Environmental Effects on Intra- and Intermolecular Photophysical

15

Environmental Effects on Intra- and Intermolecular Photophysical Processes in Cr Complexes 3+

Downloaded by UNIV OF BATH on July 3, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch015

LESLIE S. FORSTER University of Arizona, Tucson, Ariz. 85721

The effect of environment on the nonradiative processes T —--> A , T —--> E, and E—--> A is reviewed. The rate constants for E—--> A are insensitive to environment more often at low temperatures than at room temperature. The relatively few data on the T —--> A relaxation rates indicate a value of ≤ 10 sec for most complexes, but notable exceptions have been found. Some evidence indicates a variation in intersystem crossing ( T —--> E) efficiency with environment. Concentration quenching by energy transfer, where important, affects the E state rather than the T state. In general, the environmental variations in Φ occur mainly after the complexes have reached the E state. The need to collect photophysical and photochemical data under identical conditions is emphasized. 4

4

4

2

2

2

2

4

2

2

2

4

2

4

4

2

5

2

-1

4

2

2

2

4

2

2

p

V V T i t h the e x c e p t i o n o f m o l e c u l e s i n the gas p h a s e a t l o w pressures, the ™

p r o p e r t i e s o f a n y species are m e d i a t e d to some degree b y the e n v i

ronment.

I n p a r t i c u l a r , t h e p h o t o p h y s i c a l processes w i t h i n a t r a n s i t i o n

m e t a l i o n c o m p l e x are often m a r k e d l y d e p e n d e n t o n the s u r r o u n d i n g s i n w h i c h the c o m p l e x is e m b e d d e d .

Interest i n t r a n s i t i o n m e t a l i o n p h o t o -

p h y s i c s has b e e n h i g h n o t o n l y f o r the i n t r i n s i c i m p o r t a n c e (e.g., p h o s phors a n d lasers ) b u t also because p h o t o c h e m i s t r y a n d p h o t o p h y s i c s a r e i n t i m a t e l y i n t e r r e l a t e d . I t is this c o n n e c t i o n b e t w e e n p h o t o c h e m i s t r y a n d p h o t o p h y s i c s t h a t w i l l b e e m p h a s i z e d i n this discussion. T h e most s t r i k i n g contrast b e t w e e n solids, either glassy or c r y s t a l l i n e , a n d fluids is the i n h i b i t i o n o f d i f f u s i o n a l processes i n r i g i d m e d i a .

How

ever, p h o t o p h y s i c a l processes are sensitive to n o n d i f f u s i o n a l p e r t u r b a t i o n s as w e l l , a n d the effect o f e n v i r o n m e n t a l factors i n this latter category is 172 King; Inorganic Compounds with Unusual Properties Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

15.

Environmental

FORSTER

173

Effects

the subject o f this r e v i e w . W i t h f e w exceptions, p h o t o c h e m i s t r y has b e e n s t u d i e d i n fluid m e d i a , w h i l e m o s t p h o t o p h y s i c a l m e a s u r e m e n t s b e e n m a d e w h e n t h e c o m p l e x is i n a s o l i d e n v i r o n m e n t . crystalline environments are discussed:

have

T h r e e types o f

( a ) u n d i l u t e d crystals, ( b ) t h e

guest species d i l u t e d i n a n i s o s t r u c t u r a l host, a n d ( c ) d o u b l e salts. T h e n o n c r y s t a l l i n e s o l i d hosts i n c l u d e a l c o h o l - w a t e r

glasses

a n d a plastic,

poly ( methyl methacrylate ). I t is u s e f u l to d i s t i n g u i s h i o n i c c o m p l e x e s (e.g., C r m o l e c u l a r c o m p l e x e s (e.g., C r ( N H ) 3

6

3 +

3 +

:A1 0 ) 2

3

from

) w h i c h persist i n fluids as w e l l as

i n solids. I n the m o l e c u l a r c o m p l e x e s , the l i g a n d s are c o u p l e d m u c h m o r e s t r o n g l y to t h e c e n t r a l m e t a l i o n t h a n to the s u r r o u n d i n g s , a n d i t is m e a n -


i n g f u l to treat t h e system as a m o l e c u l e w i t h the e n v i r o n m e n t a c t i n g as a p e r t u r b a n t o n i n t r a m o l e c u l a r processes. M o r e o v e r , i n crystals o f m o l e c u l a r c o m p l e x e s , the b u l k y l i g a n d s p r e v e n t t h e close a p p r o a c h o f m e t a l ions that is p o s s i b l e w i t h i o n i c complexes.

