2484
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, preceding 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$0 1.5010
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 corresponding random 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
Macromolecules, Vol. 19, No. 10, 1986
Photophysics of Anthracene-Loaded 2VN-MA Copolymers 2485
combined down-chain and cross-chain mechanism available to all acceptors in alternating copolymers. This feature should be general to alternating copolymers and will be the subject of later publications. During the course of characterizing the photophysical properties of these polymers, the following anomalies or complications were observed: (1)While the steady-state fluorescence spectrum demonstrates very efficient energy transfer from naphthalene to anthracene, the naphthalene fluorescence lifetime was shortened only slightly. We interpret this as the result of a common precursor state for both the naphthalene and the anthracene fluorescent states. Some general relationships between the steadystate sensitized fluorescence and the donor fuorescence decay are discussed. (2) The fluorescence decay is multiexponential, as is common for polymer systems. In addition to using a multiexponential fluorescence decay fitting function, we have used a function of the form exp(-t/To - at"), in which n is a fraction between 0 and 1and 7o is associated with a classical unimolecular lifetime. The fluorescence decay at long times is analyzed in terms of the persistence of energy migration between the naphthalene groups. (3) In the analysis of the effect of energy transfer in the random copolymer sample, it was found that at the highest anthryl loading (ca. 4 mol %) the naphthalene fluorescence has much larger component of monomer than the other members of the series. It is proposed that the covalently bound anthracene has disturbed the chain conformation in such a way as to diminish the density of excimer-forming sites. This illustrates the potential of a "probe" molecule to disturb polymer configurations. Experimental Section A. Polymers. The P2VN-alt-MAand P2VN-co-MApolymers are like those used in a previous study: with M,, values of ca. 7.5 X lo5 and 1.4 X lo4 daltons, respectively (based on GPC elution curves against polystyrene standards). The latter polymer is ca. 45 mol % naphthalene. 9-Anthracenemethanol (Aldrich Chemical Co.) was recrystallized from ethanol and hexane. Direct esterification was used. typically, 50 mg of P2VN-alt-MA was mixed with different amounts of anthracenemethanol (at least 10 mg) in 403:3 (v/v) THF + CHCl, + acetone solution, with a few drops of HzS04 added as a catalyst, and then the mixture was refluxed for at least 24 h. Water was removed with a Dean-Stark trap. A white polymer was precipitated by adding CH30H,purified by reprecipitating from THF into water at least 3 times, washed with water followed by CH30H,and dried under vacuum at 50 "C for 4 h. The mole percentages of anthracene in the polymer were estimated by UV absorption spectra. It should be noted that without using an azeotropic solvent mixture to remove HzO, it is difficult t o obtain chemically attached anthracene for these polymers. We also found that it is impossible to achieve as high an anthracene loading on the alternating copolymer as the random copolymer (see Tables I and I11 for the mole percent values for the two sets of polymers). B. Sample Preparation and Fluorescence Measurements.
