Colloids and Surfaces in Reprographic Technology - American

1 Surface Effects in Silver Halide Photography GEORGE R. BIRD

Downloaded by 110.90.45.165 on September 9, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch001

Rutgers University, Department of Chemistry, New Brunswick, NJ 08903

Model systems of photographic sensitizing dyes and silver h a l i d e c r y s t a l s lend themselves t o b a s i c s t u d i e s o f a d s o r p t i o n and o r d e r i n g on s u r f a c e s . One of the goals o f t h i s paper is t o make a v a i l a b l e to surface scientists a t l a r g e the techniques f o r d e t e r mining the size and s t r u c t u r e o f s u r f a c e aggregates. These methods i n c l u d e the direct optical o b s e r v a t i o n o f monolayers o f dyes on silver h a l i d e crystals, the d i a g n o s t i c c a l c u l a t i o n o f the l a r g e s p e c t r a l shifts which occur when l a r g e dye aggregates form, the examination of p o l a r i z e d light a b s o r p t i o n by dyes on o r i e n t i n g faces o r f a c e t s , and the determination o f s u r f a c e c o n c e n t r a t i o n from conservation o f i n t e g r a t e d a b s o r p t i o n . The s t r u c t u r e and e l e c t r o c h e m i s t r y o f s u r f a c e aggregates o f silver atoms on silver h a l i d e s are a l s o t o p i c s open f o r b a s i c s u r f a c e s t u d i e s . Here the s u r f a c e scientist will seek simple model systems and avoid the p l e t h o r a of s u r f a c e a d d i t i v e s so essential in forming the photographic l a t e n t image. Examples will be given.

The most remarkable f e a t u r e of the silver h a l i d e photographic process is its extreme light sensitivity. This sensitivity reaches a level a t which some 10 absorbed photons c r e a t e one 3 atom silver c l u s t e r a t a f a v o r a b l e site on the individual microcrystal, and t h i s c l u s t e r acts as a c a t a l y s t f o r the r e d u c t i o n (development) of an entire crystal c o n t a i n i n g some 3·10 silver atoms. The mere f a c t t h a t one takes a film t o the local drug s t o r e t o have it developed effectively conceals the a s t o n i s h i n g performance o f the multitude o f microscopic a m p l i f i e r s i n s i d e the film. Not only is the " g r a i n " of AgBr a remarkable a m p l i f i e r , it has t o f u n c t i o n as a coincidence counter. I f g r a i n s responded t o the generation o f single thermal conduction e l e c t r o n s , f a l s e development of unexposed g r a i n s would q u i c k l y f o l l o w (1). The a c t i o n of a single photon ( o r a thermal event) produces one elec10

0097-6156/82/0200-0003$13.95/0 © 1982 American Chemical Society In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

REPROGRAPHIC TECHNOLOGY


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t r o n and one h o l e , and these both t r a p and de-trap repeatedly over a p e r i o d on the order of one second before one of the two meets the other at a t r a p , and recombination ( a n n i h i l a t i o n ) occurs. This i s the process which c l o s e s the gate unless a d d i t i o n a l photons a r r i v e d u r i n g the "open" p e r i o d and produce a d d i t i o n a l e l e c trons to f a c i l i t a t e the n u c l e a t i o n of a s i l v e r c l u s t e r , u s u a l l y on the s u r f a c e of the g r a i n . Once t h i s c l u s t e r i s formed, i t must be p r o t e c t e d a g a i n s t a t t a c k by holes (bromine atoms) and atmospheric oxygen f o r a storage p e r i o d of months i n the camera. This p r o t e c t i o n process i s a s s i s t e d by the s e n s i t i z i n g dyes and by a v a r i e t y of other s p e c i a l a d d i t i v e s . This paper w i l l not s t r e s s the f i e l d of s u r f a c e a d d i t i v e s , s i n c e t h i s f i e l d i s so s p e c i a l i z e d , p r o p r i e t a r y , and dependent on access to experimental f i l m c o a t i n g s . Residence i n a l a r g e photographic l a b o r a t o r y i s a l l but e s s e n t i a l to pursue the f i e l d of a d d i t i v e s . By c o n t r a s t , the f i e l d of sens i t i z i n g dyes and t h e i r s u r f a c e e f f e c t s can be open to those l i v ing o u t s i d e the " s i l v e r c u r t a i n " . S i m i l a r l y , there are e s s e n t i a l b a s i c experiments to be done on the p r o p e r t i e s of s m a l l s i l v e r c l u s t e r s , and many h e l p f u l experiments have a l r e a d y been done f a r from any c o a t i n g a l l e y . Some of these w i l l be d e s c r i b e d , and some avenues f o r new work w i l l be i n d i c a t e d . This w i l l not be a review paper. Herz has given a thorough d i s c u s s i o n of dye a d s o r p t i o n phenomena (2), and the chapters which bracket h i s work i n "The Theory of the Photographic Process" g i v e a broad overview of the i n t e r a c t i o n s of s e n s i t i z i n g dyes w i t h s i l v e r h a l i d e m i c r o c r y s t a l s i n photographic f i l m s ( 3 , 4 ) . Similarl y , Hamilton has t r e a t e d the formation of the l a t e n t image i n great d e t a i l (5). Here we s h a l l concentrate on the a v a i l a b i l i t y of new t o o l s f o r s u r f a c e s t u d i e s , and on the need f o r new s u r f a c e s t u d i e s to remove the l a s t shortcomings i n the s i l v e r h a l i d e phot o g r a p h i c process. The M a t e r i a l s S i l v e r bromide, s i l v e r c h l o r i d e , and the mixed h a l i d e compounds (AgBr-I and AgCl«Br are used i n preference to the pure h a l i d e s i n photography) are an a c c e s s i b l e and v e r s a t i l e e x p e r i mental system. Both AgCl and AgBr can be grown as l a r g e s i n g l e c r y s t a l s ( f . c . c ) , and are o b t a i n a b l e commercially. The m a t e r i a l s are very s o f t (Moh hardness of 1, the bottom of the s c a l e ) , and must be s l i c e d and o r i e n t e d w i t h care, cleavage being i m p o s s i b l e . O r i e n t a t i o n can be done v i s u a l l y by d e p o s i t i n g v i s i b l y ordered m i c r o c r y s t a l s of NaCl on the s i l v e r h a l i d e boule by growth from a s a t u r a t e d s a l t s o l u t i o n . The s i l v e r h a l i d e s must not be allowed to come i n contact w i t h metals other than s i l v e r , g o l d , and the platinum metals. C r y s t a l s u r f a c e s can be cleaned by a very b r i e f s t r i p p i n g etch i n water s o l u t i o n s of o r d i n a r y photographic hypo. Two aspects of AgBr behavior must be emphasized (Figure 1). the p r o d u c t i o n of p r a c t i c a l photographic "emulsions" or f i l m coat-

In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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Silver Halide

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Photography

ings the m a t e r i a l u s u a l l y c r y s t a l l i z e s i n the (111) f a c i a l form ( 6 ) , g i v i n g the f a m i l i a r t a b u l a r c r y s t a l which i s the emblem o f the S o c i e t y o f Photographic S c i e n t i s t s and Engineers. Madelung c a l c u l a t i o n s suggest t h a t t h i s face ought t o be u n s t a b l e w i t h respect t o the (100) and (110) f a c e s , but o f course these c a l c u l a t i o n s take the face i n vacuum as the r e f e r e n c e p o i n t o f energy. The formal (111) face i s a sheet o f l i k e i o n s , e i t h e r a l l A g o r a l l Br~, an e l e c t r o s t a t i c m o n s t r o s i t y . Berry has commented t h a t the face should be considered as a f u l l sheet o f one i o n (Br~) o v e r l a i d w i t h a random h a l f sheet o f the o t h e r (Ag+). This view suggests that the random h a l f l a y e r might be r e o r g a n i z e d t o accommodate ordered a r r a y s of adsorbing molecules ( 7 ) . Neglect o f t h i s p o s s i b i l i t y o f r e o r g a n i z a t i o n has produced arguments t h a t c e r t a i n dye s t r u c t u r e s cannot p o s s i b l y form on t a b u l a r AgBr ( 8 ) . Downloaded by 110.90.45.165 on September 9, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch001

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Very c l o s e l y r e l a t e d t o the s t a b i l i t y o f the (111) face i s the occurrence o f a s t r u c t u r a l coincidence between the i n o r g a n i c host and the o r g a n i c guest adsorbate ( 9 ) . I n s o f a r as one does c o n s i d e r the p l a n a r face a t a l l , the p a r a l l e l l i n e s of atoms ([110] i n t e r c e p t s ) are separated by 3.5358 on AgBr and 3.399X on AgCl. Both of these are w i t h i n acceptable coincidence w i t h the g r a p h i t i c d i s t a n c e of 3. the normal s e p a r a t i o n of p a r a l l e l aromatic planes i n g r a p h i t e and i n c r y s t a l s o f l a r g e aromatic o r h e t e r o c y c l i c compounds. I t i s h a r d l y s u r p r i s i n g t h a t the c u b i c s i l v e r h a l i d e s have an a f f i n i t y f o r p l a n a r aromatic and h e t e r o c y c l i c compounds, and one can a n t i c i p a t e t h a t ordered, c l o s e packed monolayers of adsorbed molecules w i l l f r e q u e n t l y form. This i s the case w i t h the cyanine s e n s i t i z i n g dyes. 11

