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Flourescence emission from biphenylene and related compounds
Volume 20, n u m b e r 4
PHYSICS
FLUORESCENCE
EMISSION RELATED
LETTERS
FROM COMPOUNDS
1March1966
BIPHENYLENE
AND
I.H.MUNRO, T.D.S.HAMILTON, J.P.RAY and G. F.MOORE The Physical Laboratories, Manchester, University, Manchester 13, England Received 31 January 1966
E m i s s i o n and absorption s p e c t r a for solutions of biphenylene, biphenylene oxide and fluorene, in boric acid glass and hexane are d e s c r i b e d . E m i s s i o n s p e c t r a previously attributed to biphenylene ]uminescenee have been identified with e m i s s i o n f r o m impurities in biphenylene.
A recent paper by Hilpern [I] suggested that the observed fluorescence from biphenylene originated from the second excited singlet state S 2. Subsequently Birks et al. [2] detected an additional emission band at longer wavelength than the "S2" emission. The new band was ascribed to emission from the biphenylene lowest
t t
35000
\
I
90000
25000 C m -1
i
t
l'~]
I
Fig. 2. P r o m p t f l u o r e s c e n c e s p e c t r a in boric acid glass solution. - - - - fluorene, A - biphenylene oxide, B - biphenylene.
k tt
A
singlet state Sl, and the s i m u l t a n e o u s o b s e r v a t i o n of two e m i s s i o n s p e c t r a l e d to p r o p o s a l s c o n c e r n i n g t h e d u a l i t y of e m i s s i o n in o r g a n i c m o l e c u l e s [3, 4]. In t h e p r e s e n t w o r k , b i p h e n y l e n e o b t a i n e d f r o m t h e s a m e s o u r c e a s t h a t u s e d in [1, 2] s h o w e d
c
k
x
• ,
! 35 0 0 0
\
r
x
]
32 5 0 0
30000
27500
Cr-n-1
Fig. 1. P r o m p t f l u o r e s c e n c e s p e c t r a in hexane solution. . . . . fluorene, biphenylene oxide, • • • • t!$2" e m i s sion f r o m tmpurffied biphenylene, 386
strong "S2" emission in hexane solution. However, after prolonged zone refining, no detectable emission was observed from the biphenylene either in crystal form or in hexane solution. This result supports recent work by Hochstrasser [5]. Two possible impurities in biphenylene - fluorene and biphenylene oxide - were separately zone refined and dissolved in hexane. Their prompt fluorescence spectra together with the very similar biphenylene "$2" emission, are shown in fig. i. Previously published emission spectra for fluorene and biphenylene oxide [6] are probably associated with emission from anthracene, present as an impurity in those compounds.
Volume 20, number 4
PHYSICS
LETTERS
I March 1966
..A
~
f',,
jA
> ~
tL
t
! t+ ]ll
+
Z
1]
I
25000
20000
Fig. 3. Phosphorescence spectra in boric acid glass solution. - - - - fluorene: lifetime - 5 sec. A - biphenylene oxide: lifetime - 5 sec. B - biphenylene: lifetime --: 1.5 see. P r o m p t and d e l a y e d e m i s s i o n s p e c t r a f r o m z o n e r e f i n e d b i p h e n y l e n e , f l u o r e n e and b i p h e n y l e n e o x i d e d i s s o l v e d in b o r i c a c i d g l a s s a r e s h o w n in f i g s . 2 and 3. T h e p r o m p t f l u o r e s c e n c e s p e c t r a f r o m b i p h e n y l e n e oxide and f l u o r e n e in b o r i c a c i d g l a s s (fig. 2) a r e s i m i l a r to the c o r r e s p o n d i n g s o l u t i o n s p e c t r a . H o w e v e r , in the g l a s s , b i p h e n y l e n e e x h i b i t s a p r o n o u n c e d s t r u c t u r e l e s s band with a p e a k at about 400 n m (fig. 2). T h e e x c i t a t i o n s p e c t r u m of t h i s band (at 5 x 10-3M) is i d e n t i c a l in p r o f i l e w i t h the a b s o r p t i o n s p e c t r u m of b i p h e n y l e n e in b o r i c a c i d ( s e e fig. 4). T h e a b s o r p t i o n s p e c t r a of a l l t h r e e