Electron and Energy Transfer from Phenothiazine Triplets - ACS

6 E l e c t r o n a n d Energy Transfer f r o m Phenothiazine Triplets 1

A. M. BRAUN, M.-A. GILSON, M. KRIEG, M.-T. MAURETTE, P. MURASECCO, and E. OLIVEROS 1

Downloaded by UNIV OF MELBOURNE on October 13, 2014 | http://pubs.acs.org Publication Date: May 14, 1985 | doi: 10.1021/bk-1985-0278.ch006

Institut de Chimie Physique, Ecole Polytechnique Fédérale de Lausanne, Ecublens, CH-1015 Lausanne, Switzerland

Phenothiazine derivatives have found many applications in todays chemical industry; they have been and are used primarily as dye-stuffs, antioxidants and in pharmaceutical preparations. This application is due to the discovery of their neuroleptic activity (1) (e.g. Chlorpromazine), but allergic skin reactions and ocular opacity are known to occur during therapy. Assuming that these side effects might be primarily initiated by light induced electron or energy transfers, investigations on the conditions where such transfers take place with high efficiency and with some specificity are of fundamental interest. Phenothiazine and its N-alkylated derivatives have also been of interest as electron donating substrates in early investigations of photochemically induced charge separation (2-5), necessary for a successfull photochemical energy conversion. Besides phototoxicity, which might involve oxygen, and energy conversion, just two examples of model investigations, phenothiazine derivatives are convenient probes in micelles and mixed aggregates, model systems themselves for preparative applications (6) as well as biomimetic environments (7). Phenothiazine Phenothiazine is readily oxidized when irradiated in solution with chlorinated hydrocarbones 08). The reaction has been shown to be an electron transfer generating the phenothiazine (PTH) radical cation (ΡΤΗ ') and the halogen anion as shown in equation 1. hv , PTH + RC1 • PTH + R* + Cl (1) +

+

Photooxidation may lead to a similar electron transfer yielding PTH * and superoxide, hv . PTH + 0 • PTH ' + 0"' (2) +

2

7

Current address: Laboratoire IMRCP, ERA 264, Univ. Paul Sabatier, F-31062 Toulouse, France 0097-6156/85/0278-0079$06.00/0 © 1985 American Chemical Society

In Organic Phototransformations in Nonhomogeneous Media; Fox, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

80

O R G A N I C P H O T O T R A N S F O R M A T I O N S IN N O N H O M O G E N E O U S M E D I A

i n f a c t , the n e u t r a l p h e n o t h i a z i n y l r a d i c a l ( P T * ) , produced by s u b s e quent d e p r o t o n a t i o n of P T H * ( E q u a t i o n 3 ) , has been i d e n t i f i e d by ESR spectroscopy (9). +

PTH "

•

+

PT" + H

(3)

+

The c o r r e s p o n d i n g ESR s i g n a l has a l s o been a s s i g n e d t o the n i t r o x i d e d e r i v a t i v e of p h e n o t h i a z i n e (10) which i s thought to be g e n e r a t e d by i n s e r t i o n of s i n g l e t oxygen ( 1 θ ) ( E q u a t i o n 4) i n t o t h e N-H-bond ( E q u a t i o n 5) and subsequent h o m o l y s i s of the h y d r o p e r o x i d e ( E q u a t i o n 6).


2

PTH + 0

•

2

PTH +

0

(4)

2

OOH

O*

However, the r e s u l t s of the s e n s i t i z e d o x y g e n a t i o n do n o t s u p p o r t such a sequence of r e a c t i o n s ( v i d e i n f r a ) . In a d d i t i o n t o the p o s s i b i l i t i e s o f e l e c t r o n t r a n s f e r ( E q u a t i o n 2) and energy t r a n s f e r ( E q u a t i o n 4 ) , e l e c t r o n t r a n s f e r t o s i n g l e t oxygen ( E q u a t i o n 7) and subsequent d e p r o t o n a t i o n ( E q u a t i o n 3 ) , or hydrogen t r a n s f e r t o s i n g l e t oxygen ( E q u a t i o n 8) (8) are to be t a k e n i n t o c o n s i d e r a t i o n and make an attempt of a d i f f e r e n t i a t i o n between those p o s t u l a t e d mechanisms e x t r e m e l y d i f f i c u l t . PTH +

