Semiconductor-Catalyzed Photoreactions of Organic Compounds

3 Semiconductor-Catalyzed Photoreactions of Organic Compounds KATSUMI T O K U M A R U , HIROCHIKA SAKURAGI, TATSUYA KANNO, T A K A H I D E OGUCHI, HIROAKI MISAWA, YASUO S H I M A M U R A , and YASUNAO KURIYAMA

Downloaded by IOWA STATE UNIV on October 18, 2014 | http://pubs.acs.org Publication Date: May 14, 1985 | doi: 10.1021/bk-1985-0278.ch003

Department of Chemistry, University of Tsukuba, Sakura-mura, Ibaraki 305, Japan

The actions of photoexcited semiconductor particles on organic compounds under oxygen is of significant importance from both practical and basic aspects. Semiconductors like titanium dioxide and cadmium sulfide were shown to induce oxidation of olefins and aromatic hydrocarbons under oxygen, and also to sensitize isomerization of unsaturated systems. The mechanisms of these reactions are discussed. R e c e n t l y , the p h o t o c h e m i c a l a c t i o n o f s e m i c o n d u c t o r s have been actively investigated. However, t h i s a r e a a t t r a c t e d t h e i n t e r e s t o f many workers n e a r l y t h i r t y y e a r s ago ( 1 - 5 ) . In t h o s e d a y s , L a i d l e r Ç5), C a l v e r t ( 6 , 7 ) , and Markham (8-10) s t u d i e d t h e a c t i o n o f z i n c o x i d e under i l l u m i n a t i o n i n aqueous a l c o h o l s under oxygen p r o d u c i n g c a r b o n y l compounds and hydrogen p e r o x i d e . The i n t e r e s t i n t h o s e days c a n be seen by t h e appearance o f an i n t r o d u c t o r y a r t i c l e by Markham i n J . Chem. E d . i n 1955 ( 1 1 ) . A t n e a r l y t h e same t i m e , Mashio and Kato worked on t h e p h o t o c a t a l y t i c a c t i o n of titanium d i o x i d e on o x i d a t i o n o f a l c o h o l s (12,13) and h y d r o c a r b o n s (14) i n an attempt t o e l u c i d a t e t h e mechanism by which p o l y ( v i n y l a c e t a t e ) f i b r e s dyed w i t h t h e a i d o f t i t a n i u m d i o x i d e became f r a g i l e d u r i n g use i n a i r under s u n s h i n e . They found t h a t t h e i r r a d i a t i o n o f t i t a n i u m dioxide i n i t i a t e s a u t o x i d a t i o n of the s u b s t r a t e s . In I 9 6 0 ' s , when t h e manufacture o f p o l y p r o p y l e n e by Z i e g l e r - N a t t a c a t a l y s t was s t a r t e d , d e g r a d a t i o n o f t h e polymer was found t o be a c c e l e r a t e d by t i t a n i u m compounds r e m a i n i n g i n t h e p o l y m e r , which a g a i n a t t r a c t e d a t t e n t i o n t o the p h o t o c a t a l y t i c a c t i o n o f t i t a n i u m dioxide (15-17). A t t h e end o f 1 9 6 0 s , Honda and F u j i s h i m a found t h a t i n a p h o t o c h e m i c a l c e l l employing t i t a n i u m d i o x i d e and p l a t i n u m e l e c t r o d e s , i r r a d i a t i o n o f the t i t a n i u m d i o x i d e e l e c t r o d e r e s u l t e d i n s p l i t t i n g o f w a t e r i n t o hydrogen and oxygen ( 1 8 , 1 9 ) . T h i s work has had an e x t r a o r d i n a r i l y s t r o n g impact f o r t h e r e s e a r c h on t h e p h o t o chemical a c t i o n o f various semiconductors i n d u c i n g e v o l u t i o n of hydrogen from water as w e l l as new c a t a l y t i c r e a c t i o n s ( 2 0 ) . F o r the a c t i o n o f s e m i c o n d u c t o r p a r t i c l e s on o r g a n i c compounds, Bard and h i s coworker r e p o r t e d t h a t i r r a d i a t i o n o f t i t a n i u m d i o x i d e f

0097-6156/85/0278-O043$06.00/0 © 1985 A m e r i c a n C h e m i c a l Society

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

44

ORGANIC PHOTOTRANSFORMATIONS IN NONHOMOGENEOUS MEDIA

i n aqueous a c e t i c a c i d under oxygen l e d t o the o x i d a t i o n of a c e t a t e a n i o n s t o methane and ethane ( 2 1 , 2 2 ) . A t n e a r l y t h e same t i m e , o t h e r groups i n i t i a t e d i n v e s t i g a t i o n s o f the p h o t o c a t a l y t i c a c t i o n o f s e m i c o n d u c t o r p a r t i c l e s on r e l a t i v e l y s i m p l e o r g a n i c compounds l i k e a l c o h o l s and c a r b o x y l i c a c i d s , w i t h the hope of p h o t o c h e m i c a l l y p r o d u c i n g hydrogen from water ( 2 3 - 2 5 ) . A t the end of 1970 s we attempted an i n v e s t i g a t i o n of the a c t i o n o f s e m i c o n d u c t o r p a r t i c l e s on medium s i z e d o r g a n i c compounds. In t h i s a r t i c l e , we would l i k e t o d e s c r i b e some of the work done i n our l a b o r a t o r y on the p h o t o c a t a l y t i c a c t i o n o f s e m i c o n d u c t o r s on the o x i d a t i o n of o l e f i n s and h y d r o c a r b o n s , and on the i s o m e r i z a t i o n o f unsaturated systems.


