Crystallographically Ordered Polymers - American Chemical Society

Chapter 2

Electron Spin Resonance Spectroscopy of Triplet Electron Pairs in Diacetylene Crystals 1

Hans Sixl

Downloaded by RUTGERS UNIV on May 30, 2018 | https://pubs.acs.org Publication Date: March 26, 1987 | doi: 10.1021/bk-1987-0337.ch002

Physikalisches Institut, Pfaffenwaldring 57, D-7000 Stuttgart 80, Federal Republic of Germany

Recent ESR spectra of linear chain diacetylene oligomer and polymer systems are described. The electronic properties of the polymer and of the intermediates are deduced from the t r i p l e t radical pair character of their ground and excited states. It w i l l be demonstrated that the polydiacetylenes represent a unique model system for the study of the p -electron system i n conjugated polymers. There i s a strong correlation between the p -electron structure of the intermediate states and that of the polaron and bipolaron states. The ground state of the r a d i cal electron pairs is a singlet state with S = 0. The excited states are given by t r i p lets (S = 1) and quintets (S = 2) as well as by mixed singlet-quintet states. Recent time resolved ESR spectra of the polymer are interpreted to be due to S = 1 bipolaron states. z

z

I n a s o l i d s t a t e p o l y m e r i z a t i o n r e a c t i o n monomer c r y s t a l s of d i a c e t y l e n e molecules (R-C^c-CEC-R) are converted to p o l y d i a c e t y l e n e c r y s t a l s ( 1 , 2 ) . The p r i m a r y p h o t o c h e m i c a l processes during the low-temperature photopolymerization r e a c t i o n h a v e b e e n i n v e s t i g a t e d b y ESR ( 3 , 4 ) a n d o p t i c a l a b s o r p t i o n spectroscopy ( 5 , 6 ) . A review of the s p e c t r o s copy o f t h e i n t e r m e d i a t e s t a t e s has been g i v e n by S i x l (7). A s i m p l e r e a c t i o n scheme i s shown i n F i g u r e 1. The r e a c t i o n i s c h a r a c t e r i z e d by t h e u v - p h o t o i n i t i a t i o n o f d i r a d i c a l d i m e r m o l e c u l e s . C h a i n p r o p a g a t i o n i s p e r f o r m e d by t h e r m a l a d d i t i o n o f monomer m o l e c u l e s . Thus t r i m e r , t e t r a mer, pentamer e t c . . . . m o l e c u l e s a r e o b t a i n e d . 1

Current address: Hoechst AG, Angewandte Physik, D-6230 Frankfurt am Main 80, Federal Republic of Germany 0097-6156/87/0337-0012$06.00/0 © 1987 American Chemical Society

Sandman; Crystallographically Ordered Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2.

