Excited State Geometries of Coordination Compounds - ACS

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Excited States and Reactive Intermediates Downloaded from pubs.acs.org by YORK UNIV on 12/21/18. For personal use only.

Excited State Geometries of Coordination Compounds Obtained from Vibronic Spectra and Photon Flux Fluctuation Measured by Time Resolved Spectroscopy Hans-Herbert Schmidtke Institut für Theorestische Chemie der Universität, D-4000 Düsseldorf 1, Universitätsstraβe 1, Federal Republic of Germany The intensity distributions of well resolved vibronic spectra recorded in absorption and emission at low temperature are used to de­ termine the geometric distortions of the electronically excited states of coordina­ tion compounds. In particular for complexes of lower symmetry, band analysis is neces­ sary leading to results with which bond dis­ tance changes can be calculated. For spec­ tra exhibiting no vibrational fine struc­ ture, a new technique is proposed which uses time resolved methods, considering devia­ tions from the Poisson distribution of pho­ tons by recording time intervals between two successively emitted photons. Photochemical reactivity primarily results from electron distributions which are different to those in the ground state. With a change of electron density the geometry of the excited molecule may be distorted from that of the ground state molecule. These excited states,provided with large amounts of excess energy, are short lived species which are difficult to characterize. Since quan­ tum chemistry is not able to calculate molecules of the size we are interested in ( i t also cannot satisfactorily consider cooperative effects resulting from interaction with the environment) one i s restricted to experimental investigation. In particular some spectroscopic methods are fast enough to follow the physical conversions taking place i n the molecule, by detecting the excited state species and by measuring at least some of i t s properties. Vibronic spectra reflect changes in the electronic and vibrational state of a molecule at the same time. It is possible to calculate the geometry of the excited species and the potential hypersurface close to the equi­ librium state. For this, a spectrum i s required with sufficiently well resolved vibronic structure to carry 0097-6156/ 86/ 0307-0023506.00/ 0 © 1986 American Chemical Society

