Photophysics of Polymers - American Chemical Society

Chapter 16

Configurational and Conformational Aspects of Intramolecular Excimer Formation

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch016

F. C. De Schryver, P. Collart, R. Goedeweeck, F. Ruttens, F. Lopez Arbelao, and M . Vander Auweraer Department of Chemistry, K . U . Leuven, Celestijnenlaan 200F, B3030 Heverlee, Belgium

In a polymer the photophysical behavior of the singlet excited state is influenced strongly by the configuration and the conformational distribution within each configuration. This i s i l l u s t r a t e d in this paper by the excimer formation of 2,4-diarylpentanes and bis(pyrenylalanine)peptides.

1

S i n c e t h e f i r s t r e p o r t o f t h e pyrene excimer by Fôrster intermolecular excited state interactions have become a key s u b j e c t i n p h o t o c h e m i s t r y and p h o t o p h y s i c s . From c a l c u l a t i o n s ^ * ^ a p i c t u r e o f t h e e x c i t e d s t a t e complex a r i s e s i n w h i c h t h e two chromophores, i f t h e y a r e p l a n a r , a r e a t an optimum d i s t a n c e o f 3.5 Angstroms i n a p l a n e p a r a l l e l o r i e n t a t i o n . The s t a b i l i z a t i o n o f an e x c i m e r i s much more dependent on t h e o v e r l a p between t h e two chromophores t h a n an e x c i p l e x i n w h i c h t h e c o u l o m b i c a t t r a c t i o n , e s p e c i a l l y i n non p o l a r s o l v e n t s , i s t h e main s t a b i l i z i n g factor. I n t e r m o l e c u l a r e x c i t e d s t a t e complex f o r m a t i o n c a n be d e s c r i b e d by t h e k i n e t i c scheme 1 *

A*

---1

A* A

* +_J53W1 — > M

(AM)* -~-

4

>

A + hv

>

A ( A M )

*

> A* + M 1

(AM)*

> A+ M + hv' 6

(AM)* ~->A + M k^ = k^ + k2 and kg = k$ + kg Scheme 1 0097-6156/87/0358-0186$06.00/0 © 1987 American Chemical Society

Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16.

187

Intramolecular Excimer Formation

DESCHRYVERETAL.

>x

A i s t h e l o c a l l y e x c i t e d s t a t e (LE) and (AM)* i s t h e e x c i t e d s t a t e complex ( E ) . The f l u o r e s c e n c e spectrum c o n s i s t s o f t h e e m i s s i o n o f L E , hv', a t h i g h e r e n e r g i e s and o f t h e complex E, which e m i t s a t lower e n e r g i e s , h v ,due t o t h e s t a b i l i s a t i o n and r e p u l s i o n energy terms In case o f a δ e x c i t a t i o n t h e time dependence o f t h e e m i s s i o n can be d e s c r i b e d by t h e f o l l o w i n g e q u a t i o n s " 1

k

I (t)» I LE

a

b

l [(β - X ) e x p ( - 3 t ) + ( X - β )βχρ(-β ί)]

s

2

1

ι

eq.l

2

&z - h k k [M] 5


ι

χ

κΜ

T

3

e x

abs

I

(

P "

"

e x

(

P "P2

t ) ]

e
L£., and o f t h e c o m p l e x ^ g , a n d t h e r a t i o t h e r e o f can be f o r m u l a t e d as follows: >98%) i n t h e TG c o n f o r m a t i o n . The excimer f o r m i n g s t e p i s one r o t a t i o n around one bond t o form t h e TT c o n f o r m a t i o n i n w h i c h t h e two chromophores o v e r l a p e x t e n s i v e l y . ( f i g . 2 ) c

c

Q


Intramolecular Excimer

Formation


D E S C H R Y V E R E T AL.

TG

TT

F i g . z . S p a c e f i l l i n g models r e p r e s e n t i n g t h e TG ground s t a t e and TT* e x c i t e d s t a t e . T h e a c t u a l excimer s t r u c t u r e must have t h e two pyrene groups f a r t h e r a p a r t f o r s t e r i c reasons.


