Luminescence as a Probe of Excited State Properties

13 Luminescence as a Probe of Excited

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 23, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch013

State Properties G. A. CROSBY Washington State University, Pullman, Wash. 99163

Quantitative investigations of the photoluminescence of inorganic compounds have led to experimental criteria for assigning orbital and spin labels to their low lying electronic excited states. For d compounds, chemical modification of the sequencing of ligand-field, charge-transfer, and ligand -localized excited states has been demonstrated. The capability of prescribing the lowest excited states has produced a series of materials with unusual optical properties. Details of the charge-transfer-to-ligand excited configurations that have been obtained for ruthenium(II) and osmium(II) complexes provide a new perspective on the role of spin-orbit coupling in defining the properties of the associated states. Systematic study of excited state properties indicates possibilities for dictating the pathways of photochemical reactions, for relating spectroscopy to electrochemistry, and hopefully, for correlating excited state properties with thermal reactivities. 6

m i s s i o n s p e c t r o s c o p y has a l o n g a n d v e n e r a b l e h i s t o r y of p r o v i d i n g v a l u a b l e i n f o r m a t i o n o n the n a t u r e of the l o w l y i n g e x c i t e d states of o r g a n i c molecules

(1,

2),

b u t , u n t i l r e c e n t l y , systematic use of

photo-

l u m i n e s c e n c e as a p r o b e of e x c i t e d state properties of t r a n s i t i o n m e t a l complexes was not w i d e s p r e a d . F o r complexes that c o n t a i n c e n t r a l m e t a l ions o f c e r t a i n configurations, e s p e c i a l l y d

3

a n d d , the e n e r g y l e v e l 6

schemes are p r o p i t i o u s for o c c u r r e n c e a n d d e t e c t i o n of l u m i n e s c e n c e , a n d the s t r u c t u r a l a n d e n v i r o n m e n t a l factors t h a t c o n t r o l the p r o p e r t i e s of the l o w l y i n g e x c i t e d states are b e i n g d e f i n e d t h r o u g h e m i s s i o n spectroscopy.

C r i t e r i a h a v e b e e n d e v e l o p e d f o r assigning o r b i t a l a n d s p i n

labels to e x c i t e d states of complexes ( 3 ) , a n d some p r i m i t i v e attempts t o 149 King; Inorganic Compounds with Unusual Properties Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

150

INORGANIC

COMPOUNDS WITH

UNUSUAL

PROPERTIES

engineer molecules w i t h stipulated electronic properties were (4,5).

successful

I n this p a p e r , a t t e n t i o n is focussed o n those features of the e x c i t e d

states of t r a n s i t i o n m e t a l c o m p l e x e s t h a t differentiate t h e m f r o m t h e w e l l s t u d i e d o r g a n i c ones, o n the m a g n i f i c e n t v e r s a t i l i t y i n h e r e n t i n t r a n s i t i o n m e t a l c h e m i s t r y for d e s i g n i n g m o l e c u l e s w i t h p r e s c r i b e d e l e c t r o n i c p r o p erties, a n d o n the k i n d s of d e t a i l e d i n f o r m a t i o n t h a t c a n b e

obtained

a b o u t the e x c i t e d states of t r a n s i t i o n m e t a l c o m p l e x e s b y e m i s s i o n t e c h niques.

T h e usefulness of this k i n d of i n f o r m a t i o n for other

fields

of


c h e m i c a l research is c o n s i d e r e d briefly.

