Europium Chelates as Laser Materials

12 Europium Chelates as Laser Materials DANIEL L. ROSS and JOSEPH BLANC

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RCA Laboratories, Princeton, N. J.

3+

A variety of lasers based on tetrakis β-diketonates of Eu have been developed over the last few years. A summary of the chemistry and energy transfer in these laser systems is presented, with particular attention to the salts of tetrakis benzoyltrifluoroacetone chelates of europium. Chemical effects attributed to solvents, benzene ring substitutions in the ligand, differing cations, and deuteration are consid ered. These effects manifest themselves most markedly in the variability of laser thresholds from compound to com pound and solvent to solvent. The thresholds reflect associa tion-dissociation equilibria, as well as energy transfer processes in the ligand and throughout the manifold of Eu states. 3+

/ C o n s i d e r a b l e w o r k over t h e past f e w years has b e e n s t i m u l a t e d b y ^ W e i s s m a n ' s o b s e r v a t i o n ( 3 5 ) that u l t r a v i o l e t e x c i t a t i o n of o r g a n i c chelates of e u r o p i u m b r i n g s a b o u t efficient i n t r a m o l e c u l a r energy transfer f r o m l i g a n d e x c i t e d states t o e u r o p i u m emissive levels a n d b y t h e s u g gestions of W h a n a n d C r o s b y (36) a n d S c h i m i t s c h e k (34) that e u r o p i u m chelates c o u l d b e t h e basis of laser systems. T h i s research has r e s u l t e d i n the d e m o n s t r a t i o n of laser a c t i o n b y solutions of chelates of e u r o p i u m w i t h b e n z o y l a c e t o n e (20), d i b e n z o y l m e t h a n e ( 3 0 ) , trifluoroacetylacetone (31), thenoyltrifluoroacetone (31), a n d b e n z o y l t r i f l u o r o a c e t o n e a n d i t s r i n g s u b s t i t u t e d derivatives (25, 28, 31, 33). L a s e r a c t i v i t y w a s o b s e r v e d i n i t i a l l y at temperatures near — 1 4 0 ° C . R e c e n t l y , r o o m t e m p e r a t u r e o p e r a t i o n (28) of e u r o p i u m chelate lasers has b e e n o b s e r v e d , a n d t h e p o s s i b i l i t y of a c i r c u l a t i n g , r o o m t e m p e r a t u r e l i q u i d chelate laser, a feature w h i c h s h o u l d p e r m i t l o n g t e r m , r e p e t i t i v e flashing b y e x t e r n a l l y c o o l i n g the s o l u t i o n , has b e e n d e m o n s t r a t e d (32). O n e significant a d v a n c e i n the field o f l a n t h a n i d e c h e m i s t r y r e s u l t i n g f r o m research i n this area is t h e u n e q u i v o c a l d e m o n s t r a t i o n that t h e 155

In Lanthanide/Actinide Chemistry; Fields, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

156

L A N T H A N I D E / A C T I N I D E

C H E M I S T R Y

t r i v a l e n t l a n t h a n i d e s prefer to s h o w a c o o r d i n a t i o n n u m b e r of e i g h t i n their reaction w i t h bidentate p h e n a n t h r o l i n e (1,22). the use of c o m p o u n d s species, w h e r e C case, N a

+

+

l i g a n d s s u c h as β-diketones a n d o r t h o

A l l o f t h e successful laser systems h a v e r e q u i r e d of t h e t y p e C

+

[ ( β-diketono ) E u ] " as t h e a c t i v e 4

is a c a t i o n , either s u b s t i t u t e d a m m o n i u m , or, i n one

I n t h e case of t h e benzoylacetonates

(27).

and dibenzoyl-

m e t h i d e s , t h e tris chelates h a v e b e e n s h o w n to b e i n a c t i v e a n d to h a v e q u i t e different spectroscopic properties (2, 23,


S e v e r a l articles (29)

30).

p o i n t o u t that t h e most significant p r o p e r t y of

these c o m p o u n d s w h i c h l i m i t s t h e i r usefulness as laser materials is t h e i r h i g h u l t r a v i o l e t e x t i n c t i o n coefficients, necessitating t h e use of e x t r e m e l y t h i n sections of l i q u i d ( ca. 1-6 m m . ) to a c h i e v e u n i f o r m e x c i t a t i o n of t h e solutions c o n t a i n i n g t h e c o n c e n t r a t i o n of chelate r e q u i r e d (ca.

10" M) 2

for laser a c t i o n . A t present, i t appears that n o attempts to o v e r c o m e this f u n d a m e n t a l l i m i t a t i o n b y u s i n g other e u r o p i u m d e r i v a t i v e s w i t h l o w e r e x t i n c t i o n coefficients or b y different e n e r g y transfer paths (12, 16) m e t w i t h success.

T h e w i d e range of spectroscopic

have

and chemical be

h a v i o r to b e f o u n d w i t h i n this g e n e r a l class of c o m p o u n d s

suggests t h a t

f r u i t f u l w o r k c a n b e d o n e t o w a r d o p t i m i z i n g those properties w h i c h are r e q u i r e d f o r efficient laser p e r f o r m a n c e of the tetrakis β-diketono e u r o p i u m chelates

(9,21).