All THF solutions used at room temperature were outgassed by three freeze-pump-thaw cycles at ca. lo" torr, with the solution ultimately transferred to a standard fluorescence cuvette. The naphtha1 concentration was ca. 5 X M for all solutions. For liquid nitrogen temperature studies a mixed 1:l diethyl ether-THF solvent dissolves the polymer and forms a clear glm. The solution used for low-temperaturestudies were not outgassed since oxygen quenching is essentially eliminated in low-temperature glasses. These solutions were studied in thin-walled quartz tubes (ca.4-mm 0.d.). All steady-state fluorescence studies were performed on a Spex Fluorolog 2 with a 450-W xenon lamp and a Hammamatsu R508 photomultiplier. Spectral response correction factors were obtained by comparing spectra of standard compounds with published corrected spectra. Obtaining quantum yields for low-
P2VN-ALT-MA
I1 b 310
PIVN-CO-MA .69%
360
410
460
510
nm Figure 1. Uncorrected steady-state fluorescence spectra at 77
K of P2VN-alt-MAand P2VN-co-MA. Mole percent anthracene is indicated for the solid curve. Dashed line for undoped sample. temperature glasses is often quite difficult because of strains that result in distortions in the optical path, light scattering, etc. (A cutoff filter was used to prevent excitation light scattering into the emission monochromator for steady-state and lifetime experiments). However, we found the reproducibility of the lowtemperature spectra to be excellent. As discussed in Appendix I, it is possible to obtain the value of the energy-transfer efficiency parameter (x)by a comparison of the excitation and absorption spectra. This method was used for both the solution and lowtemperature glasses. Most fluorescence lifetime measurements were made with a conventional single-photon system based on Ortec electronic components,a PRA flash lamp (Model 510),and an uncooled RCA 8850 photomultiplier. Fluorescence decay curves were analyzed by reconvoluting a multiexponential decay function with the system response function and varying the parameters of the multiexponential fitting function until the best least-squares agreement with experiment was obtained and the plot of weighted residuals vs. time was random over most of the time range. For some room-temperature studies the fluorescence decay was measured at the Center for Fast Kinetics at the University of Texas. In this installation an Ar' mode-locked laser provides a very narrow system response function with a width on the order of 400 ps. This system was used to verify the buildup rates for sensitized anthracene fluorescence in P2VN-alt-MA-co-anthracene solutions.
Results A. 77 K Experiments. A t 77 K excimer formation is essentially eliminated. Figure 1compares the fluorescence spectra for P2VN-&MA and P2VN-co-MA for roughly comparable mole fractions of anthracene. The spectra for the undoped polymers are nearly identical, although the random copolymer does have a slight shoulder to the red, which could be some residual excimer intensity (presumably by energy transfer to excimer-forming sites). It will be noted that the anthracene component is relatively stronger for the alternating copolymer for a given anthracene content. The quantum efficiency of energy transfer from naphthalene to anthracene chromophores may be defined by the following:6
UA/+N)(+N/+A) = ~ / ( -1X) (1) In eq 1I A and IN are the areas of the corrected fluorescence
Macromolecules, Vol. 19, No. 10, 1986
2486 Bai et al. Table I
x Values for Alternating and Random Copolymers
random
alternating anth mol % 0.49-a 0.49-b 0.55 0.99 1.2
77 K glass 0.29 0.24 0.34 0.53 0.62
solution 0.24 0.37 0.33 0.53 0.60
anth mol % 0.0 0.70 1.95 2.9 3.95
77 K glass
IM
0.14 0.33 0.38 0.59
0.21 0.14 0.081 0.058 0.19
solution" ID 0.79 0.32 0.54 0.50 0.42 0.37 0.57 0.57 0.24 IA
IDIIM 3.1 3.9 5.2 6.4 1.3
IAIID 0.58 1.2 1.5 2.4
