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Manipulation of Doublet Excited State Lifetimes in Chromium(III) Complexes John F. Endicott, Ronald B. Lessard, Yabin Lei, Chong Kul Ryu, and R. Tamilarasan Department of Chemistry, Wayne State University, Detroit, MI 48202 The relaxation rates observed for the lowest energy doublet excited states (2E) of chromi um (III) complexes can be represented,kre=kore + kre(T). The limiting low temperature excited state lifetime, τ=(kore)-1,is a molecular property which is nearly independent of tem perature and the condensed phase environment, but τo does decrease with such molecular properties as the number of N-H vibrational modes available to function as acceptors for the electronic excitation energy. The thermally activated decay, kre(T), is a function of the solvent and the coordinated ligands. When kre(T) is fitted to an Arrhenius function, values of ln A vary more than do the apparent activation energies, and varia tions in kre(298) are more often determined by the pre-exponential factor than by Ea. The transition between τo and thermally activated relaxation occurs at a temperature, Ttr, which is a strong function of the medium and of the coordinated ligands. The observed room tem perature 2E lifetimes are more strongly cor related with Ttr than with the apparent Arrhenius activation energy. It is suggested that the thermally activated relaxation channel(s) involves a strongly coupled, but spin forbidden surface crossing to the potential energy surface of a reaction intermediate. However, some preliminary observations suggest that there may be more than one possible decay channel. Vibrationally equilibrated electronic excited states in molecules are unique chemical species. These are metastable species with unusual electronic configurations, and on this basis alone they might be expected to exhibit unusual patterns of reactivity. However, it is probably a more striking feature 0097-6156/ 86/ 0307-0085506.00/ 0 © 1986 American Chemical Society Lever; Excited States and Reactive Intermediates ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

86

EXCITED STATES AND

REACTIVE INTERMEDIATES

o f t h e s e e l e c t r o n i c e x c i t e d s t a t e s t h a t they have s t o r e d a c o n s i d e r a b l e amount o f energy i n a p r e d o m i n a t e l y e l e c t r o n i c form, and t h a t i f t h i s e l e c t r o n i c energy were c o n v e r t e d t o o t h e r energy forms, i t would exceed the energy r e q u i r e m e n t s f o r many s i m p l e c h e m i c a l p r o c e s s e s . The c o n v e r s i o n o f e l e c t r o n i c e x c i t a t i o n energy i n t o a more u s e f u l energy form ( e . g . , v i b r a t i o n a l , e l e c t r i c a l , e t c . ) can be v e r y s e l e c t i v e . The p r i n c i p l e s g o v e r n i n g t h i s s e l e c t i v i t y are not always w e l l understood. The l o w e s t energy e x c i t e d e l e c t r o n i c s t a t e o f the c h r o mium(III) complexes c o n s i d e r e d h e r e i s d e s i g n a t e d the state ( f o r c o n v e n i e n c e , even i n low symmetry c o m p l e x e s ) . This e x c i t e d s t a t e d i f f e r s from the ground s t a t e , **&2g ( i n 0^ symmetry, see F i g u r e 1 ) , i n s p i n m u l t i p l i c i t y , but not i n o r b i t a l population. As a consequence the ^£ and states have n e a r l y i d e n t i c a l m o l e c u l a r g e o m e t r i e s , and the d i f f e r e n c e i n energy c o n t e n t o f t h e s e s t a t e s i s e n t i r e l y e l e c t r o n i c ( a s m a l l e n t r o p y c o r r e c t i o n must be made when c o n s i d e r i n g reaction driving forces). The t y p i c a l e x c i t e d s t a t e - g r o u n d s t a t e energy d i f f e r e n c e i s about 14 χ 10^ cm~l and i t exceeds the energy requirements f o r simple s u b s t i t u t i o n , i s o m e r i z a t i o n , e t c . , r e a c t i o n s o f the ground s t a t e . Thus i t i s not s u r p r i s i n g t h a t ( E ) C r ( I I I ) species are o f t e n very u n s t a b l e with respect t o l i g a n d r e p l a c e m e n t o r i s o m e r i z a t i o n r e a c t i o n s (1-3). However, d e s c r i b i n g such c h e m i c a l p r o c e s s e s i s not s i m p l e s i n c e the and e l e c t r o n i c s t a t e p o t e n t i a l energy s u r f a c e s must be v e r y s i m i l a r i n shape (1-4), a t l e a s t near t h e i r e q u i l i b r i u m n u c l e a r configurations. F u r t h e r m o r e , many o f t h e s e r e a c t i o n s have been found t o be s t e r e o s e l e c t i v e and t o o c c u r i n c o m p e t i t i o n w i t h p h o s p h o r e s c e n c e e m i s s i o n and n o n - r a d i a t i v e r e l a x a t i o n o f the e x c i t e d s t a t e (1-4). In t h i s r e p o r t we d e s c r i b e some o f the s t u d i e s which have been i n i t i a t e d t o i n v e s t i g a t e the f a c t o r s c o n t r i b u t i n g t o the b e h a v i o r o f the lowest d o u b l e t e x c i t e d s t a t e i n c h r o m i u m ( I I I ) . The p r i n c i p l e g o a l o f our work has been t o e x p l o r e t h o s e s t e r i c c o n s t r a i n t s , i n t r o d u c e d by the l i g a n d s , which g r e a t l y a l t e r the p h o t o p h y s i c a l p r o p e r t i e s o f the excited state. In p u r s u i t o f t h i s g o a l , we have r e - i n v e s t i g a t e d some f e a t u r e s o f w e l l known amine and p o l y p y r i d y l complexes i n o r d e r t o o b t a i n i n t e r n a l l y c o n s i s t e n t r e f e r e n c e systems. In p r i n c i p l e one must d e t e r m i n e o r t a k e i n t o a c c o u n t a v a r i e t y of f a c t o r s . Among t h e s e a r e : 2

1. 2.

the energy d i f f e r e n c e s , ΔΕ*, between the l o w e s t energy e l e c t r o n i c a l l y e x c i t e d q u a r t e t and d o u b l e t s t a t e s ; the energy and r o l e , i f any, o f h i g h e r energy d o u b l e t e x c i t e d s t a t e s ( e s p e c i a l l y components o f the T^, state); d i s t o r t i o n s o f the ^E e x c i t e d s t a t e p o t e n t i a l energy surface ; 2

3.