The s u r f a c e s i l v e r ions o f AgBr a l s o represent an o p p o r t u n i t y f o r chemical i n t e r a c t i o n s . When a c l e a n c r y s t a l i s exposed t o a p o t e n t i a l r e a c t a n t such as a mercaptan i n s o l u t i o n , the c r y s t a l q u i c k l y a c q u i r e s a s u r f a c e l a y e r o f the s i l v e r mercaptide. The photographic development " r e s t r a i n e r " ' , l - p h e n y l - l - H - t e t r a z o l e - 5 t h i o l ( a l i a s phenylmercaptotetrazole o r PMT) (10) i s a good examp l e of t h i s b e h a v i o r , g i v i n g m u l t i l a y e r s o f Ag PMT" t h i c k enough to y i e l d a c l e a r i n f r a r e d spectrum on o b s e r v a t i o n o f a p l a t e of AgBr or AgCl so dipped. With s m a l l molecules o f t h i s s o r t , charact e r i z e d by i n s o l u b l e s i l v e r s a l t s , a d s o r p t i o n passes e a s i l y i n t o a c t u a l m e t a t h e t i c a l r e a c t i o n , w i t h formation of AgPMT, f o r examp l e , as a separate s o l i d phase. We do w e l l t o remember that both AgCl and AgBr have been used as i n f r a r e d o p t i c a l m a t e r i a l s f o r windows and even prisms. THus, w i t h e i t h e r s c a l e expansion on a good q u a l i t y c o n v e n t i o n a l IR spectrometer o r high s e n s i t i v i t y o b s e r v a t i o n on a F o u r i e r transform (FTIR) instrument, one can observe even monolayers of adsorbed m a t e r i a l on two s i d e s of a d i s c , which may be e i t h e r s i n g l e c r y s t a l o r p o l y c r y s t a l l i n e material. This provides an unusual and much-neglected o p p o r t u n i t y to s u r f a c e chemists. Short o f o u t r i g h t r e a c t i o n by mercaptans, h e t e r o c y c l i c compounds c a r r y i n g exposed, c y c l i z e d n i t r o g e n o r s u l f u r f u n c t i o n s can form m u l t i p l e l i g a n d bonds t o adjacent s i l v e r i o n s . +


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This appears to be the case w i t h the tetraazaindene " s t a b i l i z e r s " (11) and a l s o w i t h some of the thiacarbocyanine s e n s i t i z e r s . Strong a d s o r p t i o n can occur without any such l i g a n d bonding, but one ought not to neglect t h i s a d d i t i o n a l f o r c e , e s p e c i a l l y as an o r i g i n of o r d e r i n g i n the attached l a y e r , s i n c e i t i s a short range f o r c e of a p p r e c i a b l e s t r e n g t h . The cyanine dyes are among the strongest l i g h t absorbers known to man (12). Some r e p r e s e n t a t i v e s t r u c t u r e s and s p e c t r a are shown i n Figure ( 2 ) . Oddly, although these dyes have a c a t i o n i c aromatic s t r u c t u r e , they f u n c t i o n w i t h high quantum e f f i c i e n c y as i n j e c t o r s of e l e c t r o n s from the e x c i t e d s t a t e s of the dyes to the s i l v e r h a l i d e host. F u r t h e r , the dyes can a l s o accept holes from the host, so the f i n a l t r a n s i e n t s t a t e which f o l l o w s e i t h e r d i r e c t absorption by the host (blue or UV l i g h t ) or dye a b s o r p t i o n (green, red, or IR l i g h t ) i s a conduction e l e c t r o n i n the host c r y s t a l and a trapped hole i n the dye l a y e r . We have come to the end of an era of debate on "THE" mechanism of s p e c t r a l (dye) s e n s i t i z a t i o n , and i t i s now apparent that the dominant mechanism i s one of e l e c t r o n i n j e c t i o n ( 4 ) . However, c a r e f u l experiments by Kuhn and Mobius w i t h i n s u l a t e d monolayers have shown that d i r e c t energy t r a n s f e r from dye to host i s a p o s s i b l e m i n o r i t y mechanism (13). The predominance of the e l e c t r o n i c mechanism i s underscored by the extensive c o r r e l a t i o n s between redox p o t e n t i a l s of i s o l a t e d dye molecules i n s o l u t i o n and the crossover p o t e n t i a l s f o r t r a n s i t i o n s from s e n s i t i z a t i o n to n o n - s e n s i t i z a t i o n and on to image d e s t r u c t i o n by hole i n j e c t i o n . Gilman and coworkers have c a l i b r a t e d the s o l u t i o n e l e c t r o c h e m i c a l s c a l e against the s o l i d s t a t e (vacuum = 0.0) s c a l e (14), and so f i x e d the e l e c t r o c h e m i c a l thresholds f o r a number of processes. Since t h i s i s p r i m a r i l y an e l e c t r o c h e m i c a l problem, and a problem w e l l i n hand, we pass on to the c r i t i c a l s u r f a c e phenomena i n v o l v e d i n the i n t e r a c t i o n s between AgBr and dyes. Tools f o r Surface

Science

In p r i n c i p l e , one ought to be able to use the modern surface techniques such as low energy e l e c t r o n d i f f r a c t i o n (LEED) to determine the s t r u c t u r e s of dye l a y e r s on s i n g l e c r y s t a l s of the s i l v e r h a l i d e s . Results to date have been a disappointment (15). Instead of absolute s t r u c t u r e determinations being made, a s e r i e s of techniques has been developed to gain some p a r t i a l i n f o r m a t i o n on the packing of ordered, monolayer dye s t r u c t u r e s on the s i l v e r h a l i d e s . These techniques are p r i m a r i l y o p t i c a l i n nature, and r e l y on the simple f a c t t h a t a s i n g l e monolayer of a cyanine dye has s u f f i c i e n t o p t i c a l a b s o r p t i o n to be e a s i l y observed by a modern spectrophotometer. This i s h a r d l y s u r p r i s i n g , as Kuhn and Mobius have long measured the a b s o r p t i o n of s u r f a c t a n t cyanine dyes l i f t e d from close-packed s t a t e on a water s u r f a c e onto a prepared g l a s s s l i d e (16). A s i n g l e monolayer of a cyanine dye may give from 4% to 40% peak a b s o r p t i o n of l i g h t , depending on the


BIRD

Silver Halide


Regular crystal Figure 1.

Photography

Single twinning

Double twinning

Complexity of silver bromide crystal growth without imperfections, with a single twin plane, and with a pair of twin planes.

The doubly twinned form, commonly encountered in the production of commercial photographic films, has a recessed (re-entrant) line at one of the twin planes. This is probably an active area for collection of the latent image for the action of special phototographic additives. (Reproduced, with permission, from Ref. 6. Copyright J 963, Photographische korrespondenz.)

DIPOLE STRENGTHS OF CYANINES

WAVELENGTH nm (LOG -SPACING) Figure 2. Absorption spectra of three cyanine sensitizing dyes: thiacyanine* (CH), thiacarbocyanine* (CH) , and thiadicarbocyanine* (CH) , and illustrates the progression of cyanine absorption bands by ~ lOOnm for each addition of (CH) , a carbo group. 3

S

2

This figure has been constructed with logarithmic spacing of the wavelength scale, so that the relative dipole strengths of the absorption bands of the dyes are represented by the areas under the absorption curves. Dipole strength rises with the addition of carbo groups. (Reproduced, with permission, from Ref. 12. Copyright 1980, Society of Photographic Scientists and Engineers.)


