Resonance Raman Studies of Porphyrin Radical Cations, Excited

17 Resonance Raman Studies of Porphyrin Radical Cations, Excited States, and Ligation Photodynamics 1

2

2

3

2

Dongho Kim , Lisa Miller , Oliver Su , James Terner , and Thomas G. Spiro 1

Korea Standards Research Institute, Taedok Science Town, Taejon, Cungnam, Korea Department of Chemistry, Princeton University, Princeton, NJ 08544 Department of Chemistry, Virginia Commonwealth University, Richmond, VA 23284

2

Downloaded by CORNELL UNIV on August 5, 2016 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch017

3

With a recirculating electrochemical cell i t has been possible to obtain good quality resonance Raman spectra of the radical cations of several metallo-octaethyl porphyrins, with both B- and Q-band excitation. Characteristic frequency shifts are seen upon radical formation, whose patterns clearly distinguish a from a radicals. The directions of the shifts, however, do not accord with simple bonding arguments based on orbital symmetry. The first porphyrin triplet state RR spectra are reported, for zinc, magnesium, and palladium tetraphenylporphines, using a simple cw laser technique involving a comparison of spinning and stationary (photopumped) samples. The frequency shifts upon triplet formation are very small for the porphyrin skeletal modes. There is evidence, however, for extensive broadening of a phenyl mode, possibly reflecting a heterogeneous distribution of phenyl rotamers in the excited state. The same stationary sample cw laser technique has been used to monitor excited state deligation of nickel tetraphenylporphine (NiTPP) in pyridine. Saturation behavior is observed, from which the quantum yield ratio of photoinduced ligation of NiTPP to photo-deligation of NiTPP(py) is estimated to be 0.05. NiTPP in piperidine gives evidence for the formation of a 5- as well as a 4coordinate species via photodeligation of the i n i t i a l 6coordinate adduct. At equilibrium very l i t t l e 5-coordinate adduct is formed at any piperidine concentration, but for nickel tetrakis(p-cyanophenyl)porphine a significant equilibrium fraction is present as 5-coordinate adduct in 1M piperidine. The v RR frequencies support a high-spin configuration for the 5-coordinate adduct, consistent with previous theoretical calculations. 1u

2u

2

4

Resonance Raman (RR) s p e c t r o s c o p y has been a p p l i e d e x t e n s i v e l y t o met a l l o p o r p h y r i n s and heme p r o t e i n s , and has p r o v i d e d u s e f u l i n f o r m a t i o n about m o l e c u l a r and e l e c t r o n i c s t r u c t u r e (jL) . The power o f t h e t e c h n i q u e w h i c h l i e s i n the s e n s i t i v i t y o f m o l e c u l a r v i b r a t i o n a l 0097-6156/ 86/ 0321 -0248$06.00/ 0 © 1986 American Chemical Society

Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

17.

KIM

ETAL.

