High-Energy Processes in Organometallic Chemistry - ACS Publications

Chapter 10

Electrochemiluminescence of Organometallics and Other Transition Metal Complexes

Downloaded by NORTH CAROLINA STATE UNIV on May 8, 2015 | http://pubs.acs.org Publication Date: February 26, 1987 | doi: 10.1021/bk-1987-0333.ch010

A. Vogler and H. Kunkely Universität Regensburg, Institut für Anorganische Chemie, D-8400 Regensburg, Federal Republic of Germany

A variety of transition metal complexes including organometallics was subjected to an ac electrolysis in a simple undivided electrochemical c e l l , containing only two current-carrying platinum electrodes. The compounds (A) are reduced and oxidized at the same electrode. If the excitation energy of these compounds is smaller than the potential difference of the reduced (A ) and oxidized (A ) forms, back electron transfer may regenerate the complexes in an electronically excited state (A + A -> A* + A). Under favorable conditions an electrochemiluminescence (ecl) is then observed (A* -> A + hv ). A weak ecl appeared upon electrolysis of the following complexes: Ir(III)(2-phenylpyridine-C ,N ) ; [cu(I) (pyridine)I] , Pt(II) (8-quinolinolate) , Tb(III) (TTFA) (o-phen) with TTFA = thenoyltrifluoroacetonate and o-phen = 1.10phenanthroline, Tb(III) (TTFA)-, and Eu(III) (TFFA) (o-phen). An ecl of Re(o-phen) (CO) Cl occured during the electrolysis of tetralin hydroperoxide in the presence of the rhenium compound. The mechanism of these electrochemical reactions is discussed. -

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An e l e c t r o l y s i s can be c o n s i d e r e d t o be a h i g h - e n e r g y p r o c e s s i f r e d u c t i o n and o x i d a t i o n t a k e p l a c e a t l a r g e p o t e n t i a l d i f f e r e n c e s . The p r i m a r y redox p r o d u c t s thus formed c a n p a r t i c i p a t e i n a v a r i e t y o f competing p r o c e s s e s (1_). They may be k i n e t i c a l l y l a b i l e and undergo f r a g m e n t a t i o n o r s u b s t i t u t i o n r e a c t i o n s . As an a l t e r n a t i v e a r a p i d back e l e c t r o n t r a n s f e r s h o u l d t a k e p l a c e due t o t h e l a r g e driving force. T h i s l e a d s t o t h e r e g e n e r a t i o n o f t h e s t a r t i n g compounds e i t h e r i n t h e ground s t a t e o r i n an e l e c t r o n i c a l l y e x c i t e d s t a t e i f t h e p o t e n t i a l d i f f e r e n c e exceeds t h e e x c i t a t i o n e n e r g y . At very l a r g e d r i v i n g f o r c e s e x c i t e d s t a t e generation i s expected t o be f a v o r e d o v e r ground s t a t e f o r m a t i o n i n many c a s e s a c c o r d i n g t o t h e Marcus t h e o r y (2^). V a r i o u s e x p e r i m e n t a l methods a r e a v a i l a b l e t o g a i n more i n s i g h t i n t o such h i g h - e n e r g y e l e c t r o n t r a n s f e r reactions. The d e t e c t i o n o f e x c i t e d p r o d u c t s i s p o s s i b l e i f t h e y are luminescent. Chemiluminescence r e s u l t i n g from e l e c t r o n t r a n s -

0097-6156/87/0333-0155$06.00/0 © 1987 American Chemical Society

In High-Energy Processes in Organometallic Chemistry; Suslick, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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f e r r e a c t i o n s o f t r a n s i t i o n m e t a l complexes has been o b s e r v e d i n a few c a s e s ( 3 - 1 4 ) . However, t h e e l e c t r o c h e m i c a l g e n e r a t i o n o f an a p p r o p r i a t e redox p a i r i n s i t u o f f e r s v a r i o u s a d v a n t a g e s . Under s u i t a b l e c o n d i t i o n s an e l e c t r o l y s i s w i l l t h e n be accompanied by l i g h t emission (electrochemiluminescence or electrogenerated chemil u m i n e s c e n c e , e e l ) ( 1_5). By a p p l i c a t i o n o f an a l t e r n a t i n g c u r r e n t a r e d o x p a i r i s g e n e r a t e d a t the same e l e c t r o d e . A - e A+ e A + A A*


