Photoeffects at Semiconductor-Electrolyte Interfaces - American

18 Luminescence and Photoelectrochemistry of Surfactant Metalloporphyrin Assemblies on Solid Supports Downloaded by EAST CAROLINA UNIV on November 3, 2016 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch018

JOHN E . BULKOWSKI, RANDY A . BULL, and STEVEN R. SAUERBRUNN Department of Chemistry, University of Delaware, Newark, D E 19711 Photoelectrochemical cells based on charge separation at semiconductor-liquid junctions offer promise as a practical means for converting solar energy into electricity or storable chemical fuels (1). One approach to achieving such devices involves the sensitization of wide-gap semiconductors by modifi cation of their surfaces with visible-light absorbing dyes. In this well-established spectral sensitization process (2,3,4), the light-excited dye molecules transfer majority carriers into the semiconductor, and the interfacial energetics are advantageously employed to inhibit back reactions. For instance, if the energy levels are suitably matched, dyes bound to n-type semiconductor electrodes inject electrons into the conduction band upon illumination. The electrons are swept away from the surface into the bulk of the semiconductor as a result of the space -charge layer developed beneath the semiconductor surface. In the electrochemical cell, the photooxidized dyes trapped at the surface are reduced by appropriate redox species in the electro lyte. Coupling of the photoelectrode via an external circuit to a counter electrode also immersed in the electrolyte completes the cell. For dyes absorbing in the visible region, wide-gap semi conductors are chosen to maximize electron transfer from the excited dye molecules on the surface to the solid (5,6). However, a major difficulty associated with this approach is the restriction of overall quantum conversion efficiencies due to low light absorption by thin dye layers. Thick dye layers, although they may absorb all of the light, do not result in significantly greater conversion efficiencies, since they suffer from increased quenching probabilities and large resistances. These properties of thick dye films are discouraging with regard to fabricating practical photoelectrochemical cells. Consequently, we are employing a molecular design approach to systematically examine photoinduced charge and energy transfer in highly ordered dye assemblies of a controllable architecture. The aim is to understand the fundamental sensitization processes, both within

0097-6156/81/0146-0279$05.00/0 © 1981 American Chemical Society Nozik; Photoeffects at Semiconductor-Electrolyte Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Downloaded by EAST CAROLINA UNIV on November 3, 2016 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch018

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t h e a s s e m b l i e s and a t t h e i n t e r f a c e s , i n o r d e r t o u l t i m a t e l y realize useful sensitization materials. The o b j e c t i v e o f t h i s m o l e c u l a r d e s i g n a p p r o a c h i s t h e unique three-dimensional a r r a y s c h e m a t i c a l l y represented i n F i g u r e 1. I n t h i s i d e a l i z e d s t r u c t u r e , c o o r d i n a t i o n compounds a r e o r g a n i z e d i n d i s t i n c t monomolecular l a y e r s . Each l a y e r i s c o n n e c t e d t o a d j a c e n t ones by b i f u n c t i o n a l m o l e c u l a r l i n k a g e s c o o r d i n a t e d to the a x i a l s i t e s of the m e t a l c e n t e r s . The dye m o l e c u l e s we a r e u s i n g i n o u r e f f o r t s t o s y n t h e s i z e s u c h a s t r u c t u r e a r e m e t a l l o p o r p h y r i n s , whose c o o r d i n a t i o n c h e m i s t r y and s t r u c t u r a l c h a r a c t e r i s t i c s a p p e a r t o be w e l l s u i t e d t o accomplishing the s y n t h e t i c goals. The i n t e r l a y e r l i n k a g e s t o be u s e d a r e b i f u n c t i o n a l b a s e s s u c h as p y r a z i n e , w h i c h i s shown i n t h e i l l u s t r a t i o n . Their functions are two-fold: to p r o v i d e b i n d i n g s i t e s f o r p o s i t i o n i n g s u c c e s s i v e l a y e r s i n the a r r a y , and t o p r o v i d e an a d j u s t a b l e i n n e r - s p h e r e pathway f o r p h o t o i n d u c e d charge t r a n s f e r between a d j a c e n t l a y e r s . Although i t i s not our i n t e n t to develop the d e t a i l e d r a t i o n a l e behind o u r d e s i g n h e r e , i t s h o u l d be n o t e d t h a t c o n s t r u c t i o n o f s u c h an a r r a y i n a s t e p - w i s e manner w o u l d p r o v i d e a c o n v e n i e n t means f o r t u n i n g t h e o p t i c a l and e l e c t r i c a l p r o p e r t i e s o f t h e f i l m s architecture. For example, f o r m a t i o n of a m u l t i - l a y e r e d assembly w i t h s u c c e s s i v e l a y e r s c o m p r i s e d o f m e t a l l o p o r p h y r i n s w i t h dec r e a s i n g r e d o x p o t e n t i a l s c o u l d r e s u l t i n t h e g e n e r a t i o n o f an asymmetric f i l m . The s y n t h e t i c s t r a t e g y we a r e f o l l o w i n g t o d e v e l o p t h e s e ordered f i l m s i n v o l v e s a combination of monolayer techniques and c o o r d i n a t i o n c h e m i s t r y s u b s t i t u t i o n r e a c t i o n s . T h i s s y n t h e t i c a p p r o a c h i s d e p i c t e d i n F i g u r e 2 f o r a two l a y e r e d s y s t e m i n w h i c h t h e l a y e r s a r e i n t e r c o n n e c t e d by p y r a z i n e . The f i r s t s t e p i s f o r m a t i o n o f a t e m p l a t e l a y e r on a s o l i d s u p p o r t . This i s a c c o m p l i s h e d by t r a n s f e r r i n g an o r d e r e d l a y e r o f s u r f a c t a n t m e t a l l o p o r p h y r i n s f r o m an a i r - w a t e r i n t e r f a c e o n t o a h y d r o p h o b i c a l l y t r e a t e d s u p p o r t s u r f a c e by p a s s i n g t h e s u p p o r t down through the o r i e n t e d f i l m (Step 1 ) . I t i s imperative that t h i s m o n o l a y e r i s homogeneous and t h a t t h e m o l e c u l a r p l a n e s o f e a c h p o r p h y r i n molecule are o r i e n t e d p a r a l l e l to the support s u r f a c e , l e a v i n g the m e t a l c e n t e r s exposed at the h y d r o p h i l i c i n t e r f a c e . The s e c o n d and t h i r d s t e p s i n v o l v e s u c c e s s i v e a d d i t i o n s o f t h e p y r a z i n e and n o n - s u r f a c t a n t w a t e r s o l u b l e m e t a l l o p o r p h y r i n comp l e x e s t o t h e t e m p l a t e submerged i n t h e aqueous p h a s e ( S t e p s 2 and 3, r e s p e c t i v e l y ) . The t w o - l a y e r e d a s s e m b l y i s t h e n removed from the water through a p r o t e c t i v e monolayer of s t e a r i c a c i d to prevent d i s r u p t i o n of the f i l m s t r u c t u r e (Step 4 ) . I f instead o f r e m o v i n g t h e a s s e m b l y i n S t e p 4, S t e p s 2 and 3 w e r e s u c c e s s i v e l y repeated, m u l t i - l a y e r e d assemblies having the e s s e n t i a l f e a t u r e s d e s c r i b e d i n F i g u r e 1 m i g h t be r e a l i z e d . Since achievement of template formation i s c r u c i a l to the s u c c e s s o f t h i s a p p r o a c h , much o f o u r e f f o r t t o d a t e has b e e n d i r e c t e d at e l u c i d a t i n g the monolayer p r o p e r t i e s of v a r i o u s por1