189

PHOTOPHYSICS OF POLYMERS

190

Scheme 2 hv TG TG*

> TG* > TT*

Important parameters o f the p h o t o p h y s i c s i n t a b l e 1.

o f 1 a-c are assembled

T a b l e 1 K i n e t i c and thermodynamic parameters i n t h e excimer of l a - c i n i s o o c t a n e

formation


c

parameter

l

1 a

FWMH cm" e m i s s i o n max. nm E kJmol" k 10 s~ k g ^ a t 20°C i n ns ΔΗ° k J m o l " AS° J K " m o l " a

1

3

1 0

l b

3

1

1

a)excimer

1

c

a

3400 420 16 310 37 -20±3 -17±5

l

c

b

3900 330 10 10 22 --

l

c

c

3800 485 17 60 145 -16 -42

e m i s s i o n b ) p r e e x p o n e t i a l term

S u b s t i t u t i o n o f t h e c e n t r a l methylene group by an oxygen no l o n g e r p e r m i t s t h e u s e o f t h e above mentioned nmr method t o o b t a i n i n f o r m a t i o n on t h e c o n f o r m a t i o n a l d i s t r i b u t i o n f o r t h e meso d i a stereoisomer. An a n a l y s i s o f t h e luminescence b e h a v i o r o f I Q C ^ - a s i n g l e e x p o n e n t i a l decay a t -30 °C i n i s o o c t a n e when a n a l y s e d i n t h e l o c a l l y e x c i t e d s t a t e , an a c t i v a t i o n energy f o r excimer f o r m a t i o n o f 17,6 k J m o l " i n i s o o c t a n e , t h e presence o f o n l y one excimer w i t h a k g o f 85 ns and a FWHM o f 3800 cm" - suggests a TG ground s t a t e conformation. These r e s u l t s a l l o w t h e c o n c l u s i o n t h a t i n a b i c h r o m o p h o r i c m o l e c u l e l i n k e d by t h r e e c a r b o n - c a r b o n bonds o r i n t h e e t h e r analog w i t h a w e l l d e f i n e d c o n f o r m a t i o n o f t h e c h a i n , a symmetric sub s t i t u t i o n a t t h e chromophore and i n absence o f i n t e r a c t i o n s i n t h e ground s t a t e excimer f o r m a t i o n c a n be d e s c r i b e d by a i n t r a m o l e c u l a r v e r s i o m o f t h e k i n e t i c scheme 1. Nmr a n a l y s i s o f l e^^ i n d i c a t e d t h a t t h e meso isomer i n a l k a n e s o l v e n t s a t room temperature has m a i n l y (>95%) a TG c h a i n c o n f o r m a t i o n w h i l e a t lower t e m p e r a t u r e s o n l y t h e TG c o n f o r m a t i o n i s p r e s e n t . N e v e r t h e l e s s t h e time p r o f i l e o f t h e f l u o r e s c e n c e a t -50^C can n o t be a n a l y s e d as a s i n g l e e x p o n e n t i a l b u t good f i t s a r e o b t a i n e d upon a n a l y s i s o f t h e l o c a l l y e x c i t e d s t a t e as a sum o f two exponentials. Spectral information c l e a r l y indicates that a t t h i s t e m p e r a t u r e no back d i s s o c i a t i o n o f t h e complex o c c u r s . ^ A s u b s t a n t i a l s h i f t o f t h e excimer e m i s s i o n maximum , a broadening o f t h e f u l l w i d t h a t medium h e i g h t (FWMH) o f t h e excimer e m i s s i o n and t h e a n a l y s i s o f t h e f l u o r e s c e n c e decay a c r o s s t h e excimer band 1

1

_ 1

1

c

1


16.


DESCHRYVERETAL.

c l e a r l y e s t a b l i s h e d t h e presence o f two e x c i m e r s . were o b t a i n e d f o r l e . ( t a b l e 2)

191

Analogous

results

Q

T a b l e 2 E x c i t e d s t a t e p r o p e r t i e s o f l e and l e i n i s o o c t a n e Q

