OBSERVATION OF IN SITU FORMATION OF PHTHALOCYANINE-BIS (PYRIDINE) IRON(II) BY X-RAY ABSORPTION SPECTROSCOPY

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OBSERVATION OF IN SITU FORMATION OF PHTHALOCYANINE-BIS (PYRIDINE) IRON(II) BY

X-RAY ABSORPTION SPECTROSCOPY

K.-H. Frank, E.-E. Koch, H.-W. Biester

To cite this version:

K.-H. Frank, E.-E. Koch, H.-W. Biester. OBSERVATION OF IN SITU FORMATION OF PHTHALOCYANINE-BIS (PYRIDINE) IRON(II) BY X-RAY ABSORPTION SPECTROSCOPY.

Journal de Physique Colloques, 1986, 47 (C8), pp.C8-653-C8-657. �10.1051/jphyscol:19868124�. �jpa-

00226023�

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JOURNAL DE PHYSIQUE

Colloque C8, supplkment au no 12, Tome 47, dkcembre 1986

OBSERVATION OF IN SITU FORMATION OF PHTHALOCYANINE- BIS(PYRIDINE)IRON(II) BY X-RAY ABSORPTION SPECTROSCOPY

K.-H. FRANK, E.-E. KOCH and H.-W. BIESTER*

Fritz-Haber-~nstitut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-1000 Berlin 3 3 , F.R.G.

*II.~nstitut fiir Experimentalphysik, Universitat Hamburg, D-2000 Hamburg 50, F.R.G.

A b s t r a c t

X-ray a b s o r p t i o n spectroscopy has been employed t o study i n s i t u the changes i n the l o c a l s t r u c t u r e o f t h e c e n t r a l atom o f Fe-Phthalocyanine (FePC) by t h e a d s o r p t i o n o f p y r i d i n e . A dipole-forbidden (ls+3d) pre-edge s t r u c t u r e o f the Fe K-edge has been observed which i n d i c a t e s t h e

D

symmetry o f FePC. This ls+3d t r a n s i t i o n i s missing i n the spectrum o f the ~ e ~ ~ - ~ ~ # ? d i ne compl ex. The disappearance o f t h e pre-edge s t r u c - t u r e i s i n t e r p r e t e d by the adsorption o f two p y r i d i n e molecules t o the FePC molec- u l a r plane leading t o a change o f the f o u r f o l d symmetry i n t o an octahedral geometry.

The absence.of a chemical s h i f t o f t h e Fe K-edge i n d i c a t e s t h a t t h e complex has been formed w i t h o u t a n e t t r a n s f e r o f e l e c t r o n charge between the c e n t r a l atom and the p y r i d i n e l i g a n d s .

I n t r o d u c t i o n

The metal phthalocyanines (MePcls) a r e o f considerable i n t e r e s t because the l o c a l molecular s t r u c t u r e around the c e n t r a l atom i s v e r y s i m i l a r t o t h a t i n c h l o r o - p h y l l and hemoglobin. Thus they are model substances f o r r e d o x - a c t i v e complexes i n l i v i n g organisms o r i n c a t a l y t i c systems. I n a d d i t i o n , they have i n t e r e s t i n g proper- t i e s as semiconductors and f o r device a p p l i c a t i o n s / I / . The l o c a l e l e c t r o n i c and geometric s t r u c t u r e a t t h e c e n t r a l metal atom plays 'a dominant r o l e f o r t h e i r chem- i c a l p r o p e r t i e s . The occupied valence bands o f the 3d-PC's and the b i n d i n g energies o f the core l e v e l s are w e l l known from photoemission experiments / 2 / . Recently, t h e empty molecular o r b i t a l s o f 3d-PC's have been determined by h i g h r e s o l u t i o n s o f t X-ray absorption spectroscopy (XAS) i n the range o f the carbon and n i t r o g e n K-edges and t h e 3d-metal L -edges /3/. This technique i s s i t e s p e c i f i c due t o t h e pos- s i b i l i t y t o selectl€ielBTfferent c o r e - l e v e l edges o f i n t e r e s t . Thus, i t was p o s s i b l e t o observe i n p a r t i c u l a r the unoccupied 3 d - o r b i t a l s o f the c e n t r a l atom i n the 3d MePC's /3/. The t o t a l d e n s i t y o f empty s t a t e s i s accessible by i n v e r s e photoemission spectroscopy which has been developed i n t h e l a s t few years /4/ and a p p l i e d success- f u l l y t o study t h e empty molecular o r b i t a l s o f adsorbed benzene and azabenzenes / 5 / . I n the X-ray regime the I s edges of t h e 3d metals can be reached. The r e s o l u t i o n i s reduced however by the i n t r i n s i c broadening due t o l i f e time e f f e c t s . Nevertheless, t h e measurement o f t h e near edge absorption a t the metal K-edce i n orqanometallic molecules 1 i ke 3d-MePC's contains valuable i n f o r m a t i o n about the symmetry a t the c e n t r a l atom /6/. Furthermore, these experiments can be done i n r e a c t i o n c e l l s under a r e a l chemical environment. The main f e a t u r e s o f the K-edge t r a n s i t i o n s can be r e - l a t e d t o 1s+4p and shape resonances and/or t o m u l t i s c a t t e r i n g processes. I n a d d i t i o n a small pre-edge peak has been assigned t o the ls+nd t r a n s i t i o n i n the spectra o f t r a n s i t i o n metals w i t h incomplete f i l l e d d - s h e l l s /6,7/. The i n t e n s i t y o f t h i s weak feature i s a s e n s i t i v e measure t o determine the s i t e symmetry o f t h e atom. I n a per- f e c t octahedral symmetry the pre-edge s t r u c t u r e i s completely absent. Going t o d i s - t o r t e d octahedral, square pyramidal and t e t r a h e d r a l geometries t h e amplitude i n - creases /6,7/. T h i s e f f e c t has been used t o study the s i t e symmetry i n i n o r g a n i c compounds and organometallic complexes / 8 / . I t i s w e l l known t h a t i n t h e planar YePC's Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868124