+

+

"* ~* •*

A A A* + A A + hv

anodic c y c l e cathodic cycle annihilation emission

Back e l e c t r o n t r a n s f e r t a k e s p l a c e from the e l e c t r o g e n e r a t e d reduct a n t t o the o x i d a n t n e a r the e l e c t r o d e s u r f a c e . At a s u f f i c i e n t p o t e n t i a l d i f f e r e n c e t h i s a n n i h i l a t i o n l e a d s t o the f o r m a t i o n o f e x c i t e d (*) p r o d u c t s w h i c h may e m i t l i g h t ( e e l ) o r r e a c t "photochemically" without l i g h t ( 1 , 1 6 ) . Redox p a i r s o f l i m i t e d s t a b i l i t y can be i n v e s t i g a t e d by ac e l e c t r o l y s i s . The f r e q u e n c y o f the ac c u r r e n t must be a d j u s t e d t o the l i f e t i m e o f the more l a b i l e redox partner. Many o r g a n i c compounds have been shown t o undergo e e l (17-19). Much l e s s i s known about t r a n s i t i o n m e t a l complexes d e s p i t e the f a c t t h a t t h e y p a r t i c i p a t e i n many redox r e a c t i o n s . Most o b s e r v a t i o n s o f e e l i n v o l v e R u ( b i p y ) ^ (bipy = 2 , 2 ' - b i p y r i d y l ) and r e l a t e d complexes which p o s s e s s e m i s s i v e c h a r g e t r a n s f e r (CT) m e t a l t o l i g a n d (ML) e x c i t e d s t a t e s ( 1 1 , 2 0 - 3 3 ) . The organome t a l l i c compound R e ( o - p h e n ) ( C O ) C 1 (o-phen = 1 , 1 O - p h e n a n t h r o l i n e ) i s a f u r t h e r example o f t h i s c a t e g o r y ( 3 4 ) . P a l l a d i u m and p l a t i n u m p o r p h y r i n s w i t h e m i t t i n g i n t r a l i g a n d (IL) e x c i t e d s t a t e s are a l s o eel a c t i v g ( 3 5 ) . Under s u i t a b l e c o n d i t i o n s e e l was o b s e r v e d f o r Cr(bipy)^ . In t h i s c a s e the e m i s s i o n o r i g i n a t e s from a l i g a n d f i e l d (LF) e x c i t e d s t a t e ( 2 ^ Κ F i n a l l y ^ _ i t has been shown t h a t the e l e c t r o l y s e s o f P t ( p o p ) ( 3 6 ) (pop = diphosphonate) o r M o ^ C l ^ (37_) i s a l s o accompanied by l i g h t emis sion. The r e d o x p r o c e s s e s as w e l l as the subsequent e x c i t e d s t a t e f o r m a t i o n i n v o l v e the m e t a l - m e t a l b o n d i n g o f t h e s e p o l y n u c l e a r com plexes . 3

[

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The p r e s e n t i n v e s t i g a t i o n was c a r r i e d out i n o r d e r t o e x t e n d e e l t o o t h e r t y p e s o f t r a n s i t i o n m e t a l compounds i n c l u d i n g o r g a n o m e t a l l i c s . In a d d i t i o n t o the s e a r c h f o r new systems the m o d i f i c a t i o n o f a well-known e e l was u s e d t o l e a r n more about t h e r e a c t i o n mechanism. The c h o i c e o f new complexes was g u i d e d by some s i m p l e c o n s i d e rations. The o v e r a l l e e l e f f i c i e n c y o f any compound i s the p r o d u c t o f the p h o t o l u m i n e s c e n c e quantum y i e l d and the e f f i c i e n c y of e x c i ted s t a t e formation. T h i s l a t t e r parameter i s d i f f i c u l t t o e v a l u ate. I t may be v e r y s m a l l d e p e n d i n g on many f a c t o r s . An i r r e v e r s i b l e d e c o m p o s i t i o n o f the p r i m a r y redox p a i r can compete w i t h back electron transfer. T h i s back e l e c t r o n t r a n s f e r c o u l d f a v o r the f o r m a t i o n o f ground s t a t e p r o d u c t s even i f e x c i t e d s t a t e f o r m a t i o n i s energy s u f f i c i e n t ( 1 3 , 1 4 , 3 8 , 3 9 ) . Taking i n t o account these p o s s i b i l i t i e s we s e l e c t e d complexes which show an i n t e n s e p h o t o l u m i n e s c e n c e (Φ > 0 . 0 1 ) i n o r d e r t o i n c r e a s e the p r o b a b i l i t y f o r d e t e c t i o n of e e l . In a d d i t i o n , the c h o i c e o f s u i t a b l e complexes was a l s o based on the e x p e c t a t i o n t h a t r e d u c t i o n and o x i d a t i o n would o c c u r i n an a p p r o p r i a t e p o t e n t i a l r a n g e .


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Experimental

Electrochemiluminescence of Organometallics

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Section

Materials,. The compounds Re (o-phen) (CO) C I ( 4 0 ) , I r ( 2 - p h e n y l p y r i d i n e - C ,N ) (41_), [ C u ( p y r i d i n e ) I L (42,43), P t ( 8 - q u i n o linolate) ( 4 4 ) , Tb(TTFA) (o-phen) (45,46) w i t h TTFA = t h e n o y l t r i f l u o r o a c e t o n a t e , [ N H Î C ^ ^ H T M T T F A ^ ] (45), Eu(TTFA) « (o-phen) (£5), and t e t r a l m h y d r o p e r o x i d e (47) were p r e p a r e d a c c o r ding t o published procedures. F o r t h e e l e c t r o c h e m i c a l experiments a c e t o n i t r i l e and CH C l ^ were t r i p l e vacuum l i n e d i s t i l l e d from P^O^g and d e g a s s e d by s e v e r a l f r e e z e - t h a w c y c l e s . The s u p p o r t i n g e l e c t r o l y t e Bu^NBF^ was c r y s t a l l i z e d from d r y a c e t o n e s e v e r a l times and d r i e d i n vacuo.


