- Email: [email protected]
Fluorescence of phenyl salicylate
A n a l r t t t a C l m n i t a ,'h t,t. 75 (1975) 449--452 ,~) ENcvler Scientific P u l ~ h s h m g C o m p , m y . A m s t e r d a m - P r i n t e d ,n T h e N c t h e r l a n d ~
449
SHORT COMMUNICATION Fluorescence of phenyl salicylate I MICHAEL College ol
RUTLEDGE
dtld S T E P H E N
P l u n m a c r. U n i t vr~tt I o/
G SCHULMAN
F I o r u l a . G,tmt.~l tile. F l a 3 2 6 1 0 ( U S .4 )
( R e c e i v e d 12th August 1974)
P h e n y l s a l t c y l a t e ( s a l o l ) h a s b e e n e m p l o y e d industrially as a s u n s c r e e n . e n t e r i c c o a t i n g for d e l a y e d a c t i o n t a b l e t s a n d plasticizer, a n d m a v a r i e t y o f o t h e r p h a r m a c e u t i c a l a p p l i c a t i o n s s i m i l a r to t h o s e o f a s p i r i n Its s t r u c t u r a l s~mllarlty to salicylic a c i d a n d m e t h y l s a l l c y l a t e s u g g e s t s that the s p e c t r o s c o p i c p r o p e r t i e s o f all t h r e e c o m p o u n d s s h o u l d b e similar. H o w e v e r . w h d e these c o m p o u n d s f l u o r e s c e q u i t e i n t e n s e l y t h r o u g h o u t the p H r e g i o n a n d in c o n c e n t r a t e d a c i d ~. p h e n y l s a h c y l a t e e x h i b i t s a b a r e l y d e t e c t a b l e f l u o r e s c e n c e b e l o w p H 7 ;rod In sulfuric acid a b o v e I-/o --6. In o r d e r to u n d e r s t a n d b e t t e r the s p e c t r o s c o p i c b e h a v i o r a n d to d e t e r m i n e t h e o p t l m a l c o n d , t , o n s for f l u o r i m e t n c d e t e r m i n a t i o n o f p h e n y l sallcylate, the p r e s e n t s t u d y o f t h e p H a n d s o l v e n t d e p e n d e n c e s o f the e l e c t r o m c s p e c t r a o f salol w a s undertaken.
Expermlental Absorption spectra were taken on a Beckman DB-GT spectrophotometer. F l u o r e s c e n c e s p e c t r a w e r e r e c o r d e d on a P e r k i n - E i m e r M P F 2 A f l u o r e s c e n c e s p e c t r o p h o t o m e t e r w h o s e m o n o c h r o m a t o r s w e r e c a l i b r a t e d a g a i n s t the x e n o n line e m i s s i o n s p e c t r u m a n d w h o s e o u t p u t w a s c o r r e c t e d for i n s t r u m e n t a l r e s p o n s e b y m e a n s o f a r h o d a m m e - B q u a n t u m c o u n t e r M e a s u r e m e n t o f p H w a s m a d e o n an O r i o n M o d e l 801 p H m e t e r with a B e c k m a n salver/silver c h l o r i d e - g l a s s c o m b i n a t i o n e l e c t r o d e . Phenyl s a l l c y l a t e ( M e r c k ) w a s purified b y r e c r y s t a l l l z a t l o n f r o m c h l o r o f o r m . R e a g e n t - g r a d e sulfuric a c i d . s o d m m h y d r o x i d e , s o d m m acetate, acetic acid. s o d i u m d d l y d r o g e n p h o s p h a t e a n d s o d m m m o n o h y d r o g e n p h o s p h a t e w e r e used to p r e p a r e s o l u t i o n s o f k n o w n p H a n d H a m m e t t a c i d i t y z. S p e c t r o q u a h t y c h l o r o f o r m a n d e t h a n o l ( M a t h e s o n , C o l e m a n a n d Bell. Inc.) w e r e a l s o e m p l o y e d as s o l v e n t s Result~ and discussion T h e long w a v e l e n g t h a b s o r p t z o n a n d f l u o r e s c e n c e m a x i m a o f p h e n y l s a h c y l a t e a r e p r e s e n t e d In T a b l e I. T h e shifts o f t h e l o n g w a v e l e n g t h a b s o r p t i o n r n a x z m u m o f t h e n e u t r a l m o l e c u l e ( N ) to l o w e r frequencies, o n p r o t o n a t l o n t o f o r m the c a t i o n ( C ) in c o n c e n t r a t e d acid, a n d o n d m s o c i a t l o n to f o r m the a n i o n ( A ) in d i l u t e b a s e , s u g g e s t the g r o u n d s t a t e i o n t z a t l o n s e q u e n c e d e p l e t e d in S c h e m e 1.
