Photochemistry and Photopolymerization Activity of Water-Soluble

Chapter 6

Photochemistry and Photopolymerization Activity of Water-Soluble Benzophenone Initiators Downloaded by UNIV OF NORTH CAROLINA on February 18, 2015 | http://pubs.acs.org Publication Date: December 28, 1990 | doi: 10.1021/bk-1990-0417.ch006

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Norman S.Allen ,F.Catalina ,J. L.Mateo ,R.Sastre ,W. Chen , P. N.Green ,and W. A. Green 4

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1

Department of Chemistry, Faculty of Science and Engineering, John Dalton Building, Manchester Polytechnic, Chester Street, Manchester M1 5GD, United Kingdom Instituto Plasticos y Caucho, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain Institute of Photographic Sciences, Academia Sinica, Beijing, China Ward-Blenkinsop and Company Ltd., Halebank Industrial Estate, Lower Road, Widnes, Cheshire WA8 8NS, United Kingdom 2

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4

Quantitative photophysical, photochemical and photopolymerisation data are presented on five novel water soluble benzophenone photoinitiators. Phosphorescence quantum yields, triplet lifetimes and transient formation on conventional flash photolysis correlate with the ability of the initiators to photoinduce the polymer isation of 2-hydroxyethylmethacrylate and a commercial monoacrylate resin in aqueous media. The results indicate that the lowest excited triplet state of the initiator is abstracting an electron from the tertiary amine cosynergist probably via a triplet exciplex followed by hydrogen atom abstraction. This is confirmed by a detailed analysis on the effect of oxygen, pH and the ionisation potential of the amine on transient formation and photopolymerisation. Using photocalorimetry a linear correlation is found between the photopolymerisation quantum yields of the initiators and their photoreduction quantum yields in aqueous media. One of the photoinitiators, 4-methylsulphoniumbenzophenone sodium salt, exhibited a photoreduction quantum yield of 1.5 which is associated with secondary processes due to the reaction of the alkylamino radicals with a ground-state ketone molecule. I n p r e v i o u s p a p e r s we r e p o r t e d o n t h e p h o t o c h e m i s t r y a n d p h o t o p o l y m e r i s a t i o n a c t i v i t y o f a s e r i e s o f water- and o i l - s o l u b l e thioxanthone d e r i v a t i v e s (1-6). I n t h a t work, a l l t h e t h i o x a n t h o n e s t r u c t u r e s were found t o o p e r a t e e s s e n t i a l l y by a p r i m a r y p h o t o c h e m i c a l p r o c e s s of e l e c t r o n a b s t r a c t i o n v i a an e x c i p l e x w i t h an amine c o - s y n e r g i s t . However, whereas t h e o i l s o l u b l e s t r u c t u r e s were f o u n d t o o p e r a t e s o l e l y t h r o u g h t h e i r l o w e s t e x c i t e d t r i p l e t s t a t e (1,2,6) t h e w a t e r -

0097-6156/90/0417-0072$06.00/0 ο 1990 American Chemical Society In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Downloaded by UNIV OF NORTH CAROLINA on February 18, 2015 | http://pubs.acs.org Publication Date: December 28, 1990 | doi: 10.1021/bk-1990-0417.ch006