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


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Figure 1. Schematic of the proposed dye assembly where M is coordinated in the plane of a macrocyclic ligand (e.g., a por phyrin) and pyrazine is axially coordi nated between two metals in adjacent layers

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Figure 2. Representation of the dye assembly construction procedure: (O-) stearic acid; (-%-) surfactant metalloporphyrin (MTOAPP); and (-%-) water soluble porphyrin



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p h y r i n compounds. T h i s h a s l e d t o o u r work w i t h s u r f a c t a n t p o r p h y r i n d e r i v a t i v e s o f the type s c h e m a t i c a l l y represented i n F i g u r e 3. The r e s u l t s o f o u r m o n o l a y e r a n d l u m i n e s c e n c e s t u d i e s w i t h t h e s e m o d i f i e d p o r p h y r i n s do i n f a c t i n d i c a t e t h a t t h e y f o r m w e l l - o r d e r e d monolayers a t the a i r - w a t e r i n t e r f a c e w i t h c h a r a c t e r i s t i c s s u i t a b l e f o r template formation. T h i s i s i n agreement with reported results f o r s i m i l a r surfactant porphyrin derivatives Ç7,8). The p u r p o s e o f t h i s a r t i c l e i s t o p r e s e n t t h e r e s u l t s of our l u m i n e s c e n c e and p h o t o e l e c t r i c s t u d i e s w i t h these p o r p h y r i n d e r i v a t i v e s , s i n c e t h e y d e m o n s t r a t e t h a t t h e s e compounds p r o v i d e a good o p p o r t u n i t y f o r e x a m i n i n g e l e c t r o n a n d e n e r g y t r a n s f e r phenomena a t a u n i q u e l y d e f i n e d d y e s e n s i t i z e d s e m i c o n d u c t o r electrolyte interface. The r e s u l t s a r e a l s o d i s c u s s e d w i t h r e g a r d to the molecular design o f the novel p h o t o s e n s i t i v e f i l m s . Experimental P r e p a r a t i o n and P u r i f i c a t i o n o f M a t e r i a l s . The t e t r a o c t a d e c y l a m i d e d e r i v a t i v e ( t T O A P P ) o f m e s o - t e t r a ( a , a , a , a - o - a m i n o p h e n y l ) p o r p h y r i n was p r e p a r e d b y m o d i f i c a t i o n o f t h e " p i c k e t - f e n c e " p o r p h y r i n p r o c e d u r e o f C o l l m a n and c o w o r k e r s (90. P u r i f i c a t i o n was b y c o l u m n c h r o m a t o g r a p h y o n s i l i c a g e l u s i n g g r a d i e n t e l u t i o n w i t h e t h e r - p e t r o l e u m e t h e r s o l v e n t combinations. The c h r o m a t o g r a p h i c p r o c e d u r e was r e p e a t e d u n t i l t h e p o r p h y r i n s were determined t o be g r e a t e r than 99% i s o m e r i c a l l y pure. P u r i t y was c h e c k e d u s i n g h i g h - p e r f o r m a n c e l i q u i d c h r o m a t o graphy by comparing t h e pure product t o a standard m i x t u r e o f t h e four p o s s i b l e atropisomers. Refluxing the free-base surfactant p o r p h y r i n w i t h z i n c a c e t a t e i n dimethylformamide and n i c k e l c h l o r i d e i n c h l o r o f o r m - e t h a n o l gave t h e z i n c a n d n i c k e l m e t a l l o porphyrin derivatives,respectively. O c t a d e c a n e (Eastman) a n d s t e a r i c a c i d ( A l d r i c h ) were r e c r y s t a l l i z e d from acetone and a c e t o n i t r i l e , r e s p e c t i v e l y . A l l o t h e r o r g a n i c s o l v e n t s a n d i n o r g a n i c compounds w e r e r e a g e n t grade o r b e t t e r and used a s r e c e i v e d . T r i p l y d i s t i l l e d water (from an a l l g l a s s system h a v i n g permanganate and s u l f u r i c a c i d s t a g e s ) was u s e d f o r t h e m o n o l a y e r s u b p h a s e s a n d t h e e l e c t r o l y t e solutions. S o l i d s u p p o r t s w e r e e i t h e r 25 χ 75 mm g l a s s m i c r o s c o p e s l i d e s ( F i s h e r b r a n d ) o r 25 χ 75 mm Sb doped S n 0 c o a t e d g l a s s s l i d e s ( P r a c t i c a l P r o d u c t s C o . ) . The S n 0 c o a t i n g was 3000A t h i c k a n d had a r e s i s t a n c e o f 100 Ω/Ο. O p t i c a l t r a n s m i t t a n c e was 80-85% i n t h e v i s i b l e r e g i o n above 400 nm. The g l a s s m i c r o s c o p e s l i d e s w e r e t r e a t e d w i t h h o t o r g a n i c s o l v e n t s , h o t HNOo, d i l u t e NHOH, and t h e n w e r e r i n s e d s e v e r a l t i m e s w i t h t r i p l y d i s t i l l e d w a t e r . They w e r e a i r d r i e d i n a n i n v e r t e d p o s i t i o n . The S n 0 o p t i c a l l y t r a n s p a r e n t e l e c t r o d e s (OTE's) were t r e a t e d w i t h h o t CHCI3 a n d CHOH, e t c h e d w i t h l S O , a n d t h e n washed s e v e r a l t i m e s w i t h t r i p l y d i s t i l l e d water. A conductive s i l v e r coating (Practical ?

