[acs symposium series] organic phototransformations in nonhomogeneous media volume 278 ||...

3

Semiconductor-Catalyzed Photoreactions of Organic Compounds

KATSUMI TOKUMARU, HIROCHIKA SAKURAGI, TATSUYA KANNO, TAKAHIDE OGUCHI, HIROAKI MISAWA, YASUO SHIMAMURA, and YASUNAO KURIYAMA

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.

Recent ly , the photochemical a c t i o n of semiconductors have been a c t i v e l y i n v e s t i g a t e d . However, t h i s area a t t r a c t e d the i n t e r e s t of many workers nea r l y t h i r t y years ago (1 -5 ) . In those days, L a i d l e r Ç5), Ca l ve r t (6 ,7 ) , and Markham (8-10) s tud ied the a c t i o n of z i n c oxide under i l l u m i n a t i o n i n aqueous a l coho l s under oxygen producing carbonyl compounds and hydrogen peroxide . The i n t e r e s t i n those days can be seen by the appearance of an in t roduc to ry a r t i c l e by Markham i n J . Chem. Ed. i n 1955 (11). At near ly the same time, Mashio and Kato worked on the p h o t o c a t a l y t i c a c t i o n of t i t an ium d iox ide on ox ida t i on of a l coho l s (12,13) and hydrocarbons (14) i n an attempt to e luc ida te the mechanism by which p o l y ( v i n y l acetate ) f i b r e s dyed wi th the a i d of t i t an ium d iox ide became f r a g i l e dur ing use i n a i r under sunshine. They found that the i r r a d i a t i o n of t i t an ium d iox ide i n i t i a t e s au tox ida t ion of the subs t ra te s . In I960 ' s , when the manufacture of polypropylene 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 , degradation of the polymer was found to be acce l e ra ted by t i t an ium compounds remaining i n the polymer, which aga in a t t r a c t e d a t t e n t i o n to the p h o t o c a t a l y t i c a c t i o n of t i t an ium d iox ide (15-17).

At the end of 1960 f s , Honda and Fuj ishima found that i n a photochemical c e l l employing t i t an ium d iox ide and plat inum e l e c t rodes , i r r a d i a t i o n of the t i t an ium d iox ide e lect rode r e su l t ed i n s p l i t t i n g of water in to hydrogen and oxygen (18,19) . Th is work has had an e x t r a o r d i n a r i l y s t rong impact for the research on the photo chemical a c t i o n of var ious semiconductors induc ing evo lu t i on of hydrogen from water as w e l l as new c a t a l y t i c r eac t i ons (20).

For the a c t i o n of semiconductor p a r t i c l e s on organic compounds, Bard and h i s coworker reported that i r r a d i a t i o n of t i t an ium d iox ide

0097-6156/85/0278-O043$06.00/0 © 1985 Amer i can Chemica l Society

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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 ed to the ox ida t i on of acetate anions to methane and ethane (21,22) . At nea r l y the same t ime, other groups i n i t i a t e d i n v e s t i g a t i o n s of the p h o t o c a t a l y t i c a c t i o n of semiconductor p a r t i c l e s on r e l a t i v e l y simple organic compounds l i k e a l coho l s and c a r boxy l i c a c i d s , with the hope of photochemica l ly producing hydrogen from water (23-25).

At the end of 1970 1 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 of semiconductor p a r t i c l e s on medium s i z e d organic compounds. In t h i s a r t i c l e , we would l i k e to descr ibe some of the work done i n our l abora tory on the p h o t o c a t a l y t i c a c t i o n of semiconductors on the ox i da t i on of o l e f i n s and hydrocarbons, and on the i somer i za t i on of unsaturated systems.