" I M , I A , and I D are the relative areas of the naphthalene monomer, anthracene trap, and naphthalene excimer, respectively, in the corrected fluorescence spectrum (see Figure 5). Table I1 Fluorescence Decay Fitting Parameters for Alternating Copolymer in Room-Temperature Solutions and 77 K Glasses 316-nm exc naph (340 nm) anth (420 nm) 358-nm exc Ti, (T),* Ti, (T),* anth (420 nm) sample" ai ns ns ai ns ns 7, ns 0%, RT (200 ns) 0.218 3.2 54.5 0.047 22.2 56.2 0.735 -1.436 75.4 0.5 0%, 77 K (400 ns) 60.2 1.383 88.4 1.053 1.1 29.4 4.3 0.49-a%, RT (200 ns) 53.6 -3.445 6.7 (100 ns) 0.227 29.6 3.671 12.8 0.296 47.7 62.3 0.774 0.478 0.8 -0.469 67.1 -1.022 2.2 31.0 0.49-a%, 77 K (400 ns) 19.0 1.775 36.2 0.320 71.2 1.147 55.8 0.247 0.459 1.2 45.4 -2.852 1.4 28.1 0.49-b%, RT (200 ns) 7.0 (100 ns) 12.3 0.403 32.4 3.126 0.138 66.6 43.8 0.726 -2.974 -3.021 0.49-b%, 77 K (400 ns) 66.3 1.7 33.3 11.8 (100 ns) 0.7 20.1 1.7 1.695 3.401 2.279 0.620 55.1 66.6 0.389 0.55%, RT (200 ns) 43.6 -2.894 1.7 27.9 6.6 (100 ns) 2.8 0.247 12.4 3.117 28.0 0.364 42.2 0.778 51.7 -2.127 0.55%, 77 K (400 ns) 2.7 64.9 -2.865 1.6 30.5 10.3 (100 ns) 17.2 1.808 3.5 3.057 1.319 65.3 0.807 45.8 40.9 -3.255 1.1 0.99%, RT (200 ns) 25.8 6.5 (100 ns) .369 3.1 21.7 3.570 12.2 ,211 47.4 42.9 0.685 .420 -1.114 62.0 31.7 1.0 -4.547 0.99%, 77 K (400 ne) 1.5 12.2 (200 ns) 0.565 15.8 20.1 5.142 1.549 65.4 67.9 0.406 1.2%, RT (200 ns) 1.1 38.2 0.9 23.9 7.1 (100 ns) 0.381 -3.737 9.7 12.1 0.202 3.940 42.2 39.3 0.418 0.798 1.2%, 77 K (400 ns) -0.456 1.2 62.7 -2.346 1.6 28.4 12.1 2.926 18.1 0.375 65.5 0.420 49.6 1.081 =Mole percent anthracene indicated; RT = room-temperature THF solution; 77 K = 77 K glass of 1:l THF-diethyl ether; time scale for decay curve used to obtain the fitting parameters indicated. *See eq 3 of text.
spectrum associated with the anthracene and naphthalene, respectively, and +A and +N are the corresponding fluorescence quantum yields. x may be interpreted as the fraction of photons absorbed by the naphthyl groups that are transferred to the anthracene. It is also possible to determine x by comparing the excitation and absorption spectrum (see Appendix I). Given the difficulty of measuring quantum yields, we have used the latter method for x. These values are presented in Table I for 77 K glasses and room-temperature solutions. It will be noted that the x value for alternating copolymer is higher than that of random copolymer for corresponding doping levels. The fluorescence decay curve for both polymers was obtained by exciting a t 316 nm (primarily naphthalene absorption) and observing a t 340 nm (naphthalene) and
420 nm (anthracene). The fluorescence decays were fit to a three-exponential function of the form
I(t)=
Eai i exp(-t/~~)
where a3 was constrained to 1 - a, - a2 (i.e., five independent parameters). The numerical values for ai and T~ for the alternating and random polymers are given in Tables TI and 111, respectively. A typical example of fluorescence decay data (including the best fit) for the alternating and random copolymer is presented in Figure 2. We regard the fitting function in eq 2 as a convenient mathematical form for fitting our experimental data and do not necessarily ascribe a physical interpretation to the individual 7 values (but see the Discussion). Equation 3
Macromolecules, Vol. 19, No. 10, 1986
Photophysics of Anthracene-Loaded 2VN-MA Copolymers 2487
Table I11
ANTHRACENE FLR.(77K1
Fluorescence Decay Fitting Parameters for P2VN-co-MA-co -Anth at 77 K ~~
anth (420 nm)"
naph (340 nm)" mol % anth
Tub
TIVb
a,
ns
0
-5.177 5.215 0.963
0.68
0.434 -0.008 0.574
1.96
0.682 0.037 0.281
0.4 2.6 69.3 (59.6) 7.8 62.5 76.7 (72.9) 4.2 35.3 76.6 (66.4) 4.0 66.8 108.1 (66.1) 5.0 30.2 70.3 (59.6)
2.89
0.601 0.370 0.029
3.96
0.659 0.046 0.294
a,
ns
-5.669 5.290 1.378
0.2 8.9 31.9 (20.3) 0.2 9.7 32.8 (19.0) 0.4 11.4 48.1 (22.0) 0.2 10.2 27.9 (17.5)
-3.760 3.996 0.764 -1.692 2.465 0.227 -4.916 4.761 1.156
~
100
ns
defines an average lifetime with the fluorescence yield as a weighting factor.' 1
i
400
indicated.