Consequently, intermolecular exci-

45

40

35

30

25

£

20

UJ 15

10

5

0

1

2 Dq(kK)

3

Figure 1. The variation in Cr * energy levels with ligand field (adapted from Ref. 49) 3

King; Inorganic Compounds with Unusual Properties Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

174

INORGANIC

COMPOUNDS WITH

UNUSUAL PROPERTIES

t a t i o n e n e r g y transfer m a y b e less efficient i n solids c o n t a i n i n g m o l e c u l a r complexes. B e c a u s e of the extensive l i t e r a t u r e o n the p h o t o c h e m i s t r y a n d p h o t o p h y s i c s of h e x a c o o r d i n a t e d C r

3 +

complexes

(1, 2, 3, 4, 5),

this g r o u p is

v e r y s u i t a b l e for i l l u s t r a t i n g the effect of e n v i r o n m e n t o n the rates of i n t r a a n d i n t e r m o l e c u l a r processes. T h e T 4

2

energy, r e l a t i v e to t h e g r o u n d A 4

2

state, is sensitive to Dq ( F i g u r e 1 ) , a n d c o n s e q u e n t l y t h e p o s i t i o n of the T

4

2

A ) a n d for p h o s p h o r e s c e n c e ( E 4

-»

2

2

4

fluorescence

A ). 2

I n the absence of t h e r m a l l y a c t i v a t e d E

> T 4

2

2

b a c k transfer, the

p e r t i n e n t equations a r e :

F

k + k* + k, + k

T

k + k + k + k

T

2

T

F

- i

φ

=

2

=

η

z

Φ 2E

Φρ/τ

1

ρ

}

(2)

R

h.

z

4

k k + k +

T

.

R

h

k + k + b

(3)

E

k

&

R

5

b

Tp-

4

k + k + k + k 2

=

K

R

E

k

6

R

=

k + kt + k

=

Φ &

b

E

(4)

R

2 Ε

(5)

5

W h e n b a c k transfer is significant, i t is necessary to d i s t i n g u i s h the p r o m p t fluorescence

described by Equations 1 and 2 from delayed

fluorescence

w h i c h w i l l e x h i b i t the same l i f e t i m e as p h o s p h o r e s c e n c e . T h e i n c l u s i o n of b a c k transfer leads to r a t h e r c o m p l i c a t e d

general

expressions, b u t t w o l i m i t i n g cases are i m p o r t a n t . T h e m o d i f i e d equations c o r r e s p o n d i n g to these l i m i t s (7,8) [4^-4

«

*6 + k

(fc.4 + fc +

Φ ρ

=

1

ΦΡΛΡ

2

T

= =

k

b

A

+

h

+

-

k )] T

R

:

2

( 3 , )

(h + h + fc-4 + k ) -kJc-

R

(1

for t h e steady state l i m i t

- h - h -

E

(k + kt + k + k ) 2

τρ-

are (a)

- k

E

R

5

R

Φ *)Α;-4 2

Φ 2 ^ 5


A

(40

(50

176

INORGANIC

COMPOUNDS WITH UNUSUAL

a n d ( b ) f o r t h e e q u i l i b r i u m l i m i t ( [ Γ ] / [ Ε ] = 3e~ 4

2

2

4

E

2

T

2

R

_

t

z

_

^

Γ

A £ /

* ) r

U n l e s s Δ Ε < 1000 c m " , i n t h e e q u i l i b r i u m l i m i t , Φ Λ 1

Ρ

~ fc at a m b i e n t 5

Ρ

Since the e q u i l i b r i u m limit can apply only w h e n Φ

— 1,

2Ε

2

;

(Κ>Λ

3e~

i.e. w h e n k + fc +

}

"

^

(1 +

K P

temperatures.

(3

Δ

2

Δ

Φ

kT

R

h + h + fe^ + (fc + fc» + Χ;**) ( 3 β - ^ η 1 + 3β- ^

=

K P


and E ) :

2

(h + h + k ) + (fc + k + k ) (3e-^ )

Φ ρ

P /

w h e r e Δ Ε is

,

AE/kT

the energy difference b e t w e e n the n o - p h o n o n levels o f T

PROPERTIES

« & 4 , at m o d e r a t e o r l a r g e Δ Ε E q u a t i o n 5 " w i l l

3

l e a d to essentially t h e same t e m p e r a t u r e d e p e n d e n c e as E q u a t i o n 5 ' i f Φ

2Ε

is constant.

I n p r i n c i p l e , d e t e r m i n a t i o n o f Φ / τ & as a f u n c t i o n o f t e m ρ

ρ

5

p e r a t u r e c a n b e u s e d to e v a l u a t e t h e r m a l l y i n d u c e d changes i n Φ , b u t 2Ε

k is not necessarily t e m p e r a t u r e - i n v a r i a n t . W h e n Φ τ is constant over a 5

ρ

ρ

w i d e r a n g e o f t e m p e r a t u r e , i t is l i k e l y t h a t b o t h Φ

and k

a r e also

5

2Ε

constant. I n t h e f o l l o w i n g d i s c u s s i o n , t h e effect o f e n v i r o n m e n t o n t h e i n t r a m o l e c u l a r r e l a x a t i o n rates fc , fc , a n d fc i n C r 3

6

4

3 +

c o m p l e x e s is r e v i e w e d .

I n a d d i t i o n , t h e v a r i a t i o n i n i n t e r m o l e c u l a r transfer efficiency w i t h e n v i r o n m e n t is also discussed. k

6

W i t h f e w exceptions (e.g., C r ( u r e a )

6

3 +

and C r ( H 0 ) 2

3 +

6

), τ

ρ

reaches

the l o w t e m p e r a t u r e - l i m i t i n g v a l u e ( τ ° ) at temperatures a b o v e 77 °K. ρ

I n T a b l e I a r e l i s t e d a representative selection o f l o w t e m p e r a t u r e l i f e times.