W e s h a l l discuss t h e results of i n v e s t i g a t i n g t h e effects of c h e m i c a l changes o n t h e p e r f o r m a n c e

a n d spectroscopic

p r o p e r t i e s of e u r o p i u m

chelate laser m a t e r i a l s . M u c h of o u r w o r k has b e e n d o n e w i t h t h e salts of tetrakis chelates of e u r o p i u m w i t h b e n z o y l t r i f l u o r o a c e t o n e

(BTFA).

T h e s e materials w e r e chosen f o r s t u d y because t h e h i g h r o o m t e m p e r a t u r e photoluminescent

q u a n t u m efficiency of e u r o p i u m

fluorinated

diketone

d e r i v a t i v e s , a n d t h e fact t h a t several of t h e m h a d b e e n o b s e r v e d to lase at r o o m t e m p e r a t u r e ( i n a c e t o n i t r i l e ) suggested that, b y w o r k i n g at l o w temperatures, w e m i g h t b e a l l o w e d c o n s i d e r a b l e f r e e d o m i n i n v e s t i g a t i n g the effect of v a r y i n g a n u m b e r of t h e c h e m i c a l properties of this system w i t h o u t its b e i n g p r e v e n t e d , via excessive

n o n r a d i a t i v e energy

losses,

f r o m e x h i b i t i n g laser a c t i o n . T h e c h e m i c a l effects to b e c o n s i d e r e d here are those of solvent, l i g a n d b e n z e n e - r i n g s u b s t i t u t i o n , t h e c a t i o n , a n d deuteration. F o r t h e w o r k discussed i n t h e f o l l o w i n g sections, w e h a v e u s e d as l o w t e m p e r a t u r e laser solvents f o r B T F A

chelates a 2 : 1 : 1 m i x t u r e of

β-ethoxypropionitrile, β-ethoxyethanol, a n d a c e t o n i t r i l e , ( E E A ) , forms

a clear m e d i u m to a b o u t

135 °K.,

Samelson, L e m p i c k i , a n d B r e c h e r (26),

which

a n d a mixture, reported b y

of e q u a l parts of p r o p i o n i t r i l e ,

butyronitrile, a n d isobutyronitrile ("nitrile solvent")

w h i c h seems to

r e m a i n clear at l o w e r temperatures.


12.

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A N D

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157

Chelates

Effect of Solvent T h e d i s s o c i a t i o n of tetrakis chelates i n t o m i x t u r e s of n o n l a s i n g tris chelates ( a n d p o s s i b l y l o w e r forms ) a n d free d i k e t o n a t e a n i o n i n s o l u t i o n has b e e n d i s c u s s e d extensively b y B r e c h e r , S a m e l s o n , a n d L e m p i c k i ( 5 , 26).

T h e s e w o r k e r s h a v e s h o w n that the degree of d i s s o c i a t i o n of a g i v e n

c o m p o u n d is m a r k e d l y affected b y the n a t u r e of the solvent. F o r e x a m p l e , the p i p e r i d i n i u m salt of e u r o p i u m tetrakis b e n z o y l a c e t o n a t e

was found

to be 3 7 % d i s s o c i a t e d i n t o tris or l o w e r forms i n a 3:1 e t h a n o l - m e t h a n o l Downloaded by PURDUE UNIVERSITY on March 21, 2013 | http://pubs.acs.org Publication Date: June 1, 1967 | doi: 10.1021/ba-1967-0071.ch012

s o l u t i o n at 93°K., w h i l e a d d i n g d i m e t h y l f o r m a m i d e to this system i n creased the d i s s o c i a t i o n to 8 2 % . S i m i l a r l y , the d i b e n z o y l m e t h a n e d e r i v a t i v e s h o w e d a c h a n g e f r o m 43 to 5 1 % d i s s o c i a t i o n for the t w o solvent m i x t u r e s . I n this latter case, h o w e v e r , the r e d u c t i o n i n efficiency

caused

b y the increase i n d i s s o c i a t i o n is m o r e t h a n c o m p e n s a t e d for b y a different effect—that of a n i n t e r a c t i o n b e t w e e n the d i m e t h y l f o r m a m i d e a n d the r e m a i n i n g tetrakis chelate ions ( p o s s i b l y to f o r m a n i n e - c o o r d i n a t e

com-

p l e x ) , w h i c h results i n a change i n the s y m m e t r y a b o u t the e u r o p i u m i o n a n d thus the e m i s s i o n s p e c t r u m . T h e net effect is a c o n s i d e r a b l e decrease i n the laser t h r e s h o l d of the a l c o h o l - d i m e t h y l f o r m a m i d e s o l u t i o n . [ B y the t e r m "laser t h r e s h o l d " w e m e a n that a m o u n t of energy ( i n these

cases,

the u l t r a v i o l e t l i g h t p r o v i d e d b y a x e n o n flash t u b e ) w h i c h m u s t b e d e l i v e r e d to the laser d e v i c e to b r i n g it to the p o i n t at w h i c h the onset of laser a c t i o n is o b s e r v e d . ] The

p i p e r i d i n i u m benzoyltrifluoroacetone

more interesting behavior.

