Photoelectrochemistry and Spectroscopy of Metal Phthalocyanine

8 Photoelectrochemistry and Spectroscopy of Metal Phthalocyanine Films on a Transparent

Downloaded by CORNELL UNIV on September 2, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch008

Semiconducting Electrode COOPER H . LANGFORD, BRYAN R. HOLLEBONE, and THEODORE VANDERNOOT Metal Ions Group, Chemistry Department, Carleton University, Ottawa, Canada K1S 5B6

The photoelectrochemistry of Mg, Fe, Ni, and Zn phthalo cyanine films on transparent n-type tin oxide electrodes is reported. Photocurrents of a type expected from a p-type semiconductingfilmare observed in all cases. These currents are small; slow electron transfer is indicated. There is some evidence of the retardation of the "dark couple" reaction without inhibition of the "photocouple" at the photoelectrode. The most effectivefilmis one containing the β-crystal modification of zinc phthalocyanine. This result is clarified by examination of magnetic circular dichroism spectra of metal phthalocyanine films. The broad line spectra reveal the solid state character of the films. Requirements for effi cient photoelectrochemistry in terms of electronic states are discussed.

Albery a n d A r c h e r have analyzed the i d e a l diffusion-controlled photogalvanic cell a n d indicated that i t c o u l d

achieve

c o m p a r a b l e w i t h t h a t o f g o o d p h o t o v o l t a i c devices (1,2).

a n efficiency I n i t there are

t w o r e d o x c o u p l e s , A/Β a n d Y / Z , w h e r e t h e e x c i t e d state o f A ( A * ) i s t h e p h o t o r e a c t i v e species i n t h e c e l l .

T h e reactions i n t h e c e l l c a n b e

s u m m a r i z e d as f o l l o w s . hp

A

>A*

(1)

A* + Ζ - » Β + Y o c c u r r i n g n e a r i l l u m i n a t e d electrode 0-8412-0474-8/80/33-184-139$05.00/0 © 1980 American Chemical Society Wrighton; Interfacial Photoprocesses: Energy Conversion and Synthesis Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

(2)

140

INTERFACIAL

(B +

PHOTOPROCESSES

y —> A + Z) energy wasting

(3)

B ± c" —» A occurring at illuminated electrode

(4)

Y

(5)

Z ± e~ occuring at dark electrode

F o r the o p e r a t i o n

of

a n efficient

cell, conditions

i l l u m i n a t e d electrode that c a n m e d i a t e the A/B

should include

an

c o u p l e b u t that is u n r e -

a c t i v e to the Y / Z c o u p l e p a i r e d w i t h a d a r k electrode t h a t c a n m e d i a t e the Y / Z c o u p l e .


inefficient. the

A d d i t i o n a l l y , the b a c k r e a c t i o n ( R e a c t i o n 3) m u s t b e

A n a l y s i s of the k i n e t i c s of h o m o g e n e o u s systems a i d e d

M a r c u s - W e l l e r theory

(3,4)

of

electron

transfer r e a c t i o n

suggests that these c r i t e r i a are v e r y h a r d to satisfy. W e m u s t look " n o n - M a r c u s i a n " e l e c t r o n transfer.

i n a l i m i t e d zone near

the

T o g e t h e r , a l l of these features suggest a h e t e r o

geneous system w i t h the A/B the i l l u m i n a t e d electrode. c o n d u c t o r cells (5)

for

A n a d d i t i o n a l feature is t h a t i t is

d e s i r a b l e to h a v e l i g h t a b s o r p t i o n o c c u r i l l u m i n a t e d electrode.

by

rates

c o u p l e i n c o r p o r a t e d i n t o a surface l a y e r of

S u c h a c e l l approaches the regenerative s e m i

b a s e d o n C d S or G a P a n d d i s c u s s e d elsewhere i n

this v o l u m e , b u t the use of m o l e c u l a r layers as absorbers has d i s t i n c t i v e features that l e a d us to p r e f e r m o d e l l i n g o u r discussions o n t h e p a r i s o n to h o m o g e n e o u s cells.

A third model, a

com

conductor-insulator-

c o n d u c t o r s a n d w i c h has r e c e n t l y b e e n c o n s i d e r e d b y L y o n ' s g r o u p

(6).

It turns out that there are r e l a t e d r e q u i r e m e n t s t h a t c a n be specified f o r a system that w i l l c a r r y out efficient p h o t o c h e m i c a l synthesis. E f f i c i e n t synthesis r e q u i r e s c o n c e n t r a t i o n of l i g h t a b s o r p t i o n i n t o a n a r r o w z o n e to a v o i d costly catalyst recovery. bility.

It also r e q u i r e s means of f a v o r i n g r e v e r s i

M o r e o v e r , e l e c t r o c h e m i c a l l y m e d i a t e d syntheses m a y p e r m i t a

l i g h t r e a c t i o n o n a w i d e area c o l l e c t o r to b e c o u p l e d to a m o r e

compact

a n d m a n a g e a b l e d a r k electrode r e a c t i o n w h e r e the d e s i r e d p r o d u c t is to be collected.

(It's a t t r a c t i v e to t h i n k that the m a i n l i n k to a l a r g e solar

c o l l e c t o r field c o u l d b e e l e c t r o l y t e a n d w i r e s o n l y ) . A g a i n , a n a t t r a c t i v e response

to these r e q u i r e m e n t s is to a t t e m p t to b u i l d

photosynthetic

e l e c t r o c h e m i c a l cells b a s e d o n heterogeneous systems. I t is clear i n the p h o t o g a l v a n i c c e l l case a n d p r o b a b l e i n the synthesis case, t h a t the r e q u i r e m e n t s specified l e a d to the i n v e s t i g a t i o n of w a y s of p r o d u c i n g a sort of " r e c t i f i c a t i o n ' at a p h o t o e l e c t r o d e .

T h e subject of this

c h a p t e r is o u r p r e l i m i n a r y w o r k o n s u c h r e c t i f i c a t i o n a n d its r e l a t i o n s h i p t o t h e e l e c t r o n i c p r o p e r t i e s of " s e m i s o l i d " m a t e r i a l s as r e v e a l e d b y t h e i r electronic spectra under magnetic circular dichroism ( M C D ) investiga tion.

