Picosecond Excited-State Relaxation of Some Iron Porphyrins and

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 1, 2016 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch012

Picosecond Excited-State Relaxation of Some Iron Porphyrins and Hemoproteins 1

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Karl D. Straub and Peter M. Rentzepis 1

John L. McClellan Memorial Veterans' Hospital and University of Arkansas for Medical Sciences, Little Rock, AR 72205 Department of Chemistry, University of California at Irvine, Irvine, CA 92664

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The excited state spectra and kinetics of decay of cytochrome c, myoglobin and Fe(II) protoporphyrin IX dimethyl ester (Fe(II)ProtoDME) have been studied by picosecond spectroscopy. The existence of different oxidation states, spin multiplicities, and axial ligands in these compounds affords a way to study the effects of different electronic configurations on the excited state decay. The simplest complex, FeII Proto DME, in benzene (no axial ligands) and spin state, S=2, has an excited state absorption band, 450-500 nm, which decays with a time constant of about 10 psec while the ground state Q band does not reappear until 45 psec. Similar kinetics are seen in Fe(II) cytochrome c and on a somewhat slower time scale with Fe(II) myoglobin. These kinetics indicate a rapid decay of the excited state with relaxation through the singlet manifold (S=0) or the triplet manifold (S=2) by way of π* - d or d - d intermediate transitions. Excitation of the bipyridine complex of Fe(II) Proto DME results in return to ground state in ~40 psec with formation of the ground-state 5 ligand species. Return to the 6 ligand ground state occurs in 250-300 psec. Fe(III) porphyrins regardless of spin state (S=1/2; S=5/2) have relaxation kinetics which are much faster than the resolution of our experimental conditions. The l a r g e number o f e l e c t r o n i c c o n f i g u r a t i o n o f i r o n p o r p h y r i n s w i t h o x i d a t i v e s t a t e s o f 2+ o r 3+, h i g h and low s p i n f o r m s , and charge t r a n s f e r s t a t e s w i t h d i f f e r e n t a x i a l l i g a n d s o f f e r t h e p o s s i b i l i t y o f a number o f n o n - r a d i a t i v e decay pathways (1_). In

0097-6156/ 86/ 0321 -0168S06.00/ 0 © 1986 A m e r i c a n C h e m i c a l Society

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

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STRAUB A N D RENTZEPIS

Picosecond Excited-State Relaxation

p a r t i c u l a r , t h e e f f e c t o f s p i n s t a t e on i n t e r s y s t e m c r o s s i n g r a t e and t h e a v a i l a b i l i t y o f charge t r a n s f e r s t a t e s o f t h e i r o n p o r p h y r i n s makes i t p o s s i b l e t h a t t h e r e w i l l be d i f f e r e n t r e l a x a t i o n mechanisms f o r v a r i o u s i r o n p o r p h y r i n complexes. E l e c t r o n i c e x c i t e d s t a t e s i n i r o n p o r p h y r i n s a r e thought t o r e l a x rapidly. In g e n e r a l , t h e s e compounds have been c l a s s i f i e d as " n o n - r a d i a t i v e " {2) s i n c e t h e i r luminescence i s n o t d e t e c t a b l e by o r d i n a r y means. However, by u s i n g l i n e w i d t h measurements and background f l u o r e s c e n c e from resonance Raman s p e c t r o s c o p y , Adar e t al_ (3) have been a b l e t o determine t h a t reduced cytochrome b$ has a weak luminescence (quantum y i e l d 1 0 ~ - Ι Ο " ) w h i l e o x i d i z e d cytochrome b$ and myoglobin s h o u l d have no luminescence i n t h e v i s i b l e r a n g e . D i r e c t measurement o f r e l a x a t i o n o f t h e e x c i t e d s t a t e o f i r o n p o r p h y r i n s and hemoproteins c o n f i r m s t h a t t h e F e ( I I I ) complexes have v e r y f a s t r e l a x a t i o n s ( π* t r a n s i t i o n s a r e a v a i l a b l e even w i t h o u t a x i a l l i g a n d s . Based on the temperature independent quadrapole s p l i t t i n g i n the Mossbauer s p e c t r a D o l p h i n e t al_ (iLL) c o n c l u d e d t h a t t h e r e a r e no l o w - l y i n g e x c i t e d s t a t e s i n the square p l a n a r F e ( I I ) o c t a e t h y l p o r p h y r i n s . C a l c u l a t i o n s by Z e r n e r and Gouterman (22) and Z e r n e r e t al^ (23) show t h a t the d o r b i t a l s a r e s h i f t e d i n t o such i n t e r m e d i a t e l e v e l s o n l y i f the F e atom i s d i s p l a c e d from t h e p l a n e o f the p o r p h y r i n r i n g . Such a s h i f t i n d o r b i t a l energy l e v e l s would a l s o r e s u l t i n l i f t i n g the o r b i t a l degeneracy thus a l l o w i n g the h i g h s p i n s t a t e t o e x i s t . This 4 - l i g a n d s t a t e i s known t o be h i g h s p i n , S=2, ( 1 4 j which c o n f i r m s t h a t t h e degeneracy o f the d l e v e l s i s l i f t e d . The placement o f the F e atom o u t s i d e the p l a n e o f the r i n g may a l s o reduce the symmetry from D^ t o C 4 . h

