Macromolecules 1986, 19, 2480-2484
2480
of the gels observed in this experiment are well described by eq 5. In the above treatment, the concentration of ions is taken to be proportional to the irradiatian time. This is not true at the later stage of photoirradiation as shown in Figure 7A. Almost all ions are produced in a very short time and the concentration is considered to be close to Co. Therefore, IIionmay be expressed under continuous light irradiation as follows: nion=
CH2)(copolymer), 94352-04-2; (1, X = CN, Y = N(CH&, Z =
CH=CH2)(H2C=CHCONH2)*(H2C=CHCONHCHNHCOCH=CH,) (copolymer), 94352-06-4; NaCl, 7647-14-5; KBr, ma-02-3.
References and Notes Part 7: Irie. M.: Iwavanapi. - . T.:. Tanieuchi. . Y . Macromolecules 1985, 18, 2418. Preliminary communication: Irie, M.; Kunwatchakun, D. Makromo1.- Chem., Rapid Commun. 1984,5, 829. (a) Irie, M. Molecular Models of Photoresponsiveness; Montagnoli, G., Erlanger, B. F., Eds.; Plenum: New York, 1983; 281. (b) Ciardelli, F.; Carlini, C.; Solaro, R.; Altomare, A.; Houben, J. L.; Fiss, A. Pure Appl. Chem. 1984,56, Pieroni, 0.; 329. (a) Irie, M.; Hayashi, K. J . Macromol. Sci., Chem. 1979,A13, 511. (b) hie, M.; Hirano, K.; Hashimoto, S.; Hayashi, K. Macromolecules 1981, 14, 262. (c) Blair, H. S.; Pogue, H. I.; Riodan, J. E. Polymer 1980,21, 1195. hie, M.; Menju, A.; Hayashi, K. Macromolecules 1979, 12, 1176. Irie, M.; Hosoda, M. Makromol. Chem., Rapid Commun. 1985, 6, 533. Merian, E. Text. J. 1966, 36, 612. Lovrien, R. Proc. Natl. Acad. Sci. 1967, 57, 236. van der Veen, G.; Prins, W. Nature (London) Phys. Sci. 1971, 230, 70. Smeta, G. Adu. Polym. Sci. 1983, 50, 18. Zimmermann, E. K.; Stille, J. K. Macromolecules 1986,18,321. (a) Matgjka, L.; Duiek, K.; Ilavsky, M. Polym. Bull. (Berlin) 1979,1, 659. (b) Matgjka, L.; Ilavsky, M.; DuBek, K.; Wichterle, 0. Polymer 1981, 22, 1511. Herz, M. L. J. Am. Chem. SOC.1975,97, 6777. Kuhn, W.; Ramel, A.; Walters, D. H.; Egner, G.; Kuhn, H. J. Fortschr. Hochpo1ym.-Forsch. 1960, 1, 540. Tanaka, T.; Fillmore, D. J. J. Chem. Phys. 1979, 70, 1214. Grignon, J.; Scallan, A. M. J. Appl. Polym. Sci. 1980,25,2829. RiEka, J.; Tanaka, T. Macromolecules 1984, 17, 2916. 1
1-X 1 + XRTC0
According to these equations, the gel expansion is expected to increase with increasing leucocyanide residue content, C,, in the gel network. This was not the case. It was found that the gel expansion ratio decreased when the content was increased above 2.0 mol 5%. This may be interpreted by taking into account the effect of lIcont. At high content of the leucocyanide residues, the contraction effect due to IIcontovercomes the expansion effect due to IIion. The introduction of hydrophobic bulky phenyl groups in the gel network would decrease the compatibility between the polymer segments and water, preventing gel expansion.