P h o t o c a t a l y t i c Ox idat ion of O l e f i n s

We found that e x c i t a t i o n of t i t an ium d iox ide or cadmium s u l f i d e powder suspended i n organic so lvents conta in ing o l e f i n s l i k e 1 ,1 -d iphenylethy lene (DPE) under oxygen l e d to ox i da t i on of the o l e f i n s to t h e i r epoxides and carbonyl compounds (26). In a t y p i c a l run , semiconductor powder (ca. 5 mg) was suspended i n a s o l u t i o n (ca. 2 ml) conta in ing an o l e f i n (ca. 0.3 mmol), and i r r a d i a t e d under oxygen atmosphere at room temperature with l i g h t of longer wavelengths than 350 nm for t i t an ium d iox ide and than 430 nm for cadmium s u l f i d e . The products were separated by vpc or t i c for 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 vpc. For example, DPE gave benzo-phenone and i t s epoxide, which was converted to 2 -methoxy -2 ,2 -d i -phenylethanol i n methanol s o l u t i o n .

P h . . R ' h V > °2 P h . P h . c _ c . R ' P h - C C - R " ~ ~ *^ P h ' C 0 + 0 S t " + P h - C v } S " CdS or 0

TiO„ I* R' = H, R""= OMe Ph 2C(OMe)CH 2OH

2 jMeOH R1 = R" = H, Me

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

In aqueous a c e t o n i t r i l e , CdS - s ens i t i z ed i r r a d i a t i o n of DPE gave 1 , 1 - d i p h e n y l e t h a n - l , 2 - d i o l a long wi th benzophenone, which i n d i c a t e s that the r e s u l t i n g epoxide i s hydro lyzed i n the s o l u t i o n (27).

Some r e s u l t s are summarized i n Table I (27). F igure 1 schemati c a l l y dep ic t s the r e a c t i v i t y of the o l e f i n s examined with t h e i r o x i d a t i on p o t e n t i a l s , and i n d i c a t e s that genera l l y the unreact ive o l e f i n s are those with higher ox i da t i on 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 with lower ox ida t i on p o t e n t i a l s . However, among the ox id i z ab 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 imply governed by t h e i r ox i da t i on p o t e n t i a l s as w i l l be d iscussed l a t e r .

The quantum y i e l d s for the consumption of DPE and 2 -methy1-1 ,1 -diphenylpropene were obtained as at 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 oxygenation without c o r r e c t i n g the r e f l e c t i o n of the i nc i den t l i g h t on the semiconductor su r f aces .

The above fac ts i n d i c a t e that on e x c i t a t i o n of the semiconduct o r s the o l e f i n s t r ans f e r an e l e c t r o n to the photogenerated p o s i t i v e ho le i n the i n i t i a t i o n process to give the o l e f i n r a d i c a l c a t i o n , and concur rent ly the e l e c t r o n exc i t ed to the conduct ion band i s

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3. T O K U M A R U E T A L . Photoreactions of Organic Compounds 45

Table I. Semiconductor Photocata lyzed Oxygenation of Aromatic O l e f in s (a.)

O l e f i n Semiconductor Solvent

Time /h

Conver sion/%

- Carbonyl compds/%

Epoxide or i t s deriv/%

Ph 2C=CH 2 CdS MeCN-MeOH 6 27 15 26 Ph 2C=CH 2

(1:1) p-MeOC 6H 4CH=CH 2 T i 0 2 MeCN 6 31 43 t race PhMeOCH 2 T i 0 2 MeCN 6 4 77 Ph2C=CH0Me CdS MeCN-MeOH 7 100 12 69

(1:1) Ph 2C=CMe 2 T i 0 2 MeCN 6 16 11 9 trans-PhCH=CHPh CdS MeCN 12 5 + 5 cis-PhCH=CHPh CdS MeCN 12 6 + 2 cyc looctene T i 0 2 MeCN 6 4 - 96 norbornene T i 0 2 MeCN 6 3 - 27 2-cyclohexenone T i 0 2 MeCN 8 No r e a c t i o n PhCH=CHCN T i 0 2 MeCN 8 No r e a c t i o n PhCH=CHC0Me T i 0 2 MeCN 8 No r e a c t i o n PhCH=CHC09Et T i 0 2 MeCN 8 No r e a c t i o n

a . Y i e l d s are based on the o l e f i n consumed.