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'28-654 J O U R N A L DE PHYSIQUE

i t i s p o s s i b l e t o change t h e c o o r d i n a t i o n o f t h e c e n t r a l atom by a d s o r p t i o n o f l i g a n d s w i t h donor o r a c c e p t o r c h a r a c t e r / 9 / . F o r example t h e r e e x i s t s P h t h a l o c y a n i n e - b i s ( p y - r i d i n e ) i r o n ( I I )

/ l o / .

Thus, we have t r i e d t o s t u d y i n s i t u t h e f o r m a t i o n o f t h i s com- p l e x and t h e change i n c o o r d i n a t i o n by Fe K-edge XAS.

Experimental

The XAS measurements were performed a t t h e RUM0 spectrometer a t HASYLAB (DESY, Hamburg) i n t r a n s m i s s i o n geometry u s i n g a S i ( 3 1 1 ) double c r y s t a l monochromator /11/.

The FePC sample has been evaporated o n t o a 1 um t h i c k Kapton f o i l . The t h i c k n e s s o f t h e f i l m was e s t i m a t e d t o be about 3 urn. T h i r t y o f t h e s e f o i l s were s t a c k e d t o g e t optimum a b s o r b e r - t h i c k n e s s and keeping a t t h e same t i m e a l a r g e s u r f a c e t o volume r a t i o . The sample s t a c k was t h e n t r a n s f e r r e d i n t o a vacuum chamber equipped w i t h two Kapton windows. A r e s e r v o i r f i l l e d w i t h l i q u i d p y r i d i n e was connected t o t h e chamber by a v a l v e . The p y r i d i n e had been cleaned b e f o r e by s e v e r a l f r e e z e and thaw c y c l e s . The X-ray a b s o r p t i o n spectrum o f t h e FePC was t h e n measured under vacuum c o n d i t i o n s . By opening t h e v a l v e and f i l l i n g t h e chamber w i t h p y r i d i n e vapour t h e f o r m a t i o n o f t h e FePC-pyridine complex c o u l d be observed i n s i t u by XAS d i r e c t l y .

R e s u l t s and D i s c u s s i o n

I n F i g . l a t h e t r a n s i m i s s i o n spectrum o f FePC i s shown i n t h e range o f t h e Fe K-edge. The spectrum e x h i b i t s s i x w e l l r e s o l v e d s t r u c t u r e s l a b e l l e d A1, B, C1, C and D. T h e i r e n e r g i e s a r e 1 is t e d i n t a b l e 1. The z e r o o f energy scale*$;s been f q x e d t o t h e f i r s t i n f l e c t i o n p o i n t o f t h e spectrum a t 7105 eV.

Table 1: E n e r g i e s o f t h e f e a t u r e s o f t h e Fe K-edge i n FePC r e l a t i v e t o t h e f i r s t i n f l e c t i o n p o i n t o f t h e spectrum i n F i g . l a .