S i n c e k is s e l d o m k n o w n a c c u r a t e l y , i t is necessary to estimate 5

this q u a n t i t y i n o r d e r to evaluate fc . F o r the s y m m e t r y - a l l o w e d E -> A 2

6

transition i n C r ~10"

3 +

:Al O , k 2

5

a

=

260 s e c

(19) and

1

sec ( T a b l e I ) . C o n s e q u e n t l y , k ° ~ 1 0 - 1 0

3

2

5

s y m m e t r i c complexes. centrosymmetric

T p

3

4

2

° i n C r ( o x ) " is 3

3

sec" i n n o n c e n t r o 1

I n contrast, f o r s y m m e t r y - f o r b i d d e n transitions i n

complexes

(e.g.,

Cr(CN)

6

3

" ) , fc ° ~ 10 sec" . 1

5

These

v a l u e s v a r y w i t h the extent o f v i b r o n i c c o u p l i n g b u t t h e fc ° q u a n t i t i e s i n 6

T a b l e I w e r e e v a l u a t e d b y a s s u m i n g fc ° = 1 0 a n d 10 sec" f o r n o n c e n t r o 3

5

s y m m e t r i c a n d c e n t r o s y m m e t r i c complexes,

1

respectively.

I n some c o m p l e x e s (viz., d i l u t e s o l i d solutions o f C r ( a c a c ) ) 3

quite

insensitive

Cr(D 0) 2

6

3 +

to e n v i r o n m e n t ,

and C r ( C N )

6

3

while

i n other

complexes

" ) the l a t t i c e is t h e d o m i n a n t influence

is (e.g., (Table

I ) . I n other c o m p l e x e s (e.g., C r ( o x ) " ) k ° m a y b e r e l a t i v e l y constant i n 3

3

Q

s e v e r a l hosts y e t increase m a r k e d l y i n others.

I t is significant t h a t w i t h


15.

Environmental

FORSTER

Table I. Complex

L o w T e m p e r a t u r e L i m i t i n g τ ρ ° a n d k$° k ° , seer

Ref.

475 115 17 400

2,000 8,600 58,700 2,400

9 9 10 9

460

2,100

11

910 960 900 100 180

1,000 1,000 1,000 9,900 5,500

12 IS 12

1,500 350 750 130

650 2,850 1,300 7,700

12 12 15 12

120,000 10 62 3 3,300

~o 100,000 16,000 330,000 300

12 16 16 16 17

[Cr(urea) ] I [Cr(urea) ] B r [Cr(urea) ]Cl [Cr(urea) ](C10 ) [Cr(urea)e](NO«),

160 130 240 100 240

6,200 7,600 4,100 10,000 4,100

18 18 18 18 18

[Cr(en) ](C10 ) [Cr(en) ]Cl [Cr(en) ]Br .4H 0 [Cr(en) ]I a l c o h o l - w a t e r glass

53 40 26 80 100

18,800 25,000 38,400 12,400 10,000

16 16 16 16 17

Host"

a

Cr(acac)

177

Effects

Al(acac) Cr(acac)

3

p°, μ sec

3 3

a l c o h o l - w a t e r glass poly (methyl methacrylate) plastic Cr(ox)

NaMgAl(ox) -9H 0 NaMgCr(ox) .9H 0 a l c o h o l - w a t e r glass Κ Α1(οχ) ·3Η 0 K Cr(ox) .3H 0

3

3

2


3

Cr(D 0) 2

6

3

3

2

3

3

2

C(NH ) A1(S0 ) -6D 0 A1C1 -6D 0 KA1(S0 ) -12D 0 a l c o h o l - w a t e r glass

3 +

2

3

4

3

6

3

"

2

2

3

6

6

3

6

3

6

3 +

6

6

6

3

6

3

6

3

6

Cr(en) + 3

3

4

3

4

3

3

3

3

3

3

a

2

K Co(CN) K Cr(CN) [Cr(en) ][Cr(CN) ] [Cr(NH ) ][Cr(CN) ] a l c o h o l - w a t e r glass 3

Cr(urea)

2

2

4

Cr(CN)

2

3

2

3

1

e

U 10

A c a c : acetylacetonate; en: ethylenediamine; ox: oxalate.

one e x c e p t i o n ( C r ( C N ) plexes.

6

3

~ in K C o ( C N ) ) k ° 3

A n y reduction in Φ

ρ

6

Q

»

h°

for molecular com

b e l o w one i n these systems results m a i n l y

f r o m n o n r a d i a t i v e r e a c t i v a t i o n of the E state. 2

A r a t h e r different t y p e of e n v i r o n m e n t a l effect w a s d e t e c t e d i n glassy solutions of C r ( C N )

6

3

" w h e r e e x c i t a t i o n o n t h e r e d e d g e of t h e Γ 4

a b s o r p t i o n b a n d leads to a m u l t i e x p o n e n t i a l d e c a y ( 2 0 ) .

0.8 (31a).

A

Cr :K Co(CN) , Φ /τ

1 at r o o m t e m p e r a " , is d i s s o l v e d i n

Since E

> T

2

4

E q u a t i o n 5 is v a l i d .

increases t w o - f o l d f r o m 7 7 ° - 3 0 0 ° K (12).