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U p o n traverMng the H a m m e t t a c i d i t y region f r o m 1.10--10 to H 0 - - 6 . the blue f l u o r e s c e n c e o f the c a t i o n ( C ) ~s q u e n c h e d , the m l d p o m t o f the q u e n c h i n g o c c u r r i n g at t h e g r o u n d state pK., o f - - 8 1 F r o m !-1o--6 to pH 7. u b m e l y discernible, b r o a d a r e a o f f l u o r e s c e n c e r a n g i n g f r o m 320 n m to 500 n m . occurs. A b o v e p H 7. the i n t e n s e blue f l u o r e s c e n c e o f the a n i o n a p p e a r s ( c e n t e r c d at p H = pK.,----9 7 ) a n d rises to a m a x i m u m at p H 120 A b o v e pH 13. rapid h y d r o l y s i s o f the p h c n y l estcr is q p p m e n t from tile shift o f the fluorcsccnce s p e c u um to s h o r t c r wavel e n g t h s with time. E~en at p H a~ low as 10. slow h y d r o l y s i s or the p h e n y l ester o c c u r s T h e f l u o r c s c e n c e s o r the c a t i o n a n d a m e n dcr~vcd from p h c n y l s a h c y l a t c o c c u r m the s a m e g e n e r a l spectral r e g i o n as tile f l u o r e s c c n c e s or the c o r r e s p o n d i n g species d e r i v e d f r o m s a h c y h c acid a n d m e t h y l sal~cylate ln~tmlly ~t was t h o u g h t t h a t tile fluorescences o c c u r r i n g m the region 1-1o--6 to pH 7 m~ght be d u e to I m p u r m e s m the s a m p l e . H o w e v e r . r e p e a t e d r e c r y s t a l l ~ z a u o n a n d r e s u b l l m a t l o n gaffed to alter the p d t t e r n or tile e x t r e m e l y w e a k em~ssion m the 3 2 0 - 5 0 0 n m regson M o r e o v e r . the a p p a r e n t p e a k p o s i t i o n s o f this e m i s s i o n ( T a b l e I) a r e close to the f l u o r e s c e n c e or the n e u t r a l m o l e c u l e a n d z w m e n o n d e r i v e d f r o m m e t h y l s a h c y l a t e ~ a n d t h e r e f o r e m a y r e a s o n a b l y bc : t t t n b u t e d to f l u o r e s c e n c e f r o m t h e n e u t r a l m o l e c u l e a n d z w n t e n o n d e r , v e d f r o m p h e n y l sallcylate. In e t h ; m o l the f l u o r e s c e n c e o f p h c n y l salicylate Is m o d e r a t e l y intense, m a x i m a l a t 340 n m (close to tile m a x i m u m o f o - m c t h o x y b e n z o ~ c a c i d ) ;rod t a d s o f f on the l o n g - w a v e l e n g t h side. very g r a d u a l l y to ca 500 nm. In c h l o r o f o r m , the f l u o r e s c e n c e o f p h e n y l s a h c y l a t e ~s a l s o m o d e r a t e l y intense, but m a x i m a l at 455 nm. t m h n g off g r a d u a l l y o n the s h o r t - w a v e l e n g t h s~de to ca. 320 n m T h e l a t t e r e m i s s i o n is s i m i l a r m m a x i m u m to the f l u o r e s c e n c e o f m e t h y l s a h c y l a t e m c h l o r o f o r m . T h e s e observ a t , o n s i n d i c a t e t h a t tile f l u o r c s c e n c e m a x i m u m m e t h a n o l ~s d u e to e m i s s i o n f r o m the n e u t r a l m o l e c u l e ( N ) d e r i v e d f r o m p h e n y l sallcylate w h d e tile f l u o r e s c e n c e