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P r o d u c t s Co.) was p a i n t e d a s a c o n t a c t o n one e n d o f t h e OTE, w h i c h was t h e n c u r e d i n a n o v e n a t 160°C. A f t e r c l e a n i n g a n d d r y i n g , t h e g l a s s s l i d e s a n d O T E s were s t o r e d i n s e a l e d i n d i v i d u a l g l a s s c o n t a i n e r s u n t i l use. 1


Monolayer Techniques. The g e n e r a l methods u s e d f o r f o r m i n g and m a n i p u l a t i n g mono l a y e r s a r e d e s c r i b e d b y Kuhn and c o w o r k e r s ( 1 0 ) . Pressure-area c u r v e s w e r e d e t e r m i n e d o n a p a r a f f i n c o a t e d Cenco L a n g m u i r b a l a n c e c a l i b r a t e d b y s u s p e n d i n g w e i g h t s f r o m a n a t t a c h e d s i d e arm o f known l e n g t h . The c a l i b r a t i o n was c h e c k e d p e r i o d i c a l l y b y mea s u r i n g the p r e s s u r e - a r e a curve f o r s t e a r i c a c i d (SA). The f i l m s a t t h e a i r - w a t e r i n t e r f a c e w e r e f o r m e d b y d e l i v e r i n g 100-200 μΐ o f a p p r o x i m a t e l y 10" M s o l u t i o n s o f the r e q u i r e d s u r f a c t a n t m i x t u r e s i n c h l o r o f o r m o n t o t h e aqueous s u r f a c e u s i n g a c a l i b r a t e d 200 y l s y r i n g e . The s u r f a c e - p r e s s u r e a r e a i s o t h e r m s w e r e measured by c o m p r e s s i n g t h e f i l m i n s u c c e s s i v e s t e p s a t one m i n u t e i n t e r vals. The aqueous s u b p h a s e , u n l e s s n o t e d o t h e r w i s e , was b u f f e r e d a t pH 6.5 w i t h 5 χ 10-5 M N a H C 0 a n d c o n t a i n e d 3 χ 1 0 " M C d C l . The p r e c l e a n e d g l a s s s l i d e s f o r t h e s p e c t r o s c o p i c s t u d i e s w e r e p r e c o a t e d o n one s i d e w i t h t h r e e l a y e r s o f cadmium stéarate ( f o r m e d b y a d d i t i o n o f SA t o t h e c a d m i u m - c o n t a i n i n g s u b p h a s e ) by p a s s i n g two s l i d e s p o s i t i o n e d b a c k - t o - b a c k t h r o u g h t h e monol a y e r t h r e e t i m e s a t a r a t e o f 1.0 cm/min. The s u r f a c e p r e s s u r e was h e l d c o n s t a n t a t 30 dyne/cm b y a w e i g h t and p u l l e y t r a n s f e r apparatus w i t h a motor d r i v e n l i f t (11). D e p o s i t i o n o f the s u r f a c t a n t p o r p h y r i n m i x t u r e s was b y p a s s i n g t h e stéarate c o a t e d s l i d e s down t h r o u g h t h e a p p r o p r i a t e f i l m a t a c o n s t a n t p r e s s u r e o f 20 dyne/cm. I f a s i n g l e p o r p h y r i n l a y e r was r e q u i r e d on t h e s u p p o r t , t h e p o r p h y r i n m o n o l a y e r was t h e n removed f r o m t h e s u b p h a s e s u r f a c e and a s t e a r i c a c i d f i l m was f o r m e d t h r o u g h w h i c h t h e s l i d e c o n t a i n i n g t h e p o r p h y r i n m o n o l a y e r was removed. F o r f o r m a t i o n o f two p o r p h y r i n dye l a y e r s f a c e - t o - f a c e o n t h e s u p p o r t , t h e s l i d e was s i m p l y d i p p e d a n d t h e n removed t h r o u g h the p o r p h y r i n f i l m on the subphase. D e p o s i t i o n o f s u r f a c t a n t p o r p h y r i n s o n t h e S n 0 O T E s f o r t h e p h o t o e l e c t r i c measurements was b y v e r t i c a l r e m o v a l o f t h e OTE (two s l i d e s a t a t i m e w i t h the g l a s s s u r f a c e s f a c i n g each o t h e r ) through the p o r p h y r i n m i x t u r e o n t h e aqueous s u b p h a s e . S u r f a c t a n t p o r p h y r i n (MTOAPP): s t e a r i c a c i d : o c t a d e c a n e m o n o l a y e r s i n t h e r a t i o o f 1:4:3 w e r e h e l d a t a c o n s t a n t p r e s s u r e o f 25 dyne/cm f o r t h e s e d e p o s i t i o n s . I n t h i s s u p p o r t - f i l m arrangement, the p o r p h y r i n r i n g s were p o s i t i o n e d next t o the Sn0 s u r f a c e . Coated s l i d e s were s t o r e d i n s e p a r a t e g l a s s c o n t a i n e r s i n the dark b e f o r e use. Deposition r a t i o s w e r e r o u t i n e l y measured and u s e d a s a c r i t e r i o n t o a s s e s s the q u a l i t y o f the c o a t i n g s . 4

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Luminescence Measurements.


C o r r e c t e d l u m i n e s c e n c e s p e c t r a were determined w i t h a P e r k i n E l m e r MPF-44B s p e c t r o p h o t o f l u o r i m e t e r e q u i p p e d w i t h a R-928 p h o t o m u l t i p l i e r and a DCSU-2 m i c r o p r o c e s s o r . C a l i b r a t i o n was w i t h a P e r k i n - E l m e r s t a n d a r d t u n g s t e n lamp u n i t . E x c i t a t i o n and e m i s s i o n s p e c t r a o f m o n o l a y e r s on t h e g l a s s s l i d e s were f r o m t h e f r o n t s i d e w i t h t h e s l i d e p o s i t i o n e d i n a sample h o l d e r w i t h a n o n - f l u o r e s c i n g b l a c k b a c k g r o u n d . The e x c i t i n g l i g h t was a t an a n g l e o f 30° to the normal o f the sample s l i d e w i t h d e t e c t i o n of l u m i n e s c e n c e a t r i g h t a n g l e s t o t h e i n c i d e n t l i g h t t h r o u g h a 565 nm c u t - o f f f i l t e r ( T u r n e r , S h a r p Cut #22). Photoelectrochemical

Measurements.