R e l a t i o n s of r e a c t i v i t y to spectroscopic

information on

electronic

s t r u c t u r e is a l w a y s i m p o r t a n t to effective p h o t o c h e m i c a l a d v a n c e . experiments have employed metal phthalocyanine ( P c )

dyes.

Wrighton; Interfacial Photoprocesses: Energy Conversion and Synthesis Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

These

8.

LANGFORD E T AL.

Metal

Phthaloctjanine

141

Films

T h e c h o i c e of Pc's w a s m a d e for several reasons. Pc's h a v e a t t r a c t e d a t t e n t i o n as o r g a n i c s e m i c o n d u c t o r s f r o m a n e a r l y d a t e (7,8)

and the

r i c h l i t e r a t u r e c o n c e r n i n g m e c h a n i s m of c h a r g e transport i n these films invites a t t e n t i o n . P h o t o v o l t a i c cells b a s e d o n Pc's h a v e b e e n e x p l o r e d f o r some t i m e a n d several m e c h a n i s m s for p h o t o v o l t a i c a c t i o n h a v e proposed.

been

These include models based on Shottky barrier theory

d o p e d insulators (9,10,11).

for

H o w e v e r , results h a v e b e e n r e p o r t e d t h a t

d o not i n d i c a t e a p p r o p r i a t e d e p e n d e n c e o n w o r k f u n c t i o n of

electrode

metals a n d this leads to a n a l t e r n a t i v e m o d e l b a s e d o n single c a r r i e r t h e o r y (6).

O u r p h o t o g a l v a n i c w o r k to d a t e does not l e a d to a specific


c o n n e c t i o n w i t h p h o t o v o l t a i c experiments a l t h o u g h suggestive c o n n e c t i o n s exist. T h e m a i n factors r e l e v e n t to o u r present c h o i c e of Pc's for s t u d y of heterogeneous p h o t o g a l v a n i c a n d p h o t o s y n t h e t i c effects are t h e f o l l o w i n g . F i r s t , these m a t e r i a l s are i n t e r e s t i n g s p e c t r o s c o p i c a l l y .

Second, they

b e a r a r e l a t i o n s h i p to b o t h p h o t o s y n t h e t i c p i g m e n t s a n d t h e m e t a l d i i m i n e c o m p l e x e s , w h i c h h a v e a t t r a c t e d m u c h r e c e n t a t t e n t i o n as p h o t o c h e m i c a l e l e c t r o n transfer agents. dyes.

A d d i t i o n a l l y , t h e y are c o n v e n i e n t c o m m e r c i a l

T h e results suggest

t h a t there r e m a i n s c o n s i d e r a b l e

scope

for

s y n t h e t i c i n v e s t i g a t i o n s . W e e m p h a s i z e that o n l y the barest start has b e e n m a d e i n the field of o r g a n o m e t a l l i c Theoretical

film

photoelectrodes.

Remarks

T h e characteristics of a heterogeneous system m e e t i n g the A l b e r y A r c h e r c r i t e r i a c a n b e s k e t c h e d , at least i n p a r t .

L e t ' s suppose t h a t

R e a c t i o n 2 is r e d u c t i o n of Z b y a n e x c i t e d d y e A * i n the cathode.

A

c o n v e n i e n t w a y to b l o c k r e o x i d a t i o n of Y w o u l d be for the c a t h o d e to b e a p - t y p e s e m i c o n d u c t o r w i t h the b a n d b e n t to d e p l e t e the surface l a y e r of holes. T h i s is just the c o n d i t i o n f o r a p - t y p e s e m i c o n d u c t o r to a c t as a photoreductant

(5).

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 s t r y is p r o m o t e d

w h e n t h e h o l e - e l e c t r o n p a i r c r e a t e d b y p h o t o n a b s o r p t i o n is p r e v e n t e d f r o m r e c o m b i n i n g b y m i g r a t i o n of the m a j o r i t y c a r r i e r to the i n t e r i o r a n d t h e m i n o r i t y c a r r i e r (electrons i n o u r case) to the surface to r e a c t w i t h t h e electrolyte. N o t e also, that p h o t o n s a b s o r b e d at v a r i o u s d e p t h s i n t h e space c h a n g e l a y e r c a n b e h a r v e s t e d a n d d e l i v e r e d to the r e a c t i v e site ( s u r f a c e ) i f this m e c h a n i s m is sufficiently efficient. T h e r e is a n e l e m e n t f u n c t i o n a l l y analogous to l i g h t h a r v e s t i n g i n photosynthesis. F o r B to regenerate A , i t is necessary to h a v e a n e l e c t r o n underlying the semiconducting dye an n-type semiconductor.

film.

T h e l a t t e r p o s s i b i l i t y offers t h e p r o s p e c t of a n

n - p j u n c t i o n rectifier u n d e r l y i n g the p - e l e c t r o l y t e j u n c t i o n to rectification. T h e B

source

T h i s m a y be either a m e t a l or enhance

Y w a s t i n g r e a c t i o n is also i n h i b i t e d i f B is associ

ated w i t h a hole that migrates t o w a r d the semiconductor interior a n d away from Y.


142

INTERFACIAL

PHOTOPROCESSES

A n i d e a l c e l l i s c o n c e i v a b l e as a n i l l u m i n a t e d e l e c t r o d e c o m p o s e d o f an n-type semiconductor ( o r perhaps a metal) coated b y a p-type d y e l a y e r t h a t is p h o t o c h e m i c a l l y a c t i v e ( t h e A/B c o u p l e ) i n c o n t a c t w i t h a n electrolyte containing t h e Y / Z couple.

T h e electrolyte makes

contact

w i t h a m e t a l electrode a t w h i c h t h e Y / Z c o u p l e i s r e v e r s i b l e . A n efficient cell based o n t h e diffusion of the Y / Z couple should b e t h i n a n d the m e t a l electrode is r a r e l y t r a n s p a r e n t . T h e r e f o r e , i t i s u s e f u l t o c o n s i d e r t r a n s p a r e n t n - t y p e s e m i c o n d u c t o r s c o a t e d o n glass as s u p p o r t s f o r t h e d y e l a y e r a n d e n t r y o f l i g h t to t h e i l l u m i n a t e d e l e c t r o d e f r o m t h e s u p p o r t s i d e r a t h e r t h a n t h e e l e c t r o l y t e side. T h i s h a s t h e a d d e d a d v a n t a g e o f


a l l o w i n g the use o f c o l o r e d electrolytes. I n t h e e x p e r i m e n t s d e s c r i b e d b e l o w , t h e barest s k e l e t o n o f s u c h a n i d e a l c e l l is v i s i b l e . a - C r y s t a l m o d i f i c a t i o n m e t a l p h t h a l o c y a n i n e d y e films have p-type moisture ( I I ) .

semiconducting properties w h e n equilibrated w i t h a i r / ( T h i s is i n d i c a t e d i n F i g u r e 1.) W h e n t h e y are s u b l i m e d

onto transparent n-doped conducting S n 0 a c t i v e p - t y p e s e m i c o n d u c t o r is f o r m e d .