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sharpness of the a b s o r p t i o n band(s) of the dye on the c r y s t a l . Consequently we have a monolayer system which presents i t s e l f f o r convenient o b s e r v a t i o n . We s h a l l see that the a b s o r p t i o n spectrum of a monolayer i s q u i t e d i f f e r e n t from the spectrum of a true s o l u t i o n of the same dye. This was once considered a paradox: Sens i t i z i n g dyes (on AgBr) produce a c t i o n at wavelengths where the d y e s ( i n s o l u t i o n ) are not absorbing. Yesterday's paradox becomes today's advantage, as much can be determined about the packing s t r u c t u r e of a monolayer from the wavelength s h i f t and shape of the monolayer a b s o r p t i o n band. The reader w i l l begin to sense a c e r t a i n preoccupation w i t h close-packed monolayers of dye. This has at l e a s t three o r i g i n s . Most important, the quantum e f f i c i e n c y of the s e n s i t i z a t i o n process d e c l i n e s as dye coverage reaches and exceeds one c l o s e packed monolayer (17). The mechanism(s) u n d e r l y i n g t h i s f a i l u r e of photographic and photoconductive s e n s i t i z a t i o n are not w e l l understood. This monolayer l i m i t a t i o n stands as the l a r g e s t performance gap of the s i l v e r h a l i d e s , and removal of the l i m i t a t i o n would open a route to new f i l m s of g r e a t l y enhanced r e s o l u t i o n and increased s e n s i t i v i t y (12). Some l i m i t e d progress has been r e corded i n t h i s d i r e c t i o n (18). A second o r i g i n of the preoccupation w i t h monolayers l i e s i n the mordanting power of g e l a t i n f o r s e n s i t i z i n g dyes. The s i l v e r h a l i d e s u r f a c e r e a d i l y p i c k s up the f i r s t monolayer, but s e l e c t e d s i t e s of o p p o s i t e charge i n the g e l a t i n of a photographic emuls i o n have a f f i n i t i e s * f o r the cyanine dyes which might otherwise bind weakly as a second monolayer. I t i s g e n e r a l l y observed that the c o n c e n t r a t i o n of " f r e e " monomeric dye r i s e s a b r u p t l y i n the w a t e r - g e l a t i n medium j u s t as soon as monolayer coverage i s reached. * G e l a t i n can b i n d even aggregates of s u b s t a n t i a l s i z e . Some years ago, Mr. Hector A. Rodriguez and the w r i t e r thermally c y c l e d a cyanine dye i n water between monomer and dimer at h i g h temperatures and the h i g h p o l y m e r i c J-aggregate at low temperatures. This c y c l i n g i s r e p r o d u c i b l e , and there i s a h i g h e s t temperature f o r the e x i s t e n c e of the J-aggregate f o r each p a r t i c u l a r dye conc e n t r a t i o n . On the down-cycle there was a delay of v a r i a b l e l e n g t h before the J-aggregate n u c l e a t e d . With 0.1% to 1% of g e l a t i n added to the water, the c e i l i n g temperature of the J-aggregate rose c o n s i d e r a b l y , and i t was obvious from s m a l l blue s h i f t s that the g e l a t i n was s t a b i l i z i n g aggregates of s m a l l s i z e (N~10). I t i s d i f f i c u l t to e x p l a i n t h i s s o r t of s t a b i l i z a t i o n at h i g h g e l a t i n d i l u t i o n without i n v o k i n g s p e c i f i c s i t e s (charged amino a c i d sequences) i n the g e l a t i n . R e g r e t t a b l y , we d i d not charact e r i z e the net charge on the g e l a t i n i n these experiments, and thus are unable to give a completely s a t i s f y i n g answer to a quest i o n asked by Dr. A.H. Herz. However, we do have to note t h a t a l l of the a p p r o p r i a t e s i t e s do not disappear as soon as the average charge of the g e l a t i n begins to match the charge on the dye near the i s o e l e c t r i c p o i n t .


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T h i r d and f i n a l l y , few cyanines have a molecular s t r u c t u r e s u i t a b l e f o r the formation o f s t a b l e m u l t i l a y e r s . The dyes adsorb w i t h one molecular long edge i n contact w i t h the s i l v e r h a l i d e s u r f a c e , and t h i s long edge must be f r e e from bulky groups which might i n t e r f e r e w i t h a d s o r p t i o n . The opposite long edge can bear a wide v a r i e t y of p r o j e c t i n g groups, such as Nf^Nj-diethyl-, N,N d i s u l f o b u t y l , o r N,N'-dimethyl-, without any s i g n i f i c a n t change i n e i t h e r the s u r f a c e coverage o r the s h i f t o f a b s o r p t i o n wavel e n g t h . One comes n a t u r a l l y t o a mental p i c t u r e o f the extended N,N'- s u b s t i t u e n t s as p r o j e c t i n g upward and away from the s i l v e r h a l i d e s u r f a c e and p r e s e n t i n g a d i s o r d e r e d contact a t the o u t s i d e s u r f a c e o f the dye monolayer. Downloaded by 110.90.45.165 on September 9, 2015 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch001

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A close-packed monolayer o f long, p l a n a r molecules must nece s s a r i l y c o n t a i n a t l e a s t " i s l a n d s " o f l o c a l order i n which molecules are packed plane-to-plane. One n o t i c e s that c r y s t a l s o f these dyes tend t o pack i n sheets, w i t h i n which the molecules have t h e i r long axes p a r a l l e l t o one another and p a r a l l e l t o the plane o f the sheet. The molecular s h o r t axes are n e a r l y perpend i c u l a r t o the plane o f the sheet, and the i n t e r m o l e c u l a r contacts w i t h i n the sheet tend t o occur a t the g r a p h i t i c d i s t a n c e , except f o r molecules which are bent o r t w i s t e d out o f p l a n a r i t y . Twisted molecules ("overcrowded" i n Brooker's c l a s s i f i c a t i o n ) g e n e r a l l y make poor s e n s i t i z e r s , probably because they have a f a s t route o f d e e x c i t a t i o n d i r e c t l y from the e x c i t e d s i n g l e t s t a t e t o the ground s t a t e opened by the e x i s t e n c e o f the t w i s t e d , s t r e s s e d s t r u c t u r e . Given a l l t h i s o r d e r i n g i n a monolayer sheet, one has t o ask whether an e p i t a x i a l contact - a k i n d o f l o c k and key f i t t i n g may not occur between the sheet o f dye and the s i l v e r h a l i d e s u r face ( 9 ) . Apparently t h i s happens f r e q u e n t l y , and seems t o be a c h a r a c t e r i s t i c of the best s e n s i t i z i n g dyes. I f one can be r e a sonably c e r t a i n t h a t e p i t a x i a l contact i s o c c u r r i n g , the problem of a n a l y z i n g s u r f a c e s t r u c t u r e s i s s i m p l i f i e d t o the c o n s i d e r a t i o n of a very s m a l l number o f a l t e r n a t e p o s s i b i l i t i e s . Much o f the behavior o f s t a t e - o f - t h e - a r t s e n s i t i z e r s can be e x p l a i n e d by the hypothesis o f e p i t a x i a l attachment. However, e f f i c i e n t s e n s i t i z a t i o n can occur without e p i t a x i a l contact. I n p a r t i c u l a r , the s u r f a c t a n t dyes (N^N'-dioctadecyl s u b s t i t u t e d cyanines) o f Kuhn and Mobius (16) tend not t o form e p i t a x i a l contacts on AgBr. There i s one w e l l documented case o f such a dye which attaches e p i t a x i a l l y on s i n g l e c r y s t a l AgBr but not on evaporated, p o l y c r y s t a l l i n e AgBr (19); and there are numerous examples o f n o n - e p i t a x i a l cont a c t . The forces of s t a c k i n g the p a r a l l e l o c t a d e c y l chains appare n t l y o v e r r i d e the o r d e r i n g forces a t the s u r f a c e , except f o r molecules having s t r u c t u r a l backbones e s p e c i a l l y f a v o r a b l e t o e p i t a x i a l contact. We s h a l l s h o r t l y examine some o f the evidence f o r e p i t a x i a l contacts. Now l e t us consider the kinds o f o p t i c a l observations which can be made on v a r i o u s systems o f dye and s i l v e r h a l i d e :


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1. As a c l a s s i c a l method f o r o b s e r v a t i o n of isotherms, the s i l v e r h a l i d e c r y s t a l s i n a suspension (a melted g e l a t i n "emulsion") can be c e n t r i f u g e d out, l e a v i n g the c l e a r supernatant l i q u i d a v a i l a b l e f o r q u a n t i t a t i v e s p e c t r o s copic a n a l y s i s .


2. The t o t a l r e f l e c t a n c e of an o p t i c a l l y t h i c k l a y e r of dyed p a r t i c l e s of AgBr can be measured w i t h a spectrophotometer equipped w i t h an i n t e g r a t i n g sphere. One then i n t e r p r e t s the r e f l e c t a n c e spectrum w i t h the a i d of Kubelka-Munk f u n c t i o n s (20), as i s commonly done w i t h p a i n t s and p i g ments. 3. Using a spectrometer w i t h i n t e g r a t i n g sphere, the t o t a l r e f l e c t a n c e and t o t a l transmittance s p e c t r a may be obtained from a simple emulsion c o a t i n g (21) (A photographic emuls i o n ) . This type of measurement must be done on a negat i v e which has no secondary absorber, such as the f a m i l i a r a n t i h a l o c o a t i n g . On many f i l m s t h i s c o a t i n g l i e s on the opposite s i d e of the f i l m base from the s i l v e r h a l i d e , and can be removed by l o c a l swabbing w i t h hypo or s u l f i t e s o l u t i o n . Once the s p e c t r a %R(X) and %T(X) have been o b t a i n ed, b a s e l i n e c o r r e c t i o n s are made, and the absorption of the l a y e r i s c a l c u l a t e d as %A = 100% -%R - %T. This technique i s e s s e n t i a l f o r s t u d y i n g the photochemical e f f i c iency and quantum y i e l d s of s i l v e r h a l i d e f i l m s , but i s l e s s s u c c e s s f u l than (2.) above i n determining the propert i e s of a surface l a y e r of dye. 4. The t r a n s m i s s i o n and r e f l e c t i o n s p e c t r a of a dyed s i n g l e c r y s t a l can be determined (22). Unless monolayer coverage i s s u b s t a n t i a l l y exceeded, there i s very l i t t l e change i n the r e f l e c t i o n spectrum on a p p l i c a t i o n of the dye. There i s a c e r t a i n antagonism between c r e a t i n g a f r e s h l y etched surface f o r adsorption of the dye and m a i n t a i n i n g a q u a l i t y of o p t i c a l p o l i s h on the s u r f a c e f o r o r d i n a r y t r a n s m i s s i o n measurements. We f i n d i t best to concentrate on the dye-surface c o n t a c t , and to measure the somewhat d i f f u s i n g s u r f a c e of the c r y s t a l w i t h an i n t e g r a t i n g sphere as i n ( 3 . ) , o b t a i n i n g both t r a n s m i s s i o n and r e f l e c t i o n (23). I t i s most h e l p f u l to have a computerized spectrome t e r to manipulate and c o r r e c t the data, and e s p e c i a l l y to expand the r e l a t i v e l y s m a l l a b s o r p t i o n obtained w i t h j u s t two monolayers. 5. The s i n g l e c r y s t a l method of (4.) can be repeated w i t h other host c r y s t a l s which are transparent i n the v i s i b l e or i n f r a r e d . Since NaCl i s isomorphous w i t h AgBr and AgCl and has a l a t t i c e parameter intermediate between the two s i l v e r h a l i d e s , i t i s a model host of choice f o r s t u d i e s on the (100) face (22). I t must, of course, be dyed out of nonaqueous media. Gypsum (CaSO^*2H2O) has a l s o been