Acknowledgment. We express our thanks to M. G . Tilley and L. Katsikas for kindly correcting the English in the manuscript. Registry No. (1, X = OH, Y = N(CH&, Z = CH= CH2) (H2C=CHCONH2).(H2C=CHCONHCH2NHCOCH=
I
Photoresponsive Polymers. 9.l Photostimulated Reversible Sol-Gel Transition of Polystyrene with Pendant Azobenzene Groups in Carbon Disulfide2 Masahiro Irie* and Ryuzo Iga The Institute of Scientific a n d Industrial Research, Osaka University, Ibaraki, Osaka 567, Japan. Received April 1, 1986
ABSTRACT The gel melting temperature of polystyrene-carbon disulfide gel was found to change reversibly upon ultraviolet irradiation by incorporating a small amount of azobenzene groups into the pendant groups. Photoisomerization of the pendant azobenzene groups from the trans to cis form (62% conversion) increased the gel melting temperature by as much as 9 "C when the polymer contained 10.5 mol % azobenzene groups. The gel-sol transition could be induced isothermally at -52 "C for the polymer gel having 10.5 mol % azobenzene groups and at 200 g/L by changing the irradiation wavelength. Ultraviolet irradiation (400 > X > 310 nm) converted the sol to the gel state, whereas visible irradiation (A > 450 nm) induced the transition from the gel to the sol state. The gel melting temperature dependence on the content of cis-azobenzene and the segment density in the polymer gels suggested that the dipole moment increase of the pendant groups as a consequence of the trans-to-cis isomerization reinforces the coil overlap interactions, resulting in stabilization of the gels. The stable gel reverts to a sol state when the coil overlap junctions are destroyed by the cis-to-trans isomerization with visible light.
Introduction By incorporating a small amount of photoisomerizable chromophores into polymers, we have shown that various polymer properties can be controlled reversibly by phot~irradiation.~The solubility of polystyrene in cyclohexane, for example, changes upon irradiation with light of a specific wavelength when the polymer contains less than 7 mol % azobenzene or spirobenzopyran chromo-
phores in the pendant groups; i.e., ultraviolet light caused precipitation of the polymer, while visible light resolubilized it.44 Another example is a reversible shape change of polymer gelsa7 Polyacrylamide gel with 3.1 mol % triphenylmethane leucocyanide groups dilates in water by as much as 2.2 times in each dimension upon ultraviolet irradiation, and the dilated gel in the dark contracts again to the initial size. Reversible photodissociation of the
0024-9297/86/2219-2480$01.50/00 1986 American Chemical Society
PhotoresponsivePolymers. 9 2481
Macromolecules, Vol. 19, No. 10, 1986
sample PS-A-4.7 PS-A-6.4 PS-A-8.0 PS-A-8.7 PS-A-10.5
Table I Characterization of the Copolymers mol % azobenzene groups M , x 10-40 4.7 6.4 8.0 8.7 10.5
3.2 2.7 2.3 2.2 2.1
MwIMn' 1.5 1.6 1.6 1.5 1.4
'Estimated from GPC using a calibration curve for polystyrene. leucocyanide pendant groups caused reversible dilation. In this paper, we report photostimulated isothermal sol-gel transitions of polymer gels. A polymer gel is a three-dimensional network of flexible cross-linked chains. The gels are classified into two types, irreversible and reversible gels. In the former case, the gels are cross-linked by chemical bonds and in the latter, by physical interactions between certain points on different polymer chains, such as microcrystals or helical structures.s Among many such reversible gels, the atactic polystyrene-carbon disulfide (CS,) system is of particular interest because of its sharp and completely thermally reversible gel formation characteristics. The gelation is considered to occur as a result of weak segment-segment contacts, such as coil overlap or entanglements.+'l It is to be expected that these weak intersegment interactions could be made photocontrollable by incorporating photoisomerizable pendant chromophores into the polymer. We have introduced up to 10 mol % pendant 4-((pheny1azo)anilino)carbonyl groups into polystyrene and studied the effect of the azobenzene geometrical structure on the gel melting temperature.
