6384
J. Phys. Chem. B 1997, 101, 6384-6393
Dual Exciplex Formation and Photoinduced Electron Transfer in Pyrene End-Labeled Polynorbornenes Renae D. Fossum and Marye Anne Fox* Department of Chemistry and Biochemistry, UniVersity of Texas at Austin, Austin, Texas 78712 ReceiVed: February 3, 1997; In Final Form: June 3, 1997X
Polynorbornenes substituted with 1-cyano-2-naphthyl and 2,5-dicyanophenyl groups as acceptors and terminated with pyrene as donor have been prepared by ring-opening metathesis polymerization. When excited with monochromatic light, pyrene transfers an electron, producing a radical ion pair with concurrent reduction of the acceptor. In the copolymers with one acceptor block, steady-state emission spectra reveal extensive pyrene fluorescence quenching and exciplex formation between the pyrene and the acceptor. In copolymers with two acceptor blocks, the steady-state emission spectra reveal two intramolecular exciplexes assigned to the cyanonaphthalene-pyrene and dicyanobenzene-pyrene excited-state complexes. Transient absorption spectra display an overlap of the pyrene triplet-triplet and radical cation spectra for all of the polymers. The lifetimes of the transient absorption maxima are dependent upon the identity of the acceptor and the length of the acceptor block.
Introduction
SCHEME 1
Intramolecular photoinduced electron transfer has been studied in great detail1-4 in order to minimize back-electrontransfer steps so that these materials could be useful for molecular electronic devices. A major goal has been to obtain controlled, vectorial electron transfer. In fact, it has been previously reported that electron transfer in a block copolymer donor-(acceptor or donor)-acceptor copolymer takes place when the appropriate chromophores are attached to a semirigid polynorbornene backbone and where the central donor (or acceptor) block was excited.5 In low molecular weight donordonor-acceptor triads, two-electron-transfer steps1,2,6-9 that are sometimes reversible have been observed.1 For these materials to be useful as devices, it would be optimal to arrange the graded series of chromophores so that electron transfer is possible only in one direction and that energy-wasting processes such as competing singlet or triplet energy transfer and back electron transfer are suppressed. Two sets of donor-acceptor and donor-acceptor-acceptor polymers were prepared as described earlier to attain this goal.10 Evidence is provided here that electron transfer does take place: steadystate emission spectra reveal quenching of the pyrene fluorescence in the presence of either one or two acceptors, with simultaneous formation of exciplexes between the acceptors and pyrene. Dual exciplex formation is observed in the block copolymers between pyrene and each of the flanking acceptors, suggesting that unique interactions occur as a consequence of their attachment to a polymer backbone. Synthesis and Structures of the Pyrene-Terminated Polymers The polymerization of chromophore-appended norbornene acetals and ketals (1) has been described earlier, and the regiochemistry of 1 was assigned on the basis of NOE difference experiments.10 The same general procedures were employed here: the catalyst used induces ring-opening metathesis polymerization of strained olefins to form a predominantly transpolyolefin (Scheme 1).11,12 The chain end remains reactive until a quencher such as benzaldehyde, pivaldehyde, or pyrene-1X
Abstract published in AdVance ACS Abstracts, July 15, 1997.
S1089-5647(97)00412-4 CCC: $14.00
carboxaldehyde13 is added, permitting formation of multiblock macromolecules (Scheme 1). Photophysical Results Steady-State Emission. The steady-state emission for the pyrene monomer 1d and the phenyl homopolymer terminated with pyrene (2c) are shown in Figure 1. Only fluorescence was observed upon excitation at 310 and 355 nm at room temperature in THF, benzene, or CH2Cl2 solutions or in a MTHF glass at 77 K. The pyrene monomer (1d) and homopolymer 2c show © 1997 American Chemical Society
Pyrene End-Labeled Polynorbornenes
Figure 1. Steady-state emission spectra of 1d (10-5 M), and 2c (10-5 M) in degassed THF at room temperature with excitation at 310 nm. The maxima at 375, 385, 395, and 420 nm are due to pyrene fluorescence in 1d. The maxima are shifted 20 nm in 2c because of increased conjugation with the terminal olefin.