Figure 2. Fluorescence decay at 340 nm (naphthalene)and 77 K for P2VN-&-MA and P2VN-c0-W for 0 mol % anthracene.
(7)= CUj7?/CCZiTi
300
ns Figure 3. Plot of fitting function (see text) for sensitized or directly excited anthracene fluorescence for P2VN-alt-MA (solid line) and P2VN-co-MA (dashed line) for mole percent anthracene
"Excitation at 316 nm. Time scale for fit, 200 ns. *Value in parentheses is ( 7 ) (see eq 3).
ns
200
(3)
The values of ( 7 ) clearly decrease with increasing anthracene content for both naphthalene and anthracene fluorescence in P2VN-&-MA but to an extent that depends on phase. As will be discussed, the apparent fluorescence lifetime at long times (denoted rL) is relevent to the interpretation of our experimental results. Unfortunately, this particular component is not determined very accurately since it is primarily influenced by that portion of the fluorescence decay with a small number of counts. This component may be sensitive in general to the total time interval over which the fluorescence decay is collected. As seen in Table I1 and I11 there is a tendency for the naphthyl and anthryl 7L values to decrease with anthryl content (but not linearly). This most noteworthy observations concerning the fluorescence decay are the following: (1)The fluorescence decay of all components is much less affected by increasing the anthracene content than is
the steady-state fluorescence spectrum. (2) The naphthalene component of the random copolymer has a rapidly decaying component (7 = 2-3 ns) even in the absence of anthracene. This is also observed in fluid solution (see next section). This component is definitely not the result of scattered excitation light since the experimental procedures used were identical for the alternating and random copolymers (see Experimental Section) and the former do not show this effect. The decay rates a t longer times for the naphthalene component in P2VN-co-MA do not vary systematically as a function of anthracene loading (see Table 111). (3) The naphthalene component of the alternating copolymer does display a systematically decreasing decay time constant at long time with increasing anthracene content, although the effect is small (see Table 11). (This can be seen most clearly by comparing the (7)values in Tables I1 and 111.) The long-time decay of the naphthalene component in the undoped alternating copolymer is slightly slower than the corresponding feature for the random copolymer. (4) The sensitized anthracene component for both polymers decays more slowly than directly excited anthracene, as expected from the long lifetime of the naphthalene energy donor state. The sensitized anthracene fluorescence for the alternating copolymer decays more slowly than that of the random copolymer (see Figure 3 for a comparison of the fitting functions; also compare ( T ) values in Tables I1 and 111). The long-time decay of both these sensitized emissions is faster than the corresponding long-time decay of the naphthalene component. The implications of this observation will be explored later (see Discussion). In this section we have compared the properties of the alternating and random copolymers with respect to x values and fluorescence decay properties. As has been pointed out earlier, one significant difference between the alternating and random copolymers is that for the latter pendent anthracene groups have a significant probability of being located in polymer seqeuences without a neighboring naphthyl group. However, another important difference between the alternating and random is the molecular weights. The degree of polymerization of the random of the random copolymer is on the order of 120 compared with 3500 for the alternating copolymer. Thus for low loadings there is a significant probability that some random copolymers do not contain any anthryl groups. It follows that detailed comparisons of these two polymers
2488 Bai et al.
Macromolecules, Vol. 19, No. 10, 1986 :;
PPVN-ALT-MA 1.2%
P2VN-CO-MA
350
400
nm
450
500
Figure 5. Deconvolution of corrected P2VN-co-MA spectrum into monomer (M), excimer (D), and anthracene (A) components. Mole percent anthracene is indicated.