SIXL

13

Triplet Electron Pairs in Diacetylene Crystals

The e l e c t r o n i c s t r u c t u r e o f t h e i n t e r m e d i a t e s is g i v e n by a m i x t u r e o f t h e m e s o m e r i c d i r a d i c a l (DR) and d i c a r b e n e (DC) c o n f i g u r a t i o n s o f F i g u r e 2 (a) and ( b ) . The e n e r g i e s a r e n o n - d e g e n e r a t e . The g r o u n d s t a t e e n e r g y o f t h e d i r a d i c a l D R i s g i v e n by η ε , where ε i s t h e e n e r gy d i f f e r e n c e between t h e b u t a t r i e n e s t r u c t u r e and t h e acetylene s t r u c t u r e per diacetylene u n i t . This quantity has been e s t i m a t e d t o be 0.4 eV (8,9) . The g r o u n d s t a t e e n e r g y o f t h e d i c a r b e n e D C i s g i v e n by ε = 2 . 7 e V , due t o t h e d i s r u p t i o n o f an a d d i t i o n a l τ ι - b o n d . The e n e r g i e s o f t h e p u r e D R and D C c o n f i g u r a t i o n s a r e shown i n F i g u r e 3 . As a c o n s e q u e n c e o f t h e c r o s s i n g o f e n e r g i e s , intermediates with η > 7 are almost pure dicarbenes (9). T h i s i s c o n s i s t e n t with the a c e t y l e n i c c h a i n s t r u c t u r e of p o l y d i a c e t y l e n e c r y s t a l s (_10>) . On t h e o t h e r s i d e t h e o r e t i c a l work on c o n j u g a t e d polymers p r e d i c t s r a d i c a l p a i r e x c i t a t i o n s with s i n g l e t , d o u b l e t and t r i p l e t c h a r a c t e r (J_^) . The e l e c t r o n i c s i n g l e t and t r i p l e t b i p o l a r o n c o n f i g u r a t i o n o f p o l y d i a c e t y l e n e m o l e c u l e s ( 1 β Ρ ° and B P ° , r e s p e c t i v e l y ) c a n be a p p r o x i m a t e d by t h e f o l l o w i n g s i m p l i f i e d s t r u c t u r e s o f F i g u r e 4. The B P ° s t a t e s a r e n e u t r a l ( c h a r g e Q = 0) and r e p r e s e n t e x c i t e d s t a t e s o f t h e c h a i n . The r a d i c a l e l e c t r o n p a i r i s b o u n d , due t o t h e e n e r g e t i c a l l y u n f a v o u r a b l e b u t a t r i e n e s t r u c t u r e . The s i n g l e t r a d i c a l s t a t e s t h e r e f o r e are expected to recombine a t a v e r y s h o r t time s c a l e . However t h e r e c o m b i n a t i o n o f t h e t r i p l e t s i s s p i n - f o r b i d den. T h i s p a p e r w i l l be c o n c e r n e d w i t h t h e E S R - s p e c t r o s c o p y o f S = 1 and S = 2 r a d i c a l e l e c t r o n p a i r s . T h e s e p a i r s a r e p r e s e n t i n the i n t e r m e d i a t e s as w e l l as i n the f i n a l long polydiacetylene molecules. Information con c e r n i n g t h e g r o u n d and e x c i t e d s t a t e r a d i c a l e l e c t r o n s t r u c t u r e s and e n e r g i e s a r e d e d u c e d f r o m t h e ESR f i n e s t r u c t u r e and k i n e t i c s . n

n


n

π

n

3

ESR S p e c t r o s c o p y o f

the

Reaction

Intermediates

The r a d i c a l e l e c t r o n p a i r s o f t h e d i r a d i c a l s a n d d i c a r b e n e s c a n be c o m b i n e d t o s i n g l e t s (S = 0 ) , triplets (S = 1) and q u i n t e t s (S = 2 ) . T y p i c a l ESR s p e c t r a a n d energy l e v e l diagrams of the r e a c t i o n i n t e r m e d i a t e s are shown i n F i g u r e 5 . The d i r a d i c a l s DR2 and DR3 a r e o b t a i n e d i m m e d i a t e l y a f t e r a few e x c i m e r l a s e r p u l s e s . T h e y a r e c h a r a c t e r i z e d by a two l i n e s p e c t r u m and a s m a l l f i n e s t r u c t u r e s p l i t t i n g . The d i c a r b e n e s D C a r e o b t a i n e d a f t e r t h e r m a l r e a c t i o n s . They a r e c h a r a c t e r i z e d by a f o u r l i n e s p e c t r u m and a l a r g e f i n e s t r u c t u r e s p l i t t i n g . A l l s p e c t r a c a n be d e s c r i b e d q u a n t i t a t i v e l y by an e x a c t s p i n h a m i l t o n i a n as shown by t h e l i n e p o s i t i o n s i n F i g u r e 5 a a n d by t h e e n e r g y l e v e l d i a g r a m i n F i g u r e 5 b . The c o m b i n a t i o n o f two t r i p l e t c a r b e n e s l e a d s t o m i x e d s i n g l e t - q u i n t e t s f o r t h e l i m i t o f l a r g e η (J_2) . n


CRYSTALLOGRAPHICALLY ORDERED POLYMERS

•V

X V

R

V

v

V

R

hu hu

\


i

V

-y R ftN

\

% ρ >~R > RkT V*~ -f( R

V

^

R

T

" " ' " ' i:* ' "

i

?

^

^

DIMER

TRIMER

PHOTOINITIATION

R

R

kT kT

H MONOMER

f

ADDITION

;

TETRAMER

POLYMERIZATION

Figure 1. R e a c t i o n scheme o f t h e p o l y m e r i z a t i o n tion in diacetylene crystals. Photoinitiation of d i m e r m o l e c u l e and t h e r m a l a d d i t i o n r e a c t i o n s t o trimer etc.