6. ALLEN ET A L

Activity of Water-Soluble Benzophenone Initiators

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s o l u b l e s t r u c t u r e s appeared t o operate v i a both t h e i r lowest e x c i t e d s i n g l e t and t r i p l e t s t a t e s ( 3 - 5 ) . E l e c t r o n t r a n s f e r between a s y n e r g i s t i c m i x t u r e o f a t h i o x a n t h o n e p h o t o i n i t i a t o r and an amine has r e c e n t l y been i n v e s t i g a t e d u s i n g l a s e r f l a s h s p e c t r o s c o p y and b e e n f o u n d t o be h i g h l y d e p e n d e n t on t h e p o l a r i t y o f t h e e n v i r o n ment (_7 ) . I n t h i s p a p e r we h a v e e x a m i n e d t h e p h o t o p o l y m e r i s a t i o n and p h o t o p o l y m e r i s a t i o n a c t i v i t y o f f i v e w a t e r s o l u b l e p a r a - s u b s t i t u t e d b e n z o p h e n o n e d e r i v a t i v e s o f t h e s t r u c t u r e s 1-5 shown i n T a b l e I . L u m i n e s c e n c e , p h o t o p o l y m e r i s a t i o n ( u s i n g p h o t o c a l o r i m e t r y ) and p h o t o r e d u c t i o n quantum y i e l d s have been measured f o r t h e s e i n i t i a t o r s as w e l l a s t r a n s i e n t f o r m a t i o n on c o n v e n t i o n a l f l a s h p h o t o l y s i s i n an attempt t o understand the nature of the e x c i t e d s t a t e i n v o l v e d i n t h e p h o t o i n i t i a t i o n p r o c e s s and t h e subsequent m e c h a n i s t i c f e a t u r e s o f t h e p o l y m e r i s a t i o n . The b e n z o p h e n o n e c h r o m o p h o r e i s a s t r o n g t r i p l e t s e n s i t i s e r and t h e a c t i v i t y o f t h e s e d e r i v a t i v e s i n water makes a n i n t e r e s t i n g c o m p a r i s o n , u n d e r t h e same e x p e r i m e n t a l c o n d i t i o n s , w i t h o u r e a r l i e r w o r k on t h e t h i o x a n t h o n e s w h e r e t h e n a t u r e of t h e s u b s t i t u e n t s were s i m i l a r . P r e v i o u s work on a w a t e r - s o l u b l e benzophenone sulphonium s a l t i n i n d u c i n g t h e p h o t o p o l y m e r i s a t i o n o f a c r y l a m i d e h a s shown t h a t p h o t o r e d u c t i o n o c c u r s p r i m a r i l y t h r o u g h the lowest e x c i t e d t r i p l e t s t a t e of the i n i t i a t o r (8-10). Our work h e r e w i t h 2 - h y d r o x y e t h y l m e t h a c r y l a t e and a c o m m e r c i a l w a t e r s o l u b l e monoacrylate r e s i n not only supports t h i s c o n c l u s i o n but a l s o shows t h a t i n a q u e o u s m e d i a t h e p r e s e n c e o f a n a m i n e c o - s y n e r g i s t i s i m p o r t a n t f o r e f f e c t i v e p h o t o r e d u c t i o n and p h o t o p o l y m e r i s a t i o n . I n f a c t one o f t h e p h o t o i n i t i a t o r s e x a m i n e d h e r e i s h i g h l y p h o t o a c t i v e w i t h a measured p h o t o r e d u c t i o n quantum y i e l d g r e a t e r than u n i t y . Experimental Materials. Samples o f t h e benzophenone s a l t s of t h e s t r u c t u r e s (1-5) ( T a b l e I ) w e r e s u p p l i e d b y W a r d - B l e n k i n s o p a n d Company L i m i t e d , W i d n e s , C h e s h i r e , UK a n d p a t e n t c o v e r i s p e n d i n g . The c h e m i c a l names o f t h e s a l t s a r e a s f o l l o w s : ( 1 ) 4 - ( 3 - N , N , N - t r i m e t h y l a m m o n i u m propoxy)benzophenone methylsulphonium s a l t ; (2) 4-(2-hydroxy-3-N,N,Ntrimethylammoniumpropoxy)benzophenone c h l o r o s a l t ; (3) sodium (4-benzoylJphenylmethylsulphonate; (4) 4-(3-sulphoniumpropoxy)benzo phenone sodium s a l t and (5) 4 - ( 2 - h y d r o x y - 3 - s u l p h o n i u m p r o p o x y ) b e n z o phenone sodium s a l t . The w a t e r u s e d i n t h i s s t u d y was d o u b l e d i s t i l l e d a n d t h e amines (N-methyldiethanolamine, tri-n-butylamine,triethylamine, d i e t h y l a m i n e , N - d i e t h y l m e t h y l a m i n e , d i c y c l o h e x y l a m i n e a n d Ν,Ν'd i m e t h y l a n i l i n e ) w e r e o b t a i n e d f r o m t h e A l d r i c h C h e m i c a l Company L i m i t e d , UK a s was t h e monomer 2 - h y d r o x y e t h y l m e t h a c r y l a t e . The m o n o a c r y l a t e w a t e r - s o l u b l e r e s i n RCP-1784 was s u p p l i e d b y L a n k r o C h e m i c a l s L i m i t e d , E c c l e s , m a n c h e s t e r , UK a n d u s e d a s r e c e i v e d . The 2 - p r o p a n o l ( A n a l a r g r a d e ) was o b t a i n e d f r o m F i s o n s L i m i t e d , UK. S p e c t r o s c o p i c Measurements. A b s o r p t i o n s p e c t r a were o b t a i n e d u s i n g a P e r k i n - E l m e r M o d e l 554 S p e c t r o p h o t o m e t e r and phosphorescence s p e c t r a a n d mean l i f e t i m e s w e r e o b t a i n e d a t 77 Κ u s i n g a P e r k i n - E l m e r L S - 5 L u m i n e s c e n c e S p e c t r o m e t e r c o u p l e d t o a 3600 d a t a s t a t i o n . P h o s p h o r e s c e n c e quantum y i e l d s were o b t a i n e d by t h e r e l a t i v e method u s i n g b e n z o p h e n o n e ( 0 p = 0.74 i n e t h a n o l g l a s s a t 77 K) a s a standard (11).