ρ

+

3

H o w e v e r , w h e n the same c o m p l e x , C r ( C N )

k.

back

Ρ

1 (Equation 3) and k

2

2

For

Since Φ

Ρ

is t e m p e r a t u r e - i n v a r i a n t a n d equals one i n this c r y s t a l , a l l of this increase m u s t be a s c r i b e d to the t h e r m a l e n h a n c e m e n t of fc . T h i s is q u i t e reason 5

a b l e since E - » A 4

2

ronment Φ /τ ρ

2

is s y m m e t r y - f o r b i d d e n i n this c e n t r o s y m m e t r i c e n v i

I n contrast, i n a l c o h o l - w a t e r

(32).

is constant ( ± 5 % )

ρ

2 2 5 ° K , a n d i t increases a b o u t 2 0 % 2 8 9 ° Κ (28).

solutions of

Cr(CN)

as the t e m p e r a t u r e is c h a n g e d f r o m 145°

6

3

", to

w h e n the t e m p e r a t u r e is r a i s e d to

T h e s e d a t a are consistent w i t h a d i m i n i s h e d Φ

in

2Ε

fluid

solutions at r o o m t e m p e r a t u r e , b u t , i n the absence of q u a n t i t a t i v e i n f o r m a t i o n a b o u t the t e m p e r a t u r e effect u p o n k , no definitive statement is 5

warranted. I n c i d e n t a l l y , the e v a l u a t i o n of Φ ^ f r o m E q u a t i o n 5 is f r a u g h t w i t h 2

uncertainty.

Although τ

ρ

is r e a d i l y m e a s u r e d a n d r e l i a b l e values of Φ

w e r e d e t e r m i n e d i n some cases, the e s t i m a t i o n of k» f r o m measurements is v e r y difficult (33).

C o m p u t a t i o n of k

that d e p e n d o n s u c h a c a l c u l a t i o n

2Κ

should be viewed with skepticism In alcohol-water

(4).

solutions of C r ( e n )

t e m p e r a t u r e f r o m 153° to 2 9 8 ° K (28). Cr(en)

3

is s y m m e t r y - a l l o w e d ,

3 +

dependent.

fc

5

I f one assumes t h a t k

5

8

·2Η 0)

e

2

Yet, i n aqueous solution, Φ

3

3 +

, Φ / τ „ is i n d e p e n d e n t ρ

S i n c e the E - > A 2

s h o u l d not

4

be

very

w i t h Zc . 4

However,

2

Φ

2Ε

at 2 5 ° C , Φ ^ ~ 1 for C r ( e n ) 2

4

4

3 +

is the

In a crystalline environment

data. S i n c e the p h o t o a q u a t i o n y i e l d for d i r e c t r e a c t i o n i n T i m p l y some Γ

temperature3

3 +

3

——» A 4

2

is 0.15, these

2

d e a c t i v a t i o n , i.e. fc competes f a v o r a b l y

the p h o t o c h e m i c a l

3

e v a l u a t i o n of

Φ ^ is 2

critically

d e p e n d e n t u p o n the m a g n i t u d e of the p h o t o a q u a t i o n y i e l d e x c i t e d direct E E. 2

2

m i u m β-diketonates e x h i b i t a s i n g u l a r b e h a v i o r .

R e p l a c e m e n t of m e t h y l

g r o u p s b y h y d r o g e n atoms m a r k e d l y reduces Φ / τ Downloaded by UNIV OF BATH on July 3, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch015

Ρ

a l t h o u g h Φρ/τρ for C r ( a c a c )

ρ

(35).

is t e m p e r a t u r e - i n d e p e n d e n t

3

has

3

of fc i n c r y s t a l l i n e solids is

Furthermore, i n a crystalline

host, i t decreases w i t h t e m p e r a t u r e i n n o n c r y s t a l l i n e m e d i a , i n b o t h r i g i d p l a s t i c ( F i g u r e 4 ) a n d absolute e t h a n o l solutions (28).

U n d e r some c i r

cumstances ks a p p a r e n t l y competes w i t h fc i n C r ( a c a c ) . 4

Inter molecular Energy I f measurements

3

Transfer

of Φ

ρ

and/or τ

ρ

are to b e u s e d to evaluate i n t r a

m o l e c u l a r r e l a x a t i o n rates, the effect of i n t e r m o l e c u l a r processes o n these q u a n t i t i e s must b e

assessed.

I n a d d i t i o n , the efficiency

of

excitation

e n e r g y transfer is of i n t r i n s i c interest, e s p e c i a l l y i n c o n n e c t i o n w i t h s o l i d state p h o t o c h e m i c a l a n d p h o t o p h y s i c a l processes. If e x c i t a t i o n e n e r g y is t r a n s f e r r e d to a n i d e n t i c a l center, i.e. the same species i n p r e c i s e l y t h e same e n v i r o n m e n t , t h e n a l l of the r e l a x a t i o n rates k -k 2

a n d no c h a n g e i n measureable

Q

are

unaffected,

quantities ( e x c e p t p o l a r i z a t i o n )

is ex

p e c t e d . H o w e v e r , i f transfer occurs b e t w e e n n o n - i d e n t i c a l centers, t h e n o b s e r v a b l e changes w i l l o c c u r . T w o situations c a n be d i s t i n g u i s h e d

(36):

( a ) single step d o n o r - a c c e p t o r transfer a n d ( b ) m i g r a t i o n transfer. I n c a t e g o r y a are the d i f f u s i o n - c o n t r o l l e d processes t h a t p r e v a i l i n fluid

media

——>Cr(CN) i n solids Cr(CN)

