Pure & Appl. Chem., Vol. 65,
No. 12, pp. 2593-2601, 1993.
Printed in Great Britain.
1993 IUPAC
Solute/solvent interactions and their empirical
determination by means of solvatochromic dyes
C h r i s t i a n R e i c h a r d t , * S i a m a k A s h a r i n - F a r d , A n d r e a s Blum,
M i c h a e l E s c h n e r , Abdol-Mohammad M e h r a n p o u r , P i o t r M i l a r t ,
Thomas Niem, G e r h a r d S c h a f e r , M a r t i n a W i l k
Department of Chemistry, P h i l i p p s U n i v e r s i t y ,
H a n s - M e e r w e i n - S t r a s s e , D-35032 M a r b u r g , Germany
A b s t r a c t : The
solvatochromism,
thermochromism,
piezochromism, h a l o c h r o m i s m , and p o t e n t i a l c h i r o - s o l v a t o c h r o m i s r n
of
s o l u t i o n s o f v a r i o u s p y r i d i n i u m N-phenoxide
betaine
study
UV/Visdyes have been u s e d by u s and o t h e r s t o
spectroscopically t h e dye/solvent interactions i n organic
solvents of
different polarity, different
temperature,
d i f f e r e n t e x t e r n a l pressure, d i f f e r e n t e l e c t r o l y t e concent r a t i o n , and d i f f e r e n t c h i r a l i t y .
INTRODUCTION
S o l v e n t s c a n h a v e a s t r o n g i n f l u e n c e on r e a c t i o n r a t e s , on t h e
p o s i t i o n o f c h e m i c a l e q u i l i b r i a , a s w e l l a s on t h e p o s i t i o n a n d
i n t e n s i t y of s p e c t r a l a b s o r p t i o n bands l o c a t e d i n v a r i o u s s p e c t r o s c o p i c r e g i o n s ( U V / V i s , I R , NMR, e t c . ) . Today, t h i s i s well-known
t o a l l c h e m i s t s s i n c e t h e p i o n e e r i n g work o f B e r t h e l o t a n d S a i n t G i l l e s ( P a r i s . 1862) a s w e l l a s Menschutkin ( S t . P e t e r s b u r g 1890)
on s o l v e n t e f f e c t s on r e a c t i o n r a t e s , a n d o f C l a i s e n ( A a c h e n 1 8 9 6 ) ,
Hantzsch
(Wurzburg 1 8 9 6 ) , Knorr ( J e n a 1 8 9 6 ) ,
and W i s l i c e n u s
( W u r z b u r g 1 8 9 6 ) on t h e s o l v e n t d e p e n d e n c e o f c h e m i c a l e q u i l i b r i a
( e . g . t a u t o m e r i c e q u i l i b r i a ) ( r e f . 1 ) . F o r exam l e , t h e S N 1 s o l v o l y . s i s o f 2 - c h l o r o - 2 - m e t h y l p r o p a n e i s c a . 10"
times f a s t e r i n
water than i n t h e less p o l a r s o l v e n t benzene. The e n o l c o n t e n t of
e t h y l a c e t o a c e t a t e i s 62 mol-% i n c y c l o h e x a n e , b u t o n l y 6 . 5 mol-%
i n water ( r e f . 1).
