Electrochemical and Spectrochemical Studies of Biological Redox

11

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 8, 2015 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0201.ch011

Mapping of the Energy Levels of Metallophthalocyanines via Electronic Spectroscopy, Electrochemistry, and Photochemistry A. B. P. LEVER, S. LICOCCIA, K. MAGNELL, P. C. MINOR, and B. S. RAMASWAMY York University, Department of Chemistry, 4700 Keele Street, Downsview, Ontario, Canada, M3J 1P3

The mapping of the energy levels in metallophthalocyanines is accomplished by a combination of electrochemistry, electronic spectroscopy, and photochemistry. This chapter reviews the electrochemical properties of metallophthalocyanines and includes a large amount of previously unpublished data. The results are rationalized in terms of the nature of the electron transfer, that is, redox at metal or ligand. Well-defined correlations are shown to exist between the ease of oxidation or reduction of the phthalocyanine ligand and the oxidation state, and/or polarizing power, of the metal ion. Solvent and ring substitution effects also are presented and explained. Charge transfer transition energies can be calculated directly from these data, and agreement between experiment and theory is excellent. Finally, the data are used to calculate both the photo-generated excited state redox energies and the thermodynamics of quenching by donors and acceptors.

T O h t h a l o c y a n i n e ( M P c ) c o m p l e x e s h a v e s i g n i f i c a n t i m p o r t a n c e for m a n y reasons, i n c l u d i n g t h e i r s i m i l a r i t y to the b i o l o g i c a l l y i m p o r t a n t m e t a l l o p o r p h y r i n s , t h e i r c l a s s i c a l u s e as d y e s t u f f s , a n d t h e i r d e v e l o p i n g u s e as c o m p o n e n t s o f v a r i o u s s o l a r e n e r g y c o n v e r s i o n d e vices. 0065-2393/82/0201-0237$06.00/0 © 1982 A m e r i c a n C h e m i c a l Society In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

238

BO ILOGC IAL REDOX COMPONENTS

O f p a r a m o u n t i m p o r t a n c e i n u n d e r s t a n d i n g a n d p r e d i c t i n g the p h y s i c s a n d c h e m i s t r y o f t h e m e t a l l o p h t h a l o c y a n i n e s is k n o w l e d g e o f the energy l e v e l s therein. T h i s k n o w l e d g e c a n be g a i n e d t h r o u g h study o f the electronic spectroscopy (absorption a n d emission), elec trochemistry, photochemistry (and photophysics), a n d photoelectron s p e c t r o s c o p y o f t h e s e s p e c i e s . I n t h i s c h a p t e r w e a d d r e s s t h e first t h r e e o f these t e c h n i q u e s their use.

and

consider the

information obtained

from


- 1

T h e l o w e r e x c i t e d states ( u p to at l e a s t 3 5 , 0 0 0 c m ) o f a w i d e range of metallophthalocyanine derivatives can be identified and m a p p e d . T h e s e states v a r y as a f u n c t i o n o f t h e e n v i r o n m e n t ( s o l v e n t , a x i a l c o o r d i n a t i o n , etc.), p h t h a l o c y a n i n e r i n g s u b s t i t u e n t , n a t u r e a n d s i z e o f m e t a l i o n , o x i d a t i o n state, a n d e l e c t r o n i c c o n f i g u r a t i o n . T h e e x c i t e d states m a y b e c l a s s i f i e d as π - π * a n d η - π * t r a n s i t i o n s , p r i m a r i l y l o c a t e d o n t h e p h t h a l o c y a n i n e r i n g . T h e l i g a n d ( P c r i n g ) to m e t a l c h a r g e t r a n s f e r ( L M C T ) , m e t a l to l i g a n d c h a r g e transfer ( M L C T ) , a n d d-d t r a n s i t i o n s p r i m a r i l y o c c u r o n t h e m e t a l a t o m . T h e s e transitions m a y o c c u r i n t w o s p i n manifestations. I n a d d i t i o n , s p i n c o u p l i n g c a n o c c u r b e t w e e n m e t a l i o n g r o u n d state w a v e f u n c t i o n s a n d e x c i t e d state w a v e f u n c t i o n s o f t h e p h t h a l o c y a n i n e r i n g to y i e l d a range of triplet-multiplets. T h e f o l l o w i n g three sections r e v i e w progress i n the e l e c trochemistry, electronic spectroscopy, and photochemistry o f metal l o p h t h a l o c y a n i n e s . T h e d a t a r e p o r t e d for m e t a l l o p h t h a l o c y a n i n e s m a y b e c o m p a r e d u s e f u l l y w i t h t h e d a t a o b t a i n e d for t h e p o r p h y r i n s d i s c u s s e d i n C h a p t e r 13 i n t h i s v o l u m e .

Electrochemistry M a n y r e p o r t s ( I -18) d i s c u s s t h e e l e c t r o c h e m i c a l p r o p e r t i e s o f t h e m e t a l l o p h t h a l o c y a n i n e s , b u t o n l y i n recent literature have these data coalesced into a useful w o r k i n g b o d y of knowledge. I n g e n e r a l , o x i d a t i o n a n d r e d u c t i o n are e x p e c t e d at t h e m e t a l c e n t e r a n d at t h e p h t h a l o c y a n i n e r i n g . I n e a c h c a s e , o n e o r m o r e e l e c tron transfer processes m a y b e o b s e r v e d . I n d e e d , t w o successive r i n g oxidations a n d u p to four successive r i n g r e d u c t i o n s m a y o c c u r . R i n g r e d u c t i o n s are g e n e r a l l y e l e c t r o c h e m i c a l l y r e v e r s i b l e , b u t r i n g o x i d a t i o n s u s u a l l y a r e n o t (at l e a s t o n p l a t i n u m ) . G e n e r a l l y , n o m o r e t h a n t w o r e d o x p r o c e s s e s w e r e c h a r a c t e r i z e d at a g i v e n m e t a l c e n t e r . I d e n t i f i c a t i o n o f t h e n a t u r e o f a g i v e n r e d o x p r o d u c t u s u a l l y is based on electronic spectroscopy and, where relevant, electron s p i n r e s o n a n c e ( E S R ) s p e c t r o s c o p y ( 2 , 9, 12). G e n e r a l l y , i t is p o s s i b l e to d e d u c e u n e q u i v o c a l l y w h e t h e r a r e d u c t i o n or o x i d a t i o n o c c u r r e d at t h e m e t a l c e n t e r or p h t h a l o c y a n i n e r i n g . P h t h a l o c y a n i n e a n i o n a n d

