Intramolecular Electron Transfer from Photoexcited Ru(II) Diimine

11 Intramolecular Electron Transfer from Photoexcited

Ru(II) Diimine

Downloaded by GEORGE MASON UNIV on June 18, 2016 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0226.ch011

to N,N'-Diquaternarized

Complexes

Bipyridines

Russell H . Schmehl , Chong Kul Ryu1, C. Michael Elliott , C. L . E. Headford , and S. Ferrere 1

2

2

2

Department of Chemistry, Tulane University, New Orleans, L A 70118 Department of Chemistry, Colorado State University, Fort Collins, C O 80523

1 2

A series of complexes of the type [L Ru(II)(4.x.3-DQ )] + was prepared where L is either 2,2'-bipyridine or 4,4',5 5'-tetramethyl— 2,2'-bipyridine and 4.x.3-DQ is a ligand in which a 4,4'-dimethyl— 2 2'-bipyridine links to a diquaternary 2,2'-bipyridine through a methylene chain (x). Rate constants for intramolecular electron transfer from the excited Ru(II) complex to the diquaternary 2,2'— bipyridinedecrease as the length of the bridging chain increases from x = 2 to 12. The observed electron-transfer rate exhibits an even-odd chain length alternation for x = 2 to 6. A large reorganizational barrier is obtained (1.6 V) from the temperature dependence of the electron-transfer rate for the x = 6 complex. Rate constants for the back reaction were estimated from yields for trapping the Ru(III) of the intermediate by triethylamine. A preliminary account is given of environmental effects on the intramolecular electron transfer. 2+

2

4

,

2+

,

T H E D E P E N D E N C E O F E L E C T R O N - T R A N S F E R RATES on donor-acceptor sep

aration is o f f u n d a m e n t a l interest i n charge-transfer c h e m i s t r y (1-4). T h i s d e p e n d e n c e is p a r t i c u l a r l y t r u e for electron-transfer reactions i n b i o l o g i c a l systems (2-4), w h e r e t h e distance c a n b e large ( > 1 5 Â). M o d e l systems have b e e n p r e p a r e d that l i n k a n e l e c t r o n d o n o r a n d acceptor t h r o u g h a sterically r i g i d spacer framework for w h i c h t h e d o n o r - a c c e p t o r d i s t a n c e is c l e a r l y d e f i n e d (5-13).

Several elegant p o r -

p h y r i n - q u i n o n e complexes have b e e n p r e p a r e d (14-18) i n w h i c h p h o t o 0065-2393/90/0226-0211$06.00/0 © 1990 American Chemical Society

Johnson et al.; Electron Transfer in Biology and the Solid State Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

212

E L E C T R O N TRANSFER IN BIOLOGY A N D T H E SOLID STATE

i n d u c e d e l e c t r o n transfer occurs from t h e excited p o r p h y r i n to a q u i n o n e l i n k e d to the p o r p h y r i n t h r o u g h a rigid b r i d g e . Closs a n d co-workers ( I , 5-8)

e x a m i n e d e l e c t r o n transfer from the b i p h e n y l a n i o n t h r o u g h steroid

a n d d e c a l i n links to naphthalene. A n e x p o n e n t i a l d e p e n d e n c e of the e l e c t r o n transfer rate o n t h e n u m b e r o f bonds b e t w e e n t h e d o n o r a n d acceptor is o b s e r v e d for l i n k e d derivatives i n w h i c h the stereochemistry of t h e l i n k to the b r i d g i n g a l k y l

framework

is fixed (i.e., b o t h d o n o r a n d acceptor are

equatorial). T h e results indicate that t h r o u g h - b o n d electronic c o u p l i n g is a n i m p o r t a n t c o m p o n e n t i n long-distance (thus nonadiabatic) e l e c t r o n transfer


i n this system. F a r m o r e w o r k has b e e n d o n e o n d o n o r - a c c e p t o r flexible

systems l i n k e d b y

chains o r b r i d g i n g groups w i t h several s l o w l y i n t e r c o n v e r t i n g c o n -

formers (19). M u c h o f this w o r k has b e e n d i r e c t e d t o w a r d u n d e r s t a n d i n g i n t r a m o l e c u l a r c h a i n d y n a m i c s i n flexible p o l y m e r s (20, 21). Studies o f t h e d e p e n d e n c e o f t h e rates o f i n t r a m o l e c u l a r e x c i m e r f o r m a t i o n o r e l e c t r o n transfer o n t h e n u m b e r of m e t h y l e n e units i n t h e b r i d g i n g c h a i n have y i e l d e d a m i x o f results (22-28).