c o m p o u n d s in b o r i c a c i d (fig. 4) l i e at s l i g h t e r s h o r t e r w a v e l e n g t h s t h a n the c o r r e s p o n d i n g e m i s s i o n s p e c t r a (fig. 2) a s a n t i c i p a t e d . It has b e e n s u g g e s t e d [5] that the a b s e n c e of b i p h e n y l e n e f l u o r e s c e n c e is due to a l a r g e c o n f i g u r a t i o n c h a n g e in the e x c i t e d s t a t e , w h i c h l e a d s to m i x i n g of t h e e x c i t e d and g r o u n d s t a t e v i b r a t i o n al l e v e l s and h e n c e to r a d i a t i o n l e s s d e a c t i v a t i o n . T h e o b s e r v a t i o n of f l u o r e s c e n c e in b o r i c a c i d may, t h e r e f o r e , be due to i n h i b i t i o n of t h i s c o n f i g u r a t i o n c h a n g e in the r i g i d g l a s s s o l u t i o n . T h e r e i s no significant evidence from the absorption spectra to i n d i c a t e any i n t e r a c t i o n b e t w e e n b i p h e n y l e n e and b o r i c a c i d (fig. 4). W h i l e no d e l a y e d e m i s s i o n w a s d e t e c t e d f r o m hexane solution, all three zone refined compounds e x h i b i t e d s t r o n g p h o s p h o r e s c e n c e in b o r i c a c i d g l a s s . T h e p h o s p h o r e s c e n c e l i f e t i m e s and e m i s s i o n s p e c t r a a r e shown in fig. 3.
\
~',.
16000
c m -1
45 0 0 0
I
I
I
40000
35000
30000
25000
C m -1
Fig. 4. Absorption spectra in boric acid glass solution . . . . fluorene A - biphenylene oxide B - biphenylene . . . . normalised excitation spectrum of the prompt fluorescence emission from biphenylene in boric acid glass. It a p p e a r s f r o m the p r e s e n t w o r k that " $ 2 " f l u o r e s c e n c e o b s e r v e d p r e v i o u s l y [1, 2] i s to be i n d e n t i f i e d with an i m p u r i t y f l u o r e s c e n c e , p r o b ably fluorene or biphenylene oxide or a mixture of both. T h e " S I " e m i s s i o n o b s e r v e d p r e v i o u s l y in c y c l o h e x a n e s o l u t i o n [2], f a l l s in t h e s a m e r e g i o n a s the b i p h e n y l e n e p h o s p h o r e x c e n c e (fig. 3. ). A l s o , the b i p h e n y l e n e p h o s p h o r e s c e n c e d e s c r i b e d by H i l p e r n [1] i s to be i n d e n t i f i e d with the p h o s p h o r e s c e n c e f r o m b i p h e n y l e n e oxide on the b a s i s of the s p e c t r a l and l i f e t i m e d a t a (fig. 3). We w i s h to thank D r . D. A. W e y l f o r z o n e r e f i n i n g the c o m p o u n d s u s e d , and D r . R. M. H o c h s t r a s s e r f o r a c c e s s to u n p u b l i s h e d data. One of us, G . F . M . , t h a n k s the S c i e n c e R e s e a r c h C o u n c i l f o r the p r o v i s i o n of a m a i n t e n a n c e g r a n t . 1. J . W . Hilpern, Trans. Faraday Soc. 61 (1965) 605. 2. J . B . Birks, J . M . C . C o n t e and G.Walker, Physics Letters 19 (1965) 125. 3. J . B . Birks, Physics Letters 19 (1965) 25. 4. J . B . Birks, Physics Letters 19 (1965) 214. 5. R . M . H o c h s t r a s s e r and R.D.McAlpine, to be published. 6. R . C . S a n g s t e r and J . W . I r v i n e , J. Chem. Phys. 24 (1956) 670. 387
PHYSICS
FLUORESCENCE
EMISSION RELATED
LETTERS
FROM COMPOUNDS
1March1966
BIPHENYLENE
AND
I.H.MUNRO, T.D.S.HAMILTON, J.P.RAY and G. F.MOORE The Physical Laboratories, Manchester, University, Manchester 13, England Received 31 January 1966
E m i s s i o n and absorption s p e c t r a for solutions of biphenylene, biphenylene oxide and fluorene, in boric acid glass and hexane are d e s c r i b e d . E m i s s i o n s p e c t r a previously attributed to biphenylene ]uminescenee have been identified with e m i s s i o n f r o m impurities in biphenylene.