1

0

2

PTH + i o

2

•

PTH *

+ 0~

•

PT*

m'

+

+

(7)

#

(8)

2

The i d e n t i f i c a t i o n of the major p r o d u c t s of the p h o t o o x i d a t i o n of p h e n o t h i a z i n e upon i t s d i r e c t e x c i t a t i o n does n o t s u p p o r t the i d e a o f a N - h y d r o p e r o x y i n t e r m e d i a t e ( E q u a t i o n 5) (11) . In f a c t , p h e n o t h i a z i n e - 5 - o x i d e (A) which has been f i r s t i d e n t i f i e d by comparing Rf v a l u e s i n t h i n l a y e r chromatography of i s o l a t e d and s e p a r a t e l y p r e p a r e d samples (11) r e p r e s e n t s the b e s t e v i d e n c e of a s i n g l e t oxygen r e a c t i o n ( E q u a t i o n 9) (12). Ο t

Η A


6.

B R A U N ET A L .

Phenothiazine

81

Triplets

Other i s o l a t e d p r o d u c t s c o n s i s t of the 3 H - p h e n o t h i a z i n e - 3 - o n e d e r i v a t i v e s Β ( 1 3 ) , C and D (11) , the f o r m a t i o n of which may be e x p l a i n e d by a r a d i c a l h y d r o x y l a t i o n of P T " .


Β

C

S i n c e s u p e r o x i d e r e d u c e s r e a d i l y q u i n o i d s t r u c t u r e s ( e . g . B) ( 1 4 ) , product C (7-hydroxy-3H-phenothiazine-3-one) may have a s i m i l a r g e n e s i s , and 7 - ( 1 0 - p h e n o t h i a z i n y l ) - 3 H - p h e n o t h i a z i n e - 3 - o n e (D) represents one of the many p o s s i b l e r e c o m b i n a t i o n p r o d u c t s s t a r t i n g from P T * .

D Dye s e n s i t i z e d p h o t o o x i d a t i o n of p h e n o t h i a z i n e s h o u l d r e v e a l whether i n a d d i t i o n to r e a c t i o n 9 any c o m p e t i t i v e s i n g l e t oxygen r e a c t i o n ( E q u a t i o n 5) o r e l e c t r o n t r a n s f e r p r o c e s s ( E q u a t i o n 7) i s o c cur i n g . R o s e n t h a l and Poupko (15) r e p o r t e d t h a t r a d i c a l f o r m a t i o n c o u l d be o b s e r v e d by ESR s p e c t r o s c o p y d u r i n g a methylene b l u e s e n s i t i z e d p h o t o o x i d a t i o n of p h e n o t h i a z i n e ; the s i g n a l was s u p p r e s s e d , however, upon a d d i t i o n of DABCO. S i n c e t h i s quencher does n o t e f f e c t ESR s p e c t r a o f s e v e r a l c o m m e r c i a l l y a v a i l a b l e n i t r o x i d e s , t h u s , i n d i c a t i n g t h a t a n i t r o x i d e i n t e r m e d i a t e ( E q u a t i o n 6) does not e x i s t ( 1 6 ) , i t was c l a i m e d t h a t s i n g l e t oxygen undergoes an e l e c t r o n t r a n s f e r r e a c t i o n ( E q u a t i o n 7 ) , the p r o d u c t o f which (PT* a f t e r r e a c t i o n 3) i s no l o n g e r d e t e c t a b l e when the s i n g l e t oxygen i s quenched. Benzophenone (BP) s e n s i t i z e d p h o t o o x i d a t i o n o f p h e n o t h i a z i n e l e a d s m a i n l y t o the f o r m a t i o n of p r o d u c t s A and Β (17) . In v i e w of the many d a t a a v a i l a b l e from the l i t e r a t u r e and from our own e x p e r i ments the p u b l i s h e d i n t e r p r e t a t i o n n e e d s , however, some r e v i s i o n : p r o v i d e d 3 H - p h e n o t h i a z i n e - 3 - o n e s a r e n o t p r o d u c t s of a s i n g l e t oxygen r e a c t i o n w i t h ground s t a t e p h e n o t h i a z i n e ( c f . methylphenothiazine), f o r m a t i o n of p r o d u c t s A and Β i m p l i e s two d i f f e r e n t o x i d a t i o n mechanisms. S i n c e s i n g l e t oxygen s e n s i t i z a t i o n by benzophenone ( E q u a t i o n s 10 t o 12) i s known to be v e r y i n e f f i c i e n t , r e a c t i o n sequence ( 1 0 ) , (13), ( 1 4 ) , and (9)



