Notizen 1251
Solvent Dependence of the Triplet Reactivity of Pivalophenone as Evidenced by Laser Flash Photolysis
Ikuo Naito * and Wolfram Schnabel
H a h n - M e i t n e r - I n s t i t u t f ü r K e r n f o r s c h u n g B e r l i n G m b H , B e r e i c h S t r a h l e n c h e m i e , B e r l i n
Z. N a t u r f o r s c h . 34 a, 1 2 5 1 - 1 2 5 2 (1979);
r e c e i v e d S e p t e m b e r 7, 1979
P h e n y l - t - b u t y l k e t o n e ( p i v a l o p h e n o n e ) w a s i r r a d i a t e d in m i x t u r e s of b e n z e n e a n d 2 - p r o p a n o l w i t h 2 5 n s flashes of 347.1 n m l i g h t . T r i p l e t l i f e t i m e s were m e a s u r e d b y m o n i t o r i n g t h e d e c a y of t h e T - T - a b s o r p t i o n . 2 - p r o p a n o l e x e r t s a s i g n i f i c a n t s t a b i l i z i n g i n f l u e n c e u p o n t h e t r i p l e t l i f e t i m e a n d lowers t h e t r i p l e t r e a c t i v i t y p r o b a b l y d u e t o a n, TI* - > JI, 7I* i n v e r s i o n of t h e l o w e s t t r i p l e t s t a t e .
During a recent investigation [1] of the primary events during t h e photolysis of poly(phenylisopro- penylketone) I and its model compound phenyl- t-butyl ketone II (pivalophenone) in benzene solu- tion it turned out t h a t
CHa - C H2- C
C = 0 P h
CH3
H3C—C—CH3 c = o P h I I
a-cleavage was the dominant chemical route of triplet deactivation in both cases. However, triplets of I were found to decay appreciably faster (about 4 times) t h a n those of II, which was assumed being due to a strong interaction of triplet excited carbonyl groups of I with unexcited neighboring base units. Upon investigating in benzene solution the reaction of triplets of I and I I with 2-propanol a relatively low reactivity of triplets of I I was revealed. This finding was explained by assuming t h a t molecules of I are preferentially solvated by
* O n l e a v e f r o m K y u s h u S a n g y o U n i v e r s i t y , F u k u o k a , J a p a n .
R e p r i n t r e q u e s t s t o P r o f . D r . W . S c h n a b e l , H a h n - M e i t n e r - I n s t i t u t f ü r K e r n f o r s c h u n g B e r l i n G m b H , B e r e i c h S t r a h l e n - c h e m i e , D - 1 0 0 0 B e r l i n 39.
benzene molecules. Thus, alcohol molecules are prevented to interact strongly with I but are permitted to attack occasionally excited moieties of I . On the other hand it was assumed t h a t 2-propanol is capable of interacting significantly with either the ground state or the excited state of molecules of I I . This paper reports results which yield clear evidence for an interaction between I I and 2-propanol. Pivalophenone (2.6 X 1 0- 3 mol/1) was irradiated with 25 ns flashes of 347.1 nm light produced by a frequency doubled ruby laser (Korad model K1QS2) in Ar-saturated solutions as de- scribed before [2],
Solvent mixtures containing benzene and 2-pro- panol at various ratios were used. The decay of the T-T absorption formed during the flash was monitored at the wave lengths 320 and 460 nm.
Triplet decay rate constants are plotted as a function of solvent composition in Figure 1. I t can
0 5 10
[2-Propanol] (mol/I)
F i g . 1. T h e t r i p l e t d e c a y r a t e c o n s t a n t a s a f u n c t i o n of t h e s o l v e n t c o m p o s i t i o n . P i v a l o p h e n o n e (2.6 X 10~3 mol/1) in A r - s a t u r a t e d m i x t u r e s of b e n z e n e a n d 2 - p r o p a n o l .
be seen t h a t kT increases with increasing 2-propanol content at relatively low alcohol concentrations but decreases after passing a maximum as the 2-propanol concentration is increased further. These results indicate t h a t at low concentrations of 2-propanol the latter reacts with triplets of I I relatively effectively (k= 1.6 X 105 1/mol s). However, t h e triplet lifetime is increasing again as 2-propanol is becoming the major constituent of the solvent mixture, indicating t h a t the triplet state is stabilized as the polarity of t h e solvent system is increased.
I t appears to be feasible to explain these results by an n, n* —> Jt, n* inversion of the lowest excited triplet state with increasing 2-propanol content in analogy to the explanation presented for the diminution of the reactivity and the efficiency of photo reduction of carbonyl triplet states in other systems [3, 4].
0340-4811 / 79 / 1000-1251 $ 01.00/0. - P l e a s e o r d e r a r e p r i n t r a t h e r t h a n m a k i n g y o u r o w n c o p y .
1252 Notizen [1] I . N a i t o , R . K u h l m a n n , a n d W . S c h n a b e l , P o l y m e r 20,
165 (1979).
[2] J . K i w i a n d W . S c h n a b e l , M a c r o m o l e c u l e s 8 , 4 3 0 (1975);
G. B e c k , G. D o b r o w o l s k i , J . K i w i , a n d W . S c h n a b e l , M a c r o m o l e c u l e s 8, 9 (1975).
[3] H . L u t z , E . B r e h e r d t , a n d L . L i n d q u i s t , J . P h y s . C h e m . 77, 1758 (1973).
[4] N . J . T u r r o , M o d e r n M o l e c u l a r P h o t o c h e m i s t r y , B e n - j a m i n - C u m m i n g s , Menlo P a r k CA 1978.