SllOR1 COMMUNICATION
451
n~;t,cm~um m chlorofo~ nl ~s cluc to Cml,,,,~on f r o m the z w , t t c r ~ o n ( Z ) r e s u l t i n g f r o m the p h o t o t d u t o r n c r t s m o f t h c neutral m o l e c u l e H~ the Iowc~t cxcRcd ~mglet state ( S c h e m e 2) mO'cl/O
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T h e t a d l n g o n the l o n g wa~clm~gth ~lde e l ' t h e I l u o r e s c e n c e o f ( N ) m e t h a n o l ts t h u s a t t r i b u t e d to a ,,m,'dl d e g r e e o f p h o t o t a u t o m c r l ~ m m t h a t ,,olvent w h i l e t he tailing o n t h e ,;hort w a v e l e n g t h stdc o f the f l u o r e s c e n c e o f ( Z ) tn c h l o r o f o r m i,; attl ibuted to ;i htgh degree o f p h o l o t d u t o m e t Jsm a n d d s m a l l rcstdual nuorc~ccnce f r o m th e r c m a m m g cxc~ted ilCtltr~tl ,,pCClOS In th:lt s o l v e n t T h c a p p c a r a n c c o f c o n s t a n t ~ntcns~ty f l u o r e s c e n c e o f t he n e u t r : d m o l e c u l e a n d zw'ztterlon f r o m Hf~--6 to p H 7 is u n h k e t h a t o f m e t h y l s a h c y k t t e , m w h i c h th e zw~ttm~on a l o n e f l u o r c s c c s m m o d c r a t c l v c o n c e n t r a t e d a c t d a n d b o t h the z w t t t m ~ o n :~nd n e u t r a l ~pec~cs f l u o r c s c c m t he pH r e g i o n . In the l at t er c o m p o t m d . th e f l u o r e s c e n c e o f the zw~tter~on ~n c o n c e n t r a t e d a c t d ,s a t t r ~ b u t c d to b ~ m o l c c u l a r p l ~ t o t a u t o m e n ~ m , r e q t u r m g p r o t o n a t t o n as .t first ,;top. T h e f l u o r e s c e n c e s o f the n e u t r a l spec~cs a n d zw~tter=on m t h e p H r c g t o n :~rc a t t r i b u t e d to p a r t m i m t r a m o l e c u l a r t,~t, t o m e r , ~ m , t h e c o n c e n i r a t t o n o f p r o t o n a t m g sp¢ ~es b e i n g t o o l o w m the p H r c g , o n t o p r o t o n , ~ t c t h c e xc i t e d n e u t r a l ~pec~es B e c a u s e o f t h e f a d u r e o f p h e n y l ~al~cylatc to d e m o n s t r a t e e×c~tcd s t a t e p r o t o n a t i o n , ds r e f l e c t e d b) the c o i n c i d e n c e o f th e ~nflcct~on rcg~on~ m w h i c h the d b s o r p t ~ o n a n d f l u o r e s c e n c e s p e c t r a c h a n g e . ~t m u s t be c o n c l u d e d t h a t thc p h o t o t a u t o m c r , ~ m o f the n c u t r a l speci es t o f o r m the e x c i t e d zw~ttcr~on ,scnt~rely , n t r a m o l c c u l a r Bec.~t,~e th~s p r o c e s s o c c u r s o n l y p a r t m l l y . as c v , d c n c c d b y t he occt, r, c n c c o f f l u o r c s c c n c c f r o m t he n c u t r a l spcc~cs, the r a t c c o n s t a n t for p h o t o t a u t o m c r , s m mu,d bc vcry n c a r t hc rcc~procal o f the m c a n IffcUmc o f t h e l o w e s t cxc~tcd s~nglct ~tatc. Since. w~th t hc m s t r u m c n t a t m n e m p l o y e d . f l u o r c s c c n c e s w~th IffetHnes o f 10- ~ ~-~ or" grm~ter c a n bc d c t c c t c d a n d t he f l u o r e s c e n c e s o f t he n e u t r d l ~pcc,c', a n d z w m c r ~ o n ,n wdtet ~tre v e r y ne~tr t h e l~mlt o f d e t e c t i o n o f t he f l u o r e s c e n c e s p e c t r o m e t e r . ~t m a y be c o n c l u d e d t h a t t he rat c c o n s t a n t For m t r a m o l c c u l . ' ~ r photot.'