6

[e.g., b e n z i l 6

3

~

[e.g., C o ( C N ) 3

>Cr(CN)

6

3

-

(37)

and C r ( N H ) ( N C S ) 3

2

4

a n d e n e r g y transfer b e t w e e n different

(38)]

6

3

"

> Cr(CN)

6

3

- (31)

and C r ( e n )

3

species 3 +

>

" ( 3 4 ) ] . T h i s t y p e of transfer is c h a r a c t e r i z e d b y t h e q u e n c h

i n g of d o n o r l u m i n e s c e n c e a c c e p t o r emission.

a n d i n some cases b y t h e s e n s i t i z a t i o n

of

M i g r a t i o n transfer ( c a t e g o r y b ) c a n b e v i s u a l i z e d as

a r a n d o m w a l k process that is t e r m i n a t e d either b y d e - e x c i t a t i o n of the e x c i t e d species or b y transfer to a sink. I n C r , m i g r a t i o n energy transfer 3 +

m a y t a k e p l a c e i n either T 4

4

!F

2

2

*

or E : 2

4

!T

2

4

T

2

sink •» s i n k


182

INORGANIC

COMPOUNDS

WITH

UNUSUAL PROPERTIES

T h e s i n k c a n b e at l a t t i c e defect or a n o t h e r species s u c h as a p a i r center. T h e n u m b e r of C r

sites t r a v e r s e d d e p e n d s o n t h e i n t e r a c t i o n e n e r g y a n d

3 +

t h e l i f e t i m e of the e x c i t e d state. T w o m e c h a n i s m s h a v e b e e n suggested for energy t r a n s f e r — m u l t i p o l e a n d exchange ( 3 9 ) .

M u l t i p o l e transfer i n the d i p o l e a p p r o x i m a t i o n , often

c a l l e d F o r s t e r or resonance transfer, c a n t a k e p l a c e over l a r g e distances ( 2 0 - 5 0 A ) . E x c h a n g e transfer r e q u i r e s o r b i t a l o v e r l a p b e t w e e n the i n t e r a c t i n g centers a n d is short range. T h e s i n g l e step d o n o r - a c c e p t o r

transfers

are short r a n g e as e x p e c t e d for a n exchange m e c h a n i s m , a n d t h e y a p p e a r to constitute a n i m p o r t a n t process w h e n e v e r e n e r g e t i c a l l y possible i f the species are i n close p r o x i m i t y . W h e n a c o m p a r i s o n is m a d e of

energy


transfer i n c r y s t a l l i n e a n d glassy e n v i r o n m e n t s , i t s h o u l d b e n o t e d that i n Cr

systems d o p i n g at the 0.1 m o l e %

3 +

l e v e l corresponds r o u g h l y to a

0 . 0 1 M s o l u t i o n . M i g r a t i o n energy transfer i n i o n i c solids, e.g. r u b y , occurs in

2

E b y a n exchange m e c h a n i s m (40).

energy transfer i n r u b y is

E

2

detectable i n 0 . 1 % crystals, y e t n e i t h e r τ n o r Φ of C r ( o x ) " i n N a M g ρ

Al(ox)

3

ρ

· 9 H 0 crystals is m u c h affected b y C r

3

2

0.1-100%

r a n g e (41).

3

c o n c e n t r a t i o n i n the

3 +

T h e n e a r constancy of τ a n d Φ does not neces ρ

ρ

s a r i l y r u l e out E m i g r a t i o n transfer since energy m i g r a t i o n does not l e a d 2

to q u e n c h i n g i f the sink c o n c e n t r a t i o n is s m a l l .

I n fact, t w o - f o l d v a r i a

tions i n T are o b s e r v e d i n 5 0 % crystals a n d p o w d e r s f r o m one s a m p l e to p

another.

T h e sinks i n r u b y are p a i r centers, a n d these increase r a p i d l y

w i t h concentration.

T h e 2 n d - 4 t h nearest n e i g h b o r s distances i n A 1 0 2

3

are less t h a n 3.5 A , a n d superexchange interactions via O " are v e r y i m p o r 2

tant i n this lattice. O n the other h a n d , i n oxalates the closest

Cr

3 +

-Cr

3 +

a p p r o a c h is greater t h a n 7 A , a n d superexchange is not v e r y i m p o r t a n t i n Cr

3 +

:NaMgAl(ox)

3

* 9 H 0 (41).

C o n s e q u e n t l y , the rate of m i g r a t i o n is

2

s l o w e r i n m o l e c u l a r crystals t h a n the 1 0 data

5

sec'

estimated from the r u b y

1

(40). C o n c e n t r a t i o n q u e n c h i n g of E is e v i d e n t i n C r : A l ( a c a c ) 3 +

2

especially i n C r Cr

3 +

:K Co(CN) 3

G

3 +

:K Co(CN) 3

(42).

G

T h e relatively long E

favors c o n c e n t r a t i o n q u e n c h i n g .

defects b e c o m e m o r e i m p o r t a n t w i t h i n c r e a s i n g C r P e r h a p s b u l k defects also increase.

3

(9)

and

lifetime i n

2

I n a d d i t i o n , surface 3 +

concentration

(31).

