Anisotropic Fluorescence of Donor-Acceptor- Substituted trans-Stilbenes in Solvents of Different Polarities and Low Viscosity *

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Notizen 1259

Anisotropic Fluorescence of Donor-Acceptor- Substituted trans-Stilbenes in Solvents of Different Polarities and Low Viscosity *

A. Kawski, M. Alicka, and D. Gloyna**

L u m i n e s c e n c e R e s e a r c h G r o u p , I n s t i t u t e o f P h y s i c s , U n i v e r s i t y o f G d a n s k , G d a n s k , P o l a n d

Z . N a t u r f o r s c h . 3 6 a , 1 2 5 9 - 1 2 6 0 ( 1 9 8 1 ) ; r e c e i v e d A u g u s t 2 8 , 1 9 8 1

T h e fluorescence a n i s o t r o p y ( F A ) a n d m e a n l i f e t i m e t f o f s u b s t i t u t e d trans-stilbenes w a s i n v e s t i g a t e d a t 2 9 3 K i n t h e n o n p o l a r a n d p o l a r low v i s c o s i t y solvents n - h e p t a n e , t o l u e n e , a c e t o n i t r i l e , d i m e t h y l f o r m a m i d e a n d n - p r o p a n o l . I n m o s t cases, t h e fluorescence w a s f o u n d t o b e s t r o n g l y a n i s o t r o p i c d u e t o a v e r y s h o r t l i f e t i m e o f t h e d o n o r - a c c e p t o r - s u b s t i t u t e d trans-stilbenes.

1. Introduction

Trans-stilbene and its donor-acceptor-substi- tuted derivatives exhibit not only photochemical trans-cis isomerization [1—3], but also particularly interesting fluorescent properties [4—7]. In para- position, the transition moment lies along their major axis [8, 9]. The rotational relaxation time in low-viscous solvents is comparable to the mean lifetime of the first singlet excited state, which accounts for high fluorescence anisotropy (FA) ***

observed for a series of substances in solvents with low viscosities [10, 11].

Here we present some further interesting results concerning the FA and mean fluorescence lifetime t f for eleven stilbene 1 derivatives in low-viscous nonpolar (n-heptane and toluene) and polar (aceto- nitrile, dimethylformamide DMF and n-propanol) solvents.

* C a r r i e d o u t u n d e r t h e research p r o b l e m M R . I . 5 .

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2. Experimental

Fluorescence Anisotropics (FA) were measured by automatic photoelectric recording, using an elastooptical quartz light modulator with frequency modulation of 52.4 kHz and a quartz Arago com- pensator [12]. Mean fluorescence lifetimes, TF, were obtained with a modified Bauer fluorimeter [13].

The accuracy of the TF measurement was ± 10 ps.

3. Results and Discussion

In Table 1 the obtained values of TF and FA are given. The viscosities of the nonpolar solvents n-heptane and toluene differ only slightly from those of the polar solvents DMF and acetonitrile, in contrast to the relatively large viscosity of n-propanol. The effect of the viscosity upon the FA is obvious (e.g. l a , Id, l e with TF equal in different solvents). The magnitude of the FA, however, is strongly related to the mean molecular fluorescence lifetime TF- In the case of very short TF (e.g. l a , l b , Id, I f , l h ) , high FA can be observed despite low solvent viscosities. For molecules 1 c, however, TF is very long, and hence the FA becomes very low.

The FA of molecules with equal TF in different

[ 1 ] H . G ö r n e r a n d D . S c h u l t e - F r o h l i n d e , J . P h y s . C h e m . 8 2 , 2 6 5 3 ( 1 9 7 8 ) ; B e r . Bunsenges. p h y s i k . C h e m . 8 1 , 7 1 2 ( 1 9 7 7 ) .

[ 2 ] D . J . S. B i r c h a n d J . B . B i r k s , C h e m . P h y s . L e t t e r s 3 8 , 4 3 2 ( 1 9 7 6 ) .