Chemical

Tuning

T h e l o w l y i n g e x c i t e d states of d s t r o n g field c o m p l e x e s that c o n t a i n 6

7r-conjugated l i g a n d s c a n be c o n v e n i e n t l y classified i n t o f o u r o r b i t a l p r o m o t i o n a l t y p e s : ( a ) ΤΓΤΓ* states i n w h i c h the e x c i t a t i o n energy is l o c a l i z e d essentially o n the l i g a n d s a n d the characteristics of the states s t r o n g l y reflect t h e i r l i g a n d parentage, ( b )

CZTT* states i n w h i c h the final states are

d e r i v e d f r o m a c o n f i g u r a t i o n i n w h i c h a n electron has b e e n t r a n s f e r r e d f r o m the m e t a l core to a n a n t i b o n d i n g o r b i t a l d e l o c a l i z e d over the l i g a n d 7Γ system, ( c )

ird states for w h i c h a transfer of c h a r g e f r o m t h e J i g a n d

system to the m e t a l i o n c a n be process, a n d ( d )

v i s u a l i z e d as the p r i m a r y e x c i t a t i o n

dd states that are effectively m e t a l - l o c a l i z e d e l e c t r o n i c

excitations i n w h i c h the l i g a n d s are i n v o l v e d o n l y as c o n t r i b u t o r s of t h e n o n s p h e r i c a l static p o t e n t i a l that d e t e r m i n e s

the

orbital promotional

energy. W i t h the e x c e p t i o n

of ird e x c i t e d states, d e t a i l e d analyses of

the

e m i s s i o n characteristics of d c o m p l e x e s of the s e c o n d a n d t h i r d t r a n s i t i o n 6

series h a v e l e a d to c r i t e r i a for c l a s s i f y i n g these states b y means of e m i s s i o n s p e c t r o s c o p y (3, 6).

Moreover, further investigation revealed that com

plexes c a n be e n g i n e e r e d to possess a p r e d e t e r m i n e d sequence of e x c i t e d states w i t h p r e s c r i b e d o r b i t a l types (4, 5).

This capability, designated

c h e m i c a l t u n i n g , has l e d to series of m o l e c u l e s w h o s e l o w l y i n g e x c i t e d states w e r e chosen s p e c i f i c a l l y a n d w h o s e spectroscopic

properties

were

o r d a i n e d as w e l l . O n c e the gross o r b i t a l types of the lowest e x c i t e d states of c o m p l e x e s are d e t e r m i n e d , s u b t l e alterations i n p r o p e r t i e s c a n be effected b y v a r y i n g l i g a n d substituents, b y m o d i f y i n g the e n v i r o n m e n t of the active

species,

or b y s u b j e c t i n g the m a t e r i a l s to e x t e r n a l p e r t u r b a t i o n s . T h i s fine t u n i n g of s p e c t r o s c o p i c

characteristics has p r o d u c e d a n u m b e r of u n u s u a l e l e c

t r o n i c p r o p e r t i e s w i t h p o t e n t i a l use f o r e x p l o i t a t i o n i n b o t h a f u n d a m e n t a l and a practical way.

W e d i r e c t a t t e n t i o n p a r t i c u l a r l y to charge-transfer

e x c i t e d states.

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

13.

Luminescence

CROSBY

Unusual Properties of (CTTL)

151

as a Probe

Charge-Transfer-To-Ligand

Excited States

S y s t e m a t i c investigations of series of r u t h e n i u m ( I I ) osmium (II)

( I I ) , and iridium (III)

(12)

(7,

8, 9,

c o m p l e x e s l e d to t h e e x p e r i

m e n t a l a n d t h e o r e t i c a l c h a r a c t e r i z a t i o n of C T T L e x c i t e d states. p r o p e r t i e s , d e r i v e d f r o m analyses of

10), Their

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

actions w i t h e x t e r n a l fields, differ f u n d a m e n t a l l y f r o m the e x c i t e d states


of o r g a n i c m a t e r i a l s a n d e v e n f r o m states of o t h e r o r b i t a l parentages w i t h i n t h e same m o l e c u l e . Experimental Features of CTTL

