The Role of NH-Tautomerism in the Formation of Excited States

5 The Role of NH-Tautomerism in the Formation of Excited States Porphyrins with Nonsymmetrical Substitution

Georgii P. Gurinovich, Edward I. Zenkevich, and Alexander M. Shulga Downloaded by CORNELL UNIV on September 13, 2016 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch005

The Institute of Physics, BSSR Academy of Sciences, Minsk, 220602, USSR

It has been shown that for porphyrins with isocycles (synthetic and natural compounds), bisporphyrins linked covalently via isocycles and for 2,3,12,13-tetraalkyl porphyrins additional bands due to the existence of the second tautomer are discovered in absorption and fluorescence spectra of isotropic solutions over a wide temperature range (77 to 500 K). The spacing between the S - S transitions of tautomers is as great as 380 cm- , while for S -S transitions it is 800 cm- . The difference has been discovered in the types of visible absorption spectra caused by the inversion of Q (0,0) and Q (0,0) band intensities when the NH-proton migration takes place: while the situation I (0,0) < Iy(0,0) is observed for one tautomer, Ι (0,0) > I (0,0) is character istic for the second one and vice versa. NMRH spectra show that in the basic (short-wave) tautomer of cyclopentanporphyrins NH-protons are located at the opposite pyrrole rings which are not bonded with the isocycle whereas for 2,3,12,13-tetraalkylporphyrins they are fixed at rings having alkyl groups. The investigation of dichroism of photo-induced changes in the intensities of tautomer transitions S -S under solution excitation by linearly polarized light permits us to conclude that the system of molecular oscillators X and Y mainly remains fixed at the same pyrrole rings as the H-H axis rotates through 90°. The experimental results and MO (CNDO/2) calculations explain the inversion of the Q (0,0) and Q (0,0) intensities of electronic transitions with NH-protons jumping as a consequence of the inversion of LUMO's C and C and, therefore, of the change in the configuration interaction of the Q and Β states. o

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

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Downloaded by CORNELL UNIV on September 13, 2016 | http://pubs.acs.org Publication Date: October 15, 1986 | doi: 10.1021/bk-1986-0321.ch005