A recent paper by Hilpern [I] suggested that the observed fluorescence from biphenylene originated from the second excited singlet state S 2. Subsequently Birks et al. [2] detected an additional emission band at longer wavelength than the "S2" emission. The new band was ascribed to emission from the biphenylene lowest
t t
35000
\
I
90000
25000 C m -1
i
t
l'~]
I
Fig. 2. P r o m p t f l u o r e s c e n c e s p e c t r a in boric acid glass solution. - - - - fluorene, A - biphenylene oxide, B - biphenylene.
k tt
A
singlet state Sl, and the s i m u l t a n e o u s o b s e r v a t i o n of two e m i s s i o n s p e c t r a l e d to p r o p o s a l s c o n c e r n i n g t h e d u a l i t y of e m i s s i o n in o r g a n i c m o l e c u l e s [3, 4]. In t h e p r e s e n t w o r k , b i p h e n y l e n e o b t a i n e d f r o m t h e s a m e s o u r c e a s t h a t u s e d in [1, 2] s h o w e d
c
k
x
• ,
! 35 0 0 0
\
r
x
]
32 5 0 0
30000
27500
Cr-n-1
Fig. 1. P r o m p t f l u o r e s c e n c e s p e c t r a in hexane solution. . . . . fluorene, biphenylene oxide, • • • • t!$2" e m i s sion f r o m tmpurffied biphenylene, 386
strong "S2" emission in hexane solution. However, after prolonged zone refining, no detectable emission was observed from the biphenylene either in crystal form or in hexane solution. This result supports recent work by Hochstrasser [5]. Two possible impurities in biphenylene - fluorene and biphenylene oxide - were separately zone refined and dissolved in hexane. Their prompt fluorescence spectra together with the very similar biphenylene "$2" emission, are shown in fig. i. Previously published emission spectra for fluorene and biphenylene oxide [6] are probably associated with emission from anthracene, present as an impurity in those compounds.
Volume 20, number 4
PHYSICS
LETTERS
I March 1966
..A
~
f',,
jA
> ~
tL
t
! t+ ]ll
+
Z
1]
I
25000
20000
Fig. 3. Phosphorescence spectra in boric acid glass solution. - - - - fluorene: lifetime - 5 sec. A - biphenylene oxide: lifetime - 5 sec. B - biphenylene: lifetime --: 1.5 see. P r o m p t and d e l a y e d e m i s s i o n s p e c t r a f r o m z o n e r e f i n e d b i p h e n y l e n e , f l u o r e n e and b i p h e n y l e n e o x i d e d i s s o l v e d in b o r i c a c i d g l a s s a r e s h o w n in f i g s . 2 and 3. T h e p r o m p t f l u o r e s c e n c e s p e c t r a f r o m b i p h e n y l e n e oxide and f l u o r e n e in b o r i c a c i d g l a s s (fig. 2) a r e s i m i l a r to the c o r r e s p o n d i n g s o l u t i o n s p e c t r a . H o w e v e r , in the g l a s s , b i p h e n y l e n e e x h i b i t s a p r o n o u n c e d s t r u c t u r e l e s s band with a p e a k at about 400 n m (fig. 2). T h e e x c i t a t i o n s p e c t r u m of t h i s band (at 5 x 10-3M) is i d e n t i c a l in p r o f i l e w i t h the a b s o r p t i o n s p e c t r u m of b i p h e n y l e n e in b o r i c a c i d ( s e e fig. 4). T h e a b s o r p t i o n s p e c t r a of a l l t h r e e c o m p o u n d s in b o r i c a c i d (fig. 4) l i e at s l i g h t