1

P h o t o c a t a l y t i c Oxidation of

Olefins

We found t h a t e x c i t a t i o n of t i t a n i u m d i o x i d e or cadmium s u l f i d e powder suspended i n o r g a n i c s o l v e n t s c o n t a i n i n g o l e f i n s l i k e 1 , 1 d i p h e n y l e t h y l e n e (DPE) under oxygen l e d t o o x i d a t i o n o f the o l e f i n s t o t h e i r e p o x i d e s and c a r b o n y l compounds (26). In a t y p i c a l r u n , s e m i c o n d u c t o r powder ( c a . 5 mg) was suspended i n a s o l u t i o n ( c a . 2 ml) c o n t a i n i n g an o l e f i n ( c a . 0 . 3 mmol), and i r r a d i a t e d under oxygen atmosphere a t room temperature w i t h l i g h t o f l o n g e r wavelengths t h a n 350 nm f o r t i t a n i u m d i o x i d e and than 430 nm f o r cadmium s u l f i d e . The p r o d u c t s were s e p a r a t e d by vpc o r t i c f o r t h e i r i d e n t i f i c a t i o n and q u a n t i t a t i v e l y determined by v p c . F o r example, DPE gave b e n z o phenone and i t s e p o x i d e , which was c o n v e r t e d to 2 - m e t h o x y - 2 , 2 - d i p h e n y l e t h a n o l i n methanol s o l u t i o n . Ph. Ph-

.R' °2 Ph. -R" ~ ~ *^ P h ' CdS o r TiO„ 2 = R" = H, Me = H, R""= OMe h V >

C

R R' 1

C

Ph. _ .R' Ph- v}S" 0 c

C

0

+

0

St"

+

c

C

I*

jMeOH

Ph C(OMe)CH OH or Ph C(OH)CH(OMe) 2

2

2

2

In aqueous a c e t o n i t r i l e , C d S - s e n s i t i z e d i r r a d i a t i o n o f DPE gave 1 , 1 - d i p h e n y l e t h a n - l , 2 - d i o l a l o n g w i t h benzophenone, which i n d i c a t e s t h a t the r e s u l t i n g epoxide i s h y d r o l y z e d i n the s o l u t i o n ( 2 7 ) . Some r e s u l t s are summarized i n T a b l e I (27). F i g u r e 1 schemati c a l l y d e p i c t s the r e a c t i v i t y o f the o l e f i n s examined w i t h t h e i r o x i d a t i o n p o t e n t i a l s , and i n d i c a t e s t h a t g e n e r a l l y the u n r e a c t i v e o l e f i n s a r e those w i t h h i g h e r o x i d a t i o n p o t e n t i a l s , and the r e a c t i v e o l e f i n s are those w i t h lower o x i d a t i o n p o t e n t i a l s . However, among the o x i d i z a b l e o l e f i n s t h e i r r e a c t i v i t y i s not s i m p l y governed by t h e i r o x i d a t i o n p o t e n t i a l s as w i l l be d i s c u s s e d l a t e r . The quantum y i e l d s f o r the consumption o f DPE and 2 - m e t h y 1 - 1 , 1 d i p h e n y l p r o p e n e were o b t a i n e d as a t l e a s t 0.2 and 0 . 1 , r e s p e c t i v e l y , i n T i 0 2 - s e n s i t i z e d o x y g e n a t i o n w i t h o u t c o r r e c t i n g the r e f l e c t i o n o f the i n c i d e n t l i g h t on the s e m i c o n d u c t o r s u r f a c e s . The above f a c t s i n d i c a t e t h a t on e x c i t a t i o n o f the s e m i c o n d u c t o r s the o l e f i n s t r a n s f e r an e l e c t r o n t o the p h o t o g e n e r a t e d p o s i t i v e h o l e i n the i n i t i a t i o n p r o c e s s t o g i v e the o l e f i n r a d i c a l c a t i o n , and c o n c u r r e n t l y the e l e c t r o n e x c i t e d t o the c o n d u c t i o n band i s


3.

Table I.

Olefin Ph C=CH 2

6

4

45

Compounds

Semiconductor P h o t o c a t a l y z e d of Aromatic O l e f i n s (a.)