Q

parameter a

max. 298 K nm λ max. 183 K nm FWMH(298 K ) cm" FWMHU83 K ) cm" α / α a t 450 nm c^/c^ β^ i n n s a t 450 nm 32~* i n ns at450 nm n ns a t 520 nm &C\ iit ns a t 520 nm 82'* iinn ns λ

a

a

1

a

1

b

χ

2

a

t

5


1

2

0

n

m

c

^'foJlO

1

1

€

E ( k J mol" ) *3 p > s - ')5^ E p (kJ mol" )* 3 i f o

1

0

1

1

3

Q

472 492 4800 4400 1.5 15 160 80 161 80 330 19 7 8

480 500 4300 3800 1.44 8.5 80 47 80 47 150 21 2 12

a) o f t h e excimer e m i s s i o n b) r a t i o o f t h e p r e e x p o n e n t i a l terms o f t h e two e x p o n e n t i a l decay f u n c t i o n d e s c r i b i n g t h e t i m e p r o f i l e o f t h e excimer e m i s s i o n measured a t 298 Κ i n i s o o c t a n e . c ) β exponential terms o f t h e decay f u n c t i o n . S i n c e t h e amount o f bac& d i s s o c i a t i o n a t t h i s temperature i s v e r y s m a l l t h e y can be s e t e q u a l t o k g . d ) p r e e x p o n e t i a l term o f t h e r a t e c o n s t a n t f o r f o r m a t i o n o f t h e l o n g l i v e d excimer.e) a c t i v a t i o n energy o f t h e r a t e c o n s t a n t f o r f o r m a t i o n of t h e l o n g l i v e d e x c i m e r . f ) p r e e x p o n e n t i a l term o f t h e r a t e c o n s t a n t f o r f o r m a t i o n o f t h e s h o r t l i v e d excimer.g) a c t i v a t i o n energy o f t h e r a t e constant f o r formation o f t h e s h o r t l i v e d excimer. χ 2

_ 1

The f o r m a t i o n o f two e x c i m e r s s t a r t i n g from one c h a i n con f o r m a t i o n was r e l a t e d t o t h e presence o f d i f f e r e n t rotamers o f t h e non s y m m e t r i c a l l y s u b s t i t u t e d 1 - p y r e n y l group i n t h e TG c o n f o r m a t i o n ( f i g . 3 ) . One o f t h e e x c i m e r s formed - t h e l o n g e r l i v e d one d e r i v e d from t h e T^G^ conformer - has a k g " v a l u e a t room temperature c l o s e t o t h e one o b s e r v e d f o r r e s p e c t i v e l y meso l c and meso l c and e m i t s a t l o n g e r w a v e l e n g t h s . A t low temperature i t i s t h e main c o n t r i b u t o r t o t h e excimer e m i s s i o n . T r a n s i e n t p i c o s e c o n d a b o r p t i o n s p e c t r o s c o p y ^ o f l c and l e s u b s t a n t i a t e s t h i s i n t e r p r e t a t i o n . S i n c e a TT c o n f o r m a t i o n o f l c r e s u l t s i n a f u l l o v e r l a p o f t h e two pyrene group a s i m i l a r s p a t i a l arrangement can be suggested f o r t h i s l o n g l i v e d excimer. The second excimer formed from o t h e r r o t a m e r ( s ) w i l l have o n l y p a r t i a l o v e r l a p o f t h e two chromophores. An u n e q u i v o c a l assignment o f t h e decay parameters t o t h e d i f f e r e n t rotamers f o r m i n g t h e two d i f f e r e n t e x c i m e r s c o u l d be made by t h e a n a l y s i s o f t h e decay of t h e l o c a l l y e x c i t e d s t a t e and t h e excimer decay a t 205 Κ where t h e l o n g l i v e d excimer i s t h e s o l e c o n t r i b u t o r and t h e l o c a l l y e x c i t e d s t a t e can be c o r r e l a t e d w i t h t h e growing i n o f t h i s e x c i m e r . This s u g g e s t s t h a t t h e r a t e o f r o t a t i o n o f t h e chromophore s h o u l d be s m a l l e r than t h e r a t e o f excimer f o r m a t i o n . B r o a d e n i n g o f t h e nmr s i g n a l o f t h e pyrene group a t 185K i n d i c a t e s slow r o t a t i o n o f t h i s 1

c

Q

1

Q

Q

Q




192

F i g . 3 . S c h e m a t i c r e p r e s e n t a t i o n o f two p o s s i b l e ground s t a t e c o n f o r m a t i o n s l e a d i n g t o two d i f f e r e n t e x c i m e r s of

l

c

c'


16.


DESCHRYVERETAL.