Excited States

T h e intense p h o t o l u m i n e s c e n c e e x h i b i t e d b y c o m p l e x e s t h a t d i s p l a y e m i s s i o n o r i g i n a t i n g f r o m CZTT* configurations was n o t w e l l

understood

u n t i l the o b s e r v a t i o n r a n g e w a s e x t e n d e d to t e m p e r a t u r e s b e l o w 77 °K. F o r a s i n g l e e m i t t i n g l e v e l ( or cluster of degenerate levels ), the m e a s u r e d decay time a n d the q u a n t u m y i e l d should be related b y the equation τ =

φτ

0

w h e r e τ , t h e l i m i t i n g d e c a y t i m e n e a r 0 ° K , is e x p e c t e d t o b e 0

temperature independent. ruthenium(II)

(8)

F r o m measurements of τ a n d φ at 7 7 ° Κ o n

and osmium(II)

d i c t e d for series of complexes.

complexes, v a l u e s w e r e p r e

(13)

W h e n the l o w t e m p e r a t u r e

experiments

w e r e p e r f o r m e d , h o w e v e r , i t w a s f o u n d that the m e a s u r e d d e c a y times f a r e x c e e d e d t h e t h e o r e t i c a l l i m i t s , a b e h a v i o r that is s t r o n g l y i n d i c a t i v e of a m a n i f o l d of e m i t t i n g levels, e a c h one w i t h its o w n set of r a d i a t i v e

10

20

30

40

50

60

70

e

K

Figure 1. Temperature dependence of the calculated ( ) and observed ( · · · , Χ X X j lifetimes of tris(l,10-phenanthroline)ruthenium(II) iodide and tris(l,10-phenanthroline)~ osmium(II) iodide in poly(methyl methacrylate). Energy level splittings and individual mean decay times were determined from a computer ft of the experimental lata.


152

INORGANIC

COMPOUNDS

WITH

UNUSUAL PROPERTIES

a n d radiationless d e c a y constants w h o s e separations are o n the o r d e r of kT i n t h e r a n g e of 5 ° - 5 0 ° K .

T h e s i t u a t i o n is i l l u s t r a t e d i n F i g u r e 1.

B e l o w 77 °K, the d e c a y t i m e of e a c h of t h e l u m i n e s c e n t m o l e c u l e s rises m o n o t o n i c a l l y w i t h d e c r e a s i n g t e m p e r a t u r e , a n d i t e i t h e r fails to r e a c h a l i m i t at the lowest t e m p e r a t u r e s a t t a i n e d or i t approaches a constant v a l u e that exceeds the p r e d i c t e d l i m i t b y m a n y factors.

T h e s e facts are

inconsistent w i t h the presence of a s i n g l e l u m i n e s c e n t l e v e l or a d e g e n erate set of s u c h levels. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 23, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch013

A second p e r t i n e n t feature of the d e c a y k i n e t i c s of complexes

that

d i s p l a y charge-transfer l u m i n e s c e n c e is the e x p o n e n t i a l i t y of t h e o b s e r v e d transients at a l l temperatures r e a c h e d ( ^ 1 . 5 ° K ) .

T h i s b e h a v i o r is i n

stark contrast to that e x h i b i t e d b y o r g a n i c systems at l o w temperatures. F o r the latter, single e x p o n e n t i a l decays are m a i n t a i n e d to — 1 0 °K, b u t t h e y are r e p l a c e d b y c o m p l i c a t e d k i n e t i c s at l o w e r temperatures

(2).

N o n e x p o n e n t i a l d e c a y s h a v e also b e e n o b s e r v e d at l o w t e m p e r a t u r e f o r tris complexes of r h o d i u m ( I I I ) t h a t e x h i b i t ττπ* states lowest 3

(14).

Energy Level Schemes A l t h o u g h the e m i s s i o n spectra t h a t o r i g i n a t e f r o m dw* e x c i t e d states of

complexes

have

s t r u c t u r e , the v i b r a t i o n a l b a n d w i d t h s e x c e e d

the

splittings of the e l e c t r o n i c levels. T h e r e is little s h a r p e n i n g at l o w t e m p e r a t u r e , a n d i n c o r p o r a t i o n i n t o a l a t t i c e does not a p p e a r to i m p r o v e t h e resolution (15).