In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

LEVER E T AL.

11.

Energy Levels of

239

Metallophthalocyanines

cation r a d i c a l e l e c t r o n i c spectra are quite distinct from those o f the P c ( - 2 ) s p e c i e s (12, 19), a l t h o u g h s o m e c o n f u s i o n m a y e x i s t w h e n l o w - o x i d a t i o n state t r a n s i t i o n - m e t a l i o n s , s u c h as i r o n ( I ) , a r e i n v o l v e d (9). T h e h i g h e r filled a n d l o w e r e m p t y e n e r g y l e v e l s o f a t y p i c a l m e t a l l o p h t h a l o c y a n i n e a r e i l l u s t r a t e d i n F i g u r e 1 (20-23). T h e t w o h i g h e s t filled o r b i t a l s o f r e l e v a n c e t o o u r d i s c u s s i o n a r e π - o r b i t a l s w i t h a [ 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 ( H O M O ) ] a n d (hu s y m m e t r y , r e s p e c t i v e l y ; a n d the l o w e s t e m p t y r i n g orbitals are the e [lowest u n o c c u p i e d m o l e c u l a r o r b i t a l ( L U M O ) ] a n d b 7r*-orbitals. T h e m e t a l v a l e n c e o r b i t a l s m a y b e b u r i e d i n s i d e t h e filled p h t h a l o c y a n i n e l e v e l s , or filled a n d / o r e m p t y v a l e n c e o r b i t a l s m a y o c c u r i n t h e H O M O L U M O g a p ; i n a d d i t i o n , e m p t y m e t a l o r b i t a l s m a y l i e at e n e r g i e s c o m parable to, or above, the L U M O p h t h a l o c y a n i n e l e v e l . i

2

iu


g

2u

b τ* MLCT2

MLCTI

TT

SORET

LMCT

dz^L

\LMCri

xx-

d(6g) d, t>2g 'xy

2υ

α

π

Figure 1. A simplified energy level diagram for a metallophthalo cyanine. LMCT ana MLCT are ligand to metal, and metal to ligand charge transfer transitions, respectively.

1

The phthalocyanine nomenclature used here is presented in Ref. 19. An energy level of symmetry (xy) localized on the peripheral nitrogen atoms is omitted because there is no evidence that it plays a role in the spectroscopy. 2


240



Phthalocyanine redox chemistry m a y b e classified conveniently into t w o sections: m a i n groups a n d transition groups. Main Croup Phthalocyanine Electrochemistry. . R e d o x c h e m i s t r y i n t h e m a i n g r o u p s is u s u a l l y q u i t e s t r a i g h t f o r w a r d ; g e n e r a l l y t h e m e t a l a t o m c e n t e r is u n a f f e c t e d , a n d a l l o b s e r v a b l e p r o c e s s e s o c c u r o n the p h t h a l o c y a n i n e r i n g . F o r m a i n g r o u p ions that l i e i n t h e p h t h a l o c y a n i n e p l a n e , t h e first r i n g o x i d a t i o n ( f r o m H O M O ) i s s e p a r a t e d f r o m t h e first r i n g r e d u c t i o n (to L U M O ) b y a p p r o x i m a t e l y 1 5 6 0 m V ( T a b l e I ) , w h i c h is t h e m a g n i t u d e o f t h e m o l e c u l a r b a n d g a p . T h i s v a l u e seems l a r g e l y unaffected b y the nature o f the m a i n g r o u p m e t a l , a l t h o u g h s o m e d e v i a t i o n m a y o c c u r i f t h e m e t a l is t o o large to b e a c c o m m o d a t e d b y t h e p h t h a l o c y a n i n e c e n t e r (14). H o w e v e r , t h e a b s o l u t e e n e r g i e s o f first o x i d a t i o n o r r e d u c t i o n v a r y considerably, a n d d e p e n d o n the size a n d charge o f the m e t a l i o n . T h e ease o f o x i d a t i o n or r e d u c t i o n o f the p h t h a l o c y a n i n e u n i t d e p e n d s o n t h e e l e c t r i c field g e n e r a t e d b y t h e c e n t r a l m e t a l i o n . I n d e e d , t h e r e is a clear relationship between the polarizing power o f the central i o n , e x p r e s s e d as (ze/r) a n d t h e r e d o x e n e r g y . T h e m o r e p o l a r i z i n g i s t h e ion, the easier i t is to r e d u c e the p h t h a l o c y a n i n e r i n g , a n d the more d i f f i c u l t t h e r i n g is t o o x i d i z e . A g o o d l i n e a r r e l a t i o n s h i p is o b s e r v e d b e t w e e n t h e s e q u a n t i t i e s (14) d e f i n e d b y E q u a t i o n s 1 a n d 2 : o x i d a t i o n (ze/r)(E°