Oxygenation

Time /h

Conver sion/%

CdS

MeCN-MeOH

6

27

15

Ti0 Ti0 CdS

(1:1) MeCN MeCN MeCN-MeOH

6 6 7

31 4 100

43 77 12

trace

11

9 5 2 96 27

2

2

Ph C=CMe Ti0 trans-PhCH=CHPh CdS cis-PhCH=CHPh CdS cyclooctene Ti0 norbornene Ti0 2-cyclohexenone Ti0 PhCH=CHCN Ti0 PhCH=CHC0Me Ti0 PhCH=CHC09Et Ti0 a. Y i e l d s a r e based 2

TiCL

2

2

2

2 2

2

2

2

2

(1:1) 16 MeCN 6 5 12 MeCN 6 12 MeCN 4 MeCN 6 3 MeCN 6 No MeCN 8 No 8 MeCN No MeCN 8 No MeCN 8 on the o l e f i n consumed.

Carbonyl compds/%

Epoxide or i t s deriv/% 26

Solvent

2

2

of Organic

Semiconductor

2

p-MeOC H CH=CH PhMeOCH Ph C=CH0Me


Photoreactions

T O K U M A R U ET AL.

69

+ + reaction reaction reaction reaction

CdS Oxidation

potential

Oxygenation

Ph C=CHOMe 2

t-PhCH=CHPh c-PhCH=CHPh

f

MeOC.H.CH=CH. 6 4 2 Ph OCMe 2

2

Me OCMe 2

Ph C=CH 2

2

PhMeC=CH

2

bicyclo[2.2.1]hept-2-ene cyclohexene cyclooctene PhCH=CHCN PhCH=CHCOMe

PhCH=CHC00Et cyclohexenone

vs

Figure

1.

Ag/AgN0 (0.1 3

R e a c t i v i t y of

M)

olefins

and t h e i r o x i d a t i o n

potentials.


46


t r a n s f e r r e d to oxygen t o g i v e the s u p e r o x i d e a n i o n . These r e s u l t i n g species subsequently i n i t i a t e free r a d i c a l chain r e a c t i o n s . Fox and h e r coworkers o b s e r v e d t r a n s i e n t s p e c t r a a t t r i b u t a b l e t o r a d i c a l c a t i o n s upon l a s e r e x c i t a t i o n o f powdered s e m i c o n d u c t o r i n the p r e s e n c e o f o l e f i n s ( 2 8 - 3 1 ) .


F a c t o r s C o n t r o l l i n g the R e a c t i v i t y o f O l e f i n s Oxygenation

i n the

Photocatalyzed

To examine the e l e c t r o n t r a n s f e r from o l e f i n s t o the e x c i t e d s e m i c o n d u c t o r , p h o t o e l e c t r o c h e m i c a l measurements were u n d e r t a k e n . In a c e l l c o m p r i s i n g o f two compartments d i v i d e d by a g l a s s f r i t , a t i t a n i u m d i o x i d e w o r k i n g e l e c t r o d e was immersed i n an a c e t o n i t r i l e s o l u t i o n o f an o l e f i n (0.2 M) i n one compartment, and a p l a t i n u m c o u n t e r e l e c t r o d e was put i n a c e t o n i t r i l e i n the o t h e r compartment; tetraethylammonium p e r c h l o r a t e (0.1 M) was used as a s u p p o r t i n g electrolyte. A f t e r b u b b l i n g argon i n t o the working c e l l s o l u t i o n , the t i t a n i u m d i o x i d e e l e c t r o d e was i l l u m i n a t e d (λ > 350 nm) under v o l t a g e s v a r y i n g between - 0 . 5 and 0.8 V vs Ag/AgN03 ( 0 . 