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used f o r s p e c i a l purposes (24), and many other i o n i c compounds could presumably be t r i e d . CdS has a l s o been used as a model f o r p h o t o c o n d u c t i v i t y s e n s i t i z a t i o n (25) by cyanines, and could presumably be used f o r s u r f a c e s t u d i e s . L i k e AgBr, i t i s y e l l o w (blue absorbing) and an i n f r a r e d window m a t e r i a l . 6. F o l l o w i n g Kuhn and Mobius (16), e i t h e r prepared microscope s l i d e s o r cleaved s i n g l e c r y s t a l s can be used t o l i f t compressed monolayers o f surfactant-dyes o f f a water s u r f a c e . The spectrometer developed a t Marburg and Gottingen i s o f s p e c i a l i n t e r e s t f o r t h i s work. I n i t , a h a l f - d y e d s l i d e i s o s c i l l a t e d across a s i n g l e beam o f monochromatic l i g h t . The phase-detected s i g n a l c a r r y i n g the i n f o r m a t i o n (undyed area - dyed area) i s r a t i o e d against the absolute i n t e n s i t y o f the beam t r a n s m i t t e d through the undyed area. This d i f f e r e n t i a l spectrometer has the s p e c i a l c a p a b i l i t y o f measuring f i l m s i n which the dye i s h e a v i l y d i l u t e d w i t h non-absorbing f i l m formers such as a r a c h i d i c a c i d . 7. In cases ( 4 . ) , ( 5 . ) , and ( 6 . ) , the c r y s t a l face may be s e l e c t e d t o be a n i s o t r o p i c and to have a c a p a b i l i t y o f a l i g n i n g the long axes o f dye molecules i n a s i n g l e d i r e c t i o n i n the plane (24). A l t e r n a t e l y , a n o n - d i r e c t i n g face (111) o r (100) can be cut o f f - a x i s so that e t c h i n g produces l o n g , narrow t e r r a c e s . The t e r r a c e edges can then serve as l i n e s of n u c l e a t i o n f o r a l i g n e d aggregates (25). I n e i t h e r case, p o l a r i z a t i o n s p e c t r a can be o b t a i n ed and analyzed. One caveat here i s that g r a t i n g spectrometers have an i n t e r n a l p o l a r i z a t i o n (mostly from the d i f f r a c t i o n g r a t i n g ) which i s both s t r o n g and h i g h l y v a r i a b l e w i t h wavelength. This puts a premium on having a very good p o l a r i z e r and a l i g n i n g i t p a r a l l e l t o the pref e r r e d p o l a r i z a t i o n o f the monochromator. The only proper o p t i c a l sequence i s monochromator-polarizer-sample. Examples of a l l o f the above modes of o b s e r v a t i o n w i l l be presented i n the d i s c u s s i o n below. Spectroscopic observations on adsorbed dyes would be q u i t e incomplete without a s e t of s t r u c t u r a l models and equations t o process and i n t e r p r e t the data. C l a s s i c a l a d s o r p t i o n isotherms (1.) are of value, but t e l l nothing about the d e t a i l e d s t r u c t u r e of the adsorbed dye. Figure (3.) shows the a d s o r p t i o n of a v a r i ety o f cyanine (+) and merocyanine (uncharged) dyes (26). Although the s i l v e r h a l i d e s u r f a c e area i s not given f o r t h i s f i g u r e , dyes 1,2, and 4 are f a m i l i a r s e n s i t i z i n g m a t e r i a l s , and can be seen t o be occupying s i m i l a r areas on the a v a i l a b l e s u r f a c e . Dye 4 defi n i t e l y does not give a Langmuir isotherm, as i t shows a c l e a r i n f l e c t i o n p o i n t . I f we i n f e r that the s i l v e r iodobromide d i s p e r s i o n used here had predominately the o c t a h e d r a l (111) h a b i t




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and a h a l i d e c o n c e n t r a t i o n c l o s e to pure AgBr, then the three dyes c i t e d (1,2,4) can be presumed t o be adsorbed l o n g edge-on w i t h g r a p h i t i c packing. Herz e t a l . a r r i v e d a t a coverage o f 57A /molecule f o r dye / / l i n the edge-on c o n f i g u r a t i o n (20), and t h i s compares remarkably w i t h four l a t t i c e repeats on the (111) s u r f a c e g i v i n g an area o f 4x4.083x3.535& o r 57.73& . I f t h e i r c a l i b r a t i o n area of 57& h o l d s , then the s u r f a c e area of t h i s p a r t i c u l a r iodobromide d i s p e r s i o n i s determined from the s a t u r a t i o n coverage o f 1.38mg. dye/gm. AgBr* I t o be 1.04'loVmJ/gm. U n f o r t u n a t e l y , area estimates o f t h i s s o r t are o f t e n omitted i n s u r f a c e s t u d i e s p u b l i s h e d from photographic l a b o r a t o r i e s . Dye #2 i s expected t o occupy 5x4.083x3.535& = 72.2% per molecule ( 9 ) , and from i t one would c a l c u l a t e an emulsion area o f 1.41*10^cm?/ gm. I f one has t o choose between these e s t i m a t e s , i t should be noted that dye 2 i s a p l a n a r molecule, w h i l e dye 1 has r e c e n t l y been determined t o be a s e v e r e l y t w i s t e d molecule, having the two q u i n o l i n e r i n g s i n c l i n e d a t an angle o f 50.6° t o each o t h e r (27). Thus i t i s expected t o f a i l t o f i t i n t o the simple g r a p h i t i c packing scheme, and the i d e a l i z e d f i g u r e o f 57.1% /molecule i s rendered somewhat d o u b t f u l . 2

2

2

2


2

2

Dyes 2 and 4 have c l o s e l y s i m i l a r s t r u c t u r e s , and would be expected t o occupy the same areas on the (111) face. I n f a c t , the s u r f a c e coverage o f dye 2 i s only 1.6% h i g h e r i n moles/ (wt. of s u b s t r a t e ) than the coverage o f dye 4. One can note that the r a p i d l y r i s i n g p o r t i o n s of the p l o t s i n d i c a t e that dye 2 i s much more s t r o n g l y adsorbed than dye 4, a change which might w e l l be a t t r i b u t e d t o the e x t r a a d s o r p t i o n f o r c e s generated by f o r m a t i o n of a p a i r o f S«««Ag l i g a n d bonds when dye 2 i s adsorbed edge-on. The upward c u r v a t u r e corresponding t o the t r a n s i t i o n from f l a t - o n monomeric dye t o edge-on dye i s v i s i b l e f o r the oxacarbocyanine (4) but i s obscured a g a i n s t the y - a x i s f o r the t h i a c a r b o c y a n i n e as seen i n F i g u r e ( 3 . ) . +

Dye #3 i s a p a t h o l o g i c a l case o f a r e a l l y s e v e r e l y t w i s t e d molecule. I t cannot make a f a v o r a b l e monomeric c o n t a c t w i t h AgBr nor a f a v o r a b l e i n t e r m o l e c u l a r aggregate c o n t a c t . T h i s g i v e s us a c l e a r view o f the i n i t i a l r i s e o f a d s o r p t i o n corresponding t o the b i n d i n g of i s o l a t e d molecules on the s u r f a c e . Even a f t e r the sharp break i n t o some k i n d o f aggregate s t r u c t u r e , the approach to s a t u r a t i o n coverage i n the aggregated s t r u c t u r e i s very slow. One might expect the w e i g h t - s a t u r a t i o n coverage o f dye 3 t o l i e s l i g h t l y above t h a t f o r dye 1. Dye //5 i s an e s p e c i a l l y i n t e r e s t i n g case. I t d i f f e r s only t r i v i a l l y from dye 4 i n m o l e c u l a r l e n g t h and weight, and both dyes are p l a n a r o r n e a r l y so. One wishes that we had a b e t t e r understanding of the c l e a r formation o f a double sheet of c l o s e packed molecules by dye 5. I t would be a r e a l l y s t r i k i n g improvement i n the whole a r t of dye s e n s i t i z a t i o n i f we c o u l d even get e f f i c i e n t s e n s i t i z a t i o n from a close-packed b i l a y e r . But here we confront the d e f i c i e n c i e s o f c l a s s i c a l s u r f a c e measurements. We


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are l i t e r a l l y working i n the dark, and do not know the s t r u c t u r e s of the obviously close-packed aggregates of dyes 1-5. I f , f o r example, the b i l a y e r of dye 5 has such a s t r u c t u r e that i t s o p t i c a l a b s o r p t i o n i s b l u e - s h i f t e d so f a r that i t f a l l s under the e x i s t i n g blue a b s o r p t i o n and s e n s i t i v i t y of AgBr, then t h i s r e markable s t r u c t u r e i s of no immediate use a t a l l . But before we move to the examination of o p t i c a l r e s u l t s from methods 2-7, i t w i l l be necessary to develop the conceptual models f o r e v a l u a t i n g the data.