- 1.410) = - 0 . 0 1 2

(1)

r e d u c t i o n (ze/r)(E°

+ 0.145) = - 0 . 0 1 2

(2)

w h e r e the potentials are referenced to the n o r m a l h y d r o g e n e l e c t r o d e ( N H E ) a n d t h e r a d i i u s e d o r i g i n a t e f r o m R e f . 24. T h e l i n e s a r e e s s e n tially p a r a l l e l . T h s e data i m p l y a H O M O - L U M O separation o f about 1.56 V , i n d e p e n d e n t o f t h e c e n t r a l m a i n g r o u p i o n , p r o v i d e d i t l i e s i n t h e p l a n e (14). T h i s t r e a t m e n t is d i s c u s s e d l a t e r . T h e second r e d u c t i o n process appears, o n the average, about 4 2 0 m V m o r e n e g a t i v e t h a n t h e first r e d u c t i o n ( T a b l e I) b u t t h e s e p o t e n t i a l s a r e m o r e s c a t t e r e d a n d less d i r e c t l y d e p e n d e n t o n t h e p o l a r i z i n g p o w e r o f the central ion; nevertheless, metallophthalocyanines w i t h the m o r e p o l a r i z i n g ions are g e n e r a l l y m o r e r e a d i l y r e d u c e d to the r i n g d i a n i o n . T h e t h i r d - a n d fourth-reduction processes o c c u r n e a r - 1 . 7 t o - 1 . 8 a n d - 2 . 0 V ( T a b l e I) (12) (for b o t h m a i n a n d t r a n s i t i o n g r o u p i o n s ) . T h e s e d a t a a r e d i s p l a y e d i n F i g u r e 2 . F o r t h e first t w o r e d u c t i o n processes, the ease o f r e d u c t i o n f o l l o w s the s e q u e n c e : PcH

2

2

< PcMg(II) < Pc " [Pr N] 4

2

> PcAl(III)X

T h i s sequence m a y be rationalized i n terms o f increasing negative charge on the phthalocyanine l i g a n d w h e n passing from the covalent



2

2

2

CLN DMF DMF CHC DMSO

+

1.18

1.01

1.50 1.50

++

c /c

e

0.865 1.14

1.34 0.865

0.78 0.49 0.490 0.91 0.695

0.89 0.85

1.155 1.18 1.15 1.105 1.070 1.080 0.94

1.39

+

c /c

-0.58 -0.42 -0.58 -0.285

-0.30 -0.26 -0.29 -0.415 -0.415 -0.42 -0.495 -0.475 -0.41 -0.71 -0.67 -0.68 -0.725 -0.72 -0.93 -1.06 -1.065 -0.48 -0.250 -0.82 -1.00

clc

-0.82 -0.95 -0.73

-1.31

-0.770 -0.840

-1.15 -1.04

-0.895 -0.710

-0.86

-0.74

-0.895

c lc~

-1.57

-1.69

-1.71

-1.90

-1.18

Main Group Metallophthalocyanine Electrochemistry

-1.99

-1.95

-2.34

-1.74

4

c^-lc -

c

14 16 12 t w , 14 6 15 tw, 14 14 15 16 12 tw, 14 6 15 tw, 6, 14 14 tw 9, 14, 15 tw 15 12 10 15 12 44 11

Reference

S*

«·«>·* Ο Ο

«*. 5-

ο "ΰ

r

H >

M < m w

r

e

d

0

6

1 / 2

Note: Data rounded off to 5 mV. Redox potentials, E , vs. N H E . a R = O-t-Amyl. Solvents: DMF, dimethylformamide; DMA, dimethylacetamide; DMSO, dimethyl sulfoxide; C L N , chloronaphthalene; C H C , dichloromethane. tw, this work. to PRA, N-propylammonium cation. Two-electron oxidation.

TbPcH TsPcH

2

PcPb(II) PcBa(II) PcNa(I) Pc "(PRA)V PcH

PcCd(II) PcHg(II)

PcMg(II)

PcGa(III)Cl PcIn(III)Cl

DMF DMA DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMA DMF DMF DMF DMF DMF DMF

2

PcSi(IV)R « PcAl(III)Cl

0

Solvent

Complex

Table I.


242

BIOLOGICAL REDOX COMPONENTS

—3.01—

-2.5

μ

/

A Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 8, 2015 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0201.ch011

-2.0 U

• ^ ΚC

3

/C

4

C/5

Η

Ο >

1.5

-i.o

μ

Α

μ

\ σ-/σ

Α

2

C7C "

/

-0.5

\ c/c• J

I

I 2

H, M g ^ c "

J

L 1

Al "

I

L

ι

ι

I

L

T i ^ V ' M ^ o î ' z r i ' F e ' c o Ο ο

1

Figure 2. Sketch of the variation in first-, second-, third-, and fourthring reduction potentials as a function of metal ion and oxidation state. All data vs. NHE.

N H b o n d i n P c H , to a f a i r l y e l e c t r o s t a t i c i n t e r a c t i o n w i t h t h e e l e c t r o p o s i t i v e m a g n e s i u m , a n d finally to a f u l l n e g a t i v e c h a r g e w i t h t h e p r o p y l a m m o n i u m salt. F o r a l u m i n u m ( I I I ) , t h e h i g h p o l a r i z i n g p o w e r o f t h i s i o n l e a d s to a m o r e c o v a l e n t i n t e r a c t i o n a n d a r e d u c e d n e g a t i v e charge on the p h t h a l o c y a n i n e l i g a n d . T h e s e potentials m o n i t o r the negative charge on the p h t h a l o c y a n i n e l i g a n d . 2

Transition Group Phthalocyanine Electrochemistry.