Experimental Section Polystyrene samples with pendant azobenzene chromophores were prepared by free radical copolymerization of styrene and (4-pheny1azo)methacrylanilidein benzene solution at 60 "C using AIBN as initiator. The polymer has the following structure: y
3
4 CH,CHHCH,q)ii
0
?=O
6 N*N
0 The reactivity ratios r1 and r2 were obtained to be 0.04 and 0.71, respectively,in which Ml is (4-pheny1azo)methacrylanilide. The reactivity ratios suggest that the azobenzene units are separated from each other in the copolymer at low azobenzene content. This was also confirmed by NMR measurement. The polymers possessed unimodal molecular weight distributions (M,/M,, = 1.5 i 0.1)as was seen from the gel permeation chromatogram (Toyo-Soda H-801). Weight-average molecular weights were estimated from the chromatogram curve obtained with polystyrene. The composition was determined by elemental analysis. The copolymers used in this work are listed in Table I. Irradiation was carried out with a 1-kW high-pressure mercury lamp. Ultraviolet (400 > h > 310 nm) or visible ( h > 450 nm) wavelengths were selected with the aid of Toshiba cutoff filters (UV-36c,UV31,W-47). The gel-sol transition temperature was determined by two methods: test tube tilting and the ball-drop method. In the first method, the temperature at which the solution stopped flowing w~ taken as the temperature of gelation. In the second method, which was mainly used in this work, a steel
I -L- 6 -
- 60
- 50 Temp.,
-40
-30
"C
Figure 1. Gel-sol transition of polystyrene (M,, = 1.5 X lo4)with (0) 62% cis-azobenzeneand ( 8 )100% trans-azobenzenegroups in CS2measured by the ball-drop method. Arrows indicate the gel melting temperature. The content of the azobenzenegroups in the polymer was 10.5 mol % and the polymer concentration in CS2was 200 g/L. ball was placed on top of the gel and the point at which the depth-temperature curve deviated from horizontal was taken as the gel melting temperature, T,. The temperature was raised at a rate of 0.5 "C/min and measured with a digital thermometer (Doric 410A).
Results and Discussion Geometrical Structure Effect on the Gel-Sol Transition Temperature. Polystyrene with pendant azobenzene groups is readily soluble in CS2. The polymer solution was irradiated with ultraviolet light (400 > X > 310 nm), converting some of the azobenzene groups from the trans to the cis form. The isomerization can be followed by measuring changes in the absorption spectra. The cis-form content is expressed by (1- A , / A , ) ( t t / ( t ttJ), where A, and A , are the absorbance at 350 nm at time t and before irradiation and t, and e, are the absorption coefficients of the trans and cis form at 350 nm, respectively. (1- A,/A,) gives an estimate of the fraction of the cis form, because the t, value is very small compared with Et.
4
Two tubes (diameter 8 mm), one containing CS2solutions of polystyrene with the partially photoisomerized azobenzene groups (cis content, 62%) and the other having 100% trans-azobenzene groups, were cooled slowly to -78 "C and allowed to stand for 5 h. Both solutions formed stable gels. Then a steel ball was placed on top of the gel and the temperature was raised from -78 "C at a rate of 0.5 "C/min. Figure 1shows the temperature dependence of the ball-sinking behavior of the two polymer gels. The ball on the top of the polystyrene gel having trans-azobenzene pendant groups began to sink at -56 "C, whereas the ball on the gel having partially cis-azobenzene groups still remained on the top at this temperature. In this case, the ball began to sink at -47 "C; i.e., the gel melting temperature was 9 "C higher than that of polystyrene with trans-azobenzene groups. In order to confirm that the geometrical structural change of the pendant azobenzene groups is the unique factor controlling the gel-sol transition temperature, reversibility of the temperature change was examined as follows: a CS2 solution of polystyrene with partially photoisomerized cis-azobenzenegroups (cis content 62%) was divided into two tubes and both tubes were cooled at -78 "C for 5 h. The upper half of one tube was then irradiated at -78 "C with visible light (X > 450 nm) for 20 min to convert the pendant cis-azobenzene groups into the trans form. On the basis of a control experiment, where
2482 Irie and Iga
Macromolecules, Vol. 19, No. 10, 1986
'0
LO 60 Cis fraction, %
80
Figure 3. Effect of the cis-form fraction on the gel melting temperature at constant azobenzene content in the polymer (8.0 mol %). Polymer concentration in CS2 was 150 g/L.