derivative

shows

even

I n the a l c o h o l m i x t u r e , it is c o m p l e t e l y d i s -

sociated. A d d i n g d i m e t h y l f o r m a m i d e decreases the degree of d i s s o c i a t i o n to 4 7 % , w h i l e a s o l u t i o n of this c o m p o u n d i n n i t r i l e solvents ( i n w h i c h the b e n z o y l a c t o n a t e is f o u n d to b e c o m p l e t e l y d i s s o c i a t e d ) e v e n at r o o m t e m p e r a t u r e , is less t h a n 1 0 % dissociated. efficiency of this a n d closely r e l a t e d

T h e h i g h inherent quantum

fluorinated

chelates ( 1 5 ) , a n d the

specific b e n e f i c i a l effect of n i t r i l e solvents has p e r m i t t e d laser a c t i o n to o c c u r at r o o m t e m p e r a t u r e w i t h , for e x a m p l e , p i p e r i d i n i u m ( B T F A ) E u 4

in acetonitrile

(28).

It appears that d i s s o c i a t i o n of tetrakis chelates i n s o l u t i o n to give free l i g a n d anions

( w h i c h show

respect to those of i o n

fluorescence)

phosphorescence lifetimes long (4)

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

with

is responsible for ïeports of the fluorescence

from

c e r t a i n e u r o p i u m β-diketonates, the u n e q u a l l i f e t i m e s

of

w h i c h l e d to the suggestion t h a t e n e r g y transfer to the e u r o p i u m i o n c a m e f r o m a different t r i p l e t l e v e l ( o r a h i g h e r t r i p l e t l e v e l ) t h a n that f r o m w h i c h p h o s p h o r e s c e n c e is o b s e r v e d . I n some cases, solvent m o l e c u l e s take p a r t i n a c h e m i c a l r e a c t i o n w i t h the chelate.

F r y a n d P i r i e (14)

h a v e s h o w n t h a t b o t h heat a n d


158


C H E M I S T R Y

u l t r a v i o l e t l i g h t b r o u g h t a b o u t the f o r m a t i o n of a c e t o p h e n o n e a n d e t h y l acetate

(reverse C l a i s e n c o n d e n s a t i o n of the l i g a n d s ) i n a s o l u t i o n of

e u r o p i u m tris b e n z o y l a c e t o n a t e

i n 3:1

e t h a n o l - m e t h a n o l a n d t h a t this

r e a c t i o n , p r o b a b l y the cause of the " a g i n g " of these solutions n o t e d b y L e m p i c k i a n d S a m e l s o n ( 1 9 ) , was a c c e l e r a t e d b y a d d i n g p i p e r i d i n e . Besides i n f l u e n c i n g the p r o p o r t i o n of

tetrakis chelate present i n

s o l u t i o n , solvents c a n also p l a y a role i n the n o n r a d i a t i v e d e a c t i v a t i o n of the e x c i t e d states of the chelate m o l e c u l e e s p e c i a l l y at h i g h e r t e m p e r a tures.

Before

i n t r a m o l e c u l a r energy

transfer takes p l a c e , the l i g a n d


singlet a n d t r i p l e t states are v u l n e r a b l e to q u e n c h i n g .

If the

solvent

m o l e c u l e s h a v e energy levels b e l o w those of the l i g a n d s , s i m p l e q u e n c h i n g b y energy transfer f r o m l i g a n d t r i p l e t to q u e n c h e r t r i p l e t c a n o c c u r as d e m o n s t r a t e d b y B h a u m i k a n d E l - S a y e d ( 3 ) p i p e r y l e n e q u e n c h e d the

fluorescence

w h o o b s e r v e d that cis-

of e u r o p i u m t h e n o y l t r i f l u o r o a c e t o n e

a n d trifluoroacetylacetone d e r i v a t i v e s . V i b r a t i o n a l interactions of the solvent w i t h the e l e c t r o n i c states of t h e chelate m a y also b e e x p e c t e d to b e a source of d e a c t i v a t i o n . T h i s m a y p a r t i a l l y a c c o u n t for the shorter lifetimes a n d l o w e r q u a n t u m effi ciencies r e p o r t e d for tris chelates w h i c h , l a c k i n g the f o u r t h l i g a n d , are less w e l l s h i e l d e d f r o m the solvent.

T h i s process s h o u l d increase i n

i m p o r t a n c e as the g a p increases b e t w e e n the t r i p l e t energy l e v e l a n d the u p p e r i o n l e v e l w h i c h receives this energy (see

discussion b e l o w ) .