T h e pres

ence o f a transition m e t a l i o n appears to p e r t u r b the r e d o x processes o c c u r r i n g at t h e p h t h a l o c y a n i n e u n i t ; h o w e v e r , a p a t t e r n s i m i l a r to t h e m a i n g r o u p s p e c i e s c a n b e d i s c e r n e d . I n m a n y cases, o n e or m o r e r e d o x p r o c e s s e s m a y o c c u r at t h e c e n t r a l i o n at p o t e n t i a l s l y i n g b e t w e e n r i n g o x i d a t i o n a n d r e d u c t i o n . I f the solvent, s u p p o r t i n g electro l y t e , o r a n a d d e d l i g a n d c a n b i n d t o t h e a x i a l sites o f t h e m e t a l i o n i n o n e (or m o r e ) o f its o x i d a t i o n l e v e l s , t h e n t h e o b s e r v e d r e d o x p o t e n t i a l often d e p e n d s m a r k e d l y on the c h o i c e o f solvent, e l e c t r o l y t e , or a d d e d ligand. T a b l e I I g i v e s d a t a for t r a n s i t i o n m e t a l i o n p h t h a l o c y a n i n e s w h e r e s o l v e n t c o o r d i n a t i o n is n o t s t r o n g l y p e r t u r b i n g , t h a t i s , i n w e a k d o n o r


11.

LEVER ET AL.

w

243 Energy Levels of Metallophthalocyanines

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244


solvents, or i n v o l v i n g metals that b i n d w e a k l y a l o n g the axis. T h e s e data s h o u l d be generally i n t e r p r é t a b l e w i t h o u t t a k i n g into considera t i o n s e v e r e p e r t u r b a t i o n b y s o l v e n t effects.


E l e c t r o n i c a n d E S R spectroscopy d e m o n s t r a t e d that the O T i ( I V ) , O V ( I V ) , Ni(II), C u ( I I ) , a n d Z n ( I I ) species do not u n d e r g o redox pro cesses at t h e m e t a l at p o t e n t i a l s b e t w e e n l i g a n d o x i d a t i o n a n d r e d u c tion. Iron a n d cobalt, on the other h a n d , can form M ( I ) , M ( I I ) , a n d M ( I I I ) s p e c i e s at t h e s e i n t e r m e d i a t e p o t e n t i a l s , t h a t i s , o x i d a t i o n o f t h e p h t h a l o c y a n i n e l i g a n d o c c u r r e d after t h e m e t a l w a s o x i d i z e d to M ( I I I ) , a n d r e d u c t i o n o f t h e p h t h a l o c y a n i n e l i g a n d o c c u r r e d o n l y after r e d u c t i o n o f t h e m e t a l to M ( I ) . C h r o m i u m a n d m a n g a n e s e p h t h a l o c y a n i n e s f o r m M ( I I ) a n d M ( I I I ) o x i d a t i o n states (5-13). I n p a r a l l e l w i t h m a i n g r o u p p h t h a l o c y a n i n e c h e m i s t r y , the a b i l i t y to r e d u c e a m e t a l l o p h t h a l o c y a n i n e i n c r e a s e s , t h a t is, t h e p o t e n t i a l b e c o m e s m o r e p o s i t i v e , as t h e o x i d a t i o n state o f t h e c e n t r a l i o n i n c r e a s e s . T h i s a b i l i t y c a n b e seen from the data comparisons abstracted from T a b l e II a n d s h o w n i n Tables I I I , I V , a n d F i g u r e 2. N o t s u r p r i s i n g l y , t h e p o t e n t i a l s a r e s i m i l a r to t h o s e o f m a i n g r o u p i o n s o f t h e s a m e o x i d a t i o n state a n d a p p r o x i m a t e s i z e , a l t h o u g h t h i s fact a p p a r e n t l y w a s n o t r e c o g n i z e d c l e a r l y i n t h e p a s t . B e c a u s e t h e s p r e a d i n p o t e n t i a l s for a g i v e n m e t a l o x i d a t i o n state is r e m a r k a b l y s m a l l , a n d t h e r e is a c l e a r e n o u g h s e p a r a t i o n b e t w e e n t h e r a n g e s for at

T a b l e I I I . P o t e n t i a l s , v s . N H E , for t h e F i r s t R e d u c t i o n o f t h e P h t h a l o c y a n i n e R i n g as a F u n c t i o n o f C e n t r a l I o n O x i d a t i o n S t a t e

Complex TbPcVO TbPcTiO TbPcCr TsPcCr TbPcMn TsPcMn PcMn PcFe TsPcFe PcCo TsPcCo PcNi TbPcCu PcCu PcZn

Pc(-2)M(IV)OI Pc(-3)M(IV)0

Pc(-2)M(1I)/ Pc(-3)M(II)

Pc(-2)M(I)I Pc(-3)M(I)

-0.275 -0.335 -0.630 -0.760 -0.650 -0.475 -0.450 -0.930 -0.840 -1.16 -1.050 -0.610 -0.635 -0.600 -0.650

Note: see Table II for references to the literature and abbreviations. In general, these potentials show little solvent dependence.