2

o n glass electrodes, a p h o t o

C o n t a c t of this solid a n d semi

s o l i d ( t h e d y e l a y e r ) rectifier l a y e r c a k e w i t h a n e l e c t r o l y t e c o n t a i n i n g a c o u p l e s u c h as q u i n o n e / h y d r o q u i n o n e ( i n 1 . 0 M K C 1 as s u p p o r t i n g e l e c t r o l y t e ) , i n t u r n i n contact w i t h P t , c o m p l e t e s a p o t e n t i a l r e a l i z a t i o n o f t h e " i d e a l " c e l l . A f e w o f t h e i d e a l features are r e a l i z e d i n this c e l l b u t m a n y others are not. E s p e c i a l l y , t h e r e a c t i o n o f r e d u c t i o n r e m a i n s a s l o w 200

I

FRESH CELL

20

HR. ~ 100 C*

UJ 10

O

< CO UJ

a: 2 DAYS IN I AIR

3000-A

film

sandwiched two metals.

between

4 DAYS INAIR

SUCCESSIVE MEASUREMENTS ON ONE CELL


8.

LANGFORD E T AL.

Metal

p h o t o p r o c e s s at t h e electrodes. p h o t o n e n e r g y converter.

Phthalocyanine

143

Films

T h i s c a n n o t b e t o l e r a t e d f o r a n efficient

( I t is also p o s s i b l e t h a t t h e flaw i n the i d e a l

analysis as a p p l i e d to this c e l l is t h e use of t h e m o d e l of R e f . 5 r a t h e r t h a n the m o d e l of R e f . 6.) Experimental Materials. M e t a l Pc's w e r e o b t a i n e d f r o m E a s t m a n O r g a n i c C h e m i cals a n d p u r i f i e d b y s u b l i m a t i o n . T h e d o p e d S n 0 - c o a t e d glass w a s o b t a i n e d f r o m O . H . Johns L t d . T h e resistance w a s u n d e r 100 O m~ a n d l i g h t t r a n s m i s s i o n i n the v i s i b l e r e g i o n e x c e e d e d 8 0 % . E l e c t r o l y t e c o m p o n e n t s w e r e p r e p a r e d f r o m reagent g r a d e c h e m i c a l s w i t h o u t s p e c i a l purification. W a t e r was redistilled from potassium permanganate. Electrode Preparation. F i l m s of p h t h a l o c y a n i n e w e r e d e p o s i t e d o n n - d o p e d S n 0 - c o a t e d glass ( o r i n one e x p e r i m e n t , glass) b y s u b l i m a t i o n f r o m a q u a r t z vessel i n a t u b e f u r n a c e . T h e v a c u u m l i n e w a s e v a c u a t e d w i t h a n o i l d i f f u s i o n p u m p ( m i n i m u m pressure = 1 0 ' t o r r ) except i n e x p e r i m e n t s i n t e n d e d t o p r o d u c e t h e green « m o d i f i c a t i o n of Z n P c . T h e electrode substrate w a s m o u n t e d o n a " s l e d " t h a t c o u l d b e m a g n e t i c a l l y p u s h e d past a h i n g e d b a r r i e r to the m o u t h of t h e t u b e f u r n a c e . A t t h e ( u n d e t e r m i n e d ) o p e r a t i n g t e m p e r a t u r e , films of a p p r o x i m a t e l y 5 0 0 - A thickness w e r e d e p o s i t e d i n just over 15 m i n ; 1000-A films r e q u i r e d n e a r l y 30 m i n . F i l m thickness w a s e s t i m a t e d f r o m a b s o r b a n c e at t h e w a v e l e n g t h of m a x i m u m a b s o r b a n c e u s i n g e x t i n c t i o n coefficient d a t a f r o m R e f . 12 a n d a n a s s u m e d film d e n s i t y of 1.5 g c m ' . T h u s , t h e t h i c k nesses c i t e d are q u i t e a p p r o x i m a t e . T h i s seems a p p r o p r i a t e since t h e films w e r e , u n f o r t u n a t e l y , r e c o g n i z a b l y n o n u n i f o r m . R e p r o d u c i b i l i t y f r o m one p h t h a l o c y a n i n e to another is better. Electrochemical Studies. T h e c e l l w a s f a b r i c a t e d f r o m T e f l o n w i t h a n o p e n i n g to w h i c h the t r a n s p a r e n t electrode c o u l d b e h e l d b y a brass p l a t e . E l e c t r i c a l contact to the S n 0 w a s e s t a b l i s h e d b y a brass r i n g of d i a m e t e r l a r g e r t h a n the o p e n i n g i n the c e l l . A n e o p r e n e O - r i n g a n d s i l i c o n e grease e n s u r e d a seal f r o m the electrolyte. T h e c e l l a d m i t t e d a n i t r o g e n b u b b l e r p l a c e d to sparge the electrode surface; a s a t u r a t e d c a l o m e l reference electrode w a s u s e d i n t h e p o t e n t i o s t a t i c c i r c u i t a n d a b r i g h t , l a r g e a r e a , p l a t i n u m electrode f o r m e d t h e c o u n t e r electrode. C u r r e n t - v o l t a g e c u r v e s w e r e r e c o r d e d w i t h the a i d of a P A R M o d e l 364 p o l a r o g r a p h o p e r a t e d at a scan rate of 2 m V / s e c . S m a l l b a c k g r o u n d w a v e s w e r e o b s e r v e d w h e n d y e c o a t e d electrodes w e r e a n a l y z e d i n K C 1 s o l u t i o n . T h e s e a p p e a r e d to b e t i m e d e p e n d e n t a n d l e d to a d o p t i o n of a p r o c e d u r e i n w h i c h the first scan of a n y electrode-electrolyte combination was ignored. Photochemical Aspects. T h e l i g h t source i r r a d i a t i n g t h e m e t a l p h t h a l o c y a n i n e films t h r o u g h the glass a n d S n 0 p l a t e w a s a 1 0 0 0 - W H g - X e a r c l a m p . F i l t e r s w e r e p l a c e d i n the l i g h t p a t h as f o l l o w s . A 10-cm p a t h , w a t e r - f i l l e d q u a r t z a b s o r p t i o n c e l l r e d u c e d I R a n d h e a t i n g effects; a n d a U V cutoff filter t h a t w a s tested t o e s t a b l i s h t h a t i t e l i m i n a t e d the photoeffects t h a t arise f r o m S n 0 b a n d g a p a b s o r p t i o n . T h e i n t e n s i t y of t h e l i g h t r e a c h i n g t h e film w a s e s t i m a t e d u s i n g b r o a d b a n d R e i n e c k a t e c h e m i c a l a c t i n o m e t r y ( 1 3 ) as 1.6 X 10" e i n s t e i n sec" . 2