In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

RADIATION CURING OF POLYMERIC MATERIALS

74


F l a s h P h o t o l y s i s . T r a n s i e n t 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 u s i n g a m i c r o s e c o n d k i n e t i c f l a s h p h o t o l y s i s a p p a r a t u s e q u i p p e d w i t h two 16 kV x e n o n - f i l l e d f l a s h l a m p s ( o p e r a t e d a t 10 k V ) a n d a 150 W tungsten-halogen monitoring l i g h t source. T r a n s i e n t decay p r o f i l e s w e r e s t o r e d u s i n g a T e t r o n i x DM6 s t o r a g e o s c i l l o s c o p e . S o l u t i o n s w e r e d e o x y g e n a t e d u s i n g w h i t e - s p o t n i t r o g e n g a s ( b e l o w 5 ppm o x y g e n ) . P h o t o r e d u c t i o n Quantum Y i e l d s . A b s o l u t e quantum y i e l d s o f p h o t o r e d u c t i o n (J3r) f o r t h e i n i t i a t o r s w e r e d e t e r m i n e d i n w a t e r a t 1 0 " ^ M i n t h e presence of 2-N,N-diethanolamine a t a c o n c e n t r a t i o n of 10"^ M u s i n g a n i r r a d i a t i o n w a v e l e n g t h o f 365 nm s e l e c t e d f r o m a P h i l i p s h i g h p r e s s u r e m e r c u r y l a m p (HB-CS 500 W) a n d a K r a t o s GM252 monochromator. S a m p l e c e l l s w e r e t h e r m o s t a t t e d a t 30°C a n d t h e s o l u t i o n s w e r e d e o x y g e n a t e d u s i n g n i t r o g e n g a s ( 5 ppm o x y g e n ) . The a b s o r b e d l i g h t i n t e n s i t y was m e a s u r e d u s i n g an I n t e r n a t i o n a l L i g h t M o d e l 700 r a d i o m e t e r p r e v i o u s l y c a l i b r a t e d b y A b e r c h r o m e 540 a c t i n o m e t r y (12,13) P h o t o l y s i s o f t h e b e n z o p h e n o n e s was m o n i t o r e d by m e a s u r i n g t h e c h a n g e i n UV a b s o r p t i o n o f t h e m a i n Tf-Jf* maximum a t 290 nm. Photopolymerisation. The k i n e t i c s o f p h o t o i n i t i a t e d p o l y m e r i s a t i o n of t h e m o n o a c r y l a t e r e s i n were m o n i t o r e d u s i n g a d i f f e r e n t i a l scanning p h o t o c a l o r i m e t e r d e v e l o p e d a t t h e CSIC on t h e b a s i s o f a m o d i f i e d P e r k i n - E l m e r DSC-4 c a l o r i m e t e r ( L 4 ) . The m o n o a c r y l a t e r e s i n was m i x e d w i t h 2 0 % v/w o f w a t e r f o l l o w e d b y t h e a d d i t i o n o f 1 0 % v/w o f Ν,Ν-dimethylaniline. The l a t t e r a m i n e was c h o s e n f o r s t u d y here owing t o p r a c t i c a l l i m i t a t i o n s w i t h t h i s technique f o r t h e r e s i n used. The w a t e r - s o l u b l e b e n z o p h e n o n e s w e r e d i s s o l v e d i n t h e r e s i n t o g i v e a n a b s o r b a n c e o f 0.3 a t 365 nm u s i n g a c e l l p a t h l e n g t h o f 1.5 mm. The l i g h t i n t e n s i t y was m e a s u r e d b y m o n i t o r i n g t h e t e m p e r a t u r e i n c r e a s e due t o l i g h t a b s o r p t i o n by a t h i n g r a p h i t e c o a t i n g i n a s t a n d a r d aluminium pan p l a c e d i n t h e sample h o l d e r of t h e equipment. P r i o r t o i r r a d i a t i o n samples were a l l o w e d t o e q u i l i b r a t e t o t h e o p e r a t i n g t e m p e r a t u r e o f 40°C f o r 10 m i n u t e s . The DSC t r a c e s w e r e o u t p u t t o a P e r k i n - E l m e r M o d e l 3600 d a t a s t a t i o n a n d r e c o r d e d o n a Hewlett-Packard 7470A g r a p h i c s p l o t t e r . The k i n e t i c s w e r e c r o s s c h e c k e d g r a v i m e t r i c a l l y a f t e r i r r a d i a t i o n a n d g o o d a g r e e m e n t was found. P h o t o p o l y m e r i s a t i o n e f f i c i e n c i e s were a l s o d e t e r m i n e d g r a v i m e t r i c a l l y by i r r a d i a t i n g a q u a r t z c e l l c o n t a i n i n g 10 cm o f a 50:50 v/w m i x t u r e o f 2 - h y d r o x y e t h y l m e t h a c r y l a t e - w a t e r , 0.3% w/v o f i n i t i a t o r ( 2 ) a n d 1.5% w/v o f a m i n e f o r 20 m i n u t e s . The l i g h t s o u r c e u s e d was a T h o r n h i g h p r e s s u r e Hg-W f l u o r e s c e n t l a m p ( 1 5 0 W) s e t a t a d i s t a n c e o f 10 cm a n d t h e r e a c t i o n m i x t u r e was d e o x y g e n a t e d by c o n t i n u a l l y b u b b l i n g w i t h w h i t e s p o t n i t r o g e n g a s . A f t e r i r r a d i a t i o n , t h e m i x t u r e was p o u r e d i n t o e t h a n o l f o r p r e c i p i t a t i o n , a n d t h i s was f o l l o w e d b y c e n t r i f u g i n g , f i l t r a t i o n a n d d r y i n g t o c o n s t a n t w e i g h t i n a h o t a i r o v e r a t 60°C. 3

Results

and D i s c u s s i o n

The p h o t o r e d u c t i o n q u a n t u m y i e l d s f o r t h e f i v e b e n z o p h e n o n e p h o t o i n i t i a t o r s a r e compared i n T a b l e I t o g e t h e r w i t h t h e i r r e s p e c t i v e mean t r i p l e t l i f e t i m e s .