11. LEVER ET AL.Energy Levels of Metallophthalocyanines Table IV.

TbPcTiO TbPcVO TbPcCr(II) PcMn(II) Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 8, 2015 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0201.ch011

S e c o n d R e d u c t i o n P r o c e s s for T r a n s i t i o n Metallophthalocyanines

Pc(-3)M(IV)OI Pc(-4)M(IV)0

Complex

245

Pc(-3)M(II)I Pc(-4)M(II)

Pc(-3)M(I)/ Pc(-4)M(I)

-0.780 -0.840

tw tw tw

-1.115 -1.22 -1.26 -1.115

TsPcMn(II) TsPcFe(II) PcFe(II) PcCo(II) PcNi(II) TsPcNi(II)

3 12 -1.27 -1.32 -1.56

-0.99 -0.925 (-1.69) -0.89 -0.94 - 0 . 8 7 0 (-1.655) -1.10 -1.09 (-1.82, -2.44)

PcCu(II) TsPcCu(II) TbPcCu(II) PcZn(II)

Ref.

tw tw

12 12 12 11 tw

12 11 tw

12

Note: See Tables I-III for abbreviations. Data in parentheses refer to successive reductions. Redox potentials vs. Ν HE.

l e a s t t h e first a n d s e c o n d r e d u c t i o n , t h e p o t e n t i a l s g e n e r a l l y c a n b e u s e d d i a g n o s t i c a l l y t o i d e n t i f y t h e o x i d a t i o n state o f t h e c e n t r a l t r a n s i tion metal ion. M o s t oxidations o f the phthalocyanine ring i n transition metal phthalocyanines are e l e c t r o c h e m i c a l l y irreversible, o b s c u r i n g o x i d a t i o n state t r e n d s . T r i v a l e n t a n d t e t r a v a l e n t t r a n s i t i o n m e t a l p h t h a l o c y a n i n e s g e n e r a l l y o x i d i z e a l i t t l e a b o v e 1.0 V ; t h i s t r e n d is a l s o true for the m o r e p o l a r i z i n g d i v a l e n t ions, n i c k e l ( I I ) a n d copper(II). P c ( - 2 ) Z n a l s o o x i d i z e s n e a r 1.0 V w h i l e t h e e a r l i e r first r o w t r a n s i t i o n m e t a l p h t h a l o c y a n i n e s o x i d i z e at s l i g h t l y m o r e n e g a t i v e p o t e n t i a l s . S o l v e n t effects o n t h e s e l i g a n d r e d o x p o t e n t i a l s a r e s m a l l . H o w e v e r , s o l v e n t effects o n m e t a l r e d o x p r o c e s s p o t e n t i a l s c a n b e e x t r a o r d i n a r i l y l a r g e . T a b l e V g i v e s t h e r a n g e s f o r v a r i o u s r e d o x p r o c e s s e s as a f u n c t i o n o f s o l v e n t a n d / o r s u p p o r t i n g e l e c t r o l y t e . T h e effect o f s o l v e n t d e p e n d s clearly on the electronic configurations o f the species i n v o l v e d , a n d t h e effect o f s u p p o r t i n g e l e c t r o l y t e d e p e n d s o n w h e t h e r there is b i n d i n g o f the a n i o n to either c o m p o n e n t o f t h e c o u p l e . T h e iron(II)/iron(I) a n d cobalt(III)/cobalt(II) couples b o t h i n v o l v e l o w s p i n d /d c o n f i g u r a t i o n s . S t r o n g l y b i n d i n g a x i a l l i g a n d s ( s o l v e n t m o l e c u l e s ) d e s t a b l i z e the z e l e c t r o n ( i n d ) a n d favor o x i d a t i o n to the e

7

2

7


246


Table V.

Metal Ion

Solvent/X~

6

PcCr(II) PcMn(II) PcFe(II) TsPcFe(II) PcCo(II) c

d

e

/


Solvent Effects o n Transition M e t a l l o p h t h a l o c y a n i n e

cVc

a

DMF/Py DMF/DMSO/Py DMF/DMSO/Py DMSO/KCN DCB/DMF/ DMSO/Py/ 4-EtPy

+0.04/-0.155 0.045/-0.10 0.90/-0.085 0.88/0.24

clc

c lc~

-0.45/-0.575 -0.31/-0.845 -0.98 -0.1/-0.7

-1.10/-1.28 -0.87/-1.085 -1.31 -l.il/-l.18

α

Supporting electrolytes include tetraalkylammonium chlorides, bromides, and perchlorates. Numbers indicate upper and lower boundaries for observed potentials. These data include tetrasulfonated and tetra-f-butyl derivatives. Solvents: DMF, dimethylformamide; DMSO, dimethyl sulfoxide; Py, pyridine; DCB, o-dichlorobenzene. Redox potentials vs. NHE. This work. Ref. 3. Ref. 2. Ref. 45. 'This work, Ref. 8. 6

c

d

e

6

low spin d

s p e c i e s . T h u s , i n b o t h cases t h e p o t e n t i a l s s h i f t n e g a t i v e l y

w i t h i n c r e a s i n g donor strength o f the solvent, w h i c h follows the order D M A = D M F < D M S O < pyridine

(3)

