being conducted for suitable second-generation clinical-level photosensitizers by a number of investigators. This search centers around single-component medium- to deep-redabsorbing photosensitizers that have high extinction coefficients. These are being studied because the wavelengths that activate them penetrate tissue well and can be generated with convenient lasers. One group of compounds we have investigated in connection with this search has as a common feature-the octabutoxyphthalocyaninering system, Pc(OBu), (Ford et al., 1989; Rihter er al., 1990). While the metal-free member of this group, HzPc(OBu),, I, is not particularly attractive for PDT, the Zn(I1) member, ZnPc(OBu)R, 11, is attractive in a number of respects. However, it forms aggregates when dissolved in non-coordinating solvents unless Lewis bases such as pyridine are present. This aggregation is undesirable because it *To whom correspondence should be addressed. +Abbreviations:H,Pc(dib),(OBu),, metal-free tetradibenzobarrelenooctabutoxyphthalocyanine; H,Pc(OBu),, metal-free octabutoxyphthalocyanine; PDT, photodynamic therapy; PPDME,protoporphyrin-IX dimethyl ester; ZnPc(dib),(OBu),, zinc tetradibenzobarreleno-
octabutoxyphthalocyanine.
Figure 1. A zinc tetradibenzobarrelenooctabutoxyphthalocyanine, ZnPc(dib),(OBu),.
appears, on the basis of previous work with a perylene (Ford, 1987), that aggregation of sensitizers generally reduces their ability to function. Although the formation of aggregates by I1 is not surprising, the formation of them by ZnPc(S)4(OR),-type compounds where S is a bulky non-polar ring substituent would not be expected. Accordingly, a compound of this type has the potential to combine the good features of I1 with a desirable resistance to aggregate formation. With this in mind, we have investigated such a compound and its metal-free parent. The compounds we have investigated are a metal-free tetradibenzobarrelenooctabutoxyphthalocyanine, H2Pc(dib)*(OBu),, 111, and the corresponding zinc tetradi-
677
BORISD. RIHTER er ul.
678
a
0
e
d
H2Pc(dib),(OBu),
CN Ill'
ZnPc(dib),(OBu), IVh
a) pxylene, reflux. (yield 88%);b) Cl2, AcOH (63%); c) (1) KCN, NaOH, (2) HCI (CAUTION) (82%); d) ~ B u lK2CO3, , acetone (86%); e) Li, BuOH, reflux (53%); 1) Purified by recrystallization from CH2C12-MeOH. Anal. Calcd for C120H114NeOe: C, 80.24; H. 6.40; N, 6.24. Found: C, 80.02; H, 6.50; N, 6.19. MS-HRFAB exact mass m/z cakd for C12oH114N808 (M+) 1794.8760. Found: 1794.8725; g) Zn(OAc)2*2H20, DMF (89%); h) purified by chromatography on Bio-beads S-X3 (Bio-Rad Laboratories, Hercules, CA 94547). MS-HRFAB exact mass m/z calcd for Cl2oHllzNsOeZn (M+) 1856.7895. Found: 1856.7885. Scheme I
benzobarrelenooctabutoxyphthalocyanine, or ZnPc(dib)4(0Bu)8, IV (Fig. 1).