193

group a t t h e nmr time s c a l e and hence on t h e time s c a l e o f t h e p h o t o p h y s i c a l experiment. The p a r t i a l l y o v e r l a p p i n g excimer has a lower a c t i v a t i o n energy o f f o r m a t i o n due t o t h e f a c t t h a t a s t a b i l i s i n g i n t e r a c t i o n between t h e two pyrene groups s t a r t s e a r l i e r a l o n g t h e r e a c t i o n c o o r d i n a t e decreasing the activation barrier. The a c t i v a t i o n energy o f f o r m a t i o n o f t h e f u l l o v e r l a p excimer i s i n t h e non s y m e t r i c a l l y s u b s t i t u t e d compounds l e and l e , w i t h i n e x p e r i m e n t a l e r r o r , comp a r a b l e t o t h e a c t i v a t i o n e n e r g i e s observed f o r r e s p e c t i v e l y l c and c

e

c

These r e s u l t s i n d i c a t e t h a t t h e c o m p l e x i t y o f t h e f l u o r e s c e n c e decay o f meso l e and meso l e , where t h e pyrene i s non s y m m e t r i c a l l y s u b s t i t u t e d , i s n o t due t o a c o n f o r m a t i o n a l d i s t r i b u t i o n o f t h e c h a i n but t o r o t a t i o n a l isomerism around t h e carbon carbon bond l i n k i n g t h e chromophore t o t h e c h a i n backbone p r o v i d i n g t h e p o s s i b i l i t y t o form more t h a n one excimer i n t h e s e systems.


c

The Racemic

Q

Diastereoisomers

What a r e t h e k i n e t i c consequences i f t h e chromophores a r e l i n k e d by a c h a i n f o r w h i c h more t h a n one c h a i n c o n f o r m a t i o n contributes substantially? T h i s a s p e c t can be c o n s i d e r e d by a study o f t h e racemic d i a s t e r e o - i s o m e r s o f l c ' T h e racemic d i a s t e r e i s o m e r s l c a r e systems i n w h i c h t h e c h a i n i s p r e s e n t i n d i f f e r e n t conformations namely t h e TT and t h e GG conformers ( f i g 4 ) . I n both d e u t e r a t e d cyclohexane and d e u t e r a t e d c h l o r o f o r m , a t room temperature, t h e TT c o n f o r m a t i o n o f racemic l c is predominant : 80 % TT and 73 % TT r e s p e c t i v e l y . The observed s o l v e n t e f f e c t on t h e c o n f o r m a t i o n a l d i s t r i b u t i o n i s s i m i l a r but s m a l l e r than found f o r ( l e ) ^ . These r e s u l t s agree q u i t e w e l l w i t h t h o s e o b t a i n e d on s i m i l a r compounds^» 10»17,19 ^ I n t h e NMR d a t a o f t h e racemic d i a s t e r e o i s o m e r s o f l c and l c one i m p o r t a n t d i f f e r e n c e shows up. The p o s i t i o n o f t h e s i n g l e t absorption signal of oij t h e pyrene r i n g i s s h i f t e d s i g n i f i c a n t l y t o h i g h e r Ô v a l u e s f o r l c compared w i t h l c . The p o s i t i o n o f t h e HJL p r o t o n s i n t h e ^H-NMR spectrum i s a f u n c t i o n o f t h e c o n t r i - b u t i o n o f t h e TT c o n f o r m a t i o n s i n c e t h e s e p r o t o n s a r e s h i e l d e d by t h e mutual ring current effect o f t h e pyrene chromophores i n t h e TT conformation. I t can t h e r e f o r e be concluded t h a t t h e e q u i l i b r i u m between TT and GG i n t h e case o f l c i s s h i f t e d more toward t h e GG c o n f o r m a t i o n compared t o l c . T h i s c a n be e x p l a i n e d by comparing t h e bond l e n g t h s o f t h e C-0 bond and t h e C-C bond i n t h e r e s p e c t i v e compounds. The s h o r t e r e t h e r band induces a s t r o n g e r 1,4 i n t e r a c t i o n between t h e methine hydrogens and t h e pyrene chromophores i n t h e e t h e r compound c a u s i n g a s h i f t o f t h e c o n f o r m a t i o n a l e q u i l i b r i u m from TT t o GG. The steady s t a t e f l u o r e s c e n c e s p e c t r a o f l c and l c have an excimer f l u o r e s c e n c e band t h a t i s c o m p l e t e l y superimposable at a l l t e m p e r a t u r e s i n v e s t i g a t e d w i t h t h a t o f t h e i r r e s p e c t i v e meso d i a s t e r e o i s o m e r s s u g g e s t i n g an i d e n t i c a l g e o m e t r i c a l s t r u c t u r e o f t h e excimer and hence a l s o t h e p r e s e n c e o f o n l y one excimer. The f l u o r e s c e n c e decay c u r v e s o f l c and l c m o n i t o r e d a t 377 nm and 500 nm c a n be d e s c r i b e d by t h e same decay laws used f o r t h e r e s p e c t i v e meso d i a s t e r e o i s o m e r s . x