Lever; Excited States and Reactive Intermediates ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

ENDICOTT ET AL.

3+

Doublet Excited State Lifetimes in Cr

Complexes

b. Lowest Energy Metal Centered Electronic States

Configuration Coordinate F i g u r e 1. S t a t e e n e r g i e s ( a ) and q u a l i t a t i v e p o t e n t i a l energy s u r f a c e s (b) f o r Cr(NH3)| . A l t e r n a t i v e mechanistic proposals f o r ( E ) C r ( I I I ) decay a r e i l l u s t r a t e d i n 1 ( b ) : a, back i n t e r s y s t e m c r o s s i n g ; b, d i r e c t r e a c t i o n t o y i e l d e l e c t r o n i c a l l y c o r r e l a t e d p r o d u c t s ; c, s u r f a c e c r o s s i n g t o some ground s t a t e i n t e r m e d i a t e p o t e n t i a l energy s u r f a c e . +

2


87

88

EXCITED STATES AND REACTIVE INTERMEDIATES 4.

5.

the s i g n i f i c a n c e o f v a r i a t i o n s i n the c h e m i c a l and e l e c t r o n i c p r o p e r t i e s o f C r ( I I I ) i n d u c e d by v a r i a t i o n s i n t h e ligands; the n a t u r e and c h e m i c a l b e h a v i o r o f any p h o t o g e n e r a t e d reaction intermediates.

I n p r a c t i c e , and d e s p i t e much s t u d y o f C r ( I I I ) systems, t h e r e i s o n l y a l i t t l e d e f i n i t i v e i n f o r m a t i o n about any o f t h e s e f a c t o r s . I n t h i s r e p o r t we d i s c u s s some c u r r e n t s t u d i e s o f chromium complexes w i t h s t e r i c a l l y c o n s t r a i n e d l i g a n d s . These s t u d i e s a r e p r o v i d i n g i n f o r m a t i o n m o s t l y about the f o u r t h f a c t o r l i s t e d above. Temperature

2

Dependence o f ( E ) C r ( I I I ) 2

Lifetimes

2

The l i f e t i m e s τ ( Ε ) , o f the E e l e c t r o n i c e x c i t e d s t a t e o f C r ( I I I ) complexes i s a s t r o n g f u n c t i o n o f t e m p e r a t u r e . The d e t a i l e d temperature dependence d i f f e r s from complex t o complex and from s o l v e n t t o s o l v e n t . In g e n e r a l , t ( E ) i s s t r o n g l y temperature dependent a t h i g h t e m p e r a t u r e s and temperature independent a t low t e m p e r a t u r e s ( 3 , 5, 6 ) . Thus, 2

2

[x( E)]-l

« k

r e

« k?

e

+ k

r e

(T)

(1)

where k j * ( τ ° ) " ^ i s the l i m i t i n g low temperature r a t e c o n s t a n t f o r e x c i t e d s t a t e r e l a x a t i o n and the r a t e o f r a d i a t i v e r e l a x a t i o n i s assumed t o be s m a l l . In many i n s t a n c e s the tem p e r a t u r e dependent term, k ( T ) , can be r e p r e s e n t e d as a s i m p l e A r r h e n i u s t e m p e r a t u r e dependence (6) but more o f t e n a more complex f u n c t i o n i s r e q u i r e d t o f u l l y accommodate the c u r v a t u r e ( 3 , 6, 7, 8). e

r e

2

( E ) C r ( I I I ) L i f e t i m e s i n the Low Temperature Regime. As the temperature d e c r e a s e s , k approaches a l i m i t i n g v a l u e , k j which i s u s u a l l y independent o f the condensed phase medium. The low temperature l i f e t i m e s o f Cr***N£ complexes and cyanoa m i n e - C r ( I I I ) complexes a r e u s u a l l y f o u n d t o be n e a r l y tem p e r a t u r e independent o v e r an a p p r e c i a b l e temperature range. V a l u e s o f k°- can t h e r e f o r e be r e g a r d e d as f u n c t i o n s o f s t r u c t u r a l f e a t u r e s o f the chromium complexes. S i n c e the E and p o t e n t i a l energy s u r f a c e s a r e n e s t e d , e x c i t e d s t a t e r e l a x a t i o n i n t h i s regime i s a t t r i b u t e d t o l i m i t i n g weak c o u p l i n g (9) between the e x c i t e d and ground e l e c t r o n i c s t a t e s . In such a l i m i t , the n o n - r a d i a t i v e t r a n s i t i o n between s u r f a c e s depends on n u c l e a r and e l e c t r o n i c t u n n e l i n g . The more im p o r t a n t m o l e c u l a r p a r a m e t e r s which a r e e x p e c t e d ( 9 , 10) t o c o n t r i b u t e t o r e l a x a t i o n r a t e s i n t h i s regime a r e : (1) the energy d i f f e r e n c e , Δ Ε ° , between t h e e x c i t e d and ground e l e c t r o n i c s t a t e s ; (2) t h e number o f h i g h f r e q u e n c y v i b r a t i o n a l modes a v a i l a b l e t o d i s s i p a t e t h i s energy; (3) the magnitude o f r e

e

e

2


ENDICOTT ET AL.