2

2

6

3

2

2

2

6

1


144

INTERFACIAL

PHOTOPROCESSES

M.C.D. FLOWCHART OF DATA AQUISITION POLARIZER MODULATOR MAGNET STEPPING SCAN MOTOR


SIGNAL

Data Processing

Operation Programme i ) Calculates and Sets position of Scan Motor and Modulator Control Volts ii) iii)

Reads Data at Fixed Scan Position Calculates Data Mean and S.D.

iv) Stores Data point and uses S.D. to calculate scan rate f o r ( i ) .

Figure 2.

i ) Assembles spectrum on l i n e a r energy axis with c a l i b r a t i o n points ready for p l o t t i n g i i ) Allows t r i a l f i t t i n g of spectrum with proposed assignment v i a MCDFIT iii)

Calculation of L.F. parameters from band positions

iv) Calculation of x, e t c . by Moments integration

Block diagram of the MCD spectrometer

M C D Spectra. M C D spectra were recorded o n a n i m p r o v e d design of t h e c r y o m a g n e t i n s t r u m e n t d e s c r i b e d i n R e f . 12. I t s g e n e r a l features are i n d i c a t e d i n F i g u r e 2. T h e s p e c t r a w e r e o b t a i n e d a t a field o f 4.7T. A b s o r p t i o n s p e c t r a w e r e r e c o r d e d o n a C a r y 14 spectrophotometer. M C D s p e c t r a w e r e r e c o r d e d f o r films o n q u a r t z plates. T h e t e c h n i q u e o f p r e p a r a t i o n w a s s i m i l a r t o t h a t a d o p t e d f o r electrode p r e p a r a t i o n . Survey of

Photoelectrochemistry

C u r r e n t - v o l t a g e curves have been recorded f o r n - S n 0 coated w i t h a - M g P c , a-FePc, a - N i P c , a-ZnPc, a n d £-ZnPc.

2

electrodes

I n a l l cases,

the s u p p o r t i n g e l e c t r o l y t e w a s L O O M K C 1 a n d r e a c t i v e species i n t h e


8.

LANGFORD E T A L .

Metal Phthalocyanine

145

Films

solution included the quinone/hydroquinone couple, the f e r r i c y a n i d e / f e r r i c y a n i d e c o u p l e , f m ( e t h y l e n e d i a m i n e ) c o b a l t ( I I I ) , a n d N,N' d i h e p t y l 5,5' d i p y r i d i n e ( h e p t y l v i o l o g e n ) . T h e g e n e r a l features are s i m i l a r o n a l l n-Sn0

2

electrodes; F i g u r e s 3, 4, a n d 5 p r o v i d e a s u m m a r y . A c u r v e o n

a - M g P c , w h i c h is r e p r e s e n t a t i v e f o r " t h i n " films of a p p r o x i m a t e l y 5 0 0 A , is s h o w n i n F i g u r e 3. F i g u r e 4 shows results o n N i P c t h a t p r o v i d e a c o m p a r i s o n of films of 500 A a n d 1000 A a n d F i g u r e 5 d i s p l a y s results f o r t h e ^ - c r y s t a l m o d i f i c a t i o n of Z n P c . S e v e r a l g e n e r a l observations m a y


be made. 1. P h o t o c u r r e n t s of t h e sort e x p e c t e d f o r p - t y p e s e m i c o n d u c t i n g films a r e seen i n a l l cases. I t s h o u l d b e n o t e d e s p e c i a l l y that this i n c l u d e s t h e l o w e r c o n d u c t i v i t y / J - Z n P c film.

450-

400-

Figure 3. Current-voltage curves for 5.00 X 10~ quinone/hydroqui none solutions in LOOM KCl at Mg and "green" (a) ZnPc electrodes. Curves are labelled as follows: (A) photo Zn; (B) dark Zn; (C) photo Mg; (D) dark Mg. The film thickness is near 500 A in both cases; V is in mV, I is in fiA. 4


146

INTERFACIAL

PHOTOPROCESSES

2. T h e p h o t o c u r r e n t s are s m a l l a n d t h e m o r p h o l o g y of t h e current-voltage curve implies kinetically slow electron trans fer i n a l l c a s e s — b o t h p h o t o a n d d a r k . T h i s is t h e l i m i t i n g feature of t h e p e r f o r m a n c e of these electrodes. 3. T h e p h o t o e n h a n c e m e n t of c u r r e n t s i n t h e l i m i t i n g r e g i o n suggest that t h e c a r r i e r g e n e r a t i o n process p l a y s a p a r t i n e s t a b l i s h i n g t h e l i m i t i n g c u r r e n t . T h i s is c o n f i r m e d b y l i g h t i n t e n s i t y d e p e n d e n c e t h a t is l i n e a r w i t h a slope a l i t t l e less t h a n one i n the l i m i t i n g r e g i o n .


4. T h e r e is v e r y l i t t l e d e p e n d e n c e o n t h e m e t a l i n t h e p h t h a l o c y a n i n e o v e r the r a n g e M g , F e ( I I ) , N i , a n d Z n . T h e o n l y u n i q u e m e t a l feature observed occurred w i t h F e ( I I ) . I n

-200

40-

i -200

i -400

i -600

i -800

Figure 4. Current (in fxA) vs. voltage (in mV) for the quinone/hydroquinone electrolyte described in Figure 3 at NiPc electrodes. Curve (A) photo and (B) dark were recorded on an electrode with a 500-A film. Curve (C) photo and (D) dark on a 1000-A film.