F o r comparison t h e spectrum o f t h e K-edge o f Fe i n t h e p u r e m e t a l i s shown i n F i g . I d , where t h e whole p a t t e r n o f s t r u c t u r e s A

-

D can a l s o be i d e n t i f i e d . T h e i r ener- g i e s a r e o n l y s l i g h t l y s h i f t e d , b u t t h e r e l a t i v e i n t e n s i t i e s have changed c o m p l e t e l y . The weak humps A and A2 coalesce i n t o one peak i n p u r e i r o n . They a r e a t t r i b u t e d t o t h e d i p o l e - f o r b i i d e n ls+3d t r a n s i t i o n which w i l l be d i s c u s s e d below. There i s no agreement i n t h e l i t e r a t u r e c o n c e r n i n g t h e assignment o f s t r u c t u r e B. Several ex- p l a n a t i o n s a r e discussed: ( i ) ls+4s t r a n s i t i o n /12/, ( i i ) 4p f i n a l s t a t e /13,14/, ( i i i ) t h e o n s e t o f t h e continuum /15/, ( i v ) l s 4 p shake down f e a t u r e / 1 6 / . Concern- i n g . t h e main maximum i n t h e i r o n spectrum we f o l l o w t h e i n t e r p r e t a t i o n o f Grunes /17/ and i d e n t i f y C as t h e l s 4 p t r a n s i t i o n . C and D a r e t h e n a s s i g n e d t o shape resonances and/or m b l t i s c a t t e r i n g resonances.

W$

w i l l f o c u s o u r a t t e n t i o n on t h e pre-edge peaks A

,

A i n o r d e r t o o b t a i n i n f o r m a t i o n about t h e symmetry o f t h e Fe- i o n i n t h e ~ e ~ c - b ~ r i g i n e complex.

Exposure o f t h e FePC f i l m s t o p y r i d i n e vapour changes t h e spectrum. The Fe K-edge and t h e above mentioned s t r u c t u r e s A

-

D e x h i b i t no s h i f t i n t h e i r energy p o s i t i o n s ( F i g . l b ) . However, t h e i r r e l a t i v e i n t e n s i t i e s a r e markedly a f f e c t e d by t h e a d s o r p t i o n o f p y r i d i n e t o t h e FePC molecule. The maxima C1, C2 and D a r e broadened.

C2 and D a r e a l s o reduced i n t h e i r amplitudes. The p o s t i n t e r e s t i n q e f f e c t can be observed f o r t h e ls+3d t r a n s i t i o n A2. While A i s more o r l e s s u n e f f e c t e d A 2 . i s r e - duced by t h e p y r i d i n e a d s o r p t i o n (Fig. l b ) . 1 k d i s a p p e a r s a f t e r s a t u r a t i o n w ~ t h p y r i - d i n e vapour ( F i g . l c ) . The development o f t h e s p e c t r a a, b, and c ( F i g . I ) , w i t h exposure t o p y r i d i n e vapour can be e x p l a i n e d by t h e p e n e t r a t i o n o f p y r i d i n e i n t o t h e porous FeFC-films. I n F i g . 2 an e n l a r g e d p l o t o f these s p e c t r a i s shown i n t h e r e g i o n of t h e A1, A f e a t u r e s . The spectrum o f FePC e x h i b i t s as s h o u l d e r A; on t h e l o w energy s ~ d e 8 f A separated by 2.5 eV. The disappearance o f A2 and

P i

can a l s o be c l e a r l y seen i n

Fig.

^2.

F i g . 2: E n l a r g e d p l o t o f t h e s p e c t r a o f F i g . l a , b, c i n t h e r e g i o n o f t h e

ls+3d t r a n s i t i o n .

+

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F i g . ] :

Iron K-edge X-ray absorption spectra of iron-Phthalocyanine (a) and after expo- sure to pyridine vapour

( b ) .

Spectrum (c) corresponds to the

Phthalocyanine-bis(pyri-

dine)iron(lI) complex after saturation of the FePC film with pyridine. For com- parison the K-edge of iron metal is shown

(d). See text for explanation of the nota- tion.

