17 A Comparative Study of Some Luminescence Properties of Homo- and Hetero-Bischelated Complexes of Iridium(III)
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RICHARD J. WATTS, BOULDEN GRIFFITH, and JOHN S. HARRINGTON University of California, Santa Barbara, Calif. 93106
Ligand substituent alterations affect the luminescence decay of two hetero-bischelated complexes of iridium(III). Lumi nescence decay curves of cis-dichloro-1,10-phenanthroline4,7-dimethyl-1,10-phenanthroline iridium(III) chloride com plex A) and of cis-dichloro-1,10-phenanthroline-5,6-dimethyl1,10-phenanthroline iridium(III) chloride (complex B) reveal strikingly dissimilar nonexponential behavior. While both complexes exhibit an emission wavelength dependence of the nonexponential decays, analysis of complex A on the basis of a two-level emission model yields two exponential components having lifetimes of 8.8 and 22μsec,with the short-lived level lying 300-400cm lower in energy. This contrasts with complex Β whose short- and long-lived com ponents are both longer lived than the corresponding levels of complex A and whose short-lived level lies higher in energy. -1
T ^ T u m e r o u s studies of the e m i s s i o n o f v i s i b l e a n d u l t r a v i o l e t r a d i a t i o n b y l a r g e m o l e c u l e s after t h e y h a v e b e e n p r o m o t e d to a n e x c i t e d state h a v e r e s u l t e d i n s e v e r a l b r o a d generalizations. I n l i g h t - a t o m m o l e c u l e s , s u c h as h y d r o c a r b o n s , as m a n y as t w o t h e r m a l l y n o n - e q u i l i b r a t e d e x c i t e d states m a y give rise to l i g h t emissions. T h e s e emissions a r e classified as fluorescence
a n d p h o s p h o r e s c e n c e a n d a r e associated w i t h transitions i n
w h i c h the spin q u a n t u m n u m b e r remains t h e same (fluorescence) o r changes b y o n e u n i t ( p h o s p h o r e s c e n c e ) .
P h o s p h o r e s c e n c e is g e n e r a l l y
c a u s e d b y a t r a n s i t i o n f r o m a s p i n t r i p l e t t o a s p i n singlet i n m o l e c u l e s w i t h a n e v e n n u m b e r o f electrons;
fluorescence
is g e n e r a l l y t h e r e s u l t o f
a singlet-singlet t r a n s i t i o n . T h u s , there are f o u r e m i t t i n g levels i n these 201 King; Inorganic Compounds with Unusual Properties Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
202
INORGANIC
COMPOUNDS
WITH
UNUSUAL PROPERTIES
m o l e c u l e s w h e n t h e c o m b i n e d effects of e l e c t r o n s p i n a n d o r b i t a l m o m e n t a are t a k e n i n t o c o n s i d e r a t i o n .
U n d e r the common
conditions at w h i c h
e m i s s i o n measurements of large m o l e c u l e s are m a d e ( 7 7 ° K , glassy m e d i u m ) , t h e three t r i p l e t sublevels are i n t h e r m a l e q u i l i b r i u m w i t h e a c h o t h e r whereas t h e r m a l e q u i l i b r a t i o n of t h e singlet a n d t r i p l e t m a n i f o l d s does n o t occur. W h e n h e a v y atoms are i n t r o d u c e d i n t o t h e f r a m e w o r k of a m o l e c u l e , m a r k e d changes i n t h e l i g h t e m i s s i o n characteristics occur.
I n particular,
f o r c o m p l e x m o l e c u l e s w h i c h c o n t a i n m e t a l ions f r o m t h e second a n d t h i r d t r a n s i t i o n series, o n l y a single e m i s s i o n f r o m a set of t h e r m a l l y e q u i l i b r a t e d e x c i t e d levels is g e n e r a l l y o b s e r v e d (1-7).