T i C L CdS

O x i d a t i o n p o t e n t i a l O x y g e n a t i o n

Ph 2 C=CHOMe

t -PhCH=CHPh

c -PhCH=CHPh f M e O C . H . C H = C H .

6 4 2

P h 2 O C M e 2

Me2OCMe P h 2 C = C H 2

PhMeC=CH 2

b i c y c l o [ 2 . 2 . 1 ] -h e p t - 2 - e n e

c y c l o h e x e n e

c y c l o o c t e n e

PhCH=CHCN

PhCH=CHCOMe

PhCH=CHC00Et

c y c l o h e x e n o n e

vs A g / A g N 0 3 ( 0 . 1 M)

Figure 1. R e a c t i v i t y of o l e f i n s and t h e i r ox ida t i on p o t e n t i a l s .

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t r ans f e r r ed to oxygen to g ive the superoxide an ion . These r e s u l t i n g spec ies subsequently i n i t i a t e free r a d i c a l cha in r e a c t i o n s .

Fox and her coworkers observed t r ans i en t spectra a t t r i b u t a b l e to r a d i c a l ca t ions upon l a s e r e x c i t a t i o n of powdered semiconductor i n the presence of o l e f i n s (28-31) .

Factors C o n t r o l l i n g the R e a c t i v i t y of O l e f i n s i n the Photocata lyzed Oxygenation

To examine the e l e c t r o n t r ans f e r from o l e f i n s to the exc i t ed semiconductor , photoe lect rochemica l measurements were undertaken. In a c e l l compris ing of two compartments d i v i ded by a g lass f r i t , a t i t an ium d iox ide working e l ec t rode 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 of an o l e f i n (0.2 M) i n one compartment, and a p lat inum counter e l ec t rode was put i n a c e t o n i t r i l e i n the other compartment; tetraethylammonium perch lo ra te (0.1 M) was used as a support ing e l e c t r o l y t e . A f t e r bubbl ing argon i n t o the working c e l l s o l u t i o n , the t i t an ium d iox ide e l ec t rode was i l l u m i n a t e d (λ > 350 nm) under vo l tages vary ing between - 0 . 5 and 0.8 V vs Ag/AgN03 (0.1 M). Some o l e f i n s exh i b i t ed an increase of photocurrent with increase of the app l i ed p o t e n t i a l to give a nea r l y constant current around 0.5 V; however, some o l e f i n s d id not show any photocurrent at these p o t e n t i a l s (32).

F igure 2 shows that the photocurrent measured at 0.5 V increases w i th decreas ing the ox ida t i on p o t e n t i a l s of the o l e f i n s examined; however, benzyl ideneacetone, e t c . with h igher ox ida t i on p o t e n t i a l s d i d not show any photocurrent (32).

Although the absolute amount of the photocurrents i s governed by va r ious f ac to r s such as the ox ida t i on p o t e n t i a l s of o l e f i n s and the extent of adsorpt ion of o l e f i n s on the e l e c t r ode , the above f ind ings show that the r e ac 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 oxygenation e x h i b i t photocurrents and the o l e f i n s which do not e x h i b i t photocurrents are unreact ive i n the p h o t o c a t a l y t i c oxygena t i o n . On the other hand, the o l e f i n s which e x h i b i t photocurrents are not always r e a c t i v e . For example, s t i l b e n e shows a h igher photocurrent than DPE, but i s not so r e ac t i v e as DPE. The e l e c t r o n t r ans f e r to the exc i t ed semiconductor takes p lace 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 ox ida t i on p o t e n t i a l of the former, but i n the subsequent free 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 than DPE (33).