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E v o l u t i o n o f the E x c i t e d S t a t e o f I r o n P o r p h y r i n s : In a l l the F e ( I I ) p o r p h y r i n s s t u d i e d t h e b l u e a b s o r p t i o n i n c r e a s e between 450-500 nm i s s e e n . I t s i n i t i a l l i f e t i m e i s between 20 and 40 psec and i s independent o f s p i n - s t a t e , S=0 or S=2, and the number o f a x i a l l i g a n d s 4 , 5, o r 6 . A b s o r p t i o n i n t h i s r e g i o n i s seen f o r π > π* s i n g l e t and t r i p l e t a b s o r p t i o n f o r o t h e r m e t a l l o p o r p h y r i n s ( 1 6 , 1 8 , 2 4 ) . C e r t a i n l y the assignment o f t h i s a b s o r p t i o n c o u l d be c o m p l i c a t e d by t h e f o r m a t i o n o f ir c a t i o n and anion r a d i c a l s which absorb i n the r e g i o n (25,26) and t h e r e i s a l r e a d y e v i d e n c e f o r ground s t a t e d e r e a l i z a t i o n o f the low s p i n e l e c t r o n o f F e ( I I I ) i n myoglobin CM- complex onto the p o r p h y r i n r i n g (27.). N o n e t h e l e s s , we propose t h a t t h e most l i k e l y o r i g i n o f t h i s band i s the e x c i t e d s t a t e s i n g l e t or t r i p l e t . In most o p e n - s h e l l m e t a l l o p o r p h y r i n s the f i r s t e x c i t e d s i n g l e t s t a t e i s r a p i d l y c o n v e r t e d t o the t r i p l e t s t a t e by i n t e r s y s t e m c r o s s i n g due t o s t r o n g p e r t a b a t i o n s o f the u n p a i r e d s p i n s and/or magnetic e f f e c t s o f unquenched o r b i t a l a n g u l a r momentum. Thus, paramagnetic m e t a l l o p o r p h y r i n s r a r e l y have d e t e c t a b l e f l u o r e s c e n c e (28,29) and p i c o s e c o n d s t u d i e s o f C u ( I I ) , ( 1 8 , 2 0 ) , A g ( I I ) ( 1 8 ) , and C o ( I I ) p o r p h y r i n s ( 3 0 ) , c o n f i r m t h a t the i n t e r s y s t e m c r o s s i n g l > 1 o c c u r s f a s t e r than the e x c i t a t i o n p u l s e s (6-35 psec). In a d d i t i o n t o the l a r g e e f f e c t o f u n p a i r e d e l e c t r o n s on the i n t e r s y s t e m c r o s s i n g , p i c o s e c o n d s t u d i e s have a l s o been a b l e t o demonstrate the added e f f e c t o f o r b i t a l a n g u l a r monetion i n i n c r e a s i n g the i n t e r s y s t e m c r o s s i n g i n i s o e l e c t r o n i c d i a m a g n e t i c o p e n - s h e l l e d s p e c i e s such as P d ( I I ) and P t ( I I ) [19). F o r the i r o n p o r p h y r i n s e x c i t e d s t a t e r e l a x a t i o n i s r a p i d even when the ground s t a t e i s diamagnetic and the unquenched o r b i t a l a n g u l a r momentum i s l e s s than t h a t o f the d i a m a g n e t i c P d ( I I ) complexes which have i n t e r s y s t e m c r o s s i n g r a t e s slow enough (20 psec) t o be measured. I f the i n t e r s y s t e m c r o s s i n g r a t e w i t h d i a m a g n e t i c i r o n p o r p h y r i n s i s slow then t h e o b s e r v e d correspondence i n r e l a x a t i o n t i m e s between S=0 and S=2 i r o n ( I I ) p o r p h y r i n s may not n e c e s s a r i l y i n d i c a t e t h a t the r e l a x a t i o n pathways a r e the same. 2