Introduction I t i s w e l l known t h a t i n f r e e - b a s e p o r p h y r i n s two i n n e r hydrogens, w h i c h , i n t h e i r s t a b l e c o n f o r m a t i o n , a r e bound t o o p p o s i t e n i t r o g e n s , may jump from one p a i r o f n i t r o g e n s t o a n o t h e r . A c c o r d i n g t o NMR*H s p e c t r o s c o p y d a t a ( 1 - 4 ) , t h e s e hydrogens m i g r a t e r e l a t i v e l y q u i c k l y a t room t e m p e r a t u r e , b u t a t 77 Κ t h e above tautomerism ceases when t h e p o r p h y r i n m o l e c u l e i s i n t h e ground e l e c t r o n i c s t a t e (5). The m i g r a t i o n o f t h e c e n t e r p r o t o n s may s t i l l be induced even a t l i q u i d helium t e m p e r a t u r e by p h o t o e x c i t a t i o n o f t h e p o r p h y r i n m o l e c u l e . Such p h o t o t r a n s f o r m a t i o n between two tautomers a t low temperatures has been o b s e r v e d i n two types o f e x p e r i m e n t s : i ) p h o t o - i n d u c e d r e v e r s i b l e c o n v e r s i o n o f admixture c e n t e r s i n t o one a n o t h e r f o r p o r p h y r i n - f r e e bases i n c o r p o r a t e d i n a η-octane S h p o l ' s k i i m a t r i x ( 6 , 7 ) ; i i ) v a r i a t i o n s i n t h e e m i s s i o n i n t e n s i t y and f l u o r e s c e n c e d e p o l a r i z a t i o n o f g l a s s y s o l u t i o n s o f p o r p h y r i n s a t 77 Κ d u r i n g e x c i t a t i o n by c o n s t a n t p o l a r i z e d monochromatic l i g h t (5). In symmetrical p o r p h y r i n s NH-tautomers do n o t d i s t i n g u i s h i n s t r u c t u r e and have i d e n t i c a l e l e c t r o n i c s p e c t r a . Correspondingly, i n most cases the s p e c t r a l s e p a r a t i o n between t h e i n d i v i d u a l tautomers (Δ E) does not exceed £ 100 c n r * and i s due t o c r y s t a l l i n e m a t r i x e f f e c t ( 6 , 7 ) . T h e r e f o r e , NH-tautomerism can be e f f e c t i v e l y d e t e c t e d a t 77 Κ and 4 . 2 Κ i n t h e S h p o l ' s k i i m a t r i x o n l y a t s e l e c t i v e monochromatic e x c i t a t i o n ( 5 - 7 ) . In c h l o r i n s , t h e s p e c t r a l s e p a r a t i o n between t h e two tautomers i s l a r g e : ΔΕ ~ 800-1500 cm-1 ( 8 - 1 1 ) . B u t t h e second tautomer which can be produced by p h o t o e x c i t a t i o n can be s t a b l e o n l y a t 4 . 2 Κ due t o the f a s t r e v e r s e photochemical r e l a x a t i o n ( 1 0 ) . I t has been shown f o r t h e f i r s t time by us r e c e n t l y t h a t f o r a whole c l a s s o f i s o c y c l e - c o n t a i n i n g p o r p h y r i n s s p e c t r a l c h a r a c t e r i s t i c s o f i n d i v i d u a l NH-tautomers g r e a t l y d i f f e r because o f non symmetrical s u b s t i t u t i o n caused by t h e i s o c y c l i c r i n g ( 1 2 - 1 4 ) . Comparison o f t h e f l u o r e s c e n c e and a b s o r p t i o n s p e c t r a o f two t a u tomers f o r one compound has r e v e a l e d t h e s p e c t r a l s e p a r a t i o n between t h e i r S - S i t r a n s i t i o n s t o be as g r e a t as 380 c m " , w h i l e t h a t f o r S - S 2 t r a n s i t i o n s can amount t o ~ 800 c n r . T h e r e f o r e , i n d i v i d u a l tautomers o f these compounds may be e a s i l y i n v e s t i gated u s i n g normal e l e c t r o n i c s p e c t r a i n a wide temperature range (77 - 500 K ) . Moreover, when f o u r methyl o r e t h y l groups a r e a t t a c h e d t o t h e opposed p y r r o l e r i n g s o f p o r p h y r i n m o l e c u l e s s i m i l a r p e c u l a r i t i e s o f e l e c t r o n i c s p e c t r a o f two t a u t o m e r i c forms a r e observed i n i s o t r o p i c s o l u t i o n s a t d i f f e r e n t temperatures (77 400 K) ( 1 5 - 1 7 ) . I t s h o u l d be emphasized t h a t t h e v i s i b l e a b s o r p t i o n s p e c t r a o f tautomers show n o t i c e a b l e i n v e r s i o n o f Qx(0,0) and Qy(0,0) band i n t e n s i t i e s i n e v e r y compound i n going from one tautomer t o a n o t h e r . Up t o now t h e f o u r - o r b i t a l model-based t h e o r y o f p o r p h y r i n a b s o r p t i o n e l e c t r o n i c s p e c t r a (18) has not t a k e n i n t o account the i n f l u e n c e o f NH-tautomerism i n non-symmetrical p o r p h y r i n s on t h e p o s i t i o n s and i n t e n s i t i e s o f e l e c t r o n i c t r a n s i t i o n s i n t h e v i s i b l e r e g i o n . The t h e o r e t i c a l c o n s i d e r a t i o n o f t h i s problem i n v o l v e s s o l v i n g t h e fundamental q u e s t i o n o f the a b s o l u t e o r i e n t a t i o n o f e l e c t r o n i c t r a n s i t i o n o s c i l l a t o r s f o r each t a u t o m e r . F i r s t o f a l l , 1

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the correspondence between the d i r e c t i o n o f the long-wave Q(0,0) t r a n s i t i o n o s c i l l a t o r and t h a t o f the H-H a x i s c o n n e c t i n g NH-protons must be found i n i n d i v i d u a l t a u t o m e r i c f o r m s . In t h i s p a p e r , we p r e s e n t t h e d e t a i l e d s p e c t r a l i n f o r m a t i o n o b t a i n e d f o r i n d i v i d u a l NH-tautomers and s t r a i g h t f o r w a r d e x p e r i mental arguments which p e r m i t us t o r e l a t e the r e a l p o s i t i o n o f the H-H a x i s to t h e m o l e c u l a r o s c i l l a t o r axes determined by s u b s t i t u e n t s o f d i f f e r e n t t y p e s ( i s o c y c l e o r a l k y l groups) i n two NH-tautomers. Then we used t h i s s t r u c t u r a l i n f o r m a t i o n as a b a s i s f o r a n a l y z i n g t h e i n v e r s i o n o f e l e c t r o n i c Q ( 0 , 0 ) and Qy(0,0) band i n t e n s i t i e s u s i n g the f o u r - o r b i t a l model. x