A c o m p a r i s o n of τ ° i n glasses a n d ρ

u n d i l u t e d crystals suggests, at most, a m i n o r c o n c e n t r a t i o n q u e n c h i n g of Cr(en)

3

3 +

E in [Cr(en) ]I ,

2

3

3

b u t a s o m e w h a t l a r g e r effect i n other l a t

tices. T h e n e a r constancy of τ ° for C r ( u r e a ) ρ

6

3 +

i n s e v e r a l crystals

(10)

a g a i n indicates the r e l a t i v e u n i m p o r t a n c e of E c o n c e n t r a t i o n q u e n c h i n g . 2

T h e n e p h e l a u x e t i c shift of E 2

7Γ d e r e a l i z a t i o n of the C r spectra of C r ( a c a c )

3

3 +

4

A in Cr(acac) 2

3

a n d C r ( C N ) " indicates 6

3

e x c i t a t i o n onto the l i g a n d s , a n d the E 2

4

A

2

c l e a r l y s h o w t h e splittings associated w i t h i n t e r -

m o l e c u l a r interactions (43,44).

I n the absence of ττ b o n d i n g , t h e l i g a n d s

serve as insulators a n d i n h i b i t energy transfer. A l t h o u g h E 2

concentration


15.

Environmental

FORSTER

q u e n c h i n g reduces τ

ρ

and Φ , Φ /τ ρ

t i o n , w h i c h is t r u e f o r C r Al(ox)

· 3 H 0 (14).

3

3 +

183

Effects

ρ

ρ

s h o u l d b e i n d e p e n d e n t of c o n c e n t r a

:NaMgAl(ox) · 9 H 0 3

(41)

2

and C r

O n t h e o t h e r h a n d , q u e n c h i n g of Γ 4

2

3 +

Φ w i t h o u t c h a n g i n g τ . T h e r e is some e v i d e n c e f o r a r e d u c t i o n i n ρ

ρ

w i t h increasing C r

3 +

in C r

3 +

:K Co(CN) 3

6

4

ρ

Φ /τ ρ

m i g r a t i o n energy transfer

2

b y a m u l t i p o l e m e c h a n i s m is l i k e l y to b e greater t h a n t h e rate of transfer b y a n e x c h a n g e m e c h a n i s m , b u t t h e 10" sec a n d Γ 9

4

ρ

b u t the d e c a y b e c o m e s

(42),

n o n - e x p o n e n t i a l as τ decreases. T h e rate of T

:NaCa-

w i l l reduce

2

4

T

2

E

2

l i f e t i m e is less t h a n

m i g r a t i o n p r o b a b l y c a n n o t c o m p e t e effectively w i t h fc .

2

4

C o n s e q u e n t l y , processes o r i g i n a t i n g i n the

4

Γ

2

state are u n a f f e c t e d

by


intermolecular interactions. Thexi

States

I n a c r y s t a l l i n e host, the p o t e n t i a l curves

as d r a w n i n F i g u r e 5

d e s c r i b e the t o t a l energy, c o m p l e x a n d e n v i r o n m e n t . I f v i b r a t i o n a l r e l a x a t i o n w i t h i n a n e l e c t r o n i c state is faster t h a n other c o m p e t i n g steps, t h e n photophysical and photochemical

processes o c c u r i n t h e r m a l l y e q u i l i

b r a t e d p o p u l a t i o n s . F i g u r e 5 is also a p p l i c a b l e f o r a r i g i d , n o n c r y s t a l l i n e m e d i u m , b u t as t h e solvent melts a n d solvent r e l a x a t i o n takes p l a c e d u r i n g the excited-state l i f e t i m e , a m o r e c o m p l e x r e p r e s e n t a t i o n is r e q u i r e d .

Ε

Figure 5.

Potential curves for C r

3 +

Q

I n C r , the A 3 +

4

2

a n d E states a r e b o t h d e r i v e d f r o m the f 2

t i o n , a n d t h e y h a v e n e a r l y t h e same e q u i l i b r i u m geometries.

2

3

configura

Consequently

the e q u i l i b r i u m solvent orientations are l i t t l e c h a n g e d i n t h e t w o states. Since T 4

2

is d e r i v e d f r o m the t e c o n f i g u r a t i o n , t h e e q u i l i b r i u m p o s i t i o n 2

2



184

INORGANIC

COMPOUNDS

WITH

UNUSUAL PROPERTIES

is different, a n d the p o t e n t i a l curves b e f o r e a n d after solvent r e l a x a t i o n are different. I n F i g u r e 6, the s o l i d curves c o r r e s p o n d to the solvent d i s t r i b u t i o n a p p r o p r i a t e to the g r o u n d state, w h i l e the d o t t e d curves refer t o t h e solvent o r i e n t a t i o n that p r e v a i l s i n the r e l a x e d Γ 4

state. T h e r m a l l y

2

e q u i l i b r a t e d states h a v e b e e n t e r m e d t h e x i states ( I ), a n d i t has b e e n s u g gested t h a t t h e x i states are v e r y m u c h m o r e d i s t o r t e d i n fluid t h a n i n r i g i d m e d i a . P o s s i b l e consequences of this d i s t o r t i o n i n c l u d e ( a ) i n c r e a s e d T d u e to a r e d u c e d fc , ( b ) 4

s m a l l e r e n e r g y for E — • 2

a n d ( c ) a l t e r a t i o n of the d e l a y e d

fluorescence

4

T

F

b a c k transfer,

2

spectrum.