F o r instance, t h e rate o f i n t r a m o l e c u l a r e l e c t r o n

transfer i n a l k y l - l i n k e d a n t h r a c e n e - a l k y l a m i n e systems i n p o l a r solvents is o n l y w e a k l y d e p e n d e n t o n t h e c h a i n l e n g t h a n d shows a m i n i m u m for t h e t h r e e - m e t h y l e n e - b r i d g e d c o m p l e x (28). M a t a g a a n d co-workers (29, 30) r e p o r t e d a systematic decrease o f t h e rate constant for e l e c t r o n transfer (fc ) w i t h i n c r e a s i n g c h a i n l e n g t h (two, et

four, a n d six methylenes) for l i n k e d o c t a e t h y l p o r p h y r i n - b e n z o q u i n o n e c o m plexes. I n t h e m o r e c o m p l e x b r i d g e d t e t r a p h e n y l p o r p h y r i n - b e n z o q u i n o n e systems s t u d i e d b y C o n n o l l y a n d others (31-33),

e v i d e n c e is o b t a i n e d for

ground-state Π c o m p l e x formation i n complexes c o n n e c t e d b y t w o o r three l i n k i n g m e t h y l e n e s . I n t r a m o l e c u l a r self-exchange e l e c t r o n transfer i n a l k y l l i n k e d α-naphthyl a n d IV-phthalimide moieties was s t u d i e d b y S z w a r c a n d co-workers (20). A n e x p o n e n t i a l decrease w i t h i n c r e a s i n g c h a i n l e n g t h u p to six m e t h y l e n e s was o b s e r v e d i n p r o p r i o n i t r i l e . F o r l o n g e r linkages, t h e rate was f o u n d to be essentially i n v a r i a n t w i t h solvent. I n a l l of these a l k y l - l i n k e d systems, at least o n e o f t h e t w o reactive species is u n c h a r g e d a n d ground-state c o m p l e x f o r m a t i o n is frequently o b s e r v e d . A m u c h s m a l l e r b o d y o f i n f o r m a t i o n exists for flexible c h a i n - l i n k e d d o n o r - a c c e p t o r complexes i n w h i c h b o t h reactive sites have l i k e charges. Intervalence-transfer (IT) absorption i n m i x e d - v a l e n c e c o m p l e x e s (34) (eq 1) has s h o w n that electronic c o u p l i n g b e t w e e n t h e m e t a l centers decreases r a p i d l y as t h e n u m b e r o f m e t h y l e n e s i n t h e b r i d g i n g l i g a n d ( l , n - b i s ( 4 - p y r idyl)alkanes) increases. I n fact, no I T b a n d is o b s e r v e d for t h e l , 2 - b i s ( 4 pyridyl)ethane b r i d g e d d i m e r (35, 36) [(bpy) ClRu(II)(py-(CH ) -py)Ru(III)(NH ) ] ^ 2

2

n

3

5

5 +

[(bpy) ClRu(III)(py-(CH ) -py)Ru(II)(NH )5] 2

2

n

3

w h e r e b p y is 2 , 2 ' - b i p y r i d i n e a n d p y is p y r i d i n e .


5+

(1)

11.

S C H M E H L ET AL.

Intramolecular Electron Transfer

213

H u r s t a n d co-workers (37, 38) e x a m i n e d p h o t o i n d u c e d e l e c t r o n transfer from Cu(I) olefin complexes l i n k e d to Co(III) p e n t a m m i n e t h r o u g h a l k e n o i c acids, aminoalkenes, a n d p y r i d y l a l k e n e s . F o r the a m i n o a l k e n e series, a steady decrease i n the q u a n t u m y i e l d for p h o t o r e d u c t i o n of the Co(III) c e n t e r was o b s e r v e d w i t h increasing a l k y l c h a i n l e n g t h u p to eight m e t h y l e n e b r i d g i n g carbons, w h e r e the q u a n t u m y i e l d was b e l o w the l i m i t of m e a s u r e m e n t (38). T h e y a t t r i b u t e d the c h a i n - l e n g t h d e p e n d e n c e to b o t h the effect of c o u l o m b i c r e p u l s i o n o f the two centers o n the d i s t r i b u t i o n of conformers i n solution a n d the r a p i d relaxation o f the Cu(I)-to-olefin(Tr*) m e t a l - t o - l i g a n d


charge-transfer ( M L C T ) state ( [(L) Ru(III)(4.x.3-DQ )] 2

+

4+

(2)

T h i s chapter describes the effect of c h a n g i n g the n u m b e r o f b r i d g i n g m e t h y l e n e carbons, x, o n the rate o f p h o t o i n d u c e d e l e c t r o n transfer i n c o m plexes w i t h a fixed exergonicity ( L is b p y or T M B a n d η = 3). T h e effect of e n v i r o n m e n t a l p e r t u r b a t i o n o n the electron-transfer d y n a m i c s is also p r e sented. A p r e l i m i n a r y account is g i v e n of m e a s u r e m e n t o f the r a p i d t h e r m a l Johnson et al.; Electron Transfer in Biology and the Solid State Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

214

E L E C T R O N TRANSFER IN BIOLOGY A N D T H E S O L I D STATE

back e l e c t r o n transfer (eq 3) b y t r a p p i n g the transient Ru(III) c o m p l e x w i t h triethylamine. [(L) Ru(III)(4.x.3-DQ )] +

2

[(L) Ru(n)(4.x.3-DQ )]

4+

2 +

2

(3)

4 +


Results for the f o r w a r d e l e c t r o n transfer (eq 2) show that the electron-transfer rate decreases w i t h increases i n the n u m b e r o f m e t h y l e n e carbons a n d that rates for e l e c t r o n transfer d e p e n d o n w h e t h e r the b r i d g e has an o d d or e v e n n u m b e r o f c a r b o n atoms.