RESULTS AND DISCUSSION
Ultraviolet-visible spectra of I11 and IV correspond closely to those of I and I1 respectively. MATERIALS AND METHODS Beer's Law plots for I11 and IV in benzene solution Synthesis of compounds. The reactions used to make show excellent linearity for concentrations up to 111 and IV are indicated in Scheme 1. cu 450 p M (i.e. up to near the solubility limits), Clearly, this set of reactions is just one example of a indicating that the new compounds do not form general route to metal-free and metal tetradibenzobarrelenooctaalkoxyphthalocyanines. A different route was aggregates in solution. Argon-saturated solutions of I11 in benzene upon used earlier to make the only previously reported member of this class, CuPc(dib),(OMe), (Gal'pern er al., 1983). excitation with 10 ns pulses of 355 nm light gave This earlier route appears less flexible because it probably rise to a transient absorption in the visible region will yield only Cu compounds. An important step in this new route is the reductive-cyanation that leads to 1.4- with kT"" = 590 nm. This transient decayed expodihydroxy-2.3-dicyanotriptycene,a key intermediate in the nentially with a lifetime of 58 ps and was quenched route. This reductive-cyanation is patterned after chemis- by oxygen. The bimolecular rate constant for this try developed by Reynolds and VanAllen (1964). quenching, k E , was 1.82 x lox M-l s - I (see The NMR spectra of 111 and IV are as expected. below). The same transient absorption was formed Photochemistry of compounds. The instrumentation for laser flash photolysis, coupled with UV-VIS kinetic spec- concomitantly with the decay of the TI state of trophotometry and infrared emission luminescence detec- protoporphyrin-IX dimethyl ester (PPDME) when tion has been described (Ford et 01.. 1989). solutions containing PPDME and I11 were excited
Two new sterically hindered phthalocyanines
679
Table 1
H2Pc(dib)4(OBu), ZnPc(dib)4(OBu),
58 177
590 605
44200 35714
0.222 0.372
0.26' 0.41r
(*15%)
('10%)
1.822 x 1oK 1.M+ x 10'' (510%)
*Molar extinction coefficients for the triplet states were determined using the energy transfer method with PPDMEhenzene as reference (cdW)= 3 5 0 0 M-' cm-') (Sinclair ei al.. 1980.) tQuantum yield of intersystem crossing S , -+ T,.This was determined by comparison with benzophenone in benzene (QT = 1) as standard. $Quantum yield for singlet oxygen production (02-saturatcd benzene). Determined by luminescence intensity at 1.27 )rm using benzophcnone in benzene (QA= 0.29) (Gorman er al., 1984) as standard.
at 532 nm, where absorption by PPDME was dominant. These observations suggest that the species that was absorbing at 590 nm was the TI state of 111. Similar experiments with IV indicated that it has a of ca 605 nm and a lifetime of 177 ps (Table 1). Quenching of the TI state of 111 by O2 gave rise to biphasic decay kinetics. Similar behavior has been observed with naphthalocyanines and with I, and is the consequence of reversible energy transfer (Ford et al., 1989; Rihter et al., 1990; Firey et al., 1988). Analysis of the decay profiles as a function of oxygen concentration led to values of kn, kAo, and Keq (= kn/kAG) where kT): and kAo are defined according to:
kv
kTS
T+I;
== ku;
G+A
(1)
The value of Krq obtained, 0.021, led to a AG for reaction (1) of 1.0 kcal mol-I, indicating endoergonic energy transfer to oxygen. In undergoing this endoergonic energy transfer, 111 is like 1. A parallel analysis of O2 quenching data for IV gave a value for kn for this compound. Photophysical parameters for 111 and IV are summarized in Table 1. The energy transfer between the T I state of PPDME and 111 and IV in benzene solution showed interesting behavior. T-T energy transfer is expected to be exergonic by ca 12 kcal mo1-I (Rihter et al., 1990). The bimolecular rate constants for triplet energy transfer to I and I1 were reported earlier as 2.8 X 10' M-' s-' and ca 1.7 x 10' M-I s-I respectively (Rihter et af., 1990) consistent with this energy gap. Values of 2.61 x 10' M-I s-I and 1.47 x 10' M-' s-I were obtained for 111 and IV respectively. This implies that 111 and IV and PPDME encounter severe steric difficulty in attaining the optimal approach distance for energy transfer and that the energy exchange occurs at a distance; i. e. where electronic coupling and therefore the energy transfer rate constant are reduced from the optimal values. In summary, a general route to a class of metal-