The e l e c t r o c h e m i c a l c e l l c o n s i s t e d o f t h e p o r p h y r i n t r e a t e d SnO OTE mounted as a window on a m a c h i n e d T e f l o n b l o c k w h i c h c o n t a i n e d a compartment f o r t h e e l e c t r o l y t e s o l u t i o n . The s e m i c o n d u c t o r - p o r p h y r i n f a c e c o n t a c t e d t h e e l e c t r o l y t e w i t h an a r e a o f 1.0 c m . E l e c t r i c a l c o n t a c t t o t h e S n 0 e l e c t r o d e was made v i a a c o n d u c t i v e s i l v e r c o a t i n g on one end. A p l a t i n u m w i r e and a s a t u r a t e d c a l o m e l e l e c t r o d e (SCE) s e r v e d as t h e c o u n t e r and r e f e r e n c e e l e c t r o d e s , r e s p e c t i v e l y . The SCE c o n t a c t e d t h e e l e c t r o l y t e v i a a salt bridge. The c e l l compartment was p r o v i d e d w i t h an i n l e t and o u t l e t f o r i n t r o d u c t i o n o f t h e e l e c t r o l y t e . The s u p p o r t i n g e l e c t r o l y t e was O.1 M KC1 and t h e pH was maintained a t 7.0 u s i n g a p h o s p h a t e b u f f e r . The e l e c t r o l y t e was r i g o r o u s l y deaerated w i t h a N purge. C u r r e n t v e r s u s p o t e n t i a l measurements w e r e made u s i n g c i r c u i t r y s i m i l a r t o t h a t d e s c r i b e d by Honda and coworkers (12). The p o t e n t i o s t a t and s i g n a l programmer w e r e o f c o n v e n t i o n a l d e s i g n and c o n s t r u c t e d by us f r o m c o m m e r c i a l l y a v a i l a b l e components. P h o t o c u r r e n t s were measured by a K e i t h l e y e l e c t r o m e t e r ( M o d e l 610C) and r e c o r d e d on an X-Y r e c o r d e r . The c e l l mounts and a l l e l e c t r i c a l components were c o n n e c t e d t o a common g r o u n d . T y p i c a l c e l l i n t e r n a l r e s i s t a n c e s were 20-30 k f i . The l i g h t s o u r c e f o r c u r r e n t v e r s u s p o t e n t i a l and c u r r e n t v e r s u s t i m e measurements was a 300-watt ELH lamp ( G e n e r a l E l e c t r i c ) o p e r a t e d a t an i n t e g r a t e d i r r a d i a n c e o f c a . 75 mw/cm above 350 nm. A 350 nm c u t - o f f f i l t e r was p o s i t i o n e d between t h e e l e c t r o c h e m i c a l c e l l and t h e l i g h t s o u r c e . The p h o t o c u r r e n t b a c k g r o u n d s o f S n 0 e l e c t r o d e s h a v i n g o n l y a cadmium stéarate l a y e r w e r e 10" l e s s t h a n t h o s e h a v i n g t h e p o r p h y r i n dye m o n o l a y e r s . D e t e r m i n a t i o n o f a c t i o n s p e c t r a was a c c o m p l i s h e d by p l a c i n g t h e e l e c t r o c h e m i c a l c e l l i n t h e sample chamber o f t h e P e r k i n - E l m e r MPF-44B s p e c t r o m e t e r and u s i n g t h e 1 5 0 - w a t t Xe l i g h t s o u r c e and e x c i t a t i o n o p t i c s f o r i r r a d i a t i n g the e l e c t r o d e . 2

2

2

R e s u l t s and

Discussion

Monolayer P r o p e r t i e s . P r e v i o u s s t u d i e s o f dye s e n s i t i z a t i o n w i t h p o r p h y r i n m a t e r i a l s



18.

BULKOWSKi E T A L .