~utomer~sm o f t h e neutr:tl spcc, cs to t h e e x c , t e d z w , t t c r , o n ~ a b o u t I0 ~ ~ s - ~. In th~s r c g a r d . ~t ~s r d t h e r m t c r c s t m g t hat a l t h o u g h the q u a n t u m y~clds o f f l u o r e s c e n c e (rind t h c r c f o r c t h e Iffet unes) o f t h e n e u t r : d speci es a n d z w ~ t t e r , o n a r e m u c h h~ghe~ m e t h a n o l a n d ¢hlo, o f o r m , r e s p e c u v c l y , v e r y h t t l e p h o t o t a u t o m e r ~ , , m o c c u r s m e t h a n o l a n d ne:~rly c o m p l e t e p h o t o t a u t o m e r m m o c c u r s m c h l o r o f o r m . Th~s s u g g e s t s t ha t m ~ t r o n g l y h y d r o g e n b o n d i n g s o l v e n t s (e q w a t e r a n d e t h a n o l ) , m t c r m o l e c u l a r h y d r o g e n b o n d i n g w~th t he s o l v e n t wc.'tkens t h e m i r a m o l e c u l a r h y d r o g e n b o n d bctwccn the p h c n o l , c g r o u p and the c a r b o p h c n o × y g r o u p and t h e r e b y decreases the rate of mtramolccul.'tr p h o t o t a u t o m c r ~ s m m phenyl sal~cylate M o r e o , c r . ,n w a t e r a n d ~ulfu~,c actd. t h e s t r o n g e , , t h y d r o g e n - b o n d i n g ~olvents s t u d , e d , t he ~ n t c r m o l c c u l a ! h y d r o g e n - b o n d i n g w~th t h e s o l v e n t d p p e a r s l a r g e l y to q u e n c h the f l u o r e s c e n c e o f t hc neut~:tl s p e c i e s Th~s m d y .'rise I~m~t t he
452
SHORT COM M UNICATION
efficiency o f p h o t o t a u t o m e r l s m o f the n e u t r a l spcc=es d e r t v e d f r o m p h e n y l sedlcylate in t h e s e solvents. In v~ew o f the fact t h a t p h e n y l s'.d~cylate fluoresces m o s t i n t e n s e l y m c h l o r o f o r m . tiara a p p e a r s to be t h e s o l v e n t o f c h o l c e for a n y fluor=metr~c analys~s H o w e v c r . in t h e p r e s e n c e o f o t h e r s a l t c y l a t e - t y p e d r u g s , m o s t o f w h i c h f l u o r e s c e at 400--450 rim. t h e s h o r t w a v e l e n g t h o f f l u o r e s c e n c e o f p h e n y l s a l l c y l a t e m eth~mol m a ) be a d vant.'~geous f r o m thc p o i n t o f v~ew of selcctJv~ty REFERENCES I P J Kovt, C L M d l c r , m d S G S~.hulm,ln Apacd C h m l ,l~ta 61 (1972) 7 2 M J Jorgen~en and D R [ - l a r t t c r J ,Imt, i C'hem ~'o¢ ~5 ( 1 9 6 3 ) N 7 8
449
SHORT COMMUNICATION Fluorescence of phenyl salicylate I MICHAEL College ol
RUTLEDGE
dtld S T E P H E N
P l u n m a c r. U n i t vr~tt I o/
G SCHULMAN
F I o r u l a . G,tmt.~l tile. F l a 3 2 6 1 0 ( U S .4 )
( R e c e i v e d 12th August 1974)