82

1 QBP

-i Χ

h

1 ^BP

(10)

•

~ ^BP

(11)

ISC

ΒΡ

^BP + 0


•

v

•

2

*BP +

X

0

(12)

2

^BP + *PTH

•

^BP + ^PTH

(13)

^PTH + 0

•

*PTH + O

(14)

2

l

seems l o g i c a l . F o r p r o d u c t B, e i t h e r r e a c t i o n 13 c o u l d be by r e a c t i o n 15 ^BP + PTH

•

BP"*

paralleled

+ PTH '

(15)

+

l e a d i n g i n oxygen s a t u r a t e d benzene to s u p e r o x i d e ( E q u a t i o n 1 6 ) , r e a c t i o n 7 i s s u c c e s s f u l l y competing w i t h r e a c t i o n 9 . BP""

+ 0

•

2

BP + 0~*

C o n s i d e r i n g r e a c t i o n s 3 and 17, b o t h ways a r e d u c t Β by r a d i c a l h y d r o x y l a t i o n . Ο"

-

+ H

^=ςρ

+

H0

or

(16) leading f i n a l l y to

2

pro

(17)

I t i s e v i d e n t t h a t , f o r the r e a c t i o n sequences p r o p o s e d , the q u a n t i t a t i v e a n a l y s i s o f p r o d u c t Β as a f u n c t i o n o f added DABCO cannot be used as a d e c i s i v e argument i n f a v o u r of or a g a i n s t a s i n g l e t oxygen mechanism. M o r e o v e r , the i n c r e a s e of the r a t e of p h e n o t h i a z i n o n e f o r m a t i o n w i t h the c o n c e n t r a t i o n o f p r o t i c s o l v e n t where s i n g l e t oxygen i s quenched e f f i c i e n t l y s u p p o r t s the g i v e n i n t e r p r e t a t i o n . In c o n c l u s i o n , no d i f f e r e n t i a t i o n r e g a r d i n g r e a c t i o n s 2 and 7 can be drawn from t h e a v a i l a b l e d a t a . Many e x p e r i m e n t a l d i f f i c u l t i e s a r i s i n g from the r a t h e r g r e a t number o f p r o d u c t s found due t o the d e p r o t o n a t i o n of P T H " ( r e a c t i o n 3) c a n , however, be reduced i n t a k i n g s i m p l e N - a l k y l a t e d p h e n o t h i a z i n e d e r i v a t i v e s as model compounds. In a d d i t i o n , the use of m i c r o h e t e r o g e n e o u s systems ( m i c e l l e s , m i c r o e m u l s i o n s ) as a r e a c t i o n e n v i r o n m e n t , f a v o u r i n g charge s e p a r a t i o n , might be advantageous f o r the d i f f e r e n t i a t i o n between e l e c t r o n and energy transfer reactions. +


6.

B R A U N ET A L .

Phenothiazine

83

Triplets

N-Methyl-phenothiazine T r i p l e t s o f N - m e t h y l - p h e n o t h i a z i n e (^MPT) a r e found to reduce almost quantitatively C u but t o t r a n s f e r t h e i r energy t o N i and C o ( 1 8 ) . E x p e r i m e n t s i n s o l u t i o n ( e t h a n o l / w a t e r : 1/2) and i n aqueous Cu(LS)2 m i c e l l e s show t h a t the e l e c t r o n t r a n s f e r i n f u n c t i o n a l i z e d o r g a n i z e d media i s e x t r e m e l y e f f i c i e n t . The r a t e c o n s t a n t of e l e c t r o n t r a n s f e r ( E q u a t i o n 18) i n s o l u t i o n has been measured 2 +


^MPT + C u

2 +

•

2 +

ΜΡΤ · +

+ Cu

2 +

(18)

+

to be 1 . 0 ( ± 0 . 1 ) χ 1 0 M " " s " (18) and i s , t h u s , ^40 times slower than t h a t e s t i m a t e d i n Cu(LS)2 m i c e l l e s . Energy t r a n s f e r t o N i and C o ( E q u a t i o n 1 9 ) , r e s p e c t i v e l y , i s slower by about a f a c t o r of 100; t a b l e I shows the c o r r e s p o n d i n g q u e n c h i n g r a t e c o n s t a n t s , i n d i c a t i n g 9

1

1

2 +

?MPT + C o a faster by about

Table

I.