285

( 1 3 - 1 9 ) have u s u a l l y i n v o l v e d f o r m a t i o n o f s o l i d f i l m s by t e c h niques such as s o l u t i o n e v a p o r a t i o n o r vapor d e p o s i t i o n . L i t t l e i s known a b o u t t h e s t r u c t u r e s o f s u c h f i l m s . A s a r e s u l t , we have b e e n s t u d y i n g methods f o r g e n e r a t i n g w e l l - d e f i n e d p o r p h y r i n assemb l i e s u s i n g m o n o l a y e r methods a t t h e a i r - w a t e r i n t e r f a c e ( 1 0 ) . Our a t t e m p t s t o g e n e r a t e m o n o l a y e r s o f m e t a l l o t e t r a p h e n y l p o r p h y r i n s b y s p r e a d i n g s o l u t i o n s o f them o n a n aqueous s u b p h a s e d i d not r e s u l t i n the g e n e r a t i o n o f monomolecular l a y e r s . As expected, a h i g h d e g r e e o f a g g r e g a t i o n and m i c r o c r y s t a l l i n i t y was r e a d i l y apparent. Compression o f these f i l m s r e s u l t s i n curves a t y p i c a l of monolayers w i t h e x t r a p o l a t e d area/molecule values l e s s than 15 A / m o l e c u l e a t o b s e r v a b l e s u r f a c e p r e s s u r e s . Additionof s u r f a c t a n t compounds known t o enhance m o n o l a y e r - f o r m i n g propert i e s (e.g., s t e a r i c a c i d ) i n the spreading process d i d not a l l e v i a t e the aggregation problems. Codeposition o f mixtures of v a r i o u s s u r f a c t a n t p y r i d i n e bases (e.g., an octadecylamide d e r i v a t i v e o f 4 - a m i n o p y r i d i n e ) and m e t a l l o t e t r a p h e n y l p o r p h y r i n s r e s u l t e d i n no v i s i b l e a g g r e g a t i o n . Pressure-area isotherms c h a r a c t e r i s t i c o f monomolecular l a y e r s were o b s e r v e d ; however, the p o r p h y r i n area/molecule v a l u e s ranged from 8 0 - 1 2 0 Â / m o l e c u l e d e p e n d i n g on t h e n a t u r e o f t h e m e t a l l o t e t r a p h e n y l p o r p h y r i n . T h i s suggests that the porphyrins are e i t h e r aggregated, o r i e n t e d p e r p e n d i c u l a r l y t o t h e aqueous s u r f a c e , o r t u c k e d up i n s i d e t h e surfactant layer. Although these systems are u n s u i t a b l e f o r t e m p l a t e f o r m a t i o n i n o u r a s s e m b l y p r o c e d u r e , t h e y may b e u s e f u l f o r examining dye s e n s i t i z a t i o n o r i e n t a t i o n a l e f f e c t s a t e l e c t r o d e surfaces. To a c h i e v e p o r p h y r i n m o n o l a y e r s s u i t a b l e f o r use i n o u r p r o p o s e d a s s e m b l y scheme, we examined t h e p r o p e r t i e s o f s u r f a c t a n t p o r p h y r i n s o f t h e g e n e r a l t y p e shown i n F i g u r e 3 . A t y p i c a l s u r f a c e p r e s s u r e - a r e a i s o t h e r m f o r I T O A P P o n a pH 6 . 5 b u f f e r e d NaHCO aqueous s u b p h a s e i s g i v e n i n F i g u r e 4 . T h i s c u r v e i s c h a r a c t e r i s t i c o f f o r m a t i o n o f monomolecular l a y e r s a t the a i r - w a t e r i n t e r f a c e (20). E x t r a p o l a t i o n o f the condensed phase r e g i o n to zero pressure gives a value o f ca. 160 Â / m o l e c u l e f o r the p o r p h y r i n , w h i c h i s i n agreement w i t h p r e v i o u s o b s e r v a t i o n s f o r t h i s t y p e o f compound ( 8 ) . S i m i l a r v a l u e s a r e f o u n d f o r b o t h t h e Zn and NiTOAPP d e r i v a t i v e s . This area/molecule value i s c o n s i s t e n t w i t h that expected f o r the p o r p h y r i n r i n g o r i e n t e d n e a r l y p a r a l l e l t o t h e aqueous s u r f a c e ( v a l u e s o f c a . 1 6 5 Â / molecule are c a l c u l a t e d from c r y s t a l s t r u c t u r e s o f t e t r a p h e n y l porphyrin). The d e c r e a s e i n a r e a t o c a . 1 3 0 A / m o l e c u l e upon c o m p r e s s i o n t o 2 0 dyne/cm p r o b a b l y r e s u l t s f r o m t i l t i n g o f t h e p o r p h y r i n heads a s t h e l o o s e l y p a c k e d c h a i n s ( t o t a l a r e a o f c a . 80 A / m o l e c u l e ) a r e s q u e e z e d i n t o a t i g h t e r a r r a n g e m e n t . This c o u l d r e s u l t i n some o v e r l a p o f t h e p o r p h y r i n π o r b i t a l s y s t e m . The p o r p h y r i n o r i e n t a t i o n i n d i c a t e d b y t h i s r e s u l t i s e n c o u r a g i n g w i t h r e g a r d t o u s i n g t h e s e compounds f o r t e m p l a t e f o r m a t i o n t o b u i l d up t h e p r o p o s e d p o r p h y r i n - c o n t a i n i n g a r r a y s d e s c r i b e d earlier.



286


INTERFACES

Figure 3. Schematic of a surfactant metalloporphyrin, MTOAPP

Figure 4. Surface pressure-area isotherm of H TOAPP (aqueous phase, 5 X 10~ M NaHCO ; pH 6.5; air atmosphere at 25°C) 5

2

s


18.

BULKOWSKi E T A L .


287

M i x e d m o n o l a y e r p r o p e r t i e s o f t h e MTOAPP d e r i v a t i v e s w i t h v a r i o u s c o m b i n a t i o n s o f SA and o c t a d e c a n e w e r e d e t e r m i n e d t o i n v e s t i g a t e ways f o r c o n t r o l l i n g t h e s u r f a c e c o n c e n t r a t i o n u s i n g photoinert spacers. The o b j e c t i v e was t o c h a r a c t e r i z e t h e i r s t a b i l i t y and h o m o g e n e i t y upon d i l u t i o n . Surface pressure-area i s o t h e r m s f o r d i f f e r i n g m i x i n g r a t i o s o f ZnTOAPP a n d SA a r e g i v e n i n F i g u r e 5. Two f e a t u r e s o f t h i s f a m i l y o f c u r v e s i n d i c a t e h o m o g e n e i t y o f m i x i n g ( 2 0 ) . They a r e : 1) i n c r e a s i n g breakdown p r e s s u r e a s a f u n c t i o n o f m i x i n g r a t i o , a n d 2) t h e c a l c u l a t e d curve, F i g u r e 5 c , which i s based on a d d i t i v i t y o f t h e areas o f p u r e ZnTOAPP a n d SA ( a 1:4, ZnTOAPP:SA m i x t u r e ) does n o t a g r e e w i t h t h e e x p e r i m e n t a l l y d e t e r m i n e d c u r v e , F i g u r e 5 c . Re s u l t s o b t a i n e d f o r a ZnTOAPP-octadecane m i x t u r e i n d i c a t e d t h a t a t l o w m i x i n g r a t i o s ( l e s s t h a n 1:10, ZnTOAPP:octadecane) l a y e r s a r e s t a b i l i z e d r e l a t i v e t o p u r e ZnTOAPP. Maximum f i l m s t a b i l i t y was o b s e r v e d f o r t h e 1:3 m i x t u r e o f ZnTOAPP:octadecane. I t a p p e a r s t h a t t h e o c t a d e c a n e m o l e c u l e s c a n b e accommodated b y v a c a n c i e s i n t h e l o o s e l y packed hydrophobic r e g i o n . Studies o f mixtures o f p o r p h y r i n - s t e a r i c acid-octadecane i n d i c a t e d that a 1:4:3 r a t i o o f ZnTOAPP:SA:octadecane g i v e s good, s t a b l e m o n o l a y e r s so t h i s c o m b i n a t i o n was c h o s e n f o r t h e p h o t o e l e c t r o c h e m i c a l measurements. U s i n g e s t a b l i s h e d methods ( 1 0 ) , t h e ZnTOAPP m o n o l a y e r s w e r e r e a d i l y t r a n s f e r r e d t o s o l i d supports, such as s u i t a b l y t r e a t e d g l a s s s l i d e s o r SnO o p t i c a l l y t r a n s p a r e n t e l e c t r o d e s .