where D M A , D M F , a n d D M S O represents d i m e t h y l a m i n e , d i m e t h y l formamide, and d i m e t h y l sulfoxide, respectively. I n d e e d , t h e r e is a l i n e a r c o r r e l a t i o n o f t h e s e p o t e n t i a l s w i t h G u t m a n n D o n i c i t y N u m b e r (25) o f the solvent (8). T h e cobalt(II)/cobalt(I) c o u p l e ( l o w s p i n d /d ) a l s o shifts n e g a t i v e l y w i t h i n c r e a s i n g d o n i c i t y o f t h e s o l v e n t , p r o b a b l y b e c a u s e a x i a l b i n d i n g to t h e s q u a r e p l a n a r d c o b a l t ( I ) is w e a k o r n o n e x i s t e n t . T h e i r o n ( I I I ) / i r o n ( I I ) c o u p l e ( l o w s p i n d /d ), h o w e v e r , s h o w s t h e o p p o s i t e t r e n d s h i f t i n g p o s i t i v e l y w i t h i n c r e a s i n g d o n i c i t y o f t h e s o l v e n t ( 8 , 9 ) . T h i s fact b e s t c a n b e e x p l a i n e d b y s y n e r g i s m , w h e r e strongly c o o r d i n a t i n g axial l i g a n d s favor b a c k d o n a t i o n b y the l o w s p i n d i o n i n t o p h t h a l o c y a n i n e π - a c c e p t o r o r b i t a l s . B a c k d o n a t i o n i n i r o n ( I I I ) is w e a k e r b e c a u s e o f t h e g r e a t e r c h a r g e on t h e m e t a l . T a b l e V s h o w s t h a t t h e s e s o l v e n t effects c a n b e q u i t e dramatic. For e x a m p l e , a solution o f iron(II) p h t h a l o c y a n i n e i n p y r i d i n e c o n t a i n i n g c h l o r i d e i o n is a i r s t a b l e , b u t a s i m i l a r s o l u t i o n i n D M A or D M F w i t h c h l o r i d e i o n r a p i d l y a i r o x i d i z e s to i r o n ( I I I ) p h t h a l o c y a n i n e ( 9 ) . M o r e o v e r , i f c y a n i d e i o n is a d d e d t o a s o l u t i o n o f t e t r a s u l f o n a t e d i r o n ( I I ) p h t h a l o c y a n i n e , t h e i r o n ( I I ) state is s t a b i l i z e d to a r e m a r k a b l e d e g r e e ( T a b l e V ) t h r o u g h a x i a l c o o r d i n a t i o n o f c y a n i d e i o n s . I n d e e d , u n s u b s t i t u t e d i r o n ( I I ) p h t h a l o c y a n i n e is s o l u b l e i n w a t e r i f c y a n i d e i o n s are p r e s e n t (5). E v e n m o r e r e m a r k a b l e s t a b i l i z a t i o n o f i r o n ( I I ) is s e e n w h e n i m i d a z o l e is u s e d as a n a x i a l l i g a n d ( T a b l e V ) (7). 7

8

8

5

6

6



11.

LEVER ET AL.

Energy Levels of Metallophthalocyanines

247

W h e n a series o f s u b s t i t u t e d p y r i d i n e s w e r e u s e d as s o l v e n t s ( 8 ) , b o t h the cobalt(III)/cobalt(II) a n d cobalt(II)/cobalt(I) c o u p l e s s h i f t e d n e g a t i v e l y w i t h i n c r e a s i n g ρ Κ α o f the solvent. T h i s r e s u l t m a y b e ex p l a i n e d i n t e r m s o f t h e D r a g o Ε a n d C m o d e l (26) g i v e n t h a t for t h i s series t h e e l e c t r o s t a t i c c o m p o n e n t , E , i s c h a n g i n g w h i l e t h e c o v a l e n t c o m p o n e n t , C , remains r o u g h l y constant, an observation i n a g r e e m e n t w i t h s i m i l a r p o r p h y r i n r e d o x d a t a (27). A n i n t e r e s t i n g s o l v e n t effect is o b s e r v e d w h e n c o b a l t ( I I ) P c is o x i d i z e d to c o b a l t ( I I I ) P c . B e c a u s e t h e l a t t e r s p e c i e s has a v e r y s t r o n g p r o p e n s i t y t o b e h e x a c o o r d i n a t e d w i t h t w o a x i a l l y b o u n d s o l v e n t m o l e c u l e s , t h e o x i d a t i o n p o t e n t i a l is c l e a r l y s o l v e n t d e p e n d e n t . I n a n o n c o o r d i n a t i n g s o l v e n t s u c h as d i c h l o r o b e n zene, a hexacoordinated cobalt(III) species cannot b e formed. U n d e r t h e s e c o n d i t i o n s , t h e C o ( I I I ) P c / C o ( I I ) P c c o u p l e shifts p o s i t i v e l y to a c o n s i d e r a b l e extent, s u c h that the i n i t i a l o x i d a t i o n o f the species is to f o r m a c o b a l t ( I I ) p h t h a l o c y a n i n e c a t i o n r a d i c a l (16). W h e n p y r i d i n e i s a d d e d to s u c h a s o l u t i o n , a cobalt(III) species a p p a r e n t l y is f o r m e d . P r e l i m i n a r y data for c h r o m i u m ( I I I ) p h t h a l o c y a n i n e s r e v e a l i n creased stabilization o f chromium(II) w i t h strong donor solvents. T h i s effect c o u l d b e d u e t o s t a b i l i z a t i o n o f t h e l o w s p i n d c h r o m i u m ( I I ) t h r o u g h b a c k d o n a t i o n to the p h t h a l o c y a n i n e r i n g . H o w e v e r , further d a t a a r e n e c e s s a r y t o u n d e r s t a n d t h i s p h e n o m e n o n , e s p e c i a l l y as a similar stabilization o f l o w - s p i n d manganese(II) phthalocyanine ap p a r e n t l y is n o t e v i d e n t ( T a b l e V ) . S o m e d a t a e x i s t c o n c e r n i n g t h e effect o f r i n g s u b s t i t u t i o n o n r e d o x e n e r g i e s (see T a b l e I I ) , h o w e v e r , s i g n i f i c a n t l y less so t h a n i n t h e p o r p h y r i n s e r i e s (28). G e n e r a l l y , e l e c t r o n donors favor r i n g o x i d a t i o n a n d disfavor r i n g r e d u c t i o n . A n interesting c o m p a r i s o n e x i s t s for s u l f o n i c a c i d s u b s t i t u t i o n w h e r e t h e n e u t r a l a c i d f o r m o f T s P c F e ( I I I ) r e d u c e s at - 0 . 4 0 V [vs. n o r m a l h y d r o g e n e l e c t r o d e ( N H E ) ] w h i l e t h e s o d i u m salt, w i t h f o u r n e g a t i v e c h a r g e s o n t h e p e r i p h e r y o f the m o l e c u l e , does not r e d u c e u n t i l - 0 . 6 7 V (5). T h e s e c o n d r e d u c t i o n i s s i m i l a r l y , b u t a l i t t l e less m a r k e d l y , e f f e c t e d . F o r s u b s t i t u e n t s s u c h as c h l o r i d e , m e t h y l tert-butyl, s u l f o n i c a c i d , carb o x y l i c a c i d , a n d o t h e r s , t h e shifts i n r e d o x e n e r g i e s ( e x c e p t for s p e c i a l cases s u c h as j u s t i n d i c a t e d w h e r e t h e c h a r g e o n t h e r i n g is m o d i f i e d ) r a r e l y e x c e e d 100 m V . 4