2 +

2 +

(5xlO" M)

Co

(5x10"%) 2

functionalized

micelles

M)

2 +

7.7(±0.1)

3

2

Co(LS)

(19)

2 +

Rate c o n s t a n t s of t h e energy t r a n s f e r from ^MPT (^10 to N i and C o i n s o l u t i o n ( e t h a n o l / w a t e r : 1/2) and aqueous l a u r y l s u l p h a t e m i c e l l e s (18).

Ni(LS) 2 +

*MPT + C o

t r i p l e t quenching i n a p p r o p r i a t e l y a f a c t o r of 600.

2 +

Ni

•

2 +

χ 10

6

M" s" 1

1

(2xlO" M) 2

1.4(±0.1)

(2xl0" M) 2

χ 10

7

M" s" 1

2.1(±0.1)

χ 10

7

s"

1

4.2(±0.1)

χ 10

7

s""

1

1

^PTH r e d u c e s E u ( E ° = - 0 , 4 V) a t a r a t e comparable to t h a t of r e a c t i o n 19, but o n l y t r i p l e t quenching i s o b s e r v e d when M n i s used as a q u e n c h e r . The r a t e c o n s t a n t s of the l a t t e r r e a c t i o n i n s o l u t i o n and aqueous m i c e l l a r systems c o r r e s p o n d to energy t r a n s f e r r a t e c o n s t a n t s measured w i t h N i and C o ( E q u a t i o n 19, T a b l e I). J

2 +

2 +

2 +

In c o n t r a s t t o the e l e c t r o n t r a n s f e r r e a c t i o n s i n q u e s t i o n , the e f f i c i e n c y o f which i s p r i m a r i l y d e t e r m i n e d by the d i f f e r e n c e of r e dox p o t e n t i a l s of the e x c i t e d donor m o l e c u l e and the a c c e p t o r a n d , h e n c e , by the v a r i a t i o n i n f r e e energy ( 2 1 ) , e f f i c i e n c i e s of e x o t h e r mic energy t r a n s f e r s depend s o l e l y on the l o c a l c o n c e n t r a t i o n of an a p p r o p r i a t e quencher. I f MPT i s s o l u b i l i z e d i n aqueous CTAB m i c e l l e s t o g e t h e r w i t h a h y d r o p h o b i c quencher, s t a t i s t i c a l and p r o x i m i t y e f f e c t s w i l l i n f l u ence the e f f i c i e n c y and r a t e of the energy t r a n s f e r i n the same d i r e c t i o n . Thus r e a c t i o n 20 i s enhanced i n the c o n f i n e d space of a CTAB m i c e l l e by a f a c t o r of ^10 (19) .


84


^MPT + t r a n s - s t i l b e n e

In b o t h c a s e s ,

•

where t h e quencher

where t h e quencher

*MPT + ^ t r a n s - s t i l b e n e

(20)

i s i d e n t i c a l with the c o u n t e r i o n or

i s hydrophobic,

confinement o f t h e r e a c t i o n

space

to t h e m i c e l l e s i z e i n d u c e s b o t h an enhancement o f t h e o b s e r v e d r e a c t i o n as w e l l as a r e d u c t i o n o f t h e r e a c t i o n o r d e r from two t o o n e . The o b s e r v e d r e v e r s i b l e energy t r a n s f e r energy o f ^MPT a t ^255 k J . m o l " (19).

( E q u a t i o n 21) p l a c e s t h e

1

3 1 jMPT + n a p h t h a l e n e

—


Q

^

Q

1 3 MPT + ^ n a p h t h a l e n e

(21)

S i n c e t r i p l e t N - m e t h y l - p h e n o t h i a z i n e r e d u c e s NO3, t h e redox

potential

of ^MPT must be < - l V and has been e s t i m a t e d t o be - 1 . 8 V ( 2 0 ) . The e l e c t r o n t r a n s f e r r a t e o b s e r v e d f o r r e a c t i o n 22 i n aqueous DTAC m i c e l l a r systems i s 3.5x10^ M""*s~^ a n d , t h u s , under t h e e x p e r i m e n t a l c o n d i t i o n s r e p o r t e d , about a f a c t o r o f 10^ s l o w e r than r e a c t i o n 18.

