The Journal of Physical Chemistry, Vol. 83, No. 24, 1979 3165
Disodium Salts of Free-Base Porphyrin
Chemistry and Photochemistry of the Disodium Salts of Free-Base Porphyrin C. Y. Lee and G. Levin' Department of Chemistry, State University of New York, College of Environmental Science and Forestry, Syracuse, New York 13210 (Received April 5, 1979) Publication costs assisted by the National Science Foundation
Reduction of free-base porphyrin (H,TPP) by sodium metal in tetrahydrofuran (THF) generated the corresponding dianion. Its optical spectrum was recorded. No ESR signal was detected during the reduction process, indicating that the disproportionation constant (Kdisp) of the reaction 2H2TPP-.,Na+F! HzTPP2-,2Na++ HzTPP is higher than lo6. In powerful solvents such as hexamethylphosphoramide, the radical anion (H2TPP-.,Na+) was detected by ESR spectroscopy, thus allowing calculation of the disproportionation constant (Kdisp = 0.6). The dianion was quantitatively bleached on excitation by a flash of light. Immediately following the flash a new transient was spectrophotometricallyobserved (Ama 460 nm). During the dark period the transient reverted to the dianion following a combined first- and second-order kinetics (t1lZ 1 ms). Addition of trans-stilbene as a scavenger to the photolyzed solution revealed that no electron ejection from the dianion took place. Thus, the transient was assumed to be the triplet state of the dianion, and its triplet-triplet annihilation rate constant was found to be 3 X lo9 M-' s-'. Protonation of the free-base dianion by excess methanol re-formed 70% of the parent compound as identified by its optical spectrum. The protonated mixture was further analyzed by mass spectrometry, revealing the existence of a compound with a molecular weight of 620 in addition to the parent compound (mol wt 614). No chlorin or isobacteriochlorin was detected. The unusual composition of the protonation products was interpreted as a result of consecutive protonation and electron-transfer reactions.
-
Introduction Oxidation of porphyrin and metalloporphyrin compounds to their corresponding radical cation and dication and their reduction to the radical anion and dianion are a subject of continuous investigation aiming toward a better understanding of the roles of those compounds in biological systems.' Reduction of zinc tetraphenylporphyrin (ZnTPP) in tetrahydrofuran with sodium benzophenone ketyl as an electron-transfer agent was investigated by Closs and Closs.2 Hush and Dodd3 reported that only H,TPP2-,2Na+ was observed when the free base was reduced by sodium metal in tetrahydrofuran. On the other hand, Linschitz and Felton4 reported that polarographic reduction of H z T P P in dimethyl sulfoxide (Me2SO) resulted in the formation of a radical anion which can further reduce to the corresponding dianion. During the last several years we have been investigating the chemistry and photochemistry of radical anions and dianions derived from aromatic hydrocarbon^.^^^ Therefore, we decided to investigate the dianion salts of HzTPP and to compare them to other aromatic hydrocarbon dianions previously studied. Experimental Section Tetraphenylporphyrin acquired commercially (Strem Chemicals Inc.) was pumped overnight under high vacuum and then sealed in a breakseal. Preparation of the flashed solution and the flash photolysis apparatus have been previously d e ~ c r i b e d . ~Methyl ,~ alcohol, which was used to protonate the HzTPP2-,2Na+,was deoxygenated by repeated freezing and thawing. trans-Stilbene that was used as a scavenger of electrons in the flash photolysis experiment was purified by sublimation under high vacuum into a breakseal. A saturated aqueous solution of NaNOz was used to filter the UV light. Results and Discussion The absorption spectrum of H z T P P (5 X M) in T H F was recorded and found to be similar to that in tol~ e n e The . ~ Soret band was found to be a t X 416 nm and its position remainea the same even when the concentra0022-3654/79/2083-3165$01 .OO/O
to tion was changed by a factor of lo4 (from M). On contact of the above solution with a sodium mirror, the spectrum of HzTPP gradually disappeared and a new spectrum appeared a t X 436 nm (see Figure 1). No other species was detected during the reduction process. Analysis of the sodium content confirmed the identity of HzTPP2-,2Na+( [H2TPP]:[Na+]= 1:2). More than that, no ESR signal was observed during different stages of the reduction process; therefore, the disproportionation constant of the reaction KbP
