Carbanion Photooxidation at Semiconductor Surfaces - American

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Carbanion Photooxidation at Semiconductor Surfaces M A R Y E ANNE FOX and ROBERT C. OWEN Department of Chemistry, University of Texas at Austin, Austin, T X 78712

Within the last decade, many inventive procedures for the quantum utilization of visible light by organic or inorganic ab sorbers have been developed. The most efficient of these often involve electron exchange reactions. Here, a vexing problem persists: rapid thermal recombination of the electron-hole pairs can regenerate the ground state and effectively waste the energy of the absorbed photon. We have reasoned that this back reaction could be inhibited, or at least dramatically slowed, if an ex cited anion M (eqn 1) were used as the donor rather than a neu tral molecule M (eqn 2). The reversal of equation (1), governed only by the typically low electron affinity of the photoproduced -

radical, should be less favored than the back reaction in equa tion (2), where an electrostatic attraction within the photoproduct pair favors reversion to the ground state. Indeed, in this analysis, recapture of a photo-ejected electron by the oxidized primary photoproduct formed from a dianion should be even less favorable. This approach to the inhibition of electron recap ture is therefore complementary to the use of acceptors which form metastable one electron reduction products upon photoexcita tion of an electron donor. In fact, the most effective acceptors (e.g., methyl viologen) are usually those which operate on this same electrostatic principle, i.e., where electrostatic attraction of the oxidized and reduced primary photoproducts is minimized. In the cases considered here, one might reasonably expect compar able stabilization of the primary photoproducts in equations (1) or (2) by the presence of neutral electron acceptors. Consequent ly, the relative inhibition of back electron transfer in simple processes like equations (1) and (2) should find direct parallel in the presence of neutral electron acceptors.

0097-6156/81/0146-0337$05.00/0 © 1981 American Chemical Society In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on November 3, 2015 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch021

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Further i n h i b i t i o n would a l s o be a n t i c i p a t e d i f the environment of the e l e c t r o n - h o l e p a i r i n t e r f e r e s with recombination. Separation of such p a i r s has been e f f e c t i v e l y achieved i n the e l e c t r i c f i e l d formed at the i n t e r f a c e between a semiconductor and an e l e c t r o l y t e s o l u t i o n (1). Accordingly, we have examined a s e r i e s of e x c i t e d organic anions, both i n homogeneous s o l u t i o n and at a semiconductor e l e c t r o d e i n a photoelectrochemical c e l l . As expected, redox photochemistry o c c u r r i n g at the semiconductor surface d i f f e r s s i g n i f i c a n t l y from that found under homogeneous cond i t i o n s . We have used t h i s a l t e r e d r e a c t i v i t y as a s y n t h e t i c technique f o r c o n t r o l l e d o x i d a t i v e coupling r e a c t i o n s and as an i n v e s t i g a t i v e t o o l f o r e s t a b l i s h i n g mechanism i n v i s i b l e l i g h t p h o t o l y s i s of h i g h l y a b s o r p t i v e anions. Our experimental procedure p a r a l l e l s that described e a r l i e r i n the c o n s t r u c t i o n of an a n i o n i c photogalvanic c e l l (22. A very t h i n l a y e r (-1 mm) of anhydrous s o l u t i o n containing the absorpt i v e anion i s sandwiched between an o p t i c a l f l a t and a poised semiconductor e l e c t r o d e . Upon i r r a d i a t i o n of t h i s s t i r r e d s o l u t i o n , photocurrents between the i l l u m i n a t e d e l e c t r o d e and a dark platinum counterelectrode can be monitored. (See reference 2 f o r experimental d e t a i l . ) By choosing an appropriate wavelength r e gion from the i r r a d i a t i o n source, we may e x c i t e p r e f e r e n t i a l l y e i t h e r the d i s s o l v e d (or adsorbed) anion or the semiconductor i t self. Redox r e a c t i o n s o c c u r r i n g i n the s t i r r e d s o l u t i o n may be followed i n s i t u by c y c l i c voltammetry or by withdrawing an a l i quot f o r chemical a n a l y s i s by standard spectroscopic and/or chromatographic techniques. P r e p a r a t i v e e l e c t r o l y s e s were conducted i n the same c e l l employing a PAR (Princeton A p p l i e d Research) p o t e n t i o s t a t and d i g i t a l coulometer. S o l u t i o n phase photolyses were conducted by i r r a d i a t i n g sealed, degassed, pyrex ampoules c o n t a i n i n g the r e a c t i v e anions and were analyzed as above. By e x c i t i n g the red-orange c y c l o o c t a t e t r a e n e d i a n i o n 1 i n the presence of c y c l o o c t a t e t r a e n e i n our photoelectrochemical c e l l (n-Ti02/NH3/Pt), we were able to observe photocurrents without d e t e c t a b l e decomposition of the a n i o n i c absorber Ç 2 ) . Presumably, a r a p i d dismutation of the photooxidized product i n h i b i t e d e l e c tron recombination, producing a s t a b l e hydrocarbon whose cathodic reduction at the counter e l e c t r o d e regenerates the o r i g i n a l mixture e s s e n t i a l l y q u a n t i t a t i v e l y (eqn 3 ) .