e
- 60
20
- 50
-LO
Temp ,C '
--
Figure 2. Gel-sol transition of polystyrene (M, = 1.5 X lo4)with (0) 62% cis-azobenzeneand (a) 100% trans-azobenzene groups in CS2. The 100% trans-azobenzenewere produced by preirradiation of the upper part of the tube with visible light (A > 450 nm) at -78 "C. Arrows indicate the gel melting temperature. The content of the azobenzene groups in the polymer was 10.5 mol % and the polymer concentration in CS2 was 200 g/L.
the gel was irradiated in a optical cell and the spectral change measured, visible irradiation for 20 min completely converts the cis to the trans form. The temperature of the two tubes, photoirradiated and untreated, was raised under the same conditions as before. As shown in Figure 2, the gel melting temperature of the polystyrene gel with 62% cis-azobenzene groups was -47 "C (cf. Figure l), while the melting temperature of the preirradiated gel reverted to -56 " C . This reversibility clearly indicates that the gel melting temperature increase due to ultraviolet irradiation, shown in Figure 1,is not due to irreversible photochemical reactions but due to configurational isomerization of the pendant azobenzene groups. The ball flowed in the region irradiated with visible light but stopped when it reached the region where the pendant azobenzene groups still remained in the cis form. The ball began to sink again at -49 "C. The gel melting temperature was slightly higher than the value of polystyrene with 62 % cis-azobenzene groups. The difference is ascribable to partial isomerization of cis-azobenzene in the lower region to the trans form by light scattered during the photoisomerization treatment of the upper part. The two-step sinking behavior shown in Figure 2 suggests that the gel-sol transition can be induced isothermally between -56 and -47 "C by changing the irradiation wavelength. Ultraviolet irradiation (400 > X > 310 nm) converts the sol to the gel state, whereas visible irradiation (A > 450 nm) induces the transition from the gel to the sol state. In fact, the reversible gel-sol transition was observed at -52 "C. sol
+ x
gel
A2
400
> XI > 310 nm; X, > 450 nm
Figure 3 shows the effect of cis-form fraction on the gel melting temperature at constant azobenzene content (8.0 mol % ) and constant polymer concentration (150 g/L). The gel melting temperature increased with increasing cis fraction in the copolymer. The conversion of a small amount of azobenzene groups into the cis form caused a significant increase in the temperature, while additional
100 150 Concentration
200 g/i
Figure 4. Dependence of the gel-sol transition temperature in CS2 on the polymer concentration for polystyrene with (0) 100% trans-azobenzeneand (a)partially cis-form (30%) azobenzene pendant groups. The content of the azobenzene groups in the polymer was 6.4 mol %.