1

Luminescence S t u d i e s . L u m i n e s c e n c e s p e c t r o s c o p y was u s e d a s a s e n s i t i v e t e c h n i q u e t o c h a r a c t e r i z e t h e ZnTOAPP l a y e r s a t t h e a i r - s u p p o r t i n t e r f a c e . F i g u r e 6 compares t h e e m i s s i o n ( λ > 435 nm) a n d e x c i t a t i o n (X 660 nm) s p e c t r a o f a m o n o l a y e r o f ZnTOAPP a n d SA ( 1 : 4 ) on a h y d r o p h o b i c g l a s s support (arrangement o f l a y e r s i s glass/SA/ ZnTOAPP/SA) w i t h t h o s e o f a Ι Ο " M ZnTOAPP c h l o r o f o r m s o l u t i o n . T h e r e i s a s l i g h t b a t h o c h r o m i c s h i f t ( c a . 10 nm) o f t h e m o n o l a y e r p e a k s a n d a b r o a d e n i n g o f t h e S o r e t band w h i c h q u a l i t a t i v e l y agrees w i t h r e s u l t s reported f o r vapor deposited s o l i d f i l m s o f ZnTPP on q u a r t z ( 2 1 , 2 2 ) . These s p e c t r a show t h a t o u r compounds a r e q u i t e p u r e ( e . g . , no a d d i t i o n a l p e a k s a t 630 a n d 690 nm due t o c h l o r i n i m p u r i t i e s (22)) and t h a t they a r e n o t aggregated. Even 10 M p o r p h y r i n s o l u t i o n e x c i t a t i o n s p e c t r a o f t h e s e com pounds show s e v e r a l a d d i t i o n a l bands i n t h e S o r e t r e g i o n due t o i n t e r m o l e c u l a r i n t e r a c t i o n s . A l s o , t h e b r o a d e n i n g and peak s h i f t s a r e r a t h e r i n s e n s i t i v e t o d i l u t i o n o f t h e m o n o l a y e r s w i t h SA, a l t h o u g h f l u o r e s c e n c e i n t e n s i t i e s do s i g n i f i c a n t l y d e c r e a s e . F i g u r e 7 shows t h e s e l f - q u e n c h i n g o f t h e 660 nm f l u o r e s c e n c e e m i s s i o n ( n o r m a l i z e d t o r e l a t i v e i n t e n s i t y p e r chromophore u s i n g c o n c e n t r a t i o n v a l u e s b a s e d o n o u r aqueous s u b p h a s e m o n o l a y e r d a t a ) as a f u n c t i o n o f d i l u t i o n o f t h e ZnTOAPP b y SA. A s h a r p d e c r e a s e i n t h e q u e n c h i n g o c c u r s a t ZnTOAPP:SA r a t i o s o f 1:4, a n d a c o n βχ

e m >

6



288


A R E A PER M O L E C U L E

INTERFACES

( A )

Figure 5. Surface pressure-area isotherms for ZnTOAPP.SA mixing ratios— (a) pure ZnTOAPP; (b) 1:2; (c) 1:4; (c ) 1:4 calculated; (d) 1:10; (e) 1:500; and (f) pure SA. Aqueous phase contains 5 X 10' M NaHCO buffer, pH 6.5, and 3 X 10~ M CdCl . f

5

s

4

2

100 >-

400

450

500

550

WAVELENGTH

600

650

700

(nm)

6

Figure 6. Excitation and emission spectra of a 10~ M solution of ZnTOAPP in chloroform ( ) and a monolayer of ZnTOAPP on a hydrophobically treated, Cd-SA glass support ( ). Relative intensities are arbitrarily adjusted.



18.

BULKOWSKI ET A L .


289

s t a n t l i m i t i n g v a l u e i s r e a c h e d a t a r a t i o o f c a . 1:10. This i s c o n s i s t e n t w i t h t h e f o r m u l a t i o n o f a homogeneous, t i g h t l y p a c k e d monolayer on the support s u r f a c e . A heterogeneous s u r f a c e o f i s l a n d s o f p o r p h y r i n and s p a c e r s w o u l d b e e x p e c t e d t o show l i t t l e e f f e c t upon SA d i l u t i o n . A l s o , t h i s luminescence data i n d i c a t e s t h a t energy t r a n s f e r a s s o c i a t e d w i t h s e l f - q u e n c h i n g takes p l a c e o v e r v e r y s h o r t ranges ( i . e . l e s s than 1 Â s p a c i n g s between p o r p h y r i n r i n g p e r i p h e r i e s ) . T h i s may e v e n r e q u i r e c o n t a c t o f o r b i t a l s on a d j a c e n t c h r o m o p h o r e s a s m i g h t b e e x p e c t e d f o r p o r p h y r i n s a r r a n g e d i n a t i l t e d c o n f i g u r a t i o n o n t h e s u r f a c e . By s l i g h t s e p a r a t i o n u s i n g SA, t h e q u e n c h i n g e f f e c t p e r m o l e c u l e i s d e c r e a s e d b y a f a c t o r o f 3. I n c l u s i o n o f v a r i o u s r a t i o s o f n o n - f l u o r e s c i n g NiTOAPP t r a p s i n t o t h e ZnTOAPP m o n o l a y e r (MT0APP:SA, 1:4) g i v e s t h e dependency i n d i c a t e d i n F i g u r e 8. T h i s r e s u l t i n d i c a t e s s i g n i f i c a n t e n e r g y t r a n s f e r f r o m t h e Zn c e n t e r s t o t h e t r a p s , even a t c o n c e n t r a t i o n s o f one t r a p p e r one h u n d r e d d y e c e n t e r s . The c u r v e shape a g r e e s w e l l w i t h one c a l c u l a t e d f o r a d i p o l a r - d i p o l a r c o u p l i n g mechanism (23) b e t w e e n p o r p h y r i n s . P r e l i m i n a r y evidence w i t h a two-layer s y s t e m i n w h i c h t h e ZnTOAPP m o n o l a y e r (ZnTOAPP:SA, 1:4) c o n c e n t r a t i o n was h e l d c o n s t a n t , b u t w h i c h was i n f a c e - t o - f a c e c o n t a c t w i t h a s e c o n d NiTOAPP-SA m o n o l a y e r o f v a r y i n g N i c o n c e n t r a t i o n , i n d i c a t e s a s i m i l a r i n t e n s i t y dependence o n m i x i n g r a t i o a s was f o u n d f o r t h e s i n g l e l a y e r s y s t e m . T h i s r e s u l t s u g g e s t s t h a t i n t e r l a y e r q u e n c h i n g a l s o o c c u r s v i a a d i p o l a r - d i p o l a r mechanism. However, t h e c r i t i c a l d i s t a n c e f o r q u e n c h i n g ( I / I o = O.5) was f o u n d t o b e l e s s t h a n t h a t measured f o r t h e s i n g l e l a y e r a s s e m b l y . T h i s i n d i c a t e s t h a t t h e r e i s l e s s e f f e c t i v e c o u p l i n g between w e a k l y i n t e r a c t i n g chromophores i n a d j a c e n t l a y e r s t h a n b e t w e e n m o l e c u l e s t i g h t l y p a c k e d i n i n d i v i d u a l l a y e r s . We a r e c u r r e n t l y studying these e f f e c t s i n greater d e t a i l . Photoelectrochemical

Studies.