W e h a v e o b s e r v e d ( T a b l e I I I ) that w h i l e the q u a n t u m y i e l d of c e n c e f r o m ( B T F A ) E u chelates increases b y a b o u t 4 0 % 4

fluores

in E E A

c o o l i n g f r o m r o o m t e m p e r a t u r e to 165 °K. it changes o n l y 1 0 %

on

i n the

" n i t r i l e solvent," a n d the l i f e t i m e c h a n g e is a b o u t 1 0 % i n either solvent. P a r t of this difference m a y b e c a u s e d b y greater d i s s o c i a t i o n at r o o m t e m p e r a t u r e i n E E A ( p o s s i b l y c a u s e d b y the presence of the β-ethoxye t h a n o l ) , b u t there is a s m a l l b u t m e a s u r a b l e difference i n the t r i p l e t levels (as d e t e r m i n e d b y m e a s u r i n g the p h o s p h o r e s c e n c e of t h e g a d o l i n i u m chelates) b e l o w the

5

D

2

i n the t w o solvents.

I n E E A , t h e t r i p l e t lies s l i g h t l y

E u l e v e l , b u t i n the n i t r i l e solvent there is a n essentially

exact m a t c h of these t w o levels.

The Effect of Ring Substituent We

p r e p a r e d a n u m b e r of d e r i v a t i v e s of

benzoyltrifluoroacetone

b e a r i n g substituents o n the m e t a a n d p a r a positions of the b e n z e n e r i n g a n d c o n v e r t e d these to the c o r r e s p o n d i n g e u r o p i u m tetrakis chelates. Schimitschek (33) substituting

has s t u d i e d t h e n i n e B T F A d e r i v a t i v e s o b t a i n e d b y

fluorine,

c h l o r i n e , or b r o m i n e at the ortho, m e t a , o r p a r a

p o s i t i o n of the r i n g .

T a b l e I shows t h a t substituents c a n h a v e a c o n

s i d e r a b l e effect o n the laser t h r e s h o l d of these c o m p o u n d s a l t h o u g h t h e


12.

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159

Chelates

q u a n t u m y i e l d s a n d lifetimes are less sensitive, a n d t h e effects (cf. 4fluoro ) m a y b e s m a l l e r at l o w t e m p e r a t u r e .

Table I. Effect of Ring-Substituent on Properties of E u [ B T F A ] Chelates 4

Substituent

Cation

Laser Threshold

4-CH 03,4-Cl H 4-CF 4-1 4-F 4-F 4-C1 3-C1 2-C1 3


2

3

Piperidinium Piperidinium 2,4,6-trimethylpyridinium 2,4,6-trimethylpyridinium 2,4,6-trimethylpyridinium 2,4,6-trimethylpyridinium Dimethylammonium Dimethylammonium Dimethylammonium Dimethylammonium

>4.00 3.31 1.00 3.14* 2.53 1.5V 1.0 1.2 1.2 0.5 fc 6

h

b

e

e

e

e

ϋ msec. Τ δ

0

0

—

0.75 0.70 0.70 0.70 0.74 0.67 0.67 0.67 0.67

Reference 0.56

d

d

0.63

d

d

0.58

d

e e e e

0.63 0.63 0.63 0.54

present work present work present work present work present work present work 33 33 33 33

Normalized to [ B T F A ] 4 E u pip = 1.00 with each combination of solvent and temperature. * At 168°K. in E E A . At 294°K. in acetonitrile. At 145°K. in E E A . Φ is the quantum yield of D —> F 2 E u fluorescence with the chelate irradiated in its ultraviolet absorption band; these values were obtained at room temperature in acetonitrile by comparison with the "standard" of Ref. 15. a

c

d

6

5

G

7

T h e r e a p p e a r to b e three p r i n c i p a l w a y s t h a t r i n g substituents c a n affect t h e o v e r - a l l efficiency of e n e r g y transfer f r o m l i g a n d to e u r o p i u m i o n . F i r s t , t h e substituents c a n influence t h e p o s i t i o n of t h e t r i p l e t l e v e l of t h e l i g a n d . I t has b e e n e m p h a s i z e d (6,7, 8) t h a t a g o o d m a t c h of t h e t r i p l e t l e v e l w i t h a n u p p e r l e v e l of t h e e u r o p i u m i o n ( i n these cases D ) 5

is i m p o r t a n t f o r efficient energy transfer.

2

S e c o n d l y , b y resonance a n d

i n d u c t i v e effects, t h e substituent c a n c h a n g e t h e e l e c t r o n d e n s i t y at t h e k e t o - e n o l o x y g e n atoms a n d thus affect t h e degree of c o v a l e n c y of t h e m e t a l - t o - o x y g e n b o n d a n d t h e ease of t r a n s m i s s i o n of energy t h r o u g h these b o n d s .

