Vol. 11, No. 5, Septernber-October 1978
1-Naphthyl Methacrylate-9-Vinylanthracene 925
Singlet Energy Transfer in a l-Naphthyl Methacrylate9-Vinylanthracene Copolymer J. S. Aspler,l* C. E. Hoyle,lb and J. E. Guillet* Department of Chemistry, University of Toronto, Toronto, Canada M5S 1 A l . Received April 17, 1978 ABSTRACT: Studies have been made of electronic energy transfer from the naphthalene groups in poly(l-naphthyl methacrylate) to vinylanthracene traps located a t chain ends. It is shown that the transfer is strictly intramolecular in dilute solution, and that the efficiency of transfer depends on the solvent power, being greater in solvents which tend to reduce the dimensions of the random coil. Studies of fluorescence decay times by photon counting suggest that the transfer of energy takes place in times of 1 ns or less, and temperature studies indicate that transfer occurs from naphthyl monomer rather than the excimer. Excimer formation has been observed in m a n y polymers b e a r i n g a r o m a t i c side g r o u p s (polystyrene, poly(viny1n a p h t h a l e n e ) , etc.).2-” The distinguishing f e a t u r e of M in monomer p o l y m e r excimers in d i l u t e solution groups) is that t h e y are completely intramolecular and thus t h e y are i n d e p e n d e n t of c o n c e n t r a t i o n . The excimer i n t e n s i t y i n polymers (expressed as t h e excimer t o m o n o m e r ratio, Ze/Zm) does n o t d e p e n d on t h e viscosity of the pure solvent. It does, for m o s t polymers, depend on whether the solvent is “good” (chain expanding) or “poor” (chain contracting). F o r e x a m p l e , i n poly(1n a p h t h y l m e t h a c r y l a t e ) ( P N M A ) 3 and in p o l y ~ t y r e n e , ~ excimer formation is greatest in poor polymer solvents and least in good polymer solvents. T h i s is usually explained in t e r m s of chain contraction in t h e poor solvent, resulting i n a s m a l l e r i n t e r c h r o m o p h o r e distance. T h e r e exist m a n y cases i n which excimer f o r m a t i o n is but one of several processes for t h e deactivation of singlet energy. A n o t h e r e x a m p l e of singlet energy t r a n s f e r involves t w o u n l i k e c h r o m o p h o r e s i n t h e same molecule. E a r l y s y s t e m s of t h i s t y p e a r e t h e 9 - a n t h r a c e n y l n a phthylalkanes ( n = 1,2,3).12 Only anthracene emission was seen w h e n the n a p h t h y l g r o u p was excited at 280 n m . Similar effects a r e well known in biological systems, where long-range F o r s t e r t r a n s f e r c a n a c t as a “spectroscopic ruler” for molecular d i m e n s i o n s and s t r u c t u r e . The mechanism of emission of excitation energy received b y a t r a p i n a polymer c h a i n m a y b e analogous to several well-known cases of “doped” crystals. An a n t h r a c e n e c r y s t a l w i t h a s m a l l a m o u n t of a d d e d t e t r a ~ e n e shows ’~ complete q u e n c h i n g of the expected a n t h r a c e n e emission w h e n excited in the UV. T e t r a c e n e emission only is seen. In the case of t h i s crystal system, t h e energy t r a n s f e r process arises f r o m a long-range exciton t r a n s f e r w i t h t e r m i n a t i o n at t h e t e t r a c e n e traps. F o x and co-workers h a v e s t u d i e d the energy t r a n s f e r process of polystyrene copolymerized w i t h (ca. 1 % ) 1vinylnaphthalene. T h e y f o u n d that t h e predominance of the s t y r e n e excimer t r a p s d i d not p r e v e n t transfer to t h e few naphthalene trapsea In the p r e s e n t work, the process of energy t r a n s f e r t o trap sites i n a high molecular weight polymer was studied b y observing the effect of solvent and t e m p e r a t u r e on t h e photoemission characteristics of a poly(1-naphthyl m e t h acrylate)-9-vinylanthracene copolymer (PNMA-VA).
were extracted with ether, washed with dilute NaOH and water, dried, and vacuum distilled. NMA monomer was distilled from a small amount of copper powder (which acts as an inhibitor to polymerization) and recrystallized from pentane at room temperature (NMA: mp 39-41 “C, bp 125 OC (1Torr); NIBA: mp 28-31 “C, bp 120 “C (1 Torr)). 9-Vinylanthracene (9-VA),9-methylanthracene (9-MeA) (Pfalz & Bauer), and 1-naphthol (BDH) were used as received. Methyl methacrylate monomer (MMA) (Eastman) was washed with NaOH and water, dried, and then distilled. Polymers were prepared by free-radical polymerization in benzene solution degassed to