3+


Complexes

the d i f f e r e n c e between t h e e x c i t e d s t a t e and t h e ground s t a t e n u c l e a r c o o r d i n a t e s f o r t h e s e modes; ( 4 ) t h e f r e q u e n c i e s o f t h e n u c l e a r p r o m o t i n g modes; and, ( 5 ) s p i n - o r b i t c o u p l i n g . Of these f a c t o r s , the c o n t r i b u t i o n s of the high frequency acceptor modes have been b e s t documented, (3, 11-13); e.g., i n C r ( N H 3 ) | and C r * * * ( N H 3 ) 5 X complexes, p e r d e u t e r a t i o n i n c r e a s e s τ ° by n e a r l y two o r d e r s o f magnitude, more o r l e s s c o n s i s t e n t w i t h expectation. No s i m p l e dependence on Δ Ε has emerged, b u t t h e compounds i n v e s t i g a t e d span a range o f o n l y , a t most 2000 cm""-*-, i n E ( E ) , and t h e e n e r g i e s o f t h e i r p r o m o t i n g modes p r o b a b l y v a r y o v e r a comparable range. Selected observations are summarized i n T a b l e I . +

0

2

Thermally A c t i v a t e d Relaxation

Rates

Solvent Mediation of E x c i t e d State L i f e t i m e s ; Examples from the B e h a v i o r o f P o l y p y r i d y l Complexes. Most C r ( I I I ) complexes e x h i b i t a s t r o n g s o l v e n t dependence o f t h e i r E e x c i t e d s t a t e lifetimes. The most extreme examples o f t h i s b e h a v i o r a r e p r o b a b l y found among t h e p o l y p y r i d y l complexes. F o r example, x ( E ) i s about 50 ns f o r C r ( t p y ) ^ i n water a t 25°C (14), b u t i n c r e a s e s t o 20 μβ i n t h e s o l i d s t a t e ( p e r c h l o r a t e s a l t ; 25°C) (15). T h i s k i n d o f b e h a v i o r has been e x t e n s i v e l y i n v e s t i g a t e d , e s p e c i a l l y by Kemp and co-workers (6), f o r C r ( p h e n ) ^ . The e f f e c t o f s o l v e n t on t ( E ) f o r t h i s complex appears t o be m a n i f e s t e d by compensation between temperature dependent (ΔΗ*) and t e m p e r a t u r e i n d e p e n d e n t components, ( A S * ) : V a l u e s o f τ ( Ε ) have u s u a l l y been found t o be s o l v e n t dependent i n t h e t h e r m a l l y a c t i v a t e d regime, and t o v a r y i n g d e g r e e s t h e s e s o l v e n t d e p e n d e n c i e s a r e m a n i f e s t e d by v a r i a t i o n s i n b o t h t h e A r r h e n i u s a c t i v a t i o n e n e r g i e s and p r e - e x p o n e n t i a l f a c t o r s (3, 6). Thus, i n t h e t h e r m a l l y a c t i v a t e d regime t h e l i f e t i m e o f t h e l o w e s t d o u b l e t e x c i t e d s t a t e cannot be r i g o r o u s l y r e g a r d e d as an i n t r i n s i c molecular property. Rather, values o f i ( E ) a r e de t e r m i n e d by some i n t e r a c t i o n between t h e m o l e c u l a r e x c i t e d s t a t e and i t s e n v i r o n m e n t . 2

2

+

+

2

2

2

P h o t o p h y s i c a l P r o p e r t i e s o f Simple Ammine (Amine) Complexes. Many systems have been s t u d i e d ; we w i l l o n l y c o n s i d e r a r e p r e s e n t a t i v e few. A more e x t e n s i v e r e c e n t r e v i e w o f t h e l i t e r a t u r e c a n be found e l s e w h e r e (3). In many ways t h e p h o t o p h y s i c a l b e h a v i o r o f ( E g ) C r ( N H 3 ) ^ i s p a r a d i g m a t i c o f ammine and amine c h r o m i u m ( I I I ) systems. Near ambient c o n d i t i o n s i n f l u i d s o l u t i o n , t h i s e l e c t r o n i c a l l y e x c i t e d complex has t h e f o l l o w i n g p r o p e r t i e s : ( 1 ) a h i g h l y s t r u c t u r e d e m i s s i o n ( F i g u r e 2) e x h i b i t i n g an i n t e n s e 0-0 l i n e and r e s o l v e d v i b r o n i c components; t h i s i s s t r o n g e v i d e n c e f o r s i m i l a r e q u i l i b r i u m n u c l e a r c o n f i g u r a t i o n s o f t h e e x c i t e d and ground e l e c t r o n i c s t a t e s (3, 16); ( 2 ) a s t r o n g l y t e m p e r a t u r e dependent e x c i t e d s t a t e l i f e t i m e ( F i g u r e 3 ) ; t ( E ) ~ 2 μβ a t 2

2

g


+


6

3 +

5

3

3 +

5

3

3 +

3

2

3 +

2

3

2

+

2

+

2

1

3+

ir^j?-Cr(L )(CN)

CIS-CT{)en

-Cr(L )(NH )

2

+

+

2

rra/w-CriLjMNH^*

1

+

120 (17)

22.2

52 (17) >80 (17)

18.9 (43) 20.7 (44)

14.2 (17)

14.1 (17)

14.3 (32)