cis-form azobenzene formation above 50% conversion only caused a slight temperature increase. Figure 4 shows the dependence of the gel-sol transition temperature on the polymer concentration for polystyrenes having all-trans and partially cis-form (cis fraction, 30%) azobenzene groups. The content of the pendant azobenzene groups is 6.4 mol %. The phase diagram of the polymer-CS2 system changes depending on the configuration of the pendant azobenzene groups. The gel melting temperature of polystyrene with all-trans azobenzene groups is scarcely influenced by the polymer concentration, or the density of azobenzene groups, a t relatively low azobenzene content, while the temperature increased significantly for the polystyrene with partially cis-form azobenzene groups. The difference suggests that the intermolecular segmentsegment interactions become stronger with increasing segment density when the polar cis-azobenzene groups are formed. Figure 5 shows the concentration dependence of T , for homopolystyrene and the copolymer with high pendant azobenzene content (10.5 mol % 1. Both of the polymers had a molecular weight (M,) of 1.5 X lo4. Even a t a polymer concentration of 100 g/L in CS2, no stable gel formation was observed for homopolystyrene under the experimental conditions employed. This means T , of the polystyrene gel is below -78 "C. The copolymer with high pendant azobenzene content, on the other hand, formed a stable gel even at polymer concentrations of 50 g/L. The T, of the copolymer at a concentration of 150 g/L is around 10 "C higher than the T , of polystyrene with the same molecular weight. The pronounced increase of the gel melting temperature by the introduction of azobenzene groups suggests that trans-azobenzene groups themselves
Macromolecules, Vol. 19, No. 10, 1986
PhotoresponsivePolymers. 9 2483
0
4 0 0 > AI .310nm 32 ,450nm
E
l-
Figure 7. Schematic illustration of a gel formation mechanism. ( 9 ) and ( 7 )
are trans- and cis-form azobenzene pendant groups,
respectively.
''50
100 150 Concentration, g/1
200
Figure 5. Dependence of the gel-sol transition temperature in CS2 on the polymer concentration for (0) polystyrene and ( 0 ) the copolymer with 10.5 mol % azobenzene pendant groups. Both polymers had a molecular weight (M,) of 1.5 X lo4.
1 0
I 5 10 Content of azobenzene groups. mole%
Figure 6. Dependence of the content of azobenzene pendant 100% groups in the polymer on the gel melting temperature: (0) transform azobenzene; ( 0 )30% cis-form azobenzene. Polymer concentration in CS2 was 100 g/L.
also contribute to the association sites in the system. trans-Azobenzene is reported to have a dipole moment of 0.5 D.12 Although the polarity is low in comparison with cis-azobenzene,the polarity is considered to be sufficient to increase T, in the nonpolar polystyrene-CS2 system. The azobenzene content dependence of the gel melting temperature at constant polymer concentration is shown in Figure 6. T , strongly depends on the azobenzene content in the copolymer. The T, of the copolymer with 10.5 mol % trans-azobenzene units is 11 "C higher than that of the copolymer with 4.7 mol % trans-azobenzene units. This result also suggests that trans-azobenzene groups contribute to the stabilization of the gel state. The conversion of the trans-form azobenzene groups into the cis-form further raised the gel melting temperature as shown in Figure 6. The melting temperature increased by 2-7 "C as a result of a 30% conversion to the cis form. The cis-form azobenzene is considered to reinforce the segment-segment interactions due to its strong dipole moment. Mechanism for Photoreversible Gel Formation. The molecular weight of the polystyrene and its copolymers used in these experiments was less than 3.2 X lo4, which is well below the critical molecular weight for entanglement, C, = 35000 for polystyrene." The formation of stable gels from these low molecular weight polymers in CS2 indicates that coil overlap interactions play a dominant role in the gelation mechanism. Entanglement effects are insignificant. Although the introduction of bulky and polar azobenzenes into the pendant groups may slightly affect the