Experimental

Section

S y n t h e s i s , p u r i f i c a t i o n and i d e n t i f i c a t i o n o f c y c l o p e n t a n p o r p h y r i n s and t h e i r chemical dimers c o v a l e n t l y l i n k e d v i a i s o c y c l e s have been d e s c r i b e d i n p r e v i o u s papers ( 1 7 , 1 9 - 2 3 ) . The p o r p h y r i n s w i t h the c y c l o p e n t a n o n e r i n g have been prepared by f a m i l i a r methods ( 2 4 - 2 6 ) . Our e x p e r i m e n t a l r e s u l t s were o b t a i n e d when i n v e s t i g a t i n g the f o l l o w i n g compounds: 3 , 5 - c y c l o - 3 - m e t h y l - 2 , 7 , 8 , 1 2 , 1 3 , 1 7 , 18-heptaethyl-22H,24H-porphine ( 1 ) ; S ^ i - c y c l o - S i - e x o m e t h y l en-2,7,18,12,13,17,18-heptaethyl-22H,24H-porphine (2); S ^ cyclo-3 -methyl-5 -oxy-2,7,8,12,13,17,18-heptaethyl-22H,24Hp o r p h i n e ( 3 ) ; p h y l l o e r y t h r i n methyl e s t e r ( 4 ) ; d e o x o p h y l l o e r y t h i n methyl e s t e r ( 5 ) ; 1 0 - e t h o x y - p h y l l o e r y t h r i n methyl e s t e r ( 6 ) ; pheoporphyrin-ac dimethyl e s t e r ( 7 ) ; 2-vinylpheoporphyrin-a5 dimethyl e s t e r (8); 4-vinylprotopheophytin a (9); 3 , 5 - c y c l o - 5 - e t h y l - 1 0 , 1 5 , 2 0 - t r i p r o p y l - 2 2 H , 2 4 H - p o r p h i n e ( 1 0 ) ; 3 j 5 - c y c l o d i m e r (11) which c o n s i s t s o f two m o l e c u l e s 1 and 2; 2 , 3 , 1 2 , 1 3 - t e t r a e t h y l - 2 1 H , 2 3 H - p o r p h i n e ( 1 2 ) ; 3 5 -cyclo-3 -methyl-2,12,13-triethyl-22H,24H-porphine (13); 2,3,12,13t e t r a e t h y l - 7 - b r o m o - 2 1 H , 2 3 H - p o r p P i i n e (14) and 2 , 3 , 1 2 , 1 3 - t e t r a m e t h y l - 2 1 H , 23H-porphine ( 1 5 ) . S t r u c t u r a l f o r m u l a s a r e shown i n F i g u r e 1. Compounds 1 , 2 , 3 , 5 , 1 0 , 1 1 , 1 2 , 1 3 , 1 4 were d i s s o l v e d i n EPIP ( d i e t h y l e t h e r , p e t r o l e u m e t h e r , i s o p r o p a n o l 5:5:2)whereas compounds 4 , 6 , 7 , 8 , 9 , 1 5 were d i s s o l v e d i n THF-DE ( t e t r a h y d r o f u r a n e , d i e t h y l e t h e r 1 : 1 ) . These s o l v e n t m i x t u r e s can be f r o z e n as g l a s s y samples a t 77 K. The a b s o r p t i o n s p e c t r a were r e c o r d e d on a s t a n d a r d s p e c t r o p h o t o m e t e r SF-10 o r Beckman-5270. The measurements o f f l u o r e s c e n c e e x c i t a t i o n and e m i s s i o n s p e c t r a were made w i t h the a i d o f a s p e c t r o f l u o r o m e t e r SLM-4800 w i t h automatic c o r r e c t i o n of s p e c t r a l response. Fluorescence l i f e t i m e s were measured w i t h the a i d o f a p u l s e f l u o r o m e t e r PRA-3000. Magneti c c i r c u l a r d i c h r o i s m (MCD) measurements were c a r r i e d out i n a 8 kG magnetic f i e l d u s i n g a JASC0 J - 2 0 c i r c u l a r d i c h r o m e t e r . Triplet s t a t e f o r m a t i o n was observed f o r i n v e s t i g a t e d compounds a t the exp e r i m e n t a l s e t up, whose d e t a i l e d d e s c r i p t i o n can be found i n our paper ( 2 7 ) . The o p t i c a l experiments were c a r r i e d out w i t h a p o r p h y r i n concentration of 4.10- - 4.10- m o l . I " . In NMR i n v e s t i g a t i o n s ( B r u k e r WM-360) we used h i g h e r c o n c e n t r a t i o n s ( 5 . 1 0 " m o l . l " ) and d r i e d s o l v e n t s ( C D C 1 , C S and t o l u e n e - d ) . 1

