P h o t o e x c i t e d M o l e c u l a r S t r u c t u r e o f D i p l a t i n u m ( I I ) C o m p l e x b y S i n g l e - C r y s t a l X - r a y S t r u c t u r e A n a l y s i s P h o t o e x c i t e d M o l e c u l a r S t r u c t u r e o f D i p l a t i n u m ( I I ) C o m p l e x b y S i n g l e - C r y s t a l X - r a y S t r u c t u r e A n a l y s i s D i r e c t o b s e r v a t i o n o f g e o m e t r i c a l c h a n g e s f o l l o w i n g p h o t o - e x c i t a t i o n o f m o l e c u l e s p r o v i d e s essential information on transient species such as metastable states of chemical reactions and excited states, which are sometimes difficult to describ e by m o l e c u l a r o r b i t a l c a l c u l a t i o n s . A l t h o u g h s i n g l e - c r y s t a l X - r a y s t r u c t u r e a n a l y s i s i s a p o w e r f u l a n d co nv en ti on al to ol fo r ob ta in in g ac cu ra te mo le cu la r g e o m e t r y a n d s t r u c t u r a l p a r a m e t e r s , t h e r e a r e so me di ffi cu lti es to be ov erc ome to de ter min e the structure of photo-excited molecules using ordinary d i f f r a c t o m e t e r s . B e c a u s e o f t h e e x t r e m e l y s m a l l p o p u l a t i o n s a n d s m a l l g e o m e t r i c a l c h a n g e s o f e x c i t e d - s t a t e m o l e c u l e s i n c r y s t a l s , p h o t o - e x c i t e d c r y s t a l l o g r a p h y r e q u i r e s m o r e a c c u r a t e me as ur em en ts of di ff ra ct io n in te ns it ie s. We ha ve developed a new low-temperature vacuum ( LTV ) X- ray camera installed on beamline BL02B1 ( Fig. 1 ), in addition to a special data collection system, the m u l t i p l e - e x p o s u r e I P m e t h o d f o r e x c i t e d - s t a t e cr ys ta ll og ra ph y. We re po rt he re th e in st ru me nt at io n a n d i t s f i r s t a p p l i c a t i o n t o t h e l u m i n e s c e n t d i p l a t i n u m ( I I ) [ P t 2 ( p o p ) 4 ] 4 - c o m p l e x . T h e L T V c a m e r a i s s p e c i a l l y d e s i g n e d f o r a c c u r a t e d i f f r a c t i o n m e a s u r e m e n t s w i t h h i g h S / N r a t i o s a t l o w t e m p e r a t u r e ( < 8 0 K ) . T o o b t a i n diff ract ion imag es with extr emel y low back grou nd, an image plate ( IP ) is placed in a vacuum chamber p r e v e n t i n g X - r a y s c a t t e r i n g f r o m a i r a n d v a c u u m windows. A crystal specimen is mounted on a cold head of He refrigerator and can be cooled down to 2 0 K u n d e r a h i g h v a c u u m . C r y s t a l r o t a t i o n , I P p o s i t i o n i n g , a n d t h e I P r e a d o u t s y s t e m a r e f u l l y c o m p u t e r c o n t r o l l e d f o r p h i - o s c i l l a t i o n a n d s c r e e n l e s s Weissenberg modes. An additional feature of this c a m e r a i s t h e m u l t i p l e - e x p o s u r e m o d e , i n w h i c h both diffraction patterns under light- - irradiated (light- o n ) a n d n o n - i r r a d i a t e d ( l i g h t - o f f ) c o n d i t i o n s a r e recorded on the same IP frame. The first exposure by phi -oscillation mode under the light- off condition i s f o l l o w e d b y t h e s e c o n d o n e u n d e r t h e l i g h t - o n cond itio n afte r a slig ht shif t (1 to 2 mm ) of the IP holder, and these processes are repeated several times t o minimize systematic errors in the intensity m e a s u r e m e n t s ( F i g . 2 ) . T h e d o u b l y - r e c o r d e d d i f f r a c t i o n i m a g e s c a n b e r e a d a t t h e s a m e t i m e u n d e r t h e s a m e c o n d i t i o n s , g i v i n g a c c u r a t e m e a s u r e m e n t s o f i n t e n s i t y c h a n g e f o r e a c h reflection. A laser light for photo-excitation can be i n t r o d u c e d t h r o u g h f i b e r o p t i c s i n t o t h e v a c u u m chamber and focused on the crystal ( Fig. 3 ). Th e [ Pt 2 (p op ) 4 ] 4- co mp le x io n co ns is ts of tw o Pt ( II ) atoms bridged by four pyrophosphate ligands ( F i g . 4 ) . T w o P t P 4 s q u a r e p l a n e r m o i e t i e s a r e stacked with an eclipsed configuration , and the Pt- Pt distance is 2.93 Å. Formally, there are no direct metal-metal bonds. S pectroscopic investigations of t h e c o m p l e x s h o w s t r o n g g r e e n l u m i n e s c e n c e , Fig. 1. Low-temperature vacuum X-ray camera. 