Chapter 4
Applications of Fluorescence Techniques for the Study of Polymer Solutions Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on April 2, 2018 | https://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch004
Herbert Morawetz Department of Chemistry, Polytechnic University, Brooklyn, NY 11201 Intramolecular excimer emission, the polarization of fluorescence, nonradiative energy transfer and the use of medium-sensitive fluorophores has been used to study the conformational mobility of polymers in dilute solution, the interpenetration of chain molecules, the association of polymers with each other or with small species and the cooperative transition of certain polycarboxylic acids from a compact to an expanded state. The use o f f l u o r e s c e n c e t e c h n i q u e s f o r t h e c h a r a c t e r i z a t i o n o f the b e h a v i o r o f c h a i n m o l e c u l e s i n s o l u t i o n o f f e r s a number o f important advantages: (a) S i n c e f l u o r e s c e n c e c a n be d e t e c t e d a t extreme d i l u t i o n o f t h e e m i t t i n g s p e c i e s , l a b e l i n g polymers w i t h f l u o r o p h o r e s need not r e s u l t i n s i g n i f i c a n t m o d i f i c a t i o n s o f t h e i r o t h e r p r o p e r t i e s . (b) S i n c e t h e l i f e t i m e o f t h e e x c i t e d chromophore, •t, i s g e n e r a l l y o f t h e o r d e r o f 10 ns, t h e k i n e t i c s o f f a s t p r o c e s s e s whose r e l a x a t i o n times a r e comparable t o t may be s t u d i e d by fluorescence techniques. ( c ) The phenomenon o f n o n r a d i a t i v e energy t r a n s f e r over r e l a t i v e l y long d i s t a n c e s may be used t o c h a r a c t e r i z e the s p a t i a l r e l a t i o n o f donor and a c c e p t o r l a b e l e d c h a i n m o l e c u l e s , (d) The medium s e n s i t i v i t y o f t h e e m i s s i o n from t h e " d a n s y l " l a b e l has been used t o s t u d y polymer complex f o r m a t i o n and t h e t r a n s i t i o n of c h a i n molecules from a c o n t r a c t e d t o an expanded s t a t e . Conformâti o n a l ttobj1ity. In c o n s i d e r i n g t h e r a t e a t which a f l e x i b l e c h a i n molecule changes i t s shape i n d i l u t e s o l u t i o n , we a r e f a c i n g a c o n c e p t u a l d i f f i c u l t y i l l u s t r a t e d i n F i g u r e 1. I f a h i n d e r e d r o t a t i o n around a bond i n t h e middle o f t h e c h a i n t a k e s p l a c e w i t h no change i n t h e o t h e r i n t e r n a l a n g l e s o f r o t a t i o n o r bond a n g l e s , then a long segment o f t h e c h a i n would have t o swing through t h e v i s c o u s s o l v e n t w i t h a p r o h i b i t i v e e x p e n d i t u r e o f energy. I f , on t h e o t h e r hand, we a v o i d t h i s by having two h i n d e r e d r o t a t i o n s take p l a c e s i m u l t a n e o u s l y i n what has been c a l l e d a " c r a n k s h a f t - l i k e motion", then two p o t e n t i a l energy b a r r i e r s have t o be surmounted i n t h e t r a n s i t i o n s t a t e and t h i s was i n t u i t i v e l y b e l i e v e d t o l e a d t o a d o u b l i n g o f t h e a c t i v a t i o n energy. W i t h such s t r i c t s i m u l t a n e i t y o f t h e passage over t h e two energy b a r r i e r s h i n d e r e d r o t a t i o n s i n t h e backbone o f long c h a i n 1
0097-6156/87/0358-0037$06.00/0 © 1987 American Chemical Society
Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
PHOTOPHYSICS OF POLYMERS
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on April 2, 2018 | https://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch004
38
(α)
Figure
1.
Schematic r e p r e s e n t a t i o n o f c o n f o r m a t i o n a ] t r a n s i t i o n s i n a f l e x i b l e chain molecule. (a) A s i n g l e hindered rotation, (b) Two c o r r e l a t e d r o t a t i o n s i n a " c r a n k s h a f t - l i k e mot i o n " .
Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on April 2, 2018 | https://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch004
4. MORAWETZ
39
Applications of Fluorescence Techniques
molecules would be expected t o be a t l e a s t a hundred times s l o w e r than i n analogous s m a l l molecules where o n l y a s i n g l e energy b a r r i e r has t o be passed and even i f t h e c o n d i t i o n o f s i m u l t a n e i t y i s some what r e l a x e d , a s u b s t a n t i a l d i f f e r e n c e i n t h e r a t e o f c o n f o r m a t i o n a l change o f polymers and t h e i r analogs might have been expected. The q u e s t i o n whether h i n d e r e d r o t a t i o n around a g i v e n type o f bond i s slowed down when t h i s bond o c c u r s i n t h e middle o f a long c h a i n may be r e s o l v e d unambiguously by t h e use o f a s t r u c t u r e c o n t a i n i n g two a r o m a t i c r e s i d u e s which may be made t o l i e p a r a l l e l to each o t h e r by a s i n g l e c o n f o r m a t i o n a l t r a n s i t i o n . If this tran s i t i o n o c c u r s d u r i n g t h e l i f e t i m e o f t h e e x c i t e d chromophore, excimer e m i s s i o n w i l l be observed. Dibenzylacetamides s u b s t i t u t e d i n t h e para p o s i t i o n s a r e s u i t a b l e f o r such an experiment: R
R
R
/\ * /\
*/\ 0
0
Κ/
Κ^Λ /
I WCO\ -^ \
L N
x
CH_ "* ι 3 monomer
emission
I ΝCO CH ι a
excimer
1
emission
I t was found t h a t excimer e m i s s i o n was reduced o n l y by 27% when t h e d i b e n z y l a c e t a m i d e was b u i l t i n t o t h e c e n t e r o f a long p o l y o x y e t h y l e n e c h a i n as compared t o t h e low m o l e c u l a r weight analog i n which R were methyl g r o u p s . T h i s proves t h a t h i n d e r e d r o t a t i o n i n long polymer c h a i n s t a k e s p l a c e a t s i m i l a r r a t e s as i n s m a l l m o l e c u l e s . A powerful, method f o r t h e study o f c o n f o r m a t i o n a l m o b i l i t y o f polymers u t i l i z e s polymer c h a i n s c a r r y i n g a t t h e i r ends pyrene r e s i d u e s . A l t h o u g h excimer e m i s s i o n from molecules c o n t a i n i n g two phenyl r e s i d u e s i s observed o n l y i f these phenyls a r e s e p a r a t e d by t h r e e atoms, no such r e s t r i c t i o n e x i s t s w i t h m o l e c u l e s c o n t a i n i n g two pyrene m o i e t i e s , s i n c e the e x c i t e d pyrene has a much l o n g e r l i f e t i m e . Thus, i f one o f t h e pyrenes a t t h e end o f a polymer c h a i n i s e x c i t e d , "monomer" e m i s s i o n w i l l be reduced i f t h e two c h a i n ends meet each o t h e r d u r i n g t h e e x c i t e d l i f e time t o form an excimer. T h i s p r i n c i p l e was used t o d e r i v e t h e dependence o f t h e r a t e con s t a n t o f c h a i n c y c l i z a t i o n on t h e l e n g t h o f t h e polymer chain » and the s o l v e n t power o f the medium. Another method f o r t h e study o f t h e c o n f o r m a t i o n a l m o b i l i t y o f polymers u t i l i z e d t h e dependence o f t h e p o l a r i z a t i o n o f t h e e m i t t e d l i g h t on t h e r o t a t i o n a l d i f f u s i o n o f t h e f l u o r o p h o r e . T h i s t e c h nique was p i o n e e r e d by A n u f r i e v a and h e r c o l l a b o r a t o r s who used polymers w i t h anthracene b u i l t i n t o t h e c h a i n backbone. They showed, for instance, that conformational m o b i l i t y i n polymethacrylates i s much s m a l l e r t h a n i n p o l y a c r y l a t e s . 2
3
4
5
6
Polymer C h a i n Entanglements i n S e m i d i i u t e S o l u t i o n s . S i n c e entanglements o f f l e x i b l e c h a i n molecules i n s o l u t i o n l e a d t o a decrease o f configurâtional entropy because o f t h e i n t e r dependence o f t h e c h a i n c o n f o r m a t i o n s , such interpénétration i s
Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
PHOTOPHYSICS OF POLYMERS
40
s t r o n g l y r e s i s t e d i n d i l u t e s o l u t i o n . As t h e c o n c e n t r a t i o n o f the s o l u t i o n i n c r e a s e s , a p o i n t i s e v e n t u a l l y reached where t h e s w o l l e n polymer c o i l s c a n no longer be accommodated without interpénétration. T h i s c o n c e n t r a t i o n i s g e n e r a l l y d e s i g n a t e d by c* and i t i s s a i d t o d i v i d e d i l u t e from " s e m i d i l u t e " s o l u t i o n s , marking c h a r a c t e r i s t i c changes i n t h e c o n c e n t r a t i o n dependence o f thermodynamic and hydrodynamic p r o p e r t i e s . While c* may be r e l a t e d t o t h e dimensions o f polymer c h a i n s d e r i v e d from l i g h t s c a t t e r i n g o r i n t r i n s i c v i s c o s i t y , e s t i m a t e s o f the degree o f c h a i n interpénétration a t c o n c e n t r a t i o n s beyond c* p r e s e n t a more d i f f i c u l t problem. We approached i t by employing the phenomenon o f n o n r a d j a t i v e energy t r a n s f e r . I f a system cont a i n s two f l u o r o p h o r e s such t h a t t h e f i r s t ( t h e "donor") has an e m i s s i o n spectrum which o v e r l a p s t h e a b s o r p t i o n spectrum o f t h e second ( t h e " a c c e p t o r " ) , then t h e e x c i t a t i o n energy o f t h e donor can be t r a n s f e r r e d t o t h e acceptor over r e l a t i v e l y long d i s t a n c e s . The e f f i c i e n c y o f t h i s t r a n s f e r i s g i v e n by
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on April 2, 2018 | https://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch004
7
8
Eff.
= Ro/