P h e n y l s a l t c y l a t e ( s a l o l ) h a s b e e n e m p l o y e d industrially as a s u n s c r e e n . e n t e r i c c o a t i n g for d e l a y e d a c t i o n t a b l e t s a n d plasticizer, a n d m a v a r i e t y o f o t h e r p h a r m a c e u t i c a l a p p l i c a t i o n s s i m i l a r to t h o s e o f a s p i r i n Its s t r u c t u r a l s~mllarlty to salicylic a c i d a n d m e t h y l s a l l c y l a t e s u g g e s t s that the s p e c t r o s c o p i c p r o p e r t i e s o f all t h r e e c o m p o u n d s s h o u l d b e similar. H o w e v e r . w h d e these c o m p o u n d s f l u o r e s c e q u i t e i n t e n s e l y t h r o u g h o u t the p H r e g i o n a n d in c o n c e n t r a t e d a c i d ~. p h e n y l s a h c y l a t e e x h i b i t s a b a r e l y d e t e c t a b l e f l u o r e s c e n c e b e l o w p H 7 ;rod In sulfuric acid a b o v e I-/o --6. In o r d e r to u n d e r s t a n d b e t t e r the s p e c t r o s c o p i c b e h a v i o r a n d to d e t e r m i n e t h e o p t l m a l c o n d , t , o n s for f l u o r i m e t n c d e t e r m i n a t i o n o f p h e n y l sallcylate, the p r e s e n t s t u d y o f t h e p H a n d s o l v e n t d e p e n d e n c e s o f the e l e c t r o m c s p e c t r a o f salol w a s undertaken.
Expermlental Absorption spectra were taken on a Beckman DB-GT spectrophotometer. F l u o r e s c e n c e s p e c t r a w e r e r e c o r d e d on a P e r k i n - E i m e r M P F 2 A f l u o r e s c e n c e s p e c t r o p h o t o m e t e r w h o s e m o n o c h r o m a t o r s w e r e c a l i b r a t e d a g a i n s t the x e n o n line e m i s s i o n s p e c t r u m a n d w h o s e o u t p u t w a s c o r r e c t e d for i n s t r u m e n t a l r e s p o n s e b y m e a n s o f a r h o d a m m e - B q u a n t u m c o u n t e r M e a s u r e m e n t o f p H w a s m a d e o n an O r i o n M o d e l 801 p H m e t e r with a B e c k m a n salver/silver c h l o r i d e - g l a s s c o m b i n a t i o n e l e c t r o d e . Phenyl s a l l c y l a t e ( M e r c k ) w a s purified b y r e c r y s t a l l l z a t l o n f r o m c h l o r o f o r m . R e a g e n t - g r a d e sulfuric a c i d . s o d m m h y d r o x i d e , s o d m m acetate, acetic acid. s o d i u m d d l y d r o g e n p h o s p h a t e a n d s o d m m m o n o h y d r o g e n p h o s p h a t e w e r e used to p r e p a r e s o l u t i o n s o f k n o w n p H a n d H a m m e t t a c i d i t y z. S p e c t r o q u a h t y c h l o r o f o r m a n d e t h a n o l ( M a t h e s o n , C o l e m a n a n d Bell. Inc.) w e r e a l s o e m p l o y e d as s o l v e n t s Result~ and discussion T h e long w a v e l e n g t h a b s o r p t z o n a n d f l u o r e s c e n c e m a x i m a o f p h e n y l s a h c y l a t e a r e p r e s e n t e d In T a b l e I. T h e shifts o f t h e l o n g w a v e l e n g t h a b s o r p t i o n r n a x z m u m o f t h e n e u t r a l m o l e c u l e ( N ) to l o w e r frequencies, o n p r o t o n a t l o n t o f o r m the c a t i o n ( C ) in c o n c e n t r a t e d acid, a n d o n d m s o c i a t l o n to f o r m the a n i o n ( A ) in d i l u t e b a s e , s u g g e s t the g r o u n d s t a t e i o n t z a t l o n s e q u e n c e d e p l e t e d in S c h e m e 1.