x

1

5

c

Q

c

Q

Q

c

Q

c

(

c

c

f

Q

Q




194

Fig.Λ.Space f i l l i n g r e p r e s e n t a t i o n o f t h e extended ( C 5 ) and f o l d e d ( C y ) c o n f o r m a t i o n o f 2 a e . I n t h e f o l d e d c o n f o r m a t i o n t h e pyrene group a r e d e p i c t e d i n t h e excimer geometry.



16.


DESCHRYVERETAL.

195

The f l u o r e s c e n c e decays o f t h e l o c a l l y e x c i t e d s t a t e can be a n a l y s e d as a two e x p o n e n t i a l decay f u n c t i o n i n t h e temperature domain between 298 Κ and 233 Κ . Below t h i s temperature t h e decays a r e one e x p o n e n t i a l . Furthermore e x c e l l e n t agreement between t h e decay parameters measured i n t h e l o c a l l y e x c i t e d and excimer r e g i o n of t h e e m i s s i o n spectrum i s o b t a i n e d . The m o l e c u l a r dynamics o f t h e racemic d i a s t e r e o i s o m e r upon e x c i t a t i o n a r e however more c o m p l i c a t e d than t h a t o f t h e meso diastereoisomers due t o t h e presence o f two ground state conformations. S i n c e t h e decay laws used f o r meso d i a s t e r e i s o m e r s can a l s o be a p p l i e d t o t h e racemic d i a s t e r e o i s o m e r , a l l t h e r a t e c o n s t a n t s d e s c r i b i n g t h e e q u i l i b r i u m between t h e ground s t a t e c o n f o r m a t i o n s must be l a r g e compared t o t h e r a t e c o n s t a n t o f excimer formation. The q u e s t i o n however still remains from which c o n f o r m a t i o n does excimer f o r m a t i o n t a k e p l a c e . I t i s not l i k e l y t h a t i t o c c u r s d i r e c t l y from t h e GG c o n f o r m a t i o n . T h i s would mean a s i m u l t a n e o u s r o t a t i o n around two bonds and would be a s s o c i a t e d w i t h a r e l a t i v e l y high a c t i v a t i o n b a r r i e r . One p o s s i b i l i t y i s t h a t excimer f o r m a t i o n t a k e s p l a c e from t h e TT c o n f o r m a t i o n . This p o s s i b i l i t y i s d e p i c t e d i n scheme 3. A t h i r d r o u t e c o u l d be excimer f o r m a t i o n s t a r t i n g from t h e TG conformation. T h i s i n t e r m e d i a t e c o n f o r m a t i o n between t h e TT and GG c o n f o r m a t i o n has however a r e l a t i v e l y h i g h energy c o n t e n t owing t o an u n f a v o u r a b l e i n t e r a c t i o n between t h e m e t h y l group and t h e pyrene moiety. Upon t a k i n g i n t o account t h i s p r e e q u i l i b r i u m t h e r a t e c o n s t a n t s and a c t i v a t i o n b a r r i e r s o f excimer f o r m a t i o n can be r e w r i t t e n i f scheme 3 i s c o n s i d e r e d u s i n g t h e f o l l o w i n g e q u a t i o n s : k

obs

=

f

k

e (

TT 3 K

I"TT

l

1 0

K

1 2 eq 12

=

1 + K

L

+ κ κ χ

2

Ki= k / k .