14.8 (11)

23.5 (32)

379 ± 50 (32)

5600 (32)

88 (17)

17.6 (43)

14.4 (17)

f

136 (11)

1580 (18) 116 (18)

21.4 (18)

~15 (18)

21.5 (11)

3690 ± 30 (18)

f

175 ± 5 (18)

370 ± 20 (8, 20)

24.16 (42)

14.0 (32) 22.5 (18)

196 (40)

23.6 (41)

14.2 (40)

~15 (18)

3660 (32)

540 (39)

-21.1 (38)

13.0 (38)

5000 (37)

~23.9 (34, 35)

(36)

13.7 (37)

14.8 (17)

108 ± 12 (15, 23, 24) 2560 (15)

21.88 (34, 35)

9662 (32)

15.0 (33)

100 (32)

22.2

14.7 (30, 31)

(31)

19.25 (27)

14.8 (26)

ira^Cr(L )(CN)

2

41 (28, 29)

e

3500 (15, 25)

(μβ)

2+

5 (15, 23, 24)

75 i

(μβ)

21.64 (11)

3

15.2 (16)

l

C

AN-D) '

2 +

Cr(phen) (NH )

2

Cr(tpy) *

3

Cr(phen)

Crisen) "*"

3

Cr(en)

3

Cr(NH ) CN

3

Cr(NH ) Cl

3

Cr(NH )

Complex

4 4 Ε ( T o r E) ™*1 3 (cm J\0 )

Spectroscopic Parameters o f Soae Chroaiua(III) Complexes

-1 3 (cm V H T )

Table I.


2

2

2

3

3+

3 +

2

3 +

2

+

en phen Lj L2 L3 L4 L5 L5

β

» * * = = = =

e

c

D

(12)

ethylenediaraine; sen * 4,4',4"-Ethylidynetris(3~azabutan-l-amine) 1,10-phenanthraline; tpy » t e r p y r i d i n e ; 1,4,8,1l-Tetraazacyclotetradecane(cyclam); 5,\2-meso-5,7,7,12,14,14-hexamethyl-l,4,8,11-tetraazacyclotetradecane 5,12-rac-5,7,7,12,14,14-hexamethyl-l,4,8,l1-tetraazacyclotetradecane 1,4,7-triazacyclononane; 1,4,7-tris(acetato)-l,4,7-triazacyclononane ( T C T A ) ; 1,2-bi s( 1,4, 7-triazacyclononane)-ethane (BCNE)

50 (40)

20..8 (46)

(40)

13.8

430 (40)

(21)

19.. 5

14.1 (40)

(12)

14.7

91 (40)

20..1 (40) 370 ± 30

(40)

14.7

56 (17)

19..0 (45)

(32)

(17)

94 (17)

208

>108

94 (17)

16..7 (45)

21,.6 (32)

20..0 (45)

17..4 (45)

22..8 (12)

(17)

13.8

(32)

(17)

14.1

13.8

(17)

14.4

Data from (3, 6, 8, 11, 12, 15-18). E l e c t r o n i c o r i g i n of the lowest energy doublet state, except as i n d i c a t e d . Low temperature l i m i t i n g l i f e t i m e ( i n a r i g i d glass matrix), d Coordinated amines (ammines) i n the proteo form. Coordinated amines (ammines) perdeuterated. ^ Unresolved emission spectrum (20).

a

Abbreviations:

HOTES FOR TABLE I:

6

Cr(L )

5

Cr(L )

4

Cr(L )

3

cis-Cv{L >en

A

cy -Cr(L )(NCS)

3

c7*-Cr(L )Cl

3

c^Cr(L )(CN)

+

2

+

) Cl^

tran*-Cr(L )(NCS)

trâns-Cr(

(32)

(teta); (tetb);

3200 (12)

1854

£

^

^

^ δ 2'

£

I

H >

D ο ο

EXCITED STATES AND REACTIVE INTERMEDIATES

77 Κ

15.6

15.4

15.2

15.0

cm7l0

14.8

14.6

14.4

3

F i g u r e 2.

E m i s s i o n s p e c t r a f o r C r ( N H ) ( C 1 0 4 ) 3 a t 250 Κ and 7 7 K . 3

ο

6

ο •

as

4.0

4.5 1A xid\ κ"

·

ο

·

55

5.0

1

2

+

F i g u r e 3. Temperature-dependent l i f e t i m e s of ( E)Cr(NH3>| i n DMF:CHCl3 s o l u t i o n s (upper c u r v e ) and i n Ru(NH3)|+ c r y s t a l s (Ru:Cr - 30:1); from (8).


ENDICOTT ET AL.

3+


3

Complexes

1

25°C and E - 3 . 8 x l 0 cm"- i n DMF (8); ( 3 ) a l a r g e p r o b a b i l i t y for substitution, ~ 0.5, and t h e a p p a r e n t r a t e c o n s t a n t f o r s u b s t i t u t i o n i s a t l e a s t 10* times l a r g e r f o r the E excited s t a t e t h a n f o r t h e ground s t a t e (2); ( 4 ) x ( E ) becomes n e a r l y temperature independent f o r Τ < 250 Κ and approaches a m a t r i x i n d e p e n d e n t low t e m p e r a t u r e l i m i t i n g v a l u e , x ° ( 2 ) - 75 ± 5 ]is\ ( 5 ) t h e e x c i t e d s t a t e l i f e t i m e i s i n c r e a s e d by p e r d e u t e r a t i o n even i n t h e s t r o n g l y t e m p e r a t u r e dependent regime; ( 6 ) x ( E g ) i s dependent on t h e condensed phase environment; t h i s i s most d r a m a t i c a l l y i l l u s t r a t e d by t h e c o n t r a s t between ( E ) C r ( N H ) 3 + i n D M F / C H C I 3 s o l u t i o n and doped i n t o t h e Ru(NH ) Cl s o l i d (Figure 3). a