Our i n i t i a l p h o t o e l e c t r o c h e m i c a l s t u d i e s have b e e n c o n d u c t e d w i t h m o n o l a y e r s o f ZnTOAPP:SA:octadecane m i x t u r e s i n t h e r a t i o o f 1:4:3. They a r e d e p o s i t e d d i r e c t l y on t h e SnO OTE's w i t h t h e s u r f a c t a n t p o r p h y r i n head g r o u p s i n c o n t a c t w i t h t h e e l e c t r o d e surface. The e l e c t r o l y t e c o n t a i n e d O.1 M KC1 and was m a i n t a i n e d a t pH 7.0 w i t h a p h o s p h a t e b u f f e r . The e l e c t r o l y t e was d e o x y g e n a t e d b y a N2 gas p u r g e . An a n o d i c p h o t o c u r r e n t was g e n e r a t e d under s h o r t - c i r c u i t c o n d i t i o n s which i n c r e a s e d w i t h a p p l i e d potential. The a n o d i c p h o t o c u r r e n t i s c o n s i s t e n t w i t h e l e c t r o n i n j e c t i o n t o w a r d t h e Sn02« A d d i t i o n a l l y , a n open c i r c u i t v o l t a g e was m e a s u r e d upon i r r a d i a t i o n b y t h e ELH lamp s o u r c e . The o b s e r v e d p h o t o v o l t a g e ( c a . 10 mV) was n e g a t i v e a n d c o n s i s t e n t w i t h the g e n e r a t i o n o f an anodic photocurrent. N e g l i g i b l e photoe f f e c t s were o b s e r v e d f o r a Sn02 OTE c o a t e d w i t h j u s t a cadmium stéarate l a y e r u n d e r i d e n t i c a l c o n d i t i o n s . T h i s b e h a v i o r i s c o n s i s t e n t w i t h a n e n e r g y l e v e l scheme f o r


290

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES


15

5 1 α PURE ZnTOAPP MIXING

ι

ι

1

10

1—

100

RATIO (SA/ZnTOAPP)

Figure 7. Dependence offluorescenceintensity (\ , 660 nm) of ZnTOAPP on SA.ZnTOAPP mixing ratios. Mixtures are deposited on a hydrophobically treated, Cd-SA glass slide and have a Cd-SA outer layer as represented in the insert. em

ι

MIXING

RATIO

(ΖηΤΟΑΡΡ/ΝιTOAPP)

Figure 8. Dependence offluorescencequenching on ZnTOAPP.NiTOAPP mixing ratios (MTOAPPiSA, 1:4) in a monolayer on Cd-SA-treated glass with a Cd-SA outer layer as represented in the insert



18.

BULKOWSKI ET A L .


291

e l e c t r o n t r a n s f e r i n w h i c h the e l e c t r o n donor i s s i t u a t e d a t a more n e g a t i v e p o t e n t i a l t h a n t h e c o n d u c t i o n band edge o f t h e semiconductor. The f l a t b a n d p o t e n t i a l f o r SnC a t pH 7.0 i s -O.45 V v s . SCE ( 2 4 ) . The e n e r g y l e v e l o f t h e e x c i t e d d o n o r i s approximated from i t s o x i d a t i o n redox p o t e n t i a l and e x c i t a t i o n e n e r g y ( 4 ) . The o x i d a t i o n p o t e n t i a l f o r ZnTOAPP i s t a k e n a s +O.70 V, b a s e d o n r e p o r t e d v a l u e s f o r s u b s t i t u t e d ZnTPP's ( 2 5 ) ; t h e s i n g l e t e x c i t a t i o n e n e r g y i s 1.88 eV, b a s e d o n t h e 660 nm emission. This r e s u l t s i n a c a l c u l a t e d o x i d a t i o n redox p o t e n t i a l f o r t h e e x c i t e d s i n g l e t o f -1.18 V v s . SCE. The e x c i t e d t r i p l e t p o t e n t i a l w o u l d b e e x p e c t e d t o b e no g r e a t e r t h a n O.4-O.6 V l e s s t h a n t h e s i n g l e t , b a s e d o n known Zn p o r p h y r i n t r i p l e t e n e r g i e s (26). Since these excited state l e v e l s are e n e r g e t i c a l l y higher t h a n t h e SnO c o n d u c t i o n b a n d , e l e c t r o n t r a n s f e r t o t h e s e m i c o n d u c t o r s h o u l d b e p o s s i b l e v i a e i t h e r pathway t o p r o d u c e a n o d i c photocurrents w i t h these m e t a l l o p o r p h y r i n s e n s i t i z e r s . A t y p i c a l time response f o r a s h o r t - c i r c u i t e d photocurrent i n t h e p r e s e n c e o f h y d r o q u i n o n e ( H Q ) a s a n added s o l u t i o n r e d o x s p e c i e s i s shown i n F i g u r e 9. These p h o t o c u r r e n t s w e r e s t a b l e f o r s e v e r a l hours. I n the absence o f H Q i n the e l e c t r o l y t e , t h e p h o t o c u r r e n t a l s o i n c r e a s e d r a p i d l y upon t h e o n s e t o f i l l u m i n a t i o n , b u t s u b s e q u e n t l y d e c a y e d e x p o n e n t i a l l y t o 70% o f i t s i n i t i a l v a l u e i n a h a l f - d e c a y t i m e o f c a . 25 s. T h i s b e h a v i o r i s s i m i l a r t o that observed f o r c h l o r o p h y l l monolayers d e p o s i t e d on Sn02 ( 1 2 ) . P h o t o c u r r e n t s u n d e r p o t e n t i a l l y - c o n t r o l l e d c o n d i t i o n s were a l s o s t a b l e upon i l l u m i n a t i o n , b u t e x h i b i t e d s l o w e r decay c h a r a c t e r i s t i c s when t h e l i g h t was t u r n e d o f f . T h i s e f f e c t i s unusual and i s c u r r e n t l y under f u r t h e r i n v e s t i g a t i o n . A t y p i c a l photocurrent a c t i o n spectrum i s i l l u s t r a t e d i n F i g u r e 10 t o g e t h e r w i t h t h e e x c i t a t i o n s p e c t r u m o f t h e ZnTOAPP monolayer. The good c o r r e s p o n d e n c e b e t w e e n t h e two c u r v e s i n d i c a t e s t h a t t h e dye i s , i n f a c t , t h e s o u r c e o f t h e p h o t o c u r r e n t s observed. We a r e c u r r e n t l y e x p l o i t i n g t h e u n i q u e a b i l i t y t o c o n t r o l the a r c h i t e c t u r e i n these monolayers t o f u r t h e r i n v e s t i g a t e t h e i r p h o t o e l e c t r o c h e m i c a l p r o p e r t i e s w i t h r e s p e c t t o such f a c t o r s a s l i g h t i n t e n s i t y , s o l u t i o n and f i l m r e d o x components, a s s e m b l y s t r u c t u r e , dye o r i e n t a t i o n , e t c . We a r e p a r t i c u l a r l y i n t e r e s t e d i n u s i n g t h e m o n o l a y e r s t r u c t u r a l i n f o r m a t i o n we now have t o c o r r e l a t e f i l m e l e c t r o n i c p r o p e r t i e s w i t h c h a r g e t r a n s f e r e f f e c t s a t both the d y e - s o l i d and d y e - l i q u i d i n t e r f a c e s . Acknowledgement T h i s w o r k was s u p p o r t e d b y t h e U n i v e r s i t y o f D e l a w a r e R e s e a r c h F o u n d a t i o n and b y t h e U.S. D e p a r t m e n t o f E n e r g y ( G r a n t No. DE-FG02-79ER-10533).