e r s h o r t e r w a v e l e n g t h s t h a n the c o r r e s p o n d i n g e m i s s i o n s p e c t r a (fig. 2) a s a n t i c i p a t e d . It has b e e n s u g g e s t e d [5] that the a b s e n c e of b i p h e n y l e n e f l u o r e s c e n c e is due to a l a r g e c o n f i g u r a t i o n c h a n g e in the e x c i t e d s t a t e , w h i c h l e a d s to m i x i n g of t h e e x c i t e d and g r o u n d s t a t e v i b r a t i o n al l e v e l s and h e n c e to r a d i a t i o n l e s s d e a c t i v a t i o n . T h e o b s e r v a t i o n of f l u o r e s c e n c e in b o r i c a c i d may, t h e r e f o r e , be due to i n h i b i t i o n of t h i s c o n f i g u r a t i o n c h a n g e in the r i g i d g l a s s s o l u t i o n . T h e r e i s no significant evidence from the absorption spectra to i n d i c a t e any i n t e r a c t i o n b e t w e e n b i p h e n y l e n e and b o r i c a c i d (fig. 4). W h i l e no d e l a y e d e m i s s i o n w a s d e t e c t e d f r o m hexane solution, all three zone refined compounds e x h i b i t e d s t r o n g p h o s p h o r e s c e n c e in b o r i c a c i d g l a s s . T h e p h o s p h o r e s c e n c e l i f e t i m e s and e m i s s i o n s p e c t r a a r e shown in fig. 3.
\
~',.
16000
c m -1
45 0 0 0
I
I
I
40000
35000
30000
25000
C m -1
Fig. 4. Absorption spectra in boric acid glass solution . . . . fluorene A - biphenylene oxide B - biphenylene . . . . normalised excitation spectrum of the prompt fluorescence emission from biphenylene in boric acid glass. It a p p e a r s f r o m the p r e s e n t w o r k that " $ 2 " f l u o r e s c e n c e o b s e r v e d p r e v i o u s l y [1, 2] i s to be i n d e n t i f i e d with an i m p u r i t y f l u o r e s c e n c e , p r o b ably fluorene or biphenylene oxide or a mixture of both. T h e " S I " e m i s s i o n o b s e r v e d p r e v i o u s l y in c y c l o h e x a n e s o l u t i o n [2], f a l l s in t h e s a m e r e g i o n a s the b i p h e n y l e n e p h o s p h o r e x c e n c e (fig. 3. ). A l s o , the b i p h e n y l e n e p h o s p h o r e s c e n c e d e s c r i b e d by H i l p e r n [1] i s to be i n d e n t i f i e d with the p h o s p h o r e s c e n c e f r o m b i p h e n y l e n e oxide on the b a s i s of the s p e c t r a l and l i f e t i m e d a t a (fig. 3). We w i s h to thank D r . D. A. W e y l f o r z o n e r e f i n i n g the c o m p o u n d s u s e d , and D r . R. M. H o c h s t r a s s e r f o r a c c e s s to u n p u b l i s h e d data. One of us, G . F . M . , t h a n k s the S c i e n c e R e s e a r c h C o u n c i l f o r the p r o v i s i o n of a m a i n t e n a n c e g r a n t . 1. J . W . Hilpern, Trans. Faraday Soc. 61 (1965) 605. 2. J . B . Birks, J . M . C . C o n t e and G.Walker, Physics Letters 19 (1965) 125. 3. J . B . Birks, Physics Letters 19 (1965) 25. 4. J . B . Birks, Physics Letters 19 (1965) 214. 5. R . M . H o c h s t r a s s e r and R.D.McAlpine, to be published. 6. R . C . S a n g s t e r and J . W . I r v i n e , J. Chem. Phys. 24 (1956) 670. 387