Figure 2. Steady-state emission spectra of 2a, 2b, 3a, and 3b (10-5 M) in degassed THF at room temperature with excitation at 355 nm. The maxima at 395 and 415 nm are assigned to pyrene fluorescence, that at 445 nm to cyanonaphthalene-pyrene exciplex emission and that at 510 nm to dicyanobenzene-pyrene exciplex emission.
similar spectra at room temperature and 77 K. The fluorescence maxima for pyrene are visible at 375, 385, 395, and 420 nm in 1d. In 2c, an identical spectrum is observed but red-shifted by 20 nm, reflecting pyrene’s conjugation with the terminal olefin in the polymer backbone. Upon excitation at 355 nm, the fluorescence of each of the homopolymers substituted with 1-cyanonaphthalene or 1,4dicyanobenzene groups and terminated with pyrene (2a and 2b) is substantially quenched and exciplex emission is observed (Figure 2). In the block copolymers, pyrene fluorescence is also quenched, and two emissions assigned to the cyanonaphthalene-pyrene (CN-pyr) and dicyanobenzene-pyrene (DCBpyr) exciplexes (λmax 445 and 510 nm, respectively) are observed. As the length of the cyanonaphthalene block is increased, the intensity of the pyrene fluorescence and the cyanonaphthalene exciplex emission in 3 are increased qualitatively (Figure 3). Fluorescence spectra for 2 and 3 measured in MTHF glasses at 77 K showed pyrene fluorescence to be quenched less efficiently than at room temperature and that exciplex formation was completely suppressed. Presumably,
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6385
Figure 3. Steady-state emission spectra of 3a-d (10-5 M) in degassed THF at room temperature. Excitation at 355 nm. The maxima are the same as those in Figure 2.
the exciplexes require a particular conformation that arises from the dynamic motion of the chromophores that is not accessible in a frozen glass. The fluorescence quantum yields14 are shown in Table 1 for 1d, 2, and 3 in THF, benzene, and CH2Cl2 at room temperature. Because the spectra for 2a and 2b are a combination of pyrene fluorescence and exciplex emission, the spectra were each corrected by subtraction of a normalized spectrum of 2c to reveal the deconvoluted exciplex emission alone (CN-pyr and DCBpyr shown in Figure 4A). The spectra for 3 were similarly corrected for pyrene fluorescence, permitting two exciplexes to be resolved by deconvolution in 2a or 2b. A representative example is shown in Figure 4B for 3b. Table 1 shows the total emission quantum yield, Φem, together with the quantum yields for pyrene fluorescence, Φem(pyr), with λmax at 395 nm; CNpyr exciplex emission, Φem(CN-pyr), with λmax at 445 nm; and DCB-pyr exciplex emission, Φem(DCB-pyr), with λmax at 510 nm. With the quantum yield for the phenyl polymer 2c serving as reference, pyrene fluorescence is quenched more efficiently in 2b than in 2a in all solvents, suggesting that dicyanobenzene is a better quencher than cyanonaphthalene. This order is expected from the relevant redox potentials: there is a greater driving force for electron transfer from excited pyrene (*Eox ) -2.14 eV)15 to dicyanobenzene (Ered ) -1.60 eV)15 than to cyanonaphthalene (Ered ) -1.98 eV).15 Interestingly, the relative contributions of the three deconvoluted fluorescent components (pyrene fluorescence, and emission from the CN-pyr and DCB-pyr exciplexes) depend on the solvent. In general, exciplex emission is greatest