T h i s b e h a v i o r is
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a p p a r e n t l y associated w i t h a s p i n - o r b i t c o u p l i n g effect, a n d , i n d e e d , i t has b e e n suggested that this effect is so large as to r e n d e r s p i n labels i n a p p r o p r i a t e i n s e c o n d - a n d t h i r d - r o w m e t a l complexes
(8).
A l t h o u g h t h e n u m b e r of e m i t t i n g levels w h i c h a r e t h e r m a l l y p o p u l a t e d at 77° Κ i n h e a v y m e t a l complexes is n o t g e n e r a l l y k n o w n , several studies of complexes of t h e d m e t a l ions, R u ( I I ) a n d I r ( I I I ) , h a v e c o n 6
c e n t r a t e d o n this q u e s t i o n (4-7).
T h e s e studies, w h i c h use measurements
of 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 l u m i n e s c e n c e l i f e t i m e s ,
indicate that
complexes w i t h charge-transfer emissions l u m i n e s c e f r o m f o u r t h e r m a l l y e q u i l i b r a t e d levels at 77 °K.
T h e s e levels l i e i n a n e n e r g y r e g i o n of 1 0 0 -
200 c m " f r o m e a c h other. T h e n a t u r e of these levels has b e e n r e s e a r c h e d 1
i n considerable detail i n the ruthenium trisbipyridyl i o n a n d i n related ions of D s y m m e t r y . I n these c o m p l e x ions, t w o of the f o u r e m i t t i n g levels 3
are degenerate a l t h o u g h this d e g e n e r a c y s h o u l d b e r a i s e d i n complexes w i t h reduced symmetry. T h e c o n t r a s t i n g l u m i n e s c e n c e b e h a v i o r of l i g h t - a t o m a n d h e a v y - a t o m m o l e c u l e s indicates t h e v i t a l role of s p i n - o r b i t c o u p l i n g i n d e t e r m i n i n g t h e m e c h a n i s m s b y w h i c h e l e t c r o n i c energy is c o n v e r t e d to v i b r a t i o n a l energy i n the molecular framework.
I t is w e l l k n o w n h o w e v e r t h a t s p i n - o r b i t
c o u p l i n g alone w i l l n o t ensure r a p i d e n e r g y
transfer (9-11).
m o l e c u l a r parameters, s u c h as energy gaps (12, 13), q u e n c i e s (14),
Other
vibrational fre
a n d t h e o r b i t a l n a t u r e of t h e e x c i t e d states ( 1 5 , 16), a r e
i m p o r t a n t i n d e t e r m i n i n g e n e r g y transfer rates i n l i g h t a t o m
molecules
a n d m u s t at least h a v e c o m p l e m e n t a r y roles to s p i n - o r b i t c o u p l i n g i n heavy-atom
complexes
(17).
U n f o r t u n a t e l y , t h e roles of these
other
p a r a m e t e r s a p p e a r , i n g e n e r a l , to b e s u b m e r g e d b y t h e d o m i n a n t r o l e of s p i n - o r b i t c o u p l i n g , w h i c h causes t h e r a p i d energy transfer processes l e a d i n g to a single e m i s s i o n f r o m a set o f t h e r m a l l y e q u i l i b r a t e d levels i n h e a v y - m e t a l complexes. S e v e r a l r a r e exceptions
to t h e g e n e r a l l u m i n e s c e n c e
b e h a v i o r of
heavy-atom molecules described above have recently been reported.
It
has b e e n f o u n d t h a t h e t e r o - t r i s c h e l a t e d complexes o f R h ( I I I ) w i t h 2,2'-
King; Inorganic Compounds with Unusual Properties Advances in Chemistry; American Chemical Society: Washington, DC, 1976.
17.
Hetero-Bischelated
WATTS E T A L .
Complexes
203
of Iridium(IH)
b i p y r i d i n e ( b i p y ) a n d 1,10-phenanthroline ( p h e n ) h a v e n o n - e x p o n e n t i a l l u m i n e s c e n c e d e c a y curves (18).