Chemists h a v e u s u a l l y a t t e m p t e d t o u n d e r s t a n d s o l v e n t e f f e c t s
i n terms o f t h e ' p o l a r i t y o f t h e s o l v e n t ' . B u t w h a t d o e s s o l v e n t
p o l a r i t y r e a l l y mean ? The s i m p l i c i t y o f i d e a l i z e d e l e c t r o s t a t i c
models f o r t h e d e s c r i p t i o n of s o l v a t i o n of i o n s o r d i p o l a r molecules, c o n s i d e r i n g s o l v e n t s a s a non-structured continuum, has l e d
t o t h e u s e o f t h e r e l a t i v e p e r m i t t i v i t y (Er), t h e p e r m a n e n t d i p o l e
or functions thereof moment ( p ) , a n d t h e r e f r a c t i v e i n d e x ( n )
a s macroscopic, physical solvent p o l a r i t y parameters. B u t s o l u t e /
s o l v e n t i n t e r a c t i o n s t a k e p l a c e on a m o l e c u l a r - m i c r o s c o p i c l e v e l ,
w i t h individual solvent molecules surrounding t h e i o n s o r molecules
of t h e s o l u t e . Therefore, t h e simple e l e c t r o s t a t i c approach often
f a i l e d b e c a u s e i n t e r m o l e c u l a r f o r c e s between s o l u t e and s o l v e n t
include i n addition t o t h e nonspecific coulombic, d i r e c t i o n a l ,
i n d u c t i v e , and d i s p e r s i o n i n t e r a c t i o n s a l s o s p e c i f i c i n t e r a c t i o n s
such a s hydrogen bonding, e l e c t r o n - p a i r donor ( E P D ) / e l e c t r o n - p a i r
-
2593
2594
C. REICHARDT e t a /
a c c e p t o r ( E P A ) , a n d s o l v o p h o b i c i n t e r a c t i o n s . H e n c e , f r o m a more
p r a g m a t i c p o i n t o f v i e w , i t s e e m s t o be more f a v o r a b l e t o d e f i n e
' s o l v e n t p o l a r i t y ' a s t h e overall solvation capability ( s o l v a t i o n
power) f o r ( i ) r e a c t a n t s and p r o d u c t s (+
chemical e q u i l i b r i a ) ,
( i i ) r e a c t a n t s a n d a c t i v a t e d c o m p l e x e s (-+ r e a c t i o n r a t e s ) , a n d
( i i i ) m o l e c u l e s i n t h e e l e c t r o n i c g r o u n d a n d e x c i t e d s t a t e (+
l i g h t a b s o r p t i o n ) , w h i c h i n t u r n d e p e n d s on t h e a c t i o n o f all
p o s s i b l e , n o n s p e c i f i c and s p e c i f i c , i n t e r m o l e c u l a r i n t e r a c t i o n s
between s o l u t e and s o l v e n t m o l e c u l e s - e x c l u d i n g s u c h i n t e r a c t i o n s
leading t o d e f i n i t e chemical a l t e r a t i o n s of t h e s o l u t e molecules
by t h e s o l v e n t a s consequence of a normal chemical r e a c t i o n b e t ween t h e m ( r e f s . 1 , 2 ) .
O b v i o u s l y , s o l v e n t p o l a r i t y s u c h d e f i n e d c a n n o t be m e a s u r e d b y
means o f m a c r o s c o p i c , p h y s i c a l p a r a m e t e r s s u c h a s p e r m i t t i v i t y o r
d i p o l e moment. O t h e r i n d i c e s of s o l u t e / s o l v e n t i n t e r a c t i o n s on a
molecular-microscopic s c a l e a r e sought. For t h i s reason, empirical
p a r a m e t e r s o f s o l v e n t p o l a r i t y have been i n t r o d u c e d , u s i n g a w e l l selected, strongly solvent-dependent, p a r t i c u l a r chemical reaction
( t h e r a t e o r e q u i l i b r i u m of which a r e s t u d i e d ) o r s p e c t r a l a b s o r p t i o n a s s u i t a b l e models f o r a l l o t h e r s o l v e n t - d e p e n d e n t p r o c e s s e s .
I f one c a r e f u l l y s e l e c t s an a p p r o p r i a t e , s u f f i c i e n t l y s o l v e n t dependent r e f e r e n c e process ( o r s i m i l a r i t y model; c f . r e f . 3 ) ,
an e m p i r i c a l m e a s u r e o f s o l v e n t p o l a r i t y c a n be d e r i v e d f r o m i t ,
w h i c h i s b e l i e v e d t o p r o v i d e a more c o m p r e h e n s i v e m e a s u r e o f t h e
o v e r a l l s o l v a t i o n c a p a b i l i t y o f t h e s o l v e n t s t h a n do t h e i r i n d i v i d u a l physical data.