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(4) R R - H,R -CH (2) R D2, i . e . , the d i c h r o i s m o f longwavelength Q bands o f two tautomers i s o p p o s i t e i n s i g n (d^ = . 9 p e r c e n t , d2 = +6 p e r c e n t ) . T h i s remarkable r e s u l t , as seen from F i g . 8 seems t o have o n l y one e x p l a n a t i o n : the o s c i l l a t o r o f longwave a b s o r p t i o n band o f tautomer 2, which appears a f t e r p o l a r i z e d i r r a d i a t i o n i n the Q band o f tautomer 1, must be p a r a l l e l t o the e x c i t i n g l i g h t v e c t o r E, a n d , c o r r e s p o n d i n g l y , t o t h e o s c i l l a t o r o f the longwavelength Q band o f tautomer 1. Hence, ugder c e n t r a l p r o t o n d i s p l a c e m e n t the H-H a x i s r o t a t e s through 90 whereas the longwavelength t r a n s i t i o n o s c i l l a t o r X remains presumably f i x e d a t the same p y r r o l e r i n g s f o r both t a u t o m e r s . As f a r as f o r each tautomer o f the same compound the e l e c t r o n i c d e n s i t y d i s t r i b u t i o n i s not i d e n t i c a l ; t h e r e f o r e , i t i s not e x c l u d e d t h a t the mutual d i s p l a c e m e n t o f o s c i l l a t o r s X and Y may be s l i g h t l y d i f f e r e n t i n them. But as a whole the system o f o s c i l l a t o r s X and Y does not r o t a t e through 90° i n g o i n g from tautomer 1 t o tautomer 2 i n the compounds c o n s i d e r e d . Such s i t u a t i o n i s c o n s i s t e n t w i t h MCD d a t a . As seen from F i g . 9 t h e r e a r e two w e l l d i s t i n g u i s h e d bands i n the r e d r e g i o n o f a b s o r p t i o n s p e c t r a f o r compounds 6 and 10 even a t 293 K. These bands belong t o Q(0,0) t r a n s i t i o n s o f i n d i v i d u a l NHt a u t o m e r s . But a t the same time t h e r e i s no s i g n i n v e r s i o n o f MCD bands i n t h i s r e g i o n ; we observe o n l y one band whose maximum i s s i t u a t e d a p p r o x i m a t e l y between tautomer a b s o r p t i o n maxima. T h i s r e s u l t shows t h a t longwave t r a n s i t i o n o s c i l l a t o r s i n both tautomers a r e presumably f i x e d a t t h e same p o s i t i o n a g a i n s t t h e m o l e c u l a r s t r u c t u r e o f each compound. F i n a l l y , the changing o f s i g n p a t t e r n i n MCD s p e c t r a o f compound 6 i n comparison w i t h t h a t o f compound 10 0

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F i g u r e 8 · A b s o r p t i o n o n s e t experiments f o r p h o t o i n d u c e d c o n v e r s i o n o f compound 10 NH-tautomers i n EPIP a t 77 K: (A): I n i t i a l spectrum; ( B , C ) : Monitored spectra a f t e r i r r a d i a t i o n (At « 30 min) by the p o l a r i z e d l i g h t ( x = 638 nm). C o n d i t i o n s f o r p o l a r i z e d e x c i t a t i o n and p o l a r i z e d m o n i t o r i n g a r e shown by a r r o w s . The d i s p l a c e ment o f i n n e r p r o t o n s i n tautomers conforms t o e x p e r i m e n t a l d a t a . The d i r e c t i o n o f X - a x i s i s chosen f o r tautomer 1 to be p a r a l l e l t o v e c t o r Ε o f e x c i t i n g l i g h t . (Reproduced w i t h p e r m i s s i o n from R e f . 15. C o p y r i g h t 1985 N o r t h - H o l l a n d P h y s i c s P u b l i s h i n g Company.)


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F i g u r e 9. A b s o r p t i o n and magnetic c i r c u l a r d i c h r o i s m s p e c t r a o f compounds 6(a) and 10(b) i n THF-DE a t 293 K. O p t i c a l path i s 2mm. (Reproduced w i t h p e r m i s s i o n from Ref. 1 5 . C o p y r i g h t 1985 N o r t h H o l l a n d P h y s i c s P u b l i s h i n g Company.)