3

Equipment and Methods. The ac e l e c t r o l y s e s were c a r r i e d o u t under argon i n 1-cm q u a r t z s p e c t r o p h o t o m e t e r c e l l s which were e q u i p p e d w i t h two p l a t i n u m f o i l e l e c t r o d e s d i r e c t l y c o n n e c t e d t o a Krôncke Model 1246 s i n e wave g e n e r a t o r as an ac v o l t a g e s o u r c e . E e l was d e t e c t e d and s p e c t r a l l y a n a l y z e d by s e v e r a l p r o c e d u r e s . The f i r s t d e t e c t i o n was a c h i e v e d by c o n n e c t i n g t h e s p e c t r o p h o t o m e t e r c e l l d i r e c t l y w i t h a p h o t o m u l t i p l i e r (Hamamatsu 1 P21). This arrangement was a l s o u s e d t o o b t a i n maximum e e l i n t e n s i t y by v a r i a t i o n o f the t e r m i n a l ac v o l t a g e and t h e ac f r e q u e n c y . A crude s p e c t r a l a n a l y s i s o f t h e e e l was a c c o m p l i s h e d by p l a c i n g a p p r o p r i a t e b r o a d band i n t e r f e r e n c e f i l t e r s and c u t - o f f f i l t e r s between t h e c e l l and the p h o t o m u l t i p l i e r . The i n t e r f e r e n c e f i l t e r s ( B a l z e r ) K3, K4, K5, and K7 t r a n s m i t t e d maximum i n t e n s i t y a t λ = 510, 565, 610, and 700 nm. The S c h o t t c u t - o f f f i l t e r KV 550 t r a n s m i t t e d l i g h t o f λ > 530 nm. E e l s p e c t r a were r e c o r d e d on a H i t a c h i 850 F l u o r e s c e n c e Spectrophotometer. Results As r e p o r t e d p r e v i o u s l y ac e l e c t r o l y s e s were c a r r i e d o u t i n a s i m p l e u n d i v i d e d e l e c t r o c h e m i c a l c e l l c o n t a i n i n g o n l y t h e two c u r r e n t c a r r y i n g e l e c t r o d e s ( 1_6 ). Most compounds i n v e s t i g a t e d i n t h e p r e s e n t s t u d y showed o n l y v e r y weak e e l i n t e n s i t i e s . F i r s t experi ments were c a r r i e d o u t by p l a c i n g t h e e e l c e l l d i r e c t l y i n f r o n t o f a photomultiplier. By t h i s simple p r o c e d u r e t h e l o w e s t l i g h t i n t e n s i t i e s c o u l d be d e t e c t e d . Under comparable e x p e r i m e n t a l condi t i o n s i n t e g r a t e d e g l i n t e n s i t i e s were d e t e c t e d w ^ i c h were r o u g h l y by a f a c t o r o f 10 lower than t h a t o f R u ( b i p y ) ^ . These measurements were used t o a d j u s t t h e e x p e r i m e n t a l parameters such as ac v o l t a g e and f r e q u e n c y t o maximum e e l i n t e n s i t y . A qualita t i v e a n a l y s i s o f t h e s p e c t r a l d i s t r i b u t i o n o f e e l was a c h i e v e d by i n s e r t i n g broad-band i n t e r f e r e n c e f i l t e r s and c u t - o f f g l a s s f i l t e r s between t h e c e l l and t h e p h o t o m u l t i p l i e r . F i n a l l y , complete e e l s p e c t r a were r e c o r d e d on a l u m i n e s c e n c e s p e c t r o m e t e r . Measurements by c y c l i c voltammetry were c a r r i e d o u t by A. H a i m e r l ( 4 8 ) . For the i n d i v i d u a l compounds t h e f o l l o w i n g e x p e r i m e n t a l d e t a i l s o f t h e e e l experiments a r e g i v e n : s o l v e n t , c o n c e n t r a t i o n o f the supporting e l e c t r o l y t e , c o n c e n t r a t i o n o f t h e compound s u b j e c t e d t o e l e c t r o l y s i s , t e r m i n a l ac v o l t a g e , ac f r e q u e n c y , c u r r e n t , i n t e g r a t e d e e l i n t e n s i t y i n arbitrary units not corrected for photomultiplier r e s p o n s e , and wavelength o f maximum l i g h t i n t e n s i t y , t r a n s m i t t e d by appropriate f i l t e r s .


HIGHE -NERGY PROCESSES IN ORGANOMETALLC I CHEMS ITRY

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Re(o-phen)(CO) CI. CH CN, 0.1 M Bu.NBF^, 3x10 M com plex, 2 ^, 30 HzT 1.1 mA, 40 units, = 610 nm. Upon addition of 3x10~ M tetralin hydroperoxide the eel intensity increased to 120 units. 2 1 Ir(ppy) with ppy = 2-phenylpyridine-C ,N , CH^CN, 0.05 M Bu^NBF, 10 M complex, 4 V, 10 Hz, 9 mA, 4 units, X =510 nm. [Cu(py)l3. with py = pyridine. CH C1 , 0.05 M Bu^NBF^, 2x10" M complex, 5 V, 1 Hz, 10 mA, 20 units, A = 700 nm. max 3

m a x

4

2


K

4

Pt(QO) with QU « 8-quinolinolate. CH CN, 0.005 M Bu^BF , 3x10" M complex, 4 V, 30 Hz, 8 mA, 10 units, λ > 530 nm; at δ V, 30 Hz, and 21 mA the eel intensity increasecPtio 200 units. Tb(TTFA)^(o-phen) with TTFA - thenoyltrifluoroacetonate. CH CN, 0.05 m Bu NBF 2.9x10 M complex, 4 V, 300 Hz, 20 mA, 50 units, λ = 565 nm. During electrolysis a solid _ max ^ separates and covers the electrodes. 3

4

4>

4

[NH(C H ) ]Tb(TTFA) . CH CN, 0.05 M Bu^NBF^, 3x1 θ" M complex, 4 V, 300 Hz, 20 mA, 2 units, λ = 565 nm. max Eu(TTFA) (o-phen). CH CN, 0.005 M Bu NBF 2.7x1 θ" M complex, 4 V, 30 Hz, 8.7 mA, 60 units, = 610 nm. Electrodes are covered by a solid during the electrolysis. 2

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Discussion The ac electrolyses of this work were carried out in an undivided electrochemical cell containing only the two current-carrying elec trodes. This simple apparatus has certainly its limitations but was appropriate for the detection of new ecl-active compounds. The eel intensity of most systems studied here was only very small. Generally, there may be several explanations for this observation. In some cases the reduced and oxidized species formed at the elec trodes are not very stable as revealed by cyclic voltammetry. Only a small fraction of these reactive molecules may undergo the desired annihilation reaction competing with an irreversible decay. Moreover, the back electron transfer could favor the formation of ground state products even i f excited state generation is energy sufficient (13,14,38,39). Finally, for some complexes i t is d i f f i cult to obtain the materials free of impurities. In other cases the complexes are thermally not completely stable and dissolution is accompanied by a small degree of decomposition. These impuri ties may either interfere with the desired electrode process or act as quenchers for the excited molecules undergoing eel. The main goal of the present study was to discover new ecl-ac tive complexes. But the f i r s t example may demonstrate that comple xes known to show eel can serve to gain more insight into the mechanism of electron transfer processes.