1 M). Some o l e f i n s e x h i b i t e d an i n c r e a s e o f p h o t o c u r r e n t w i t h i n c r e a s e o f the a p p l i e d p o t e n t i a l t o g i v e a n e a r l y c o n s t a n t c u r r e n t around 0.5 V; however, some o l e f i n s d i d n o t show any p h o t o c u r r e n t a t t h e s e potentials (32). F i g u r e 2 shows t h a t the p h o t o c u r r e n t measured a t 0.5 V i n c r e a s e s w i t h d e c r e a s i n g t h e o x i d a t i o n p o t e n t i a l s o f the o l e f i n s examined; however, b e n z y l i d e n e a c e t o n e , e t c . w i t h h i g h e r o x i d a t i o n p o t e n t i a l s d i d n o t show any p h o t o c u r r e n t (32). A l t h o u g h the a b s o l u t e amount o f t h e p h o t o c u r r e n t s i s governed by v a r i o u s f a c t o r s such as the o x i d a t i o n p o t e n t i a l s o f o l e f i n s and the e x t e n t o f a d s o r p t i o n o f o l e f i n s on the e l e c t r o d e , the above f i n d i n g s show t h a t the r e a c t i v e o l e f i n s i n the p h o t o c a t a l y t i c o x y g e n a t i o n e x h i b i t p h o t o c u r r e n t s and the o l e f i n s w h i c h do n o t e x h i b i t p h o t o c u r r e n t s are u n r e a c t i v e i n the p h o t o c a t a l y t i c o x y g e n ation. On the o t h e r hand, the o l e f i n s w h i c h e x h i b i t p h o t o c u r r e n t s are n o t always r e a c t i v e . F o r example, s t i l b e n e shows a h i g h e r p h o t o c u r r e n t than DPE, but i s not so r e a c t i v e as DPE. The e l e c t r o n t r a n s f e r to the e x c i t e d s e m i c o n d u c t o r t a k e s p l a c e more e f f i c i e n t l y from s t i l b e n e than from DPE due to the lower o x i d a t i o n p o t e n t i a l o f the former, but i n the subsequent f r e e r a d i c a l r e a c t i o n s , s t i l b e n e i s l e s s r e a c t i v e t h a n DPE ( 3 3 ) . T h e r e f o r e , i t can be c o n c l u d e d t h a t f o r the p h o t o c a t a l y t i c o x y g e n a t i o n t o o c c u r , the e l e c t r o n t r a n s f e r from the o l e f i n t o the p o s i t i v e h o l e has t o take p l a c e , but the o v e r a l l r e a c t i v i t y o f the o l e f i n s i s governed by the e f f i c i e n c y o f f r e e r a d i c a l p r o c e s s e s as exemplified i n Table II. In view o f the above r e s u l t s , i n p h o t o c a t a l y t i c o x i d a t i o n o f a s e r i e s o f 4 - s u b s t i t u t e d d i p h e n y l e t h y l e n e s , an i n c r e a s e i n r e a c t i v i t y w i t h d e c r e a s i n g Hammett s sigma c o n s t a n t s (31) seems t o a r i s e n o t o n l y from the l o w e r i n g o f t h e o x i d a t i o n p o t e n t i a l s o f the o l e f i n s i n t h i s sequence b u t a l s o from the g e n e r a l t r e n d o f t h e i n c r e a s e i n the r e a c t i v i t y o f o l e f i n s toward p e r o x y l r a d i c a l s w i t h i n c r e a s i n g the e l e c t r o n donating a b i l i t y of o l e f i n s (33). To get i n s i g h t i n t o the r e a c t i v i t y o f o l e f i n r a d i c a l c a t i o n s toward oxygen, a n o d i c o x i d a t i o n of o l e f i n s under oxygen was attempted. DPE was e l e c t r o l y z e d at 1.5 V vs SCE i n a m i x t u r e o f f