free and metallated tetradibenzobarrelenooctaalkoxyphthalocyanines has been developed. The metal-free and Zn(I1) octabutoxy members of this class of compounds have been prepared and characterized. The steric hindrance provided by the barreleno groups leads to compounds that have little tendency to aggregate in solution, even near 500 pm. Triplet energy transfer from PPDME is sterically hindered. However, the barreleno groups do not affect the So + S, gap. These factors suggest that the zinc compound and several other metal compounds containing Pc(dib)4(0R)Hring systems should be excellent deep-red absorbing photosensitizers for photodynamic applications. Acknowledgements-We are grateful for helpful advice from Dr. W. E. Ford, for support from the National Institutes of Health through grant CA 46281, and for support from the Center for Photochemical Sciences, Bowling Green State University. Mass spectra determinations were done by the Midwest Center for Mass Spectroscopy at Lincoln, Nebraska, a National Science Foundation Regional Instrumentation Facility (grant CHE 8620117). REFERENCES
Firey, P. A., W. E. Ford, J. R. Sounik, M. E. Kenney and M. A. J . Rodgers (1988) Silicon naphthalocyanine triplet state and oxygen: a reversible energy-transfer reaction. J . Am. Chem. SOC. 110. 7626-7630. Ford, W. E. (1987) Photochemistry of 3,4,9.10-perylenetetracarboxylic dianhydride dyes: visible absorption and fluorescence of the di(glycy1)imide derivative monomer and dimer in basic aqueous solutions. J . Photochem. 37. 189-204. Ford, W. E., B. D. Rihter. M. E. Kenney and M. A. J . Rodgers (1989) Photoproperties of alkoxy-substituted phthalocyanines with deep-red optical absorbance. Phoiochem. Phorobiol. 50. 277-282. Gal'pern, M. G., V. K. Shalaev. T. A. Shatskaya, L. S. Shishkanova, V. R. Skvarchenko and E. A . Luk'yanets ( 1983) Phthalocyanines and related compounds. XXIII tetra-2,3-tryptycenoporphyrazine and its metal cornplexes. J. Gen. Chem. USSR (Engl. Transl.) 53, 2346-235 I . Gorman. A. A., 1. Hamblett and M. A. J. Rodgers (1984) Time-resolved luminescence measurements of tripletsensitized singlet-oxygen production: variation in energy-transfer efficiencies. J. Am. Chem. SOC.106,
680
BORISD. RIHTERel ul.
4679-4682. Kessel, D. (1990) Drug development and photodynamic therapy. Spectrum 3. 13-15. Rihter. B. D., M. E. Kenney, W. E. Ford and M. A . J . Rodgers (1990) Synthesis and photoproperties of diamagnetic octabutoxyphthalocyanines with deep red optical absorbance. 1.Am. Chem. Soc. 112,8064-8070.
Reynolds, G. A. and J. A. VanAllen (1964) The reactions of 2.3-dichloro-l ,.l-naphthoquinone with alkali metal cyanides. 1. Org. Chem. 29, 3591-3593. Sinclair, R. S., D. Tait and T. G. Truscott (1980) Triplet states of protoporphyrin IX and protoporphyrin IX dimethyl ester. 1. Chem. SOC. Furuduy Truns. I 76, 417-425.
pp. 677-680, 1992
Printed in Great Britain. All rights rcscrved
Copyright
003 1-8655/92 $05 .00+0.00 1992 Pergamon Press pic
0
TWO NEW STERICALLY HINDERED PHTHALOCYANINES: SYNTHETIC AND PHOTODYNAMIC ASPECTS BORISD. RIHTER',MARIAD. BOHORQUEZ~, MICHAEL A. J. RODGERS~* and MALCOLM E. KENNEY'* 'Department of Chemistry, Case Western Reserve University, Cleveland, O H 44106, USA and T e n t e r for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403. USA (Received 16 October 1991; accepted 4 November 1991) Abstract-The synthesis and the properties of 1,4-dibutoxy-2,3-dicyanotriptycene, of a metal-free tetradibenzobarrelenooctabutoxyphthalocyanine,and of the corresponding zinc phthalocyanine are described. The two phthalocyanines do not aggregate when dissolved in benzene at concentrations up to 450 pV. For the metal-free and the zinc compounds, the red band maxima are at 736 and 757 nm, the triplet maxima are at 590 and 605 nm, the triplet state lifetimes are 58 and 177ps, and the protoporphyrin-IX dimethyl ester-to-compound bimolecular rate constants for triplet energy transfer are 2.61 X 108 and 1.47 x lox M - ' s-l. Triplet energy transfer from the metal-free compound to O2 is endoergonic by 1.0 kcal mol-I. The potential of the zinc compound for photodynamic therapy is touched upon.