EXCITED STATES AND

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REACTIVE INTERMEDIATES

out a b a n d a n a l y s i s . T h i s s h o u l d a l l o w f o r d e c o m p o s i t i o n o f t h e bands i n t o d i f f e r e n t e l e c t r o n i c t r a n s i t i o n s and a f u r t h e r r e s o l u t i o n of the v i b r a t i o n a l f i n e s t r u c t u r e . U s u a l l y , the e l e c t r o n i c s p e c t r a of t r a n s i t i o n metal coor­ d i n a t i o n compounds i n s o l u t i o n , o r i n t h e s o l i d s t a t e , e x h i b i t r e l a t i v e l y b r o a d o v e r l a p p i n g b a n d s , w h i c h do n o t show any s i g n o f v i b r a t i o n a l f i n e s t r u c t u r e . However, t h e r e a r e many c a s e s known where d i s t i n c t v i b r a t i o n a l s t r u c t u r e i s d e t e c t e d although o f t e n , the s t r u c t u r e i s not completely r e s o l v e d . T h i s has l e d some a u t h o r s , on the b a s i s o f such s p e c t r a , to e x p l a i n t h e i r r e s u l t s , i n an i n c o r r e c t f a s h i o n . To a v o i d m i s i n t e r p r e t a t i o n one i s urged to consider only those s p e c t r a w i t h optimal reso­ l u t i o n . T h i s i s , f o r i n s t a n c e , a c h i e v e d when t h e v i b r a ­ t i o n a l quanta measured from a band p r o g r e s s i o n i n a l u m i n e s c e n c e s p e c t r u m a g r e e w i t h t h e f u n d a m e n t a l modes o f a v i b r a t i o n a l spectrum taken from the ground s t a t e (Raman o r I R ) . The MIME e f f e c t ( m i s s i n g mode e f f e c t ) , i . e . t h e a b s e n c e o f n o r m a l modes i n t h e v i b r a t i o n a l p r o ­ g r e s s i o n i n t e r v a l s (1 « 2 ) , may e x i s t on v a r i o u s occasions where t h e damping i n t h e d i s s i p a t i v e s y s t e m i s t o o l a r g e t o d e t e c t s e p a r a t e modes. However, one c a n n o t e x c l u d e t h e p o s s i b i l i t y t h a t an u n u s u a l p r o g r e s s i o n a l f r e q u e n c y i s s i m u l a t e d by i n c o m p l e t e r e s o l u t i o n i m p o s e d on t h e s y s t e m by i n s u f f i c i e n c i e s i n t h e e x p e r i m e n t . To o b t a i n i n g h i g h q u a l i t y , w e l l r e s o l v e d a b s o r p t i o n o r e m i s s i o n s p e c t r a , v a r i o u s t e c h n i q u e s h a v e b e e n ap­ plied. e.g. ( l ) d e c r e a s i n g t h e t e m p e r a t u r e o f t h e p r o b e , i f n e c e s s a r y b e l o w t h e A p p o i n t o f l i q u i d He (^2 K) investigating single crystals c o n s i d e r i n g d o p e d chromophore m a t e r i a l s w i t h (inert) host c r y s t a l s ( 4 ) d i l u t i n g t h e chromophore t o be i n v e s t i g a t e d w i t h appropriate counter ions or l o o k i n g at double salts e, t.g. [CoiNH U { j r ( C N ) ] Q ) or £Cr(NH^) J(C10 ) Cl.KCl (4) 5) u s i n g p o l a r i z e d l i g K t ' ! 6) i m p r o v i n g t h e a p p a r a t u s t o be u s e d (monochromators, d e t e c t i o n systems, photon c o u n t i n g , micro-optics etc.). However, i n many c a s e s , t h e s e e f f o r t s may n o t l e a d t o any r e s o l u t i o n o f t h e v i b r a t i o n a l f i n e s t r u c t u r e . T h i s o b v i o u s l y has a p h y s i c a l r e a s o n . S i n c e condensed systems a r e i n v e s t i g a t e d , i n t e r a c t i o n w i t h the e n v i r o n ­ ment i s i n v o l v e d i n t h e t r a n s i t i o n . The chromophore i s an open s y s t e m w h i c h d i s s i p a t e s v i b r a t i o n a l e n e r g y i n t o t h e s u r r o u n d i n g medium by i r r e v e r s i b l e p r o c e s s e s . T h i s phenomenon c a n be u s e d f o r d e t e c t i n g f i n e s t r u c t u r e f r o m t h e t i m e r e s o l v e d measurements o f p h o t o n e v e n t s , by m o n i t o r i n g t h e c o r r e l a t i o n s between s u c c e s s i v e l y e m i t t e d p h o t o n s . T h i s new t e c h n i q u e w i l l be r e p o r t e d i n t h e second p a r t of t h i s a r t i c l e .