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may be e x p l a i n e d as a r e s u l t o f t h e changing o f t h e r e l a t i v e magnitudes o f t h e o r b i t a l energy d i f f e r e n c e s between t h e two h i g h e s t o c c u p i e d (ΔΗΟΜΟ) and t h e two l o w e s t unoccupied (ALUMO) m o l e c u l a r o r b i t a l s ( 3 5 , 3 6 ) . Experiments and t h e o r e t i c a l MO c a l c u l a t i o n s i n t h i s f i e l d a r e performed i n o u r l a b o r a t o r y . I n t e r p r e t a t i o n o f NH-tautomer v i s i b l e a b s o r p t i o n s p e c t r a . The d a t a summarized i n T a b l e 1 r e v e a l t h a t each o f t h e NH-tautomers e x h i b i t s f o u r v i s i b l e bands which a r e g e n e r a l l y c h a r a c t e r i s t i c o f f r e e - b a s e p o r p h y r i n s . But t h e observed i n t e n s i t y p a t t e r n d r a m a t i c a l l y changes when p a s s i n g from one tautomer t o a n o t h e r f o r each compound; w h i l e t h e s i t u a t i o n I ( Q ) < I (Qy) i s o b s e r v e d f o r one tautomer t h e s p e c t r a l p i c t u r e I ( Q ) > 1 (Qy) o c c u r s f o r the second one and v i c e v e r s a . In t h e f o u r - o r b i t a l model ( 1 8 ) , l o w - l y i n g ΤΓ-ΊΓ* s t a t e s o f f r e e base p o r p h y r i n s (symmetry D2fo) a r e c o n s i d e r e d as r e s u l t i n g from s i n g l e e l e c t r o n e x c i t a t i o n from a p a i r o f nondegenerate h i g h e s t o c c u p i e d m o l e c u l a r o r b i t a l s ( b i , b2) t o a p a i r o f nondegenerate l o w e s t unoccupied m o l e c u l a r o r b i t a l s (c\ e g ) . In t h e case o f symmetry D2h» m u t u a l l y p e r p e n d i c u l a r e l e c t r i c t r a n s i t i o n d i p o l e s X and Y a r e n o t e q u i v a l e n t a n d , t h e r e f o r e , i n t h e v i s i b l e a b s o r p t i o n s p e c t r a o f f r e e - b a s e p o r p h y r i n s two d i f f e r e n t e l e c t r o n i c bands Q ( 0 , 0 ) and Q ( 0 , 0 ) a r e observed ( T a b l e 1 and F i g . 1 0 ) . The q u e s t i o n o f r e l a t i v e i n t e n s i t i e s o f Q and Qy t r a n s i t i o n s which i s a m a t t e r o f f o c u s i n t h e p r e s e n t p a r t o t t h e paper has been s o l v e d by Gouterman (31) u s i n g p e r t u r b a t i o n t h e o r y . If i t i s assumed t h a t s t a t e s o f d i f f e r e n t p o l a r i z a t i o n ( Q and Qy) a r e not m i x e d , the Q s t a t e s o f an a r b i t r a r y f r e e - b a s e p o r p h y r i n a r e c o n s t r u c t e d as f o l l o w s : x


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Qy = Qy + XyBy, Xy = [ e ( C l ) - e ( b l ) ] - [ e ( C 2 ) - e ( b 2 ) ]

/2Δ

where Δ i s an i n i t i a l energy gap between Bo and Qo bands, e(b-i) and e ( C j ) a r e o r b i t a l e n e r g i e s . The i n t e n s i t i e s o f Q and Qy s t a t e s a r e p r o p o r t i o n a l t o ;? and X y . Hence, t h e amounts o f a b s o r p t i o n i n t e n s i t y g a i n e d i n t h e i n i t i a l l y f o r b i d d e n Q and Q^ t r a n s i t i o n s o f a r b i t r a r y f r e e - b a s e p o r p h y r i u n by b o r r o w i n g from a p p r o p r i a t e B£ and B$ S o r e t s t a t e s (by t h e s o c a l l e d configuration i n t e r a c t i o n ) are r e l a t e d to differences i n o r b i t a l e n e r g i e s o f s t a t e s c r e a t e d by x - o r y - p o l a r i z e d s i n g l e t e l e c t r o n e x c i t a t i o n . That i s , f o r each p o l a r i z a t i o n , as t h e e q u a l i t y o f t h e t r a n s i t i o n e n e r g i e s b r e a k s down ( o r c o n f i g u r a t i o n i n t e r a c t i o n becomes w e a k e r ) , t h e c o r r e s p o n d i n g v i s i b l e band becomes allowed. In o r d e r t o c o n s i d e r t h e i n v e r s i o n o f Q ( 0 , 0 ) and Qy(0,0) e l e c t r o n i c t r a n s i t i o n i n t e n s i t i e s i n NH-tautomers o f non-symmetrical f r e e - b a s e p o r p h y r i n s we c a l c u l a t e d t h e g r o u n d - s t a t e o r b i t a l e n e r g i e s o f t h e i n v e s t i g a t e d m o l e c u l e s by a CND0/2 method u s i n g t h e symmetrized c r y s t a l geometry o f p o r p h y r i n m o l e c u l e ( 3 7 , 3 8 ) . On t h e b a s i s o f the above e x p e r i m e n t a l r e s u l t s we must i n t r o d u c e a m o t i o n l e s s system o f m o l e c u l a r X and Y a x e s , i d e n t i c a l l y f i x e d i n both t a u t o m e r s . Then u s i n g t h e o r e t i c a l MO c a l c u l a t i o n s and t h e a n a l y s i s x

x

x

x



-8.5

-8.0h

-0.6

-0.2

E.ev

500

b

2

500

-C.