N o n e t h e l e s s , i t is possible to o b t a i n the l e v e l s p l i t t i n g s

f r o m the d e c a y d a t a .

I f one assumes a m a n i f o l d of e x c i t e d states i n

t h e r m a l e q u i l i b r i u m at a l l temperatures, e a c h d e c a y i n g w i t h t e m p e r a t u r e i n d e p e n d e n t d e c a y constants, one arrives at a n a n a l y t i c a l expression f o r 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 the m e a n d e c a y t i m e of t h e

ensemble

(8,9,10):

A c o m p u t e r fit of this expression y i e l d s the ki's f o r the levels.

Analyses

of this t y p e w e r e m a d e f o r series of drr* emitters a n d the e n e r g y l e v e l schemes d e r i v e d f r o m t h e d e c a y curves are i n c l u d e d i n F i g u r e 1. T h e l e v e l schemes f o r [ R u ( p h e n ) ] 3

1,10-phenanthroline)

2 +

and [ O s ( p h e n ) ] 3

+ 2

(phen

—

are r e p r e s e n t a t i v e of the clusters of l o w l y i n g elec

t r o n i c states t h a t arise f r o m άπ* configurations of m a n y r u t h e n i u m ( I I ) , o s m i u m ( I I ) , a n d i r i d i u m ( I I I ) complexes.

T h e y are h i g h l y u n u s u a l s i n c e

t h e y h a v e d e c a y p a r a m e t e r s t h a t he b e t w e e n t h e ranges e x p e c t e d

for

c o n v e n t i o n a l singlet a n d t r i p l e t states a n d b e c a u s e of the m a g n i t u d e s of t h e s p l i t t i n g s themselves.

T h e s e p a r a m e t e r s c o n t r o l t h e n a t u r e of dm*


13.

153

Luminescence as a Probe

CROSBY

p h o t o l u m i n e s c e n c e a n d are r e s p o n s i b l e f o r its h i s t o r y o f a s s i g n m e n t a n d reassignment ( 1 6 ) . Coupling Model In order to rationalize the behavior of the luminescence f r o m nd complexes 6

observed

that d i s p l a y ίίπ* e m i s s i o n a n d t o a c c o u n t f o r t h e

exceptional splittings a n d decay parameters obtained for them, a m o d e l


for t h e e x c i t e d states has e m e r g e d t h a t emphasizes t h e r o l e o f s p i n - o r b i t c o u p l i n g i n c o n t r o l l i n g t h e i r p r o p e r t i e s ( 1 7 , I S ) . A n e x c i t e d άπ* c o n figuration

is v i e w e d as a n a b o r t i v e o x i d a t i o n o f the d c o m p l e x that p r o 6

d u c e s a system w i t h a d

core a n d a p r o m o t e d

5

(optical)

electron dis

t r i b u t e d i n a n a n t i b o n d i n g o r b i t a l e n c o m p a s s i n g the π-conjugated l i g a n d s . T h e core is v i e w e d as a K r a m e r s i o n w i t h a set o f e l e c t r o n i c states w h o s e positions a r e d e f i n e d b y electrostatic, s p i n - o r b i t , a n d l i g a n d - f i e l d i n t e r actions. T h e s e c o r e states define a v e c t o r space.

A second vector space

is d e f i n e d b y the set o f m o l e c u l a r s p i n o r b i t a l s o p e n t o the e x c i t e d e l e c t r o n r e s i d i n g o n t h e l i g a n d s . T h e final eigenspace

f o r t h e t o t a l e x c i t e d d?

system is d e t e r m i n e d b y d i a g o n a l i z i n g t h e d i r e c t p r o d u c t space o f t h e t w o subsystems u n d e r t h e f u l l H a m i l t o n i a n o f t h e six-electron p r o b l e m . F o r t h e l o w e s t

Luminescence as a Probe of Excited State Properties

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