TOKUMARU ET AL.

Table

II.

Olefin PhCH=CHPh Ph C=CH PhCH=CHC0Me 2

2

Phoîoreactions

of Organic

Reactivity of Olefins Oxygenation

Electron transfer good medium poor

Compounds

i n Photocatalyzed

Efficiency Radical chain oxidation poor good poor

Overall medium good poor

American Chemfcaf Society Library 1155

16ih St. N. W.

In Organic Phototransformations in Nonhomogeneous Media; Fox, M.; Washington. . C. 20036 ACS Symposium Series; American0Chemical Society: Washington, DC, 1985.


48


a c e t o n i t r i l e and methanol ( 1 : 3 by volume) i n the p r e s e n c e of t e t r a ethylammonium p e r c h l o r a t e as a s u p p o r t i n g e l e c t r o l y t e under oxygen. The e l e c t r o l y s i s c a r r i e d out t o n e a r l y 70% c o n v e r s i o n o f the o l e f i n a f f o r d e d a 1 , 2 - d i m e t h o x y l a t e d p r o d u c t as the main p r o d u c t a l o n g w i t h a s m a l l amount o f benzophenone, but 2 , 2 - d i p h e n y l - 2 - m e t h o x y e t h a n o l , w h i c h had been e x p e c t e d to a r i s e from o x y g e n a t i o n o f t h e r a d i c a l c a t i o n i n m e t h a n o l , was formed i n a v e r y low y i e l d ( 2 7 ) . These r e s u l t s i n d i c a t e t h a t t h e DPE r a d i c a l c a t i o n i s n o t h i g h l y r e a c t i v e w i t h oxygen. For generation of o l e f i n r a d i c a l c a t i o n s , r e c e n t l y s e v e r a l a t t e m p t s have been made to use 9 , 1 0 - d i c y a n o a n t h r a c e n e (DCA) as an e l e c t r o n a c c e p t i n g s e n s i t i z e r under oxygen. On e x c i t a t i o n o f DCA, the r e s u l t i n g DCA s i n g l e t e x c i t e d s t a t e a c c e p t s an e l e c t r o n from an o l e f i n t o g i v e the r a d i c a l a n i o n of DCA and the r a d i c a l c a t i o n o f the o l e f i n , and the former a n i o n can t r a n s f e r an e l e c t r o n t o oxygen t o give the superoxide a n i o n . F o r example, D C A - s e n s i t i z e d o x y g e n a t i o n o f DPE was r e p o r t e d to g i v e a m i x t u r e o f v a r i o u s o x i d a t i o n p r o d u c t s i n v o l v i n g benzophenone as the main p r o d u c t a l o n g w i t h the c o r r e s p o n d i n g epoxide and 1 , l - d i p h e n y l - 2 - m e t h o x y e t h a n o l , e t c . , and the p r o d u c t s were p r o p o s e d t o a r i s e from the r e a c t i o n between the o l e f i n r a d i c a l c a t i o n and s u p e r o x i d e a n i o n ( 3 4 - 3 7 ) . Among o l e f i n r a d i c a l c a t i o n s s t u d i e d o n l y adamantylideneadamantane (ADA) i s e s t a b l i s h e d to r e a c t f a c i l e l y w i t h oxygen t o g i v e an oxygen adduct which s u b s e q u e n t l y a c c e p t s an e l e c t r o n from ADA to g i v e t h e c o r r e s p o n d i n g d i o x e t a n e and to r e g e n e r a t e the ADA r a d i c a l c a t i o n , thus a c c o m p l i s h i n g a c h a i n r e a c t i o n (38). T h e r e f o r e , i t seems r e a s o n a b l e to suppose t h a t i n the s e m i c o n d u c t o r c a t a l y z e d o x i d a t i o n o f DPE, the r e s u l t i n g r a d i c a l c a t i o n o f the o l e f i n c o u l d r e a c t w i t h the s u p e r o x i d e a n i o n t o g i v e a p e r o x y e t h y l - l , 4 - d i r a d i c a l which s u b s e q u e n t l y r e a c t s w i t h the o l e f i n and oxygen through t h e f r e e r a d i c a l c h a i n p r o c e s s e s . A u t o x i d a t i o n o f a r o m a t i c o l e f i n s i n i t i a t e d by a r a d i c a l c h a i n i n i t i a t o r l i k e a z o b i s i s o b u t y r o n i t r i l e (AIBN) or by p h o t o i r r a d i a t i o n under 1 atm oxygen u s u a l l y g i v e s c a r b o n y l compounds as the main p r o d u c t s a l o n g w i t h e p o x i d e s , and the p r o d u c t i o n o f the e p o x i d e s tends t o i n c r e a s e w i t h d e c r e a s e o f oxygen p r e s s u r e ( 3 3 , 3 9 ) . Accordi n g l y , the p r o d u c t i o n of a c o n s i d e r a b l e amount o f the e p o x i d e accomp a n i e d by benzophenone i n t h e s e m i c o n d u c t o r p h o t o c a t a l y z e d o x y g e n a t i o n o f DPE under 1 atm oxygen shows t h a t i n the s e m i c o n d u c t o r c a t a l y s i s the r e a c t i o n might p r o c e e d through a mechanism s l i g h t l y d i f f e r e n t from the a u t o x i d a t i o n , p o s s i b l y due to t h e p a r t i c i p a t i o n o f the DPE r a d i c a l c a t i o n and s u p e r o x i d e a n i o n i n p l a c e o f n e u t r a l r a d i c a l s and oxygen i n the c h a i n i n i t i a t i o n s t e p . Photocatalyzed Isomerization

of Unsaturated

Systems

The s e n s i t i z i n g a c t i o n o f s e m i c o n d u c t o r a f f e c t s i s o m e r i z a t i o n o f olefins. On e x c i t a t i o n o f CdS o r T1O2 i n the p r e s e n c e o f t r a n s - o r c i s - s t i l b e n e under a r g o n , the i s o m e r i z a t i o n o c c u r r e d o n l y from the c i s - to t r a n s - i s o m e r ; the t r a n s - i s o m e r s c a r c e l y underwent i s o m e r i z a t i o n i n t o the c i s - i s o m e r . I r r a d i a t i o n under oxygen r e s u l t e d i n the o x y g e n a t i o n to g i v e benzaldehyde and t r a n s - s t i l b e n e o x i d e , i r r e s p e c t i v e o f the c o n f i g u r a t i o n o f the s t a r t i n g o l e f i n , a l o n g w i t h the i s o m e r i z a t i o n from the c i s - to trans*-isomer (40). The i s o m e r i z a t i o n t a k i n g p l a c e o n l y from c i s - to t r a n s - s t i l b e n e