Resonance Raman Studies

f r e q u e n c i e s to changes i n s t r u c t u r e , and the s e l e c t i v e enhancement of a l i m i t e d number of v i b r a t i o n a l modes v i a c o u p l i n g w i t h the e l e c t r o n i c t r a n s i t i o n w i t h w h i c h the l a s e r e x c i t a t i o n i s i n resonance. RR s p e c t r o s c o p y i s a p p l i c a b l e as w e l l to e x c i t e d s t a t e s o f p o r p h y r i n s and to p r i m a r y e l e c t r o n t r a n s f e r p r o d u c t s , p r o v i d e d t h a t a p p r o p r i a t e methods are used f o r the g e n e r a t i o n and s a m p l i n g o f these u n s t a b l e s p e c i e s . The t e c h n i q u e can p r o v i d e s t r u c t u r a l i n f o r m a t i o n about t h e s e h i g h energy m o l e c u l e s and can be a p p l i e d as a m o n i t o r t o probe t h e i r i n v o l v e m e n t i n r e a c t i n g systems. We have examined RR s p e c t r a of a s e r i e s of r a d i c a l c a t i o n s gene r a t e d from m e t a l l o - o c t a e t h y l p o r p h y r i n s (OEP's). OEP's resemble phy s i o l o g i c a l p o r p h y r i n s i n h a v i n g a l i p h a t i c s u b s t i t u e n t s a t the p y r r o l e p o s i t i o n s and hydrogen atoms a t the methine p o s i t i o n s . T h e i r r a d i c a l c a t i o n s show an i n t e r e s t i n g dichotomy r e s u l t i n g from the near degene r a c y of the h i g h e s t f i l l e d a i and a2u o r b i t a l s ( 2 ) . Depending on the c e n t r a l m e t a l and i t s l i g a n d s , the h o l e can r e s i d e on one or the o t h e r of these o r b i t a l s , g i v i n g r i s e to markedly d i f f e r e n t EPR (3) and a b s o r p t i o n s p e c t r a ( 4 ) . We f i n d t h a t the RR f r e q u e n c y s h i f t p a t t e r n s are s i m i l a r l y d i s t i n c t i v e f o r these two c l a s s e s of r a d i c a l c a t i o n s , but t h a t the d i r e c t i o n s of the s h i f t s a r e not what one would e x p e c t from s i m p l e o r b i t a l symmetry and bonding arguments ( 5 ) . M e t a l l o p o r p h y r i n e x c i t e d s t a t e s have an e x t r e m e l y wide range of l i f e t i m e s . When d - o r b i t a l v a c a n c i e s on a c e n t r a l m e t a l l i e a t e n e r g i e s lower than the π-π* s t a t e s , the l i f e t i m e s become e x c e e d i n g l y s h o r t , and f a s t p u l s e d methods are needed f o r t h e i r d e t e c t i o n . Down s h i f t s and b r o a d e n i n g i n the p i c o s e c o n d RR spectrum o f the oxy-hemog l o b i n p h o t o p r o d u c t have been i n t e r p r e t e d as a r i s i n g from u n r e l a x e d e l e c t r o n i c e x c i t a t i o n of the deoxy-heme ( 6 ) . When d o r b i t a l v a c a n c i e s a r e u n a v a i l a b l e , however, the l i f e t i m e of the l o w e s t t r i p l e t π-π* s t a t e , formed by e f f i c i e n t i n t e r s y s t e m c r o s s i n g from the i n i t i a l l y e x c i t e d s i n g l e t l e v e l s , can be q u i t e l o n g (microseconds to m i l l i s e c o n d s ) . We have found t h a t , i n f a v o r a b l e c a s e s , RR s p e c t r a of m e t a l l o p o r p h y r i n t r i p l e t s t a t e s can be o b t a i n e d w i t h cw l a s e r e x c i t a t i o n by the s i m p l e e x p e d i e n t of e x c i t i n g s t a t i o n a r y samples; the ground s t a t e RR spectrum, o b t a i n e d by s p i n n i n g the same sample, s e r v e s as a r e f e r e n c e ( 7 ) . Frequency s h i f t s upon t r i p l e t f o r m a t i o n f o r z i n c , magnesium and p a l l a d i u m TPP*s are found to be s m a l l , but t h e r e i s e v i d e n c e f o r a heterogeneous d i s t r i b u t i o n o f rotamers of the p h e n y l r i n g , c o n s i s t e n t w i t h i n c r e a s e d p h e n y l c o n j u g a t i o n i n the excited states. The same s t a t i o n a r y sample cw t e c h n i q u e has been used to s t u d y p h o t o d e l i g a t i o n dynamics f o r NiTPP i n p y r i d i n e (8). The RR s p e c t r a show t h a t the e q u i l i b r i u m between 6- and 4 - c o o r d i n a t e s p e c i e s i s s h i f t e d i n c r e a s i n g l y toward the l a t t e r w i t h i n c r e a s i n g l a s e r power; the e f f e c t c o l l a p s e s when the sample i s spun. The d a t a i n d i c a t e a much lower quantum y i e l d f o r l i g a t i o n than f o r d e l i g a t i o n v i a the p h o t o e x c i t e d s p e c i e s . For NiTPP i n p i p e r i d i n e t h e r e i s RR e v i d e n c e t h a t p h o t o d e l i g a t i o n produces a 5- as w e l l as 4 - c o o r d i n a t e p r o d u c t . The e q u i l i b r i u m f r a c t i o n o f a 5 - c o o r d i n a t e adduct i s v e r y s m a l l a t any p i p e r i d i n e c o n c e n t r a t i o n ; but when para-cyano groups a r e s u b s t i t u t e d on the TPP p h e n y l r i n g s t h e r e b y i n c r e a s i n g the a x i a l l i g a n d a f f i n i t y , the 5 - c o o r d i n a t e f r a c t i o n becomes r e a d i l y d e t e c t a b l e a t 1M p i p e r i d i n e ( 9 ) . The RR f r e q u e n c i e s p o i n t t o a h i g h - s p i n c o n f i g u r a t i o n f o r the 5 - c o o r d i n a t e a d d u c t , c o n s i s t e n t w i t h t h e o r e t i c a l e x p e c t a t i o n s based on the c a l c u l a t i o n s o f Ake and Gouterman (10). u