There fore , i t can be concluded that fo r the p h o t o c a t a l y t i c oxygenation to occur , the e l e c t r o n t r ans f e r from the o l e f i n to the p o s i t i v e hole has to take p l a ce , but the o v e r a l l r e a c t i v i t y of the o l e f i n s i s governed by the e f f i c i e n c y of f ree r a d i c a l processes as exempl i f i ed i n Table I I .

In view of 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 ox ida t i on of a s e r i e s of 4 - sub s t i t u t e d d ipheny le thy lenes , an increase i n r e a c t i v i t y wi th decreasing Hammett fs sigma constants (31) seems to a r i s e not only from the lowering of the ox ida t i on p o t e n t i a l s of the o l e f i n s i n t h i s sequence but a l so from the general trend o f the increase i n the r e a c t i v i t y of o l e f i n s toward peroxy l r a d i c a l s wi th 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 of o l e f i n r a d i c a l ca t ions toward oxygen, anodic ox ida t i on 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 mixture of

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TOKUMARU ET AL. Phoîoreactions of Organic Compounds

Table I I . R e a c t i v i t y of O l e f i n s i n Photocata lyzed Oxygenation

O l e f i n E f f i c i e n c y

O l e f i n E l e c t r o n t r ans f e r

Rad i ca l chain ox ida t i on

O v e r a l l

PhCH=CHPh good poor medium Ph 2C=CH 2 medium good good PhCH=CHC0Me poor poor poor

American Chemfcaf Society Library

1155 16ih St. N. W. Washington. 0. C. 20036

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a c e t o n i t r i l e and methanol (1:3 by volume) i n the presence of t e t r a -ethylammonium perch lo ra te as a support ing 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 to nea r l y 70% convers ion of the o l e f i n a f forded a 1,2-dimethoxylated product as the main product a long wi th a smal l amount of benzophenone, but 2 ,2 -d ipheny l -2 -methoxyethano l , which had been expected to a r i s e from oxygenation of the r a d i c a l c a t i o n i n methanol, was formed i n a very low y i e l d (27). These r e s u l t s i n d i c a t e that the DPE r a d i c a l c a t i o n i s not h i gh l y r e ac t i v e wi th oxygen.

For generat ion of o l e f i n r a d i c a l c a t i on s , r e c e n t l y s eve r a l attempts have been made to use 9,10-dicyanoanthracene (DCA) as an e l e c t r o n accept ing s e n s i t i z e r under oxygen. On e x c i t a t i o n of 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 ta te accepts an e l e c t r o n from an o l e f i n to g ive the r a d i c a l anion of DCA and the r a d i c a l c a t i on of the o l e f i n , and the former anion can t r ans f e r an e l e c t r o n to oxygen to g ive the superoxide an ion . For example, DCA - sens i t i z ed oxygenation of DPE was reported to g ive a mixture of var ious ox ida t i on products i n v o l v i n g benzophenone as the main product a long wi th the c o r r e sponding epoxide and 1 , l -d ipheny l -2 -methoxyethano l , e t c . , and the products were proposed to a r i s e from the r e ac t i on between the o l e f i n r a d i c a l c a t i o n and superoxide anion (34-37) .

Among o l e f i n r a d i c a l ca t ions s tud ied only adamantylideneadaman-tane (ADA) i s e s t ab l i shed to react f a c i l e l y with oxygen to give an oxygen adduct which subsequently accepts an e l e c t r o n from ADA to g ive the corresponding dioxetane and to regenerate the ADA r a d i c a l c a t i o n , thus accompl ishing a cha in r e a c t i o n (38).

There fore , i t seems reasonable to suppose that i n the semiconductor ca ta lyzed ox ida t i on of DPE, the r e s u l t i n g r a d i c a l c a t i o n of the o l e f i n cou ld react wi th the superoxide anion to give a peroxy-e t h y l - l , 4 - d i r a d i c a l which subsequently reacts wi th the o l e f i n and oxygen through the free r a d i c a l cha in processes .