B y t h e same m e c h a n i s m , t h e e l e c t r o n i c effects of t h e s u b

stituents c a n influence t h e c h e m i c a l s t a b i l i t y of t h e chelate a n d affect t h e p o s i t i o n of t h e d i s s o c i a t i o n e q u i l i b r i u m i n s o l u t i o n . O r t h o substituents c a n p r e s u m a b l y p l a y a n a d d i t i o n a l r o l e o w i n g to t h e i r steric effects. T h e l a r g e r t h e ortho substituent, t h e m o r e i t c o u l d b e e x p e c t e d to influence the r e l a t i v e positions of t h e f o u r l i g a n d s a r o u n d t h e e u r o p i u m a t o m . O r t h o substituents, b y some c o m b i n a t i o n of these m e c h a n i s m s , seem t o s h o w t h e most p r o n o u n c e d effects o n laser p e r f o r m a n c e o f B T F A chelates. S c h i m i t s c h e k ( 3 3 ) has s h o w n that b y i n t r o d u c i n g a n ortho h a l o g e n a t o m i n t o t h e b e n z e n e r i n g of these c o m p o u n d s t h e r o o m - t e m p e r a t u r e


160


laser t h r e s h o l d c a n be m a r k e d l y l o w e r e d , efficiencies

even

though

C H E M I S T R Y

the

quantum

of the r e s u l t i n g c o m p o u n d s are l o w e r t h a n that of the u n -

s u b s t i t u t e d B T F A d e r i v a t i v e . H e interprets the l o w e r q u a n t u m efficiency of these c o m p o u n d s as c a u s e d i n p a r t b y a shift of the energy of the l i g a n d t r i p l e t state a n d suggests that the l o w e r laser t h r e s h o l d m a y e x p l a i n e d b y the o b s e r v a t i o n that of the t w o p r i n c i p a l D —> F 5

b a n d s f o u n d at —

0

7

2

be

emission

6120 a n d 6140A. at l o w t e m p e r a t u r e , the r e l a t i v e

i n t e n s i t y of the l a s i n g , shorter w a v e l e n g t h t r a n s i t i o n is i n c r e a s e d m a r k e d l y i n the ortho s u b s t i t u t e d chelates.

S u c h a subtle c h a n g e i n the details of


the emission s p e c t r u m is exactly w h a t one w o u l d expect as a result of a m i n o r r e p o s i t i o n i n g of the l i g a n d s a r o u n d the e u r o p i u m i o n

brought

a b o u t b y steric effects. T h e i n t e r p l a y of the v a r i o u s effects of substituent is p o o r l y u n d e r s t o o d at present a n d deserves f u r t h e r study.

The Effect of Cation W e have reported

(I)

that the emission spectra of s o l i d C

k e t o n o ) E u ] ~ salts c a n be i n f l u e n c e d b y the n a t u r e of the c a t i o n . 4

+

[diInas-

m u c h as e a c h tetrakis chelate a n i o n m u s t h a v e a c a t i o n as its near n e i g h b o r i n the c r y s t a l lattice, it w o u l d be e x p e c t e d that the size a n d o t h e r steric features of the c a t i o n c o u l d affect the p r e f e r r e d geometric a r r a n g e m e n t of the f o u r l i g a n d s a r o u n d the e u r o p i u m i o n so as to m i n i m i z e c a t i o n - a n i o n interactions. fluorescence

S i n c e the s t r u c t u r a l details of the

europium

are d e t e r m i n e d b y the s y m m e t r y at the e u r o p i u m i o n site,

slight rearrangements of the r e l a t i v e positions of the l i g a n d s w i l l c h a n g e these features of the emission s p e c t r u m . W e w e r e l e d to the c o n c l u s i o n that i n s o l u t i o n as w e l l , at least at l o w t e m p e r a t u r e , there is significant i n t e r a c t i o n b e t w e e n the c a t i o n a n d the chelate a n i o n , p e r h a p s b y w a y of some sort of i o n p a i r i n g . A l t h o u g h it s h o w e d o n l y a b o u t 1 0 % m o r e d i s s o c i a t i o n a n d n o a p p r e c i a b l e difference i n its u s u a l s p e c t r o s c o p i c properties f r o m the p i p e r i d i n i u m salt, the t e t r a p r o p y l a m m o n i u m salt of the b e n z o y l a c e t o n e chelate d i d not u n d e r g o laser a c t i o n . A c a t i o n effect is s h o w n b y the d a t a l i s t e d i n T a b l e I I w h e r e the t e t r a p r o p y l a m m o n i u m i o n shows a n adverse effect o n the laser b e h a v i o r of B T F A chelates as w e l l . A possible w a y i n w h i c h changes i n the c a t i o n m i g h t influence the chelate solutions, other t h a n b y a n i o n p a i r i n g m e c h a n i s m , is b y c h a n g i n g the p o s i t i o n of the e q u i l i b r i u m : C [ L E u ] - ^± L , E u + C L ; +

4

if C is R N H : +

3

+

R N H L " ^± R N + L H 3

+

3


12.

