Chapter 27
Exciton Migration in Copolymers of Acenaphthylene
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W. R. Cabaness, S. A. Zamzam, and C. T. Chen Department of Chemistry, The University of Texas at El Paso, El Paso, T X 79968-0513
Excited states and energy migration by way of excitons have been studied in random copolymers of acenaphthylene (ACN) with acrylonitrile (AN), methacrylonitrile (MAN) and 2-vinyl napthalene (2VN). Also ACN was copolymerized with vinyl acetate and hydrolyzed to give free hydroxyl groups. The latter were reacted with acyl chlorides: 1-naphthoyl, 2-naphthoyl, benzoyl and cinnamoyl. Exciton migration lengths, L, quenching rate constants, k , and efficiencies of energy transfer were calculated based on steady state fluorescence spectra using oxygen or dimethylaniline as quenchers. The average exciton migration lengths are: 92.5 poly(ACN-co-AN), 130 poly(ACN-co-MAN), 100.5 poly(ACN-co-2VN) and 79.4 Åfor polymers containing ester groups. qe
The absorption of UV energy by fluorophores attached to a polymer chain and the subsequent energy migration by way of excitons are subjects of theoretical interest (1-3). Other workers (4) have described polymers containing pendant chromophores as "antenna polymers" which may function as energy gathering devices or allow singlet energy to proceed to a trap. More fundamental questions as the exact mechanism of exciton migration between fluorophores, the required orientation and maximum distance remain unanswered for many systems. We have studied excited states in polyacenaphthylene (PACN) and in random copolymers of acenaphthylene with the following comonomers: 2-vinylnaphthalene (2VN), acrylonitrile (AN), methacrylonitrile (MAN) and N-vinylcarbazole (VCz) (5.6). Also ACN was copolymerized with vinyl acetate, and the acetate groups were removed by hydrolysis. The hydroxylated copolymers, poly(acenaphthylene-co-vinyl alcohol), were reacted with the following acyl chlorides: 1-naphthoyl, 2-naphthoyl, benzoyl and cinnamoyl. The latter polymers contain an ester group as a spacer between the main chain and one of the fluorophores. The choice of an ester group as a connecting unit between the polymer chain and fluorophore is probably a poor one since i t is known that carbonyl groups easily form triplets and photo-Fries rearrangements can occur (7). A copolymer containing a dimethylsiloxy spacer was prepared by reacting poly(acenaphthylene-co-vinyl alcohol) with dimethyldichlorosilane followed by reaction with 2-naphthol. (See Figure 1). 0097-6156/87/0358-0358$06.00/0 © 1987 American Chemical Society
Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
27.
CABANESS ET AL.
Exciton Migration in Acenaphthylene
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Two types of copolymers of ACN were investigated i n t h i s work. The f i r s t type contains ACN units separated by photo-inactive groups in the main chain such as AN or MAN. This permits a comparison of the homopolymer, PACN, to polymers containing smaller numbers of chromophores and the l a t t e r being randomly spaced along the polymer chain. The second type i s a copolymer of ACN and another monomer containing different chromophore. In the f i r s t type only excited s i n g l e t , A*, and excimers, (AA)* are important states while i n the second type two excimers, (AA) and (BB)*, and the exciplex (AB)* are possible. Exciton hopping to the same chromophore and using equi-energetic states depend on dipole-dipole orientation and the interaction distance, R . 0
*
A
.
ο
+
A.
ο
A.
*
+
A.