7100 7150 7200

PHOTON

ENERGY

( e V )

7090 7100 7110 7120

PHOTON ENERGY (eV1

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C8-656 JOURNAL DE PHYSIQUE

It i s w e l l known /6,7/ t h a t t h e i n t e n s i t y o f the d i p o l e - f o r b i d d e n pre-edge peak i s c o r r e l a t e d t o t h e s i t e symmetry o f t h e adatom. I n a p e r f e c t octahedral s i t e t h e ls+3d t r a n s i t i o n i s o n l y quadrupole allowed / 7 / . Thus i t i s very weak. Lowering t h e s i t e s y m e t r y t o d i s t o r t e d octahedral, square pyramidal and tetrahedal geometry t h e pre-edge i n t e n s i t y increases /6,7/. The pre-edge a b s o r p t i o n becomes d i p o l e allowed because o f stronger 3d-4p mixing and h y b r i d i s a t i o n o f the 3d o r b i t a l s w i t h the p- l e v e l s o f t h e l i g a n d s . I n a p l a n a r f o u r f o l d s i t e t h e r e e x i s t s a l s o a pre-edge s t r u c - t u r e /18/. Therefore we i n t e r p r e t t h e disappearance o f peaks A and A' by t h e ad- s o r p t i o n of two p y r i d i n e molecules t o the FePC molecule below $nd abo6e i t s molec- u l a r plane. The s i t e symmetry o f t h e Fe atom i s then octahedral. Most probably the p y r i d i n e s a r e deposited w i t h t h e i r n i t r o g e n end t o t h e FePC /19/. This geometry i s shown i n F i g . 3. A d e t a i l e d a n a l y s i s o f the Fe-N d i s t a n c e cannot be obtained from our measurements. But a p e r f e c t o r o n l y s l i g h t l y P 8 i s t o r t e d octahedral s i t e can be expected. This i s i n agreement w i t h ab i n i t i o MO c a l c u l a t i o n s o f S a i t o and Kashiwagi /19/ f o r t h i s FePC ( p y r i d i n e ) complex. The authors c a l c u l a t e d f o r t e low s p i n F e ( I 1 ) s t a t e o f t h e F ~ P C - p y r i g i n e complex a Fe-N d i s t a n e o f 2.19

k ^,

which i s about 10 % l a r g e r than the i n p l a n e Fe-N distancep8f 2.02

i .

The occurrance o f t h e low s p i n F e ( I 1 ) s t a t e f o r FePC and FePC-Py has been proven by Mossbauer s t u d i e s o f Hudson and W h i t f i e l d /20/. T h i s r e s u l t may be checked e x p e r i m e n t a l l y by EXAFS measurements o f t h e complex from which the nearest neighbour distances and the number o f nearest neighbours can be e x t r a c t e d .

The l o c a l geometry o f i r o n i n FePC(Py) i s very s i m i l a r t o t h e s i t e symmetry o f i r o n i n hemoglobin /21/. Indeed t h i s c l o s e Zorrespondence i s r e f l e c t e d by the s i m i - l a r i t y o f t h e XAS o f hemoglobin /22/ and F ~ P C ( P Y ) ~ .

F i g . 3: S i x f o l d octahedral geometry o f t h e Phthalocyanine-bis(pyridine)iron(II) complex.

As mentioned above we have n o t observed any chemical s h i f t o f the X-ray ab- s o r p t i o n edges o f the f r e s h l y evaporated FePC and the i n s i t u prepared FePC-pyridine complex. Transfer o f n e t e l e c t r o n charge between the F e ( I 1 ) ions /20/ and the n i t r o - gen atoms o f the p y r i d i n e molecules does n o t occur by the adsorption. This f i n d i n g i s i n agreement w i t h XPS r e s u l t s /23/ and t h e Mossbauer s t u d i e s /20/. They have found F e ( I 1 ) i o n s i n FePC as w e l l as i n t h e complex. F i n a l l y , we note t h a t exposure o f t h e FePC-sample t o O2 d i d n o t l e a d t o an observable change i n the XAS w i t h i n the d e t e c t - i o n l i m i t s o f t h e present experiment.

Acknowledgement

We wish t o thank G. M a t e r l i k f o r h i s v a l u a b l e support i n the i n i t i a l stages o f t h i s work, B. Lengeler f o r v a l u a b l e discussions and I. Chernotiorsky and P. Obel f o r t h e i r very s k i l l f u l l t e c h n i c a l support.

T h i s work has been supported i n p a r t by t h e Bundesministerium fiir Forschung und Technologie (BMFT) from funds f o r research w i t h synchrotron r a d i a t i o n (BMFT) 06 390 FXg/C3-09).

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