6. ALLEN ET A L Table

Reference

I.

E x t i n c t i o n C o e f f i c i e n t s ( E ) , T r i p l e t L i f e t i m e s ( T ) and P h o t o r e d u c t i o n Quantum Y i e l d s (J3r) o f t h e Water S o l u b l e Benzophenones

4-Substituent

E365 (M" cm 38.46 1


1

+

-OCH CH CH N (CH )3SO3-CH3 2

2

2

3

+

2

-OCH2CH(OH)CH2N (CH3)3CI"

3

-CH2S03~Na

4 5

75

ActivhyofWaUr-SolubkBenzophemmlnuHators

+

-OCH2CH(OH)CH2S03"Na -OCH2CH CH S03"Na 2

+

2

+

J3r

Τ ) (ms) 17.4

0.60

38.78

12.4

0.40

25.65

6.4

1.50

48.68

8.8

0.93

55.78

11.6

0.73

T h e s e d a t a show t w o i n t e r e s t i n g f e a t u r e s . F i r s t l y , a l l t h e p h o t o r e d u c t i o n quantum y i e l d s a r e h i g h , t h e most s i g n i f i c a n t b e i n g a v a l u e o f 1.5 f o r s t r u c t u r e 3. T h i s i n t e r e s t i n g b e h a v i o u r has been a s s o c i a t e d w i t h t h e f o l l o w i n g secondary r e a c t i o n paths between a g r o u n d - s t a t e k e t o n e m o l e c u l e and an a l k y l a m i n o r a d i c a l w h i c h f o l l o w s t h e p r i m a r y p r o c e s s o f h y d r o g e n atom a b s t r a c t i o n f r o m t h e a m i n e by t h e p h o t o e x c i t e d t r i p l e t s t a t e o f t h e c a r b o n y l g r o u p s (15,16). p-R-Ar-CO~-Ar

+ CH(CH

+

)=N R

U p-R-Ar-CO-Ar

+ *C(CH

)HNR 3

2

|2 p-R-Ar-C-Ar

I 0-CH(CH )NR 3

2

(1)

The f i r s t s t e p ( 1 ) i n v o l v e s e l e c t r o n a b s t r a c t i o n f r o m t h e a l k y l amino r a d i c a l by t h e g r o u n d s t a t e k e t o n e t o g i v e a c a r b o n y l r a d i c a l a n i o n a n d a n immonium i o n w h i l e t h e s e c o n d s t e p ( 2 ) i n v o l v e s t h e f o r m a t i o n o f an a d d i t i o n p r o d u c t . The s e c o n d i n t e r e s t i n g f e a t u r e o f t h e r e s u l t s i s t h e c l e a r d i s t i n c t i o n between t h e c a t i o n i c and a n i o n i c t y p e p h o t o i n i t i a t o r s . The l a t t e r s t r u c t u r e s e x h i b i t g r e a t e r a c t i v i t y a n d o n e p o s s i b l e e x p l a n a t i o n , i s t h a t t h e c a t i o n i c s t r u c t u r e s e x h i b i t a much h i g h e r d e g r e e o f i n t e r m o l e c u l a r and i n t r a m o l e c u l a r s e l f - q u e n c h i n g owing t o t h e p r e s e n c e o f t h e q u a t e r n a r y amine s a l t . This e f f e c t , although r e a s o n a b l e , d o e s n o t a p p e a r t o be r e f l e c t e d by t h e i r l o n g e r t r i p l e t lifetimes. Photopolymerisation A c t i v i t y . F r o m t h e DSC c u r v e s t h e q u a n t i t a t i v e d a t a i n T a b l e I I were o b t a i n e d f o r each o f t h e i n i t i a t o r s . The p o l y m e r i s a t i o n r a t e s Rp w e r e c a l c u l a t e d f r o m t h e i n i t i a l s l o p e s o f t h e DSC c u r v e s u s i n g a l e a s t - s q u a r e s a p p r o a c h . These d a t a were t h e n c o n v e r t e d i n t o a c t u a l p h o t o p o l y m e r i s a t i o n q u a n t u m y i e l d s (J3m) by d i v i d i n g by t h e r e a l a b s o r b e d l i g h t i n t e n s i t y o f e a c h s a m p l e ( l a ) , t h e v a l u e s o f J3m a r e e x p r e s s e d a s t h e number o f grammes o f monomer



76


p o l y m e r i s e d by 1 m o l e o f a b s o r b e d p h o t o n s . I n agreement w i t h t h e p h o t o r e d u c t i o n quantum y i e l d s , i t i s seen t h a t t h e c a t i o n i c s t r u c t u r e s a r e much l e s s a c t i v e t h a n t h e a n i o n i c t y p e s , a l t h o u g h t h i s d i s a g r e e s w i t h r e c e n t work o n a c r y l a m i d e p h o t o p o l y m e r i s a t i o n (_T7). The l a t t e r e f f e c t may be d u e t o t h e n a t u r e o f t h e monomer. From t h e d a t a i n Table I I i t i s seen t h a t t h e r e i s a c o r r e l a t i o n between t h e p h o t o r e d u c t i o n and p h o t o p o l y m e r i s a t i o n quantum y i e l d s . This confirms our e a r l i e r c o n c l u s i o n s w i t h r e g a r d t o t h e i m p o r t a n c e o f a p h t o r e d u c t i o n p r o c e s s i n p h o t o p o l y m e r i s a t i o n i n v o l v i n g an amine c o - s y n e r gist. T h e s t r u c t u r e o f t h e b e n z o p h e n o n e a p p e a r s t o be v e r y i m p o r t a n t in controlling this relationship.