Redox and Spectroscopic Properties of [(L) Ru(4.x.3-DQ )] + 2+

2

4

T h e b r i d g i n g ligands w e r e p r e p a r e d as r e p o r t e d e a r l i e r (39, 40, 42) a n d d i q u a t e r n a r i z e d w i t h 1,3-dibromopropane to y i e l d 4 . x . 3 - D Q (x = 2, 3, 4, 5, 6, o r 12). T h e b r i d g i n g l i g a n d ( 4 . x . 3 - D Q ) was t h e n h e a t e d (175 °C) w i t h a n excess of [ R u ( L ) C l ] ( L is b p y or T M B ) i n e t h y l e n e g l y c o l u n d e r N for 1 h to p r o d u c e the r e s u l t i n g c o m p l e x , [(L) R u ( 4 . x. 3 - D Q ) ] . P u r i f i c a t i o n of the c o m p l e x was a c h i e v e d b y repeated c h r o m a t o g r a p h y o n silica g e l w i t h 5:4:1 acetonitrile:water:saturated aqueous K N 0 (39, 40). 2 +

2 +

2

2

2

2 +

2

4 +

3

T h e redox b e h a v i o r for the series of complexes w i t h fixed L was i d e n t i c a l ; one-electron oxidation o f the c o m p l e x (eq 4) was r e v e r s i b l e b y c y c l i c v o l t a m m e t r y a n d corresponds to the m e t a l - l o c a l i z e d , R u ( I I I / I I ) , c o u p l e (40). [(L) Ru(III)(4.x.3-DQ )] 2+

2

5+

[(L) Ru(H)(4.x.3-DQ )] 2 +

2

4 +

(4)

F i v e r e v e r s i b l e r e d u c t i v e waves w e r e o b s e r v e d b y c y c l i c v o l t a m m e t r y (40). T h e first two o c c u r (eq 5) at - 0 . 6 4 a n d - 0 . 9 2 V vs. the sodium-saturated c a l o m e l electrode ( S S C E ) a n d c o r r e s p o n d to two sequential reductions o f the 4 . x . 3 - D Q . 2 +

[(L) Ru(II)(4.x.3-DQ )]

4+

^

[(L) Ru(II)(4.x.3-DQ )]

3+

(5a)

[(L) Ru(II)(4.x.3-DQ )]

3+

^

[(L) Ru(II)(4.x.3-DQ°)]

2+

(5b)

2+

2

+

2

2

2

+

T h e r e m a i n i n g reductions c o r r e s p o n d to s e q u e n t i a l r e d u c t i o n o f the d i i m i n e ligands c o o r d i n a t e d to the Ru(II) center. T h e first of these r e d u c t i o n s is at - 1 . 3 6 V for [ ( b p y ) R u ( 4 . 2 . 3 - D Q ° ) ] , v i r t u a l l y i d e n t i c a l to the first r e d u c t i o n p o t e n t i a l of [ ( b p y ) R u ( D M B ) ] , w h i c h is b p y l o c a l i z e d . T a b l e I s u m m a r i z e s redox data for complexes i n w h i c h L is e i t h e r b p y o r T M B . 2+

2

2 +

2

T h e absorption a n d e m i s s i o n properties o f Ru(II) d i i m i n e complexes have b e e n w i d e l y s t u d i e d (43). T h e spectroscopic characteristics o f the [(L) Ru(4.x.3-DQ )] series closely p a r a l l e l those of the parent c o m p l e x , [ ( L ) R u ( D M B ) ] , except that the e m i s s i o n q u a n t u m y i e l d s are s m a l l e r a n d 2 +

2

2

4 +

2 +


11.

215


S C H M E H L E T AL.

Table I. Physical Properties of Complexes with the Ligand 4 . 6 . 3 - D Q * 2

Property

[(bpy) Ru(L)] +

[(TMB) Ru(L)] +

1.24 -0.64 -1.36 624 0.008 8.1 x 10 0.36

1.06 -0.64 -1.51 634 0.003 8.3 x 10 0.46

2

E° [Ru(III/II)] E° [ D Q )] E° (U/L'-) E (298 K), n m (298 K ) k, s ΔΕ, V e

( 2 + / +

e

b

em

r

c

c

em

4

_1

d

4

2

4

4

Potentials, E°, vs. SSCE in C H C N . V is bpy for [(bpy) Ru(L)] and D M B of the 4.6.3-DQ for [(TMB) Ru(L)] . I n deaerated C H C N . Emission maxima, E and quantum yields, Φ em, are uncorrected for detector response. ^Determined from the approximate E , E° [ D Q ] , and E° [Ru(III/II)] using ΔΕ = Eoo - E° [Ru(III/II)] + E° [ D Q ]. 3


b

4+

2

2

2+

4+

3

em>

( 2 + / + )

m

( 2 + / + )

the l u m i n e s c e n c e decay rates are m u c h faster t h a n those o f t h e p a r e n t c o m plex. T h e absorption is Ru(II) to L ( i r * ) M L C T . 3