450
SHORT ® (~ O , ~ c / / O H
oK a - - 8 1
~O~c/~'O
(c)
DKa - 9 7
~O'c//O
(N)
( Scheme
COMMUNICATION
(A)
1 )
TABLI_ I LONG WAVI'LENGTII ABSORP]ION (r,)AND SALICYLATE IN VARIOUS SOLVENTS
Soht,.t .
.
C,(cm'
.
.
.
.
.
.
.
.
.
S u l l u n ¢ A c i d ( I!. Wdlcr(pH=I()) EIh,mol Cldort~form W , i t e r ( p H I I 0) .
.
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U p o n traverMng the H a m m e t t a c i d i t y region f r o m 1.10--10 to H 0 - - 6 . the blue f l u o r e s c e n c e o f the c a t i o n ( C ) ~s q u e n c h e d , the m l d p o m t o f the q u e n c h i n g o c c u r r i n g at t h e g r o u n d state pK., o f - - 8 1 F r o m !-1o--6 to pH 7. u b m e l y discernible, b r o a d a r e a o f f l u o r e s c e n c e r a n g i n g f r o m 320 n m to 500 n m . occurs. A b o v e p H 7. the i n t e n s e blue f l u o r e s c e n c e o f the a n i o n a p p e a r s ( c e n t e r c d at p H = pK.,----9 7 ) a n d rises to a m a x i m u m at p H 120 A b o v e pH 13. rapid h y d r o l y s i s o f the p h c n y l estcr is q p p m e n t from tile shift o f the fluorcsccnce s p e c u um to s h o r t c r wavel e n g t h s with time. E~en at p H a~ low as 10. slow h y d r o l y s i s or the p h e n y l ester o c c u r s T h e f l u o r c s c e n c e s o r the c a t i o n a n d a m e n dcr~vcd from p h c n y l s a h c y l a t c o c c u r m the s a m e g e n e r a l spectral r e g i o n as tile f l u o r e s c c n c e s or the c o r r e s p o n d i n g species d e r i v e d f r o m s a h c y h c acid a n d m e t h y l sal~cylate ln~tmlly ~t was t h o u g h t t h a t tile fluorescences o c c u r r i n g m the region 1-1o--6 to pH 7 m~ght be d u e to I m p u r m e s m the s a m p l e . H o w e v e r . r e p e a t e d r e c r y s t a l l ~ z a u o n a n d r e s u b l l m a t l o n gaffed to alter the p d t t e r n or tile e x t r e m e l y w e a k em~ssion m the 3 2 0 - 5 0 0 n m regson M o r e o v e r . the a p p a r e n t p e a k p o s i t i o n s o f this e m i s s i o n ( T a b l e I) a r e close to the f l u o r e s c e n c e or the n e u t r a l m o l e c u l e a n d z w m e n o n d e r i v e d f r o m m e t h y l s a h c y l a t e ~ a n d t h e r e f o r e m a y r e a s o n a b l y bc : t t t n b u t e d to f l u o r e s c e n c e f r o m t h e n e u t r a l m o l e c u l e a n d z w n t e n o n d e r , v e d f r o m p h e n y l sallcylate. In e t h ; m o l the f l u o r e s c e n c e o f p h c n y l salicylate Is m o d e r a t e l y intense, m a x i m a l a t 340 n m (close to tile m a x i m u m o f o - m c t h o x y b e n z o ~ c a c i d ) ;rod t a d s o f f on the l o n g - w a v e l e n g t h side. very g r a d u a l l y to ca 500 nm. In c h l o r o f o r m , the f l u o r e s c e n c e o f p h e n y l s a h c y l a t e ~s a l s o m o d e r a t e l y intense, but m a x i m a l at 455 nm. t m h n g off g r a d u a l l y o n the s