K . H . Meyer ( M u n i c h 1 9 1 4 ) i n t r o d u c e d t h e s o - c a l l e d desmotropic
constant L a s e m p i r i c a l m e a s u r e o f t h e e n o l i z a t i o n c a p a b i l i t y o f
s o l v e n t s , u s i n g t h e solvent-dependent t a u t o m e r i z a t i o n of e t h y l
a c e t o a c e t a t e a s r e f e r e n c e r e a c t i o n ( r e f . 4 ) , whereas W i n s t e i n and
G r u n w a l d ( L o s A n g e l e s 1 9 4 8 ) d e v e l o p e d t h e Y-values a s m e a s u r e s o f
t h e s o l v e n t i o n i z i n g power, w i t h t h e S N 1 , s o l v o l y s i s r e a c t i o n o f
2-chloro-2-methylpropane a s s t a n d a r d r e a c t i o n ( r e f . 5 ) . Meanwhile,
a c o n s i d e r a b l e number o f e m p i r i c a l s o l v e n t p a r a m e t e r s h a v e b e e n
i n t r o d u c e d , b a s e d on v a r i o u s s o l v e n t - d e p e n d e n t r e a c t i o n s a n d
a b s o r p t i o n s ( r e f s . 1 - 3 , 6 - 9 ) , sometimes combined t o m u l t i p a r a meter c o r r e l a t i o n e q u a t i o n s w i t h d i f f e r e n t e m p i r i c a l parameters
f o r t h e v a r i o u s s o l u t e / s o l v e n t i n t e r a c t i o n s ( r e f s . 1, 1 0 , 11).
1. Qualitative
representation of
the solvent effect
o n the C T absorption
of betaine d y e 3
(JJ
> p e ) a n d the
desinition of E, (30)
values as molar'
transition energies.
Fig.
Nonpolar
Polar
*
Solvent
Numerous opplicotions to other
solvent -dependent processes
Ernpiricol Parameter
@
Known for >300 solvents
2595
Solutelsolvenf inferactions and their empirical determination
SOLVATOCHROMIC BETAINE DYES
The first suggestion that solvatochromic dyes such as the zwitterionic 4-hydroxystyryl-pyridinium d y e 1 c a n serve as empirical
indicators of solvent polarity was m a d e by Brooker et al.
(Rochester 1951; ref. 12), but Kosower (New York 1958) was the
first to set u p a real UV/Vis spectroscopic solvent scale, called
Z-scale, based on the intermolecular charge-transfer ( C T ) absorption of the 4 - m e t h o x y c a r b o n y l - 1 - e t h y l p y r i d i n i u m iodide 2 as
solvent-sensitive reference process (ref. 13). B y virtue of its
exceptionally large negative solvatochromism, the 2,6-diphenyl-4( 2 , 4 , 6 - t r i p h e n y l - l - p y r i d i n i o ) - p h e n o l a t e 3 ( a n d its more lipophilic, even in hydrocarbons soluble penta-tert-butyl-substituted
derivative 4 ) has been proposed by us as a new spectroscopic indicator of solvent polarity (Marburg 1963; refs. 14 - 1 6 ) , called
the ET(30)-scale (ref. 17 a n d Fig. 1).
Q
’
6
A,,
:91L
\less
8
basic
nm
]AA:389nm
Am,=
525nm
_
I
_
Rocemtorm
Q ( n = 0.1.2)
Scheme 1.
meso-Form
(R.Rl+ IS,Sl
IR.SI
10
11
Negatively Solvatochrornic D y e s 1 - 11
(1: Brooker; 2: K o s o w e r ; 3 - 11: Reichardt)
2596
C. REICHARDT et a/ .
T h e following peculiar properties o f 3 are obviously r e s p o n s i b l e
f o r t h e solvent-mediated stabilization o f its highly d i p o l a r e l e c t r o n i c ground s t a t e , r e l a t i v e t o its l e s s d i p o l a r excited s t a t e ,
t h e l a t t e r resulting f r o m an i n t r a m o l e c u l a r c h a r g e t r a n s f e r by
l i g h t absorption within t h e visible s p e c t r a l r e g i o n (cf. F i g , 1):
(i) it exhibits a l a r g e permanent d i p o l e m o m e n t , s u i t a b l e f o r t h e
registration o f d i p o l e / d i p o l e and dipole/induced d i p o l e i n t e r a c t i o n s ; (ii) it possesses a l a r g e polarizable V - e l e c t r o n s y s t e m
s u i t a b l e f o r t h e r e g i s t r a t i o n o f dispersion i n t e r a c t i o n s ; and
(iii) with t h e phenoxide oxygen atom it e x h i b i t s a highly basic
electron-pair d o n o r c e n t r e , s u i t a b l e f o r i n t e r a c t i o n s with weak
Bronsted a c i d s (H-bonding) and L e w i s a c i d s (EPD/EPA-bonding). T h e
positive c h a r g e of t h e pyridinium moiety is delocalized and s t e r i cally shielded. T h e r e f o r e , t h e C T absorption of 3 d e p e n d s s t r o n g l y
on t h e electrophilic s o l v a t i o n power o f t h e s o l v e n t s , i. e. on
t h e i r HBD ability and L e w i s acidity ( = EPA behaviour), r a t h e r t h a n
on t h e i r nucleophilic s o l v a t i o n capability ( = EPD behaviour).