in benzene, since exciplexes are preferred in nonpolar solvents.16 The intensity of exciplex emission is the lowest in CH2Cl2 but so also is the overall emission. Because the exciplex accounts for much of the light emitted, it is important to determine whether the exciplex is an intermediate on the way to charge separation or whether it is an energetically dissipative secondary pathway.17 As shown in Figures 2 and 3, the CN-pyr exciplex emission is most intense in homopolymer 2a (Figure 2) and increases in the block copolymers 3 as the number of repeat units in the cyanonaphthalene block increases (Figure 3). The Φem(pyr) increases and Φem(DCB-pyr) decreases as the number of cyanonaphthalene repeat units increases in both THF and benzene. This finding suggests that the second exciplex between
6386 J. Phys. Chem. B, Vol. 101, No. 33, 1997
Fossum and Fox
TABLE 1: Emission Quantum Yields,a Φem, for 1d, 2, and 3
1d 2a 2b 2c 3a 3b 3c 3d
Φem (THF)
Φem (pyr)b
0.571 0.457 0.057 0.346 0.082 0.158 0.175 0.218
0.571 0.109 0.003 0.346 0.023 0.056 0.062 0.070
Φem (CN-pyr)c
Φem (DCB-pyr)d
0.348 0.054 0.017 0.055 0.078 0.132
0.042 0.046 0.036 0.016
Φem (benzene)
Φem (pyr)b
0.318 0.127 0.097 0.310 0.184 0.193 0.227 0.258
0.318 0.039 0.010 0.310 0.027 0.056 0.066 0.081
Φem (CN-pyr)c
Φem (DCB-pyr)d
0.088 0.087 0.013 0.006 0.016 0.057
0.144 0.131 0.145 0.120
Φem (CH2Cl2)
Φem (pyr)b
Φem (CN-pyr)c
0.255 0.075 0.003 0.137 0.015 0.028 0.031 0.058
0.255 0.014
0.061
Φem (DCB-pyr)d
0.003 0.137 0.004 0.007 0.004 0.009
0.001 0.004 0.009 0.015
0.010 0.017 0.018 0.034
a Excitation at 310 nm for 1d and at 355 nm for 2 and 3 in degassed THF at room temperature. b Quantum yield assigned to emission from isolated pyrene group fluorescence after correction. c Quantum yield assigned to emission from an intramolecular cyanonaphthalene-pyrene exciplex (CN-pyr). d Quantum yield assigned to emission from an intramolecular dicyanobenzene-pyrene exciplex (DCB-pyr).
Figure 4. (A) Steady-state emission spectra of 2a, 2a deconvoluted by subtraction of a normalized spectrum of 2c, 2b and 2b deconvoluted by subtraction of a normalized spectrum of 2c. (B) Steady-state emission spectra of 3b, 3b deconvoluted by subtraction of a normalized spectrum of 2c, and 3b deconvoluted by subtraction of a normalized, corrected spectrum of 2a. All solutions (10-5 M) were in degassed THF at room temperature with excitation at 355 nm.
pyrene and dicyanobenzene could be hindered by the facile formation of CN-pyr exciplex or by structural changes induced upon increasing the number of repeat units. Formation of an emissive CN-pyr exciplex is favored by attachment of cyanonaphthalene to the polymer backbone: bimolecular quenching of 2c (10-5 M) by 1-cyanonaphthalene could be observed at a quencher concentration of 10-3 M, although no exciplex emission could be detected (Figure 5A). Bimolecular quenching of the fluorescence of 2c with 1,4dicyanobenzene also revealed pyrene fluorescence quenching, and the growth of a small amount of DCB-pyr exciplex
Figure 5. (A) Steady-state emission spectra of 2c (10-5 M) with 0, 100, 300, and 500 µL of 1-cyanonaphthalene (0.1 M) in degassed THF at room temperature with excitation at 310 nm. (B) Steady-state emission spectra of 2c (10-5 M) with 0, 30, 70, and 100 µL of 2,5dicyanobenzene (0.08 M) in degassed THF at room temperature with excitation at 310 nm.