D i r e c t e v i d e n c e for h i g h l y d i s t o r t e d t h e x i states is difficult to o b t a i n . I n t e r s y s t e m c r o s s i n g m i g h t p r e c e d e solvent r e o r i e n t a t i o n a n d Φ

2Ε

then

reflects o n l y b e f o r e - r e l a x a t i o n i n t e r s y s t e m crossing. H o w e v e r , i f a s i g n i f i cant f r a c t i o n of the e x c i t e d m o l e c u l e s r e l a x i n T 4

t h e t h e x i state, t h e n a r e d u c t i o n i n Φ / τ ρ

of the r i g i d glass solvent.

ρ

2

and if Φ

is s m a l l e r i n

2Ε

w o u l d a c c o m p a n y the m e l t i n g

W i t h i n the p r e c i s i o n of o u r

( 5 % ), n o s u c h c h a n g e w a s o b s e r v e d f o r either C r ( C N )

6

measurements 3

" or C r ( e n )

3

3 +

(28). A d e f i n i t i v e test for a n i n c r e a s e d l i f e t i m e i n the r e l a x e d state is to m o n i t o r the excited-state a b s o r p t i o n , w i t h p u l s e d e x c i t a t i o n , as a f u n c t i o n of m e d i u m r i g i d i t y . Cr

3 +

T h i s e x p e r i m e n t has not yet b e e n r e p o r t e d for a

complex. T h e Stokes shift for d e l a y e d

fluorescence

is another m e a s u r e

of

excited-state d i s t o r t i o n . E v e n i n c r y s t a l l i n e e n v i r o n m e n t s , t h e Stokes shift c a n v a r y w i t h t h e host as m u c h as 1500 c m " , e.g. C r ( u r e a ) 1

T h e r e are f e w d i r e c t d a t a o n the d e p e n d e n c e of the d e l a y e d

6

3 +

(30).

fluorescence


15.

Environmental

FORSTER

185

Effects

s p e c t r u m o n solvent r i g i d i t y , b u t i n o n e i n s t a n c e , viz. C r ( u r e a )

e

in

3 +

m e t h a n o l - D M F , n o m a r k e d s p e c t r a l c h a n g e is o b s e r v e d a t t h e glass point

(45).

Conclusions A l t h o u g h n o n r a d i a t i v e r e l a x a t i o n of t h e Γ 4

e n v i r o n m e n t , t h e m a j o r v a r i a t i o n s of Φ t h e E state.

ρ

state m a y b e sensitive t o

2

w i t h environment originate i n

T h i s w o u l d result i n the lack of correlation between a n y

2

p h o t o c h e m i s t r y t h a t occurs i n t h e Γ 4

2

state a n d E - » A 4

2

F r o m t h e p h o t o p h y s i c a l d a t a amassed f o r C r

3 +

2

luminescence.

c o m p l e x e s as a f u n c

t i o n o f e n v i r o n m e n t a n d t e m p e r a t u r e , one salient p o i n t emerges. I f p h o t o -


p h y s i c a l d a t a are to b e u s e d to i n f e r p h o t o c h e m i c a l

mechanisms, the

p h o t o p h y s i c a l d e t e r m i n a t i o n s m u s t i n c l u d e , b u t n o t b e l i m i t e d to, m e a s u r e m e n t s m a d e u n d e r p r e c i s e l y t h e same c o n d i t i o n s as t h e p h o t o c h e m i c a l d e t e r m i n a t i o n s . T h i s r e q u i r e m e n t has b e e n f u l f i l l e d i n a f e w i n s t a n c e s — e.g. C r ( C N )

6

3

- i n D M F (46),

Cr(en)

3

in H

3 +

( N C S ) ~ i n a H 0 - a l c o h o l m i x t u r e (47)—but 4

2

2

0 (8),

and C r ( N H ) 3

2

the widespread availability

of p u l s e d e x c i t a t i o n i n t h e n a n o s e c o n d d o m a i n n o w m a k e s i t p o s s i b l e to r e c o r d t h e faster d e c a y s t h a t p r e v a i l u n d e r p h o t o c h e m i c a l l y

meaningful

c o n d i t i o n s , e.g. E - » A e m i s s i o n i n a q u e o u s solutions at r o o m t e m p e r a 2

t u r e (48).

4

2

H o w e v e r , t h e a m b i g u i t i e s associated w i t h b a c k transfer r e q u i r e

t h a t p h o t o p h y s i c a l d a t a b e r e c o r d e d over a r a n g e of t e m p e r a t u r e s . T h e d e t e r m i n a t i o n of i n t e r s y s t e m c r o s s i n g efficiencies, e.g. Φ , 2Ε

will

c o n t i n u e to b e difficult, b u t a c o m p a r i s o n of results f r o m d i r e c t a n d s e n s i t i z e d photolyses a n d m e a s u r e m e n t s of Φ Λ w i l l b e v e r y u s e f u l . Ρ