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SCHMD ITKE

25 Excited State Geometries of Coordination

Conventional Vibronic

Compounds

Spectra

H e r e we w i l l d i s c u s s t h e v i s i b l e a n d UV a b s o r p t i o n a n d e m i s s i o n s p e c t r a o f some s e l e c t e d t r a n s i t i o n m e t a l a n d m a i n g r o u p c o o r d i n a t i o n compounds* T h e s p e c t r a a r e due to e l e c t r o n i c t r a n s i t i o n s and e x h i b i t e x t e n s i v e v i b r a ­ t i o n a l f i n e s t r u c t u r e i n l o n g band p r o g r e s s i o n s super­ i m p o s e d on e a c h o t h e r . The t r a n s i t i o n metal d-d t r a n s i ­ t i o n s become a l l o w e d b y a c o m p l i c a t e d c o u p l i n g mechanism, which mixes l e v e l s o f d i f f e r e n t p a r i t y by a v i b r o n i c cou­ p l i n g operator, and d i f f e r e n t s p i n s t a t e s by a s p i n - o r b i t coupling operator. These intermixings prepare t h e i n i t i a l and f i n a l s t a t e s o f t h e t r a n s i t i o n f o r a p p r o p r i a t e sym­ metry s e l e c t i o n r u l e s . V i b r a t i o n a l p r o g r e s s i o n s and broad band spectra (in cases where r e s o l u t i o n i s n o t a c h i e v e d ) a r e explained, i n g e n e r a l , by s h i f t s o f t h e p o t e n t i a l energy curves along s p e c i f i c n u c l e a r coordinates, along which l a r g e c h a n g e s o f bond p r o p e r t i e s may o c c u r b y e x c i t a t i o n of t h e molecule t o a h i g h e r s t a t e (Figure 1). These s t a t e s a r e l i k e l y t o be p h o t o a c t i v e . The t h e o r e t i c a l a n a l y s i s o f t h e s p e c t r u m t o o b t a i n i n g i n f o r m a t i o n a b o u t t h e e x c i t e d s t a t e s t r u c t u r e , must s t a r t f r o m a d e f i n i t e a s s i g n m e n t o f b a n d components a n d a r e l i ­ a b l e band a n a l y s i s c a r r i e d out b y r e s o l v i n g t h e band p r o ­ g r e s s i o n i n t o g a u s s i a n o r l o r e n t z i a n p r o f i l e s . The impor­ t a n c e o f g o o d b a n d a n a l y s e s s h o u l d n o t be u n d e r e s t i m a t e d i f t h e i n t e n s i t y d i s t r i b u t i o n o f t h e measured s p e c t r a i s compared w i t h t h e t h e o r e t i c a l b a n d p r o f i l e f u n c t i o n . A band system which remains u n d e t e c t e d under t h e main p r o ­ g r e s s i o n may l e a d t o d i f f e r e n t r e s u l t s when c a l c u l a t i n g geometry d i s t o r t i o n s . T h e o r e t i c a l band p r o f i l e s a r e u s u a l l y o b t a i n e d from Franck-Condon f a c t o r s which a r e a d j u s t e d t o t h e band p e a k s o f v i b r a t i o n a l components i n t h e p r o g r e s s s i o n . In o u r b a n d a n a l y s e s , we a r e u s i n g d i s t r i b u t i o n f u n c t i o n s I j (ιη,η;Δ, β) w h i c h c o l l e c t F r a n c k - C o n d o n i n t e g r a l s a n d Herzberg-Teller f a c t o r s i n t o a comprehensive f u n c t i o n w h i c h h a s more g e n e r a l a p p l i c a b i l i t y t h a n e a r l i e r b a n d analysis procedures. T h e method c a n b e u s e d f o r j - f o l d d e g e n e r a t e v i b r a t i o n s , where t h e v i b r a t i o n a l q u a n t a may be d i f f e r e n t f r o m t h e g r o u n d s t a t e ^ = *? / φ 1 and f o r v i b r a t i o n a l e x c i t a t i o n i n t h e i n i t i a l s t a t e m>0, b y w h i c h t e m p e r a t u r e dpendence w i l l be i n t r o d u c e d i n t o t h e .band p r o f i l e f u n c t i o n (5-7) · T h e p a r a m e t e r Δ = (/oM/ii) ' ΔΟ. s u p p l i e s t h e s h i f t o f t h e p o t e n t i a l c u r v e minimum o f t h e e x c i t e d s t a t e compared t o t h e g r o u n d s t a t e . We s h a l l a p p l y t h e s e f u n c t i o n s t o some o f t h e s p e c t r a discussed below. e

Emission Spectra. The chance t o o b t a i n v i b r a t i o n a l f i n e s t r u c t u r e i s u s u a l l y h i g h e r f o r luminescence than f o r absorption spectra since i n emission, absolute l i g h t i n t e n s i t i e s w i t h h i g h s e n s i t i v i t i e s a r e measured r a t h e r than i n t e n s i t y d i f f e r e n c e s . However, few compounds e x ­ h i b i t l u m i n e s c e n c e i n t e n s e enough t o measure a r e l i a b l e

26

EXCITED STATES AND REACTIVE INTERMEDIATES

F i g u r e 1· P o t e n t i a l e n e r g y c u r v e s i n t h e t o t a l l y sym­ m e t r i c s u b s p a c e Q-j i n t h e c a s e o f o c t a h e d r a l d^ s y s t e m s and r e s u l t i n g band shapes o f o p t i c a l s p e c t r a .

3.