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The assignment o f r e l a x a t i o n pathway f o r t h e S=0 s t a t e might be l > d 0 'P s t a t e i n v o l v e d . The w*-d CT t r a n s i t i o n s would be c h i e f l y r e s p o n s i b l e f o r quenching t h e l i f e t i m e o f S\. The resonance Raman l i n e w i d t h measurements o f Adar e t a l {3} i n d i c a t e t h a t cytochrome b$ and by analogy o t h e r s i m i l a r cytochromes such as cytochrome c have f i r s t e x c i t e d s t a t e l i f e t i m e s which a r e on o r d e r o f magnitude l o n g e r than h i g h - s p i n deoxyhemoglobin. I t i s p o s s i b l e , t h e r e f o r e , t h a t f o r t h e S=0 i r o n p o r p h y r i n s the r e l a x a t i o n o c c u r s through t h e s i n g l e t m a n i f o l d . We have p o s t u l a t e d t h e same mechanism f o r n o n - r a d i a t i v e decay t o account f o r o u r o b s e r v a t i o n s o f p i c o s e c o n d s p e c t r a o f d i m a g n e t i c N i ( I I ) p r o t o p o r p h y r i n DME. The b l e a c h i n g i n t h e ground s t a t e Q bands can be a s s o c i a t e d w i t h t h e d i s a p p e a r a n c e o f t h e ground s t a t e and t h e decrease i n t h e b l e a c h i n g i s then a s s o c i a t e d w i t h r e t u r n o f t h e ground s t a t e . It s h o u l d be n o t e d , however, t h a t d-d t r a n s i t i o n s may o c c u r on t h e way t o t h e ground s t a t e w i t h o u t b e i n g seen i n t h e v i s i b l e spectrum (31h I t i s d i f f i c u l t to unambigiously assign the r e l a x a t i o n of the 520 - 570 nm b l e a c h i n g t o r e f o r m a t i o n o f t h e ground s t a t e , b u t we b e l i e v e t h a t t h i s i s t h e most l i k e l y e v e n t . Thus, t h e decay pathway o f t h e h i g h s p i n (S=2) F e ( I I ) p o r p h y r i n s would be π » π* (So - S j ) which undergoes i n t e r s y s t e m c r o s s i n g t o t h e t r i p l e t or m u l t i p l e t s t a t e w i t h i n the pulsewidth of the e x c i t a t i o n followed by a decay t o t h e metal p o r p h y r i n charge t r a n s f e r s t a t e s π* > d i n ~ 20 psec and s u b s e q u e n t l y g e n e r a t i o n o f t h e ground s t a t e i n 40 p s e c . The s l o w e r decay o f both s t a t e s i n myoglobin may be due t o s h i f t s i n t h e o r b i t a l e n e r g i e s i n myoglobin which i n c r e a s e t h e energy gaps i n t h e d o r b i t a l s . A l t e r n a t i v e l y t h e 5 t h l i g a n d o f nryoglobin c o u l d be e j e c t e d d u r i n g t h i s t i m e . Loss o f t h e 5 t h l i g a n d would g e n e r a t e t h e 4 l i g a n d s t a t e seen w i t h F e ( I I ) p r o t o p o r p h y r i n DME i n benzene. The Q bands s h o u l d be seen d u r i n g the time t h e l i g a n d i s r e l e a s e d (40-60 psec) b u t no Q band a b s o r p t i o n i s seen o v e r t h a t time u n l e s s i t i s h i d d e n by t h e f i l t e r a t 530 nm. The r e l a x a t i o n k i n e t i c s o f F e ( I I ) p r o t o p o r p h y r i n DME i n p y r i d i n e a r e c o n s i s t a n t w i t h a l i g a n d d i s s o c i a t i o n from 6 l i g a n d t o 5 l i g a n d s t a t e d u r i n g t h e f i r s t 40 psec and w i t h f o r m a t i o n o f t h e 5 l i g a n d ground s t a t e which undergoes r e c o m b i n a t i o n w i t h p y r i d i n e t o r e g e n e r a t e t h e 6 l i g a n d complex i n 250 - 300 psec as has been shown t o be t h e case f o r F e porphyrin with other basic nitrogenous l i g a n d s by Dixon e t al^ ( 8 ) . However, t h e b i p h a s i c absorbance a t 450 nm has a l o n g ^ i v e d T i f e t i m e o f t h e same o r d e r as t h e p o s t u l a t e d 5 l i g a n d ground s t a t e . E i t h e r t h e 450 absorbance i s p a r t o f t h e r e d s h i f t e d S o r e t band o f t h e 5 l i g a n d ground s t a t e o r t h e r e i s a l o n g - l i v e d e x c i t e d s t a t e . We p r e f e r t h e former e x p l a n a t i o n so t h a t t h e 450 absorbance r e p r e s e n t s two d i f f e r e n t s p e c i e s - f i r s t a π > ir* t r a n s i t i o n which l a s t s 40 psec i n a c c o r d w i t h the o t h e r F e ( I I ) p o r p h y r i n s s t u d i e d and s e c o n d l y t h e l o n g e r decay r e p r e s e n t i n g t h e d i s a p p e a r a n c e o f a r e d - s h i f t e d ground s t a t e S o r e t band o f t h e 5 l i g a n d s p e c i e s as t h e 6 l i g a n d ground s t a t e reappears. s