[ 3 ] J . B . B i r k s , C h e m . P h y s . L e t t e r s 3 8 , 4 3 7 ( 1 9 7 6 ) . [ 4 ] M . A l i c k a , R . K . B a u e r , a n d A . K a w s k i , Z . N a t u r -

forsch. 3 5 a , 8 9 6 ( 1 9 8 0 ) .

[ 5 ] I . G r y c z y n s k i , D . G l o y n a , a n d A . K a w s k i , Z . N a t u r - forsch. 3 5 a , 7 7 7 ( 1 9 8 0 ) .

[ 6 ] D . G l o y n a , A . K a w s k i , a n d I . G r y c z y n s k i , Z . N a t u r - forsch. 3 5 a , 1 1 9 2 , 1 4 1 1 ( 1 9 8 0 ) .

[ 7 ] D . G l o y n a , I . G r y c z y n s k i , a n d A . K a w s k i , Z . N a t u r - forsch. 3 6 a , 6 2 6 ( 1 9 8 1 ) .

solvents having equal viscosities and different di- electric constants (e.g. l a , I d and l e , in n-heptane and acetonitrile) reveals a distinct effect of e upon the FA, the FA increases with growing e, contrary to the phenomenon observed previously [14]. It is difficult to determine unequivocally the effect of e upon the FA for the remaining 1 substances, as the influence of such quantities as TF and rj is sub- stantial.

As shown in Table 1, in the polar solvents DMF and acetonitrile, TF is not monotonic function of the acceptor activity of the R' substituent, in contrast to the nonpolar solvent case (n-heptane and toluene [6]). In both polar solvents, TF attains a minimum value for the substituted halogenoderivatives l h and l i in the series l g , l b , l h — l k , which has already been observed qualitatively for the series of substances mentioned above, both in non-polar and polar solvents, by measuring the FA, i.e. by an independent method (compare the values of TF and r in Table 1).

Compound l b is an exception, it having a too large ratio r\xp. This can be accounted for, com- paring the remaining substances of the series l g , l b , l h — l k , by the fact that l b exhibits a slower rotational motion in these solvents. Molecule l b with its large tetrahedral diphenylphosphinyl group serving as an acceptor, differs most strongly from the other substances of this series, due to its ge- ometry.

We thank Professor A. Schmillen (University of Giessen) for his valuable remarks.

[ 8 ] A . K a w s k i , I . G r y c z y n s k i , C h . J u n g , a n d K . - H . H e c k - n e r , Z . N a t u r f o r s c h . 3 2 a , 4 2 0 ( 1 9 7 7 ) .

[ 9 ] W . L i p t a y , D i p o l e M o m e n t s a n d P o l a r i z a b i l i t i e s o f M o l e c u l e s i n E x c i t e d E l e c t r o n i c S t a t e s , i n E x c i t e d S t a t e s V o l . 1, A c a d . Press, L o n d o n 1 9 7 4 , p . 1 2 9 - 2 2 9 . [ 1 0 ] W . L i p t a y , H . J . S c h u m a n n , a n d F . P e t z k e , C h e m .

P h y s . L e t t e r s 3 9 , 4 2 7 ( 1 9 7 6 ) .

[ 1 1 ] A . K a w s k i a n d M . A l i c k a , Z . N a t u r f o r s c h . 3 4 a , 1 3 7 1 ( 1 9 6 9 ) ; 3 5 a , 7 7 5 ( 1 9 8 0 ) .

[ 1 2 ] A . K a w s k i , Z . K o j r o , a n d M . A l i c k a , Z . N a t u r f o r s c h . 3 5 a , 1 1 9 7 ( 1 9 8 0 ) .

[ 1 3 ] R . K . B a u e r a n d K . J . R u d i k , A c t a P h y s . P o l o n . 3 5 , 2 5 9 ( 1 9 6 9 ) .

[ 1 4 ] A . K a w s k i , J . K u k i e l s k i , a n d J . K a m i r i s k i , Z . N a t u r - forsch. 3 3 a , 1 2 2 8 ( 1 9 7 8 ) .

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