D R

n

reac the the

c=c=c=c =C=C=C*> C=C-C=C^

R

n-2 s

R

DC,

^C-CEC-C C-C=C-C^ . N

C-CEC-C^

2

*R

S 0 S - 1 S = 2

N

R

Figure 2. D i r a d i c a l D R and D i c a r b e n e D C (n > 2) c o n f i g u r a t i o n s o f t h e i n t e r m e d i a t e s . The s p i n m u l t i p l i c i t i e s a r e g i v e n b y s i n g l e t s (S = 0 ) , t r i p l e t s (S = 1) and q u i n t e t s (S = 2 ) . n

n


2.

15


SIXL

5

.Χ ω

> ο cr

Ε ( D C ) = ε*

3

* 2


LU

^

n

/

Δ

E(DR ) = η·ε n

1

0

I 1

ι 2

ι

3

Α

ι

5

ι

6

ι

CHAIN LENGTH η

7

ι 8

ι 9

ι

ι 10

—

Figure 3. E n e r g i e s o f t h e d i r a d i c a l and d i c a r b e n e configurations. ε i s the energy o f a π - b o n d , ε i s the e n e r g y d i f f e r e n c e o f t h e b u t a t r i e n e and a c e t y l e n e c o n f i g u r a t i o n s per u n i t c e l l . π

Figure 4. P r i n c i p a l t r i p l e t and s i n g l e t b i p o l a r o n configurations of the polydiacetylene c h a i n .



B

0

II χ


(a)

DR

2

0 200 400 MAGNETIC FIELD B [ mT ] 0

F i g u r e 5. Energy l e v e l s p l i t t i n g i n a magnetic f i e l d B a n d Am = 1 E S R - t r a n s i t i o n s a t a m i c r o w a v e f r e q u e n cy o f 9.30 GHz. The f o l l o w i n g parameters a r e used i n t h e c a l c u l a t i o n D R : D / h c = 0.0393 c m " , E / h c = - 0.0016 c m " . D C : D / h c = 0.2960 c m " , E / h c = - 0.0040 cm" and e / h c = 4.5 c m " . 0

1

2

1

1

1 0

1

1

c n


2.

17


SIXL

The t h e r m a l r e a c t i o n o f t h e d i r a d i c a l s o f F i g u r e 5 a a t 90 Κ i s shown i n F i g u r e 6. The c u r v e s a r e c a l c u l a t e d using the d i f f e r e n t i a l equations of a thermal a d d i t i o n r e a c t i o n ( i n t e r m e d i a t e (n) + monomer -*· i n t e r m e d i a t e (n + 1 ) ) . The t r i p l e t s c h a n g e t o q u i n t e t s a t η = 7. S t a r t i n g f r o m η = 7 t r i p l e t s and q u i n t e t s a r e p r e s e n t s i m u l t a n e o u s l y a s shown by Neumann and M u l l e r ( V 3 ) · The r e a c t i o n r a t e s o f t h e t h e r m a l r e a c t i o n o f t h e d i f f e r e n t intermediates η are s t r o n g l y temperature depen d e n t and c a n be d e s c r i b e d by an A r r h e n i u s l a w , g i v e n by k = k e x p ( - E ( n ) / k T ) . The r e s u l t i n g a c t i v a t i o n e n e r g i e s f o r t h e r m a l a d d i t i o n r e a c t i o n s E ( n ) a r e shown i n F i g u r e 7 together w i t h the f i n e s t r u c t u r e parameters D / h c , the h y p e r f i n e s t r u c t u r e p a r a m e t e r s A-j a n d A and t h e s i n g l e t t r i p l e t energy s e p a r a t i o n e ( n ) . As seen from F i g u r e 7 t h e change f r o m d i r a d i c a l s t o d i c a r b e n e s a t η = 6 i m p l i e s g r o s s changes i n t h e p h y s i c a l p r o p e r t i e s o f the i n t e r m e d i a t e s . O n l y d i r a d i c a l and d i c a r b e n e t r i p l e t s t a t e p r o p e r t i e s a r e shown i n t h e F i g u r e . The c o m b i n a t i o n o f t h e sp - r a d i c a l e l e c t r o n i n t h e d i r a d i c a l s l e a d s t o a s i n g l e t g r o u n d s t a t e and t o a t h e r m a l l y a c t i v a t e d t r i p l e t s t a t e . The s p and p - r a d i c a l e l e c t r o n s i n t h e carbene form a t r i p l e t ground s t a t e and a s i n g l e t e x c i t e d s t a t e . F i n a l l y , t h e c o m b i n a t i o n o f two t r i p l e t ground s t a t e carbenes a g a i n l e a d s to a s i n g l e t g r o u n d s t a t e and t o t h e r m a l l y a c t i v a t e d t r i p l e t and q u i n t e t s t a t e s . The c o n s e q u e n c e i s t h a t a l l ESR s p e c t r a a r e t h e r m a l l y a c t i v a t e d and t h e r e f o r e d i s a p p e a r a t e x t r e m e l y low t e m p e r a t u r e s . The s i n g l e t - t r i p l e t and s i n g l e t - q u i n t e t energy s e p a r a t i o n s e and e Q c a n be d e t e r m i n e d f r o m t h e t e m p e r a t u r e d e p e n d e n c e o f t h e ESR i n t e n s i t i e s . They a r e g i v e n i n F i g u r e 7. A g a i n t h e d i r a d i c a l p r o p e r t i e s w i t h η < 6 a r e d i f f e r e n t from t h o s e o f t h e d i c a r b e n e s f o r η > 7. n