LANGFORD E T AL.

8.

Metal

Phthalocyanine

Films

147

a n i r r e g u l a r ( a n d u n d i a g n o s e d ) event, i l l u m i n a t e d elec trodes e x h i b i t e d a c a t a s t r o p h i c c u r r e n t increase a c c o m p a n i e d b y loss o f t h e i l l u m i n a t e d z o n e o f t h e film. W e suspect conversion to F e ( I I I ) accompanied b y axial ligation m a y be involved. T h e m a j o r v a r i a b l e s l e a d i n g t o s i g n i f i c a n t changes o f b e h a v i o r w e r e film

thickness a n d phthalocynaine crystal modification. Increasing

film

t h i c k n e s s decreases t h e r m a l c u r r e n t s as h i g h e r resistance m i g h t i m p l y . B u t t h e decrease o f p h o t o c u r r e n t i s n o t p r o p o r t i o n a l t o t h e decrease i n dark current. A l s o , larger open circuit photogalvanic potentials were


r e c o r d e d f o r t h e 1 0 0 0 - A films. A c h a n g e f r o m the a - t o ^ - c r y s t a l m o d i f i c a -200" -180-

-160 -

20

40 600

400

200

-200 - 4 0 0

l -600

I I - 8 0 0 -1000

Figure 5. Current (in fiA) vs. voltage (in mV) for electrolytes at a "blue" (P) ZnPc electrode. Curves (A) photo and (B) dark are for the quinone/ hydroquinone electrolyte described above. Curves (C) photo and (D) dark are for a similar concentration of K Fe(CN) K Fe(CN) in KCl. Although the present scale suppresses detail in (C) and (D), limiting regions at cathodia potential were observed. s

6

i

6

American Chemical Society Library 1155 16th St. N. W.

Wrighton; Interfacial Photoprocesses: Energy Conversion and Synthesis D. C. 20036 Advances in Chemistry;Washington, American Chemical Society: Washington, DC, 1980.

148

INTERFACIAL

t i o n p r o d u c e d s i m i l a r b u t e m p h a s i z e d effects.

PHOTOPROCESSES

T h e photogalvanic

open

c i r c u i t v o l t a g e v a r i e d f r o m 4 0 - 8 0 m V o n a films. I t r e a c h e d 180 m V o n t h e p films. T h i s leads to the most i n t e r e s t i n g issue, w h a t is t h e a d v a n t a g e of ^ - c r y s t a l m o d i f i c a t i o n ? W e r e t u r n to this q u e s t i o n i n the spectroscopic discussion below. A

final

e x p e r i m e n t is w o r t h r e c o r d i n g .

T o attempt evaluating the

r o l e of n - S n 0 a film of Z n P c w a s d e p o s i t e d o n glass. I n this e x p e r i m e n t , 2

t h e e l e c t r i c a l contact, w h i c h w a s the brass ring u s e d i n other e x p e r i m e n t s , w a s at the same front surface of the film t h a t w a s i n contact w i t h the electrolyte. T h e d e f e c t o n this e x p e r i m e n t a l a r r a n g e m e n t is t h a t t h e

film


w a s p o l a r i z e d r a d i a l l y . N o n e t h e l e s s , t h e r e s u l t is i n t e r e s t i n g ; t h e p o o r l y c o n d u c t i n g (11)

^ - c r y s t a l m o d i f i c a t i o n acts as a p h o t o c a t a l y s t for b o t h

o x i d a t i o n a n d r e d u c t i o n of the q u i n o n e / h y d r o q u i n o n e c o u p l e . T h e o p e n c i r c u i t p h o t o g a l v a n i c p o t e n t i a l is essentially zero.

T h i s emphasizes

the

m o l e c u l a r p h o t o c h e m i s t r y of w h i c h this film is c a p a b l e . T r e a t i n g o r g a n o m e t a l l i c films as s e m i c o n d u c t o r s is o n l y p a r t of a n a p p r o p r i a t e p e r s p e c t i v e (see

R e f . 6 ) . T h i s p o i n t is c o n f i r m e d b y the l i m i t e d ( i f large w i d t h o f )

s p e c t r a l b a n d to b e discussed b e l o w . A suggestion for f u t u r e d e v e l o p m e n t is c o n t r o l l e d d o p i n g of the films to t r y to p r o d u c e b o t h n - a n d p - s e m i c o n d u c t i n g properties. T h e r e d u c t i o n of the ' V i o l o g e n " r e l a t i v e is also significant. R e d u c e d v i o l o g e n s c a n c a t a l y t i c a l l y ( c o l l o i d a l P t ) generate H

2

(14)

from water.

O n e electrode for the p h o t o e l e c t r o l y s i s of w a t e r is i m p l i e d . Spectroscopy and

Structure

C o m p a r i s o n of a b s o r p t i o n a n d M C D spectra of p h t h a l o c y a n i n e s i n s o l u t i o n w i t h those i n the s o l i d state reveals m a n y s u b s t a n t i a l differences. C h a n g e s o c c u r i n b a n d centers, w i d t h s , a n d r e l a t i v e intensities t h a t e x c e e d those associated w i t h a state c h a n g e of a n i s o l a t e d c h r o m o p h o r e .

Several

different regions of these s p e c t r a h a v e b e e n e x a m i n e d i n d e t a i l a n d t h e differences b e t w e e n gas p h a s e o r s o l u t i o n s p e c t r a o n one h a n d a n d s o l i d state spectra o n t h e other h a v e b e e n a t t r i b u t e d to p o l y m e r i z a t i o n i n v a r i o u s forms i n the s o l i d state

(15).

T h e s e effects are r e a d i l y o b s e r v e d i n the Q b a n d , a series of a d s o r p tions i n the r e d r e g i o n . I n s o l u t i o n spectra a s h a r p s t r o n g t r a n s i t i o n , often l a b e l l e d a, at l o w e n e r g y is f o l l o w e d b y one or m o r e less intense b a n d s , l a b e l l e d 0,

to the h i g h e n e r g y side.

T h e s e are u s u a l l y assigned as a

v i b r o n i c series a n d M C D s p e c t r u m shows t h a t the 0 - 0 t r a n s i t i o n leads to a degenerate

e x c i t e d state w h i l e t h e 0 - 1 , 0 - 2 , etc. b a n d s y i e l d a

n o n d e g e n e r a t e system ( 1 6 ) .

