EXCITON-POLARITON AND CHARGE TRANSFER STATES IN POLYMORPHS OF QUASI-METALLIC CRYSTALS OF DYES

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EXCITON-POLARITON AND CHARGE TRANSFER

STATES IN POLYMORPHS OF QUASI-METALLIC

CRYSTALS OF DYES

M. Tristani-Kendra, J. Delaney, C. Eckhardt

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C7, supplriment au nOIO, Tome 46, octobre 1985 _{page C7-425}

EXCITON-POLARITON A N D C H A R G E T R A N S F E R S T A T E S

I N

P O L Y M O R P H S OF QUASI-METALLIC C R Y S T A L S OF D Y E S

If. Tristani-Kendra, J. Delaney and C.J. Eckhardt

Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304,

U.

S. A.

A b s t r a c t

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The r e f l e c t i o n spectra o f d i f f e r e n t c r y s t a l l i n e forms o f two dyes, pseudoisocyanine i o d i d e (PIC) and 2,4-bis(4-diethylami no-2-hydroxy

phenyl ) c y c l o b u t a d i e n e d i y l i um-1 , 2 - d i o l a t e (HEAPS), have been i n v e s t i g a t e d i n order t o g a i n understanding o f dye aggregation. The study i n v o l v e s c l a r i f i c a t i o n of e a r l i e r c o n f l i c t i n g r e s u l t s f o r PIC and t h e examination o f polymorphs o f HEAPS. The discovery of another c r y s t a l l i n e form o f PIC i s a l s o reported. C r y s t a l spectra a r e presented which i n d i c a t e the p o s s i b l e r e l a t i o n t o the J-band observed i n dye aggregation.

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INTRODUCTION

From the i n i t i a l s t u d i e s o f J e l l e y ( I ) , the phenomenon o f dye aggregation has been of spectroscopic i n t e r e s t . The d e s c r i p t i o n o f the J-band observed when aggregation occurs has been w e l l r a t i o n a l i z e d by using e x c i t o n theory. U n f o r t u n a t e l y , i t has r e s i s t e d complete e x p l a n a t i o n and remains y e t o f i n t e r e s t . One o f the primary con- s i d e r a t i o n s i s t h e s t r u c t u r e o f t h e aggregate i t s e l f since t h i s w i l l have s i g n i f i - cant bearing on t h e e n e r g e t i c s and dynamics o f the excitons.

Since the e x c i t o n approach seems t o be t h e most e f f e c t i v e , i t has been n a t u r a l t o l o o k t o t h e spectra o f c r y s t a l s o f dyes t o see i f t h e J-band appears i n t h e spectra of t h e s o l i d . A seminal i n v e s t i g a t i o n pursuing t h i s idea was focussed on PIC by M a r c h e t t i and co-workers ( 2 ) . The r e f l e c t i o n spectra o f t h e dye c r y s t a l were studied. While an i n t e n s e r e f l e c t i o n was observed a t the frequency o f t h e PIC J- band, i t d i d n o t possess the narrow bandwidth o r expected stopping band behavior. I n a l a t e r study, Tanaka and co-workers ( 3 ) repeated and extended t h e previous r e - f l e c t i v i t y measurements on PIC t o o t h e r faces. This r e s u l t e d i n discovery of a b-axis t r a n s i t i o n o f the monoclinic c r y s t a l which i m p l i e d a l a r g e l y "out-of-plane" p o l a r i z a t i o n . Because i t appeared as t h e o n l y v i s i b l e t r a n s i t i o n i n the f a c t o r group, had n o t been p r e v i o u s l y observed,and came a t the frequency o f t h e dye aggregate J-band, i t was proposed t h a t t h i s was the e q u i v a l e n t t r a n s i t i o n t o t h a t e l u s i v e t r a n s i t i o n . Further, i t was suggested t h a t t h e t r a n s i t i o n was charge t r a n s - f e r i n c h a r a c t e r because i t s p o l a r i z a t i o n suggested i n t e r a c t i o n between t h e r i n g s o f two separate molecules i n t h e c r y s t a l . The r e f l e c t i v i t y o f t h e t r a n s i t i o n , nor i t s i n t e n s i t y obtained from Kramers-Kronig transform, i s n o t as h i g h as would have been expected.

C7-426 JOURNAL DE PHYSIQUE

These s t u d i e s prompted an i n v e s t i g a t i o n o f t h e PIC system as w e l l as s e a r c h i n g a n o t h e r dye c r y s t a l system f o r p o s s i b l e r e l a t i o n s h i p t o J-bands.The l a t t e r , HEAPS, was known t o be t h e f i r s t dye c r y s t a l which d i s p l a y e d q u a s i - m e t a l l i c r e f l e c t i o n f o r two f a c t o r group symmetries ( 4 ) . HEAPS a l s o d i s p l a y e d a charge t r a n s f e r t r a n s i t i o n i n one o f i t s polymorph's spectrum.

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RESULTS AND DISCUSSION

The c r y s t a l s p e c t r a o f HEAPS have been r e p o r t e d i n d e t a i l elsewhere ( 4 ) . I n t h a t work, a n a t t e m p t was made t o o b t a i n J-banding b u t i t was u n s u c c e s s f u l . D e t a i l e d c a l c u l a t i o n on t h e r e f l e c t i o n s p e c t r a i n v o l v i n g m o l e c u l a r e x c i t o n - p o l a r i t o n t h e o r y p r o v i d e d good agreement o n l y when f r e q u e n c y dependent damping was invoked. S i n c e t h e t r i c l i n i c and m o n o c l i n i c polymorphs o f t h e dye showed e x t r e m e l y d i f f e r e n t o p t i - c a l responses, t h e p o s s i b i l i t y o f f i n d i n g a J-band response i n o n l y one o r p o s s i b l y b o t h o f f e r e d an o p p o r t u n i t y t o e x t e n d u n d e r s t a n d i n g o f t h e a g g r e g a t i o n .