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i s c o n s i s t e n t w i t h the i s o m e r i z a t i o n through the o l e f i n r a d i c a l cation. F i g u r e 3 d e p i c t s the e n e r g i e s o f the r a d i c a l c a t i o n s o f c i s and t r a n s - s t i l b e n e . In the ground s t a t e , c i s - s t i l b e n e i s n e a r l y 3 k c a l / m o l ( c a . 0.13 eV) h i g h e r i n energy than the t r a n s - i s o m e r (41). S i n c e the o x i d a t i o n p o t e n t i a l s o f t r a n s - and c i s - s t i l b e n e a r e 1.15 and 1.23 V vs Ag/AgNU3 ( 0 · ^ )> r e s p e c t i v e l y , the c a t i o n r a d i c a l o f c i s - s t i l b e n e l i e s 0.21 eV h i g h e r i n energy than t h a t o f t r a n s stilbene. The i n e f f i c i e n t p r o d u c t i o n of the c i s - i s o m e r from the t r a n s - i s o m e r r u l e s out a p o s s i b i l i t y f o r the p a r t i c i p a t i o n o f the t r i p l e t s t a t e o f s t i l b e n e (2.1 eV over the t r a n s - i s o m e r ) . T h i s means t h a t the p a i r o f the t r a n s r a d i c a l c a t i o n w i t h the e l e c t r o n i n the c o n d u c t i o n band o f T1O2 ( c a . - 0 . 6 V vs Ag/AgN0 ) or CdS ( c a . - 1 . 1 V) l i e s i n the e n e r g i e s ( c a . 1.8 and 2.3 eV, r e s p e c t i v e l y o v e r the t r a n s - i s o m e r ) n o t much e x c e e d i n g the t r i p l e t s t a t e . A c c o r d i n g l y , i t i s r e a s o n a b l e t o c o n c l u d e t h a t i n the p h o t o c a t a l y z e d i s o m e r i z a t i o n o f o l e f i n s , the c i s r a d i c a l c a t i o n t w i s t s t o the more s t a b l e t r a n s o r t r a n s - l i k e t w i s t e d r a d i c a l c a t i o n ( 4 2 - 4 6 ) , which s u b s e q u e n t l y r e c e i v e s an e l e c t r o n from the e x c i t e d s e m i c o n d u c t o r t o g i v e the t r a n s - i s o m e r . S i m i l a r i s o m e r i z a t i o n u s i n g CdS was r e p o r t e d r e c e n t l y (47). On the o t h e r h a n d , the " t r a n s " r a d i c a l c a t i o n r e s u l t i n g from o x i d a t i o n o f the t r a n s - i s o m e r cannot i s o m e r i z e t o the l e s s s t a b l e c i s r a d i c a l c a t i o n . The f o r m a t i o n o f t r a n s s t i l b e n e o x i d e i r r e s p e c t i v e o f the c o n f i g u r a t i o n o f the s t a r t i n g i s o m e r can be a t t r i b u t e d to the r e a c t i o n o f the " t r a n s " r a d i c a l c a t i o n w i t h oxygen g i v i n g a p e r o x y l r a d i c a l c a t i o n which s u b s e q u e n t l y c o l l a p s e s t o the t r a n s - e p o x i d e . The b e h a v i o r o f s t i l b e n e r a d i c a l c a t i o n s i n the s e m i c o n d u c t o r c a t a l y s i s i s i n k e e p i n g w i t h the r e s u l t o f p h o t o i s o m e r i z a t i o n o f o t h e r o l e f i n s l i k e (3-methylstyrene s e n s i t i z e d by e l e c t r o n a c c e p t o r s l i k e c h l o r a n i l i n p o l a r s o l v e n t s (48). The s e m i c o n d u c t o r p h o t o c a t a l y z e d i s o m e r i z a t i o n o f s t r a i n e d c y c l o b u t a n e s to s t r a i n e d d i e n e s ( i s o m e r i z a t i o n o f q u a d r i c y c l e n e to n o r b o r n a d i e n e and s i m i l a r r e a c t i o n s o f complex cage compounds (49)) i s r e l a t e d to the o l e f i n i s o m e r i z a t i o n d i s c u s s e d above. M


3

P h o t o c a t a l y t i c Oxygenation

of

Hydrocarbons

I r r a d i a t i o n o f powdered t i t a n i u m d i o x i d e suspended i n s o l u t i o n s c o n t a i n i n g a r o m a t i c compounds and w a t e r under oxygen has r e c e n t l y been shown to i n d u c e h y d r o x y l a t i o n o f a r o m a t i c n u c l e i g i v i n g p h e n o l i c compounds and o x i d a t i o n of s i d e c h a i n s o f the a r o m a t i c compounds ( 5 0 - 5 5 ) . These r e a c t i o n s have been assumed to p r o c e e d through hydroxy1 and o t h e r r a d i c a l i n t e r m e d i a t e s , but the mechanism f o r t h e i r g e n e r a t i o n , whether r e a c t i v e f r e e r a d i c a l s r e s u l t from o x i d a t i o n o f w a t e r , from r e d u c t i o n o f oxygen, o r from o x i d a t i o n o f the s u b s t r a t e s on the s u r f a c e s o f the e x c i t e d t i t a n i u m d i o x i d e , has not been c l e a r . An attempt was made to r e v e a l the mechanism f o r the f o r m a t i o n o f f r e e r a d i c a l s upon i r r a d i a t i o n o f t i t a n i u m d i o x i d e i n the p r e s e n c e o f benzene and t o l u e n e . C a r e f u l e x a m i n a t i o n o f the e f f e c t s o f oxygen and w a t e r showed t h a t the p r e s e n c e o f oxygen i s e s s e n t i a l f o r the r e a c t i o n , and t h a t under oxygen the o x i d a t i o n o f w a t e r c o n t r i b u t e s t o the a r o m a t i c h y d r o x y l a t i o n and the o x i d a t i o n o f t o l u e n e as a s u b s t r a t e l e a d s to o x i d a t i o n o f i t s s i d e c h a i n ( 5 6 ) . Powdered t i t a n i u m d i o x i d e (30 mg) was suspended i n an a c e t o -



50


1.36

1.23 +

1.15

0.13

trans

trans

eV

Figure

3.