Conceptual Models: The s t r e n g t h of a b s o r p t i o n of a monomeric dye, measured as d i p o l e s t r e n g t h , i s conserved t o a good approximation upon going i n t o an aggregated monolayer s t r u c t u r e . The shape and the peak e x t i n c t i o n of an a b s o r p t i o n band of a dye w i l l change completely on aggregation, but the d i p o l e s t r e n g t h w i l l not. This leads to a k i n d of i n t e g r a l v a r i a n t of Beer's Law(22): j ^ > f

-

(1

f

>

Here A(A) i s the measured absorbance of a f i l m of dye on a s u r f a c e , A i s the wavelength i n any convenient dimensional unit, C i s the c o n c e n t r a t i o n of dye on the s u r face given i n millimoles/cm? (This i s the c o r r e c t u n i t of surface c o n c e n t r a t i o n to use w i t h the o r d i n a r y s o l u t i o n e x t i n c t i o n c o e f f i c i e n t w i t h u n i t s of ( m o l e s / l i t e r ) " c m } or ( m i l l i m o l e s / c m ? ) " c m . " ) . e(A) i s , of course, the measured s o l u t i o n e x t i n c t i o n c o e f f i c i e n t . The f a c t o r p i s an o r i e n t a t i o n f a c t o r , t a k i n g account of the s e l e c t i v e arrangement of molecules on the s u r f a c e . For a f i l m having complete p a r a l l e l o r i e n t a t i o n o f a l l molecular long axes, p = 3 f o r one sense of p o l a r i z e d l i g h t and p = 0 f o r the other. For p i a n o - o r i e n t a t i o n , the arrangement most commonly found, i n which there are i s l a n d s of aggregates o r i e n t e d i n s e v e r a l d i r e c t i o n s i n the s u r f a c e plane, p = 3/2. The d i p o l e s t r e n g t h does not appear i n t h i s equation, s

1

1

1

but the r i g h t hand ( s o l u t i o n ) i n t e g r a l I = J e-y- need only 39

be m u l t i p l i e d by the constant 9.185*10" t o give the d i p o l e s t r e n g t h i n esu cm. As may be seen i n f i g u r e (2) i t i s o f t e n convenient to give the value of the simple s o l u t i o n i n t e g r a l I i n u n i t s of molar e x t i n c t i o n c o e f f i c i e n t . This permits convenient comparisons between the peak e x t i n c t i o n c o e f f i c i e n t s of dyes and t h e i r i n t e g r a t e d e x t i n c t i o n s . This s o l u t i o n i n t e g r a l can be approximated s a t i s f a c t o r i l y by a sum I = AA Z e (2) 2

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done w i t h a computerized spectrophotometer o r simply by hand c a l c u l a t o r from an e x i s t i n g spectrum. 2. An aggregate s t r u c t u r e does not have A s i n g l e spectrum, but r a t h e r a size-dependent a r r a y of s p e c t r a . For simple l i n e a r column aggregates, an approximate equation f o r the large spectral s h i f t s i s Av = A v ^ N - D / N where N = number of molecules (28). For the two-dimensional aggregates more commonly seen, the observed s p e c t r a l s h i f t depends on both the l e n g t h and the width o f the aggregate. The general method of c a l c u l a t i n g size-dependent s p e c t r a l s h i f t s w i l l be g i v e n below. Even s o , one r o u t i n e l y observes s p e c t r a l s h i f t i n g w i t h s i z e , and the equation j u s t g i v e n i s a q u a l i t a t i v e conceptual a i d . This i s c e r t a i n l y a s i m p l e r and more d i r e c t method f o r a n a l y z i n g the s i z e s of aggregate i s l a n d s than f u l l s c a l e e l e c t r o n d i f f r a c t i o n methods.


N

3. The d i r e c t i o n o f the a b s o r p t i o n d i p o l e s of i n d i v i d u a l molecules i s a l s o conserved as the molecules become i n c o r porated i n t o an aggregate ( 9 ) . The aggregate has a twodimensional u n i t c e l l , and the number and arrangement o f molecules i n t h i s c e l l determine the number of e l e c t r o n i c t r a n s i t i o n s which w i l l be seen. When there i s only a s i n g l e molecule i n the c e l l , we observe the s i n g l e sharp a b s o r p t i o n of an H- (hypsochroraic o r b l u e - s h i f t e d ) aggregate o r a J-aggregate (red s h i f t e d ) . When two o r more molecules l i e i n the c e l l w i t h t h e i r long axes p a r a l l e l , m u l t i p l e a b s o r p t i o n bands w i l l be seen, a l l having the same sense o f l i n e a r p o l a r i z a t i o n on a d i r e c t i n g substrate. When the m u l t i p l e molecules i n the c e l l are n o n - p a r a l l e l (herringbone, V- o r X-packing), the m u l t i p l e a b s o r p t i o n bands w i l l have two p e r p e n d i c u l a r senses o f p o l a r i z a t i o n (29). I n the s i m p l e s t case of herringbone packing of two molecules i n the u n i t c e l l , the r a t i o of i n t e n s i t i e s of the two p e r p e n d i c u l a r l y p o l a r i z e d a b s o r p t i o n bands w i l l be given simply by I|/*w t a n 6 , where 0 i s the angle between the l i n e o f advance o f the herringbone and the i n d i v i d u a l molecular long axes. The spectrum of a herringbone s t r u c t u r e and the corresponding h y p o t h e t i c a l s t r u c t u r e are shown i n Figures (8a,c). Note, however, the above simple i n t e n s i t y equation r e f e r s ONLY t o the i n t e g r a t e d band i n t e n s i t i e s and not t o the peaks. Since the two bands u s u a l l y o v e r l a p , i t may r e q u i r e a b i t o f i m a g i n a t i o n t o e x t r a c t an accurate value of 0 . This i s c e r t a i n l y the s i m p l e s t p o s s i b l e route t o a s t r u c t u r a l parameter that one can imagine. =

2

4. There i s a r e l i a b l e method o f c a l c u l a t i n g the s p e c t r a l s h i f t s of aggregates of known s t r u c t u r e . The o v e r s i m p l i f i e d p o i n t - d i p o l e approach t o t h i s problem f a i l s q u a n t i -



t a t i v e l y ( 9 ) , but the p o i n t - d i p o l e expression f o r a s i n gle column of aggregated molecules i s i n s t r u c t i v e :


AE(N)

= —.

2

(1 - 3cos a)(N- 1)/N

(3 )

Here D i s the d i p o l e s t r e n g t h , h i s Planck's constant, r i s the s e p a r a t i o n between centers of adjacent molecules, a i s the angle between the l i n e of centers of molecules and the long a x i s of any molecule, and N i s the s i z e of the aggregate. Figure (4.) represents the p r e d i c t i o n of (3.) f o r a dimer aggregate. We see from t h i s simple equation that the s p e c t r a l s h i f t w i l l be to the blue f o r a p e r p e n d i c u l a r s t a c k of molecules (deck-of-cards i n the box) but to the red i f the molecules r o t a t e s u f f i c i e n t l y about the l i n e of centers ( s l i p p e d deck-of-cards). This can e x p l a i n the s p e c t r a l s h i f t s of +100nm from the s o l u t i o n monomer peak seen w i t h H- and J-aggregates. What must be s u b s t i t u t e d f o r the p o i n t - d i p o l e theory i s a gene r a l quantum mechanical p e r t u r b a t i o n theory of s p e c t r a l s h i f t i n g i n ordered c l u s t e r s of molecules. This t r e a t ment has been given by Norland, Ames, and T a y l o r (30), and has been e l a b o r a t e d by Reich (29,31). To c a l c u l a t e the s p e c t r a l s h i f t i n g of an aggregate, one must f i r s t produce the ground s t a t e and f i r s t e x c i t e d s i n g l e t s t a t e wavefunctions of an i s o l a t e d cyanine dye molecule. This can be done w i t h i n c r e a s i n g l e v e l s of s o p h i s t i c a t i o n by Hiickel LCA0-M0, P a r i s e r - P a r r - P o p l e , or CNDO methods. The t r a n s i t i o n d i p o l e f o r the SQ S\ absorption i s c a l c u l a t e d and compared w i t h the observed d i p o l e s t r e n g t h . The discrepancy i s u s u a l l y only a few percent when working w i t h cyanine chromophores, and i s used to r e f i n e the r e s u l t of the next s t e p . Using the c a l c u l a t e d wavef u n c t i o n s , the t o t a l t r a n s i t i o n d i p o l e i s d i v i d e d i n t o t r a n s i t i o n d e n s i t i e s l o c a l i z e d on the atoms of the molecule. P h y s i c a l l y , these t r a n s i t i o n d e n s i t i e s represent the f r a c t i o n a l e l e c t r o n i c charges which flow to and then away from p a r t i c u l a r atoms during s u c c e s s i v e h a l f c y c l e s of the r a d i a t i o n f i e l d which d r i v e s the a b s o r p t i o n process. The i n t e r a c t i n g molecules are now arranged i n the aggregate geometry, and an instantaneous Coulombic i n t e r a c t i o n energy i s c a l c u l a t e d . Figure (5) shows the two p o s s i b l e e x c i t e d s t a t e i n t e r a c t i o n s between a p a i r of s t i c k - m o l e c u l e s , and Figure (6) shows the t r a n s i t i o n dens i t i e s c a l c u l a t e d f o r a model t h i a c a r b o c y a n i n e . The c a l c u l a t i o n of i n t e r a c t i o n energies i s accomplished by t r e a t i n g the t r a n s i t i o n d e n s i t i e s as though they were s t a t i c Coulombic charges and a l l o w i n g a l l p o s s i b l e atomto-atom i n t e r a c t i o n s to occur between molecules according to the Coulombic i n t e r a c t i o n between p o i n t charges. The m a t r i x of i n t e r a c t i o n energies i s d i a g o n a l i z e d , g i v i n g



1.