( M L C T ) state that is f o r m e d

E m i s s i o n arises from the

following intersystem crossing

from

the

* ( M L C T ) state. F o r the parent c o m p l e x , the i n t e r s y s t e m crossing efBeiency is u n i t y (44). T h e ( M L C T ) state of [ ( b p y ) R u ( D M B ) ] 3

2

is q u e n c h e d efficiently b y the

2 +

d i q u a t e r n a r y 2 , 2 ' - b i p y r i d i n e . F l a s h photolysis a n d steady-state t r a p p i n g (of transient Ru(III)) studies indicate that the q u e n c h i n g occurs b y

electron

transfer from the e x c i t e d c o m p l e x to the d i q u a t e r n a r y 2 , 2 ' - b i p y r i d i n e 45,

The luminescence

46).

a n d redox characteristics o f the Ru(II)

(43, com-

p l e x - d i q u a t e r n a r y b i p y r i d i n e systems s t u d i e d h e r e are g i v e n i n T a b l e I. T h e exergonicity o f excited-state e l e c t r o n transfer i n [ ( L ) R u ( 4 . 6 . 3 - D Q ) l 2 +

2

4 +

is

e s t i m a t e d from the R u ( I I I / I I ) a n d D Q ( 2 + / + ) potentials a n d the z e r o - z e r o e m i s s i o n energy,

of [ ( L ) R u ( D M B ) ] 2

2 +

(39-41). E x e r g o n i c i t i e s i n the

range o f - 0 . 3 6 to - 0 . 4 6 V indicate that the reactions are i n t h e n o r m a l free e n e r g y r e g i o n (45, 46, 47,

48).

Chain-Length Dependence of Photoinduced Electron Transfer T h e l u m i n e s c e n c e lifetimes o f the complexes i n the series [ ( b p y ) R u ( 4 . x . 3 2

DQ

2 +

)]

4 +

i n C H C N at 298 Κ are g i v e n i n T a b l e I I . T h e rate o f i n t r a m o l e c u l a r 3

e l e c t r o n transfer from the ( M L C T ) state o f t h e Ru(II)complex to t h e l i n k e d 3

d i q u a t e r n a r y b i p y r i d i n e (eq 2) is o b t a i n e d from the difference i n l u m i n e s cence decay rates o f the d i q u a t e r n a r y b i p y r i d i n e - c o n t a i n i n g c o m p l e x (fc ) obs

a n d the parent c o m p l e x of the series, [ ( b p y ) R u ( D M B ) ] , k (eq 6). 2

2 +

0


216


Table II. Luminescence Lifetimes and Intramolecular Electron-Transfer Rate Constants for [(bpy) Ru(L)] Complexes i n C H C N at 298 Κ 2

3

Ligand L

τ, ns

DMB 4.2.3-DQ

2 +

4.3.3-DQ

2 +

4.4.3-DQ

2 +

4.5.3-DQ 4.6.3-DQ 4.12.3-DQ

2+


2+

2+

690 1.7 42 18 128 99 373

± ± ± ± ± ± ±

k* x I 0 , 8'

Number of C-C Bonds

590 22 54 6.4 8.7 1.2

3 4 5 6 7 13

1

6

5 0.5 3 3 3 2 6

T h i s rate assumes that e l e c t r o n transfer occurs e x c l u s i v e l y from the ( M L C T ) 3

state a n d that the i n t e r s y s t e m crossing efficiency, t\

is u n i t y for t h e d i

isc9

q u a t e r n a r y b i p y r i d i n e - c o n t a i n i n g complexes.

I f r e d u c t i o n o f the d i q u a t e r

n a r y b i p y r i d i n e occurs from the ^ M L C T ) state, t\

isc

Because the l u m i n e s c e n c e l i f e t i m e (r ), em

m u s t b e less t h a n 1.

emission quantum y i e l d

( ), em

T) are r e l a t e d (eq 7), r e l a t i v e changes i n the i n t e r s y s t e m crossing efficiency isc

can b e d e t e r m i n e d from q u a n t u m y i e l d a n d l i f e t i m e data i f i t is a s s u m e d that the radiative decay rates, k , o f the p a r e n t c o m p l e x a n d the d i q u a t e r T

nized-species d i q u a t e r n a r y 2 , 2 ' - b i p y r i d i n e - c o n t a i n i n g c o m p l e x are the same.