58 light-off light-on (with offset) Multiple-exposed image + w h i c h h a s b e e n a t t r i b u t e d t o 3 A 2 u → 1 A 1 g p h o s p h o r e s c e n c e [ 1 ] . E m i s s i o n s p e c t r a o f t h e B a 2 + s a l t s a t 5 K e x h i b i t f i n e s t r u c t u r e s w i t h a sp ac in g of 15 0 cm -1 , wh ic h ar e as si gn ed to Pt -P t s t r e t c h i n g v i b r a t i o n i n t h e 3 A 2 u e x c i t e d s t a t e [ 2 ] . T h e f r e q u e n c y o f t h e v i b r a t i o n a l s t r u c t u r e i n t h e excited states is higher than that in the ground state ( 1 1 6 c m - 1 ) , s u g g e s t i n g a n e f f e c t i v e P t - P t b o n d f o r m e d i n t h e e x c i t e d s t a t e s . T h e 1 A 1 g → 1 A 2 u a b s o r p t i o n c o r r e s p o n d s t o t h e 5 d σ * → 6 p σ e l e c t r o n i c transitions. A s i n g l e c r y s t a l o f ( n - B u 4 N ) 2 H 2 [ P t 2 ( p o p ) 4 ] s i t u a t e d o n t o p o f c a r b o n f i b e r s u p p o r t s w a s mounted on the camera, and was kept at 54 K. A He-Cd blue CW laser (442 nm / 100 mW ) was used for photo-excitation, continuously illuminating during t h e l i g h t - o n p e r i o d t o m a i n t a i n t h e c r y s t a l i n a p s e u d o - s t e a d y s t a t e . T h e m u l t i p l e - e x p o s u r e o scillation photo comprising alternating light-off and light-on periods for 24 seconds each was repeated t e n t i m e s t o r e c o r d o n e I P f r a m e . 5 2 f r a m e s o f o s c i l l a t i o n p h o t o w e r e s u b j e c t e d t o i n t e n s i t y d a t a p r o c e s s i n g . S t a n d a r d c r y s t a l s t r u c t u r e a n a l y s e s w e r e p e r f o r m e d u s i n g e a c h l i g h t - o n a n d l i g h t - o f f intensity data set s independently. Lattice constants f o r t h e l i g h t - o n d a t a s e t w e r e s l i g h t l y l a r g e r t h a n th at fo r th e li gh t- of f se t. Ho we ve r, st an da rd s t r u c t u r a l a n a l y s i s s h o w e d t h a t t h e r e w e r e n o di ff er en ce s in at om ic pa ra me te rs ex ce pt an in cr ea s e i n t e m p e r a t u r e f a c t o r s f o r t h e l i g h t - o n d a t a s e t . T h e s e r e s u l t s i n d i c a t e t h a t t h e h e a t i n g e f f e c t b y l a s e r i r r a d i a t i o n h a s c a u s e d a s m a l l t e m p e r a t u r e rise tha t led to expansion of the unit-cell volume in the light-on condition . Fig. 2. Multiple-exposure IP method. The first X-ray exposure is followed by the second one after a 1 - 2 mm shift of the IP holder without readout or erasing processes. The doubly-recorded images are read at one time. X-ray IP slit IP translation Laser Crystal laser coupler laser shutter laser focuser vacuum chamber fiber optics Fig. 3. Schematic layout of the LTV camera with laser irradiation optics. 59 Fig. 4. Difference Fourier map (superimposed with a molecular diagram) of | F on | – | F off | in a plane containing Pt (1) – Pt (2) vector and coordinated four P atoms of the ligands. Continuous lines and dashed lines indicate positive and negative density, drawn at every 0.2 e/Å 3 , respectively. F i g . 4 References [ 1 ] D . M . R o u n d h i l l e t a l . , A c c . C h e m . R e s . 2 2 (1989) 55. [2] A. P. Zipp, Coord. Chem. Rev. 84 (1988) 47. [3] Y. Ozawa et al. , J. Appl. Cryst. 31 (1998) 128. [ 4 ] Y o s h i k i O z a w a , M a d o k a T e r a s h i m a , M i n o r u M i t s u m i , K o s h i r o T o r i u m i , N o b u h i r o Y a d u d a , Hidehiro Uekusa and Yuji Ohashi, Chem. Lett. 32 (2003) 62. Yoshiki Ozawa and Koshiro Toriumi Himeji Institute of Technology E-mail: toriumi @ sci.himeji-tech.ac.jp T o r e v e a l s m a l l c h a n g e s o f c r y s t a l s t r u c t u r e b e t w e e n t h e l i g h t - o n a n d l i g h t - o f f d a t a s e t s , d i f f e r e n c e F o u r i e r s y n t h e s e s w e r e p e r f o r m e d f o r | F on | – | F of f | us in g th e ph as e fa ct or s ca lc ul at ed by the atomic parameters of the light-off data set. The o b s e r v e d e l e c t r o n d e n s i t y m a p ( ) s h o w s po si ti ve an d ne ga ti ve pe ak s wi th he ig ht s of 1 - 2 e / Å 3 n e a r t h e P t a t o m s , i n d i c a t i n g t h a t a s m a l l p o r t i o n o f t h e m e t a l a t o m s m o v e s t o w a r d t h e p o s i t i v e p e a k s i n t h e l i g h t - o n c r y s t a l . T h e P t ( 1 ) atom shifts toward the Pt (2) position, while the Pt (2) a t o m s h i f t s w i t h i n t h e p l a n e p e r p e n d i c u l a r t o t h e Pt -Pt vector. These results clearly reflect shrinkage in the Pt-Pt distance. Changes in the geometrical p a r a m e t e r s o f t h e P t a t o m s w e r e a n a l y z e d qu an ti ta ti ve ly by le as t- sq ua re s ca lc ul at io ns ba se d on the response ratio (defined as η = ( I on – I off ) / I off ) [3 ]. Th e po si ti on al an d oc cu pa nc y pa ra me te rs of th e Pt at om s in th e ex ci te d st at es re ve al th at th e Pt (1 *) –P t (2 *) di st an ce in th e ex ci te d st at e is 2. 70 ( 4 ) Å , w h i c h i s 0 . 2 3 Å s h o r t e r t h a n t h a t o f t h e ground state ( Pt (1) –Pt (2) 2.9289 (2) Å) [4]. 60 P t 1 P t 2 P 3 P 5 P 1 P 7
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