249


P O R P H Y R I N S : E X C I T E D STATES A N D D Y N A M I C S

250


Experimental Metalloporphyrins were purchased from Mid-Century Chemicals and p u r i f i e d by chromatography on alumina (A540) to remove fluorescent im p u r i t i e s . Radical cations were prepared i n s i t u v i a controlled po t e n t i a l e l e c t r o l y s i s i n dichloromethane with tetrabutylammonium perchlorate supporting e l e c t r o l y t e . A standard photoelectrochemical c e l l was modified with connections f o r v i t o n tubing connected to a glass c a p i l l a r y serving as a Raman c e l l , through which the solution was flowed v i a a p e r i s t a l t i c pump. The RR spectra were recorded at 90° from the c a p i l l a r y . The extent of oxidation was monitored by coulometry and UV-vis absorption spectroscopy. The t r i p l e t state and photodeligation studies were carried out with a standard NMR spinning c e l l i n backscattering. Raman spectra were recorded with a computer-controlled Spex 1401 double monochromator equipped with a cooled photomultiplier and pho ton counting e l e c t r o n i c s . Excitation was provided by Spectra Physics K r and A r cw l a s e r s . +

+

Results and Discussion OEP Radical Cations. Figure 1 shows RR spectra, with 406.7nm e x c i t a t i o n , of ZnOEP during i t s gradual conversion to r a d i c a l cation v i a e l e c t r o l y s i s (5). ZnOEP fluoresces from i t s lowest excited s i n g l e t state and RR spectra with excitation i n the Q-band are obscured by the emission, but the background at 406.7nm i s n e g l i g i b l e . This wavelength i s nearly coincident with the ZnOEP Β absorption band (X =404nm). For ZnOEP- this band i s at 391nm and i s only 0.6 times as strong; consequently the enhancement i s considerably lower, as evidenced by the higher noise l e v e l of the spectrum. Nevertheless, the r e l a t i v e i n t e n s i t i e s of the bands are similar for the r a d i c a l and the neutral porphyrin, and the frequencies are readily correlated. Large (19-24cm~ ) up- and downshifts are seen for V 4 and V 2 , at 1373 and 1580cm i n ZnOEP (the porphyrin s k e l e t a l mode assignments are from reference (11), while smaller downshifts are seen for V 3 (1483cm" ) and Vi$ (1614cm- ); V u , at 1557cm" , becomes hidden by the downshifted \>2 band of t h e r a d i c a l cation. The EPR spectrum of ZnOÇP* shows i t to have a hole i n the a i o r b i t a l , while that of NiOEP· i s assignable to an a2 r a d i c a l (3). Figure 2 compares the B-band excited spectra of NiOEP and NiOEP^. Large s h i f t s are again observed for the dominant bands, V 4 (1382cm ) and V 2 ( 1600cm"" ) , but now they are i n the opposite d i r e c t i o n s , down and up, respectively, to those shown by ZnOEP·. Smaller downshifts are again seen f o r V 3 (1519cm" ) and V 1 0 (1655cm" ). For NiOEP and i t s r a d i c a l cation i t was possible to obtain Q-band excited spectra, at 514.5 and 568.2nm, allowing i d e n t i f i c a t i o n and c o r r e l a t i o n of the non-totally symmetric A2 , Big and B2g modes (1) i n the same f r e quency region. Their s h i f t s were found to be quite small. RR spectra were obtained for several other OEP r a d i c a l cations for which EPR and/or o p t i c a l spectra had indicated an a ^ or a 2 assignment: 03,4·) MgQEP- ( a i ) , CuOEP. (a2u) , Co 0EP · · (Br") ( a l ) and max

1

-1

1

1

1

+

u

u

1

1

1

g

u

u

i:[I

u

2

u

T h e

Co OEP- (C&04"")2 ( a 2 ) » frequencies and s h i f t s on r a d i c a l formation are given In Table I. u


KIM E T A L .