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Re(o-phen)(CO)^Cl and Tetraline Hydroperoxide In 1978 Wrighton and his group showed that the complex Re(o-phen)(CO)^Cl undergoes eel from its lowest excited state which lies about +2.3 eV above the ground state (34). The annihilation is energy sufficient. The oxidation of the neutral complex occurs at E^ . = 1-3 V vs. SCE while the reduction takes place at -1.3 V. In 1981 we found that Re(o-phen)(CO) C1 shows an intense chemiluminescence during the catalytic decomposition of tetralin hydroperoxide (THPO) in boiling tetraline (12). Downloaded by NORTH CAROLINA STATE UNIV on May 8, 2015 | http://pubs.acs.org Publication Date: February 26, 1987 | doi: 10.1021/bk-1987-0333.ch010

3

THPO

It was suggested that the mechanism of this reaction can be explai ned on the basis of a "chemically initiated electron exchange lumi nescence (CIEEL)" (49,50) according to the following scheme: +

Re(o-phen)(CO) CI + THPO - Re(o-phen)(C0) C1 + THPO" THPO~ -* α-tetralone" + H 0 Re(o-phen)(CO) C l + α-tetralone" - Re(o-phen)(CO) Cl* + a-tetralone Re(o-phen)(C0) C1* - Re(o-phen)(CO) C1 + hv 3

2

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The f i r s t step is an activated electron transfer which takes place only at higher temperatures (T > 400 K). In the second step the reduced hydroperoxide is converted to the tetralone anion by elimination of water. This ketyl radical anion is strongly reducing ( E ^ ^ = -1.12 V) vs. SCE) (51_). Electron transfer to the complex cation provides enough energy (~ 2.4 eV) to generate Re(o-phen)(CO) C1 in the emitting excited state. The overall process can be described as a catalyzed decomposition of tetralin hydroperoxide. The rhenium complex serves as an electron transfer catalyst which finally takes up the decomposition energy of the peroxide. Apparently the same reaction sequence takes place when THPO and Re(o-phen)(CO) C1 are electrolyzed in acetonitrile at room temperature. The electrolysis replaces only the f i r s t activated electron transfer step of the CIEEL mechanism. At an ac frequency of 30 Hz and a voltage larger than 2.6 V the eel of Re(o-phen)(CO) Cl was very intense. If the voltage dropped below 2.6 V the efficiency of the electrolysis decreased. At 2 V the eel was very weak. Upon addition of equimolar amounts of THPO the eel intensity increased by a factor of ~ 3. The hydroperoxide which is known to undergo an irreversible reduction at E ^ = -0.73 V vs. SCE (52) is apparently reduced during the cathodic cycle while the complex is oxidized during the anodic 3

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cycle. The subsequent reactions are assumed to be the same as those of the CIEEL mechanism. The overall reaction is an electrocatalyzed decomposition of THPO. The complex acts as an electrocatalyst.

Ir(2-phenylpyridins-C^,N')^ With regard to transition metal complexes thg majority of eel studies have been carried out with Ru(bipy)^ and i t s deriva tives (11,20-33). Recently, King, Spellane, and Watts reported on t£e emission properties of Ir(ppy)^ with ppy = 2-phenylpyridineC ,N (41_) which can be considered to be an organometallic counterpart of Ru(bipy)^


+

The lowest excited state (~ 2.5 eV) of the iridium complex which is also of the MLCT type undergoes an efficient emission. The quantum yield was about 0.4 in deoxygenated toluene at room temperature. The complex can be oxidized at E^ = +0.7 V vs. SCE. The reduction was not reported but can be estimated to occur at Ε « = -1.9 V. This potential was obtained for the reduction of P-cippy)^ (53^)· In this case the reduction was also assumed to take place at the ortho-metalated ppy ligand. The potential difference for reduction and oxidation (ΔΕ ~ 2.6 V) provides sufficient energy to generate an excited Ir complex in the annihilation reaction. At an ac voltage of 4 V and 10 Hz we observed a weak eel of Ir(ppy) in acetonitrile. The following reaction sequence may explain this observation: 2

+

Ir(ppy) - e~ "* Ir(ppy) anodic Ir(ppy) + e~" Ir(ppy) ~" cathodic Ir(ppy) + Ir(ppy) " ~* Ir(ppy) * + Ir(ppy) annihilation Ir(ppy) * -* Ir(ppy) + η ν emission 3

3

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3

3

3

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2+

Compared to the efficient eel of Ru(bipy) the low eel inten sity of the Ir complex is rather surprising. 3