a

eq 13

K = k. /k

b

eq 14

a

2

b

ΔΗ° E

obs

=

E

2

+ α+Κ )ΔΗ° 2

1 e c

3 +

l

1 5

1 + K + κ κ In these equations k and E a r e t h e observed r a t e c o n s t a n t and a c t i v a t i o n b a r r i e r o f excimer f o r m a t i o n . The f r a c t i o n s o f t h e TT or TG c o n f o r m a t i o n s a t a g i v e n temperature a r e r e p r e s e n t e d by f-p^ and TG" I t c o u l d be s h o w n t h a t s i n c e t h e f r a c t i o n o f t h e TT c o n f o r m a t i o n i s c o n s i d e r a b l e and decreases w i t h i n c r e a s i n g temperature Scheme 3 s h o u l d l e a d t o a n e g a t i v e d e v i a t i o n from l i n e a r i t y i n t h e A r r h e n i u s p l o t o f t h e r a t e c o n s t a n t o f excimer f o r m a t i o n . T h i s c o u l d be e x p e r i m e n t a l l y v e r i f i e d p r o v i n g t h e v a l i d i t y o f scheme 3 The k i n e t i c and thermodynamic d a t a o f l c and l c a r e sum m a r i z e d i n t a b l e 3. I f the linear part i n the Arrhenius p l o t i s l

χ

o b s

2

o b s

f

1 S

13

c

Q




196


16.


DESCHRYVERETAL.

197

e x t r a p o l a t e d t o h i g h e r t e m p e r a t u r e s , an e s t i m a t e o f f ^ can be made. T h i s assumes t h a t t h e e x t r a p o l a t e d r a t e c o n s t a n t s a t a l l g i v e n t e m p e r a t u r e s a r e e q u a l t o t h e v a l u e o f k . The v a r i a t i o n o f f ^ w i t h temperature i n t h e h i g h temperature r e g i o n (303 K-343 K) can t h e n be used t o determine Δ Η and A S f o r the conformational e q u i l i b r i u m betwee TT and GG o f t h e r a c e m i c d i a s t e r e o i s o m e r . I n t h e case o f l c t h e r e s p e c t i v e v a l u e s a r e 12,2 k J m o l " and 35 Jmol K " . In the case of l c these values are 5 k J mol" and 12 J m o l ^ K " respectively. The c o n f o r m a t i o n a l d i s t r i b u t i o n between TT and GG can t h e n be c a l c u l a t e d and e q u a l s 70 % TT/ 30% GG f o r l c ' and 61 % TT/ 39 % GG f o r l c a t room t e m p e r a t u r e . 3

σ

Q

0

L

1

1

1

1

Q

c

Q

Table 3. K i n e t i c and thermodynamic parameters l c and l c i n i s o octane ( a ) : p r e - e x p o n e n t i a l f a c t o r i n t h e A r r h e n i u s e q u a t i o n ; (b) determined i n t h e l i n e a r p o r t i o n o f t h e A r r h e n i u s p l o t a t low temperature (243 K-203 K)


c

,

X

1

E ( k J m o l " ) (b) °obs ~ ) ) o b s

K

E

( s

( a

χ

1

1

C

Ί1

40 (± 1 3

8)

3 (± 2 ) x l 0

-14 (± 7) -47 (± 12)

0

1

U

5.0 (± D x i O

ΔΗ ( k J mol" ) ASoUmol" K" )

0

22 (± 2) 3.3 (±0.3)χ10

34 (± 5)

1

( s " ) (a)

5

X

8

( k J mol" )

5

C

20 ( ± 2 ) · (±0.2)xlO

1

k°

C

Q

1 4

-18 (± 10) -57 (± 30)

T h e r e s u l t s o b t a i n e d f o r t h e r a c e m i c d i a s t e r e o i s o m e r s l c ' and l c i n d i c a t e t h a t even when t h e r a t e o f c o n f o r m a t i o n a l change between t h e d i f f e r e n t c h a i n c o n f o r m a t i o n s i s much f a s t e r t h a n t h e r a t e o f excimer f o r m a t i o n , r e s u l t i n g i n f l u o r e s c e n c e decays s i m i l a r t o t h o s e o f t h e meso d i a s t e r e o i s o m e r s i t i s p o s s i b l e t o e x t r a c t some i n f o r m a t i o n on t h e c o n f o r m a t i o n a l d i s t r i b u t i o n o f t h e c h a i n from t h e obtained data. t

c

Q

BIS(PYRENYLALANINE)PEPTIDES By c h o o s i n g another l i n k between t h e two chromophpores i t i s p o s s i b l e t o slow down t h e r a t e o f c o n f o r m a t i o n a l i n t e r c h a n g e as t o be i n t h e same o r d e r o f magnitude as t h e i n v e r s e o f t h e f l u o r e s c e n c e l i f e t i m e o f t h e chromophore. T h i s was r e a l i z e d i n b i s ( p y r e n y l a l a n i n e ) p e p t i d e s o f t h e g e n e r a l s t r u c t u r e 2. CH CO-NH CHCO-NH CHCO-XCH 3