Piezomodulation s p e c t r a o f t h e two polymorphs show r a t h e r d r a m a t i c a l l y t h a t t h e two d i f f e r e n t c r y s t a l s ' r e f l e c t i o n s p e c t r a must be comprised o f two o s c i l l a t o r s i n t h e case o f t h e m o n o c l i n i c c r y s t a l , and t h r e e o r p o s s i b l y f o u r f o r t h e t r i c l i n i c . To f u r t h e r i n v e s t i g a t e . t h i s , t h e 5K r e f l e c t i o n spectrum o f t h e HEAPS m o n o c l i n i c c r y s t a l was determined. The p h y s i c a l p r o p e r t i e s o f t h e t r i c l i n i c c r y s t a l p r e v e n t e d a s i m i -

l a r s t u d y . T h i s spectrum i s p r e s e n t e d i n F i g . 1 f o r t h e (2077 f a c e . The b - a x i s spectrum shows some l o w e r i n g o f r e f l e c t i v i t y i n t h e l o w energy, s i n g l e p a r t i c l e r e g i o n b u t e s s e n t i a l l y l i t t l e change f r o m t h e h i g h t e m p e r a t u r e s p e c t r a . However, t h e p e r p e n d i c u l a r t o b - a x i s d i r e c t i o n has d r a m a t i c a l l y sharpened and i s o f q u i t e h i g h r e f l e c t i v i t y . S i n c e t h i s band (12 950 cmil-)is s i g n i f i c a n t l y t o t h e r e d o f t h e s o l u t i o n t r a n s i t i o n a t 15 530 c m q i t i s a p o s s i b l e J-band c a n d i d a t e . I t i s p o l a r - i z e d i n t h e p l a n e o f t h e m o l e c u l e b u t n o t w e l l accounted f o r by t h e o r e t i c a l c a l c u l a - t i o n .

For PIC, t h e (100) f a c e s p e c t r a were measured and a r e shown i n F i g . 2. These r e - f l e c t i v i t i e s a r e much c l o s e r t o t h o s e o f Tanaka e t a l . t h a n M a r c h e t t i e t a l . b u t a r e s i g n i f i c a n t l y h i g h e r t h a n t h o s e r e p o r t e d by b o t h groups. N e v e r t h e l e s s , t h e band shapes a r e i n good agreement.

The o r t h o r h o m b i c - f o r m of PIC i s n o t a t r u e polymorph b u t a s o l v a t e ( 5 ) . The spec- trum f o r t h e (101) f a c e i s d i s p l a y e d i n F i g . 3. None o f t h e t r a n s i t i o n s i n t h e o r t h o r h o m b i c s p e c t r a match t h e i s o l a t e d t r a n s i t i o n o f t h e b - a x i s seen i n t h e mono- c l i n i c form. However, t h e r e i s weak s t r u c t u r e a t 17 600 c m L h e r e t h e J-band i s observed. Of course, i n t e n s i t y arguments can o n l y be s a f e l y made f r o m t h e t r a n s - forms o f t h e r e f l e c t i o n d a t a b u t t h e s t r u c t u r e appears o f t o o l o w r e f l e c t i v i t y t o o f f e r t h e p o s s i b i l i t y o f s i g n i f i c a n t i n t e n s i t y . A l t h o u g h t h e f o u r m o l e c u l e s i n t h e u n i t c e l l appear i n d e f i n i t e p a i r s , one l y i n g on t o p o f t h e o t h e r , t h e f a c t t h a t t h e r e i s no evidence o f charge t r a n s f e r i s o f i n t e r e s t . Such i s p r e d i c t e d by Tanaka e t a l . s i n c e such an arrangement does n o t p e r m i t l o c a l i z a t i o n o f t h e charge i n a p a r t i c u l a r r i n g .

I 1 1

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CONCLUSIONS

The s t u d y o f d i f f e r e n t c r y s t a l l i n e forms o f t h e same dye a f f o r d s an approach t o e l u c i d a t i n g t h e n a t u r e o f t h e J-band i n dye aggregates. S t r u c t u r e s a r e found t o e x i s t i n some o f t h e s p e c t r a w h i c h a r e s u g g e s t i v e o f a J-band b u t n o t i n t h e s p e c t r a o f o t h e r forms. T h i s i s a c t u a l l y t o be sought s i n c e i t would n o t be expected t h a t t h e J-band i s independent o f s t r u c t u r e . T h i s method o f s t u d y a l s o a d m i t s t h e p o s s i - b i l i t y o f o b t a i n i n g more d i r e c t i n f o r m a t i o n on aggregate s t r u c t u r e s h o u l d agreement w i t h t h e c r y s t a l s p e c t r a be found.

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ACKNOWLEDGEMENTS

JOURNAL DE PHYSIQUE

ENERGY

(lC?

cm"1

Fig. 3 . Reflection s p e c t r a of orthorhombic PIC (10:) f a c e taken a t 300K along t h e b-axis ( ) and t h e perpendicular t o b-axis ( -

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-) d i r e c t i o n s .

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