SCHMD ITKE

27 Excited State Geometries of Coordination Compounds

s p e c t r u m . A l s o , i n many c a s e s , o n l y e m i s s i o n f r o m t h e l o w e s t e l e c t r o n i c s t a t e can be o b s e r v e d , due t o r a d i a t i o n l e s s d e a c t i v a t i o n from h i g h e r e x c i t e d s t a t e s . t r f R h py^Clg] CI: The powder e m i s s i o n s p e c t r u m o f t h i s compound a t 2 Κ (immersed i n l i q u i d H e l i u m ) has b e e n r e c o r d e d by C r o s b y and c o w o r k e r s (8)· A s i n g l e progres­ s i o n i n 350 cm"" has b e e n f o u n d , w h i c h does n o t c o r r e ­ spond t o any o f t h e v i b r a t i o n a l f r e q u e n c i e s o b t a i n e d f r o m a v i b r a t i o n a l spectrum of the ground s t a t e . FranckCondon a n a l y s i s o f t h e s p e c t r u m , c a r r i e d out by t h e s e a u t h o r s on t h e b a s i s o f an e l a b o r a t e t h e o r y i n c l u d i n g a n h a r m o n i c e f f e c t s , w a s a b o u t t o be r e v i s e d when a b e t t e r r e s o l v e d s p e c t r u m was o b t a i n e d w h i c h e x h i b i t e d t h r e e superimposed p r o g r e s s i o n s w i t h equal v i b r a t i o n a l quanta (2). The s e r i e s o f b a n d peaks ( F i g u r e 2) f o l l o w t h e formula 1

i n which a r e u n g e r a d e p r o m o t i n g modes i n d u c i n g t h e d-d t r a n s i t i o n s w h i c h b y v i b r o n i c c o u p l i n g become e l e c ­ t r i c dipole allowed. The i n t e r v a l s P s 370 cm"" , V> 2 = 251 cm*" , and V ^ s 176 cm" now a g r e e w i t h q u a n t a o b t a i n e d from the f a r i n f r a r e d spectrum. A l s o the p r o g r e s s i o n a l i n t e r v a l V~ = 295 cm" compares w e l l w i t h t h e Raman b a n d \?1 (a-jg) = 2o9 cm"" ( a t room t e m p e r a t u r e ) . With these data, t h e v i b r a t i o n a l s t r u c t u r e o f t h e s p e c t r u m c a n be w e l l understood. ^PtCl^j A t 1.9 Κ t h e e m i s s i o n s p e c t r u m o f c u b i c s i n g l e c r y s t a l s ( F i g u r e 3) e x h i b i t s t h r e e progressions due t o p r o m o t i n g modes ^3("t 1 u) » ^4(*1u) ^o(*2u) w i t h b a n d i n t e r v a l s a g r e e i n g w i t h i n f r a r e d ( t - j ) and t h e o r e t i ­ c a l d a t a (t2u) · P r o g r e s s i o n a l i n t e r v a l s are i n each c a s e i?g s 323 cm"' corresponding to a ^ ( g ) 320 cm" f u n d a m e n t a l v i b r a t i o n r e p o r t e d f r o m a Raman measurement (10). E m i s s i o n o c c u r s from the lowest e x c i t e d e l e c t r o n i c f ^ j s t a t e w h i c h i s one o f t h e ^T-|g(*2g g ) l split by s p i n - o r b i t c o u p l i n g ( j j j . T h i s l e v e l i s degenerate and s u b j e c t t o a J a h n - T e l l e r e f f e c t d i s t o r t i n g t h e m o l e ­ c u l e e i t h e r by e~ o r t2g v i b r a t i o n s . S i n c e a p r o g r e s s i o n i n eg i s o b s e r v e d , we c a n c o n c l u d e t h a t t h i s v i b r a t i o n a l mode i s p r e d o m i n a n t l y J a h n - T e l l e r a c t i v e , A n a l y s i s o f the band p r o f i l e by f i t t i n g the spectrum to the t h e o r e t i ­ c a l f u n c t i o n f o r t h e t r a n s i t i o n r a t e (band p r o f i l e f u n c ­ t i o n ) i n the z e r o - t e m p e r a t u r e l i m i t (maO) (5-7) y i e l d s a p p r o p r i a t e f i t t i n g p a r a m e t e r s A and β f r o m w h i c h a d i s t o r t i o n o f Az = 0.19JÎ and Ax. = Ay = -0.095 & i s c a l c u ­ l a t e d f o r one o f t h e p o s s i b l e Γ3 p e r t u r b a t i o n s ( and t h e t o t a l H a m i l t o n i a n i s t r a n s f o r m e d b y ÎTs DHD*" = Uq t o t h e l o c a l s y s t e m H a m i l ­ tonian ( 2 1 , 2 2 ) . To d e s c r i b e t h e t i m e d e v e l o p m e n t i n t h e new r e p r e s e n t a t i o n , t h e e l e c t r o n d e n s i t y $ = I ^ X ^ t l g