Ross

where C

A N D

+

Europium

B L A N C

Chelates

161

is the c a t i o n , L f is the d i k e t o n a t e a n i o n , a n d L H is the n e u t r a l

β-diketone. T h e t h e r m o d y n a m i c s t a b i l i t y of C L " c o u l d c l e a r l y influence t h e p o s i +

t i o n of this e q u i l i b r i u m . T h e i m p o r t a n c e of this effect is s h o w n b y the w o r k of R e i d e l a n d C h a r l e s ( 2 3 )

w h o p r e p a r e d salts of [ ( B T F A ) E u ] ~ 4

w i t h 15 different s u b s t i t u t e d a m m o n i u m cations a n d o b s e r v e d

a

more

t h a n t h r e e f o l d c h a n g e i n laser t h r e s h o l d at 0 ° C . o n g o i n g f r o m p i p e r i d i n i u m t h r o u g h q u i n o l i n i u m . T h e increase i n t h r e s h o l d r o u g h l y p a r a l l e l s a n increase i n the p K Downloaded by PURDUE UNIVERSITY on March 21, 2013 | http://pubs.acs.org Publication Date: June 1, 1967 | doi: 10.1021/ba-1967-0071.ch012

those c o m p o u n d s

b

of the a m i n e f r o m w h i c h the c a t i o n is d e r i v e d for

w i t h other t h a n q u a t e r n a r y a m m o n i u m ions, a n d for

the q u i n o l i n i u m salt these w o r k e r s s h o w e d t h a t the extent of d i s s o c i a t i o n to tris chelate is a b o u t 4 0 % , as c o m p a r e d

with 10%

or less for

the

piperidinium compound. Table II.

Effect of Cation on Threshold of C [ B F T A ] E u

Cation

+

Threshold"

Piperidinium 2,4,6-Trime thy lpy ridinium Imidazolium Tetramethylammonium n-Butylammonium Pyridinium Triethylammonium Tetraethylammonium Tetrapropylammonium Tetrapropylammonium Quinolinium

4

Reference 24; present w o r k present w o r k present w o r k

1.00 1.00 1.09 1.16 1.33 1.50 1.83 3.00 3.00* 1.62 >3.0 b

b

b

24 24 24 24 24 24

e

c

e

e

e

present

b

work

24

e

These thresholds fall into two groups : the set reported in ref. 24, measured in aceto nitrile at 273°K. and those of the present work, done in E E A at 140°K.; within each group, the values are normalized to [ B T F A ] E u pip = 1.00 although the absolute value of the threshold is, of course, different under the two experimental conditions. At 140°K. in E E A . At 273°K. in acetonitrile. At 243°K. in acetonitrile; presumably the figure compared with the piperidinium salt at the same temperature will be higher. a

4

b e

d

I n one case, a c h a n g e of the c a t i o n has b e e n o b s e r v e d to shift the f r e q u e n c y of the laser emission. L e m p i c k i a n d c o - w o r k e r s ( 2 7 )

observed

that a d d i n g s o d i u m acetate to a s o l u t i o n of p i p e r i d i n i u m tetrakis ( b e n z o y l a c e t o n o ) e u r o p i u m i n a n a l c o h o l i c s o l u t i o n c h a n g e d the laser f r e q u e n c y f r o m 6131 to 6114A. a n d suggested t h a t here, a n e w species, " a n a d d u c t of the tetrakis chelate a n d the c a t i o n , " is f o r m e d w i t h a different symmetry. I n o r d e r to test for the p o s s i b i l i t y of i o n p a i r i n g i n s o l u t i o n , w e w i s h e d to c h a n g e the c a t i o n i n a w a y that w o u l d not d i s t u r b t h e d i s s o c i a t i o n e q u i l i b r i u m . B y s u b s t i t u t i n g the d e u t e r a t e d p i p e r i d i n i u m i o n , i n w h i c h the p i p e r i d i n e c a r b o n - b o u n d

hydrogen

atoms

are r e p l a c e d


by

162


C H E M I S T R Y

d e u t e r i u m , for n o r m a l p i p e r i d i n i u m , gives a n e w c o m p o u n d ( " [ B F T A ] E u 4

p i p " ) i n w h i c h the steric effects, base s t r e n g t h , a n d degree of s o l v a t i o n

d

10

of the c a t i o n s h o u l d b e as n e a r l y as possible the same as i n the u n d e u t e r a t e d salt. I f d i r e c t interactions of t h e chelate e l e c t r o n i c states w i t h t h e c a t i o n w e r e to b e i m p o r t a n t , a c h a n g e i n the s p e c t r o s c o p i c p r o p e r t i e s or laser b e h a v i o r s h o u l d b e

noted.

Table III. Effect of Deuteration on Properties of Piperidinium [ B F T A ] E u Chelates


4

298°

168°

298°

168°

Relative Laser Threshold (168°K.)

0.62 0.63 0.66 0.70 0.63 0.65 0.63 0.69 0.62 0.69

0.68 0.70 0.64 0.66 0.70 0.68 0.66 0.66 0.72 0.71 0.73 0.70

0.35

0.59

1.00

0.59 0.59 0.59

0.64 0.59 0.47

5

Chelate

Solvent

[ B T F A ] E u pip [ B T F A ] E u pip [ B T F A ] E u pip [ B T F A ] E u pip [BTFA] Eu d pip [d BTFA] Eupip 4

4

4

4

w

4

5

4

[4BTFA] Eu d

w

4

[d BTFA] [ τ a n d k —> ·/> are m u c h faster 8

T

t h a n a n y c o m p e t i n g processes, w e c o n c l u d e that, at l o w t e m p e r a t u r e i n undissociated europium

(BTFA)

chelates, t h e o v e r - a l l q u a n t u m effi

4

c i e n c y of p o p u l a t i o n of t h e D l e v e l is n e a r l y u n i t y . T a b l e s I a n d III 5