INTRODUCTION
Photodynamic therapy (PDT)t is a promising experimental treatment for neoplastic disease. An essential feature of this therapy is a photosensitizer that is activated by visible or near infrared radiation. In the studies done on the clinical level so far, two mixtures, hematoporphyrin derivative and Photofrin 11, have been used as the photosensitizer (Kessel, 1990). Currently, a search is being conducted for suitable second-generation clinical-level photosensitizers by a number of investigators. This search centers around single-component medium- to deep-redabsorbing photosensitizers that have high extinction coefficients. These are being studied because the wavelengths that activate them penetrate tissue well and can be generated with convenient lasers. One group of compounds we have investigated in connection with this search has as a common feature-the octabutoxyphthalocyaninering system, Pc(OBu), (Ford et al., 1989; Rihter er al., 1990). While the metal-free member of this group, HzPc(OBu),, I, is not particularly attractive for PDT, the Zn(I1) member, ZnPc(OBu)R, 11, is attractive in a number of respects. However, it forms aggregates when dissolved in non-coordinating solvents unless Lewis bases such as pyridine are present. This aggregation is undesirable because it *To whom correspondence should be addressed. +Abbreviations:H,Pc(dib),(OBu),, metal-free tetradibenzobarrelenooctabutoxyphthalocyanine; H,Pc(OBu),, metal-free octabutoxyphthalocyanine; PDT, photodynamic therapy; PPDME,protoporphyrin-IX dimethyl ester; ZnPc(dib),(OBu),, zinc tetradibenzobarreleno-
octabutoxyphthalocyanine.
Figure 1. A zinc tetradibenzobarrelenooctabutoxyphthalocyanine, ZnPc(dib),(OBu),.
appears, on the basis of previous work with a perylene (Ford, 1987), that aggregation of sensitizers generally reduces their ability to function. Although the formation of aggregates by I1 is not surprising, the formation of them by ZnPc(S)4(OR),-type compounds where S is a bulky non-polar ring substituent would not be expected. Accordingly, a compound of this type has the potential to combine the good features of I1 with a desirable resistance to aggregate formation. With this in mind, we have investigated such a compound and its metal-free parent. The compounds we have investigated are a metal-free tetradibenzobarrelenooctabutoxyphthalocyanine, H2Pc(dib)*(OBu),, 111, and the corresponding zinc tetradi-
677
BORISD. RIHTER er ul.
678
a
0
e
d
H2Pc(dib),(OBu),
CN Ill'
ZnPc(dib),(OBu), IVh
a) pxylene, reflux. (yield 88%);b) Cl2, AcOH (63%); c) (1) KCN, NaOH, (2) HCI (CAUTION) (82%); d) ~ B u lK2CO3, , acetone (86%); e) Li, BuOH, reflux (53%); 1) Purified by recrystallization from CH2C12-MeOH. Anal. Calcd for C120H114NeOe: C, 80.24; H. 6.40; N, 6.24. Found: C, 80.02; H, 6.50; N, 6.19. MS-HRFAB exact mass m/z cakd for C12oH114N808 (M+) 1794.8760. Found: 1794.8725; g) Zn(OAc)2*2H20, DMF (89%); h) purified by chromatography on Bio-beads S-X3 (Bio-Rad Laboratories, Hercules, CA 94547). MS-HRFAB exact mass m/z calcd for Cl2oHllzNsOeZn (M+) 1856.7895. Found: 1856.7885. Scheme I
benzobarrelenooctabutoxyphthalocyanine, or ZnPc(dib)4(0Bu)8, IV (Fig. 1).