Q

a

a


2

S T

2

2

z

S T

ESR-Spectroscopy of

S

E x c i t a t i o n s on t h e

Polymer Chain

The o p t i c a l a b s o r p t i o n and t h e c o r r e s p o n d i n g prompt f l u o rescence s p e c t r a o f d i a c e t y l e n e polymer molecules are shown i n F i g u r e 8. The s p e c t r a a r e t y p i c a l f o r a l m o s t a l l d i f f e r e n t l y s u b s t i t u t e d p o l y d i a c e t y l e n e polymer molecules. T h e y a r e due t o (S = 0) s i n g l e t F r e n k e l e x c i t o n s o f t h e q u a s i o n e - d i m e n s i o n a l s y s t e m . Upon e x c i t a t i o n a t much higher energies a r e l a t i v e l y broad delayed emission a p p e a r s , w h i c h i s s t r o n g l y r e d s h i f t e d a s compared t o t h e e x c i t o n e m i s s i o n . The e x c i t a t i o n and t h e c o r r e s p o n d i n g d e l a y e d e m i s s i o n s p e c t r a a r e a l s o shown i n F i g u r e 8. The o b s e r v e d 720 n m - e m i s s i o n a n d t h e c o r r e s p o n d i n g h i g h - e n e r g y a b s o r p t i o n w i t h maximum a t 380 nm a r e t e n t a t i v e l y e x p l a i n e d by e l e c t r o n - h o l e p h o t o e x c i t a t i o n , l a t t i c e - r e l a x a t i o n t o p o l a r o n p a i r s P and P" and p o l a r o n r e c o m b i n a t i o n l u m i n e s c e n c e e m i s s i o n . The r e s u l t i n g t r i p l e t b i p o l a r o n s B P ° w i t h a l i f e t i m e o f a b o u t 80 \is c a n be d e t e c t e d i n a t r a n s i e n t a b s o r p t i o n e x p e r i m e n t d e s c r i b e d by O r e n s t e i n e t a l ( 1_4) . +

3




-ι

0

1

2.5

1

1

5.0

7.5

TIME t [ h ]

τ10.0

F i g u r e 6. Thermal r e a c t i o n k i n e t i c s o f the different r e a c t i o n i n t e r m e d i a t e s . The u p p e r p a r t shows t h e triplets (n = 2 , 3 , 4 and 5 a r e p u r e d i r a d i c a l s ) . The l o w e r p a r t shows t h e q u i n t e t s (n > 7 a r e p u r e d i c a r benes).



SIXL


2

4

6

8

CHAIN LENGTH [ MONOMER UNITS ]

F i g u r e 7. Fine s t r u c t u r e c o n s t a n t s D , h y p e r f i n e s t r u c ture constants A , a c t i v a t i o n energies of the thermal r e a c t i o n and s i n g l e t t r i p l e t e n e r g y s e p a r a t i o n o f t h e d i r a d i c a l and d i c a r b e n e t r i p l e t s t a t e s . T h e r e i s a c h a r a c t e r i s t i c change o f a l l p r o p e r t i e s when t h e s t r u c t u r e changes from d i r a d i c a l s t o d i c a r b e n e s .


20


Wavelength λ [nm] 1000

800

700

600

500

400

100 J r-? 80 J

40 J

V)

c

prompt emission

absorption

a> Downloaded by RUTGERS UNIV on May 30, 2018 | https://pubs.acs.org Publication Date: March 26, 1987 | doi: 10.1021/bk-1987-0337.ch002

c

20 0 100 J

Crystallographically Ordered Polymers - American Chemical Society

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