Table

Reference 1 2 3 4 5

I I . Q u a n t i t a t i v e P h o t o c a l o r i m e t r i c Data f o r the Benzophenone I n i t i a t o r s

Time a t Peak 1.42 1.52 1.49 1.67 1.65

(g L

-5

P

s 0.64 0.74 0.87 0.77 0.77

-1

) (10

5

1

Ά

-1 einsteins L 3.23 3.78 3.55 3.52 3.78

0m

-1 s )

19700 19520 24463 21886 20372

The r e s u l t s i n T a b l e I I I show t h e e f f e c t o f v a r i o u s a m i n e s o f d i f f e r e n t i o n i s a t i o n p o t e n t i a l o n t h e p h o t o p o l y m e r i s a t i o n o f 2h y d r o x y e t h y l m e t h a c r y l a t e i n n i t r o g e n s a t u r a t e d water i n i t i a t e d by t h e b e n z o p h e n o n e w i t h t h e s t r u c t u r e 2. A s f o u n d e a r l i e r f o r t h e w a t e r s o l u b l e t h i o x a n t h o n e s (6), the percentage photoconversion decreases w i t h i n c r e a s i n g i o n i s a t i o n p o t e n t i a l of t h e amine. Ina l l o f t h e s e e x p e r i m e n t s , o x y g e n h a d a s t r o n g q u e n c h i n g e f f e c t on t h e photopolymerisation. I t would a p p e a r , t h e r e f o r e , t h a t i n aqueous m e d i a t h e p h o t o i n d u c e d p o l y m e r i s a t i o n o f t h e a c r y l a t e monomer o c c u r s s o l e l y v i a t h e l o w e s t e x c i t e d t r i p l e t s t a t e of t h e benzophenone mole c u l e t o f o r m an e x c i p l e x w i t h t h e a m i n e c o - s y n e r g i s t (Scheme I ). This c o n t r a s t s s i g n i f i c a n t l y with our e a r l i e r photopolymerisation r e s u l t s o n t h e w a t e r - s o l u b l e t h i o x a n t h o n e s (6_) w h e r e p h o t o c o n v e r s i o n o f t h e same monomer was u n a f f e c t e d by t h e p r e s e n c e o f o x y g e n , i n d i c a t i n g the p o s s i b i l i t y of p h o t o a c t i v i t y v i a the lowest e x c i t e d singlet state.

Table

I I I . Photopolymerisation of 2-Hydroxyethylmethacrylate i n N i t r o g e n S a t u r a t e d W a t e r by I n i t i a t o r 2 i n t h e P r e s e n c e o f V a r i o u s Amines

Amine N-Methyldiethanolamine Tri-n-butylamine Triethylamine Diethylamine Dicyclohexylamine

Ionisation Potential (eV) 7.20 7.40 7.85 8.40 9.20

%

Conversion (20 m i n s ) 2.31 2.06 0.10 0.03 0.01


6. ALLEN ET A L

hv

BZ

ActivhyofWaUr-SolubkBenzophemnelnUiators

BZ

(BZ A) Exciplex

BZ~

i

77

BZ-/AÎ

ΒΖ· + A-


BZH(Intramolecular Α·-Η

+ M

BZH* M

—

AM*

nM

+ A--H hydrogen

atom

abstraction

Polymer

AM-n+1

Polymer

BZ = B e n z o p h e n o n e ; A = A m i n e Scheme I

Spectroscopic Properties. The p h o s p h o r e s c e n c e q u a n t u m y i e l d s o f t h e i n i t i a t o r s a t 77 Κ i n 2 - p r o p a n o l a r e c o m p a r e d i n T a b l e I V . T h e s e w e r e o b t a i n e d u p o n e x c i t a t i o n a t t h e a b s o r p t i o n maximum o f 290 nm f o r a l l t h e compounds. B o t h t h e a b s o r p t i o n and p h o s p h o r e s c e n c e s p e c t r a o f t h e i n i t i a t o r s w e r e s i m i l a r t o t h o s e o f b e n z o p h e n o n e . The q u a n t u m y i e l d s on t h e o t h e r h a n d w e r e v a r i a b l e . The p h o s p h o r e s c e n c e e m i s s i o n s p e c t r a o f a l l t h e compounds h a d maxima a t 4 1 4 , 443 a n d 474 nm respectively. No f l u o r e s c e n c e was o b s e r v e d w h i c h i s t y p i c a l o f aromatic ketones i n d i c a t i n g a high rate of intersystem c r o s s i n g . The h i g h r a t e i s f u r t h e r s u p p o r t e d b y t h e v a r i a b l e b u t h i g h p h o s p h o r e s c e n c e quantum y i e l d s .