RESULTS AND DISCUSSION
Ultraviolet-visible spectra of I11 and IV correspond closely to those of I and I1 respectively. MATERIALS AND METHODS Beer's Law plots for I11 and IV in benzene solution Synthesis of compounds. The reactions used to make show excellent linearity for concentrations up to 111 and IV are indicated in Scheme 1. cu 450 p M (i.e. up to near the solubility limits), Clearly, this set of reactions is just one example of a indicating that the new compounds do not form general route to metal-free and metal tetradibenzobarrelenooctaalkoxyphthalocyanines. A different route was aggregates in solution. Argon-saturated solutions of I11 in benzene upon used earlier to make the only previously reported member of this class, CuPc(dib),(OMe), (Gal'pern er al., 1983). excitation with 10 ns pulses of 355 nm light gave This earlier route appears less flexible because it probably rise to a transient absorption in the visible region will yield only Cu compounds. An important step in this new route is the reductive-cyanation that leads to 1.4- with kT"" = 590 nm. This transient decayed expodihydroxy-2.3-dicyanotriptycene,a key intermediate in the nentially with a lifetime of 58 ps and was quenched route. This reductive-cyanation is patterned after chemis- by oxygen. The bimolecular rate constant for this try developed by Reynolds and VanAllen (1964). quenching, k E , was 1.82 x lox M-l s - I (see The NMR spectra of 111 and IV are as expected. below). The same transient absorption was formed Photochemistry of compounds. The instrumentation for laser flash photolysis, coupled with UV-VIS kinetic spec- concomitantly with the decay of the TI state of trophotometry and infrared emission luminescence detec- protoporphyrin-IX dimethyl ester (PPDME) when tion has been described (Ford et 01.. 1989). solutions containing PPDME and I11 were excited
Two new sterically hindered phthalocyanines
679
Table 1
H2Pc(dib)4(OBu), ZnPc(dib)4(OBu),
58 177
590 605
44200 35714
0.222 0.372
0.26' 0.41r
(*15%)
('10%)
1.822 x 1oK 1.M+ x 10'' (510%)
*Molar extinction coefficients for the triplet states were determined using the energy transfer method with PPDMEhenzene as reference (cdW)= 3 5 0 0 M-' cm-') (Sinclair ei al.. 1980.) tQuantum yield of intersystem crossing S , -+ T,.This was determined by comparison with benzophenone in benzene (QT = 1) as standard. $Quantum yield for singlet oxygen production (02-saturatcd benzene). Determined by luminescence intensity at 1.27 )rm using benzophcnone in benzene (QA= 0.29) (Gorman er al., 1984) as standard.