^ e m

Ήΐ8ο^ι·Τ"βιη

F o r the c o m p l e x [ ( b p y ) R u ( 4 . 6 . 3 - D Q ) ] , 2 +

2

4 +

CO

the e m i s s i o n q u a n t u m y i e l d

i n C H C N is 0.008 ± 0.001 a n d the l u m i n e s c e n c e l i f e t i m e is 99 ns, g i v i n g 3

a value of 8 Χ 1 0 s 4

1

for η J f e . T h i s v a l u e is close to t h e [ ( b p y ) R u ( D M B ) ] r

2

2 +

radiative decay rate o f 8.3 Χ 1 0 . T h e s i m i l a r i t y i m p l i e s that e l e c t r o n transfer 4

from

the * ( M L C T ) state o f [ ( b p y ) R u ( 4 . 6 . 3 - D Q ) ] 2

2 +

4 +

is not a major decay

p a t h . B i m o l e c u l a r q u e n c h i n g can also b e e x c l u d e d as a decay p a t h for the 3

M L C T state u n d e r the e x p e r i m e n t a l conditions u s e d . T h e c o m p l e x

c e n t r a t i o n was l o w ( < 2

Χ

10"

5

con

M ) a n d the b i m o l e c u l a r q u e n c h i n g rate

constant is less t h a n diffusion-controlled (2 Χ 1 0 M " 8

1

s" ). 1

F i g u r e 1 illustrates the d e p e n d e n c e o f In (fc ) o n the n u m b e r o f c a r et

b o n - c a r b o n b o n d s b e t w e e n the two redox centers. A l t h o u g h the data are l i m i t e d , i t is clear that, for o d d n u m b e r s o f b o n d s u p to 7, an e x p o n e n t i a l decrease i n fc occurs. T h e e x p o n e n t i a l falloff is e x p e c t e d i f the p r e d o m i n a n t et

conformation o f the c o m p l e x e s i n s o l u t i o n is f u l l y e x t e n d e d a n d c o n f o r m a t i o n a l f o l d i n g is slow o n t h e t i m e scale o f t h e e x p e r i m e n t . T h e

observed

alternation of fc t w i t h the n u m b e r o f b o n d s is also consistent w i t h e l e c t r o n e

transfer from f u l l y e x t e n d e d conformers. T h e o b s e r v e d e x p o n e n t i a l decrease for the series b r i d g e d b y a n o d d n u m b e r o f b o n d s suggests that the e l e c t r o n -


and


11.



Ο

2

6

4

8

217

10

12

14

# C-C bonds Figure 1. Plot of In (k^) vs. the number of C-C bonds between the Ru(II)coordinated bipyridine and the diquaternized species diquaternary 2,2'-bi pyridine. The solid line represents the least-squares fit to data from complexes with an odd number of C-C bonds (O, odd; A, even). The dashed line rep resents the decay rate of [(bpy) Ru(DMB)] . 2+

2

transfer rate w i l l b e slower t h a n t h e decay rate o f the u n q u e n c h e d c o m p l e x (dashed line) w h e n m o r e t h a n n i n e bonds separate t h e Ru(II) c o m p l e x from the d i q u a t e r n i z e d species d i q u a t e r n a r y 2 , 2 ' - b i p y r i d i n e . F o r t h e 13-bond b r i d g e d c o m p l e x , t h e m e a s u r e d fc is greater t h a n (k )~ , a n i n d i c a t i o n that conformational changes f o l l o w i n g excitation o f the c o m p l e x result i n e l e c t r o n transfer. 0

et

l

Studies o f W i n n i k (21) show that the d y n a m i c s of conformational changes i n l i n e a r a l k y l p o l y m e r s are o n t h e 10-100-ns t i m e scale. T h e l u m i n e s c e n c e lifetimes o f the [ ( b p y ) R u ( 4 . a : . 3 - D Q ) ] series are i n t h e range o f 1 - 1 0 0 ns (except for t h e 4.12.3 complex). T h e r e f o r e , t h e o b s e r v e d rates are l i k e l y to b e a function o f b o t h t h e rate o f conformational change a n d e l e c t r o n i c t u n n e l i n g from t h e d o m i n a n t solution conformers. F l e x i b l e c h a i n - b r i d g e d d o n o r - a c c e p t o r complexes are poor models for e x a m i n i n g distance effects o n electron-transfer reactions. H o w e v e r , such effects m a y b e seen i n systems 2+

2

4+

that have electron-transfer rates that are faster than conformational r e a r rangement rates (fc > 1 0 s" ) or have steric o r electrostatic factors that h i n d e r conformational motions. 7

et

1

T h e excited-state e l e c t r o n transfer i n these complexes exhibits t e m p e r ature d e p e n d e n c e b e t w e e n 180 a n d 300 K . L u m i n e s c e n c e lifetimes o f [(TMB) Ru(4.6.3-DQ )] and [ ( T M B ) R u ( D M B ) ] i n 4:1 e t h a n o l : m e t h a n o l above t h e liquid-to-glass transition t e m p e r a t u r e are s h o w n i n F i g 2