polymer flexibility, the physical properties of the copolymer are similar to those of polystyrene at relatively low azobenzene group content (510mol %). Stable gel formation of the copolymer in CS2 at lower segment density in comparison with polystyrene indicates that the coil overlap interactions are enhanced by the presence of the azobenzene groups. The azobenzene groups in the surface region of the polymer coil are considered to assist the overlap interaction between the coils. When the azobenzenes are converted to the more polar cis form, the interactions are reinforced. It is inferred from these results that the segment-segment interaction in the coil surface region becomes stronger in the following order: -(styrene)vs. -(styrene)- < -(trans-azobenzene)- vs. -(trans-azobenzene)- < -(trans-azobenzenel- vs. -(cis-azobenzene)< -(cis-azobenzene)- vs. -(cis-azobenzene)-, where -( )denotes the segments. According to the above scheme, the gel melting temperature is considered to depend not on the total concentration of azobenzenes but on the amount of azobenzene groups in the surface region. The amount of the azobenzenes in the surface region, C,,is roughly expressed as where C, is the total concentration of the azobenzenes in a coil. Although the azobenzene in the surface region increases with increasing total concentration, the increasing ratio is less than one. I t has a tendency of saturation at high concentrations. The relation suggests that introduction of a small amount of azobenzene groups (less than 10 mol % ) to the pendant groups is sufficient to build up the gel network junctions, as is observed in Figure 6. The relation between the fraction of cis-azobenzene and the gel melting temperature, shown Figure 3, can also be interpreted by eq 1. Cis-form azobenzenes formed in the surface region at low conversion of the azobenzene groups are effective in building intermolecular segment-segment cross-linking sites, while additional conversion above 50% only contributes slightly to the melting temperature increase. All of these considerations lead us to an isothermal gel formation mechanism as schematically shown in Figure 7. Before photoirradiation in the dark, the pendant transazobenzene groups are in weak contact with each other. The interactions are, however, too weak to make a stable gel above Tmtof the gel having trans-azobenzene groups. The system is in a sol state. The conversion of the trans-azobenzene to the cis form reinforces the weak intersegment interactions and results in a gel state below TmC of the gel having cis-azobenzene groups. The photogenerated cis-cis interactions are strong enough to make a stable gel. The stable gel reverts again to a sol state when the junctions are destroyed by the cis-to-trans isomerization. This is an isothermal photostimulated gel-sol transition at the temperature Tmt< T < T,".
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M a c r o m o l e c u l e s 1986, 19, 2484-2494
The reversible change of the gel-sol transition temperature by photoirradiation is, so far as the authors know, the first example of a photocontrolled phase transition in a polymer system. The reversible physical property change in the polymer system has potential applications to optical data storage or display devices. Registry No. (4-C6H5N=NC6H4NHCOC(CH,)=CH,),(C6H5CH=CH2) (copolymer), 35176-66-0; 4-C6H5N= NC&HdNHCOC(CHs)=CH,, 2615-08-9; Cp,H&H=CH,, 100-42-5.
References and Notes (1) Part 8: hie, M.; Kunwatchakun, D. Macromolecules, preced-
ing paper in this issue.
(2) Preliminary communication: Irie, M.; Iga, R. Makromol. Chem., Rapid Commun. 1985, 6, 403.
(3) Irie, M. Molecular Models of Photoresponsiveness;Montagnoli, G., Erlanger, B. F., Eds.;Plenum: New York, 1983;p 291. (4) Irie, M.; Tanaka, H. Macromolecules 1983, 16, 210. (5) hie, M.; Schnabel, W. Macromolecules 1985, 18, 394. (6) hie, M.; Iwayanagi,T.; Taniguchi, Y. Macromolecules 1985,18, 2418. (7) hie, M.; Kunwatchakun, D. Makromol. Chem., Rapid Commun. 1984, 5, 829. (8) de Gennes, P.-G. Sculling Concepts i n Polymer Physics; Corne11 University: Ithaca, NY, 1979. (9) Wellinghoff, S.; Show, J.; Baer, E. Macromolecules 1979, 12, 932. (10) Tan,H.; Moet, A.; Hiltner, A.; Baer, E. Macromolecules 1983, 16, 28. (11) Boyer, R. F.; Baer, E.; Hiltner, A. Macromolecules 1985, 18, 427. (12) Bulloch, D. J. W.; Cumper, C. W. N.; Vogel, I. J . Chem. SOC. 1965, 5316.