h o r t - w a v e l e n g t h s~de to ca. 320 n m T h e l a t t e r e m i s s i o n is s i m i l a r m m a x i m u m to the f l u o r e s c e n c e o f m e t h y l s a h c y l a t e m c h l o r o f o r m . T h e s e observ a t , o n s i n d i c a t e t h a t tile f l u o r c s c e n c e m a x i m u m m e t h a n o l ~s d u e to e m i s s i o n f r o m the n e u t r a l m o l e c u l e ( N ) d e r i v e d f r o m p h e n y l sallcylate w h d e tile f l u o r e s c e n c e
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T h e t a d l n g o n the l o n g wa~clm~gth ~lde e l ' t h e I l u o r e s c e n c e o f ( N ) m e t h a n o l ts t h u s a t t r i b u t e d to a ,,m,'dl d e g r e e o f p h o t o t a u t o m c r l ~ m m t h a t ,,olvent w h i l e t he tailing o n t h e ,;hort w a v e l e n g t h stdc o f the f l u o r e s c e n c e o f ( Z ) tn c h l o r o f o r m i,; attl ibuted to ;i htgh degree o f p h o l o t d u t o m e t Jsm a n d d s m a l l rcstdual nuorc~ccnce f r o m th e r c m a m m g cxc~ted ilCtltr~tl ,,pCClOS In th:lt s o l v e n t T h c a p p c a r a n c c o f c o n s t a n t ~ntcns~ty f l u o r e s c e n c e o f t he n e u t r : d m o l e c u l e a n d zw'ztterlon f r o m Hf~--6 to p H 7 is u n h k e t h a t o f m e t h y l s a h c y k t t e , m w h i c h th e zw~ttm~on a l o n e f l u o r c s c c s m m o d c r a t c l v c o n c e n t r a t e d a c t d a n d b o t h the z w t t t m ~ o n :~nd n e u t r a l ~pec~cs f l u o r c s c c m t he pH r e g i o n . In the l at t er c o m p o t m d . th e f l u o r e s c e n c e o f the zw~tter~on ~n c o n c e n t r a t e d a c t d ,s a t t r ~ b u t c d to b ~ m o l c c u l a r p l ~ t o t a u t o m e n ~ m , r e q t u r m g p r o t o n a t t o n as .t first ,;top. T h e f l u o r e s c e n c e s o f the n e u t r a l spec~cs a n d zw~tter=on m t h e p H r c g t o n :~rc a t t r i b u t e d to p a r t m i m t r a m o l e c u l a r t,~t, t o m e r , ~ m , t h e c o n c e n i r a t t o n o f p r o t o n a t m g sp¢ ~es b e i n g t o o l o w m the p H r c g , o n t o p r o t o n , ~ t c t h c e xc i t e d n e u t r a l ~pec~es B e c a u s e o f t h e f a d u r e o f p h e n y l ~al~cylatc to d e m o n s t r a t e e×c~tcd s t a t e p r o t o n a t i o n , ds r e f l e c t e d b) the c o i n c i d e n c e o f th e ~nflcct~on rcg~on~ m w h i c h the d b s o r p t ~ o n a n d f l u o r e s c e n c e s p e c t r a c h a n g e . ~t m u s t be c o n c l u d e d t h a t thc p h o t o t a u t o m c r , ~ m o f the n c u t r a l speci es t o f o r m the e x c i t e d zw~ttcr~on ,scnt~rely , n t r a m o l c c u l a r Bec.