ET(30)-values a r e s i m p l y defined a s m o l a r t r a n s i t i o n e n e r g i e s
(in kcal/mol) o f betaine dye 3 , dissolved i n t h e s o l v e n t u n d e r
study (cf. Figs. 1 and 2 ) . A high ET(30)-value c o r r e s p o n d s t o high
s o l v e n t polarity. ET(30)-values r a n g e f r o m 30.7 f o r t e t r a m e t h y l silane (the l e a s t polar solvent) up to 63.1 k c a l l m o l f o r w a t e r
t h e most polar solvent); t h e y are known f o r more t h a n 300 o r g a n i c
s o l v e n t s and numerous binary s o l v e n t m i x t u r e s (ref. 1). Only for
t w o g r o u p s o f s o l v e n t s are ET(30)-values
n o t directly a v a i l a b l e :
acidic s o l v e n t s and perfluorohydrocarbons. I n a c i d i c s o l v e n t s ,
dye 3 is protonated and t h e solvatochromic C T a b s o r p t i o n band disappears, and i n perfluorohydrocarbons t h e betaine d y e s 3 and 4 a r e
not soluble. I n order t o i n c r e a s e t h e betaine solubility i n nonpolar s o l v e n t s , t h e l i p o p h i l i c hepta-tert-butyl-substituted d y e 5
and t h e bis-(1-adamanty1)-tris-tert-butyl-substituted
dye 6 as
w e l l a s t h e 'fluorophilic' eicosafluoro-substituted d y e 7 and t h e
penta-trifluoromethyl-substituted d y e 8 have been r e c e n t l y s y n t h e sized (ref. 18). U n f o r t u n a t e l y , a l l f o u r betaine d y e s 5 - 8 a r e not
soluble i n perfluorohydrocarbons s u c h a s perfluorooctane and perfluorodecalin. H o w e v e r , t h e introduction o f electron-withdrawing
groups r e d u c e s t h e basicity o f t h e phenoxide o x y g e n and permits
t h e direct determination of ET-values o f 7 i n s l i g h t l y a c i d i c solvents s u c h a s hexafluoro-2-propanol. By m e a n s o f a particular
Negative Sdvotochromism
Solvent
t
change
-
Thermo
Solvo tochromism
Genuine
Holochromism
-
Chiro
Solva tochromism
Fig. 2 . Different responses ('chromisrns') of betaine dye 3 to
various environmental changes (i.e. solvent, temperature,
external pressure, addition of salts, chiral solvents).
Soluteholvent interactions and their empirical determination
2597
d i l u t i o n t e c h n i q u e , E T ( 3 0 ) - v a l u e s o f 3 f o r 55 s u b s t i t u t e d p h e n o l s
h a v e been d i r e c t l y o b t a i n e d ( r e f . 1 9 ) . O n t h e o t h e r h a n d , t h g
s o l u b i l i t y o f b e t a i n e d y e 3 i n w a t e r i s a l s o v e r y low ( < 2.10
m o l / l ) . T h e r f f o r e , by i n t r o d u c t i o n o f h y d r o p h i l i c s u b s t i t u e n t s
, SO C H 3 ) i n t o 3 , i t s s o l u b i l i t y i n w a t e r h a s b e e n
( e . g . CO2:Na
s l i g h t l y increase2 (ref. 20). Quite recently, a reasonable estimate
f o r t h e ET(30)-value o f b e t a i n e dye 3 a s i s o l a t e d molecule i n t h e
g a s p h a s e ( = 2 7 . 1 k c a l / m o l ) h a s b e e n made ( r e f s . 1 4 b , 2 1 ) .