A l t h o u g h this is c h a r a c t e r i s t i c of l i g h t
e m i s s i o n f r o m s e v e r a l t h e r m a l l y n o n - e q u i l i b r a t e d states, t h e i n d i v i d u a l l u m i n e s c e n c e s p e c t r a of the n o n - e q u i l i b r a t e d levels c a n n o t b e r e s o l v e d b y c o n v e n t i o n a l e m i s s i o n spectroscopy. near degeneracy
T h i s is a p p a r e n t l y the r e s u l t of a
of the e m i t t i n g levels.
I n these c o m p l e x e s ,
the n o n -
e q u i l i b r a t e d l u m i n e s c e n t levels are t h o u g h t to arise f r o m e x c i t e d states w h i c h h a v e t h e i r energy l o c a l i z e d i n e i t h e r the 2 , 2 ' - b i p y r i d i n e or phenanthroline ligand
1,10-
(18).
S e v e r a l h e t e r o - b i s c h e l a t e d complexes
of I r ( I I I )
with
1,10-phenan
t h r o l i n e a n d s u b s t i t u t e d 1,10-phenanthroline h a v e also b e e n r e p o r t e d to have non-exponential luminescence decay
curves
(19).
A l t h o u g h the
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i n d i v i d u a l e m i s s i o n s p e c t r a of the n o n - e q u i l i b r a t e d levels of these c o m plexes are a g a i n too close to resolve b y c o n v e n t i o n a l e m i s s i o n spectros c o p y , p a r t i a l r e s o l u t i o n has b e e n a c c o m p l i s h e d b y t i m e - r e s o l v e d e m i s s i o n s p e c t r o s c o p y via b o x - c a r a v e r a g i n g t e c h n i q u e s (20).
Complete resolution
has b e e n a c c o m p l i s h e d b y c o m p u t e r analysis of l u m i n e s c e n c e d e c a y curves as a f u n c t i o n of e m i s s i o n w a v e l e n g t h (20).
I n these c o m p l e x e s , t h e l u m i
nescent levels a p p e a r t o arise f r o m b o t h l i g a n d - l o c a l i z e d (πττ* ) states a n d c h a r g e - t r a n s f e r (dw*)
states.
I n this p a p e r w e first r e v i e w the e x p e r i m e n t a l d a t a w h i c h c h a r a c terizes the l u m i n e s c e n c e of s e v e r a l complexes f o r m e d b y the b i n d i n g of t w o c h l o r i d e ions a n d t w o b i d e n t a t e l i g a n d s to I r ( I I I ) . this e x p e r i m e n t a l i n f o r m a t i o n , w e
O n the basis of
present a s i m p l e m o l e c u l a r o r b i t a l
m o d e l w h i c h describes the o r b i t a l p a r e n t a g e of the l u m i n e s c e n t states of these complexes.
T h i s m o d e l is u s e d to i n t e r p r e t t h e n o n - e x p o n e n t i a l
l u m i n e s c e n c e of h e t e r o - b i s c h e l a t e d complexes of I r ( I I I ) . Review of Experimental
Results
T h e l u m i n e s c e n c e of c i s - d i c h l o r o b i s - 1,10-phenanthroline i r i d i u m ( I I I ) c h l o r i d e , [ I r C l ( p h e n ) ] C l , has b e e n s t u d i e d i n several l a b o r a t o r i e s 2
22).
2
(21,
A l t h o u g h t h e r e is g e n e r a l a g r e e m e n t t h a t a s i n g l e e m i s s i o n f r o m a
set of t h e r m a l l y e q u i l i b r a t e d levels occurs u p o n e x c i t a t i o n of the c o m p l e x at 77° Κ i n glassy m e d i a , there is some d i s p u t e c o n c e r n i n g t h e o r b i t a l p a r e n t a g e of the e m i t t i n g levels.
W h e r e a s C r o s b y a n d C a r s tens h a v e
a s s i g n e d the e m i t t i n g levels to a άπ* o r b i t a l p a r e n t a g e (21), a n d H i l l i s (22)
c o n t r i b u t i o n f r o m b o t h