4067 Â

EX.


ZnODOEPt

J

1300

ι

I

1500 cm"

I

J

1700

1

Figure 1. RR spectra with 406.7nm excitation for ZnOEP (0.5mM, bottom) i n CH2CJI2 (50mW incident laser power) and for solutions with increasing amounts of monocation r a d i c a l formed by elec t r o l y s i s at 0.85V vs. SCE (15mW). Each scan was obtained with fresh sample (Reproduced from Ref. 5„ Copyright 1986 American Chemical Society ) 0



252

4 0 6 7 AE X .

1623 ^

1

1

1

1


P O R P H Y R I N S : E X C I T E D STATES A N D

260

DYNAMICS

NiTPP in Pyridine 406.7 nm Ex. CD

O)

CO

CO

J

1


*

1300

1340

S

1380

H Raman Shift(cm ) F i g u r e 5. V 4 Raman peaks f o r a s t a t i o n a r y sample o f NiTPP (0.2mM) i n p y r i d i n e , w i t h 406.7nm e x c i t a t i o n . Arrows i n d i c a t e the d e c r e a s i n g (1346cm" ) and i n c r e a s i n g (1369cm" ) r e l a t i v e band i n t e n s i t i e s a t i n c r e a s i n g l a s e r power l e v e l s (1.5, 5, 10, 15, 20, 25, 50, 75, 90,125, and 140mW). I n the absence o f an i n t e r n a l s t a n d a r d ( p y r i d i n e peaks a r e obscured by those o f NiTPP) the s p e c t r a were s c a l e d t o w h i c h e v e r peak was s t r o n g e r . (The i n t r i n s i c s c a t t e r i n g f a c t o r s a r e not the same f o r the two peaks.) S p e c t r a were o b t a i n e d v i a b a c k s c a t t e r i n g (135°)from a NMR sample tube, u s i n g a Spex 1401 double monochromator: 7cm" s l i t w i d t h , l s e c time c o n s t a n t , 0.5cm /sec scan r a t e . (Reproduced from Refο 8ο C o p y r i g h t 1986 American Chemical S o c i e t y . ) 1

1

1

_1


Gouterman et al.; Porphyrins ACS Symposium Series; American Chemical Society: Washington, DC, 1986. 0

C

1

1

F i g u r e 6. The 1369αη~ /1346αιΓ Raman peak h e i g h t r a t i o f o r NiTPP i n p y r i d i n e v s . 406.7nm l a s e r power f o r a s t a t i o n a r y sample (*) (see s p e c t r a i n F i g u r e 1) and a s p i n n i n g sample ( 0 ) . (Reproduced from R e f 8 C o p y r i g h t 1986 American C h e m i c a l S o c i e t y . )




262

Figure 7. V 4 Raman peaks f o r a stationary sample of NiTPP (0.5mM) i n piperidine, with 406.7nm excitation. Arrows indicate the increasing r e l a t i v e i n t e n s i t i e s of the 1356 and 1369cm" peaks with increasing laser power (5, 15, 25, 50, 75, 100, 125, and 150mW): 5cm" s l i t w i d t h , lsec time constant, O.Scm^^/sec scan rate. The spectra were scaled to the 1346cm"" peak. CReproduced from Ref« 8„ Copyright 1986 American Chemical Society.) 1

1

1


KIM ETAL.


413.1 nm Ex. Ni[TPP(CN) ]/GH CI + Pip 2

2


4

1300 ΔΡ/cm"

1

Figure 8. V 4 region of the RR spectrum of Ni[TPP(CN)4] (0.5mM, spinning sample) i n CH2C£2> with piperidine added to a con centration of 1 and 2M, and i n pure piperidine (10M).