2 +

4 +

2

2 +


218


u r e 2. B o t h complexes show t e m p e r a t u r e - d e p e n d e n t l u m i n e s c e n c e decays. T h e behavior of [ ( T M B ) R u ( D M B ) ] 2

complexes


3

is s i m i l a r to that o f o t h e r d i i m i n e Ru(II)

2 +

that e x h i b i t t h e r m a l l y activated i n t e r n a l c o n v e r s i o n

from

the

( M L C T ) state to a m e t a l - c e n t e r e d , ( M C ) , state (43). 3

T h e t e m p e r a t u r e d e p e n d e n c e of i n t r a m o l e c u l a r e l e c t r o n transfer (eq 2) can b e e x a m i n e d b y u s i n g e q 6 to d e t e r m i n e fc at each t e m p e r a t u r e . A c tivation parameters o b t a i n e d from E y r i n g plots y i e l d an activation e n t h a l p y (AH*) o f 4.7 k c a l / m o l a n d an activation e n t r o p y (AS*) of - 8 . 9 e u . F o r i n t r a m o l e c u l a r e l e c t r o n transfer i n p o l y p r o l i n e - l i n k e d O s - R u a m m i n e s , the ac tivation e n t h a l p y , a n d h e n c e the reorganizational e n e r g y , has b e e n s h o w n to d e p e n d o n the distance b e t w e e n the two redox active centers (49). et

F u r t h e r , it was s h o w n that the distance d e p e n d e n c e o f the n u c l e a r factor is greater than that of the electronic factor i n these systems. I f i t is assumed that A S = 0 for the reaction a n d that t h e r e is no t e m p e r a t u r e d e p e n d e n c e of solvent d i e l e c t r i c p r o p e r t i e s , t h e n the reorganizational e n e r g y , λ , c a n b e d e t e r m i n e d from A f f * ; i n [ ( T M B ) R u ( 4 . 6 . 3 - D Q ) ] , X = 1.6 V . E s t i m a t e s of λ o f a p p r o x i m a t e l y 0.8 V have b e e n o b t a i n e d from studies o f b i m o l e c u l a r excited-state e l e c t r o n transfer b e t w e e n Ru(II) b i p y r i d y l complexes a n d a series of d i q u a t e r n a r y b i p y r i d i n e s a n d viologens (45-48). I n this c o m p l e x , the larger λ may result from the increased distance o r from conformational changes r e q u i r e d p r i o r to e l e c t r o n transfer. 2

2 +

4 +

I n p r i n c i p l e , the rate of back e l e c t r o n transfer from the r e d u c e d d i q u a ternary b i p y r i d i n e s to the Ru(III) (eq 3) can b e d e t e r m i n e d e i t h e r from flash


11.



219

photolysis o r steady-state p h o t o l y s i s , i n w h i c h o n e o f t h e transient species is c h e m i c a l l y t r a p p e d . S u b n a n o s e c o n d [(L) Ru(4.2.3-DQ )] 2 +

2

flash photolysis studies (39) o f

s h o w e d that t h e back e l e c t r o n transfer is faster t h a n

4 +

t h e rise t i m e o f t h e apparatus u s e d . T h u s , a l o w e r l i m i t for t h e rate constant was e s t i m a t e d to b e 3 X 1 0

s

1 0

(39).

_ 1

O n e a p p r o a c h to c h e m i c a l l y t r a p p i n g o n e of t h e i n t e r m e d i a t e ions f o r m e d i n e q 2 is r e d u c t i o n o f the Ru(III) b y a m i l d r e d u c t a n t s u c h as t r i e t h y l a m i n e or t r i e t h a n o l a m i n e . T h i s a p p r o a c h has b e e n w i d e l y u s e d i n schemes to g e n erate r e d u c e d viologens o r d i q u a t e r n a r y b i p y r i d i n e s for studies o f h y d r o g e n


e v o l u t i o n from w a t e r u s i n g c o l l o i d a l catalysts (50-52). T h e s c h e m e for t r a p p i n g Ru(III) b y t r i e t h y l a m i n e i n t h e series [ ( L ) R u ( 4 . x . 3 - D Q ) 2 +

2

is s h o w n

4 +

i n eqs 8 - 1 2 . [(L) Ru(II)(4.x.3-DQ )]

4 +

[(L) Ru(III)(4.x.3-DQ )]

4+

2 +

2

+

2

[(L) Ru(III)(4.x.3-DQ )] +

2

+

4+

[(L) Ru(III)(4.x.3-DQ )] +

2

[(L) Ru(II)(4.x.3-DQ )] 2 +

2

2

5

3

+

2

2

5

3

+ (C H ) Nt

3+

2

5

(10)

3

+ ( C H ) N ^ (C H ) NH+ + (C H ) N(CHCH )

+

2

[(L) Ru(II)(4.x.3-DQ )] 2 +

2

5

3

2

+

4 +

5

3

2

5

2

(11)

3

(C H ) N(CHCH ) 2

5

2

3

[(L) Ru(II)(4.x.3-DQ )] +

2

The

(9)