The e m p i r i c a l s o l v e n t p o l a r i t y p a r a m e t e r E T ( 3 0 ) h a s f o u n d m a n i f o l d a p p l i c a t i o n s some o f w h i c h a r e c o m p i l e d i n Scheme 2 . T h e a p p l i c a t i o n of t h e ET(30)-values t o chemical r e a c t i v i t y ( r e f . 22)
and a n a l y t i c a l chemistry ( r e f . 23) h a s been r e v i e w e d .
I n a d d i t i o n t o i t s n e g a t i v e solvatochromism, pyridinium N-pheno x i d e b e t a i n e d y e s s u c h a s 3 e x h i b i t a l s o t h e phenomena o f t h e r m o s o l v a t o c h r o m i s m , p i e z o - s o l v a t o c h r o m i s m , g e n u i n e h a l o c h r o m i s m , and
p o s s i b l y c h i r o - s o l v a t o c h r o m i s m , a s s c h e m a t i c a l l y shown i n F i g . 2 .
( r e f . 1 6 ) . That i s , t h e p o s i t i o n o f t h e long-wavelength CT a b s o r p t i o n b a n d o f d i s s o l v e d 3 d e p e n d s on t h e s o l u t i o n t e m p e r a t u r e ( r e f .
2 4 ) , on e x t e r n a l p r e s s u r e ( r e f . 2 5 ) , on t h e a d d i t i o n o f s a l t s ( r e f .
2 0 b , 2 6 ) , a n d , i n t h e c a s e o f c h i r a l b e t a i n e d y e s s u c h a s 10 and
11 p o s s i b l y on t h e c o n f i g u r a t i o n o f e n a n t i o m e r i c a l l y p u r e s o l v e n t s i n which t h e y a r e d i s s o l v e d ( r e f . 2 7 ) .
P o l a r i t y o f ca. 300 S o l v e n t s
and N u m e r o u s B i n a r y S o l v e n t
Mixtures
T
Q u a n t i t a t i v e A n a l y s i s o f Binary
Mixtures of Solvents with
Different Polarity
Aqueous Electrolyte Solutions
I’
/
LFE Correlations with Other
Solvent-Dependent Reactions
and Absorptions
Solutions
€T (30 ) -VCI Iues
-
Liquid S a l t s
(Pienta; P o o l e ; Reichardt;
,
Polarity o f Alumina Surfaces in
A d s o r p t i o n Liquid C h r o m a t o g r a p h y
(Dorsey)
f
P o l a r i t y of S u p e r c r i t i c a l - F l u i d
(SCF) S o l v e n t s (e.g. C 0 2 , F r e o n )
( B e r g e r ; Hyatt)
-
M o b i l e P h a s e P o l a r i t y in R e v e r s e d P h a s e L i q u i d C h r o m a t o g r a p h y (RPLC)
(Carr: Dorsev)
P o l a r i t y o f t h e I n t e r f a c i a l MicroEnvironment of Micro-Heterogeneous
S o l u t i o n s (e.g. m i c r o - e m u l s i o n s ,
micelles, vesicles, phospholipid
bilayers, surfactants, tensides)
(Baumgartel; Drummond; Evans;