-Ci

550 600 A,nm 500

7

-7.5

-6.6

ai

0.6 £

3

«2= C H

550 600 650

tautomer i

B

5 0 0 5 5 0 6 0 0 650

tautomer 2 M Τ H

F i g u r e 10: S t r u c t u r e s , v i s i b l e a b s o r p t i o n s p e c t r a and f o u r - o r b i t a l diagram o f MO's f o r NH-tautomers o f p o r p h y r i n s w i t h t h e non-symmetrical s u b s t i t u t i o n . ( A ) : compound 1 2 ; ( B ) : compound 1 0 . (Reproduced w i t h p e r m i s s i o n from R e f . 1 5 . C o p y r i g h t 1985 N o r t h H o l l a n d P h y s i c s P u b l i s h i n g Company.)

550 6 0 0

tautomer i


b

4

\nm

5.


ΝH-Tautomerism in the Formation of Excited States

o f t h e i r symmetry p r o p e r t i e s t h e o r d e r i n g o f o r b i t a l s b\ c i and d e t e r m i n e d . F i g u r e 10 i l l u s t r a t e s t h a t , s i m i l a r l y t o t h e case o f symmetrical p o r p h y r i n s (18.), t h e o r d e r i n g o f c j and eg o r b i t a l s changes as NH-protons d i s p l a c e . I t i s essential t h a t i n symmetrical p o r p h y r i n s t h e X a x i s would always keep t h e H-H a x i s d i r e c t i o n ( 1 8 , 3 3 ) . T h e r e f o r e , as t h e H-H a x i s r o t a t e s through 90° ( g o i n g from one tautomer t o a n o t h e r ) t h e o r b i t a l c o m b i n a t i o n s (b-jCj) d e t e r m i n i n g X- and Y - p o l a r i z e d e x c i t a t i o n s must be changed. F o r example, t h i s change f o r X - p o l a r i z e d e x c i t a t i o n s must be p r e s e n t e d as f o l l o w s : ( b ^ ) » ^ c ^ ) > (b]Ci) (b2C2). As a r e s u l t , t h e c o n f i g u r a t i o n i n t e r a c t i o n e n e r g i e s o f X- and Y - p o l a r i z e d e x c i t a t i o n do n o t change i n g o i n g from one tautomer t o a n o t h e r and c o n s e q u e n t l y , t h e i r a b s o r p t i o n s p e c t r a a r e i n d i s t i n g u i s h a b l e from one a n o t h e r . However, f o r non-symmetrical p o r p h y r i n s due t o f i x e d X and Y axes t h e o r b i t a l c o m b i n a t i o n s (b -Cj) c o r r e s p o n d i n g t o X- and Y - p o l a r i z e d e x c i t a t i o n s remain unchanged under t h e NH-proton m i g r a t i o n . Then, as seen from F i g . 10, t h e d i f f e r e n c e s i n o r b i t a l e n e r g i e s o f ttie s t a t e s reached by X and Y e x c i t a t i o n r e a l l y d i f f e r f o r each p o l a r i z a t i o n when tautomer 1 > tautomer 2 t r a n s i t i o n o c c u r s . Such v a r i a t i o n o f t h e c o n f i g u r a t i o n i n t e r a c t i o n e n e r g i e s f o r each p o l a r i z a t i o n must r e s u l t i n p r i n c i p a l changes i n r e l a t i v e i n t e n s i t i e s o f Q ( 0 , 0 ) and Qy(0,0) a b s o r p t i o n bands i n two tautomers o f t h e same m o l e c u l e . A c t u a l l y such a s i t u a t i o n i s e x p e r i m e n t a l l y observed f o r compounds 12 and 15 ( F i g . 5 and T a b l e 1 ) : e t i o - t y p e spectrum o f tautomer 1 t r a n s f o r m s i n t o d e o x o p h y l l o t y p e spectrum o f tautomer 2 . And o u r r e s u l t s , i n f a c t , a r e i n agreement w i t h t h e o r e t i c a l c o n s i d e r a t i o n s o f Gouterman on p a r a l l e l i s m o f H-H and methyl axes ( 1 8 ) , b u t t h a t i s t h e c a s e o n l y f o r tautomer 1. Moreover, t h e l o w e s t Q band o f t h e compound 15 a b s o r p t i o n spectrum (18) a c t u a l l y b e l o n g s t o tautomer 2 , as i s seen from T a b l e 1. B e s i d e s , a t 293°C compounds 12 and 15 show j u s t an e t i o t y p e spectrum i n c o n t r a s t t o a rhodotype s p e c t r u m , used i n paper ( 1 8 ) . Our e x p e r i m e n t a l d a t a (NMR, mass-spectrometry and a b s o r p t i o n s p e c t r a ) p e r m i t us t o c o n f i r m t h a t a rhodo spectrum belongs t o compound 14 which i s t h e accompanying p r o d u c t d u r i n g t h e s y n t h e s i s o f compounds 12 and 1 5 . T a b l e 1 shows t h a t t h e B r - d e r i v a t i v e has a l s o two tautomers w i t h different spectra. These t y p e s o f arguments have been used by us f o r e x p l a n a t i o n o f c h a r a c t e r i s t i c p e c u l i a r i t i e s i n s p e c t r a o f compound 10 NHtautomers ( F i g . 10) and were a p p l i e d t o u n d e r s t a n d t h e p r i n c i p a l changes i n t h e NH-tautomer v i s i b l e a b s o r p t i o n o f i n v e s t i g a t e d p o r p h y r i n s w i t h non-symmetrical s u b s t i t u t i o n . I t may be c o n c l u d e d t h a t d i f f i c u l t i e s i n the t h e o r e t i c a l treatment o f s p e c t r a l p e c u l i a r i t i e s o f p o r p h y r i n s w i t h ττ-acceptor s u b s t i t u e n t s (33) a r e caused by t h e absence o f d e t a i l e d a b s o r p t i o n and f l u o r e s c e n c e e x c i t a t i o n s p e c t r a o f i n d i v i d u a l tautomers o f t h e s e compounds. t