264

a t 1355cm"" , i n a d d i t i o n t o t h e 1348 and 1 3 7 3 c m V4 bands o f t h e 6- and 4 - c o o r d i n a t e s p e c i e s ( 9 ) . The 1 3 5 5 c m band reaches a m a x i mum a t 1M p i p e r i d i n e , and d e c r e a s e s a g a i n a t h i g h e r p i p e r i d i n e conc e n t r a t i o n s , l e a v i n g a weaker band a t 1360cm" . The l a t t e r band, w h i c h i s d e p o l a r i z e d , i s a s s i g n e d t o mode v g (B2g) (J-Sa) o f 6 - c o o r d i n a t e complex. The superimposed 1355cm"" band seen i n 1M p i p e r i d i n e , which i s p o l a r i z e d , i s assigned to V 4 of the 5-coordinate adduct. The f r e q u e n c y o f t h i s band i s much c l o s e r t o t h a t o f t h e 6 - c o o r d i n a t e than o f t h e 4 - c o o r d i n a t e complex, as expected i f the 5as w e l l as the 6 - c o o r d i n a t e s p e c i e s has a h i g h - s p i n c o n f i g u r a t i o n , c o n s i s t e n t w i t h t h e c a l c u l a t i o n s o f Ake and Gouterman ( 1 0 ) . A l a r g e d e c r e a s e i n the V4 f r e q u e n c y upon a d d i n g p i p e r i d i n e l i g a n d s i s b e l i e v e d t o be a s s o c i a t e d w i t h an e x p a n s i o n o f p o r p h y r i n c o r e due t o p r o m o t i o n o f an e l e c t r o n t o t h e i n - p l a n e a n t i b o n d i n g d 2 - y 2 o r b i t a l i n the h i g h - s p i n c o n f i g u r a t i o n ; p o r p h y r i n s k e l e t a l mode f r e q u e n c i e s a r e known t o be s e n s i t i v e t o t h e c o r e - s i z e , as w e l l as t o t h e e l e c t r o n i c e f f e c t s ( 2 3 ) . The p o r p h y r i n c e n t e r - t o - p y r r o l e n i t r o g e n d i s t a n c e (C^-N) i s ^ 1.95Â i n 4 - c o o r d i n a t e N i p o r p h y r i n s (24) , b u t 2.038Â i n the b i s - i m i d a z o l e adduct o f N i - t e t r a k i s ( m e t h y l p y r i d y l ) p o r p h i n e ( 2 9 ) . No s t r u c t u r e i s a v a i l a b l e f o r a 5 - c o o r d i n a t e N i p o r p h y r i n , b u t t h e 7cm"" i n c r e a s e i n V4 r e l a t i v e t o t h a t o f t h e 6 - c o o r d i n a t e ( p i p ) N i T P P s u g g e s t s p a r t i a l r e l a x a t i o n o f the c o r e s i z e due t o the o u t - o f - p l a n e d i s p l a c e m e n t o f the N i atom expected f o r a 5 - c o o r d i n a t e h i g h - s p i n adduct, by analogy w i t h the r e l a x a t i o n seen f o r (2-MeImH)FeTPP (2.045À C -N; Fe 0.5Â o u t - o f - p l a n e ) (30) r e l a t i v e t o h i g h - s p i n (THF) FeTPP (2.057Â C -N, Fe i n - p l a n e ) ( 3 1 ) . 1

-1

-1

1

t

n

e

2

1


x

1

2

t

2

t

Acknowledgments T h i s work was s u p p o r t e d by g r a n t DE-AC02-81ER10861 from the U.S. Department o f Energy. We thank P r o f e s s o r Dewey H o l t e n f o r h e l p f u l discussions.

Literature Cited 1. Spiro, T. G. "Iron Porphyrins"; Lever, A. B. P., and Gray, H. B., Eds.; Addison-Wesley: Reading, MA, 1983, Part Two, pp. 85-159. 2. Gouterman, M. In "The Porphyrins"; Dolphin, D., Ed.; Academic Press, New York, 1978, Vol. III, Chapter 1. 3. Fajer, J.; Davis, M. S. In "The Porphyrins"; Dolphin, D., Ed.; Academic Press, New York, 1978, Vol. IV, pp. 197-256. 4a. Dolphin, D.; Muljiani, Z.; Rousseau, K.; Borg, D. C.; Fajer, J.; Felton, R. H. Ann. N.Y. Acad. Sci. 1973, 206, 177. 4b. Fuhrhop, J.-H.; Mauzerall, D. J. Am. Chem. Soc. 1969, 91, 4174. 5. Kim, D. H.; Miller, L. Α.; Rakhit, G.; Spiro, T. G. J. Phys. Chem. 1986 (in press). 6. Terner, J.; Voss, D. S.; Paddock, C.; Miles, R. B.; Spiro, T. G. J. Phys. Chem. 1982, 86, 859.