Photon-Harvesting Polymers: Singlet Energy Transfer in Anthracene-Loaded Alternating and Random Copolymers of 2-Vinylnaphthalene and Methacrylic Acid Fenglian Bai,?C.-H. Chang, and S. E. Webber* D e p a r t m e n t of Chemistry and Center for Polymer Research, University of T e x a s at A u s t i n , A u s t i n , T e x a s 78712. Received February 12, 1986
ABSTRACT: Alternating and random copolymers of 2-vinylnaphthalene and methacrylic acid have been loaded with small amounts (0-4 mol %) of anthracene by direct esterification with 9-anthracenemethanol. Energy transfer from the singlet state of naphthalene to anthracene was studied in 77 K glasses and roomtemperature solutions. In all cases the quantum efficiency ( x ) of energy transfer was higher for the anthryl-loaded alternating copolymer than for the random copolymer. On the basis of the steady-state value of x and that derived from the naphthalene fluorescence decay, it is suggested that both the naphthalene and anthracene singlet states can be populated by a common precursor state in addition to sensitization of the anthracene singlet by energy transfer from the naphthalene singlet. It is propwed that reasonably efficient energy’migration between naphthalene groups occurs a t longer time, but ultimately excitation self-trapping may occur. For the alternating copolymer in room-temperature solution the fluorescence decay of the naphthalene in the presence of anthryl acceptors can be fit to a function of the form exp(-t/so - at”) as is expected for a “fractal structure”. On the basis of the deconvolution of the room-temperature fluorescence spectrum of the random copolymer into monomer, excimer, and anthracene components, it is proposed that a t higher loading of anthracene the host polymer conformation is perturbed, leading to a decrease of excimer-forming sites.
Introduction The study of the photophysics and photochemistry of polymers has become a very active area in polymer science in recent years.’ One aspect of this field has been the study of “photon-harvesting by which is meant the capture of a photon by one species of chromophore on the polymer backbone followed by transfer of this energy to an intrapolymer energy trap. This process is also referred to as the “antenna effect” by Guillet and co-~orkers.~ One may speculate that this phenomenon may find future application in sensitizing useful photochemical processes at the energy trap, analogous to photosynthetic systems. Quite a lot of effort has been directed to elucidating the mechanisms of energy transfer and trapping in naphthalenic polymers containing anthryl energy traps because this system is experimentally convenient (i.e., a variety of naphthalene-containing polymers can be synthesized, and naphthalene can be excited essentially independently of the anthracene trap4). Thus if one can understand the important features of prototype polymeric naphthaleneanthracene systems, then molecular design of general ‘Permanent address: Institute of Chemistry, Academia Sinica, Beijing, China. 0024-9297 / 86 / 22 19-2484$01.5010
photon-harvesting polymers might be put on firm ground. The present paper presents results for an alternating copolymer of 2-vinylnaphthalene and methacrylic acid (P2VN-&MA) and the correspondingrandom copolymer (P2VN-co-MA)loaded with various mole fractions of covalently bound 9-methoxyanthracene as an energy trap. The quantum efficiency (x) of energy transfer from naphthalene to the anthracene and the time dependence of fluorescence of the naphthalene and anthracene gorups have been measured in low-temperature glasses and room-temperature solutions. It has been found that anthracene sensitization for the alternating copolymer is more efficient than for the random copolymer for a comparable anthracene mole fraction. One contributing factor to this enhancement in fluid solution is that excimer formation is essentially absent in P2VN-alt-MA, such that one photophysical pathway that competes with anthracene sensitization is eliminated. A additional factor that favors sensitization in alternating copolymers is that the anthryl groups are always obliged to have a pair of neighboring naphthalenes while in random copolymers the anthracene can be located in sequences of methacrylic acid groups. For such isolated energy acceptors the only possible mechanism of sensitization is cross-chain Forster-type transfer, which certainly should be less efficient than a 0 1986 American Chemical Society