5

Electronic Spectroscopy G o u t e r m a n e t a l . (20-23) first d e t a i l e d t h e e l e c t r o n i c s t r u c t u r e o f m e t a l l o p h t h a l o c y a n i n e s , s h o w i n g that the t w o p r i n c i p a l b a n d s (both l

l

E

u

e ( π * ) (Q g

e„ ( π * ) ( S o r e t b a n d n e a r 3 5 0 n m ) .

U n l i k e t h e p o r p h y r i n s y s t e m (28), t h e a

lu

and a

2u

orbitals are fairly

w e l l - s e p a r a t e d i n energy, a n d these t w o transitions d o not,

therefore,


248


m i x a p p r e c i a b l y . E m i s s i o n data r e v e a l that the s p i n triplet c o m p o n e n t o f t h e Q b a n d l i e s a b o u t 5 0 0 0 - 5 5 0 0 c m b e l o w t h e s p i n s i n g l e t (29). I n s y s t e m s l a c k i n g c h a r g e t r a n s f e r a b s o r p t i o n , s u c h as m o s t o f t h e m a i n g r o u p s p e c i e s , i t is t h i s s p i n t r i p l e t t h a t i s l i k e l y t o b e p h o t o a c t i v e w h e n t h e p h t h a l o c y a n i n e i s u t i l i z e d as a p h o t o c a t a l y s t . T h e *@ state (fluorescence) has a l i f e t i m e o f o n l y a f e w nanoseconds, w h i l e t h e t r i p l e t state l i f e t i m e i s i n t h e m i c r o s e c o n d - m i l l i s e c o n d r e g i o n at l i q u i d n i t r o g e n t e m p e r a t u r e (29, 30). W h e r e p a r a m a g n e t i c i o n s s u c h as c o p p e r ( I I ) a r e c o n c e r n e d , t h i s l o w e s t state m o s t l i k e l y i s a t r i p l e t m u l t i p l e t , l y i n g at r o u g h l y t h e s a m e e n e r g y as 3Q (28, 31).


- 1

H o w e v e r , the situation c a n change dramatically w h e n charge transfer transitions are present. S u c h transitions o c c u r w h e n e v e r m e t a l d - l e v e l s l i e at e n e r g i e s i n s i d e t h e H O M O - L U M O b a n d g a p o f t h e p h t h a l o c y a n i n e (or c l o s e t o , b u t a b o v e t h e L U M O e n e r g y ) . S u c h t r a n s i t i o n s w e r e d i s c u s s e d i n d e p t h (31 ) a n d o n l y a s u m m a r y o f t h e s e d a t a is p r o v i d e d here. W i t h m o d e r a t e l y o x i d i z i n g i o n s s u c h as m a n g a n e s e ( I I I ) a n d c h r o m i u m ( I I I ) , charge transfer absorption from b o t h t h e p h t h a l o c y a n i n e a a n d a2 o r b i t a l s i n t o e (d) o r b i t a l s o n t h e m e t a l i s a l l o w e d electronically a n d observed readily, the former transition l y i n g i n the n e a r I R r e g i o n ( F i g u r e 1, T a b l e V I ) . lu

U

g

C o n s i d e r t h e charge transfer transition: 2

3

P c ( - 2 ) ( a ) C r ( I I I ) d -> P c ( - l ) ( a l u

labeled L M C T 1

l t t

)Cr(II)d

4

i n F i g u r e 1. T h i s r e a c t i o n m a y b e c o n s t r u e d as t h e

s u m o f t w o redox processes, v i z : Pc(-2)(a

2

l t t

Pc(-2)(a

) Cr(III)d 2

l M

) Cr(II)d

3

2

+ e~ ^ Pc(-2)(a ) Cr(ll)d* lu

4

?± P c ( - l ) ( a

l t t

)Cr(II)d

4

+ e~

(4) (5)

3

whose potentials [ - 0 . 4 0 a n d ~(+0.70)V] can b e s u m m e d to y i e l d a t r a n s i t i o n e n e r g y o f 1.10 e V , t h a t is, a p r e d i c t e d c h a r g e t r a n s f e r e n e r g y o f 8 8 7 0 c m , i n s a t i s f a c t o r y a g r e e m e n t w i t h a n o b s e r v e d t r a n s i t i o n at 7900 c m " . B o t h chromium(III) a n d manganese(III) exhibit charge transfer b a n d s i n t h e n e a r I R r e g i o n , a l t h o u g h for m a n g a n e s e ( I I ) a n d c h r o m i u m ( I I ) species these L M C T b a n d s are b l u e - s h i f t e d to a p p r o x i m a t e l y 11,000 c m " . A treatment s i m i l a r to the one i n d i c a t e d i n E q u a tions 3 - 5 a l l o w s p r e d i c t i o n o f t h e energies o f these charge transfer transitions g e n e r a l l y to w i t h i n a n accuracy o f a b o u t 1 0 0 0 c m " (Table V I ) . - 1

1

1

3

Estimated potential is given in Ref. 31.