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161

LCu(pyridine)I]^


B i n u c l e a r and p o l y n u c l e a r compounds w i t h d i r e c t m e t a l - m e t a l i n t e r a c t i o n c o n s t i t u t e a l a r g e c l a s s o f t r a n s i t i o n m e t a l complexes which p l a y an i m p o r t a n t r o l e a l s o i n o r g a n o m e t a l l i c c h e m i s t r y . G e n e r a l l y , the f r o n t i e r o r b i t a l s o f t h e s e compounds a r e engaged i n m e t a l - m e t a l bonding. C o n s e q u e n t l y , redox p r o c e s s e s a f f e c t t h e m e t a l - m e t a l interaction. The same i s t r u e f o r t h e l u m i n e s c e n c e o f such comple xes s i n c e i t i n v o l v e s a l s o t h g f r o n t i e r o r b i t a l s . The b i n u c l e a r complex Pt^Cpop) ,^36) (pop ~ = d i p h o s p h o n a t e ) and the c l u s t e r Mo^CI (31) a r e r a r e examples o f compounds which c o n t a i n m e t a l - m e t a l bonds and show p h o t o l u m i n e s c e n c e a t ambient c o n d i t i o n s . Both complexes a r e a l s o e e l a c t i v e . In the p r e s e n t s t u d y t h e t e t r a m e r i c complex C c u ( p y ) l ] w i t h py = p y r i d i n e as a n o t h e r compound o f t h i s t y p e was i n v e s t i g a t e d . The c o l o r l e s s t e t r a m e r C c u ( p y ) l ] i s f a i r l y s t a b l e o n l y i n non- o r weakly c o o r d i n a t i o n s o l v e n t s such as benzene, C H ^ C l ^ j o r a c e t o n e . A t room temperature i n s o l u t i o n t h i s copper complex shows an i n t e n s e r e d p h o t o l u m i n e s c e n c e (Φ ~ 0·04^ aj^ = 698 nm (54). The e m i t t i n g s t a t e i s a m e t a l - c e n t e r e d 3d 4s e x c i t e d s t a t e which i s s t r o n g l y m o d i f i e d by C u d ) - C u ( I ) i n t e r a c t i o n i n t h e tetramer. T h i s c o n s i s t s o f a (Cul) cubane c o r e . A t a t e r m i n a l v o l t a g e o f 5 V and a f r e q u e n c y o f 1 Hz t h e com plex Ccu(py)l] i n CH^Cl^ showed a weak e e l which was c l e a r l y i d e n t i f i e d as t h e r e d e m i s s i o n o r i g i n a t i n g from t h e l o w e s t e x c i t e d s t a t e o f t h e complex. I t seems f e a s i b l e t h a t t h e e e l o c c u r s a c c o r d i n g t o t h e u s u a l mechanism. R e d u c t i o n and o x i d a t i o n o f t h e complex i s f o l l o w e d by the a n n i h i l a t i o n and l u m i n e s c e n c e . However, t h e r e must be an e f f i c i e n t c o m p e t i t i o n by o t h e r p r o c e s s e s s i n c e t h e e e l i n t e n s i t y i s r a t h e r low compared t o t h e p h o t o l u m i n e s c e n c e . As i n d i c a t e d by CV measurements t h e r e d u c t i o n a t E^ . - -0.7 V and -1 .6 V and o x i d a t i o n a t +0.8 V v s . SCE a r e l a r g e l y a s s o c i a t e d w i t h i r r e v e r sible reactions. Hence, t h e r e d u c e d and o x i d i z e d forms o f t h e complex seem t o be n o t s t a b l e . The e e l i n t e n s i t y i s then low because o n l y a s m a l l f r a c t i o n o f t h e e l e c t r o g e n e r a t e d redox p a i r e s c a p e s an i r r e v e r s i b l e decay and undergoes an a n n i h i l a t i o n . I t i s a l s o p o s s i b l e t h a t t h e back e l e c t r o n t r a n s f e r i s n o t q u i t e e n e r g y s u f f i c i e n t f o r t h e f o r m a t i o n o f e x c i t e d [ c u ( p y ) l ] ^ (Ε ~ 2 eV) s i n c e t h e p o t e n t i a l d i f f e r e n c e between f i r s t r e d u c t i o n and o x i d a t i o n i s o n l y 1.5 V. F i n a l l y , t h e l a r g e v o l t a g e o f 5 V required f o r the observation of the e e l could also i n d i c a t e that the s o l v e n t CH^Cl which i s r e d u c e d a t E^ . = -2.33 (55) p a r t i c i p a t e s i n t h e e l e c t r o l y s i s and g e n e r a t i o n o f e x c i t e d [Cu(py)l3 . 4

4

A

m

a

x

4

Pt(8-quinolinolate)^ The e x c i t e d s t a t e s which a r e r e s p o n s i b l e f o r t h e e e l o f t h e p r e v i ous examples a r e o f t h e CTML type o r i n v o l v e d i n m e t a l - m e t a l b o n d i n g o f p o l y n u c l e a r complexes. Photoluminescence, o r e e l i n our c a s e , can a l s o o r i g i n a t e from i n t r a l i g a n d (IL) e x c i t e d s t a t e s p r o v i d e d t h e s e s t a t e s a r e t h e l o w e s t e x c i t e d s t a t e s o f such com plexes. I L e m i s s i o n s a r e c h a r a c t e r i s t i c f o r many t r a n s i t i o n m e t a l


162



porphyrins due to the low energy of the ππ* transitions of the porphyrin ligand (56). In 1974 Tokel-Takvoryan and Bard observed eel from porphyrin IL excited states of Pd(II) and Pt(II) tetraphenylporphyrin (35). In the present study we investigated the eel of Pt(QO) (QO = 8-quinolinolate) which is associated with an IL state of this Pt(II) chelate.