Autox ida t i on of aromatic 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 cha in 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 usua l l y gives carbonyl compounds as the main products a long wi th epoxides, and the product ion o f the epoxides tends to increase with decrease of oxygen pressure (33,39) . Acco rd i n g l y , the product ion of a cons iderab le amount of the epoxide accompanied by benzophenone i n the semiconductor photocata lyzed oxygena t i o n of DPE under 1 atm oxygen shows that i n the semiconductor c a t a l y s i s the r eac t i on might proceed through a mechanism s l i g h t l y d i f f e r e n t from the au tox ida t i on , po s s i b l y due to the p a r t i c i p a t i o n of the DPE r a d i c a l c a t i o n and superoxide anion i n p lace of n e u t r a l r a d i c a l s and oxygen i n the cha in i n i t i a t i o n s tep .

Photocata lyzed I somer izat ion of Unsaturated Systems

The s e n s i t i z i n g a c t i o n of semiconductor a f f ec t s i s omer i z a t i on of o l e f i n s . On e x c i t a t i o n of CdS or T1O2 i n the presence of t r a n s - or c i s - s t i l b e n e under argon, the i s omer i z a t i on occurred only from the c i s - to t rans - i somer ; the t rans - i somer s ca r ce l y underwent i somer 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 oxygenation to give benzaldehyde and t r a n s - s t i l b e n e oxide , i r r e s p e c t i v e of the con f i gu ra t i on of the s t a r t i n g o l e f i n , a long wi th the i s omer i z a t i on from the c i s - to trans*-isomer (40).

The i s omer i z a t i on tak ing p lace only from c i s - to t r a n s - s t i l b e n e

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3. TOKUMARU ET AL. Photoreactions of Organic Compounds 49

i s cons i s tent with the i s omer i z a t i on through the o l e f i n r a d i c a l c a t i o n . F igure 3 dep icts the energies of the r a d i c a l ca t ions of 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 ea r l y 3 kcal/mol (ca. 0.13 eV) h igher i n energy than the t rans - i somer (41). Since the ox ida t i on p o t e n t i a l s of t r a n s - and c i s - s t i l b e n e are 1.15 and 1.23 V vs Ag/AgNU3 (0·^ M)> r e s p e c t i v e l y , the ca t i on 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 higher i n energy than that of t r a n s -s t i l b e n e . The i n e f f i c i e n t product ion of the c i s - i s o m e r from the t rans - i somer r u l e s out a p o s s i b i l i t y for the p a r t i c i p a t i o n of the t r i p l e t s ta te of s t i l b e n e (2.1 eV over the t r ans - i somer ) . Th is means that the p a i r o f the trans r a d i c a l c a t i on with the e l e c t r o n i n the conduction band of T1O2 (ca. - 0 . 6 V vs Ag/AgN0 3) or CdS (ca. - 1 . 1 V) l i e s i n the energies (ca. 1.8 and 2.3 eV, r e s p e c t i v e l y over the t rans - i somer ) not much exceeding the t r i p l e t s t a t e .

Acco rd ing ly , i t i s reasonable to conclude that i n the photoca ta lyzed i s omer i z a t i on of o l e f i n s , the c i s r a d i c a l c a t i o n tw i s t s to the more s tab l e trans or t r a n s - l i k e twisted r a d i c a l c a t i o n (42-46) , which subsequently rece ives an e l e c t r o n from the exc i t ed semiconductor to g ive the t r ans - i somer . S im i l a r i s omer i za t i on us ing CdS was reported r ecent l y (47). On the other hand, the " t r a n s " r a d i c a l c a t i on r e s u l t i n g from ox ida t i on of the t rans - i somer cannot i somerize to the l e s s s t ab l e c i s r a d i c a l c a t i o n . The formation of t r a n s -s t i l b e n e oxide i r r e s p e c t i v e of the con f i gu ra t i on of the s t a r t i n g isomer can be a t t r i b u t e d to the r e ac t i on of the " t r a n s " r a d i c a l c a t i on wi th oxygen g i v i n g a peroxy l r a d i c a l c a t i o n which subsequently co l l apses to the t rans -epox ide .