1

2

g

2

g

E g

2

2

3

3

6

3

2

+

The t h e r m a l l y a c t i v a t e d b e h a v i o r o f ( E ) C r ( N H ) 3 , ( 1 5 ) , ( E)Cr(NH ) CN ( 1 7 ) , and C r ( N H ) C 1 + (17) p r o v i d e some i n structive contrasts. The 77 Κ l i f e t i m e o f t h e hexammine i s b r a c k e t e d by t h o s e o f t h e pentammines (75, 100 and 42 μ β , r e s p e c t i v e l y ) . The room t e m p e r a t u r e l i f e t i m e s a r e i n t h e same o r d e r , b u t span many o r d e r s o f magnitude ( i n H 2 O ca: 2, 14 and CI . The much s h o r t e r ambient s o l u t i o n l i f e t i m e s found f o r t h e c h l o r o complexes r a i s e s t h e p o s s i b i l i t y o f t h e i n t e r v e n t i o n o f second r e l a x a t i o n c h a n n e l ; e.g., such a c h a n n e l c o u l d i n v o l v e d i r e c t p a r t i c i p a t i o n o f t h e l o w e s t energy q u a r t e t e x c i t e d s t a t e s , which a r e p r o b a b l y r e a s o n a b l y c l o s e i n energy t o t h e £ s t a t e i n t h e s e c h l o r o complexes. Any a d d i t i o n a l r e l a x a t i o n c h a n n e l would r e q u i r e m o d i f i c a t i o n o f e q u a t i o n 1, so that t

r

2

k

T

re< >

β

k

?e

(

T

)

+

k

r e

(

T

)

(

2

)

I f t h e two c h a n n e l s were t r u l y d i s t i n c t , t h e n d o m i n a t i o n o f k (T) by t h e c h l o r i d e m e d i a t e d r e l a x a t i o n c h a n n e l (designated "b") s h o u l d l e a d t o a r e p r e s s i o n o f ( a n d u l t i m a t e l y e l i m i n a t e ) c h a r a c t e r i s t i c features o f r e l a x a t i o n channel " a " . I f the l o n g e r l i f e t i m e f o r the trans- than f o r t h e c j ^ - g e o m e t r i e s be t a k e n as a c h a r a c t e r i s t i c f e a t u r e o f k ( T ) , t h e n t h i s re f e a t u r e does appear t o be r e p r e s s e d when t h e c o o r d i n a t i o n o f c h l o r i d e i n c r e a s e s k ( T ) by more than t h r e e o r d e r s o f mag nitude. T h i s argues t h a t t h e c h l o r i d e m e d i a t e d r e l a x a t i o n pathway i s d i s t i n c t . A p o s s i b l e means f o r a c c o u n t i n g f o r t h e s e observations i s that the E r e l a x a t i o n involves crossing to the p o t e n t i a l energy s u r f a c e s o f r e a c t i o n i n t e r m e d i a t e s (i.e., i n t o some, n o t n e c e s s a r i l y t h e l o w e s t e n e r g y , r e a c t i o n c h a n n e l o r c h a n n e l s o f t h e e l e c t r o n i c ground s t a t e ) which have q u a r t e t spin m u l t i p l i c i t y . Such e l e c t r o n i c a l l y f o r b i d d e n crossings would be f a c i l i t a t e d by s p i n o r b i t c o u p l i n g , and ^ E - ^ - J ^ s p i n o r b i t c o u p l i n g i n c r e a s e s as Δ Ε * E ( ^ T ) - E ( E ) d e c r e a s e s . r

e

a

r e

2

β

2

2

L i g a n d s w i t h a Tendency Towards T r i g o n a l D i s t o r t i o n s The observations summarized i n t h e p r e c e d i n g s e c t i o n seem t o i n d i c a t e t h a t t h e n u c l e a r d i s t o r t i o n s which e f f e c t r e l a x a t i o n o f t h e ( E ) C r ( I I I ) e x c i t e d s t a t e a r e more e a s i l y a c c o m p l i s h e d from a cz'.i?-Cr* -*-(N4)X2 complex t h a n from a trans-Cr^^(H^)X2 complex. T h i s s u g g e s t s t h a t l i g a n d s which f a c i l i t a t e c e r t a i n t y p e s o f n u c l e a r motions s h o u l d reduce t h e e x c i t e d s t a t e lifetimes. W i t h t h i s h y p o t h e s i s i n mind, we have been i n v e s t i gating the p o t e n t i a l f o r h i g h l y s t r a i n e d ( i n the e l e c t r o n i c ground s t a t e ) l i g a n d s t o i n d u c e e x c i t e d s t a t e d i s t o r t i o n s by 2