In copolymers the exciton migration length i s limited by the polymer structure a v a i l a b l e , and most importantly, by polymer conformation. When exciton migration involves excimers or exciplexes, long sequences of excimer-forming-sites (EFS) must be correctly orientated for maximum distance. Poly(ACN) has a r i g i d chain structure yet can form excimers with alternate units along the chain ( 8 ) , or by stacking i n a h e l i c a l con formation. Excimer formation has been reported for alternate copoly mers of ACN with styrene (9) and for ACN with maleic anhydride (10). The situation i s different for 2-vinylnaphthalene since alternating copolymers of 2VN with methyl methacrylate or methacrylic acid did not form excimers, yet random copolymers of the same systems showed excimer fluorescence (11). Only random copolymers of ACN were prepared i n t h i s work. Quenching was carried out using oxygen or Ν,Ν-dimethylaniline to obtain steady state fluorescence spectra. Then exciton migration lengths were calculated by three methods described below with the assumption that the fluorophores show exponential decay. An arbitrary comparison between the migration length and the polymer's end-to-end distance was made. Since the migration length i s based on a one-dimensional random walk, a more meaningful comparison i s between migration lengths i n copolymers and homopolymers. In equation 1 the singlet exciton migration length, L, i s related to the Stern-Volmer constant, K , and the Forster encounter radius, R , and N i s Avogadro's number (12). s v
0
0
L = Dv^N^i
(1)
Webber and co-workers have proposed two equations which allow the calculation ofA /D where Λ i s the singlet energy migration rate and D i s the diffusion constant for the quencher (13). In equation 2, k|q i s the bimolecular rate constant for quenching of the polymer while k™ refers to a model compound o f low molecular weight such as ethylnapnthalene. e
Λ /D = [ ΐ ζ - ( l / 2 ) k ^ / ( l / 2 ) k ^
(2)
A similar equation 3 gives theA/D r a t i o when the model system i s a polymer containing only a small percentage of chromophore (11).
Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
PHOTOPHYSICS OF POLYMERS
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m
(3)
qe
Then the exciton migration length, L, assuming a one-dimensional ran dom walk, i s given by equation 4:
L = [2Λτ ]1
(4)
Ρ
where r
i s the l i f e t i m e of the excited state in the polymeric system.
Experimental Copolymers of ACN and AN were prepared with varied compositions by emulsion polymerization in water-ethanol (4:1 by vol) degassed with nitrogen. A surfactant was added and potassium persulfate was the initiator. Polymerizations were carried out at c a . 90 C for 24 h. Copolymers were precipitated into methanol and p u r i f i e d by subsequent solution i n benzene and p r e c i p i t a t i o n into methanol. The preparation of poly(ACN-co-2VN) was reported previously ( 6 ) . A l l samples of ACN used in polymerizations were r e c r y s t a l l i z e d twice from methanol-pentane (3:1). AN and MAN were d i s t i l l e d prior to use. Copolymers of ACN and MAN were prepared by solution poly merization in benzene at 60 C for 36 h. AIBN was the i n i t i a t o r , and the polymers were isolated by p r e c i p i t a t i o n into 600 mL of methanol followed by two subsequent precipitations from benzene into methanol. Vinyl acetate and ACN were copolymerized in dry benzene at 60°C for 36 h using AIBN as i n i t i a t o r . The molar r a t i o of monomers, v i n y l acetate to ACN was 20 to 1. Polymers were isolated by p r e c i p i t a t i o n into methanol, followed by two subsequent precipitations from benzene into methanol. Poly(ACN-co-vinyl acetate) (0.5 g) was dissolved in 100 mL of THF and mixed with 25 mL of 1 Ν methanolic NaOH. After heating to 60°C for 10 h, the mixture was precipitated into 600 mL of methanol. The polymer was p u r i f i e d by dissolving i n THF and p r e c i p i tation i n methanol. Poly(ACN-co-vinyl alcohol) (0.4 g) was dissolved i n 50 mL of pyridine and treated with benzoyl chloride in benzene. The mixture was s t i r r e d at room temperature for 10 h and precipitated into 600mL of methanol. The copolymer was p u r i f i e d by two p r e c i p i t a tions from THF into methanol. A similar procedure was used for reacting other acyl chlorides with poly(ACN-co-vinyl a l c o h o l ) . PolyLacenaphthyylene-co-dimethyl-(2-naphthoxy)-vinyloxysilaneJ was prepared by treating poly(ACN-co-vinyl alcohol)(0.4 g) i n 50 mL of THF with dichlorodimethylsilane (0.8 mL) and triethylamine (0.5 mL) at 60°C for 10 h. Then, 2-naphthol (1.0 g) i n 20 mL of THF was added and heating was continued for another 10 h at 60°C. The mixture was cooled and poured into 600 mL of diethyl ether. The polymer was p u r i f i e d by two precipitations from THF into ether. Copolymer compositions of poly(ACN-co-AN) and poly(ACN-co-MAN) were determined by elemental nitrogen analyses. The percentages of ACN in hydrocarbon copolymers were determined by UV spectroscopy, using the measured o p t i c a l density for ACN i n PACN at 310 nm i n 1,2-dichloroethane as solvent. For poly(ACN-co-2VN) the compositions were checked by NMR spectroscopy (300 MHz). In a l l cases, conversions were held to ten percent or less i n order to avoid d r i f t i n copolymer composition. e