17.

KIM ET A L .


265

7. Kim, D.; Terner, J.; Spiro, T. G. J. Am. Chem. Soc. 1986 (in press). 8. Kim, D.; Spiro, T. G. J. Am. Chem. Soc. 1986 (in press). 9. Kim, D.; Su, Y. O . , ; Spiro, T. G. (submitted). 10. Ake, R. L.; Gouterman, M. Theoret. Chim. Acta 1970, 17, 408. 11a. Kitagawa, T.; Abe, M.; Ogoshi, H. J. Chem. Phys. 1978, 69, 4516. 11b. Abe, M.; Kitagawa, T.; Kyogoku, Y. J. Chem. Phys. 1978, 69, 4526. 12. Maggiora, G. M. J. Am. Chem. Soc. 1973, 95, 982. 13a. Yamaguchi, H.; Nakano, M.; Itoh, K. Chem. Lett. 1982, 1397. 13b. Yamaguchi, H.; Soeta, Α.; Toeda, H.; Itoh, K. J. Electroanal. Chem. 1983, 159, 347. 14. Spaulding, L. D.; Eller, P. G.; Bertrand, J. Α.; Felton, R. H. J. Am. Chem. Soc. 1974, 96, 982. 15. Barkigia, K. M.; Spaulding, L. D.; Fajer, J. Inorg. Chem. 1983, 22, 349. 16. Darwent, J. R.; Douglas, P.; Harriman, Α.; Porter, G.; Richoux, M.-C., Coord. Chem. Rev. 1982, 44, 83. 17. Harriman, A. J. Chem. Soc., Faraday Trans. 2 1981, 97, 3660. 18a. Stein, P.; Ulman, Α.; Spiro, T. G. J. Phys. Chem. 1984, 88, 369-374. 18b. Burke, J. M.; Kincaid, J. R.; Spiro, T. G. J. Am. Chem. Soc. 1978, 100, 6077. 19. LaMar, G. N.; Eaton, G. R.; Holm, R. H.; Walker, F. A. J. Am. Chem. Soc. 1973, 95, 63. 20. Eaton, S. S.; Eaton, G. R. J. Am. Chem. Soc. 1975, 97, 3660. 21. Walker, F. Α.; Hui, E.; Walker, J. M. J. Am. Chem. Soc. 1975, 97, 2390. 22a. Kim, D.; Kirmaier, C.; Holten, D. Chem. Phys. 1983, 75, 305. 22b. Kim, D.; Holten, D. Chem. Phys. Lett. 1983, 98, 584. 23. Choi, S.; Spiro, T. G.; Langry, K. C.; Smith, K. M.; Budd, D. L.; LaMar, G. N. J. Am. Chem. Soc. 1982, 104, 4345. 24. 1.929Å in tetragonal (D2d, ruffled) (25) and 1.958Å in triclinic (D4h, planar) (26) Ni octaethylporphyrin; 1.960Å in Ni deuteroporphyrin (27), 1.957Å in Ni etioporphyrin (28). 25. Meyer, E. F., Jr. Acta Crystallogr. Sect. Β. 1972, 28, 2162. 26. Gullen, D. L.; Meyer, E. F., Jr. J. Am. Chem. Soc. 1974, 96, 2095. 27. Hamor, T. Α.; Caughey, W. S.; Hoard, J. L. J. Am. Chem. Soc. 1965, 87, 2305. 28. Fleischer, Ε. B. J. Am. Chem. Soc. 1963, 85, 146. 29. Kirner, J. F.; Garofalo, J., Jr.; Scheldt, W.R. Inorg. Nucl. Chem. Lett. 1975, 11, 107. 30. Hoard, J.L.; Scheldt, W. R. Proc. Natl. Acad. Sci., USA, 1973, 70, 3913; 1974, 71, 1578. 31. Reed, C. Α.; Mashiko, T.; Scheldt, W. R.; Spartalian, K.; Lang, G. J. Am. Chem. Soc. 1980, 102, 2302. R E C E I V E D April 21, 1986


Resonance Raman Studies of Porphyrin Radical Cations, Excited

Recommend Documents