Table IV.

Initiator 1 2 3 4 5

Phosphorescence

P r o p e r t i e s of t h e Benzophenone i n 2-Propanol

P h o s p h o r e s c e n c e E m i s s i o n S p e c t r a nm 414,443,474 n

" " __"

Initiators

J3p ( s e e s ) 0.98 0.41 0.79 0.67 0.76



78


I n f a c t , s t r u c t u r e 1 e x h i b i t s the h i g h e s t r e c o r d e d quantum y i e l d o f 0.98 w h e r e a s s t r u c t u r e 2 e x h i b i t s t h e l o w e s t a t 0.41. Comparison of the p h o s p h o r e s c e n c e quantum y i e l d s w i t h the p h o t o p o l y m e r i s a t i o n q u a n t u m y i e l d s shows a g o o d c o r r e l a t i o n . Thus, f o r the c a t i o n i c t y p e s s t r u c t u r e 1 has a h i g h e r p h o s p h o r e s c e n c e quantum y i e l d t h a n s t r u c t u r e 2 and t h i s i s c o n s i s t e n t w i t h i t s h i g h e r p h o t o p o l y m e r i s a t i o n activity. The same c o r r e l a t i o n a p p l i e s t o t h e t h r e e a n i o n i c s t r u c t ures. From t h e s e r e s u l t s i t w o u l d appear t h a t a h i g h t r i p l e t y i e l d i s c o n s i s t e n t w i t h h i g h p h o t o r e a c t i v i t y , a l t h o u g h i t s h o u l d be p o i n t e d out t h a t the y i e l d s of other competing processes are important i n any o v e r a l l a s s e s s m e n t . However, as d i s c u s s e d above, the p r e s e n c e o f an a m i n e was i m p o r t a n t f o r p h o t o p o l y m e r i s a t i o n t o o c c u r , w h i c h s u g g e s t s t h a t t h e r e a c t i v i t y i s more r e l a t e d t o t h e y i e l d o f t r i p l e t exciplex. The t r i p l e t e x c i p l e x w i l l t h e n d i s s o c i a t e a s i n d i c a t e d i n Scheme I t o g i v e t h e r e a c t i v e a l k y l a m i n o r a d i c a l . F l a s h P h o t o l y s i s . End-of-pulse t r a n s i e n t a b s o r p t i o n s p e c t r a of t h e b e n z o p h e n o n e i n i t i a t o r s i n w a t e r and 2 - p r o p a n o l a r e s u m m a r i s e d i n T a b l e s V and V I . t h e l a t t e r shows t h e e f f e c t o f t h e p r e s e n c e o f t r i e t h y l a m i n e on t r a n s i e n t f o r m a t i o n . I n b o t h s o l v e n t s , weak t r a n s i e n t f o r m a t i o n was o b s e r v e d w i t h a b r o a d maximum b e t w e e n 500 and 600 nm and a much s t r o n g e r b a n d b e l o w 350 nm. Apart from s t r u c t u r e 1, t h e t r a n s i e n t i s s t r o n g e r i n 2 - p r o p a n o l t h a n i t i s i n w a t e r and i s c o n s i s t e n t w i t h e a r l i e r w o r k on b e n z o p h e n o n e w h e r e t h e same t r a n s i e n t was a s s o c i a t e d w i t h t h e k e t y l r a d i c a l f o r m e d by t h e l o w e s t e x c i t e d t r i p l e t s t a t e of the benzophenone a b s t r a c t i n g a hydrogen a t o m f r o m t h e s o l v e n t ( 1 8 , 1 9 ) . T r a n s i e n t f o r m a t i o n due t o h y d r o g e n a b s t r a c t i o n f r o m w a t e r i s u n u s u a l and may w e l l be due t o i n t e r m o l e c u l a r and/or i n t r a m o l e c u l a r h y d r o g e n atom a b s t r a c t i o n f r o m t h e molecules themselves. The p r e s e n c e o f t r i e t h y l a m i n e e n h a n c e s t r a n s i e n t f o r m a t i o n i n b o t h s o l v e n t s and i n d u c e s a r e d s h i f t i n t h e s p e c t r u m o w i n g t o t h e r a d i c a l a n i o n f o r m e d by e l e c t r o n a b s t r a c t i o n v i a t h e t r i p l e t e x c i p l e x shown i n Scheme I. The r a d i c a l a n i o n w i l l t h e n u n d e r g o i n t e r m o l e c u l a r h y d r o g e n atom a b s t r a c t i o n f r o m t h e a m i n e and e n h a n c e k e t y l r a d i c a l f o r m a t i o n . The r e d s h i f t i n t h e t r a n s i e n t spectrum i s analogous to that reported e a r l i e r f o r the benzophenone r a d i c a l a n i o n (18,19).

Table

Initiator

1 2 3 4 5

V.