.-._ 310
360
410 nm
460
510
Figure 4. Uncorrected steady-statefluorescence spectra at room
temperature (THF solvent) for P2VN-&-MA and P2VN-co-MA. Mole percent anthracene is indicated. Dashed line for undoped
sample.
may be misleading, especially with respect to the time dependence of fluorescence. What we should like to emphasize from the results of this section is that the alternating naphthalene copolymer with anthryl traps is a very efficient photon-harvesting polymer. Detailed consideration of the fluorescence decay function (see Discussion) implies that significant singlet energy migration between naphthalene groups must be occurring despite the methacrylic acid spacer group. B. Room-TemperatureSolution Experiments. Examples of the alternating and random copolymer fluorescence spectra in T H F are presented in Figure 4. Because of the absence of an excimer emission the anthracene component of the spectrum for the alternating copolymer is clearly seen. For the random copolymer there is extensive overlap of the excimer and anthryl emission (Figure 4). These spectra may be deconvoluted into monomer, excimer, and anthryl components as illustrated in Figure 5 (for corrected spectra). The trends in the relative areas of these three components will be analyzed in the Discussion. The overall energy-transfer efficiency (x)for the alternating copolymer can be calculated following the method in Appendix I. These values are listed in Table I along with the 77 K data. Also presented in Table I are the relative areas of the monomer, anthracene, and the excimer fluorescence for corrected spectra of the random copolymer. We do not present x values for the random copolymer because of the quantum yields of the monomer and excimer components are almost certainly not constant. We will return to this point in the Discussion. The fluorescence decay was studied in detail only for the alternating copolymer because of the absence of an overlapping excimer-anthracene fluorescence. The naphthalene component could be fit satisfactorily to a oneor two-exponential fit in most cases, although in Table I1 are presented the best parameters for a three-exponential fit (see eq 2). As in the case of the 77 K data, we regard this fitting function as an empirical fit with no particular significance assigned to the individual lifetimes (although see Discussion). Inspection of the values of the ( 7 ) in Table I1 demonstrates that the naphthalene average lifetime shortens steadily as the anthracene content is in-
creased. There is a more significant decrease in ( 7 ) for the alternating polymer in THF solution than in 77 K glasses. For the alternating copolymer the anthracene component displays a rise time and initial rate of decay that are essentially independent of the anthracene content. Since rise times can be very difficult to deconvolute from a ca. 5-ns-wide excitation pulse, the early portion of the anthracene decay was measured independently at the Center for Fast Kinetics using an experimental system with a much narrower response function (see Experimental Section). The rise time component was in excellent agreement with the data presented in Table 11. The decay rate at long times tends to increase with anthracene content, similar to the naphthalene component. The anthracene decay always has one relatively long-lived component, which could be the result of spectral overlap of the naphthalene fluorescence with that of anthracene. This possiblity can be eliminated as follows: By inspection of the steady-state spectra (Figure 4) one can see that there is very little naphthalene fluorescence at 420 nm. The steady-state fluorescence is related to the fluorescence decay function as follows:
(4) in which the multiexponential fitting function for I ( t ) was assumed in the integral. If the decay function contains a component of naphthalene fluorescence from spectral overlap, i.e.
+
I ( t ) = Ca,’e-t/Ti uNe-f/rh
(5)
(where a / are the modified coefficients required in going from eq 2 to eq 5 ) , then the fraction of the total steadystate fluorescence that arises from the naphthalene is given by fN
=
aNTN/Car7,
(6)
The experimental value of f~ is very low (