292

PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES


eg

1

,

I

ι

I

Ο

1

2

3

4

TIME

.

I

I

5

6

(sec)

Figure 9. Short-circuited photocurrent vs. time for a monolayer of ZnTOAPP (ZnTOAPP.SA, 1:4) directly on a Sn0 OTE (O.1M KCl, pH 7.0,O.05MH Q, N purged) 2

2

2

ι ~~

Γ

WAVELENGTH

(nrrO

Figure 10. Action spectrum of a monolayer of ZnTOAPP (ZnTOAPP.SA :octadecane, 1:4:3) directly on a Sn0 OTE: O.1M KCl, pH 7.0, electrode potential is +O.3 V vs. SCE, Ν purged; ( ) the excitation spectrum (\ m, 660 nm) of ZnTOAPP monolayer at SnO -electrolyte interface 2

2

e

2


18.

BULKOWSKI


Literature

ET AL.


293

Cited

1. Wrighton, M.S. Acc. Chem. Res., 1979,9,303. 2. Gerischer, H. in "Physical Chemistry: An Advanced Treatise," Eyring, H.; Henderson, D.; Jost, W., Eds.; Academic Press, New York, 1970; Vol. 9A, Chapter 5. 3. Memming, R. in "Electroanalytical Chemistry," A.J. Bard, Ed.; Marcel Dekker, New York, Vol. 11; 1978. 4. Gerischer, H. Topics in Current Chemistry, 1976, 61, 31. 5. Memming, R. Photochem. Photobiol., 1972, 16, 325. 6. Spitler, M.T.; Calvin, M. J. Chem. Phys., 1977, 66, 4294. 7. Whitten, D.G.; Eaker, D.W.; Horsey, B.E.; Schmehl, R.H.; Worsham, P.R. Ber. Bunsenges. Phys. Chem., 1978, 82, 858. 8. Mercer-Smith, J.A.; Whitten, D.G. J. Am. Chem.Soc.,1979, 101, 6620. 9. Collman, J.P.; Gagne, R.R.; Reed, C.A.; Halbert, T.R.; Lang, G.; Robinson, W.T. J. Am. Chem. Soc., 1975, 97, 1427. 10. Kuhn, H.; Mobius, D.; Bucher, H. "Physical Methods of Chemistry," Vol. 1; Part 3B; Weissburger, Α.; Rossiter, B., Eds.; Wiley, New York, 1972; p. 588. 11. Bucher, H.; Eisner, O.; Mobius, D.; Tillmann, P.; Wiegand, J. Z. Phys. Chem., 1969, 65, 152. 12. Miyasaka, T.; Watanabe, T.; Fujishima, Α.; Honda, K. J. Am. Chem. Soc., 1978, 100, 6657. 13. Meier, H. Topics in Current Chemistry, 1976, 61, 85. 14. Wang, J.H. P r o c . Natl. Acad. Sci., U.S.A., 1969, 62, 653. 15. Adler, A.D. J. Polymer Sci. Part C, 1970, 29, 73. 16. Adler, A.D.; Varadi, V.; Wilson, N. Ann. N.Y. Acad. Sci., 1975, 244, 685. 17. Umezawa, Y.; Yamamura, T. J. Chem. Soc. Chem. Comm., 1978, 1106. 18. Tang, C.W. U.S. Patent No. 4,164,431, August, 1979. 19. Umezawa, Y.; Yamamura, T. J. Electroanal. Chem., 1979, 95, 113. 20. Gaines, G.L., Jr. "Insoluble Monolayers at Liquid-Gas Interfaces"; Wiley Interscience, New York, 1966. 21. Tanimura, K.; Kawai, T.; Sakata, T. J. Phys. Chem., 1979, 83, 2639. 22. Tanimura, K.; Kawai, T.; Sakata, T. J. Phys. Chem., 1980, 84, 751. 23. Kuhn, H. J. Photochem., 1979, 10, 111. 24. Mollers, F.; Memming, R. Ber. Bunsenges. Phys. Chem., 1975, 76, 469. 25. Wolberg, A. Isr. J. Chem., 1975, 12, 1031. 26. Hopf, F.R.; Whitten, D.G. in "Porphyrins and Metalloporphyrins," Smith, K.M., Ed.; Elsevier, New York, 1975; Chapter 16. RECEIVED October 3, 1980.


Photoeffects at Semiconductor-Electrolyte Interfaces - American

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