T r a n s i e n t A b s o r p t i o n Maxima o f Benzophenone I n i t i a t o r s (5 χ 10~ M) i n t h e a b s e n c e and p r e s e n c e o f T r i e t h y l a m i n e ( 1 0 ~ M) i n N i t r o g e n S a t u r a t e d W a t e r

No A m i n e Absorbance Wavelength (nm) 0.012 520 0.010 540 0.010 540 0.012 560 560 0.006

Amine Absorbance Wavelength (nm) 0.030 640 0.015 620 0.013 620 0.023 540 0.016 560


6.

ALLEN ET A L


Table

VI.

Initiator

1 2 3 4 5

Activity ofWater-Soluble Benzophenone Initiators

79

T r a n s i e n t A b s o r p t i o n Maxima of Benzophenone I n i t i a t o r s (5 χ 10 M) i n t h e a b s e n c e and p r e s e n c e o f T r i e t h y l a m i n e (10 M) i n N i t r o g e n S a t u r a t e d 2 - P r o p a n o l

No A m i n e Absorbance Wavelength (nm) 0.008 535 0.015 500 0.017 560 0.018 585 0.017 500 0.028 580 0.014 540

Amine Absorbance Wavelength (nm) 0.022 560 0.028 530 0.022 590 0.050 560 0.024 500 0.027 580 0.027 550

The m o s t i n t e r e s t i n g f e a t u r e o f t h e r e s u l t s i s t h e g r e a t e r enhancement i n the t r a n s i e n t s p e c t r u m i n t h e p r e s e n c e of the amine f o r t h e more p h o t o a c t i v e i n i t i a t o r ( 1 ) c o m p a r e d w i t h t h a t o f t h e l e s s e f f e c t i v e i n i t i a t o r ( 2 ) . Because of v a r i a t i o n s i n a b s o r p t i o n maxima, such c o r r e l a t i o n s a r e d i f f i c u l t but n e v e r t h e l e s s p r o v i d e u s e f u l i n f o r m a t i o n w i t h r e g a r d t o the i n v o l v e m e n t of the t r i p l e t exciplex. F u r t h e r c o n f i r m a t i o n of the f o r m a t i o n of the r a d i c a l a n i o n i s shown by a number o f e x p e r i m e n t a l o b s e r v a t i o n s . The r e s u l t s i n T a b l e V I I show t h e e f f e c t o f d i f f e r e n t a m i n e s on t r a n s i e n t a b s o r p t i o n f o r i n i t i a t o r (1) i n 2 - p r o p a n o l . Here t r a n s i e n t a b s o r p t i o n a t 540 nm i n c r e a s e d w i t h d e c r e a s i n g i o n i s a t i o n p o t e n t i a l o f t h e a m i n e . F i n a l l y , t h e r e s u l t s i n T a b l e V I I I show t h e e f f e c t o f pH on t r a n s i e n t a b s o r p t i o n i n w a t e r f o r i n i t i a t o r ( 1 ) . H e r e t h e pH was v a r i e d u s i n g d i l u t e HC1 and NaOH s o l u t i o n s . I n a g r e e m e n t w i t h e a r l i e r w o r k on b e n z o p h e n o n e s ( 18_) and t h i o x a n t h o n e s ( 6_), t h e t r a n s i e n t a b s o r p t i o n i n c r e a s e s and u n d e r g o e s a s i g n i f i c a n t r e d s h i f t w i t h i n c r e a s i n g pH f r o m 2 t o 12, c o n f i r m i n g t h e i n v o l v e m e n t of t h e r a d i c a l a n i o n . The r o l e o f e x c i t e d s t a t e c o m p l e x e s i n p h o t o p o l y m e r i s a t i o n has b e e n t h e s u b j e c t o f some c o n t r o v e r s y . A l t h o u g h many w o r k e r s h a v e supported the involvement of a c h a r g e - t r a n s f e r complex between a r o m a t i c c a r b o n y l compounds and t e r t i a r y a m i n e s t h r o u g h w h i c h e l e c t r o n t r a n s f e r and s u b s e q u e n t h y d r o g e n a b s t r a c t i o n may t a k e p l a c e (20-22), o t h e r s b e l i e v e a mechanism i n v o l v i n g d i r e c t hydrogen a b s t r a c t i o n f r o m t h e a m i n e by t h e p h o t o e x c i t e d t r i p l e t s t a t e o f t h e c a r b o n y l compound ( 2 3 ) . A l t h o u g h t h e r e s u l t s p r e s e n t e d h e r e do n o t d i r e c t l y i m p l i c a t e a t r i p l e t e x c i p l e x , they n e v e r t h e l e s s appear to i n d i c a t e i n v o l v e m e n t o f an e l e c t r o n t r a n s f e r p r o c e s s w h i c h s t r o n g l y s u p p o r t s this hypothesis.



80 Table

VII.

T r a n s i e n t A b s o r p t i o n M a x i m a a t 540 nm f o r I n i t i t i a t o r 1 (10 M) C o m p a r e d w i t h t h e I o n i s a t i o n P o t e n t i a l o f V a r i o u s A m i n e s ( 1 . 4 8 χ 10 M) i n N i t r o g e n Saturated 2-Propanol


Amine Tri-n-butylamine Triethylamine N-Diethylmethylamine Diethylamine Dicyclohexylamine

Table

VIII.