w


91

a

s

n

x

Conclusions We b e l i e v e t h a t d e t a i l e d a n a l y s i s o f a b s o r p t i o n , f l u o r e s c e n c e and f l u o r e s c e n c e e x c i t a t i o n s p e c t r a p e r m i t s us t o i n v e s t i g a t e t h e main s p e c t r o s c o p i c f e a t u r e s o f i n d i v i d u a l NH-tautomers i n p o r p h y r i n s w i t h non-symmetrical s u b s t i t u t i o n . Then, o b t a i n e d i n f o r m a t i o n


92

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

t o g e t h e r w i t h NMR*H data and s e l e c t i v e e x c i t a t i o n s p e c t r o s c o p y r e s u l t s a t 4.2 Κ p r o v i d e t h e p o s s i b i l i t y t o e s t a b l i s h which t y p e s o f a b s o r p t i o n s p e c t r a o f i n d i v i d u a l tautomers c o r r e s p o n d t o r e a l d i s t r i b u t i o n o f NH-protons i n every compound. MO c a l c u l a t i o n s based on t h e f o u r - o r b i t a l model and c a r r i e d out w i t h account o f t h e a b s o l u t e o r i e n t a t i o n o f longwave band o s c i l l a t o r s i n NH-tautomers e x p l a i n t h e unexpected t y p e s o f t r a n s f o r m a t i o n o f v i s i b l e a b s o r p t i o n s p e c t r a under NH-proton m i g r a t i o n .