In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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We hope to confirm the p r i n c i p l e o f t h i s mechanism by demonstra t i n g that c h a r a c t e r i z a b l e chemical r e a c t i o n s might ensue i f the i n i t i a l photooxidation ( u n l i k e eqn 3) were i r r e v e r s i b l e . Previous work i n our l a b o r a t o r y (3) and i n others (A) has e s t a b l i s h e d that the primary photoprocess i n a v a r i e t y of e x c i t e d carbanions i n v o l v e s e l e c t r o n e j e c t i o n . T h i s photooxidation w i l l generate a r e a c t i v e free r a d i c a l i f recapture o f the e l e c t r o n i s i n h i b i t e d . P a r a l l e l generation of these same carbon r a d i c a l s by e l e c t r o c h e m i c a l o x i d a t i o n r e v e a l s an i r r e v e r s i b l e anodic wave, c o n s i s t e n t with r a p i d chemical r e a c t i o n by the o x i d i z e d organic species (5). L i t t l e chemical c h a r a c t e r i z a t i o n o f the products has been attempted, however (6). A t y p i c a l c y c l i c voltammetric t r a c e f o r the anodic o x i d a t i o n of the f l u o r e n y l anion 2 at platinum i s shown i n F i g u r e 1. The o x i d a t i o n p o t e n t i a l f o r t h i s and s e v e r a l other resonance s t a b i l i z e d carbanions l i e s conveniently w i t h i n the band gap of n-type T 1 O 2 i n the non-aqueous s o l v e n t s , and hence i n a range s u s c e p t i b l e to photoinduced charge t r a n s f e r . Furthermore, dimeric products (e.g., b i f l u o r e n y l ) can be i s o l a t e d i n good y i e l d (55-80%) a f t e r a one Faraday/mole c o n t r o l l e d p o t e n t i a l (+1.0 eV vs Ag q u a s i reference) o x i d a t i o n at platinum. I f an e l e c t r o n acceptor i s a v a i l a b l e i n homogeneous s o l u t i o n , photochemical r e a c t i o n can be observed. For example, when 2 i s excited (λ 350 nm) i n anhydrous d i m e t h y l s u l f o x i d e (DMSO), methylation occurs, u l t i m a t e l y g i v i n g r i s e to 9,9-dimethylf l u o r e n e i n >80% y i e l d . By analogy with T o l b e r t ' s mechanism f o r photomethylation i n DMSO (4), such a process may be i n i t i a t e d by e l e c t r o n t r a n s f e r to DMSO to form a caged r a d i c a l - r a d i c a l anion pair from which subsequent C-S cleavage occurs (eqn 4 ) .

I f no acceptor i s present, recapture o f the photoejected e l e c t r o n w i l l be r a p i d and the photon energy w i l l be l o s t . In accord with t h i s p r e d i c t i o n , a f t e r overnight p h o t o l y s i s of a tetrahydrofuran s o l u t i o n o f 2, under c o n d i t i o n s i d e n t i c a l to those described above, 2 can be recovered e s s e n t i a l l y unchanged. E x a c t l y p a r a l l e l r e s u l t s have been obtained with t e t r a p h e n y l cyclopentadienide (3). That a semiconductor e l e c t r o d e can prevent b a c k - e l e c t r o n t r a n s f e r f o l l o w s from d e t e c t i o n of dimer (dihydrooctaphenyl-



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eV (vs.

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I Ag)

Figure 1.

1.5

1.0

-O.5

O.5

-1.0

Electrochemical oxidation offluorenyllithium (DMSO, LiClO (O.1M), room temperature, scan rate: 50 mV s' ) k

1


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f u l v a l e n e ) (7) by HPLC and NMR, when l i t h i u m t e t r a p h e n y l c y c l o pentadienide i s excited at the surface of doped η-type TiU2 c r y s t a l held a t O.0 eV (vs Ag) and connected with a platinum e l e c trode i n DMSO containing LiC10i+ (8) as i n e r t e l e c t r o l y t e (eqn 5 ) .