I


EXCITED STATES AND

REACTIVE INTERMEDIATES

employing a s e r i e s o f s u b s t i t u t e d 1 , 4 , 7 - t r i a z a c y c l o n o n a n e complexes. These N - s u b s t i t u t e d l i g a n d s t e n d t o promote a t r i g o n a l p r i s m a t i c geometry, a p o t e n t i a l d i s t o r t i o n which might open a r e a c t i o n c h a n n e l not a v a i l a b l e i n the t e t r a g o n a l complexes d i s c u s s e d above. ^ The p a r e n t , Cr( [ Î J j a n e ^ ^ , i s r e l a t i v e l y l o n g l i v e d under ambient c o n d i t i o n s ( 1 2 ) . The t r i - a c e t a t o d e r i v a t i v e , ( E ) C r ( T C T A ) , and ( E ) C r ( [ 9 ] a n e N ) | have comparable v a l u e s o f k° , but ( E ) C r ( T C T A ) has an e x c e p t i o n a l l y s h o r t l i f e t i m e under ambient c o n d i t i o n s . These v a r i a t i o n s i n k ( T ) a r e not m a n i f e s t e d i n E , but i n v e r y d i f f e r e n t v a l u e s o f T (or A). The TCTA l i g a n d has a tendency t o f a v o r a t r i g o n a l p r i s m a t i c geometry, but Cr(TCTA) i s o n l y s l i g h t l y d i s t o r t e d from an a n t i p r i s m a t i c geometry ( 2 1 ) . I f a t r i g o n a l t w i s t i n g mechanism promoted r e l a x a t i o n , then one would e x p e c t a s m a l l e r v a l u e o f E f o r ( E ) C r ( T C T A ) than f o r ( E ) C r ( [ 9 ] a n e N ) | . T h i s i s not our o b s e r v a t i o n . Once a g a i n t h e r e are l a r g e d i f f e r e n c e s i n the l o w e s t q u a r t e t e x c i t e d s t a t e e n e r g i e s , and the arguments d e v e l o p e d i n the p r e c e d i n g s e c t i o n , f o r a c h l o r i d e m e d i a t e d pathway, may a l s o be a p p l i c a b l e h e r e . The b e h a v i o r o f the Cr(BCNE) complex i s more i n l i n e w i t h e x p e c t a t i o n based on the tendency o f a s t r a i n e d l i g a n d to f a c i l i t a t e e n t r y i n t o a r e l a x a t i o n channel. The A r r h e n i u s a c t i v a t i o n energy i s e x c e p t i o n a l l y s m a l l and a r e l a t i v e l y b r o a d d o u b l e t e m i s s i o n i s o b s e r v e d a t 77 Κ (fwhh ca 340 cm"-*-). Thus, i t would appear t h a t some o f the ground s t a t e s t r a i n energy i s r e l a x e d t h r o u g h e x c i t e d s t a t e d i s t o r t i o n s , and t h a t t h e s e d i s t o r t i o n s reduce the b a r r i e r f o r the r e l a x a t i o n p r o c e s s . The c o n t r a s t i n b e h a v i o r o f the C r ( e n ) and Cr(sen) complexes might be a t t r i b u t e d t o the e f f e c t s o f a t r i g o n a l l y s t r a i n e d ground s t a t e . Thus, E(^T2> i s l a r g e r , but τ(298 Κ) i s much s m a l l e r i n the sen complex. These complexes d i f f e r o n l y i n the c a p p i n g o f one t r i g o n a l f a c e o f the sen complex. The c a p p i n g C H ^ C i C ^ - ) ^ m o i e t y i s a p p r e c i a b l y s t r a i n e d i n the ground s t a t e . However, the e n t h a l p i c component o f t h i s s t r a i n does not make a c l e a r c o n t r i b u t i o n to the d i f f e r e n c e i n v a l u e of k ( 2 9 8 K ) . Once a g a i n the e f f e c t appears to be "entropie" (manifested i n T or A) and the e m i s s i o n band w i d t h s are comparable. T h i s does not r u l e out the p o s s i b i l i t y t h a t the d i f f e r e n c e i n l i g a n d s t r a i n dominates v a r i a t i o n s i n τ(298 Κ) f o r t h e s e two complexes, but the s t r a i n c o n t r i b u t i o n i s a p p a r e n t l y not m a n i f e s t e d i n an e x c i t e d s t a t e d i s t o r t i o n i n Cr(sen) . +

2

2

+

3

2

r e

a

t r

2

2

+

a

3

3 +

3 +

3 +

3

r e

t r

3 +

2

An Attempt t o E l u c i d a t e the R o l e o f the T t , S t a t e . We have some p r e l i m i n a r y o b s e r v a t i o n s which may bear on the r o l e o f the T i e x c i t e d s t a t e i n the r e l a x a t i o n p r o c e s s . T h i s s t a t e can o f t e n be found 200 t o 1000 cm" above the ( E ) s t a t e i n s i m p l e amine and ammine complexes ( 2 2 ) . As the symmetry o f the complexes d e c r e a s e s , the Τ χ s t a t e s p l i t s i n t o two o r t h r e e 2

1

2

2


ENDICOTT ET AL.

3+


Complexes

components, and t h e energy o f t h e lowest o f these can approach E( E). F o r example E ( E ) - E ( T ) - 600 cm" i n C r ( p h e n ) ^ , b u t i n Cr(phen)2(NH3>| t h e lowest component o f T ^ o r i g i n appears a t about 150 cm" h i g h e r energy ( F i g u r e 5 a ) . A t 77 Κ both o f e l e c t r o n i c o r i g i n s are r e s o l v e d , but the v i b r o n i c s t r u c t u r e i s weak enough t h a t o n l y t h e s t r u c t u r e a s s o c i a t e d with the E emission i s e a s i l y detected ( F i g u r e 5a). As t h e temperature i s i n c r e a s e d t h e h i g h e r energy e l e c t r o n i c o r i g i n i s populated s u f f i c i e n t l y that the v i b r o n i c s t r u c t u r e associated w i t h i t i s superimposed on t h e s t r u c t u r e d ( E ) e m i s s i o n (222 Κ spectrum) r e s u l t i n g i n a n e t , broadened e m i s s i o n spectrum. A s i m i l a r e f f e c t appears i n t h e more complex spectrum o f C r ( t e t a ) ( C N ) + ( F i g u r e 5b). I n t h e 222 Κ e m i s s i o n spectrum o f t h i s complex t h e s u p e r p o s i t i o n o f t h e components o f e m i s s i o n s from t h e d i f f e r e n t e l e c t r o n i c o r i g i n s g i v e s a n e t spectrum which appears t o be Stokes s h i f t e d a t ambient t e m p e r a t u r e s . 2