In 1978 Scandola and his group observed that solutions of PtiQO)^ exhibit an intense photoluminescence (Φ ~ 0.01) at λ = 650 nm under ambient conditions (44). More details on the pBotiophysics and photochemistry of this compound were reported later (57-59). The emitting excited state was assigned to the lowestenergy IL triplet of the chelate ligand. At a terminal ac voltage of 4 V and a frequency of 30 Hz we observed a weak eel which was clearly identified as the IL emission of PtiQO)^- It is assumed that the ac electrolysis generates a redox pair Pt(QO) Pt(QO) ~. The subsequent annihilation leads to the formation of electronically excited PtiQO)^The low eel intensity may be associated with the observation that the electrochemical oxidation and reduction of Pt(QO) is largely irreversible. CV measurements revealed an oxidation at E. = +0.9 and a reduction at -1.7 V vs. SCE. These redox reactions are probably ligand-based processes. The redox pair generated in the ac electrolysis decays irreversibly to a large extent. Only a small fraction undergoes the annihilation. The potential diffe rence between Pt(QO) and Pt(QO) ~ is 2.6 V. This is certainly sufficient to populate the emitting IL state which lies around 2.0 V above the ground state. 2

2

2

2

Terbium and Europium Complexes The previous examples of eel were interpreted on the basis of a relatively simple mechanism. In these cases the back electron transfer generates directly the emitting excited state (annihila tion). However, in more complicated systems back electron trans fer and formation of an emitting state may be separate processes



10.

VOGLER AND K U N K E L Y


163

(17-19). The back e l e c t r o n t r a n s f e r l e a d s t o a n o n - e m i t t i n g e x c i t e d s t a t e which undergoes energy t r a n s f e r t o a l u m i n e s c e n t s t a t e . T h i s s t a t e does n o t p a r t i c i p a t e i n t h e redox r e a c t i o n . The energy t r a n s f e r can o c c u r as an i n t r a - o r i n t e r m o l e c u l a r p r o c e s s . An i n t e r m o l e c u l a r s e n s i t i z a t i o n o f t h i s t y p e i n v o l v i n g a m e t a l complex was s t u d i e d by Bard and h i s group (60). An europium c h e l a t e s e r v e d as e m i t t i n g energy a c c e p t o r . In t h e p r e s e n t work we s t u d i e d e e l which i n v o l v e s i n t r a m o l e c u l a r energy t r a n s f e r . The back e l e c t r o n t r a n s f e r leads t o a non-emitting e x c i t e d state a t a c e r t a i n part of a molecule. Energy t r a n s f e r p o p u l a t e s an e m i t t i n g e x c i t e d s t a t e a t a different part. We s e l e c t e d r a r e e a r t h m e t a l c h e l a t e s f o r t h i s s t u d y s i n c e t h e y are w e l l known t o undergo i n t r a m o l e c u l a r energy t r a n s f e r from e x c i t e d I L s t a t e s t o e m i t t i n g m e t a l - c e n t e r e d f - l e v e l s upon I L l i g h t a b s o r p t i o n (61-64). Moreover, a t l e a s t T b ( I I I ) i s r a t h e r redox i n e r t and does c e r t a i n l y n o t p a r t i c i p a t e i n e l e c t r o n t r a n s f e r p r o c e s s e s a t moderate p o t e n t i a l s . Tb(thenoyltrifluoroacetonate)

(o-phen)

The complex Tb(TTFA)^(o-phen) w i t h TTFA = t h e n o y l t r i f l u o r o a c e t o n a t e i s a o c t a c o o r d i n a t e r a r e e a r t h c h e l a t e which c o n t a i n s one o-phen and t h r e e TTFA l i g a n d s (45-46). The l a t t e r a r e r e l a t e d t o acetylacetonate.

The l o n g e s t wavelength a b s o r p t i o n band o f Tb(TTJA)^(o-phen) appears a t λ = 336 nm. T h i s i n t e n s e (ε « 10 ) and b r o a d band i s a s s i g n e d t!o an I L t r a n s i t i o n o f TTFA (65). The m e t a l c e n t e r e d f - f bands a r e a l l narrow and o f low i n t e n s i t y . L i g h t a b s o r p t i o n by t h i s I L band caused t h e t y p i c a l g r e e n e m i s s i o n o f T b ( I I I ) . The main f e a t u r e o f t h i s sgructur^ed emmission spectrum § 543 nm i s a s s i g n e d t o t h e ~* F^ t r a n s i t i o n o f t h e Tb i o n (63,64). I n a n a l o g y t o many o t h e r r a r e e a r t h c h e l a t e s an energy transfer o c c u r s from t h e l i g a n d t o t h e e m i t t i n g D s t a t e o f Tb a t 20500 cm" o r 2.54 eV. The donor s t a t e f o r energy t r a n s f e r i s t h e l o w e s t TTFA t r i p l e t a t 20660 cm" o r 2.56 X

a

A

+

=

m

a

x

1

eV (63,64).


164


A t a t e r m i n a l v o l t a g e o f 4 V and an ac f r e q u e n c y o f 300 Hz T b ( T T F A ) ( o - p h e n ) showed a weak e e l which was d e f i n i t e l y i d e n t i f i e d as t h e e m i s s i o n from t h e D„ s t a t e o f T b ( I I I ) . The 4 3

f o l l o w i n g scheme may d e s c r i b e t h e e e l mechanism. Tb(III)(TTFA) (o-phen)

- e~ - T b ( I I I ) ( T T F A ) ( o - p h e n )

3

+

3

anodic T b ( l I I ) ( T T F A ) ( o - p h e n ) + e~ - T b ( I I I ) ( T T F A ) ( o - p h e n ) "


3

3

cathodic

Tb(III)(TTFA) (o-phen)

+

3

-> T b ( I I I ) [ ( T T F A )

+ Tb(III)(TTFA)

(o-phen)"

( o - p h e n ) ] * + Tb(TTFA)