emission (Figure 5B). Intermolecular exciplex formation between both pyrene and cyanonaphthalene and pyrene and dicyanobenzene has been observed previously at concentrations greater than 10-2 M in toluene.18 Dilution studies show that the emission observed from exciplexes in 2a, 2b, and 3 must be intramolecular because it is not affected by a decrease in concentration (when normalized to the pyrene fluorescence). A comparison of the quantum yields for 3 (Table 1) shows that the intensity of the DCB-pyr exciplex fluorescence
Pyrene End-Labeled Polynorbornenes
Figure 6. Transient absorption spectra of 1d (10-5 M, 34 µs after the laser pulse), 2c (10-5 M, at 30 µs after the laser pulse), and the corrected difference spectrum in degassed THF at room temperature with excitation at 355 nm.
decreases with an increase in the number of cyanonaphthalene repeat units. Thus the DCB-pyr exciplex probably arises from a conformation induced by the structure of the polymer backbone. In 3a the cyanonaphthalene block is short, so a kink in the chain would position the terminal pyrene in the vicinity of a DCB repeat unit, whereas with 20 repeat units, a kink in the chain would position the pyrene close to a cyanonaphthalene, thus giving rise to more intense CN-pyr exciplex emission. Exciplex emission can also be disrupted by increasing the polarity of the solvent. The polymers are not soluble in CH3CN, but a sample of 2b in THF diluted with CH3CN shows a decrease in the intensity of exciplex emission observed. Furthermore, a comparison of fluorescence quantum yields of 2 and 3 in different solvents shows that in THF the fractional share of DCB-pyr exciplex emission is decreased compared to that obtained in benzene. The reverse is true for the CNpyr exciplex, possibly because the CN-pyr exciplex can act more as a “mixed excimer” rather than a direct charge-transfer complex, as has been suggested for the bimolecular DCB-pyr exciplex.18 If so, the CN-pyr exciplex emission would be less sensitive to increases in solvent polarity than the DCB-pyr exciplex emission. Nano- and Picosecond Transient Absorption. The transient absorption spectra obtained using a nanosecond laser pulse also reveal electron transfer in pyrene end-labeled polymers. In 1d, the pyrene triplet-triplet absorption was observed at 420 nm19 upon excitation at 355 nm. In the control polymer 2c, this maximum was shifted to 440 nm, as is consistent with increased conjugation with the terminal olefin. A subtraction of the normalized spectrum of 1d revealed a small residual peak at 450 nm which is assigned to the pyrene radical cation absorption produced by photoionization (Figure 6).20 Therefore, the observed spectrum is a superposition of the triplet-triplet and radical cation absorptions of pyrene. In the remaining block copolymers, similar spectra were observed with maxima at 440 nm accompanied by a large bleach (assigned to the ground-state absorbance of pyrene) at 370 nm, indicative of the triplet-triplet absorption of pyrene, the pyrene radical cation absorption, or a combination of both. Electrontransfer would also produce the radical anions of cyanonaphthalene and dicyanobenzene (λmax, respectively at 375 and 345 nm),21 but because their absorptions overlap with the groundstate bleach, it is not surprising that they are not directly observed in our spectra. However, if the spectra are deconvo-
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6387
Figure 7. Transient absorption spectra of 2c, 3a, and (3a-normalized 2c, 10-5 M) at 60 µs after the laser pulse in degassed THF at room temperature with excitation at 355 nm.
Figure 8. Transient absorption spectra of 2c (10-5 M) at 0.7 ns, 3.6 ns, 4.6 ns, and 34 µs after the laser pulse in degassed THF at room temperature with excitation at 355 nm. Transient absorption spectrum of 1d (10-5 M) at 30 µs after the laser pulse is displayed to show the maximum of the pyrene triplet-triplet absorption.