F r o m this i t is c l e a r t h a t t h e 0 - 0 b a n d is the

" l o n g a x i s " t r a n s i t i o n p o l a r i z e d i n the p l a n e , w h i l e t h e 0 - 2 are " s h o r t


LANGFORD E T AL.

8.

Metal

Phthalocyanine

Films

149

-200"

H80-


-160-

20-

40I

I

600

400

I

200

I

0

-200

I

I

-400

-600

I

1

-800 H000

Figure 6. Absorption (upper) and MCD (lower) Q band region of a-NiPc on a glass support. The resolution of the envelope (after Ref. 12) is shown. a x i s " or p e r p e n d i c u l a r t r a n s i t i o n s . T h e 0-0 b a n d c a n t h u s b e a s s i g n e d as A

1

lg

-»

i n the D

4 h

point group.

T h i s is u s u a l l y f u r t h e r i n t e r p r e t e d ,

f r o m e x t e n d e d H i i c k e l c a l c u l a t i o n s , as a ( a m ) c o n t r i b u t i o n f r o m other configurations.

1

(e y g

excitation w i t h little

(17).

T h e s o l i d state spectra are q u i t e different; F i g u r e 6 presents a - N i P c Q b a n d as representative.

F i l m s p r e p a r e d at l o w

the

temperature

a n d pressure s h o w a s i n g l e b r o a d a d s o r p t i o n i n t h e Q r e g i o n w i t h a low energy shoulder.

T h i s r e v e r s a l of r e l a t i v e intensities is a c c o m p a n i e d

r

by

e n e r g y shifts a n d a c o n s i d e r a b l e b r o a d e n i n g f r o m a f e w h u n d r e d to a few thousand wave numbers.

R e c e n t M C D results (12)

show that two

p a i r s of transitions o c c u r u n d e r this envelope.

O n e p a i r of transitions is

r e p r e s e n t e d b y A terms i m p l y i n g degenerate

e x c i t e d states w h i l e t h e

s e c o n d p a i r , d i s p l a y i n g B t e r m s , are nondegenerate.


150

INTERFACIAL

PHOTOPROCESSES

T h i s o b s e r v a t i o n c a n b e i n t e r p r e t e d m o s t easily as a s t r o n g i n t e r a c t i o n b e t w e e n m o l e c u l a r p a i r s o n i d e n t i c a l s y m m e t r y sites i n t h e u n i t c e l l .

The

states of s u c h p a i r s are r e a d i l y d e r i v e d u s i n g a D a v y d o v m o d e l .

The

degenerate

states o f t h e m o n o m e r s m a y b e l i n e a r l y c o m b i n e d

p r o v i d e t w o degenerate states i n the site g r o u p of t h e u n i t c e l l .

to The

n o n d e g e n e r a t e states m a y b e r e c o m b i n e d s i m i l a r l y , p r o d u c i n g t w o states of t h e d i m e r . T h e s p l i t t i n g b e t w e e n states i n e i t h e r p a i r c a n b e p a r a m e t e r i z e d i n a point dipole m o d e l or treated more completely i n a molecular orbital c a l c u l a t i o n . W h i l e the l a t t e r is u s u a l l y a better a p p r o x i m a t i o n , t h e p o i n t


d i p o l e h i g h l i g h t s t h e r o l e p l a y e d b y r e l a t i v e o r i e n t a t i o n of t h e d i m e r m o l e c u l e s a n d is u s e f u l i n p r e d i c t i n g s t r u c t u r a l features of the s o l i d . I n the s i m p l e s t m o d e l

using two molecules per unit c e l l on

(18)

i d e n t i c a l sites, the m a g n i t u d e of t h e s p l i t t i n g b e t w e e n p a i r s of

states

becomes; V —

^ ab

[sin 0 sin cos (0 — ) — 2cos 6 cos ]

a

i n w h i c h p a n d p are the t r a n s i t i o n d i p o l e m o m e n t s f o r m o l e c u l e s a a n d a

b

9

b

is the distance b e t w e e n d i p o l e centers, a n d 6 a n d are t h e angles

f o r m e d b y t h e d i p o l e s o n a a n d b, r e s p e c t i v e l y , w i t h t h e d i s p l a c e m e n t vector. W h e n 0 =

=

0 ° , the d i p o l e s are c o l i n e a r a n d w h e n 0 =

7r/2, t h e y are c o p a r a l l e l . A t the c o l i n e a r l i m i t V ~ ~ = transition dipoles become; p = +

0 a n d p. =2

Pa

p pb/ a n d the 9

a

w h e r e p+ is the d i p o l e of

t h e t r a n s i t i o n to the s y m m e t r i c c o m b i n a t i o n w h i l e p. refers to t h e a n t i s y m m e t r i c state. I n contrast at the c o p a r a l l e l l i m i t e d V = dipoles become; p = +

2

P a

a n d p. =

b

=

p pb/d a

3

a n d the t r a n s i t i o n

0.

T h u s a n e a r l y c o p a r a l l e l or " s a n d w i c h " d i m e r w o u l d d i s p l a y t w o b a n d s , one s t r o n g to h i g h e n e r g y of a m o n o m e r t r a n s i t i o n b y V = one w e a k to l o w energy.

ft

and

I n contrast, a n e a r l y c o l i n e a r d i m e r w o u l d

y i e l d t w o b a n d s one w e a k to h i g h e n e r g y b y V " ^ a n d one s t r o n g to l o w energy.

Since V =

6

=

V i V ~ ~ for i d e n t i c a l m o l e c u l e s (a =

& ) , the dimer

geometries n o t o n l y y i e l d a r e v e r s a l of intensities b u t t h e i n t e r a c t i o n e n e r g y d o u b l e s o n c h a n g i n g f r o m t h e c o p a r a l l e l to c o l i n e a r l i m i t s .

Be

t w e e n these l i m i t s the a n g l e b e t w e e n d i p o l e s c a n b e e s t i m a t e d f r o m t h e c o m p l e t e expression for V

a &

. E s t i m a t e s of angles f r o m r e l a t i v e i n t e n s i t y

measurements are c o m p l i c a t e d b y v i b r o n i c e n h a n c e m e n t of

forbidden

d i p o l e terms. A p p l i c a t i o n of this m o d e l to the l o w t e m p e r a t u r e c r y s t a l f o r m of p h t h a l o c y a n i n e s , u s u a l l y l a b e l l e d a, i n d i c a t e s that i t is a c o p a r a l l e l d i m e r c o n f o r m a t i o n i n the u n i t c e l l ( 1 2 ) .