2

2

1

+

X

+

2

1

2

2

2

S i n c e t ( E ) i s e s s e n t i a l l y independent o f temperature f o r Cr( t e t a ) ( C N ) j J , i t i s c l e a r t h a t t h e r m a l p o p u l a t i o n o f low l y i n g e l e c t r o n i c components o f t h e h i g h e r energy d o u b l e t s t a t e do n o t p r o v i d e an e f f i c i e n t r e l a x a t i o n pathway i n t h i s system. Some I n f e r e n c e s About t h e Mechanism f o r S o l v e n t M e d i a t e d , Thermally A c t i v a t e d , Non-Radiative R e l a x a t i o n o f ( ^ E ) C r ( T l I ) . The ( E ) C r ( I I I ) r e l a x a t i o n r a t e has been f o r m u l a t e d , i n e q u a t i o n 1, as a composite o f t h e c o n t r i b u t i o n s o f mechan i s t i c a l l y d i s t i n c t pathways. The temperature independent c o n t r i b u t i o n , k ° seems w e l l behaved, i n a c c o r d w i t h e x re p e c t a t i o n f o r n e s t e d e x c i t e d and ground s t a t e p o t e n t i a l energy s u r f a c e s . The temperature dependent component, k ° ( T ) , e x h i b i t s a number o f i m p o r t a n t g e n e r a l f e a t u r e s : z

1.

k ° ( T ) tends t o v a r y w i t h t h e s o l v e n t medium, and t h e s e v a r i a t i o n s appear i n b o t h E and A f o r t h e t h e r m a l l y activated relaxation rates; a

2.

2

E

l

s

quenched by base f o r many o f t h e amine compounds, and t h i s s e n s i t i v i t y i s most s t r i k i n g l y m a n i f e s t e d i n v a r i a tions i n T ; A r r h e n i u s p r e - e x p o n e n t i a l f a c t o r s (A) v a r y o v e r a con s i d e r a b l e range; t h e overwhelming m a j o r i t y o f compounds s t u d i e d i n a s i n g l e s o l v e n t have v e r y s i m i l a r A r r h e n i u s a c t i v a t i o n e n e r g i e s ( E - (3.6 ± 0.5) χ 1 0 cm" i n DMS0-H 0, b u t many o f t h e A r r h e n i u s p l o t s a r e c u r v e d ( 3 , 6, 8); t h e r e i s no c o r r e l a t i o n o f E w i t h ΔΕ*; l i g a n d s i n which t h e n u c l e a r motions a r e e i t h e r s t e r i c a l l y i n h i b i t e d o r promoted have p r o f o u n d e f f e c t s on t h e h i g h temperature l i f e t i m e s , m a n i f e s t e d m o s t l y as v a r i a t i o n s i n T (or A); d e u t e r a t i o n o f c o o r d i n a t e d amines (ammines) r e s u l t s i n a d e c r e a s e i n k ( T ) i n s e v e r a l systems. t r

3. 4.

3

1

a

5. 6.

2

a

t r

7.

r e


EXCITED STATES AND REACTIVE INTERMEDIATES

f

1

χ ίο , κ 3

1

2

F i g u r e 4. A c i d dependence o f the ( E ) C r ( t e t a ) ( C N ) + l i f e t i m e , i n water w i t h a c i d o r base added: 3 χ 10~ M NaOH, t r i a n g l e s ; 3 χ 10~ M HC1, open c i r c l e s ; 2.5 χ 10" M HC1, c l o s e d c i r c l e s . 6

6

6

1.46

1.42 1.38 _ 1

cm /10

1.40 _ 1

cm /10

Cr(phen) (NH ) 2

1.46

4

3

3 + 2

1.34 4

Cr(tet) a ) ( C N )

3 + 2

F i g u r e 5. D o u b l e t e m i s s i o n s p e c t r a o f C r ( P H e u ) ( N H ) 3 + and C r ( t e t a ) ( C N ) + a t 222 Κ and 77 K. 2

3

2


3+


ENDICOTT ET AL.

Complexes

These o b s e r v a t i o n s f o r c e us t o c o n c l u d e t h a t l a r g e n u c l e a r d i s p l a c e m e n t s make a major c o n t r i b u t i o n t o k ( T ) . That t h e e f f e c t s o f s o l v e n t a r e m a n i f e s t e d i n b o t h t h e temperature de pendent and t h e temperature independent components o f k ( T ) i s reminiscent o f the behavior of c l a s s i c a l s o l v o l y s i s r e a c t i o n s or c o n f o r m a t i o n a l rearrangements. Thus, t h e r e l a x a t i o n dynamics might b e t t e r be d i s c u s s e d i n terms o f f r e e energy c o n t r i b u t i o n s than i n terms o f p o t e n t i a l e n e r g i e s . I n such a view t h e v a r i a t i o n i n t h e A r r h e n i u s p r e - e x p o n e n t i a l term t r a n s l a t e s i n t o a ± 24 J K" m o l " (2σ v a l u e ) v a r i a t i o n around a mean AS* « 1 J K" m o l " ; o n l y f o r complexes w i t h c o n s t r a i n e d l i g a n d s a r e t h e AS* v a l u e s f o r e n t r i e s i n T a b l e I I s i g n i f i c a n t l y n e g a t i v e ; i . e . , f o r trans-CriN^)(CN)^ and f o r Cr([9]aneN )| . These a r e a l s o t h e complexes w i t h t h e h i g h e s t energy q u a r t e t e x c i t e d s t a t e s . r e