2H2TPP--,Na+?H2TPP2-,2Na++ HzTPP in T H F was calculated to be &isp > lo6. Hush and Dodd3 arrived at the same conclusion. They reported that the initial product of the reduction of HzTPP in tetrahydrofuran or 1,2-dimethoxyethane by sodium is the disodium salt. Previous studies6 of disproportionation of radical ions derived from aromatic hydrocarbons indicated that several factors contribute to the equilibrium constant of disproportionation; for example, solvation of the counterions will decrease the disproportionation constant due to the fact that the counterions of the dianion are less solvated than the counterion of the radical anion. Hence, in a solvent with a high solvation power the disproportionation is very low. For example, the disproportionation constant of the sodium salt of the tetracene radical anion in T H F is of the order of lo4 as compared to lo-' in diethyl ether. Another factor that affects the disproportionation constant is the electrostatic interaction between the negative charge which delocalized on the aromatic hydrocarbons and the positively charged cations. Such an effect will result in a higher disproportionation constant for aromatic hydrocarbons with a more localized charge.'O We believe that the electrostatic interaction is the major factor contributing to the high disproportionation constant of H2TPP-.,Na+. The preferred formation of the dianion over the radical anion is probably due to the strong Coulombic interaction exerted between the disodium cations and the high-density negative charge residing on the nitrogens. More than that, the interaction could be further enhanced if the sodium
0 1979 American Chemical Society
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The Journal of Physical Chemistry, Vol. 83, No. 24, 1979
Lee and Levin 05
04
0.3
OD YXI
403
m
600
700
800
A * m
Flgure 1. Optical spectrum of the sodium salts of free-base tetraphenyiporphyrin dianion (H,TPP2-,2Na+) in tetrahydrofuran.
02
TABLE I: Effect of Dielectric Constant of the Solvent on the Disproportionation Constant solvent
dielectric const
disproportionation const
THF HMPA Me,SO
7.39 29.6 49.6
>lo6 0.6 10-5
cations occupy the empty core in the free-base porphyrin. Supporting evidence for this assumption is given in the fact that, when the two hydrogens attached to the nitrogens are replaced with a divalent cation such as Zn, the disproportionation constant decreases considerably. In this case the core is occupied by the zinc and thus the electrostatic interaction is reduced. For example, Closs and Closs2 reported that two-distinguishable-electron reduction can be observed during the reaction of ZnTPP with a sodium mirror in tetrahydrofuran. This kind of observation is indicative of a disproportionation constant much lower than 1. More than that, our analysis of the combined spectrum of the ZnTPP, its radical anion, and its dianion at about 50% conversion to the radical anion shows that the disproportionation constant in T H F is less than Additional supporting evidence results when the tetrahydrofuran (THF) is replaced with a more powerful solvent such as hexamethylphosphoramide; the disproportionation constant then drastically decreases to 0.6 from approximately lo6 measured in THF. It is worth comparing the disproportionation constant in three different solvents. A comparison of this kind is instructive and appears in Table I. This can serve as another indication for the importance of the electrostatic interaction, i.e., the higher the dielectric constant the lower the disproportionation constant. The disproportionation constant in HMPA was determined in the following way: The absorption spectrum of a mixture of H2TPP (2.3 X lo4 M) and H2TPP2-,2Na+(2.4 x low4M) in T H F was recorded and then the solvent was pumped out under high vacuum and replaced by the same volume of highly purified HMPA. The concentrations of the dianion (Ama 434) and the neutral HzTPP (A, 415) were decreased, and a new band with a low intensity located approximately in the region of 450 nm appeared (see Figure 2). The above solution was analyzed by an ESR technique. A narrow (15 G) signal was detected and found to be in agreement with that reported by Linschitz and Feltona4The concentration of the radical anion in HMPA was estimated by comparing its ESR absorption spectrum to that of perylene radical anion of known concentration. The concentration of the radical anion formed amounts
01
340
360
380
400
420
440
460
480
500
A , nm Flgure 2. Absorption spectra of a mixture of H2TPP2-,2Na+and H2TPP: (.-e) in THF; (-) in HMPA.