The behavior of s t i l b e n e r a d i c a l ca t ions i n the semiconductor c a t a l y s i s i s i n keeping with the r e s u l t o f photo i somer izat ion of other 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 acceptors l i k e c h l o r a n i l i n po l a r so lvents (48). The semiconductor photocata l yzed i s omer i z a t i on of s t r a i n e d cyclobutanes to s t r a i n e d dienes ( i somer i za t ion of quadr icyc lene to norbornadiene and s i m i l a r react ions of complex cage compounds (49)) i s r e l a t e d to the o l e f i n i s omer i z a t i on d iscussed above.

Pho toca t a l y t i c Oxygenation of Hydrocarbons

I r r a d i a t i o n of powdered t i t an ium dioxide suspended i n s o l u t i on s conta in ing aromatic compounds and water under oxygen has r e cen t l y been shown to induce hydroxy la t i on of aromatic n u c l e i g i v ing pheno l i c compounds and ox ida t i on of s ide chains of the aromatic compounds (50-55). These react ions have been assumed to proceed through hydroxy1 and other r a d i c a l in te rmedia tes , but the mechanism for t h e i r generat ion , whether r eac t i ve free r a d i c a l s r e s u l t from ox ida t i on of water, from reduct ion of oxygen, or from ox ida t i on o f the substrates on the sur faces of the e x c i t e d t i t an ium d iox ide , has not been c l e a r .

An attempt was made to revea l the mechanism for the formation of f ree r a d i c a l s upon i r r a d i a t i o n of t i t an ium diox ide i n the presence of benzene and to luene . Ca re fu l examination of the e f f e c t s of oxygen and water showed that the presence of oxygen i s e s s e n t i a l for the r e a c t i o n , and that under oxygen the ox ida t i on of water cont r ibutes to the aromatic hyd roxy l a t i on and the ox i da t i on of toluene as a substrate leads to ox ida t i on of i t s s ide cha in (56).

Powdered t i t an ium diox ide (30 mg) was suspended i n an ace to -

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1.23 1.36

+ 1.15 t r a n s

0.13 t r a n s

eV

Figure 3 . Energies of s t i l b e n e r a d i c a l c a t i on s .

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3. TOKUMARU ET AL. Photoreactions of Organic Compounds 5 1

n i t r i l e s o l u t i o n (2 ml) conta in ing benzene or toluene (16 vol%) and vary ing amounts of water (0-8 vol%), and i r r a d i a t e d with l i g h t longer than 350 nm for 15 h at 20°C with and without oxygen.

F igure 4 dep ic t s t y p i c a l r e s u l t s o f the ox i da t i on of benzene under oxygen and without oxygen wi th va ry ing concentrat ions of water. As F i g . 4 c l e a r l y shows, only a t race amount of phenol was produced, even i n the presence o f water without oxygen, whi le under oxygen the y i e l d of phenol i s s t i l l low i n the absence of water, but increases with i n c r e a s i n g concent ra t ion of water. For the r e ac t i on of toluene under c a r e fu l removal of oxygen, on ly t race amounts o f ox i da t i on products were produced even i n the presence of water, as was found for benzene.

I t i s no t i ceab l e that i n the ox ida t i on o f toluene 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 nuc lea r hydroxy l a t i on of the substrate are produced i n very low y i e l d s i n the absence of water, but tend to increase with i nc r ea s ing concent ra t ion o f water; on the cont ra ry , benzaldehyde and benzy l a l c o h o l a r i s i n g from ox ida t i on of the s ide cha in are produced i n much h igher y i e l d s than c r e so l s i n the absence of water, but t h e i r y i e l d s do not vary with water concent ra t i on .