1

LEVER ET AL.

11.

Table V I .

Energy Levels of Metallophthalocyanines

Complex TsPcCr(II)

Observed Energy (cm ~')

Calculated Energy ( cm ~')

11,780

11,935 13,710 24,925 8,870 22,590 23,710 10,970 15,725 26,180 6,535 19,740 22,100

? Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 8, 2015 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0201.ch011

249

O b s e r v e d a n d C a l c u l a t e d Charge Transfer D a t a ( D M F )

TsPcCr(III)

25,510 7,900 20,080

PcMn(II)

10,910

TbPcMn(III)

26,300 7,630 20,120

? ?

?

Assignment LMCT1 MLCT2 LMCT2 LMCT1 LMCT2 MLCT2 LMCT1 MLCT2 LMCT2 LMCT1 LMCT2 MLCT2

a

T h e charge transfer transitions concerned are a —>eJd) LMCT1; a -* e (d) LMCT2 and e (d) -*b (w*) MLCT2 (31 ). The counterion for chromium (III) is a sulfonic acid residue, and for manganese(III) it is acetate. lu

g

2u

0

lu

F u r t h e r p r o o f o f the a s s i g n m e n t is o b t a i n e d b y l o c a t i o n o f a s e c o n d c h a r g e transfer b a n d ( L M C T 2 ) , a r i s i n g f r o m a t o e„(d) ( F i g u r e 1), l y i n g b e t w e e n the Q a n d Soret bands. M o s t significantly, the energy separation b e t w e e n these t w o charge transfer b a n d s is a l m o s t e x a c t l y e q u a l to t h e e n e r g y s e p a r a t i o n b e t w e e n t h e Q a n d S o r e t b a n d s (31) . V i r t u a l l y a l l t h e a n t i c i p a t e d L M C T b a n d s i n t h e first-row t r a n s i t i o n m e t a l p h t h a l o c y a n i n e s c a n b e assigned a n d c a l c u l a t e d b y this s i m p l e p r o c e d u r e . M L C T b a n d energies also c a n b e c a l c u l a t e d , b u t a p p e a r to b e too w e a k to b e o b s e r v e d . T h e energies o f o r b i t a l l y f o r b i d d e n L M C T transitions also c a n b e d e r i v e d b y this t e c h n i q u e , a l l o w i n g the pre s u m e d d e t e c t i o n o f states t h a t c a n n o t b e o b s e r v e d d i r e c t l y b y e l e c t r o n i c s p e c t r o s c o p y (see T a b l e V I ) . 2u

3

Surprisingly, such a s i m p l e relationship b e t w e e n electronic ab sorption b a n d s a n d r e d o x p o t e n t i a l energies is successful. E v i d e n t l y , the entropy differences b e t w e e n the various c o m p o n e n t s o f the c o u p l e a r e v e r y s m a l l . M o r e o v e r , t h e L M C T t r a n s i t i o n s a p p e a r to b e ( 0 - 0 ) i n vibrational character, e l i m i n a t i n g another possible source o f disagree m e n t b e t w e e n c a l c u l a t e d r e d o x e n e r g y a n d o b s e r v e d d a t a (31 ). P r e c e d e n t for s u c h a g r e e m e n t b e t w e e n c h a r g e t r a n s f e r e n e r g i e s a n d s u m s o f r e d o x p o t e n t i a l s e x i s t s ( 3 2 , 3 3 ) . B e c a u s e t h e s e L M C T t r a n s i t i o n s fre q u e n t l y l i e at e n e r g i e s b e l o w t h e Q b a n d , t h e y (or h i g h e r s p i n v e r s i o n s ) are l i k e l y to b e p h o t o c h e m i c a l l y a c t i v e . H o w e v e r , l i f e t i m e data are n o t yet available. H e n c e , a combination o f electronic absorption spectroscopy a n d


250



electrochemical measurement can m a p the energy levels o f a metallophthalocyanine w i t h considerable accuracy, a n d provide a measure o f t h e r e d o x p o t e n t i a l s o f t h e v a r i o u s e x c i t e d states, i n f o r m a t i o n o f considerable value i n understanding photochemical behavior. A l though f e w heavy transition metal i o n phthalocyanines have been i n v e s t i g a t e d t o d a t e , t h e i r b e h a v i o r is n o t e x p e c t e d to v a r y g r e a t l y f r o m the details p r e s e n t e d i n this chapter ( p r o v i d e d the metals l i e i n s i d e the phthalocyanine macrocycle ring). I n general, their d-levels w i l l b e b u r i e d b e l o w t h e p h t h a l o c y a n i n e H O M O l e v e l a n d r e d o x p r o c e s s e s at the m e t a l are not e x p e c t e d .

Photochemistry A r a n g e o f m a i n g r o u p , first r o w , a n d l a t e r t r a n s i t i o n m e t a l i o n s w e r e s c r e e n e d r e c e n t l y for t h e i r a b i l i t y t o g e n e r a t e r e d u c e d m e t h y l v i o l o g e n ( M V ) w h e n i r r a d i a t e d (into the Q b a n d ) i n the presence o f m e t h y l v i o l o g e n ( M V ) a n d a d o n o r s u c h as t r i e t h a n o l a m i n e (34). Species containing M g , T i O , Cr(II), Fe(II), Zn(II), Rh(III), a n d Ru(II) generated r e d u c e d m e t h y l viologen, albeit i n s m a l l y i e l d (

Electrochemical and Spectrochemical Studies of Biological Redox

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