BIRD

Silver Halide

17

Photography

ALIGNMENT

OF TRANSITION D l P O L E S

(OR M O L E C U L A R LONG

AXES)

R E L A T I V E TO LINE - O F - C E N T E R S Figure 4. Qualitative predictions from the point-dipole expression (Equation 3) for spectral shifting of dimers. Dimer formation splits the excited state into two levels, one which allows transition to the ground state, and one which is silent for the case of parallel molecules in the dimer. The spectral shift depends on the stacking angle of the pair of molecules. The slipped structure is the precursor of the red shifted J-aggregate, a desirable structure for photographic sensitization. (Reproduced, with permission, from Ref. 9. Copyright 1968, Society of Photographic Scientists and Engineers.)


18


IN PHASE MOOE

OUT OF PHASE MOOE


AMPLtTUDE|

M PHASE + OUT OF PHASE Figure 5. "Stick" molecules in the possible modes of interaction during an optical transition, with transition density represented by vertical displacement from the horizontal sticks. Right, the transition densities cancel each other and are silent, and left, the transition densities add so that the dimer becomes a supermolecule with a single allowed optical transition having the combined dipole strength of the two participating molecules. (Reproduced, with permission, from Ref. 30. Copyright 1973, Society of Photographic Scientists and Engineers.)

Y

BTCC Figure 6. Transition density values calculated for a thiacarbocyanine sensitizer by the Hiickel LCAO MO method. Transition densities are localized on the conjugated atoms of the chromophore. The numbers represent the values in units of electron charge. The transition densities on the opposite side of the molecule differ only by a change of sign, and have not been reproduced. The transition densities alternate small, large, small, large, etc. along the chromophore; one of the values is actually being negative and cancelling some of the overall dipole strength of the transition. (Reproduced, with permission, from Ref. 30. Copyright 1973, Society of Photographic Scientists and Engineers.)


1.

5/7ver Halide

BIRD

Photography

the set o f N e x c i t e d s i n g l e t energies f o r an aggregate o f N molecules. As shown by Norland e t a l . t h i s d i a g o n a l i z e d m a t r i x a l s o s u p p l i e s the c o e f f i c i e n t s which give the c o n t r i b u t i o n s (+ o r -) o f the i n d i v i d u a l molecules t o the t r a n s i t i o n d i p o l e o f the whole a r r a y . Thus the c a l c u l a t i o n gives the energy s h i f t s (wavelength s h i f t s ) and the i n d i v i d u a l d i p o l e strengths o f the array o f N p o s s i b l e S -** S\ t r a n s i t i o n s belonging t o the N-molecule aggregate.


0

The t r a n s i t i o n d e n s i t y of a l a r g e aggregate w i l l o f t e n concentrate i n one o r a few o f the N t r a n s i t i o n s . When t h i s happens, the aggregate i s behaving as a k i n d o f super-molecule. We note, f o r example, t h a t the r a d i a t i v e l i f e t i m e f o r such an aggregate t r a n s i t i o n may be s h o r t e r by a f a c t o r n e a r l y N" than the r a d i a t i v e l i f e t i m e o f a s i n g l e molecule f o r aggregates o f modest s i z e . This c a l c u l a t i o n o f energy s h i f t s and i n t e n s i t i e s gives s u r p r i s i n g l y good r e s u l t s , r e l a t i v e l y independent o f the d e t a i l s of the molecular o r b i t a l s used t o generate the wavef u n c t i o n s . This seems t o be a k i n d of t h e o r e t i c a l reward for c o n c e n t r a t i n g on t r u l y allowed t r a n s i t i o n s o f simple, non-degenerate molecules- the p r e c i s e s i t u a t i o n i n which p e r t u r b a t i o n c a l c u l a t i o n s ought t o work w e l l . A program for e x e c u t i n g t h i s k i n d o f aggregate c a l c u l a t i o n may be found i n the appendix o f Reich's t h e s i s (32). This work r e q u i r e s a l a r g e d i g i t a l computer, but the cost o f comput a t i o n need not be high unless one i n s i s t s on doing l a r g e aggregates. 1

5. The e p i t a x i a l hypothesis becomes i n t e r l o c k e d w i t h t r a n s i t i o n d e n s i t y c a l c u l a t i o n s when one considers the cost and the p r a c t i c a l d i f f i c u l t i e s of " f i s h i n g f o r an unknown aggregate s t r u c t u r e by doing an extended s e r i e s o f c a l c u l a t i o n s on d i f f e r e n t s t r u c t u r e s . I f e p i t a x i a l attachment of a dye has occurred, there w i l l be only a handful of possible structures (just 3 for a primitive unit c e l l a l i g n e d along [110] i n t e r c e p t s on the (111) face o f AgBr, where the g r a p h i t i c coincidence favors a p a r t i c u l a r o r i e n t a t i o n o f molecular axes). I n t h i s s i t u a t i o n , one does a l i m i t e d number o f t r a n s i t i o n d e n s i t y c a l c u l a t i o n s f o r the a l t e r n a t e p o s s i b i l i t i e s . However, t h i s i s no guarantee that e p i t a x i a l attachment a c t u a l l y has occurred. I t does seem to be a common occurrence, and some o f the conf i r m a t o r y i n d i c a t i o n s o f e p i t a x i a l attachment are: 1 1

a. The dye spectrum s h i f t s o r s p l i t s when dye i s adsorbed on a new face of a f a m i l i a r host. (AgBr (100)) (29,31).


20


b. The dye spectrum s h i f t s i n a p r e d i c t a b l e way when the host l a t t i c e parameter i s changed, as i n comp a r i s o n s p e c t r a on the (100) faces of AgCl and AgBr ( 3 3 ) .


c. The dye tends t o be attached more f i r m l y i n the e p i t a x i a l mode, and i t becomes more d i f f i c u l t t o reorganize s m a l l i s l a n d s i n t o l a r g e sheet aggregates. Consequently, s m a l l s p e c t r a l s h i f t s are observed when steps such as i n c u b a t i o n at s l i g h t l y e l e v a t e d temperature are taken ( 2 1 ) . d. Phase t r a n s i t i o n s are sometimes observed, c o r r e s ponding t o g l i d i n g movements o f the close-packed molecules (21). These movements produce s t r i k i n g changes i n the a b s o r p t i o n spectrum, but g i v e no change a t a l l i n s u r f a c e coverage and l i t t l e change i n the s t r e n g t h of attachment. When these phase t r a n s i t i o n s do occur on w e l l - c h a r a c t e r i z e d f a c e s , the s p e c t r a l s h i f t observed i n the phase t r a n s i t i o n i s p r e d i c t e d by the t r a n s i t i o n d e n s i t y c a l c u l a t i o n s and the e p i t a x i a l hypothesis (31). e. Once one begins to understand the nature of these g l i d i n g phase t r a n s i t i o n s , i t becomes p o s s i b l e t o understand the c r i t i c a l r o l e of "indexing ' groups. These are minor chemical s u b s t i t u e n t s which protrude out of the molecular plane and so prevent the formation of one o r more o f the e p i t a x i a l structures. 1

In the d i s c u s s i o n s below, we s h a l l see numerous examples of the r e s u l t s of e p i t a x i a l attachment. Figure (7) shows the r e f l e c t i o n s p e c t r a o f t h i c k , nontransm i t t i n g l a y e r s of A g B r - g e l a t i n d i s p e r s i o n s c o n t a i n i n g the same amounts of AgBr and i n c r e a s i n g amounts o f dye (2,20). The r e f l e c tance s p e c t r a (here given as -Log^R^) may be converted back t o R^, and processed according t o the Kubelka-Munk equation (4) . K _

(1-Rco)

s"

-251

2

e

n

.

( 4 )

-°s

Here Ro, i s the r e f l e c t a n c e (at wavelength A) from an o p t i c a l l y t h i c k l a y e r o f a pigment coated w i t h the s e n s i t i z i n g dye, c i s a volume c o n c e n t r a t i o n of dye i n the suspension, K and S are the a b s o r p t i o n and s c a t t e r c o e f f i c i e n t s o f the suspension (as d e t e r mined by o b s e r v a t i o n of R** . Thus, a t a given wavelength, Roo i s measured, g i v i n g K./S, and S may be determined f o r the same emuls i o n by adding a non-adsorbing dye of known e . A study of t h i s equation shows that i t does l e a d to d e t e r m i n a t i o n o f reasonable values of £agg. f ° aggregate s p e c i e s . C e r t a i n a b s o r p t i o n bands r


Silver Halide

Photography


BIRD

400

500 Wavelength

Figure 7a.