The above r e s u l t s wi th benzene and toluene show that the presence o f oxygen i s necessary for the formation of ox i da t i on products , but the presence of water alone i s not enough to induce the ox ida t i on of the subs t ra te s . These facts i n d i c a t e that water can be ox id i zed to hydroxy l r a d i c a l s by the photogenerated p o s i t i v e holes wi th concurrent removal of e l ec t rons i n the conduction band by oxygen, presumably adsorbed on the semiconductor sur faces . The r e s u l t i n g hydroxy l r a d i c a l s w i l l hydroxylate the aromatic n u c l e i .

or CH Χ X

ρ -Η C 6 H 5 C H 3 • (C 6 H 5 CH 3 ) • C 6 H 5 C H 2 '

°2 • • C 6 H 5 CH 2 OH, C 6H 5CHO

H +

0 2 • 0 2 · • H0 2 ·

Under oxygen i n the absence of water, toluene w i l l t r ans f e r an e l e c t r o n to the p o s i t i v e ho l e , concurrent ly with e l e c t r on t r ans f e r from the conduction band to oxygen, to give a toluene r a d i c a l c a t i o n . On the other hand, i n the presence of water, both toluene and water w i l l t r ans f e r an e l e c t r o n to the p o s i t i v e h o l e s . The r e s u l t i n g toluene r a d i c a l c a t i on may subsequently lose a proton a f f o rd ing a benzy l r a d i c a l , which w i l l be ox id i zed wi th oxygen or the superoxide anion to benzy l a l coho l and benzaldehyde, as proposed fo r the react ions of Fenton 's reagent wi th toluene (57).

There fore , i t i s reasonable to conclude that upon i r r a d i a t i o n of t i t an ium dioxide under oxygen, the e l e c t r o n t r ans f e r from water to the p o s i t i v e hole mainly r e s u l t s i n hydroxy l a t i on of the aromatic

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ORGANIC PHOTOTRANSFORMATIONS IN NONHOMOGENEOUS MEDIA

Figure 5. Ox idat ion of toluene i n a c e t o n i t r i l e under oxygen; p roducts : benzaldehyde ( φ ) , benzy l a l c o h o l ( φ ) , o - c r e s o l ( Ο ) , m-creso l ( Q ) , p - c r e s o l ( φ ) .

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3. T O K U M A R U E T A L . Photoreactions of Organic Compounds 53

n u c l e i and the e l e c t r o n t r ans f e r from toluene mainly leads to ox ida t i on of i t s s ide cha in , and that oxygen cont r i bu tes to these reac t ions by concur rent ly accept ing an e l e c t r o n from the conduction band of semiconductor.

T i tan ium d iox ide photocata lyzed ox ida t i on of neat t e t r a l i n was p r ev i ous l y reported to g ive i t s hydroperoxide (14). Re inve s t i g a t i on showed that t e t r a l o l and te t r a l one are a l so formed i n a c e t o n i t r i l e po s s i b l y through e l e c t r o n t r ans f e r from t e t r a l i n to the p o s i t i v e holes (27).

Concluding Remarks

Semiconductors act as photocata lys t s on var ious organic compounds. The e s s e n t i a l features of the mechanism are becoming c l e a r . However, how the i n i t i a t i o n steps proceed, p a r t i c u l a r l y how the species on the semiconductor sur face react wi th water and oxygen, i s not yet we l l understood. Moreover, another area to 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 ca t ions of the substrates undergo t ransformat ion to products .

Acknowledgments

The authors express t h e i r thanks to M i n i s t r y o f Educat ion , Sc ience , and Cu l ture i n Japan for the s p e c i a l p ro j ec t research on energy under grant i n a i d of s c i e n t i f i c research and to Japan-US Cooperat i v e Research on Photoconversion and Photosynthesis for enab l ing d i scuss ions wi th the s c i e n t i s t s concerned.

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RECEIVED February 21, 1985

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[acs symposium series] organic phototransformations in nonhomogeneous media volume 278 ||...

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