(o-phen) back e l e c t r o n

Tb(III)[(TTFA) (o-phen)]* 3

transfer

Tb(III)*(TTFA) (o-phen) 3

i n t r a m o l e c u l a r energy

Tb(III)*(TTFA) (o-phen) 3

- Tb(III)(TTFA) (o-phen) 3

transfer

+ hv emission

The complex T b ( T T F A ) ( o - p h e n ) underwent a r e d u c t i o n a t Ε = - 1 . 5 V v s . S C E which was p a r t i a l l y r e v e r s i b l e . An o x i d a t i o n was n o t o b s e r v e d below +2 V. A l l redox r e a c t i o n s s h o u l d be l i g a n d based p r o c e s s e s . The p o t e n t i a l d i f f e r e n c e o f Δ Ε > 3 . 5 V i s energy s u f f i c i e n t t o g e n e r a t e t h e I L t r i p l e t a t 2 . 5 6 eV. The low e e l i n t e n s i t y c o u l d be due t o a competing i r r e v e r s i b l e decay o f t h e p r i m a r y redox p a i r . 3

Tb(thenoyltrifluoroacetonate)^~ The a b s o r p t i o n (λ = 335 nm) and e m i s s i o n (λ = 543 nm) spectrum o f Tb( are very s i m i l a r t o those o f Tb(TTFA) (o-phen). The I L e x c i t a t i o n o f T b ( T T F A ) ^ " i s c e r t a i n l y a l s o f o l l o w e d by energy t r a n s f e r t o t h e e m i t t i n g D f - l e v e l of Tb(III). A t a t e r m i n a l v o l t a g e o f 4 V and a f r e q u e n c y o f 300 Hz t h e a n i o n T b ( T T F A ) ^ " shows a v e r y weak e e l . I t was i d e n t i f i e d by a broad-band i n t e r f e r e n c e f i l t e r t o appear around 565 nm. The i n t e n s i t y was t o o low t o r e c o r d t h e e e l spectrum on t h e e m i s s i o n spectrometer. The e e l mechanism i s assumed t o be t h e same as t h a t o f Tb(TTFA) (o-phen). However, t h e v e r y low e e l i n t e n s i t y o f Tb(TTFA) r e q u i r e s an e x p l a n a t i o n . The f i r s t r e d u c t i o n o f t h i s a n i o n t a k e s p l a c e a t Ε . = - 1 . 7 5 V v s . S C E and i s p a r t i a l l y 5

3


10.


VOGLER AND KUNKELY

165

reversible. However, an o x i d a t i o n a t +0.85 V c o u l d be due t o t h e f r e e l i g a n d . I n s o l u t i o n r a r e e a r t h complexes a r e n o t v e r y s t a b l e w i t h r e g a r d t o l o s s o f l i g a n d s . The f r e e l i g a n d i n t e r f e r e s then w i t h t h e redox p r o c e s s e s o f t h e complex i n t h e ac e l e c t r o l y s i s .


Eu(thenoy1tri

fluoroacetonate)^(o-phen)

In a n a l o g y t o T b ( I I I ) s i m i l a r E u ( I I I ) complexes show an i n t e n s e m e t a l - c e n t e r e d p h o t o l u m i n e s c e n c e i n v o l v i n g t h e f - l e v e l s (61-65). An e e l o f an e u r o p i u m ( I I I ) c h e l a t e was r e p o r t e d b e f o r e (60_). The complex was n o t i n v o l v e d i n t h e e l e c t r o l y s i s . Excited organic compounds formed e l e c t r o c h e m i c a l l y underwent an i n t e r m o l e c u l a r energy t r a n s f e r t o t h e e m i t t i n g Eu compound. Interestingly, i n the absence o f t h e r e d o x - a c t i v e o r g a n i c compounds an e e l o f t h e europium c h e l a t e was n o t o b s e r v e d ^ 2+ W h i l e Tb i s redox i n e r t Eu c a n be r e d u c e d t o Eu at r a t h e r low p o t e n t i a l s (~ -0.5 V) (66,67). T h i s adds a f u r t h e r c o m p l i c a t i o n t o any p o s s i b l e e e l mechanism i n v o l v i n g E u ( I I I ^ complexes. F u r t h e r m o r e , t h e back e l e c t r o n t r a n s f e r from Eu to an o x i d i z i n g l i g a n d r a d i c a ^ g e n e r a t e d i n t h e e l e c t r o l y s i s i s a s p i n - a l l o w e d p r o c e s s i f Eu i s formed i n i t s ground s t a t e ^ j 6 8 ) . However, t h e f o r m a t i o n o f t h e e m i t t i n g e x c i t e d s t a t e g f Eu i s spin-forbidden. I t f o l l o w s t h a t t h e g e n e r a t i o n o f Eu i n the ground s t a t e c o u l d be f a v o r e d even i f t h e e x c i t e d s t a t e f o r m a t i o n i s an e n e r g y - s u f f i c i e n t p r o c e s s . But i t was p o i n t e d o u t t h a t t h e s p i n - s e l e c t i o n r u l e may n o t be i m p o r t a n t due t o t h e heavy atom e f f e c t o f europium. W h i l e t h e a b s o r p t i o n spectrum o f Eu(TTFA) (o-phen) i s n e a r l y i d e n t i c a l t o t h a t o f t h e c o r r e s p o n d i n g Tb complex, t h e i n t e n s e r e d e m i s s i o n i s c h a r a c t e r i s t i c f o r Eu . The main band o f t h e structured spectrum appeals a t λ = 613 nm and i s a s s i g n e d t o the m e t a l - c e n t e r e d D ~+ t r a n s i t i o n (63-6^). The emitting D s t a t e has an energy o f 17150 cm o r 2.13 eV w h i l e t h e l o w e s t I L e x c i t e ^ s t a t e which i s a t r i p l e t o f t h e TTFA l i g a n d o c c u r s a t 20450 cm" o r 2.54 eV. The i n i t i a l l y e x c i t e d l i g a n d undergoes an e f f i c i e n t i n t r a m o l e c u l a r energy t r a n s f e r t o t h e emitting D s t a t e o f Eu A t a t e r m i n a l ac v o l t a g e o f 4 V and a f r e q u e n c y o f 30 Hz E u ( T T F A ) ( o - p h e n ) shows^ a weak e e l which was i d e n t i f i e d as t h e t y p i c a l e m i s s i o n o f Eu at λ = 6 1 3 nm. W i t h o u t e x t e n s i v e s p e c u l a t i o n i t i s d i f f i c u l t t o propose an e e l mechanism due t o t h e c o m p l i c a t i o n s d i s c u s s e d above. Eu(TTFA) (o-phen) was n o t o x i d i z e d below +2 V y s . S C E a s i n d i c a t e d by CV measurements. The r e d u c t i o n o f Eu t o Eu i s e x p e c t e d t o o c c u r around -0.5 V. A c l e a r r e d u c t i o n wave was n o t observed i n t h i s r e g i o n . A n o t h e r r e l a t e d E u ( I I I ) c h e l a t e (6£) was a l s o n o t r e d u c e d n e a r t h i s p o t e n t i a l w h i l e some E u ( I I I ) cryptâtes (66,67) undergo a r e v e r s i b l e r e d u c t i o n i n t h i s r a n g e . The complex Eu(TTFA)^(o-phen) showed two i r r e v e r s i b l e r e d u c t i o n s a t -1.3 and -1.63 V. These a r e c e r t a i n l y l i g a n d - b a s e d p r o c e s s e s . The i r r e v e r s i b i l i t y may be due t o a d i r e c t d e c o m p o s i t i o n o f t h e r e d u c e d complex. As an a l t e r n a t i v e t h e r e d u c e d l i g a n d c o u l d r a p i d l y t r a n s f e r an e l e c t r o n t o E u . The E u ( I I ) complex may undergo a f a c i l e l i g a n d displacement. A l l these complications a t v a r i o u s stages o f +