Literature Cited 1. Storm, C.B.; Teklu, Y. J. Am. Chem. Soc., 1972, 94, 1745-7. 2. Eaton, S.S.; Eaton, G.R. J. Am. Chem. Soc., 1977, 99, 1601-4. 3. Hennig, J.; Limbach, H.-H. J. Chem. Soc. Faraday Trans., 1979, 2, 752-66. 4. Abraham, R.J.; Hawkes, G.E.; Smith, K.M. Tetrahedron Lett., 1974, 1483-6. 5. Zalesski, E.I.; Kotlo, V.N.; Sevchenko, A.N.; Solov'ev, K.N.; Shkirman, S.F. Dokl. Akad. Nauk SSSR, 1972, 207, 1314-7. 6. Korotaev, O.N.; Personov, R.I. Opt. i Spektroskopiya, 1972, 32, 400-2. 7. Solov'ev, K.N.; Zalesski, I.E.; Kotlo, V.N.; Shkirman, S.F. Pis'ma ν ZhoTPh, 1973, 17, 463-6. 8. Shkirman, S.F.; Arabey, S.M.; Egorova, G.D. Zh. Prikl. Spektroskopii, 1979, 31, 817-21. 9. Yolker, S.; Macfarlane, R.M. Liquid Cryst., 1979, 50, 213-6. 10. Arabey, S.M.; Egorova, G.D.; Solov'ev, K.N.; Shkirman, S.F. Zh. Prikl. Spektroskopii, 1984, 40, 92-8. 11. Mauring, K.; Avarmaa, R. Chem. Phys. Lett., 1981, 81, 446-9. 12. Zenkevich, E.I.; Shulga, A.M.; Chernook, A.V.; Gurinovich, G.P. Chem. Phys. Lett., 1984, 109, 306-11. 13. Zenkevich, E.I.; Shulga, A.M.; Chernook, A.V.; Gurinovich, G.P. Dokl. Akad. Nauk SSSR, 1984, 28, 1082-5. 14. Zenkevich, E.I.; Shulga, A.M.; Chernook, A.V.; Gurinovich, G.P.; Sagun, E.I. Zh. Prikl. Spektroskopii, 1985, 42, 772-9. 15. Zenkevich, E.I.; Shulga, A.M.; Filatov, I.V.; Chernook, A.V.; Gurinovich, G.P. Chem. Phys. Lett., 1985, 120, 63-8. 16. Zenkevich, E.I.; Shulga, A.M.; Filatov, I.V.; Chernook, A.V.; Gurinovich, G.P. Dokl. Akad. Nauk SSSR, 1985, 285, No. 1. 17. Shulga, A.M.; Gladkov, L . l . ; Stanishevski, I.V.; Starukhin, A.S. Teoreticheskaya i Experimentalnaya Khimiya, 1985, 5, 554-60. 18. Gouterman, M. J. Mol. Spectrosc., 1961, 6, 138-63. 19. Ponomarev, G.V.; Shulga, A.M. Khimija Heterocycl. soedinenii, 1984, 479-84. 20. Ponomarev, G.V.; Shulga, A.M. Khimija Heterocycl. soedinenii, 1984, 485-9. 21. Shulga, A.M.; Ponomarev, G.V. Khimija Heterocycl. soedinenii, 1984, 922-7. 22. Ponomarev, G.V.; Shulga, A.M. Dokl. Akad. Nauk SSSR, 1983, 271, 365-7. 23. Shulga, A.M. Khimija Heterocycl. soedinenii, 1985, 132-3.



5.


ΝH- Tautomerism in the Formation of Excited States

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24. Kenner, G.W.; McCombie, S.W.; Smith, K.M. J. Chem. Soc. Perkin Trans., 1973, 1, 2517-23. 25. Kenner, G.W.; McCombie, S.W.; Smith, K.M. J. Chem. Soc. Perkin Trans., 1974, 1, 527-30. 26. Housier, C.; Sauer, S. Biophys. Biochem. Acta, 1969, 172, 492-502. 27. Gurinovich, G.P.; Zenkevich, E.I., Sagun, E.I. Dokl. Akad. Nauk SSSR, 1981, 257, 70-4. 28. Gurinovich, G.P.; Sevchenko, A.N.; Solov'ev, K.N. "Spektroskopiya khlorofilla i rodstvennykh soedinenii", Nauka i tekhnika, Minsk, 1968. 29. Gouterman, M. In "The Porphyrins"; Dolphin, D., Ed.; New York, 1978, III, 1-165. 30. Zenkevich, E.I.; Shulga, A.M.; Sagun, E.I.; Chernook, A.V.; Gurinovich, G.P. Zh. Prikl. Spektroskopii, 1985, 43, 455-61. 31. Gouterman, M. J. Chem. Phys., 1959, 30, 1139-61. 32. Anex, B-G.; Umans, R.S. J. Am. Chem. Soc., 1964, 86, 5026-7. 33. Djerassi, C.; Lu, Y.; Waleh, A. et al. J. Am. Chem. Soc., 1984, 106, 4241-58. 34. Simpson, W.T. J. Chem. Phys., 1949, 17, 1218. 35. Michl, J. J. Am. Chem. Soc., 1978, 100, 6812-8. 36. Keegan, J.D.; Stolzenberg, A.M.; Lu, Y.-C.; Linder, R.E.; Barth, G.; Moscowitz, Α.; Bunnenberg, E.; Djerassi, C. J. Am. Chem. Soc., 1982, 104, 4305-16. 37. Caughey, W.S.; Ibers, J.A. J. Am. Chem. Soc., 1977, 99, 6639-43. 38. Fischer, M.S.; Templeton, D.H.; Zalkin, Α.; Calvin, M. J. Am. Chem. Soc., 1972, 94, 3613-9. RECEIVED

June 10, 1986


The Role of NH-Tautomerism in the Formation of Excited States

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