2

s h o w , h o w e v e r , that e v e n u n d e r t h e most f a v o r a b l e c o m b i n a t i o n s of s o l vent, t e m p e r a t u r e , a n d c h e m i c a l structure, t h e o v e r - a l l q u a n t u m efficiency for

5

D —> F emission, w h i c h comprises m o r e t h a n 9 5 % of t h e t o t a l 7

0

2

l u m i n e s c e n c e f o r these c o m p o u n d s , never exceeds a b o u t 6 0 % . W i n d s o r a n d his co-workers

(10, I I ) have shown, b y direct excita

t i o n of t h e D levels i n a n u m b e r of e u r o p i u m c o m p o u n d s , 5

q u a n t u m y i e l d of cessively excites of

5

f )

5

D

fluorescence

G

that t h e

decreases p r o g r e s s i v e l y as one suc

D , D i , a n d D . W h i l e the y i e l d o n direct excitation 5

0

5

2

D is q u i t e d e p e n d e n t o n t h e p a r t i c u l a r c o m p o u n d a n d m e d i u m ( t h e 0

v a l u e is 0.82 f o r t h e thenoyltrifluoroacetone chelate i n acetone s o l u t i o n ) , the p r o p o r t i o n of t h e energy lost b e t w e e n e.g. compound

is r a t h e r insensitive to changes

n o n r a d i a t i v e processes f r o m

5

r ,

D a n d D within a given r >

2

0

i n the environment.

These

D a n d D i , w h o s e n a t u r e is n o t u n d e r s t o o d , r ,

2

w o u l d a p p e a r to b e responsible f o r most of t h e ca. 4 0 % energy loss i n these m a t e r i a l s . O n c e t h e e n e r g y arrives at D , i t is e m i t t e d to r >

ciency.

G

N o n r a d i a t i v e processes f r o m

r >

D

0

7

F w i t h h i g h effi 2

are almost c o m p l e t e l y l a c k i n g

i n ( B T F A ) E u chelates i n n i t r i l e solvents as is i n d i c a t e d b y t h e r e l a t i v e 4

i n s e n s i t i v i t y of t h e l i f e t i m e of t h e

r ,

D —» F 7

0

2

e m i s s i o n to changes i n

environment a n d temperature. T h e last step w e c o n s i d e r is t h e r e t u r n of t h e e u r o p i u m i o n to t h e g r o u n d state, F 7

2

7

F . T h e rate, k -+ F2

0

F o

, does n o t influence t h e q u a n

t u m y i e l d or l i f e t i m e of D ~"* F emission, b u t i t c a n b e of p a r a m o u n t n

importance

7

0

2

i n d e t e r m i n i n g laser p e r f o r m a n c e .

m a t h e m a t i c a l analysis, i t is clear that i f k + F2r

E v e n without detailed

F is m u c h s l o w e r t h a n q

k

Do

_*.F2> n o n e t i n v e r s i o n of D c a n b e o b t a i n e d u n d e r steady state c o n d i 5

0

tions since t h e energy w i l l t e n d to p i l e u p i n t h e F l e v e l . T h e r e is n o 7

2

d i r e c t e x p e r i m e n t a l e v i d e n c e d e a l i n g w i t h t h e m a g n i t u d e of k -+ , Fr

Fo

but

the o n l y w a y w e h a v e b e e n a b l e to r a t i o n a l i z e t h e decreases i n laser


12.

Ross

Europium

A N D B L A N C

167

Chelates

t h r e s h o l d b r o u g h t a b o u t b y d e u t e r a t i o n of ( B T F A ) E u p i p is t o assume 4

that i t is this rate w h i c h is c o n t r o l l i n g t h e o v e r - a l l process. T h e s e p a r a t i o n between

7

F a n d F is 900 c m . . I t is n o t u n r e a s o n a b l e t o expect 2

7

Ιϊρ^—*-

1

D

F

Q

to b e s i g n i f i c a n t l y affected b y t h e a v a i l a b i l i t y of m o l e c u l a r v i b r a t i o n a l m o d e s w i t h this energy.

I n t h e case of t h e d B T F A chelates, w e h a v e 5

o b s e r v e d t h e a p p e a r a n c e of n e w absorptions i n t h e i n f r a r e d s p e c t r u m at 875, 842, a n d 830 c m . " s h i f t e d t o these values f r o m a r o m a t i c C - H i n - p l a n e 1

d e f o r m a t i o n s seen at 1105, 1082, a n d 1070 c m . " i n t h e u n d e u t e r a t e d 1

compounds. W e h a v e n o t b e e n a b l e to a r r i v e at t h e o r e t i c a l values f o r k ^ , Downloaded by PURDUE UNIVERSITY on March 21, 2013 | http://pubs.acs.org Publication Date: June 1, 1967 | doi: 10.1021/ba-1967-0071.ch012

Fr

Fo

but

if o u r i n t e r p r e t a t i o n is correct, t h e n f o r t h e u n d e u t e r a t e d c o m p o u n d i t m u s t b e o f t h e same o r d e r as K -* , Do

F2

a n d l a r g e r ( b y a factor of t w o o r m o r e )

i n t h e d B T F A chelates. 5

I n c o n c l u s i o n , a l t h o u g h a d e t a i l e d o u t l i n e of b o t h t h e c h e m i s t r y a n d e n e r g y transfer processes i n β-diketonates of E u

3 +

has e m e r g e d i n t h e last

f e w years, i t is c l e a r that m u c h 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 w o r k needs to b e done, e s p e c i a l l y o n chelate-solvent interactions a n d energy

loss

mechanisms w i t h i n the europium ion manifold.

Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)

Bauer, H., Blanc, J., Ross, D. L., J. Am. Chem. Soc. 86, 5125 (1964). Bhaumik, M . L., et al., J. Phys. Chem. 68, 1490 (1964). Bhaumik, M. L., El-Sayed, Μ. Α., J. Chem. Phys. 42, 787 (1965). Bhaumik, M. L., Ferder, L., El-Sayed, Μ. Α., J. Chem. Phys. 42, 1843 (1965). Brecher, C., Samelson, H., Lempicki, Α., J. Chem. Phys. 42, 1081 (1965). Crosby, G. Α., Whan, R. E., Naturwiss. 47, 276 (1960). Crosby, G. Α., Whan, R. E., J. Chem. Phys. 32, 614 (1960). Crosby, G. Α., Whan, R. E., Alire, R. M., J. Chem. Phys. 34, 743 (1961). Crosby, G. Α., Mol. Crystals 1, 37 (1966). Dawson, W. R., Kropp, J. L., J. Opt. Soc. Am. 55, 822 (1965). Dawson, W. R., Kropp, J. L., Windsor, M . W., J. Chem. Phys. 45, 2410 (1966). El-Sayed, Μ. Α., Bhaumik, M. L., J. Chem. Phys. 39, 2391 (1963). Freeman, J. J., Crosby, G. Α., Lawson, Κ. E., J. Mol. Spectry. 13, 390 (1964). Fry, F. H., Pirie, W. R., J. Chem. Phys. 43, 3761 (1965). Gudmundsen, R. Α., Marsh, O. J., Matovich, E., J. Chem. Phys. 39, 272 (1963). Heller, Α., Wasserman, E., J. Chem. Phys. 42, 949 (1965). Kropp, J. L., Windsor, M . W., J. Chem. Phys. 39, 2769 (1963). Kropp, J. L., Windsor, M. W., J. Chem. Phys. 42, 1599 (1965). Lempicki, Α., Samelson, H., "Proceedings of the Symposium on Optical Masers," p. 347, Polytechnic Press, Brooklyn, Ν. Y., 1963. Lempicki, Α., Samelson, H., Phys. Letters 4, 133 (1963). Levine, A. K., ed., "Lasers, a Series of Advances," Chap. 3, Marcel Dekkar, Inc., New York, 1966. Melby, L. R., Rose, N . J., Abramson, E., Caris, J. C., J. Am. Chem. Soc. 86, 5117 (1964).


168

(23) (24) (25) (26) (27) (28) (29)


(30) (31) (32) (33) (34) (35) (36) (37) (38)

LANTHANIDE/ACTINIDE

CHEMISTRY

Metlay, M., J. Chem. Phys. 39, 491 (1963). Riedel, E. P., Charles, R. G., J. Appl. Phys. 36, 3954 (1965). Ross, D. L., Blanc, J., Pressley, R. J., Appl. Phys. Letters 8, 101 (1966). Samelson, H., Brecher, C., Lempicki, Α., Proc. Rare Earth Conf. 5th, Ames, Iowa, 1, 41 (1965). Samelson, H., Brophy, V. Α., Brecher, C., Lempicki, Α., J. Chem. Phys. 41, 3998 (1964). Samelson, H., Lempicki, Α., Brecher, C., Brophy, V. Α., Appl. Phys. Letters 5, 173 (1964). Samelson, H., Lempicki, Α., Brophy, V. Α., Brecher, C., J. Chem. Phys. 40, 2553 (1964). Schimitschek, E. J., Nehrich, R. B., J. Appl. Phys. 35, 2786 (1964). Schimitschek, E. J., Nehrich, R. B., Trias, J. Α., J. Chem. Phys. 42, 788 (1965). Schimitschek, E. J., Nehrich, R. B., Trias, J. Α., Appl. Phys. Letters 9, 103 (1966). Schimitschek, E. J., Nehrich, R. B., Trias, J. Α., J. Chim. phys. 64, 673 (1967). Schimitschek, E. J., Schwarz, E. G. K., Nature 196, 832 (1962). Weissman, S. I., J. Chem. Phys. 10, 214 (1942). Whan, R. E., Crosby, G. Α., J. Mol. Spectry. 8, 315 (1962). Windsor, M . W., TRW Systems, Redondo Beach, California (private communication). Yuster, P., Weissman, S. I., J. Chem. Phys. 17, 1182 (1949).

RECEIVED

October 18, 1966.


Europium Chelates as Laser Materials

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