4 +

(C H ) N-^ [(L) Ru(II)(4.x.3-DQ )]

(C H ) N

(8)

4+

+ (C H ) N = C H C H

3+

2

5

2

(12)

3

radical c a t i o n o f t r i e t h y l a m i n e f o r m e d i n e q 10 reacts w i t h a s e c o n d

m o l e o f ( C H ) N to p r o d u c e a radical that c a n r e d u c e a s e c o n d d i q u a t e r n a r y 2

5

3

b i p y r i d i n e (eq 12). T h u s , a single p h o t o n w i l l p r o d u c e t w o r e d u c e d d i q u a ternary bipyridines. T h e overall quantum y i e l d

for t h e process is g i v e n

(# b ) 0

s

b y e q 13. 4>obs = 2^ [kJ(k isc

The

+

+ k

r

n

fc )]{fc [(C H ) ]/(fc et

t

2

5

3

+

b

fc [(C H ) N])} t

2

(13)

5 3

i n t e r s y s t e m crossing efficiency, T| , is a s s u m e d to b e u n i t y , a n d t h e ISC

electron-transfer efficiency, k /(k et

+ k

r

n

+ fc ), η et

φ

is o b t a i n e d from t h e

e m i s s i o n lifetimes o f [ ( L ) R u ( 4 . x . 3 - D Q ) ] , T , a n d [ ( L ) R u ( D M B ) ] , τ , 2 +

2

4 +

c

2 +

2

0

as 1 - ( T / T ) . A n estimate o f t h e i n t r a m o l e c u l a r back-electron-transfer rate C

may

0

be obtained

fc [(C H ) N]), t

2

from

t h e efficiency o f t r a p p i n g ,

& [(C2H ) N]/(fc t

a n d k n o w l e d g e o f the t r a p p i n g rate, k . t

5 3

complexes [ ^ p y ) R u ( 4 . x . 3 - D Q ) ] 2

2 +

4 +

5

3

+

b

Photolysis of the

i n C H C N c o n t a i n i n g 0.5 M ( C H ) N 3

2

5

3

leads to t h e p r o d u c t i o n o f t h e r e d u c e d d i q u a t e r n a r y b i p y r i d i n e , [ ( b p y ) R u 2

( 4 . x . 3 - D Q ) ] , after 1 0 - 3 0 m i n o f photolysis for t h e 5-, 6-, a n d 12-carbon +

3 +

b r i d g e d species. T a b l e I I I lists o b s e r v e d q u a n t u m y i e l d s for D Q

+

production and ap-


220


Table HI. Quantum Yields for Trapping of [(bpy) Ru(4.x.3-DQ )] with (C H ) N +

2

2

Complex 2

2

2

3

k*, s-

V

[(tmb) Ru(4.6.3-DQ )] [(bpy) Ru(4.5.3-DQ )] [(bpy) Ru(4.6.3-DQ )] [(bpy) Ru(4.12.3-DQ )] 2

5

1

2+

2+

2+

1.3 1.0 2.0 3.0

0.003 0.003 0.002 0.007

0.95 0.87 0.84 0.45

2+

x x x x

10 10 10 10

10

10 10

9

NOTE: These values assume that = 1 and k = 2 Χ 1 0 M ' V . See text eq 13. "Fraction of excited states quenched by electron transfer. ^Observed quantum yield for diquaternized species formation. Relative error is ± 5 0 % . 7


t

1

proximate i n t r a m o l e c u l a r back-electron-transfer rates w i t h an assumed t r a p p i n g rate constant o f 2 Χ 1 0 M " s" (46). T h e margins o f e r r o r are large because o f the uncertainty i n t h e l u m i n e s c e n c e lifetimes o f the complexes, c o u p l e d w i t h t h e fact that t h e r e d u c e d d i q u a t e r n a r y b i p y r i d i n e slowly d e composes. A s a result, t h e calculated back-electron-transfer rate constants represent u p p e r l i m i t i n g values. Results from b i m o l e c u l a r t r a p p i n g o f p h o toproducts from photolysis o f [ R u ( b p y ) ] a n d diquaternary bipyridine i n aqueous solution (46) y i e l d values o f 1-4 X 1 0 M s" for t h e geminate back-electron-transfer rate. 7

1

1

3

2+

1 0

-

1

1

A l t h o u g h l i m i t e d data are available h e r e , t h e results are q u a l i t a t i v e l y comparable w i t h distance-dependence effects o b s e r v e d for the f o r w a r d e l e c t r o n transfer (eq 2). Back-electron-transfer rates that are m u c h faster t h a n q u e n c h i n g rates are expected because t h e reorganizational energies o f the forward a n d back electron transfer s h o u l d b e comparable. G i v e n t h e a c t i vation parameters o b t a i n e d for the forward process, the back reaction s h o u l d be close to t h e activationless l i m i t (λ = 1.6 V a n d A G = - 1 . 8 8 V ) .