T h e two in-plane dipoles display a

s t r o n g h i g h energy a n d w e a k l o w energy s p e c t r u m ; t h e t w o p e r p e n d i c u l a r


LANGFORD E T AL.


8.

Figure

7.

Metal

The absorption

Phthalocyanine

151

Films

(upper) and MCD (lower) spectra of in the Q band region.

fi-ZnPc

b a n d s h a v e r e v e r s e d intensities a n d a s e p a r a t i o n 1.5 t i m e s as l a r g e as the i n - p l a n e transitions. A

second

c r y s t a l m o d i f i c a t i o n g r o w n at h i g h e r t e m p e r a t u r e s

pressures, u s u a l l y l a b e l l e d /?, d i s p l a y s a v e r y different s p e c t r u m .

and Here

t h e i n - p l a n e t r a n s i t i o n intensities are t h e reverse of those o b s e r v e d i n t h e a f o r m a n d the s e p a r a t i o n is l a r g e r (15).

T h i s implies a more colinear

a r r a n g e m e n t of the d i m e r , w h i c h is p e r h a p s d e r i v e d f r o m t h e a f o r m b y m a i n t a i n i n g a constant v a l u e of d

ab

b u t r e d u c i n g the angles 0 a n d . T h i s

cannot, i n c i d e n t a l l y , b e c o n f i r m e d b y c r y s t a l l o g r a p h y since t h e a p h a s e is a l w a y s m i c r o c r y s t a l l i n e . I n a n y case the i n c r e a s e d i n t e r a c t i o n e n e r g y i n d i c a t e s t h a t t h e

0

m o d i f i c a t i o n is a m o r e c o m p a c t l a t t i c e t h a n the a a n d the s t a c k i n g a n g l e $ =

=

46° has b e e n e s t i m a t e d f o r the p f o r m (15).

t h e observations of T a c h i k a w a a n d F a u l k n e r (11)

T h i s complements

o n t h e h i g h resistance

of the /3 f o r m .


152

INTERFACIAL

PHOTOPROCESSES

I n a l l present experiments films h a v e b e e n a g e d b y exposure to t h e atmosphere.

T h i s a g i n g increases the c o n d u c t i v i t y of s u b l i m e d

films.

T h e m e c h a n i s m f o r this increase is not w e l l u n d e r s t o o d . A s i m p l e m o d e l w o u l d b e b a s e d o n o x i d a t i o n of some p h t h a l o c y a n i n e m o l e c u l e s i n t h e film

c r e a t i n g c o n d u c t i n g holes i n the l a t t i c e . E v i d e n c e of t h i s is n o t

observed

i n the

M C D spectra.

A

substantial concentration

of

such

o x i d i z e d species w o u l d l e a d to t e m p e r a t u r e - s e n s i t i v e m o l a r e l l i p t i c i t i e s c a l l e d C terms associated w i t h t h e s p i n degenerate g r o u n d states. S p e c t r a of a g e d a - m o d i f i c a t i o n

films

at 6 a n d 300 K s h o w e d

no

appreciable

differences.


H o w e v e r , it is c l e a r f r o m present results t h a t t h e c o n d u c t i v i t y of a g e d films d e p e n d s o n the l a t t i c e m o d i f i c a t i o n . T h e c o n d u c t i v i t y of t h e f$ films e x a m i n e d is s u b s t a n t i a l l y r e d u c e d i n c o m p a r i s o n w i t h the a It seems c l e a r that w h a t e v e r t h e a t m o s p h e r i c c o n d u c t i v i t y d e p e n d s u p o n p e n e t r a t i o n of 0

films

reaction may

(II).

be,

that

a n d H 0 m o l e c u l e s i n t o the

2

2

l a t t i c e a n d that the m o r e c o m p a c t /? f o r m is m o r e resistant to s u c h attack. Spectroscopy and Photochemical

Properties

I n the absence of the dopants, t h e M C D spectra of the p u r e c a n g i v e some i n d i c a t i o n of the t y p e of p h o t o e l e c t r i c expected.

T h e v a l e n c e b a n d corresponds

films

b e h a v i o r to

be

to the m o l e c u l a r *Aig state,

w h i l e p r o m o t i o n to the first c o n d u c t o r b a n d corresponds to e x c i t a t i o n to the *£„ state i n the i s o l a t e d m o l e c u l e .

T h e effects of the l a t t i c e i n this

state h a v e just b e e n discussed b u t it is c l e a r f r o m the d i s t r i b u t i o n of c h a r g e i n the 0 - » 0 e x c i t e d state of the i s o l a t e d species t h a t t h e electrons w o u l d r e m a i n l o c a l i z e d i n the s o l i d . T h e t r a n s i t i o n d i p o l e resides i n the p l a n e of the r i n g a n d p r o v i d e s l i t t l e m o v e m e n t of e l e c t r o n d e n s i t y a w a y f r o m the p l a n e t o w a r d s the n e i g h b o r i n g m o l e c u l e .

This in turn

means

t h a t the l a t t i c e is a g o o d i n s u l a t o r i n b o t h the v a l e n c e a n d c o n d u c t i o n b a n d s . I n contrast to this analysis of t h e 0 - 0 b a n d t h e v i b r o n i c overtones o b s e r v e d i n the M C D are nondegenerate. v i b r a t i o n , or other p e r t u r b a t i o n , of a n e

g

T h i s must i m p l y an active symmetry.

A l l nondegenerate

t r a n s i t i o n m o m e n t s b e a r some out of p l a n e c o m p o n e n t s a n d t h e o n l y w a y s u c h c o m p o n e n t s c a n arise is f r o m the c o u p l i n g , i n d i r e c t p r o d u c t n o t a t i o n , J

E

U

X e c Aiu + g

A

2 u

+

Biu +

B

2 u

. T h i s out-of-plane transition moment

is c a p a b l e of m o v i n g c h a r g e t o w a r d s n e i g h b o r i n g m o l e c u l e s a n d h e n c e r e d u c i n g film resistance. R e c e n t t h e o r e t i c a l analysis of t r a n s i t i o n m o m e n t s i n v i b r o n i c t r a n s i tions of

s i m p l e c h r o m i u m complexes

show

that the overall v i b r o n i c

s e l e c t i o n r u l e s h o u l d b e at least o c t u p o l a r i n c h a r a c t e r ( 1 9 ) .