A

B

3

CH2

1

i

2 i f H^,Hg i s r e p l a c e d by a methylgroup 2 ( M E o r ME ) R = 1-pyrenyl = a R = 2-pyrenyl = b a

b

Structure 2


198


N - a c e t y i - b i s ( l - p y r e n y l ) a l a n i n e m e t h y l e s t e r 2a c a n occur i n two d i a s t e r e o i s o r a e r i c forms- t h r e o ( t ) and e r y t h r o ( e ) - w h i c h c o u l d be s e p a r a t e d by c h r o m a t o g r a p h y ^ . i i n e r t solvents,not capable of f o r m i n g hydrogen bonds,the r a t i o o f excimer e m i s s i o n over e m i s s i o n from t h e l o c a l l y e x c i t e d s t a t e i s s u b s t a n t i a l l y h i g h e r t h a n i n s o l v e n t s which can accept hydrogen bonds. The d e c r e a s e o f t h e emission ratio c a n be c o r r e l a t e d w i t h the T a f t basicity parameter.. These o b s e r v a t i o n s and t h e f a c t t h a t 3ae always has a h i g h e r e f f i c i e n c y o f excimer f o r m a t i o n than 3 a t were e x p l a i n e d by a c o n s e c u t i v e k i n e t i c scheme i n v o l v i n g t h e l o c a l l y e x c i t e d s t a t e o f pyrene i n two d i f f e r e n t c o n f o r m a t i o n s o f t h e p e p t i d e c h a i n : an extended c h a i n c o n f o r m a t i o n , C 5 , w i t h a l t e r n a t e s i d e c h a i n s ( f i g . 4 ) and a f o l d e d c o n f o r m a t i o n , C y , w i t h quasi p a r a l l e l side chains(fig.4) and supported by an i n t r a m o l e c u l a r hydrogen bond between the carbony1 f u n c t i o n o f the a c e t y l p r o t e c t i n g group and the amine f u n c t i o n o f t h e second p y r e n y l a l a n i n e r e s i d u e . Only t h e Cy c o n f o r m a t i o n a l l o w s r o t a t i o n o f t h e a r o m a t i c s i d e groups t o a p a r t i a l l y o v e r l a p p i n g excimer geometry w i t h i n t h e l i f e t i m e o f the e x c i t e d pyrene moeiety. T h i s p o i n t c o u l d be proven by t h e s u b s t i t u t i o n o f t h e hydrogen i n v o l v e d i n t h e hydrogen bond formation i n t h e Cy c o n f o r m a t i o n by a m e t h y l g r o u p 2(ME^)a r e s u l t i n g i n t h e d i s s a p e a r e n c e o f t h e excimer band i n t h e e m i s s i o n spectrum. The f l u o r e s c e n c e decay o f t h e l o c a l l y e x c i t e d s t a t e a t temper a t u r e s were t h e excimer does not d i s o c i a t e back c o u l d be a n a l y s e d as a sum o f two e x p o n e n t i a l s - * .That t h i s i s n o t due t o r o a t i o n a l i s o m e r i s m o f t h e 1-pyrenyl group c o u l d be proven by t h e a n a l y s i s o f 2b w h i c h showed analogous behavior.The a n a l y s i s o f t h e f l u o r e s c e n c e decay a c c o r d i n g t o scheme 4 p e r m i t s t h e d e t e r m i n a t i o n o f the r a t i o o f 2