Scheme 2. A p p l i c a t i o n s o f ET(30)-values
2598
C. REICHARDT ef a/.
Are a q u e o u s o r n o n a q u e o u s e l e c t r o l y t e s o l u t i o n s m o r e p o l a r
t h a n t h e p u r e s o l v e n t s ? The a d d i t i o n of s a l t s t o s o l u t i o n s o f
b e t a i n e dye 3 causes hypsochromic s h i f t s of its solvatochromic
CT a b s o r p t i o n b a n d , c o r r e s p o n d i n g t o a n i n c r e a s e o f t h e E T ( 3 0 ) v a l u e . F o r e x a m p l e , t h e a d d i t i o n o f L i I , N a I , KI, R b I , CsI, C a I ,
S r 1 2 , a n d Ba12 t o s o l u t i o n s o f 3 i n a c e t o n i t r i l e l e a d s t o a d i g f e r e n t i a l hypsochromic band s h i f t w i t h t h i s e l e c t r o l y t e (ionophore)
o r d e r , i.e. with i n c r e a s i n g charge d e n s i t y (= i o n charge/Pauling
c a t i o n r a d i u s ) o f t h e c a t i o n (ref. 2 6 a ) . The hypsochromic s h i f t s
a r e concentration-dependent, t h e y i n c r e a s e w i t h i n c r e a s i n g s a l t
concentration, Surprisingly, there exists a linear correlation
between t h e ET(30)-values of t h e e l e c t r o l y t e s o l u t i o n s s t u d i e d
and t h e c h a r g e d e n s i t y o f t h e c a t i o n s of t h e s a l t s a d d e d , a s shown
i n F i g . 3. O b v i o u s l y , l o o s e i o n - p a i r f o r m a t i o n b e t w e e n t h e phenoxide oxygen atom and t h e metal c a t i o n i n c r e a s e s t h e i o n i z a t i o n
energy o f t h e electron-donor moiety of 3, whereas t h e electrona c c e p t o r p a r t of 3 ( t h e p y r i d i n i u m m o i e t y ) r e m a i n s u n c h a n g e d ( n o
a s s o c i a t i o n w i t h t h e a n i o n o f t h e s a l t s a d d e d ) . As r e s u l t , t h e
i n t r a m o l e c u l a r CT a b s o r p t i o n b a n d is h y p s a c h r o m i c a l l y s h i f t e d ,
corresponding t o an increase i n t h e ET(30)-value and, hence, t o
a n i n c r e a s e o f t h e p o l a r i t y o f t h e s o l v e n t by t h e a d d i t i o n of
salts.
Fig. 3 . L i n e a r c o r r e l a t i o n
between E (30) o f b e t a i n e
d y e 3 , measured i n a c e t o -
n i t r i l e , a f t e r addition of
an excess o f s a l t , and t h e
effective charge of t h e
respective metal cation M ~ +
o f t h e s a l t added ( s l o p e =
0.992; r = 0.989) ( r e f . 2 6 a ) .
T h e s a l t - d e p e n d e n t s o l u t i o n s p e c t r a o f 3 c o n s t i t u t e a new t y p e
of genuine halochromism, i n c o n t r a s t t o t h e t r i v i a l halochromism
f i r s t f o u n d by B a e y e r a n d V i l l i g e r (Munich 1 9 0 2 ) . E x a m p l e s o f t h i s
t r i v i a l halochromism a r e acid/base r e a c t i o n s i n s o l u t i o n i n which
a colourless reactant forms a coloured product during a chemical
r e a c t i o n , a s shown i n Scheme 3. T h e g e n u i n e h a l o c h r o m i s m o f 3 ,
h o w e v e r , i s p r o d u c e d w i t h o u t a n y c h e m i c a l a l t e r a t i o n of t h e h a l o c h r o m i c d y e ! We h a v e r e c e n t l y p r o p o s e d t o s p e a k o f n e g a t i v e
( p o s i t i v e ) g e n u i n e h a l o c h r o m i s m t h e n o n l y when t h e U V / V i s a b s o r p t i o n band of a d i s s o l v e d compound i s h y p s o c h r o m i c a l l y ( b a t h o c h r o m i c a l l y ) s h i f t e d o n a d d i t i o n of a n e l e c t r o l y t e , a n d when t h i s
b a n d s h i f t i,s n o t a c c o m p a n i e d b y a c h e m i c a l a l t e r a t i o n o f t h e
chromophore (refs. 20b, 26a).
2599
So/ute/so/ventinteractions and their empirical determination
,,Trivial
"
Halochromism.
A. Baeyer, 1! Vill;ger, Ber. Dtsch.Chem. Ges.
Am,,.
331 nm (EtOH)
Amox-
U 7 nrn (H,SO,
95% 1
orange -red
yellowish
Scheme 3 . E x a m p l e s f o r t h e
(1902) 1189 ; e.g.
'trivial'
halochromism o f Baeyer.