Environmental Effects on Intramolecular Electron Transfer A f e w s i m p l e experiments illustrate t h e relative i m p o r t a n c e o f c o u l o m b i c a n d d y n a m i c conformational changes o n excited-state e l e c t r o n transfer i n [ ( b p y ) R u ( 4 . x . 3 - D Q ) ] . A d d i t i o n of t e t r a e t h y l a m m o n i u m perchlorate (0.1 M ) to acetonitrile solutions o f b o t h the 6- a n d 12-carbon b r i d g e d complexes results i n an increase i n fc , as shown i n T a b l e IV. T h e o b s e r v e d effect is consistent w i t h a strong influence o f electrostatic factors o n t h e d i s t r i b u t i o n of conformers i n solution p r i o r to electron transfer. 2

2 +

4 +

et

Viscosity effects o n electron transfer i n these complexes d e p e n d o n t h e particular m e d i u m . F o r example, as shown i n T a b l e IV, t h e r e is o n l y a factor of 2.2 decrease i n t h e l u m i n e s c e n c e lifetime o f [ ( T M B ) R u ( 4 . 6 . 3 - D Q ) ] i n c h a n g i n g t h e viscosity, η , o f the m e d i u m from g l y c e r o l at 273 Κ (η = 220,000 cp) to 4:1 ethanol:methanol at t h e same t e m p e r a t u r e (η = 1.2 cp). H o w e v e r , i n p o l y ( m e t h y l methacrylate) ( P M M A ) glasses at r o o m t e m p e r a t u r e , t h e l u m i n e s c e n c e lifetime o f [ ( b p y ) R u ( 4 . 6 . 3 - D Q ) ] becomes b i 2 +

2

2

2 +

4 +


4 +

11.



221

Table IV. Effects of M e d i u m on Intramolecular Excited-State Electron Transfer Complex

[(bpy) Ru(4.12.3-DQ )] C H C N C H C N , 0.1 M T E A P * [(bpy) Ru(4.6.3-DQ )] C H C N C H C N , 0.1 M T E A P PMMA 2

4+

3

3

2+

2

4+

3

f e

3

C

[(bpy) Ru(4.6.3-DQ )] 2+

2

4+

C H OH-CH OH glycerol e H OH-CH OH glycerol 2


2

5

5

Ί\ ns

Temperature,Κ

Medium 2+

3

3

373 93 99 57 608 1460 36 99 63 141

298 298 298 298 298 298 298 276 273

± ± ± ± ± ± ± ± ± ±

6 3 2 4 5 (33%) 8 (67%) 3 4 3 6

"τ is luminescence lifetime of M L C T emission. *ΤΕΑΡ is tetraethylammonium perchlorate. T M M A is poly(methyl methacrylate) glass. 3

exponential a n d the longer of the two decays approaches that of [(bpy) Ru(DMB)] i n P M M A . T h e nearly c o m p l e t e disappearance of i n t r a m o l e c ular electron transfer i n P M M A may result from slow d i e l e c t r i c relaxation of the m e d i u m (53-55). T h e s e effects are c u r r e n t l y b e i n g investigated i n greater d e t a i l . 2

2 +

Summary T h i s w o r k demonstrates that excited-state electron transfer i n flexible c h a i n l i n k e d d o n o r - a c c e p t o r complexes w i t h a large c o u l o m b i c r e p u l s i o n b e t w e e n the d o n o r a n d acceptor can show effects s i m i l a r to those o b s e r v e d i n rigid b r i d g e d systems. F o r the series [ ( b p y ) R u ( 4 . x . 3 - D Q ) ] , b o t h electronic a n d nuclear factors are i m p o r t a n t i n d e t e r m i n i n g the rate of i n t r a m o l e c u l a r e l e c t r o n transfer, as reflected b y the large distance d e p e n d e n c e a n d o b servable temperature d e p e n d e n c e . I o n r e c o m b i n a t i o n i n this c o m p l e x series is r a p i d ( > 1 0 s ) , e v e n i n complexes w i t h l o n g a l k y l bridges. F u r t h e r , examination of excited-state electron transfer of these complexes i n viscous a n d rigid m e d i a illustrates that d i e l e c t r i c relaxation effects can significantly affect the electron-transfer rate, particularly w h e n the m e d i u m d i e l e c t r i c relaxation rate is slower than the excited-state relaxation rate. 2

i 0

2 +

4 +

1

Acknowledgments R . H . S c h m e h l acknowledges the support of the P e t r o l e u m Research F u n d , a d m i n i s t e r e d b y the A m e r i c a n C h e m i c a l Society, a n d the L o u i s i a n a E d u cational Q u a l i t y S u p p o r t F u n d , a d m i n i s t e r e d b y the L o u i s i a n a State B o a r d of Regents. C M . E l l i o t t thanks the N a t i o n a l Science F o u n d a t i o n (Grant N o . C H E 8516904) for support.


222


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Intramolecular Electron Transfer from Photoexcited Ru(II) Diimine

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