n


2 1

2 2

2 2

2

c /c . 7

5

Scheme 4. j^fol

^ex

-fol -ex I n i n e r t s o l v e n t s the m o l e c u l e i s s t a b i l i s e d by f o l d i n g l e a d i n g to a h i g h Cy p o p u l a t i o n and hence a more i n t e n s e excimer e m i s s i o n than i n hydrogen bonding s o l v e n t s t h a t s h i f t the c o n f o r m a t i o n a l d i s t r i b u t i o n i n the ground s t a t e more t o t h e C5 conformer .When compaied i n t h e same s o l v e n t t h e t h r e o d i a s t e r e o i s o m e r has a lower f o l d e d p o p u l a t i o n then t h e e r y t h r o i n p a r t due t o i n c r e a s e d s t e r i c hinder a n c e and i n p a r t due t o t h e absence o f a s t a b i l i s i n g N-H pyrene interaction The s o l v e n t induced s h i f t o f the c o n f o r m a t i o n a l e q u i l i b r i u m was c o n f i r m e d , i n t o l u e n e a t -20°C a r a t i o o f 3 and 0.75 was c a l c u l a t e d f o r 2ae and 2 a t r e s p e c t i v e l y w h i l e i n e t h y l a c e t a t e these values d e c r e a s e t o 0.8 and 0.4. k

k

CONCLUSIONS I n t h i s c o n t r i b u t i o n we attempt t o e v a l u a t e t h e i n f l u e n c e o f c o n f o r m a t i o n a l and c o n f i g u r a t i o n a l a s p e c t s ,both on excimer f o r m a t i o n r a t e s



16.

D E S C H R Y V E R E T AL.


199

and on excimer p r o p e r t i e s . F r o m t h e r e s u l t s p r e s e n t e d , we c o n - c l u d e that the difference i n photophysical b e h a v i o r o f two d i f f e r e n t c o n f i g u r a t i o n s c a n be r e l a t e d t o t h e c o n f o r m a t i o n a l d i s t r i b u t i o n o f each d i a s t e r e o i s o m e r . I f o n l y one c o n f o r m a t i o n i s p r e s e n t complex f l u o r e s c e n c e decay s t i l l c a n a r i s e by t h e non s y m m e t r i c a l s u b s t i t u t i o n o f t h e chromophore l i n k e d t o t h e c h a i n . I f more t h e n one c h a i n c o n f o r m a t i o n i s p r e s e n t w i l l t h i s l e a d t o complexity i n the fluorescence decay o n l y i f t h e r a t e o f conf o r m a t i o n a l change i s comparable t o t h e r a t e o f excimer f o r m a t i o n a in the dipeptides. I n absence o f t h i s c o m p l i c a t i o n i t i s however n e c e s s a r y t o i n t e r p r e t t h e o b t a i n e d r e s u l t s t a k i n g i n t o account t h e f a s t p r e e q u i l i b r i u m between t h e d i f f e r e n t conformers. The n a t u r e o f t h e excimer formed and i t s p r o p e r t i e s w i l l f o r a g i v e n chromophore depend on t h e d i a s t e r e o i s o m e r . The b i n d i n g energy of t h e excimer w i l l be a f f e c t e d by s t a b i l i s i n g o r d e s t a b e l i s i n g e f f e c t s of the chain. Furthermore i f r o t a t i o n a l isomerism o f t h e chromophore i s p o s s i b l e and i n t e r c o n v e r s i o n between t h e rotamers i s slow compared t o excimer f o r m a t i o n t h e o c c u r e c e o f more then one e x c i t e d s t a t e complex i s p o s s i b l e and depends on t h e r e s p e c t i v e s t a b i l i s a t i o n o f t h e d i f f e t r e n t complexes.The r e l a t i v e c o n t r i b u t i o n of each a t a g i v e n t e m p e r a t u r e w i l l depend on t h e r o t a t i o n a l d i s t r i b u t i o n and t h e r e s p e c t i v e r a t e o f excimer f o r m a t i o n

Acknowledgments The a u t h o r s w i s h t o thank t h e B e l g i a n N a t i o n a l S c i e n c e F o u n d a t i o n f o r c o n t i n o u s s u p p o r t t o t h e laboratory.Dr.M.VdA i s a Reseach A s s o c i a t e of t h e NFWO.Dr.Boens i s thanked f o r h i s e s s e n t i a l c o n t r i b u t i o n t o t h e development o f t h e s o f t w a r e used i n t h e a n a l y s i s o f t h e f l u o r e s c e n c e decays o b t a i n e d by t i m e c o r r e l a t e d s i n g l e photon c o u n t i n g . REFERENCES

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Photophysics of Polymers - American Chemical Society

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