This implies

that w h i l e the 0 - 0 b a n d , w h i c h is p u r e l y e l e c t r o n i c , is a l l o w e d to d i p o l a r , the s u c c e e d i n g v i b r o n i c p r o g r e s s i o n m u s t d i s p l a y the


be

octupole

Metal

LANGFORD E T A L .

8.

Phthalocyanine

153

Films

t r a n s i t i o n r u l e . U n d e r this a s s u m p t i o n t h e generative l a b e l ( 1 9 ) t o b e u s e d t o c h a r a c t e r i z e t h e c o m p l e t e v i b r a t i o n a l basis v e c t o r s h o u l d b e a t least 2, a c o n c l u s i o n c o m p l e t e l y consistant w i t h the use of a n e v i b r a t i o n . g

T h i s does m e a n , h o w e v e r , t h a t the p r o p e r a s s i g n m e n t o f t h e v i b r a t i o n a l p r o g r e s s i o n is p r o b a b l y

J

A

^(O-O),

l g

1

E ( 0 - 2 ) , E ( 0 - 4 ) , . . . i n first u

1

u

o r d e r , a s s u m i n g the 0 - » o d d t r a n s i t i o n m o m e n t s are n e g l i g i b l e since t h e y w o u l d r e q u i r e g - » g transitions. T h i s c o n c l u s i o n p r o v i d e s a n a n s w e r t o a l o n g standing difficulty i n the vibronic assignment of phthalocyanines. T h e o b s e r v a b l e s e p a r a t i o n i n the p r o g r e s s i o n is 1100 c m ' is v e r y h i g h f o r a r i n g v i b r a t i o n assignment.

1

or more, w h i c h

C l e a r l y this v i b r a t i o n m a y

n o w b e assigned the m o r e reasonable v a l u e o f a p p r o x i m a t e l y 550 c m " . Downloaded by CORNELL UNIV on September 2, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0184.ch008

1

If t h e phthalocyanine

film

is t o b e m o d i f i e d to i m p r o v e t h e c o n

d u c t i v i t y o f the c o n d u c t a n c e b a n d , t w o factors e m e r g e f r o m the f o r e g o i n g analysis. F i r s t , the n a t u r e o f the

state c o u l d b e c h a n g e d b y p r e p a r a

t i o n o f h e a v y m e t a l d e r i v a t i v e s . T h i s w o u l d p r o v i d e stronger d e r e a l i z a t i o n o f c h a r g e i n t h e e * o r b i t a l o f the r i n g t h r o u g h o v e r l a p w i t h g

filled

dir* o r b i t a l s of t h e m e t a l . S e c o n d l y , t h e a m p l i t u d e o f t h e e v i b r a t i o n g

c o u l d b e i n c r e a s e d b y h e a v y n o n m e t a l s u b s t i t u e n t groups o n the p y r o t e r i n g s . E a c h of these substitutions w o u l d increase t h e m a g n i t u d e o f t h e i n d i v i d u a l t r a n s i t i o n m o m e n t s , w h i c h w o u l d i n t u r n b e reflected i n t h e m a g n i t u d e of the o u t - o f - p l a n e m o m e n t f o r m e d as the v e c t o r cross p r o d u c t . S u c h effects w o u l d b e c l e a r l y v i s i b l e i n M C D s p e c t r u m o f t h e e x c i t o n structure. I n p a r t i c u l a r , the h i g h energy nondegenerate t r a n s i t i o n w o u l d increase i n r o t a t i o n a l s t r e n g t h because o f i n c r e a s e d m a g n e t i c a n d e l e c t r i c d i p o l e s i n the a n t i s y m m e t r i c state. Acknowledgment W e t h a n k t h e donors of t h e p e t r o l e u m r e s e a r c h f u n d f o r s u p p o r t . Literature 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Cited

Albery, W. J.; Archer, M. D. J. Electrochem. Soc. 1977, 124, 688. Albery, W. J.; Archer, M. D. Electrochim. Acta 1976, 21, 1155. Marcus, R. A. Anna. Rev. Phys. Chem. 1964, 15, 155. Rehm, D.; Weller, A. Ber. Bunsenges. Phys. Chem. 1969, 73, 834. Tributsch, H.; Gerischer, H. Ber. Bunsenges. Phys. Chem. 1969, 73, 251, 850. Hall, K. J.; Bauham, J. S.; Lyons, L. E. Austr. J. Chem. 1976, 31, 1661. Gutman, G. F.; Lyons, L . E . "Organic Semiconductors"; Wiley: New York, 1967. Lever, A. B. P. Adv. Inorg. Chem. Radiochem. 1967, 7, 27. Usov, N. N.; Benderskii, V. A. Sov. Phys.—Semicond. (Engl. Transl.) 1968, 2, 580 Ghosh, A. K.; Feng. T. J. Appl. Phys. 1973, 44, 2781. Tachikawa, H.; Faulkner, L. R. J. Am. Chem. Soc. 1978, 100, 4379. Hollebone, B. R.; Stillman, M. J. J. Chem. Soc., Faraday Trans. II 1978.


154

INTERFACIAL PHOTOPROCESSES

13. Wegner, E . E.; Adamson, A. W. J. Am. Chem. Soc. 1966, 88, 394. 14. Moradpour, A.; Amouyal, E.; Keller, P.; Kagan, H. "Abstracts of Papers," 2nd International Conference on Photochemical Conversion and Storage of Solar Energy, Cambridge, August 1978, p. 31. 15. Sharp, J. H.; Lardon, M. J. Phys. Chem. 1968, 72, 3230. 16. Stillman, M. J.; Thomas, A. J. J. Chem. Soc., Faraday Trans.II1974, 70, 805. 17. Schaffer, A. M.; Gouterman, M. Theor. Chim. Acta 1972, 25, 62. 18. Davydov, A. S. "Theory of Molecular Excitons"; McGraw Hill: New York, 1962. 19. Hollebone, B. R. Theor. Chim. Acta, in press.


RECEIVED October 2, 1978.


Photoelectrochemistry and Spectroscopy of Metal Phthalocyanine

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