C
o
Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
361
Exciton Migration in Acenaphthylene
27. CABANESS ET AL.
Steady state fluorescence spectra were obtained on an Amino-Bowman J4 instrument using a Hg-Xe l i g h t source. Samples were prepared immediately before use with 1,2-dichloroethane. Several excitation wavelengths 290, 325 and 353 nm were t r i e d with each series of ACN copolymers to obtain emission bands of highest inten sity. For quenching experiments using oxygen, samples were degassed with nitrogen, a i r and oxygen i n sequence (14). The concentrations of Ν,Ν-dimethylaniline when used as a quencher were zero, 2.3 and 12.9 χ 10" M. The d i f f u s i o n c o e f f i c i e n t for oxygen in 1,2-dichloroethane at 298°K was calculated to be 8.79 χ 10 cm /sec by the Stokes eguation (15). The following values were used in c a l c u l a t i o n s : τ , excimer in P2VN, i s 45 ns (17); k , model polymer, i s 1.52 χ 10 1 m - i s (11), k g , 2-ethylnaphthalefll, i s 4.96 χ 10 1 m-ls-1 (11). ~ 3
β
2
9
1
9
e
^
Results and Discussion Stern-Volmer plots were made from fluorescence spectral data and K values c a l c u l a t e d . Exciton migration lengths were calculated using equation (1) and equations (2), (3) and subsequently equation (4). This procedure gave three values of the migration length for comparison. Migration lengths in random copolymers of ACN with AN are not greatly affected by composition. Copolymers containing 47 and 80% ACN had the longest migration length of 70A compared to 63 A for PACN using equation (1). The above percentages correspond to an o v e r a l l monomer composition of 1:1 and 4 : 1 . Random poly(ACN-co-AN) showed monomeric fluorescence at 355 nm and excimer fluorescence at 405 nm. The IQ/IM r a t i o s were 0.88 and 0.61 for the 47 and 80% ACN copolymers, respectively, and the same value for PACN was 0.52. The incorporation of photo-inactive units such as AN in the copolymer increase chain f l e x i b i l i t y and allows more excimer formation. Exciton migration lengths are l i s t e d in Table I . For a series of poly(ACN-co-MAN) copolymers the amount of chro mophore, ACN, was limited to f i v e , seven and 14 percent to determine the e f f e c t on migration length and excimer formation. The r e s u l t s are given in Table I I including the end-to-end distances for com parison. A copolymer containing five percent ACN had the longest exciton migration distance of 149 A which exceeds i t s end-to-end distance of 31 A. The migration lengths decreased as the ACN content increased from five to 14 percent but in a l l cases were longer than the end-to-end distances. The fluorescence spectra of poly(ACN-co-MAN) showed monomer emission at 346 nm and excimer emission at 405 nm. The intensity of the excimer emission band decreased as the percentage of ACN decreased from 14 to 5% and shifted to a longer wavelength at 412 nm. Corresponding IQ/IM r a t i o s decreased from 0.33 ro 0.13, r e s p e c t i v e l y . The second type of copolymer studied in t h i s work contained ACN and another monomer which was also a fluorophore. A series of copoly mers of ACN and 2VN were prepared which varied in composition from 5.5 to 94.5% ACN. Calculated exciton migration lengths and K values are given in Table I I I . A copolymer containing 53.5% ACN or c a . 1:1 S v
s v
Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
PHOTOPHYSICS OF POLYMERS
362
(c)
(d)
Figure 1. Copolymers of acenaphthylene: (a) poly(ACN-co-MAN); (b) poly(ACN-co-2VN); (c) poly(ACN-co-vinyl ester) where acyl i s benzoyl, cinnamoyl, 1-naphthoyl and 2-naphthoyl; (d) poly [ACN-co-dimethyl-(2-naphthoxy)-vinyloxysilane].
Table I Compositions, Quenching Constants and Migration Lengths in Poly(ACN-co-AN) ,a b c d > " >A >A
mol % ACN in copolymer
K
M
36 47 66 76 78 80 90 95 100 a
b
K
s v
157 168 134 113 139 169 131 138 137
L
68 70 63 58 64 70 62 64 63
101 107 87 72 90 108 85 90 89
62 68 46 24 50 69 44 50 49
for excimer emission at 405 nm.
L = &