Energies

of s t i l b e n e

radical

cations.



3.

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of Organic

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51

n i t r i l e s o l u t i o n (2 ml) c o n t a i n i n g benzene o r t o l u e n e (16 vol%) and v a r y i n g amounts o f w a t e r (0-8 v o l % ) , and i r r a d i a t e d w i t h l i g h t l o n g e r than 350 nm f o r 15 h a t 20°C w i t h and w i t h o u t oxygen. F i g u r e 4 d e p i c t s t y p i c a l r e s u l t s o f the o x i d a t i o n o f benzene under oxygen and w i t h o u t oxygen w i t h v a r y i n g c o n c e n t r a t i o n s o f w a t e r . As F i g . 4 c l e a r l y shows, o n l y a t r a c e amount o f p h e n o l was p r o d u c e d , even i n t h e p r e s e n c e o f w a t e r w i t h o u t oxygen, w h i l e under oxygen the y i e l d o f p h e n o l i s s t i l l low i n the absence o f w a t e r , but i n c r e a s e s with increasing concentration of water. F o r the r e a c t i o n o f t o l u e n e under c a r e f u l removal o f oxygen, o n l y t r a c e amounts o f o x i d a t i o n p r o d u c t s were produced even i n the p r e s e n c e o f w a t e r , as was found f o r benzene. I t i s n o t i c e a b l e t h a t i n the o x i d a t i o n o f t o l u e n e under oxygen ( F i g . 5) o - , m-, and p - c r e s o l s r e s u l t i n g from n u c l e a r h y d r o x y l a t i o n o f the s u b s t r a t e a r e produced i n v e r y low y i e l d s i n the absence o f water, but tend to i n c r e a s e with i n c r e a s i n g c o n c e n t r a t i o n o f water; on the c o n t r a r y , benzaldehyde and b e n z y l a l c o h o l a r i s i n g from o x i d a t i o n of the s i d e c h a i n are p r o d u c e d i n much h i g h e r y i e l d s than c r e s o l s i n the absence o f w a t e r , b u t t h e i r y i e l d s do n o t v a r y w i t h water c o n c e n t r a t i o n . The above r e s u l t s w i t h benzene and t o l u e n e show t h a t the p r e s e n c e o f oxygen i s n e c e s s a r y f o r the f o r m a t i o n o f o x i d a t i o n p r o d u c t s , but the p r e s e n c e o f w a t e r a l o n e i s n o t enough t o i n d u c e the o x i d a t i o n o f the s u b s t r a t e s . These f a c t s i n d i c a t e t h a t w a t e r can be o x i d i z e d t o h y d r o x y l r a d i c a l s by the p h o t o g e n e r a t e d p o s i t i v e h o l e s w i t h c o n c u r r e n t removal o f e l e c t r o n s i n the c o n d u c t i o n band by oxygen, presumably a d s o r b e d on the s e m i c o n d u c t o r s u r f a c e s . The r e s u l t i n g h y d r o x y l r a d i c a l s w i l l h y d r o x y l a t e the a r o m a t i c n u c l e i .

or CH

Χ

X

ρ C H CH 6

5

-Η •

3

(C H CH ) 6

5

•

3

C H CH ' 6

5

2

°2 •

•

C H C H O H , C H CHO 6

5

2

H 0

2

•

0 · 2

6

5

+

•

H0 · 2

Under oxygen i n the absence o f w a t e r , t o l u e n e w i l l t r a n s f e r an e l e c t r o n t o the p o s i t i v e h o l e , c o n c u r r e n t l y w i t h e l e c t r o n t r a n s f e r from the c o n d u c t i o n band t o oxygen, t o g i v e a t o l u e n e r a d i c a l cation. On the o t h e r h a n d , i n the p r e s e n c e o f w a t e r , b o t h t o l u e n e and w a t e r w i l l t r a n s f e r an e l e c t r o n t o the p o s i t i v e h o l e s . The r e s u l t i n g t o l u e n e r a d i c a l c a t i o n may s u b s e q u e n t l y l o s e a p r o t o n a f f o r d i n g a b e n z y l r a d i c a l , w h i c h w i l l be o x i d i z e d w i t h oxygen o r the s u p e r o x i d e a n i o n to b e n z y l a l c o h o l and b e n z a l d e h y d e , as proposed f o r the r e a c t i o n s o f F e n t o n ' s reagent w i t h t o l u e n e (57). T h e r e f o r e , i t i s r e a s o n a b l e to c o n c l u d e t h a t upon i r r a d i a t i o n o f t i t a n i u m d i o x i d e under oxygen, the e l e c t r o n t r a n s f e r from water to the p o s i t i v e h o l e m a i n l y r e s u l t s i n h y d r o x y l a t i o n o f the a r o m a t i c




F i g u r e 5. O x i d a t i o n o f t o l u e n e i n a c e t o n i t r i l e under oxygen; p r o d u c t s : benzaldehyde ( φ ) , b e n z y l a l c o h o l ( φ ) , o - c r e s o l (Ο), m-cresol ( Q ) , p-cresol (φ).