3

Phi+

Phi*

Photooxidation occurs at p o t e n t i a l s w e l l negative o f the anodic wave. As dimer formation occurs photo-currents (O.1-3.9 μΑ) can be detected, implying photoinduced change t r a n s f e r . It i s a l s o s i g n i f i c a n t to note that dimeric products a r e formed whether the anion or the semiconductor i s e x c i t e d . I t i s p o s s i b l e to s e l e c t i v e l y e x c i t e d the anion by f i l t e r i n g out l i g h t of wavelengths shorter than the onset of band gap i r r a d i a t i o n i n n-Ti02, t.e.yby cut o f f f i l t e r s . For very t h i n l a y e r s o f i r r a d i a t e d s o l u t i o n i n our c e l l , incomplete l i g h t absorption by the solution/adsorbate occurs when u n f i l t e r e d l i g h t i s used as the e x c i t a t i o n source. Under these c o n d i t i o n s , where s i g n i f i c a n t e x c i t a t i o n of the semiconductor occurs simultaneously with anion e x c i t a t i o n , g r e a t l y enhanced photocurrents a t t r i b u t a b l e to band gap i r r a d i a t i o n i n the presence of donors are observed. Thus o x i d a t i o n i s achieved r e s p e c t i v e l y e i t h e r i f an e l e c t r o n i s i n j e c t e d i n t o the conduction band from the a n i o n i c e x c i t e d s t a t e or i f an e l e c t r o n i s t r a n s f e r r e d to the photogenerated hole i n the valence band. As we have shown p r e v i o u s l y , s e n s i t i z a t i o n by adsorbed o r chemically attached dyes (9) should t h e r e f o r e ex tend the wavelength response of l a r g e band gap semiconductors i n a process p a r a l l e l to that observed here. Analogous r e s u l t s are being obtained f o r the f l u o r e n y l anion and other resonance s t a b i l i z e d anions. Although the mechanistic d e t a i l s are s t i l l under i n v e s t i g a t i o n and w i l l be discussed e l s e where, these experiments demonstrate that the semiconductor i n t e r face i s e f f e c t i v e i n i n h i b i t i n g e l e c t r o n recapture i n anion p h o t o l y s i s , i n e s t a b l i s h i n g mechanism i n p o s s i b l e charge t r a n s f e r photoreactions, and i n a c t i n g as a c a t a l y t i c s u r f a c e i n u s e f u l o x i d a t i v e photocoupling r e a c t i o n s . Acknowledgement. We are g r a t e f u l to the U.S. Energy f o r F i n a n c i a l support.

Department o f


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References and Notes 1.

(a) Bard, A.J., J. Photochem. 1980, 10, 59; (b) Nozik, A.J., Ann. Rev. Phys. Chem. 1978, 29, 189; (c) Memming, R. in "Electroanalytical Chemistry", A.J. Bard, ed., M. Dekker, N.Y. Vol. II, 1978; (d) Archer, M.D., J. Appl. Electrochem. 1975, 5, 17; (e) Gerischer, H. in "Physical Chemistry - An Advanced Treatise", H. Eyring, D. Henderson, and W. Jost, eds., Academic Press, N.Y., 463 (1970).

2.

Fox, M.A.; Kabir-ud-Din,

3.

Fox, M.A.; Singletary,

4.

For examples, see Tolbert, L.M., J. Am. Chem. Soc. 1978, 100, 3952; Dvorak, V.; Michl, J., ibid. 1976, 98, 1080; and Fox, M.A., Chem. Rev. 1979, 79, 259.

5.

Lochert, P.; Federlin,

6.

For exceptions to this generalization, see Schafer, H.; Azrak, A.A., Chem. Ber. 1972, 105, 2398; Borhani, K.J.; Hawley, M.D., J. Electroanal. Chem., 1979, 101, 407.

7.

Pauson, P.L.; Williams, B.J., J. Chem. Soc., 1961, 4158.

8.

A control experiment established that LiClO has no effect on the homogeneous photolysis of the lithium salt of 2. At conversions of anion greater that ~20%, the efficiency of dimer formation decreases. Whether this can be attributed to further oxidation of the anion derived from dimer or to cathodic cleavage of dimer is under investigation.

9.

Fox, M.A.; Nobs, F.J.; Voynick, T.A., J. Amer. Chem. Soc. 1980, 102, 4029, 4036.

J. Phys. Chem. 1979, 83, 1800. N.J.,

Solar Energy, 1980, in press.

P., Tetrahedron Lett. 1973, 1109.

4

R E C E I V E D October 15, 1980.


Carbanion Photooxidation at Semiconductor Surfaces - American

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