^MPT + N0~

•

M P T " + NO "* +

(22)

2

T h i s i s i n agreement w i t h t h e p r e d i c t i o n s based on t h e Marcus t h e o r y (21) , t h a t t h e r a t e o f r e a c t i o n s h o u l d be c o n s i d e r a b l y below t h e d i f f u s i o n - c o n t r o l l e d l i m i t due t o t h e r e l a t i v e l y s m a l l d e c r e a s e i n f r e e energy i n t h e r e d u c t i o n o f NO3. I t i s i n t e r e s t i n g t o n o t e t h a t t h e r a t e o f r e a c t i o n 22 i s e n hanced i n c a t i o n i c m i c e l l a r systems w i t h r e s p e c t t o a n i o n i c m i c e l l a r systems b u t remains slower by a f a c t o r o f 10 t h a n i n homogeneous s o l u t i o n ( w a t e r - e t h a n o l ) . T h i s has been i n t e r p r e t e d i n terms o f r e a c t i o n e n e r g e t i c s a t t r i b u t i n g t o QMPT and ^MPT s o l u b i l i z e d i n m i c e l l a r systems h i g h e r redox p o t e n t i a l s t h a n i n homogeneous s o l u t i o n ( 2 0 ) . T a b l e I I shows, however, t h a t t h e ground s t a t e redox p o t e n t i a l o f N m e t h y l - p h e n o t h i a z i n e i s found s m a l l e r i n m i c e l l a r a g g r e g a t e s (SLS) and t h e s u b s t a n c e hence a b e t t e r r e d u c t a n t under t h e s e c o n d i t i o n s (22) . Table

II.

S t a n d a r d redox p o t e n t i a l s o f MPT i n s o l u t i o n and i n m i c e l l a r aggregates.

System

[MPT] ΙΟ"

3

10-*

EtOH/H 0 2

E°

(2/1)

1 0 M SLS i n H 0 _ 1

2

0.62

Remarks [LiC10 ] 4

:

4.8xlO" M 2

0.43

The d i f f e r e n c e o f t h e s e redox p o t e n t i a l s i s too b i g t o be e x p l a i n e d by a c o m p l e x a t i o n w i t h t h e s u r f a c t a n t a g g r e g a t e ( 2 3 ) . A d e t a i l e d s t u dy o f t h e dependence o f t h e redox p o t e n t i a l o f MPT from t h e n a t u r e and t h e c o n c e n t r a t i o n o f t h e s u r f a c t a n t l e a d s t o t h e c o n c l u s i o n t h a t MPT * i s i n t e r a c t i n g w i t h the a n i o n i c s u r f a c t a n t s y s t e m , i n t e r a c t i o n most pronounced w i t h t h e monomeric s u r f a c t a n t e n t i t i e s ( 2 4 ) . +


6.

BRAUN ET AL.

Phenothiazine

85

Triplets

E x p e r i m e n t a l e v i d e n c e f o r t h i s i n t e r p r e t a t i o n ' s g i v e n by ESR s p e c t r o s c o p y , and the UV s p e c t r a i n d i c a t e t h a t MPT * i s l o c a l i z e d i n a highly polar

region

of t h e a g g r e g a t e ,

groups of the s u r f a c t a n t . Similar t r i p l e t energies

t h a t i s now the a n i o n i c head

( 1 8 ) and ground s t a t e

donor

r i s t i c s o f MPT and PTH i n s o l u t i o n i m p l y t h a t t r i p l e t a l s o f the two s u b s t a n c e s must a l s o be c l o s e .

characte

redox

potenti


Based on t h e s e r e s u l t s o f model e l e c t r o n and energy t r a n s f e r r e a c t i o n s JMPT must produce s u p e r o x i d e by b o t h d i r e c t ( E q u a t i o n 2 3 ) , as w e l l as p r o t o n a s s i s t e d t r a n s f e r ( E q u a t i o n 2 4 ) . ^MPT + 0

•

2

1

MPT""* + 0~*

Ε

ÎjWr

+ 0

2

+ Η

+

°ο /ο-·

- -°·

2

•

(23)

3 3

Electron and Energy Transfer from Phenothiazine Triplets - ACS

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