+

5

Q

3

3

2

3 +


166


t h e ac e l e c t r o l y s i s c a n c o n t r i b u t e t o t h e low e e l i n t e n s i t y o f Eu(TTFA) (o-phen). 3


Conclusion F o r t r a n s i t i o n m e t a l complexes an i n t e n s e e e l as i t was o b s e r v e d for Ru(bipy). seems t o be r a t h e r an e x c e p t i o n . I t i s c e r t a i n l y d i f f i c u l t t o draw d e f i n i t e m e c h a n i s t i c c o n c l u s i o n s based on s m a l l e e l e f f i c i e n c i e s because e e l may o r i g i n a t e from s i d e r e a c t i o n s i n t h e s e c a s e s . However, o u r r e s u l t s do show t h a t e l e c t r o n t r a n s f e r r e a c t i o n s w i t h l a r g e d r i v i n g f o r c e s can g e n e r a t e e l e c t r o n i c a l l y e x c i t e d t r a n s i t i o n m e t a l complexes as a r a t h e r g e n e r a l phenomenon. Acknowledgments We thank A. Merζ and A. H a i m e r l f o r measurements and t h e d i s c u s s i o n o f e l e c t r o a n a l y t i c a l d a t a . F i n a n c i a l s u p p o r t o f t h i s work by t h e Deutsche F o r s c h u n g s g e m e i n s c h a f t and t h e Fonds d e r Chemischen I n d u s t r i e i s g r a t e f u l l y acknowledged.

Literature Cited 1. Vogler, Α.; Kunkely, H.; Schäffl, S. In "The Chemistry of Excited States and Reactive Intermediates"; Lever, A. B. P., Ed.; ACS Symposium Series No. 307, American Chemical Society: Washington, D. C., 1986, p. 120. 2. Siders, P.; Marcus, R. A. J. Am. Chem. Soc. 1981, 103, 748. 3. Lytle, F. E.; Hercules, D. M. Photochem. Photobiol. 1971, 13, 123.

4. Martin, J. E.; Hart, E. J.; Adamson, A. W.; Halpern, J. J. Am. Chem. Soc. 1972, 94, 9238.

5. Gafney, H. D.; Adamson, A. W. J. Chem. Ed. 1975, 52, 480. 6. Jonah, C. D.; Matheson, M. S.; Meisel, D. J. Am. Chem. Soc. 1978, 100,

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7. Bolletta, F.; Rossi, Α.; Balzani, V. Inorg. Chim. Acta 1981, 53, L 23. 8. Vogler, Α.; El-Sayed, L.; Jones, R. G.; Namuath, J.; Adamson, A. W. Inorg. Chim. Acta 1981, 53, L 35. 9. Balzani, V.; Bolletta, F. J. Photochem. 1981, 17, 479. 10. Bolletta, F.; Balzani, V. J. Am. Chem. Soc. 1982, 104, 4250. 11. Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 512. 12. Vogler, Α.; Kunkely, H. Angew. Chem. Int. Ed. Engl. 1981, 20, 469.

13. Ghosh, P. K.; Brunschwig, B. S.; Chou, M.; Creutz, C.; Sutin, N.

J. Am. Chem. Soc. 1984, 106, 4772.

14. Liu, D. K.; Brunschwig, B. S.; Creutz, C.; Sutin, N. J. Am. Chem. Soc. 1986, 108, 1749. 15. Faulkner, L. R.; Bard, A. J. In "Electroanalytical Chemistry"; Bard, A. J., Ed.; Marcel Dekker Inc.: New York, 1977; Vol. 10, p. 1. 16. Kunkely, H.; Merz, Α.; Vogler, A. J. Am. Chem. Soc. 1983, 105, 7241.



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