Ρ

Literature Cited 1. Fleischauer, P. D., Adamson, A. W., Sartori, G., in "Inorganic Reaction Mechanisms," J. O. Edwards, Ed., Part II, p. 1, Interscience, New York, 1972. 2. Schlafer, H. L., Z. Chem. (1970) 10, 9. 3. Balzani, V., Carassiti, V., "Photochemistry of Coordination Compounds," Academic, London, 1970. 4. Forster, L. S., Transition Met. Chem. (1969) 5, 1. 5. Kirk, A. D., Mol. Photochem. (1973) 5, 127. 6. Tanabe, Y., Sugano, S.,J.Phys. Soc. Jpn. (1954) 9, 753. 7. Porter, G. B., in "Concepts in Inorganic Photochemistry," A. Adamson and P. Fleischauer, Eds., chap. 2, Wiley, New York, 1975. 8. Ballardini, R., Varani, G., Wasgestian, H. F., Moggi, L., Balzani, V., J. Phys. Chem. (1973) 77, 2947. 9. Targos, W., Forster, L. S.,J.Chem. Phys. (1966) 44, 4342. 10. Otto, H., Yersin, H., Gliemann, G., Z. Phys. Chem. (NF) (1974) 92, 193. 11. Camassei, F. D., Saldinger, J., unpublished data. 12. Camassei, F. D., Forster, L. S.,J.Chem. Phys. (1969) 50, 2603. 13. Coleman, W. F., Forster, L. S.,J.Lumin. (1971) 4, 429. 14. Coleman, W. F.,J.Lumin. (1975) 10, 72.



186

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15. Goldsmith, G. J., Shallcross, F. V., McClure, D. S.,J.Mol.Spectrosc. (1965) 16, 296. 16. Zander, H. U., Dissertation, Frankfurt/Main, 1969. 17. Chatterjee, K. K., Forster, L. S., Spectrochim. Acta (1965) 20, 1603. 18. Yersin, H., Otto, H., Gliemann, G., Theor. Chim. Acta (1974) 33, 63. 19. Nelson, D. F., Sturge, M. D., Phys. Rev. A (1965) 137, 1117. 20. Castelli, F., Forster, L. S.,J.Am. Chem. Soc. (1973) 95, 7223. 21. Reich, S., Raziel, S., Michaeli, I.,J.Phys. Chem. (1973) 77, 1378. 22. Schläfer, H., Gausmann, H., Witzke, H., J. Chem. Phys. (1967) 46, 1423. 23. Everett, P. N., J. Appl. Phys. (1971) 42, 2106. 24. Castelli, F., Forster, L. S., Phys. Rev. Β (1975) 11, 920. 25. Glass, A. M., J. Chem. Phys. (1969) 50, 1501. 26. Reynolds, M. L., Hagston, W. E., Garlick, G. F. J., Phys. Status Solidi (1968) 30, 113. 27. Castelli, F., Forster, L. S.,J.Am. Chem.Soc.,in press. 28. Castelli, F., unpublished data. 29. Englman, R., Jortner, J., Mol. Phys. (1970) 18, 145. 30. Laver, J. L., Smith, P. W., Aust. J. Chem. (1971) 24, 1807. 31. Castelli, F., Forster, L.S.,J.Phys. Chem. (1974) 78, 2122. 31a. Sabbatini, N., Scandola, Μ. Α., Carassiti, V., J. Phys. Chem. (1973) 77, 1307. 32. Condrate, R., Forster, L. S.,J.Chem. Phys. (1968) 48, 1514. 33. Forster, L. S., in "Concepts in Inorganic Photochemistry," A. Adamson and P. Fleischauer, Eds., chap. 1, Wiley, New York, 1975. 34. Castelli, F., Forster, L. S., Chem. Phys. Lett. (1975) 30, 465. 35. DeArmond, M. K., Forster, L. S., Spectrochim. Acta (1963) 19, 1687. 36. Ganrud, W.B.,Moos, H. W.,J.Chem. Phys. (1968) 49, 2170. 37. Binet, D. J., Goldberg, E. L., Forster, L. S., J. Phys. Chem. (1968) 72, 3017. 38. Chen, S., Porter, G.B.,J.Am. Chem. Soc. (1970) 92, 3196. 39. Dexter, D. L., J. Chem. Phys. (1953) 21, 836. 40. Birgenau, R. J.,J.Chem. Phys. (1969) 50, 4282. 41. Castelli, F., Forster, L. S.,J.Lumin. (1974) 8, 252. 42. Kirk, A. D., Ludi, Α., Schläfer, H. L., Ber. Bunsenges. Phys. Chem. (1969) 73, 669. 43. Armendarez, P. X., Forster, L. S.,J.Chem. Phys. (1964) 40, 273. 44. Courtois, M., Forster, L. S., J. Mol Spectrosc. (1965) 18, 396. 45. Klassen, D. M., Schläfer, H. L., Ber. Bunsenges. Phys. Chem. (1968) 72, 663. 46. Wasgestian, H. F.,J.Phys. Chem. (1972) 76, 1947. 47. Adamson, Α.,J.Phys. Chem. (1967) 71, 798. 48. Kane-Maguire, N. A. P., Langford, C. H., Chem. Commun. (1971) 895. 49. Liehr, A. D.,J.Phys. Chem. (1963) 67, 1314. 50. Porter, G. B., Schläfer, H. L., Z. Phys. Chem. (NF) (1963) 37, 109. RECEIVED February 6, 1975. Work supported by the National Science Founda tion.


Environmental Effects on Intra- and Intermolecular Photophysical

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