r e

1

1

1

1

+

3

In summary, o u r p h o t o p h y s i c a l s t u d i e s i n d i c a t e t h a t t h e t h e r m a l l y a c t i v a t e d r e l a x a t i o n pathways o f ( E ) C r ( I I I ) v e r y l i k e l y i n v o l v e £-to-^(intermediate) surface c r o s s i n g . These ^ ( i n t e r m e d i a t e s ) c a n be a s s o c i a t e d w i t h some, n o t n e c e s s a r i l y the l o w e s t energy, t r a n s i t i o n s t a t e ( o r t r a n s i t i o n s t a t e s ) f o r ground s t a t e s u b s t i t u t i o n . The A r r h e n i u s a c t i v a t i o n b a r r i e r s f o r t h e r m a l l y a c t i v a t e d r e l a x a t i o n a r e r e m a r k a b l y s i m i l a r from complex t o complex, b u t they can be a l t e r e d i n systems w i t h highly strained ligands. Some o f t h i s work i n d i c a t e s t h a t t h e s t e r i c and e l e c t r o n i c p e r t u r b a t i o n s o f t h e l i g a n d s d i c t a t e t h e c h o i c e among p o s s i b l e r e l a x a t i o n c h a n n e l s . 2

2

ACKNOWLEDGMENTS We a r e g r a t e f u l t o P r o f e s s o r K a r l Wieghardt f o r p r o v i d i n g samples o f t h e s u b s t i t u t e d [ 9 ] a n e N l i g a n d s . P r o f e s s o r N. A. P. Kane-Maguire k i n d l y p r o v i d e d us w i t h a number o f u s e f u l d e t a i l s about t h e c y c l a m complexes p r i o r t o p u b l i c a t i o n . The r e s e a r c h d e s c r i b e d i n t h i s paper has been g e n e r o u s l y s u p p o r t e d by t h e N a t i o n a l S c i e n c e F o u n d a t i o n . 3



6

3 +

5

3

2

2

3 +

3

3+

χ

)(CN) *

r

1

2

+

3

si

/ Y \

3+

2

2

2

^/?5-Cr(L )(CN) "*"

cjjj-Cr( Lj )en

^

+

2

trans-Cr(1^)(NCS)

cj ^Cr(L )Cl

1

cj>-Cr(L )(NH )

3 +

3

2

3+ trans-Cr( 1> ) ( NH >

trans-Crih

Cr(phen) (NH )

Cr(tpy)

Cr(phen) ^

6.7

4.3

4.2

2.3

0.7

~4

~5

7.7

7.0

5.8

7.5

5.0

+

5.0

2+

Cr(sen)

2.0

4

4.4

5

3

1

6

δ

C

3

J

A

380

80

2

(lxl0" )

(8xl0" )

1*

136

361

3.6

8

g

g

(1χ10~ )

126

(0.02)

1.2

14

(2χ10~* )

2.2

(μβ)

x(298K)

8

331

243

156

168

~235

~308

314

233

178

235

202

234

287

170

244

(K)

i

3.7

2.1

4.1

3.6

5.6

i

2.9

4.0

3.2

3.5

3.4

4.3

4.5

3.2

a

3

Ε - l (cm V I C T )

C β

2

2

0.4

(2xl0)

5

4xl0

0.03

2x10

4

3

8

6xl0~

90

1.2

88

(2xl0)

0.1

Α

8 h

Photophysical Parase t e r s f o r SomeChroaiua(III) Complexes

(ca/lO »' )

2 +

3

3

Cr(en>

3

Cr(NH ) CN

3

Cr(NH ) Cl

3

Cr(NH )

Complex

Table I I .

h

(32)

(32)

(17)

(17)

(17)

(18)

(18)

(32)

(39)

(39)

(39)

(17)

(15)

(32)

(17)

(8)

Ref


2

3 +

X

3+

5.2

3 +

2

4

See note a Table I for abbreviations.

7

3

2.8 2.5 1.5

140 134

8

8

3.5

260

196

DMF-CHCI3

Arrhenius a c t i v a t i o n parameters

Temperature dependence seems c o r r e l a t e d with OH" quenching.

1

J 293 Κ

Data c o l l e c t e d i n a r i g i d glass.

n

See f i g u r e 4.

8 Value extrapolated from thermally a c t i v a t e d behavior at very low temperatures

f

e

!

n>

S'

g£

^

6

c

&

m ζ D Ô Ο H H m H >

g

3

(40)

(40)

(12)

(24)

for Cr(NH )

3

(17)

(32)

In DMSO-water (or H 2 O ) except as noted

3

0.025

38

l.lxlO"

150

0.1

2.3

181

E ) obtained r e l a t i v e to 6E set equal to 4 χ 10

(6xl0~ )

3

(5xl0~ )

40

8

4.2

3.8

237

^ Temperature of the t r a n s i t i o n between temperature dependent and temperature independent regimes of τ ( i n DMSO-H2O except as noted)

c

4

3

(2xl0~ )

2.1

assuming band widths do not change.

b Relative values of ΔΕ* » E ( E ) - E ( T or

a

HOTES FOR TABLE I I :

6

Cr(L )

5

2

3.0

4

/τ

Cr(L )

„

+

5.7

•

2

5.4

CIS-CT{L^)en Cr(L )

3

Manipulation of Doublet Excited State Lifetimes in ... - ACS Publications

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