to twice as much as the decrease of the dianion or the neutral free base. This ascertains the correct stoichiometry of the disproportionation reaction. Photochemical Behavior of H2TPP2-,2Na+.The photochemical behavior of the dianions was investigated by a flash photolysis technique. In previous studies6 flash photolysis of aromatic hydrocarbon dianions resulted in photoejection of electrons and formation of the corresponding radical anions and the sodium electron pair (e-,Na+). For example, in the case of perylene the dianion was bleached to form its radical anion and electron sodium pair in a ratio of l:l:l, and no other species were observed immediately after the flash (25 k s ) . A different situation occurred when the disodium salts of free-base porphyrin was subjected to a flash of light. The dianion which absorbed a t X 436 nm was quantitatively bleached. Immediately after the flash a new transient appeared (Amm 460 nm). During the dark period the transient decayed with the same rate as the bleached dianion recovered. However, no absorption was detected around 800 nm, indicating the absence of a Na+,e- pair (see Figure 3). The possibility of the photolysis of the dianion leading to electron ejection was checked by adding a scavenger to the flashed solution. The choosing of the right scavenger should be considered very carefully. For example, higher electron affinity of the chosen scavenger may result in electron transfer from the dianion to the scavenger not by capturing of the ejected electrons but by oxidation of the dianion in its excited state. trans-Stilbene was chosen as a scavenger of electrons in this reaction. Addition of trans-stilbene (2.4 x M) to the photolyzed dianion (lo-' M) revealed in a conclusive way that no electron ejection occurred. The transient spectra of the photolyzed solutions in the presence or absence of trans-stilbene are identical; i.e., the ratio of the AOD a t the maximum
The Journal of Physical Chemistry, Vol. 83, No. 24, 7979 3167
Disodium Salts of Free-Base Porphyrin 01
0
AOD 01
A = 460 nm 1
-3 2
:
:
-07
XI0
450
400
550
600
6M
4.0
20
3.0
15
2.0
IO
1.0
50
7W
A , nm
Figure 3. Transient spectrum of H,TPP2-,2Na+ 0.5 ms after the flash.
20
I
40
30
.In [AOD]
IO
05
IO
15
20
25
I0
20
15
25
30
TIME, ms
Figure 5. Kinetics of the dark reaction for [H,TPP2-,2Na+] = 3.6 X 10-7 M.
20
0
05
30
TIME, ms
Figure 4. Kinetics of the dark reaction for [H,TPP2-,2Na+] = 1.1 X 10-7 M.