at 532 nm, where absorption by PPDME was dominant. These observations suggest that the species that was absorbing at 590 nm was the TI state of 111. Similar experiments with IV indicated that it has a of ca 605 nm and a lifetime of 177 ps (Table 1). Quenching of the TI state of 111 by O2 gave rise to biphasic decay kinetics. Similar behavior has been observed with naphthalocyanines and with I, and is the consequence of reversible energy transfer (Ford et al., 1989; Rihter et al., 1990; Firey et al., 1988). Analysis of the decay profiles as a function of oxygen concentration led to values of kn, kAo, and Keq (= kn/kAG) where kT): and kAo are defined according to:
kv
kTS
T+I;
== ku;
G+A
(1)
The value of Krq obtained, 0.021, led to a AG for reaction (1) of 1.0 kcal mol-I, indicating endoergonic energy transfer to oxygen. In undergoing this endoergonic energy transfer, 111 is like 1. A parallel analysis of O2 quenching data for IV gave a value for kn for this compound. Photophysical parameters for 111 and IV are summarized in Table 1. The energy transfer between the T I state of PPDME and 111 and IV in benzene solution showed interesting behavior. T-T energy transfer is expected to be exergonic by ca 12 kcal mo1-I (Rihter et al., 1990). The bimolecular rate constants for triplet energy transfer to I and I1 were reported earlier as 2.8 X 10' M-' s-' and ca 1.7 x 10' M-I s-I respectively (Rihter et af., 1990) consistent with this energy gap. Values of 2.61 x 10' M-I s-I and 1.47 x 10' M-' s-I were obtained for 111 and IV respectively. This implies that 111 and IV and PPDME encounter severe steric difficulty in attaining the optimal approach distance for energy transfer and that the energy exchange occurs at a distance; i. e. where electronic coupling and therefore the energy transfer rate constant are reduced from the optimal values. In summary, a general route to a class of metal-
free and metallated tetradibenzobarrelenooctaalkoxyphthalocyanines has been developed. The metal-free and Zn(I1) octabutoxy members of this class of compounds have been prepared and characterized. The steric hindrance provided by the barreleno groups leads to compounds that have little tendency to aggregate in solution, even near 500 pm. Triplet energy transfer from PPDME is sterically hindered. However, the barreleno groups do not affect the So + S, gap. These factors suggest that the zinc compound and several other metal compounds containing Pc(dib)4(0R)Hring systems should be excellent deep-red absorbing photosensitizers for photodynamic applications. Acknowledgements-We are grateful for helpful advice from Dr. W. E. Ford, for support from the National Institutes of Health through grant CA 46281, and for support from the Center for Photochemical Sciences, Bowling Green State University. Mass spectra determinations were done by the Midwest Center for Mass Spectroscopy at Lincoln, Nebraska, a National Science Foundation Regional Instrumentation Facility (grant CHE 8620117). REFERENCES
Firey, P. A., W. E. Ford, J. R. Sounik, M. E. Kenney and M. A. J . Rodgers (1988) Silicon naphthalocyanine triplet state and oxygen: a reversible energy-transfer reaction. J . Am. Chem. SOC. 110. 7626-7630. Ford, W. E. (1987) Photochemistry of 3,4,9.10-perylenetetracarboxylic dianhydride dyes: visible absorption and fluorescence of the di(glycy1)imide derivative monomer and dimer in basic aqueous solutions. J . Photochem. 37. 189-204. Ford, W. E., B. D. Rihter. M. E. Kenney and M. A. J . Rodgers (1989) Photoproperties of alkoxy-substituted phthalocyanines with deep-red optical absorbance. Phoiochem. Phorobiol. 50. 277-282. Gal'pern, M. G., V. K. Shalaev. T. A. Shatskaya, L. S. Shishkanova, V. R. Skvarchenko and E. A . Luk'yanets ( 1983) Phthalocyanines and related compounds. XXIII tetra-2,3-tryptycenoporphyrazine and its metal cornplexes. J. Gen. Chem. USSR (Engl. Transl.) 53, 2346-235 I . Gorman. A. A., 1. Hamblett and M. A. J. Rodgers (1984) Time-resolved luminescence measurements of tripletsensitized singlet-oxygen production: variation in energy-transfer efficiencies. J. Am. Chem. SOC.106,
680
BORISD. RIHTERel ul.
4679-4682. Kessel, D. (1990) Drug development and photodynamic therapy. Spectrum 3. 13-15. Rihter. B. D., M. E. Kenney, W. E. Ford and M. A . J . Rodgers (1990) Synthesis and photoproperties of diamagnetic octabutoxyphthalocyanines with deep red optical absorbance. 1.Am. Chem. Soc. 112,8064-8070.
Reynolds, G. A. and J. A. VanAllen (1964) The reactions of 2.3-dichloro-l ,.l-naphthoquinone with alkali metal cyanides. 1. Org. Chem. 29, 3591-3593. Sinclair, R. S., D. Tait and T. G. Truscott (1980) Triplet states of protoporphyrin IX and protoporphyrin IX dimethyl ester. 1. Chem. SOC. Furuduy Truns. I 76, 417-425.