600 nm

Optical data of microcrystalline suspensions with added dye.

AgBr is dispersed in gelatin at constant concentration, and the dyes are added as shown. Reflectance is measured from a dispersion layer so thick that no radiation emerges from the far side giving R . The red-shifted aggregate is the dominant species until a saturation coverage is approached. Beyond this saturated and close-packed monolayer, additional dye simply dissolves in the suspending medium. (Reproduced, with permission, from Ref. 2. Copyright 1977, MacMillan Publishing Co., Inc.) x


22



/ / / / / / /

400

500

600

Wavelength (nm) Figure 7b.

Optical data of microcrystalline suspensions with added dye.

AgBr is dispersed in gelatin at constant concentration, and the dyes are added as shown. Reflectance is measured from a dispersion layer so thick that no radiation emerges from the far side, giving R . This dye, with six methyl groups projecting out of the chromophobe plane, absorbs first as isolated monomer molecules (M„), and then forms an aggregate with red- and blue-shifted bands at higher total dye concentrations. (Reproduced, with permission, from Ref. 2. Copyright 1977, MacMillan Publishing Co., Inc.) x


1.

BIRD

Silver Halide

Photography

23

( e s p e c i a l l y the r e d - s h i f t e d " J " band) do l e v e l o f f o r s a t u r a t e i n i n t e n s i t y at some r a t i o o f m i l l i m o l e s o f dye per mole o f subs t r a t e . I n t h i s way o p t i c a l a d s o r p t i o n isotherms can be d e t e r mined, and used t o show the s a t u r a t i o n coverage and the s u r f a c e area per molecule, as f o r the red s h i f t e d " J " aggregate o f 1,1'diethy1-2,2'cyanine iodide~ i n f i g u r e 7a.


+

The pseudoindole carbocyanine o f f i g u r e (7b) i s a good examp l e of a dye having too many non-planar groups to be able t o pack w e l l i n a g r a p h i t i c aggregate. I t thus shows a s u r f a c e monomer band ( f l a t - o n adsorbed dye on AgBr, w i t h a modest red s h i f t from the s o l u t i o n monomer. This red s h i f t i s caused by i n t e r a c t i o n o f the i s o l a t e d molecules w i t h the o p t i c a l d i e l e c t r i c constant (34) of the s i l v e r bromide adsorbent). At much higher c o n c e n t r a t i o n s i t then shows a m u l t i p l e banded aggregate which i s not a simple H- o r J - s t r u c t u r e . Figure (8) shows a b s o r p t i o n s p e c t r a of AgBr d i s p e r s i o n s as obtained from measuring the r e f l e c t i o n and t r a n s m i s s i o n of a c t u a l photographic coatings (method 3). The most s t r i k i n g d i f f e r e n c e between the a d s o r p t i o n o f t h i s dye on c u b i c and on o c t a h e d r a l faces i s t h a t the r a t i o o f i n t e n s i t i e s o f the two a b s o r p t i o n bands i s constant on the cubic face but v a r i e s markedly on the o c t a h e d r a l face (35). This suggests that a s i n g l e aggregate i s formi n g on the cube face, whereas the f a m i l i a r " J " aggregate w i t h reds h i f t e d a b s o r p t i o n i s forming on the o c t a h e d r a l face a t higher dye c o n c e n t r a t i o n s . Simpson has confirmed by d e t a i l e d luminescence measurements t h a t the cubic a b s o r p t i o n peaks a t 590nm and 525nm match a p a i r of e x c i t a t i o n peaks which give a common output spectrum and i d e n t i c a l luminescence decay curves. T r a n s i t i o n d e n s i t y c a l c u l a t i o n s by Reich have shown that these double a b s o r p t i o n bands can be described i n d e t a i l by assuming V-packed "herringbone" s t r u c t u r e s f o r e p i t a x i a l l y attached aggregates (29). His r e s u l t s f o r a r e l a t e d dye, 3,3'-dicarboxyethyl- 5 , 5 ' - d i c h l o r o t h i a c a r b o cyanine, are shown i n Table I . The f a c t that a c l a s s o f t h i a c a r bocyanine dyes shows t h i s common s e n s i t i v i t y t o the p a r t i c u l a r c r y s t a l face o f the AgBr s u b s t r a t e has always seemed t o us t o be a proof o f e p i t a x i a l attachment. Reich's f i n d i n g o f a p a r t i c u l a r dye which gives double-banded a b s o r p t i o n s p e c t r a on both cubic and o c t a h e d r a l f a c e s , but w i t h a l a r g e s h i f t i n a b s o r p t i o n maxima (Table I) strengthens t h i s c o n c l u s i o n . The probable presence o f more than one aggregate s t r u c t u r e i n e p i t a x i a l attachment on the (111) face o f AgBr l a y s open the poss i b i l i t y that monolayer phase t r a n s i t i o n s might occur. The three most obvious s t r u c t u r e s a v a i l a b l e t o a 5 , 5 ' - d i s u b s t i t u t e d dye on t h i s face are shown i n F i g u r e (9). These are c h a r a c t e r i z e d by s l i p angles o f 60°, 30°, and 19°06' and by p r e d i c t e d a b s o r p t i o n maxima o f 477nm, 643nm, and 653nm as c a l c u l a t e d by the t r a n s i t i o n d e n s i t y method (31) r e l a t i v e t o a monomer a b s o r p t i o n peak at 579nm ( i n a h y p o t h e t i c a l medium o f o p t i c a l d i e l e c t r i c constant matching



I

i

I

1

I

1

500

550

600

650


a

400

450

Wovelength (nm) -7"

1

"7 "

1

i

i

mg dye/Ag mole

b /

U — 800

^

^^H-200 \^-VVl00

400

1 450

1 500

i

*!

550

600

> 650

Wavelength (nm) Figure 8. Wavelength vs. absorptance for a normal photographic coating of a thiacarbocyanine dye on cubic crystals (a), and on octahedral crystals of AgBr • / (2.5% I) (b). The rising absorption to the left of each plot is the intrinsic absorption of the silver halide. (Reproduced, with permission, from Ref. 35. Copyright 1974, Society of Photographic Scientists and Engineers.)



1.

BIRD

Silver Halide

25

Photography

AgBr (100) Figure 8c. A cubic herringbone structure for a thiacarbocyanine dye unsubstituted in the 5,6,5',6' positions. (Reproduced, with permission, from Ref. 29. Copyright 1974, Society of Photographic Scientists and Engineers.)

Table I C a l c u l a t e d and Observed S p e c t r a of Herringbone Aggregates on Various Faces of AgBr Crystal Face

Molecular Length i n L a t t i c e Repeats

Slip Angle

*max (calc)

Rel. Int. (calc)

582 669

3.09 1

593 649

2.9 1

60°

433 485

1 3.04

(111)

60°

441 473

1 3

(100)

35°50

(100)

35°50

(111)

30°

(111)

30°

(Ill)

6

f

f

550(avg.) 598

2.15 1

560(avg.) 592

2.05 1

r(obs)

Rel. Int.

600 645

525 590

Table I i s computed from the r e s u l t s o f r e f e r e n c e s 29,32, and 35. The term " ( a v g . ) i n d i c a t e s that the o r i g i n a l c a l c u l a t i o n produced a c l u s t e r of c l o s e d spaced a b s o r p t i o n s . Since t h i s c l u s t e r i s an a r t i f a c t of the p a r t i c u l a r aggregate geometry c a l c u l a t e d (2 columns of herringbones of s i z e 2x10 molecules on the (100) f a c e ) , a weighted average wavelength i s given. 11



26


Figure 9.

The octahedral (111) face of AgBr covered with three possible ordered, epitaxial monolayers of adsorbed dye.

The dots represent Ag' ions in lattice positions, with nearest neighbors being separated by 4.083A. The lines of Ag* ions are separated by 3.535A, a distance very close to the normal packing distance of 3.4A between aromatic planes in crystals, including graphite. Thus, planar molecules can form any of these structures while preserving their optimal packing dimension. The top structure is predicted to give a blue shift of absorption, the middle structure to give a red shift, and the bottom structure a larger red shift. The molecular length shown is appropriate to a 5,6-unsubstituted thiacarbocyanine dye, which covers 5 X 4.083 X 3.535A — 72.2A . These structures can explain many of the observations on thiacarbocyanine red sensitizers on octahedral surfaces, including the observation of gliding phase transitions among the structures. (Reproduced, with permission, from Ref. 8. Copyright 1974, Society of Photographic Scientists and Engineers.) 2