~t,~e th~s p r o c e s s o c c u r s o n l y p a r t m l l y . as c v , d c n c c d b y t he occt, r, c n c c o f f l u o r c s c c n c c f r o m t he n c u t r a l spcc~cs, the r a t c c o n s t a n t for p h o t o t a u t o m c r , s m mu,d bc vcry n c a r t hc rcc~procal o f the m c a n IffcUmc o f t h e l o w e s t cxc~tcd s~nglct ~tatc. Since. w~th t hc m s t r u m c n t a t m n e m p l o y e d . f l u o r c s c c n c e s w~th IffetHnes o f 10- ~ ~-~ or" grm~ter c a n bc d c t c c t c d a n d t he f l u o r e s c e n c e s o f t he n e u t r d l ~pcc,c', a n d z w m c r ~ o n ,n wdtet ~tre v e r y ne~tr t h e l~mlt o f d e t e c t i o n o f t he f l u o r e s c e n c e s p e c t r o m e t e r . ~t m a y be c o n c l u d e d t h a t t he rat c c o n s t a n t For m t r a m o l c c u l . ' ~ r photot.'~utomer~sm o f t h e neutr:tl spcc, cs to t h e e x c , t e d z w , t t c r , o n ~ a b o u t I0 ~ ~ s - ~. In th~s r c g a r d . ~t ~s r d t h e r m t c r c s t m g t hat a l t h o u g h the q u a n t u m y~clds o f f l u o r e s c e n c e (rind t h c r c f o r c t h e Iffet unes) o f t h e n e u t r : d speci es a n d z w ~ t t e r , o n a r e m u c h h~ghe~ m e t h a n o l a n d ¢hlo, o f o r m , r e s p e c u v c l y , v e r y h t t l e p h o t o t a u t o m e r ~ , , m o c c u r s m e t h a n o l a n d ne:~rly c o m p l e t e p h o t o t a u t o m e r m m o c c u r s m c h l o r o f o r m . Th~s s u g g e s t s t ha t m ~ t r o n g l y h y d r o g e n b o n d i n g s o l v e n t s (e q w a t e r a n d e t h a n o l ) , m t c r m o l e c u l a r h y d r o g e n b o n d i n g w~th t he s o l v e n t wc.'tkens t h e m i r a m o l e c u l a r h y d r o g e n b o n d bctwccn the p h c n o l , c g r o u p and the c a r b o p h c n o × y g r o u p and t h e r e b y decreases the rate of mtramolccul.'tr p h o t o t a u t o m c r ~ s m m phenyl sal~cylate M o r e o , c r . ,n w a t e r a n d ~ulfu~,c actd. t h e s t r o n g e , , t h y d r o g e n - b o n d i n g ~olvents s t u d , e d , t he ~ n t c r m o l c c u l a ! h y d r o g e n - b o n d i n g w~th t h e s o l v e n t d p p e a r s l a r g e l y to q u e n c h the f l u o r e s c e n c e o f t hc neut~:tl s p e c i e s Th~s m d y .'rise I~m~t t he
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efficiency o f p h o t o t a u t o m e r l s m o f the n e u t r a l spcc=es d e r t v e d f r o m p h e n y l sedlcylate in t h e s e solvents. In v~ew o f the fact t h a t p h e n y l s'.d~cylate fluoresces m o s t i n t e n s e l y m c h l o r o f o r m . tiara a p p e a r s to be t h e s o l v e n t o f c h o l c e for a n y fluor=metr~c analys~s H o w e v c r . in t h e p r e s e n c e o f o t h e r s a l t c y l a t e - t y p e d r u g s , m o s t o f w h i c h f l u o r e s c e at 400--450 rim. t h e s h o r t w a v e l e n g t h o f f l u o r e s c e n c e o f p h e n y l s a l l c y l a t e m eth~mol m a ) be a d vant.'~geous f r o m thc p o i n t o f v~ew of selcctJv~ty REFERENCES I P J Kovt, C L M d l c r , m d S G S~.hulm,ln Apacd C h m l ,l~ta 61 (1972) 7 2 M J Jorgen~en and D R [ - l a r t t c r J ,Imt, i C'hem ~'o¢ ~5 ( 1 9 6 3 ) N 7 8