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EXCITED STATES AND REACTIVE INTERMEDIATES

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0.9H · · 3 / η ) t r a n s f o r m e d b y D(+L) a n d a l l o t h e r op­ e r a t o r s b y D ( - L ) , l e a d i n g t o t i m e o p e r a t o r s T(+L) a n d 7^(-L) w h i c h , due t o t h e LtO~ symmetry, were shown t o be s u b s t i t u t e d b y T(+L) = where T° i s t h e o d d f u n c t i o n a l p a r t T ° ( - L ) S - ^ ( + L ) o f T. W i t h t h e " i n t r i n s i c t i m e " f ° t h e t o t a l t i m e e v o l u t i o n o f t h e s y s t e m h a s two c o n t r i b u ­ tions i

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One r e s p r e s e n t s e v o l u t i o n w i t h r e s p e c t t o u n i v e r s a l t i m e c o n t r o l l e d b y t h e l o c a l s y s t e m L i o u v i l l i a n LQ, t h e o t h e r part d e s c r i b e s e v o l u t i o n w i t h r e s p e c t t o i n t r i n s i c time b e i n g t h e i r r e v e r s i b l e component o f t h e p r o c e s s d e t e r ­ mined b y t h e i n t e r a c t i o n L i o u v i l l i a n L y . The average ( t ) / ? T ^ then corresponds t o any d e v i a t i o n o f t h e u s u a l d e n s i t y c h a n g e s due t o t h e p o p u l a t i o n o r d e p o p u l a ­ t i o n (decay) o f t h e s t a t e s . F o r a s t a t i o n a r y luminescence experiment, t h i s term t h e r e f o r e d e s c r i b e s the photon f l u c t u a t i o n s from a v i b r o n i c s t a t e o f t h e l o c a l system HQ, c a u s e d b y e n v i r o n m e n t a l interactions. These photon f l u x f l u c t u a t i o n s a r e a l s o observed i n the time r e s o l v e d s p e c t r a o f other systems. Two more examples r e f e r t o Sn^+ d o p e d i n K I ( w h i c h i s a l s o a n s e l e c t r o n i c s y s t e m ) a n d | R u ( b i p y ) 3 U C I 2 i n aqueous s o l u t i o n (Ru2+ t o b i p y r i d i n e charge t r a n s f e r t r a n s i t i o n ) ( 2 0 ) . F i g u r e s 8 a n d 9 show c ( £ t ) p l o t s f o r t h e s e s y s t e m s exhibiting d i s t i n c t band s t r u c t u r e s f o r s m a l l time limits. Assignments t o superimposed p r o g r e s s i o n s and t o several emitting states are p o s s i b l e ; the obtained reso­ l u t i o n d o e s n o t a l l o w , however, a d e t a i l e d a n a l y s i s . W i t h t h e equipment a v a i l a b l e t o u s , t h e method i s l i m i t e d t o compounds w h i c h e x h i b i t a r e l a t i v e l y l a r g e e m i s s i o n i n t e n ­ sity. T h e r e f o r e t h e z e r o phonon r e g i o n s w h i c h e v e n t u a l l y show t h e most p r o n o u n c e d f i n e s t r u c t u r e , c o u l d n o t be investigated. S i n c e t r a n s i t i o n group elements a r e r a t h e r low e m i t t e r s o n l y few measurements ( e . g . f o r ^ P t C l ^ ) were s u c c e s s f u l u n t i l now, s u p p l y i n g b e t t e r r e s o l v e d v i b r a t i o n a l s t r u c t u r e s than those obtained by u s u a l emission or absorption spectroscopy. Acknowledgment I am g r a t e f u l t o D r . A . B. P. L e v e r f o r h i s a s s i s t a n c e t o prepare the manuscript.

editorial

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EXCITED STATES AND REACTIVE INTERMEDIATES

Figure 8. C o r r e s p o n d i n g p l o t s as i n F i g u r e 7 f o r K I : S n ^ s i n g l e c r y s t a l s f o r v a r i o u s g i v e n time l i m i t s A t compared t o t h e e m i s s i o n s p e c t r u m ^ Ι > · +

3.

SCHMIDTKE

Excited State Geometries of Coordination Compounds

F i g u r e 9 · E m i s s i o n i n t e n s i t y a n d c ( ^ t ) p l o t s f o r p l u ( b i p y ) ^ J c i 2 i n aqueous s o l u t i o n a t n o r m a l t e m p e r a ­ ture. E x c i t a t i o n wave l e n g t h λ = 358 nm.

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EXCITED STATES AND REACTIVE INTERMEDIATES

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