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the i n t e r i o r of an aggregate). Rosenoff e t a l . have d e s c r i b e d a set o f o b s e r v a t i o n s on a p a i r o f dyes which d i f f e r from Reich's herringbone-forming dye only by the a d d i t i o n o f 9 - e t h y l o r 9methyl groups (21). As observed by combined r e f l e c t a n c e and transmittance measurements on photographic c o a t i n g s , the 9 - e t h y l dye was seen t o be a w e l l behaved red s e n s i t i z e r , always g i v i n g the r e d - s h i f t e d J-aggregate a t 648-650nm, as seen i n F i g . (10). In t o t a l c o n t r a s t , the 9-methyl dye had gained a c e r t a i n n o t o r i e t y by g i v i n g i r r e p r o d u c i b l e r e s u l t s which came t o be known as "the day of the week e f f e c t " . T h i s p e c u l i a r behavior was shown t o be due t o a g l i d i n g phase t r a n s i t i o n from s m a l l b l u e - s h i f t e d 60° aggregates t o r e d - s h i f t e d aggregates, now assigned t o the more s t r o n g l y s h i f t e d 19° s t r u c t u r e . I t was a l s o shown t h a t the growth of the 19° aggregates f o l l o w e d a u t o c a t a l y t i c k i n e t i c s , g i v i n g i n i t i a l growth curves which were concave upward. Since the i n t e r molecular c o n t a c t s between the f i r s t two members o f a 19° J aggregate form only a fragmentary c o n t a c t between halves o f the molecules, one might expect that the s m a l l aggregates would be unstable w i t h respect t o both the 60° aggregate and the l a r g e , nested multicolumn 19° J-aggregate. I t ' s i n t e r e s t i n g t o note that t h i s s o r t o f g l i d i n g phase t r a n s i t i o n occurs w i t h no change i n the s u r f a c e area occupied by the dye. Thus l i t t l e o r no change would be seen on an isotherm as i n F i g . ( 3 ) . The 5 , 5 ' - d i c h l o r o s u b s t i t u t e d dyes have molecu l a r lengths o f 21.lX ( i n c l u d i n g the 1.8& van der Waals r a d i i o f the t e r m i n a l c h l o r i n e s ) and so must occupy 6_ x 4.083& along the [110] edge on the (111) face of AgBr ( 7 ) . Given an edge separat i o n o f 3.5358, the p r e d i c t e d s u r f a c e coverage on (111) AgBr i s 86.60R f o r any of the s t r u c t u r e s shown. Reich e t a l . have s u r veyed the a v a i l a b l e s t r u c t u r e s on the (111) and (100) faces of AgBr and have assigned the o c t a h e d r a l a b s o r p t i o n s a t 620nm and 648nm as the 30° and 19°06' s t r u c t u r e s f o r 6 u n i t dyes respect i v e l y (31). The l o n g e s t observed a b s o r p t i o n wavelength (660nm) i s assigned t o the cube face. Here i t i s amusing t o note that two h y p o t h e t i c a l aggregate s t r u c t u r e s on the cube face p r e d i c t almost e x a c t l y the same aggregate a b s o r p t i o n wavelength and the same s u r f a c e coverage. One o f these i s a s t r u c t u r e propagating along the [310] edge proposed by Smith (36), the other i s propag a t i n g along the [110] edge as proposed by the w r i t e r (37). One ought not t o be too dogmatic as t o which e p i t a x i a l s t r u c t u r e may be o c c u r r i n g , even though the f a c t s s t r o n g l y support e p i t a x i a l attachment i n a p a r t i c u l a r case. The two cube s t r u c t u r e s a r e p r e d i c t e d t o occupy e x a c t l y the same area of 3a£ = 95.4A* per molecule. Both s t r u c t u r e s may one day be e s t a b l i s h e d as o c c u r r i n g f o r p a r t i c u l a r dyes. At the moment we have no means f o r d i f f e r e n t i a t i n g between them. 2

2

The molecular d e t a i l s o f the s u r f a c e contacts between AgBr and these same carbocyanine dyes have been e x p l o r e d through the use o f a c r y s t a l s t r u c t u r e d e t e r m i n a t i o n by Potenza and Mastro-




401

500

too

;oo

WAVELENGTH (nm) Figure 10a. Results of coating matching octahedral AgBr 1(1% I) emulsions on clear acetate film base and measuring %R, %T. On incubation at 38°C, the red-shifted aggregates are simply ordered increasingly with time, and a modest red shift occurs. The central ethyl group prevents formation of the 60° structure. (Reproduced, with permission, from Ref. 21. Copyright 1970, Society of Photographic Scientists and Engineers.)


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WAVELENGTH (nm) Figure 10b. Results of coating matching octahedral AgBr 1(1% I) emulsions on clear acetate film base and measuring %R, %T, which undergoes a gliding phase transition presumed to be from the 60° structure to the 19° structure. Figure 10b differs from Figure 10a by the central 9-ethyl group found on the structure of Figure 10a. (Reproduced, with permission, from Ref. 21. Copyright 1970, Society of Photographic Scientists and Engineers.)



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paolo (38). Given the p a r t i a l l y random atomic c o n f i g u r a t i o n of t h i s s u r f a c e , contact s t r u c t u r e s can be proposed which accommodate a v a r i e t y of d i f f e r e n t l y charged dyes and r e s t o r e the e l e c t r i c a l n e u t r a l i t y of the combined AgBr-dye s u r f a c e ( 7 ) , as shown i n F i g . (11). The use of the hypothesis of e p i t a x i a l attachment has been c r i t i c i z e d on the grounds that seemingly continuous a b s o r p t i o n s h i f t s occur as the host or the dye i s changed ( 2 ) . This i s part i c u l a r l y l i k e l y to occur when a host i s taken through a domain of f a c i a l i n s t a b i l i t y (62). One can, indeed, observe continuous s p e c t r a l s h i f t s under these circumstances, but the continuous s h i f t s can be e x p l a i n e d i n terms of a confusing s u p e r p o s i t i o n of s i z e s h i f t s (N-l)/N and of the a c t u a l f a c i a l t r a n s f o r m a t i o n s h i f t . In a p o o r l y c h a r a c t e r i z e d f a c i a l system i t i s e s s e n t i a l l y impossible to prove or disprove e p i t a x i a l attachment. It i s also i n c o r r e c t to argue f o r continuous s p e c t r a l s h i f t i n g from observ a t i o n s of thiacarbocyanines on the (111) face when the cyanines are of such molecular l e n g t h that d i f f e r e n t members of the set occupy d i f f e r e n t numbers of l a t t i c e p o s i t i o n s . In g e n e r a l , the e p i t a x i a l hypothesis i s a very economical e x p l a n a t i o n of a l a r g e set of phenomena. However, these are very r e a l exceptions to the a c t i o n of the hypothesis and many of these are discussed i n t h i s paper. The use of s i n g l e c r y s t a l s of the s i l v e r h a l i d e s as model systems f o r the study of a d s o r p t i o n and s u r f a c e o r d e r i n g has a l ready been suggested. With c r y s t a l s which have been c a r e f u l l y prepared to maximize the displacement of conduction e l e c t r o n s (schubweg), the d y e - s e n s i t i z e d p h o t o c o n d u c t i v i t y of the c r y s t a l s can be measured, and r e l a t i v e quantum y i e l d s f o r charge c a r r i e r generation can be determined (39). The y i e l d s are r e l a t i v e i n the sense that absolute quantum y i e l d s can be determined f o r d i r e c t a b s o r p t i o n of blue and UV l i g h t i n the body of the c r y s t a l , and then the dye a b s o r p t i o n spectrum and c a r r i e r generation can be compared to the i n t r i n s i c y i e l d s . Figure (12) shows the r e s u l t s of Piechowski on two dyeings at q u i t e d i f f e r e n t coverages w i t h a w e l l known dye, 1 , 1 ' , 3 , 3 - t e t r a e t h y l - 5 , 5 , 6 , 6 ' - t e t r a chlorobenzimidazolocarbocyanine" ~ i o d i d e " (40). The f i r s t of these i s dyed to a coverage of a s i n g l e close-packed monolayer, and shows s u b s t a n t i a l l y p e r f e c t p a r a l l e l i s m between the a b s o r p t i o n spectrum and the current y i e l d ( a c t u a l l y measured as a chargedisplacement s i g n a l i n a c a p a c i t o r ) . This i s c l e a r l y an e p i t a x i a l monolayer, as t h i s dye tends to form a J-aggregate w i t h somewhat d i f f e r e n t a b s o r p t i o n maximum (594nm) on l e s s d i r e c t i n g s u r f a c e s . ,

l

,

The second dyeing i s e q u i v a l e n t to about four monolayers, and the s h i f t of a b s o r p t i o n maximum toward 594nm can be noted. Gardner and Herz have noted that m u l t i l a y e r formation w i t h t h i s dye i s s t r o n g l y favored by the presence of I " (40b). The mechanism of a s s i s t a n c e of m u l t i l a y e r i n g i s not c l e a r . I t could be a



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0 [

m

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-

A|Br

Figure 11. Hypothetical packing arrangement for a thiacarbocyanine dye on a [110] edge on the (111) face of AgBr from a crystal structure determination. The circles represent surface positions for Ag* ions. In the absence of the dye, half of the Ag* positions are filled randomly. When an attractive ligand interaction is established between the heterocyclic sulfurs of the dye and two Ag" ions, position, A,A' are filled, and B,B' must be emptied to remove a repulsive interaction between the 7,7' protons of the dye and the Ag" ions. Positions C,C will be emptied to maintain local electrical near-neutrality if the dye is a cationic carbocyanine. If a charge-compensated betaine dye (e.g. monosulfopropyl) is added, one of the C C positions may he filled, or if an anionic dye (N W-disulfopropyl) is added, both may be filled. (Reproduced, with permission, from Ref. 1. Copyright 1974, Society of Photographic Scientists and Engineers.) %

t


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Colloids and Surfaces in Reprographic Technology - American

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