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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

D i s s o c i a t i o n o f a x i a l l i g a n d s has been f o l l o w e d by p i c o s e c o n d s p e c t r o s c o p y f o r a number o f m e t a l l o p o r p h y r i n s . F o r t h e well-known p h o t o d i s s o c i a t i o n o f Ο2 and CO from hemoglobin and myoglobin t h e photoproducts appear v e r y e a r l y < 10 p s e c . D i s s o c i a t i o n o f b a s i c a x i a l l i g a n d s such as p y r i d i n e and p i p e r a d i n e o c c u r s w i t h i n t h e l i f e t i m e o f t h e e x c i t e d s t a t e f o r N i ( I I ) , C o ( I I I ) as w e l l as f o r F e ( I I ) p o r p h y r i n s . Whether t h e e j e c t e d s p e c i e s i s " h o t " w i t h energy from t h e e l e c t r o n i c d e a c t i v a t i o n o f t h e p o r p h y r i n i s n o t known, b u t t h e d i s s o c i a t i o n process does n o t appear t o be dependent upon t h e wavelength o f t h e e x c i t a t i o n p u l s e ( 3 0 , 3 2 ) . The ease o f f o r m a t i o n o f p o r p h y r i n r a d i c a l s and t h e e x i s t e n c e o f ground s t a t e charge t r a n s f e r i n t o t h e a x i a l l i g a n d s (33) makes i t necessary t o keep i n mind t h e p o s s i b i l i t y o f l i g a n d e j e c t i o n accompanied by o x i d a t i o n - r e d u c t i o n d u r i n g t h e e x c i t e d s t a t e s o f metalloporphyrins. From o u r o b s e r v a t i o n s , we f e e l i t i s u n l i k e l y t h a t p h o t o - i n d u c e d redox r e a c t i o n s a r e o c c u r r i n g i n t h e p r e s e n t s t u d i e s . However, a t wavelength < 300 nm p h o t o d i s s o c i a t i o n o f oxyhemoglobin r e s u l t s i n the f o r m a t i o n o f t h e 0^" r a d i c a l a n i o n ( 3 4 ) .

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18. Kobayashi, T., Huppert, D., Straub, K.D., Rentzepis, P.M. J. Chem. Phys., 1979, 70, 1720-26. 19. Kobayashi, T., Straub, K.D., Rentzepis, P.M. Photochem. Photobiol., 1979, 29, 925-31. 20. Kim, D., Holten, D., Gouterman, M. J. Am. Chem. Soc., 1984, 106, 2793-98. 21. Dolphin, D., Sams, J.R., Tsin, T.B., Wong, K.L. J. Am. Chem. Soc., 1976, 98, 6970-75. 22. Zerner, Μ., Gouterman, M. Theor. Chim. Acta, 1960, 4, 44-63. 23. Zerner, M., Gouterman, M., Kobayashi, H. Theor. Chim. Acta, 1966, 6, 363-400. 24. Magde, D., Windsor, M.W., Holten, D., Gouterman, M. Chem. Phys. Letts., 1974, 29, 183-88. 25. Dolphin, D., Muljiani, Z., Rousseau, K. Ann. NY Acad. Sci., 1973, 206, 177-98. 26. Felton, R.H. In "The Porphyrins"; Dolphin, D., Ed.; Academic; New York, 1978; Vol. 5, p. 53. 27. Shulman, R.G., Glarum, S.H., Karplus, M. J. Mol. Biol., 1971, 57, 93-115. 28. Gouterman, M., Mathies, R.A., Smith, B.E., Caughey, W.S. J. Chem. Phys., 1970, 52, 3795-3802. 29. Eastwood, D., Gouterman, M. J. Mol. Spectroscopy, 1970, 35, 359-75. 30. Tait, D.C., Holten, D., Gouterman, M. Chem. Phys. Letts., 1983, 100, 268-72. 31. Ake, R.L., Gouterman, M. Theor. Chim Acta, 1969, 15, 20-42. 32. Kim, D., Holten, D. Chem. Phys. Letts., 1983, 98, 584-89. 33. Bajdor, K., Kincaid, J.R., Nakamoto, K. J. Am. Chem. Soc., 1984, 106, 7741-47. 34. Demma, L.S., Salhany, J.M. J. Biol. Chem., 1977, 252, 1226-30. RECEIVED

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Picosecond Excited-State Relaxation of Some Iron Porphyrins and

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