A p a r t i c u l a r c a t i o n - s e l e c t i v e genuine halochromism has been
r e c e n t l y o b s e r v e d i n s o l u t i o n s o f t h e new c r o w n - e t h e r s u b s t i t u t e d
b e t a i n e d y e s 9 . As s h o w n i n F i g . 4 , a d d i t i o n o f s o d i u m i o d i d e t o
a s o l u t i o n o f 9 (n = 0) i n a c e t o n i t r i l e l e a d s t o a family+ o f
c u r v e s w h i c h m e e t a t a n i s o s b e s t i c p o i n t . F o r [ 9 ( n = 0 ) + Na I-]
i n a c e t o n i t r i h e , t h e complex f o r m a t i o n c o n s t a n t K amounts t o ca.
8008 ( m o l / l ) As s h o w n i n F i g . 5 , a d d i t i o n o f 6 . 0 1 M p o t a s s i u m
i o d i d e t o a 0 . 0 0 1 M s o l u t i o n o f 9 ( n = 1) i n a c e t o n i t r i l e g i v e s
r i s e t o a c o l o u r change f r o m v i o l e t t o d a r k - r e d . A g a i n , an enf o r c e d i o n - p a i r f o r m a t i o n between a l k a l i m e t a l c a t i o n and t h e
phenoxide p a r t , supported by t h e crown-ether r i n g , i s r e s p o n s i b l e
f o r t h i s r a t h e r l a r g e h a l o c h r o m i c s h i f t o f t h e i n t r a m o l e c u l a r CT
band o f 9 . T h i s n e g a t i v e halochromism depends on t h e r a d i u s o f t h e
c a t i o n o f t h e added s a l t as w e l l as on t h e s i z e o f t h e crown-ether
r i n g . T h e r e f o r e , t h e new b e t a i n e d y e s 9 r e p r e s e n t a new c l a s s o f
cation-selective
chromo-ionophores,
p o s s i b l y u s e f u l as c a t i o n
i n d i c a t o r s ( r e f . 26b).
.
A
= 583nm (CH,CN)
A = 529 nm lCH3CN+ KI 1
c
Colour change from
violet to dark-red
Alnm
-
Fig. 4 . U V / V i s s p e c t r a o f d y e 9
( n = 0), m e a s u r e d in a c e t o n i t r i l e
b e f o r e ( c u r v e 1) a n d a f t e r a d d i t i o n o f i n c r e a s i n g amounts o f N a I
( c u r v e s 2 - 9); I s o s b e s t i c p o i n t
a t c a . 5 3 4 nm ( r e f . 2 6 b ) .
(ld-Crown-5I-Betaine
in CH3CN [c = 10-3molll)
A chromo-ionophoric dye
F i g . 5 . The h a l o c h r o m i c b e t a i n e d y e 9
(n = 1 ) as c a t i o n ( p o t a s s i u m ) s e l e c t i v e i n d i c a t o r (chromo-ionophore)
( r e f . 26b).
2600
C. REICHARDT e t a / .
F i n a l l y , t h e r a c e m i c b e t a i n e d y e 10 a n d i t s a c h i r a l c o u n t e r p a r t 11 ( m e s o - F o r m ) s h o u l d b e a b l e t o form d i a s t e r e o m o r p h i c s o l vates i n p a i r s o f homochiral s o l v e n t s [e.g. ( R ) - and (S)-butane1,2-diol],
g i v i n g r i s e t o a phenomenon c a l l e d c h i r o - s o l v a t o c h r o m i s m
( r e f . 27, 28).
The f i r s t p r e l i m i n a r y r e s u l t s w i t h s u c h c h i r a l
betaine dyes are promising ( r e f . 27).
Acknowledgements
I s i n c e r e l y t h a n k my c o w o r k e r s f o r t h e i r e n t husiam and t h e i r e x e l l e n t c o l l a b o r a t i o n . Ig r a t e f u l l y acknowledge
t h e f i n a n c i a l s u p p o r t f r o m t h e Deutsche Forschungsgemeinschaft,
Bonn, a n d t h e F o n d s d e r C h e m i s c h e n I n d u s t r i e , F r a n k f u r t ( M a i n ) .
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2601
Solute/solvent interactions and their empirical determination
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