bleaching of the dianion (A,, 436 nm) to the AOD a t A 480 nm (maximum absorption of the trans-stilbene radical anion) remains constant with or without trans-stilbene. The kinetics of the transient disappearance during the dark period is very useful in establishing the identity of the transient. For example, the bleached dianion was recovered in a time scale of 2 ms. This long lifetime ruled out the possibility that the transient is the singlet excited state of the dianion. More than that, the transient disappearance obeys a combination of first- and second-order kinetics; a t low concentration of the dianion (1.1 X M) the return as seen in Figure 4 is first order with a slope of 425 SK Increasing I. the concentration of the bleached dianion to about 3.6 X M leads to a large deviation from first-order kinetics in such a way that about 70% of the reaction obeyed second-order kinetics (see Figure 5). The above observation led us to believe that the transient observed with A,, 460 nm is the excited triplet dianion
where the rate constant of triplet-triplet annihilation is about 3 X lo9 M-' The first-order kinetics of the triplet annihilation was found to be 425 s-l and might correspond to self-quenching or quenching by impurities. Protonation of HzTPP2-,2Na+. Protonation of H2TPP2-,2Na+in THF was carried out under high vacuum by degassed methanol. More than a 1000-fold excess of methanol over the dianion concentration was used to quantitatively convert the dianion to the protonation products. Spectrophotometric analysis of the products indicated that about 70% of the original H z T P P was recovered. The protonation mixture did not include any chlorin (H,TPC) or isobacteriochlorin (H2TPisoB). This conclusion was drawn by comparing the absorption spectra of the protonation products with the respective absorption spectrum of H2TPC and H,TPisoR published elsewhere.12 Mass spectrometric analysis of the protonation products after evaporation of the solvent indicated the existence of a molecular peak a t mle 620 and a bigger peak at mle 614. The last one is the molecular peak of the free-base porphyrin which was previously identified in the protonation mixture by its optical absorption. The first molecular peak a t mle 620 is the protonated product of the dianion in which six extra hydrogens were added to the parent molecule. The existence of both H2TPP and H8TPP requires some comments. One possibility is that the source of H z T P P might be due to the liberation of H 2 from the initial product of the protonated dianion, i.e. H2TPP2-,2Na++ 2CH30H [H,TPP] + 2CH30-,Na+ HdTPP
-
H2TPP
+ H2
The uncondensable gas products of this reaction were analyzed by a Toepler technique. No H2 or other gases with a boiling point below that of liquid air were detected. An alternative explanation is that during the protonation the
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The Journal of Physical Chemistry, Vol. 83,
No. 24, 1979
dihydro free-base porphyrin obtained as an intermediate might participate in an electron-transfer reaction by accepting electrons from the unprotonated dianion, thus forming the dianion of the dihydro compound and the free-base porphyrin. This electron transfer and subsequent protonation may continue further, i.e. H2TPP2-,2Na++ 2CH30H
+ 2CH30-,Nae H4TPP2-,2Na"+ H2TPP
-
- +
H4TPP + HzTPP2-,2Na+
H4TPP
H4TPP2-,2Na++ 2CH30H
+ 2CH30-,Na+,etc.
or a disproportionation reaction might take place to form H2TPP and H8TPP, for example, H2TPP2-,2Na++ CH30H H3TPP-.,Na+ CH30-,Na+ --t
-
+
H4TPP + H2TPP2-,2Na+
-
H4TPP + HzTPP2-,2Na+ H4TPP2-,2Na+t CH,OH
T h e nature of t h e protonation products of H2TPP2-,2Na+ is completely different from that of ZnTPP2-,2Na+ reported by Closs and Clom2 They observed the formation of phlorin and a subsequent slow transformation to chlorin. The protonation experiment of ZnTYP2-,2Na+was repeated in our laboratory and the results were consistent with those reported by Closs. We cannot suggest any explanation for the difference in the behavior toward protonation between H2TPP2-,2Na+and ZnTPP2-,2Na a t this moment. However, we are planning to investigate it in the future. +
HsTPP
2H3TPP-.,Na+