Ionisation Potential Transient Absorption 540 nm (eV) 0.039 7.40 0.036 7.85 0.035 8.10 0.034 8.40 0.031 9.20

T r a n s i e n t A b s o r p t i o n Maximum o f I n i t i a t o r (5 χ 10 M) a t V a r i o u s pH V a l u e s i n N i t r o g e n S a t u r a t e d Water

Wavelength 540 560 560 560 600 620

pH 2 4 6 8 10 12

1

Absorbance 0.0019 0.0057 0.0072 0.0073 0.0197 0.0368

(nm)

Conclusions The r e s u l t s c l e a r l y show t h a t t h e p h o t o i n i t i a t i n g a c t i v i t y o f t h e w a t e r s o l u b l e b e n z o p h e n o n e i n t h e p r e s e n c e o f an a m i n e i s m a i n l y a s s o c i a t e d w i t h the a b i l i t y of the lowest e x c i t e d t r i p l e t - * state t o a b s t r a c t an e l e c t r o n f r o m t h e a m i n e c o - s y n e r g i s t v i a an i n t e r m e d i a t e e x c i p l e x shown i n Scheme I . The r a d i c a l a n i o n w i l l t h e n i n d u c e h y d r o g e n a t o m a b s t r a c t i o n t o g i v e a k e t y l r a d i c a l and an alkylamino radical. The l a t t e r i s m a i n l y r e s p o n s i b l e f o r i n d u c i n g p o l y m e r i s a t i o n o f t h e a c r y l i c monomer and s u p p o r t s e a r l i e r w o r k on t h e b e n z o p h e n o n e - t r i e t h y l a m i n e tetramine-induced photopolymer i s a t i o n o f m e t h y l m e t h a c r y l a t e d u r i n g w h i c h t e r m i n a l amine groups w e r e d e t e c t e d ( 2A_). Acknowledgments The a u t h o r s t h a n k NATO S c i e n t i f i c travel grant. Literature 1. 2.

Allen, Polym. Allen, Polym.

Affaires,

Brussels, for a

Cited N. S., Catalina, F., J., 1986, 22, 347. N. S., Catalina, F., J., 1985, 21, 841.

Green,

P.

N.,

Green,

W.

Α.,

Eur.

Green,

P.

Ν.,

Green,

W.

Α.,

Eur.


6. ALLEN ET AL. Activity of Water-Soluble Benzophenone Initiators 3. 4. 5. 6. 7.


8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

18. 19. 20. 21. 22. 23.

24.

81

Allen, Ν. S., Catalina, F., Moghaddam, Β. M., Green, P. Ν., Green, W. Α., Eur. Polym. J., 1986, 22, 691. Allen, Ν. S., Catalina, F., Green, P. Ν., and Green, W. Α., Eur. Polym. J., 1986, 22, 793. Allen, Ν. S., Catalina, F., Green, P. Ν., Green, W. Α., Eur. Polym. J., 1986, 22, 871. Allen, Ν. S., Catalina, F., Green, P. Ν., Green, W. Α., J. Photochem., 1987, 36, 99. Fouassier, J. P., Lougnot, D. J., Paverne, Α., Wieder, F., Chem. Phys. Letts., 1987, 135, 30. Fouassier, J. P., Lougnot, D. J., Zuchowiez, I., Eur. Polym. J., 1986, 22, 933. Fouassier, J. P., Lougnot, D. J., Polym. Photochem., 1983, 3, 79. Fouassier, J. P., Lougnot, D. J., Jacques, P., J. Photochem., 1982, 19, 59. Demas, J. Ν., Crosby, G. Α., J. Phys. Chem., 1971, 75, 91. Heller, H. G., Lanagan, J. R., J. Chem. Soc., Perkin Trans. 1., 1981, 341. Heller, H. G., British Patent, 7/1464603. Sastre, R., Catalina, F., Conde, Μ., Mateo, J. L., Revs. Plast. Mod., in the press. Cohen, S. G., Parola, Α., Parson, G. H., Chem. Revs., 1973, 73, 141. Merlin, Α., Lougnot, D. L., Fouassier, J. P., Polym. Bull., 1980, 3, 1. Fouassier, J. P., Lougnot, D. L., Zuchowiez, I., Green, P. Ν., Timpe, H. J., Kronfield, Κ. P., Muller, U., J. Photochem., 1987, 36, 347. Beckett, Α., Porter, G., Trans. Faraday Soc., 1983, 59, 2038. Porter, G., Wilkinson, F., Trans. Faraday Soc., 1961, 57, 1686. Granchak, S. Μ., Kondratenko, P. Α., Dilung, I. I., Toer. Eskp. Khim., 1984, 20, 43. Granchak, S. M., Chermerskaya, Z. F., Dilung, I. I., Vysokomol, Soedin. Ser. Α., 1985, 27, 276. Guttenplan, J. B., Cohen, S. G., Tetrahedron Letts., 1972, 2163. Merlin, Α., Lougnot, D. J., Fouassier, J. P., Polym. Bull., 1980, 3, 1. Ghosh, P., Bandyopadhyay,

A. R., Eur. Polym. J., 1984, 11, 1117.

RECEIVED September 13, 1989


Photochemistry and Photopolymerization Activity of Water-Soluble

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