20 Photochemical ReactionsinMicroemulsions and Allied Systems
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 6, 2015 | http://pubs.acs.org Publication Date: March 27, 1985 | doi: 10.1021/bk-1985-0272.ch020
S. ATIK, J. KUCZYNSKI, B. H. MILOSAVLJEVIC, K. CHANDRASEKARAN, and J. K. THOMAS Department of Chemistry, University of Notre Dame, Notre Dame, IN 46556
Photophysical and photochemical studies of polymerized microemulsions and colloidal semiconductors are investigated. Photochemical studies of pyrene incorporated into polymerized microemulsions indicate that electron or energy transfer occurs via a tunneling or Förster mechanism, respectively. Steady state and pulsed laser excitation techniques have been used to investigate photo-induced processes at the surface of CdS and TiO 2 colloids. The luminescence of CdS and TiO 2 is strongly dependent on either the excitation intensity and the nature of the adsorbed species, or the colloid crystallinity. CdS luminescence is quenched rapidly ( τ 10 mM) of these quenchers. Amorphous T i 0 , ( T i 0 ) , prepared at 35°C does not show any emission a t room temperature. ++
2
2
3
A
+
Photoreduction of Μ ν * on TiQ~ C o l l o i d s . Pulse photolysis of solutions containing ( T i 0 ) and MV"*" leads to instantaneous formation of MV , ( F i g . 10), which is characterized by i t s absorption spectrum with maxima at 395 nm and 605 nm. These data can be explained by d i r e c t electron transfer from excited ( T i 0 ) to adsorbed MV . Benzylviologen, a derivative of MV is not adsorbed on the surface, and hence is not reduced on photoexcitation. The charge separated energy-rich products do not l i v e long enough so that any stored energy can be u t i l i z e d subsequently. Itie photoreduced MV is s t i l l adsorbed on the ( T i 0 ) surface and r e a d i l y reacts with the hole giving back the starting materials. MV adsorbed on ( T i 0 ) shows an i n i t i a l fast decay (τ y ^ 0 ns) followed by a slower decay with a l i f e t i m e of 1 ys. The f r a c t i o n of the i n i t i a l decay depends on the pH of the medium. In acidic medium (pH 1) the i n i t i a l fast decay accounts for a l l of the process, whereas in basic medium only about 60% of the signal decays by the f a s t process, while the remaining MV is stable. I t should be noted that more MV is adsorbed on ( T i 0 ) in basic media than in a c i d i c media. Amorphous Ti0 /MV systems behave quite d i f f e r e n t l y upon photo-excitation. MV is not adsorbed on amorphous T i 0 and so instantaneous formation of MV is not observed. However,inthe presence of electron donors such as polyvinyl alcohol (PYA), or halide ions, a slow growth of MV formation is observed over a period of several \JS ( F i g . 10). In the case of PVA, a permanent reduction of MV is observed as reported by Grâ'tzel et a l . (16), but in the case of C l ~ no permanent reduction is observed. Pulse photolysis studies show large yields of i n i t i a l l y formed MV , but rapid back reaction of MV and C l " y i e l d the s t a r t i n g materials. In O.1 M HC10 the reduction of MV is not e f f i c i e n t , as electron transfer from C l 0 " to the T i 0 hole is not e f f i c i e n t . The results can be explained as follows: 2
2
c
+
2
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 6, 2015 | http://pubs.acs.org Publication Date: March 27, 1985 | doi: 10.1021/bk-1985-0272.ch020
2+
2+
2+
2
c
+
2
c
+
2
c
2
2+
2
+
+
2+
+
+
2
2+
4
4
2
(TiO) 2 a
h
V
e" + MV
h
+
RCH -0H 2
>
+
TiO (e~, h ) 2
++
>
>
+
MV
RCH-OH
+ H 0
2+ MV
+
RCH-OH
>
+ MV +
1 1
+ RCH + H
In Macro- and Microemulsions; Shah, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
c
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 6, 2015 | http://pubs.acs.org Publication Date: March 27, 1985 | doi: 10.1021/bk-1985-0272.ch020
320
MACRO- AND MICROEMULSIONS
A
Ι
I
Ο.4
O.8
1.2
1.6
TIME [ / i s ]
2
+
>
co 3
3
+
co " 3
+
o
co CO Net reaction
> >
+
2e 2
CO ~
β
o
2
2
C0 " 3
co
CO
2H — ~
>
+
3
>
2
e"
co -
e"
3
+
2
2
>
2
3
(e , h )
2
C0 " ; 0
o -
+
C0 " Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 6, 2015 | http://pubs.acs.org Publication Date: March 27, 1985 | doi: 10.1021/bk-1985-0272.ch020
Ti0
+
+ o"
3
2
o
2
+
>
h
'
> H
HCHO HCHO
+ 0
+
The important aspect of the reaction i s that both hole and electron are used to drive the net chemical reaction. The free energy stored in t h i s reaction i s 138 kcals/mole. Conclusions The foregoing studies show the unique features introduced into photochemical systems by the use of microemulsions which assemble reactants and promote reaction features of i n t e r e s t . The use of large surface areas, as with semiconductor c o l l o i d s , also leads to unique photochemical reactions which are quite d i f f e r e n t to bulk reactions. F i n a l l y , c o l l o i d a l systems provide excellent vehicles for studies i n i n t e r f a c i a l photochemistry. Acknowledgments The authors wish to thank the Army Research Office, the National Science Foundation and the Petroleum Research Foundation of the American Chemical Society, f o r support of this work. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.
Fendler, J. H. "Membrane Mimetic System". Wiley, Ν. Y. (1983). Turro, N. ; Braun, A; Gratzel, M. Angew. Chem. 1980, 19, 675. Thomas, J. K. Chem. Rev. 1980, 80, 283. Atik, S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 3550. Atik, S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 4279. Atik, S.; Thomas, J. K. J. Am. Chem. Soc. 1982, 104, 5868. Atik, S.; Thomas, J. K. J. Am. Chem. Soc. 1983, 105, 4515. Atik, S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 4367.
In Macro- and Microemulsions; Shah, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
324
9. 10. 11. 12. 13. 14.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 6, 2015 | http://pubs.acs.org Publication Date: March 27, 1985 | doi: 10.1021/bk-1985-0272.ch020
15. 16. 17. 18. 19.
MACRO- AND MICROEMULSIONS
Krautler, Β . ; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 4317. Duonghong, D.; Borgarello, E.; Grätzel, M. J. Am. Chem. Soc. 1981, 103, 4685. Kuczynski, J.; Thomas, J. K. Chem. Phys. Letts. 1982, 88, 445. Kuczynski, J.; Milosavljevic, Β. H.; Thomas, J. K. J. Phys. Chem., in press. Kuczynski, J. P.; Milosavljevic, Β. H.; Thomas, J. K. J. Phys. Chem. 1983, 87, 3368. Farrington, J. Α.; Ebert, M.; Land, E. J.; Fletcher, Κ. B.B.A. 1973, 314, 372. Patterson, L. K.; Small, R. D.; Scaiano, J. C. Rad. Res. 1977, 72, 218. Duonghong, D.; Ramsden, J.; Grätzel, M. J. Am. Chem. Soc. 1982, 104, 2977. Henglein, A. Ber. Bunenges. Phys. Chem. 1982, 86, 241. Grossweiner, L. S.; Matheson, M. S. J. Phys. Chem. 1957, 61, 1089. Chandrasekaran, K.; Thomas, J. K. Chem. Phys. Lett. 1983, 99, 7.
RECEIVED June 8, 1984
In Macro- and Microemulsions; Shah, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