Marcoux, van Swaay, and Setser
Acknowledgment. The support of this study by the National Science Foundation is gratefully acknowledged. Also, we thank Pr. H. Wang for his valuable suggestions during the experimental phase of this research. References and Notes K. M. Smith, Ed., "Porphyrins and Metalloporphyrins", Elsevier, Amsterdam, 1975. G. L. Closs and L. E. Closs, J . Am. Chem. Soc., 85, 818 (1963). J. W. Dodd and N. S. Hush, J . Chem. Soc., 4607 (1964). R. H. Feiton and H. Linschitz, J. Am. Chem. SOC.,88, 1113 (1966). M. Szwarc and G. Levin, J . Photochem., 5, 119 (1976). M. Szwarc and G. Levin in "Protons and Ions Involved in Fast Dynamic Phenomena", Elsevier, Amsterdam, 1978. G. Ramme, M. Fisher, S. Claesson, and M. Szwarc, Proc. R . SOC. London, Ser. A , 327, 467 (1972). G. Levin, J . Phys. Chem., 82, 1584 (1978). L. Pekkarinen and H. Linschitz, J. Am. C h m . Soc., 82, 2407 (1960). G. Levin, E. E. Hoiloway, and M. Szwarc, J . Am. Chem. SOC.,98, 5706 (1976). E. D. Lillie, D. Van Ooteghem, G. Levin, and M. Szwarc, Chem. Phys. Lett., 41, 216 (1976). Y. Harel, Thesis, submitted to the Scientific Council of the Weizmann Institute of Science, Rehovot, Israel, July 1978. G. Levin, J. Jagur Grodzinski, and M. Szwarc, J . Am. Chem. Soc., 92, 2269 (1970). N. H. Vetthorst, Doctoral Thesis, Free University of Amsterdam, 1963.
+
H4TPP2-,2Na+ HzTPP
H5TPP--,Na++ CH30-,Na+
2H5TPP-.,Nat -* H8TPP
+ H21'PP, etc.
Protonation of the dianion derived from an aromatic hydrocarbon followed by subsequent electron transfer or partial protonation followed by disproportionation was previously reported by Szwarc et al.I3 and Ve1th0rst.l~ Analysis of the protonation products in conjunction with the calculated ones using charge balance required that the reaction products include 662/3% of H2TPP and 331/3% of H'TPP. Actually, 70% of' H2TPP was found which is in good agreement with the expected percentage.
Vibrational Relaxation of CO+(A*n,), CS(A'II), and C2(A311,) in Helium P. J. Marcoux,+ M. van Swaay, and D. W. Setser* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received May 7, 1979) Publication costs assisted by the Department of Defense
Nonequilibrium vibrational distributions of CO+(A2n,u'=0-6),CS(A1rI,u'=&5),and Cz(A3TI,u'=M)were prepared by collisional processes in a 300-I< helium flowing-afterglow apparatus. The vibrational band intensities of the electronic emission systems were used to obtain the steady-state vibrational distributions from 0.8 to 15 torr. Extensive vibrational relaxation by collisions with He was observed for CO"(A) and CS(A), but not for C,(A), over this pressure range, Electronic quenching of CS(A)probably is competitive with vibrational relaxation, even in helium. The data were fitted to relaxation models based upon Au = 1 collisional transitions by using the steady-state master equation formulation. The Au = 1 relaxation cross sections for CO+(A)and CS(A) with He are in the range of 0.01 of the gas kinetic values. The upper limit to the 4 u = -1 relaxation cross section for C,(A) is 5 X of the gas kinetic value. Studies of the relaxation of CS(A'n) in Ar were attempted, but electronic quenching appeared to dominate over vibrational relaxation. These results are compared to vibrational-translational relaxation of other electronically excited states.
Introduction Vibrational to translational energy transfer in the ground electronic state has been extensively and is reasonably well understood, There is much less information about V-T energy transfer in electronically excited states, but much of the available dat~5-15suggest that V-T relax-
'Hewlett-Packard Laboratories, Palo Alto, CA
94304.
0022-3654/79/2083-3168$01 .OO/O
ation can be much faster for excited electronic states than for the ground state, even for molecules with large energy spacings between vibrational levels. In orle case, Li,(B), vibrational relaxation even competed successfully with rotational relaxation.' 111 the Present work the m&astable rare gas atom flowing-afterglow technique was utilized to produce CO+(A211),CS(A'II), and Cz(A311) in nonequilibrium vibrational levels and vibrational refaxation was ob0 1979 American Chemical Society