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53

n u c l e i and t h e e l e c t r o n t r a n s f e r from t o l u e n e m a i n l y l e a d s t o o x i d a t i o n o f i t s s i d e c h a i n , and t h a t oxygen c o n t r i b u t e s t o t h e s e r e a c t i o n s by c o n c u r r e n t l y a c c e p t i n g an e l e c t r o n from t h e c o n d u c t i o n band o f s e m i c o n d u c t o r . T i t a n i u m d i o x i d e p h o t o c a t a l y z e d o x i d a t i o n o f n e a t t e t r a l i n was p r e v i o u s l y reported to give i t s hydroperoxide (14). Reinvestigation showed t h a t t e t r a l o l and t e t r a l o n e are a l s o formed i n a c e t o n i t r i l e p o s s i b l y through e l e c t r o n t r a n s f e r from t e t r a l i n t o the p o s i t i v e holes (27).


C o n c l u d i n g Remarks S e m i c o n d u c t o r s a c t as p h o t o c a t a l y s t s on v a r i o u s o r g a n i c compounds. The e s s e n t i a l f e a t u r e s o f t h e mechanism a r e becoming c l e a r . However, how the i n i t i a t i o n s t e p s p r o c e e d , p a r t i c u l a r l y how the s p e c i e s on the s e m i c o n d u c t o r s u r f a c e r e a c t w i t h w a t e r and oxygen, i s not y e t w e l l understood. Moreover, a n o t h e r a r e a t o be i n v e s t i gated i s the mechanisms through which the r e s u l t i n g r a d i c a l c a t i o n s o f the s u b s t r a t e s undergo t r a n s f o r m a t i o n t o p r o d u c t s . Acknowledgments The a u t h o r s e x p r e s s t h e i r thanks t o M i n i s t r y o f E d u c a t i o n , S c i e n c e , and C u l t u r e i n Japan f o r the s p e c i a l p r o j e c t r e s e a r c h on energy under g r a n t i n a i d o f s c i e n t i f i c r e s e a r c h and t o Japan-US C o o p e r a t i v e R e s e a r c h on P h o t o c o n v e r s i o n and P h o t o s y n t h e s i s f o r e n a b l i n g d i s c u s s i o n s w i t h the s c i e n t i s t s c o n c e r n e d .

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Freund, T; Gommes, W. P. Catal. Rev. 1970, 3, 1. Egerton, G. S. J . Text. Inst. 1948, 39, 293, 305. Treiber, E. Kolloid Z. 1953, 130, 39. Jacobsen, E. Ind. Eng. Chem. 1949, 41, 523. Markham, M. C.; Laidler, K. J . J . Phys. Chem. 1953, 57, 363. Rubin, T. R.; Calvert, J . G.; Rankin, G. T . ; MacNevin, W. M. J . Am. Chem. Soc. 1953, 75, 2850. Calvert, J . G.; Theurer, K.; Rankin, G. T.; MacNevin, W. M. J . Am. Chem. Soc. 1954, 76, 2575. Markham, M. C.; Hannan, M. C.; Paternostro, R. M.; Rose, C. B. J . Am. Chem. Soc. 1958, 80, 5394. Kuriacose, J . C.; Markham, M. C. J . Catalysis 1962, 1, 498. Markham, M. C.; Upreti, M. C. J . Catalysis 1965, 4, 229. Markham, M. C. J . Chem. Ed. 1955, 32, 540. Mashio, F . ; Kato, S. Japanese Patent 252 007, 1959. Mashio, F . ; Kato, S. U.S. Patent 2 910 415, 1959. Kato, S.; Mashio, F. Kogyo Kagaku Zasshi 1964, 67, 1136. Allen, N. S.; Kellar, J . F . ; Phillips, G. O.; Wood, D. G. M. J . Polym. Sci. 1974, Β 12, 241. Allen, N. S.; Kellar, J . F . ; Phillips, G. O.; Chapman, C. B. J. Polym. Sci. 1974, Β 12, 723. For chalking, for example, Pappas, S. P.; Fischer, R. M. J . Paint Tech. 1974, 46, 65. Fujishima, Α.; Honda, K.; Kikuchi, S. Kogyo Kagaku Zasshi 1969, 72, 108.


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RECEIVED February 21, 1985


Semiconductor-Catalyzed Photoreactions of Organic Compounds

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