luted from a normalized spectrum of 2c, the pyrene radical cation absorption maximum is observed at 420 nm together with a shoulder at 370 nm, which may be assigned to the cyanonaphthalene radical anion, and a peak at 340 nm that may be assigned to the dicyanobenzene radical anion (Figure 7). The transient absorption spectra induced by a picosecond laser pulse confirms that the pyrene radical cation overlaps with the pyrene singlet-singlet absorption, and in 2a,b and 3, a new species is observed at 520 nm. The transient absorption spectra for 2c is shown in Figure 8 and has a broad maxima at 420 nm. Comparison of the picosecond transient absorption spectra of 2c to the transient absorption spectrum of 1d at several microseconds after the laser pulse shows that the triplet-triplet absorption of the monomer overlaps with the spectrum for 2c. The maxima at 0.7, 3.6, and 4.6 ns in the spectrum of 2c represent the pyrene singlet-singlet absorption since the pyrene monomer singlet has been measured to be on the order of 200 ns and the intersystem crossing rate for pyrene is 106 s-1. Since the pyrene singlet-singlet absorption has previously been observed at 460 nm,22 we conclude that the conjugation of pyrene with the terminal olefin in the backbone shifts the singlet-singlet spectrum. The spectra at 3.6 and 4.6 ns in Figure 8 also revealed a superposition of the pyrene radical
6388 J. Phys. Chem. B, Vol. 101, No. 33, 1997
Figure 9. Transient absorption spectra of 2b at 0.8 ns, 1.4 ns, 3.3 ns, 5.3 ns, 5.9 ns and 60 µs after the laser pulse in degassed THF at room temperature with excitation at 355 nm.
Fossum and Fox
Figure 11. Transient absorption spectra of 3a at 0.3, 0.7, 0.8, 4.0, and 5.3 ns after the laser pulse in degassed benzene at room temperature with excitation at 355 nm.
TABLE 2: Transient Absorption Lifetimes,a τabs, for Pyrene Monomer 1d and Pyrene End-terminated Polymers 2 and 3 1d 2a 2b 2c 3a 3b 3c 3d
contribution (%)
τabs (µs) (440 nm)
100 100 100 100 21 79 19 81 18 82 29 71
430 ( 9b 407 ( 10 317 ( 5 603 ( 15 143 ( 12 568 ( 10 44 ( 2 177 ( 3 101 ( 39 364 ( 16 183 ( 28 647 ( 4
a
b
Figure 10. Transient absorption spectra of 3a at 0.7 ns, 0.8 ns, 1.3 ns, 4.0 ns, 5.3 ns and 12 µs after the laser pulse in degassed THF at room temperature with excitation at 355 nm.
cation absorption at 440 nm as determined from comparison to the transient absorption spectrum of 2c at several microseconds after the laser pulse. In the homopolymer 2b, the same species are observed as in 2c, but an additional peak is present at 520 nm which is assigned to a charge-transfer complex between dicyanobenzene and pyrene (Figure 9). This new species is also present in 3: representative spectra are shown in Figure 10 for 3a. It is interesting to note that the pyrene radical cation grows in as the charge-transfer absorption decays. A comparison of the spectra at time segments 1.3 and 4.0 ns to that at 12 µs (Figure 10) shows that the pyrene radical cation originates from the charge-transfer absorption with maxima at 520 nm. The presence of a charge-transfer complex agrees well with the exciplexes observed in the steady-state emission spectra and suggests that the exciplex is an intermediate on the way to charge separation. From the quantum yields for emission in Table 1, a large fraction of the total emission came from exciplexes. Therefore if the exciplexes were a separate pathway from the electron transfer, the efficiency of electron transfer would be reduced. The steady-state emission spectra of the polymers 2 and 3 having been shown to be different in benzene than THF, dictated that the picosecond transient absorption spectra of 2 and 3 also be measured in benzene. In the control polymer 2c, the spectra looked very similar to that obtained in THF, with maxima at 420 and 520 nm. Interestingly for 3, the charge-transfer absorption at 520 nm in THF was also present in the spectra
Excitation at 355 nm in degassed THF at room temperature. Maximum at 420 nm.
measured in benzene. The spectra were slightly different in the two different solvents: the charge-transfer absorption was broader in benzene than THF and did not decay at times as long as 5.3 ns (Figure 11). In benzene, the pyrene radical cation was not clearly visible. This result was expected since it is known from the steadystate measurements that, in benzene, the DCB-pyr exciplex was strongly favored. Charge separation to the radical ion pair would therefore likely be more difficult because of the increased stability of the exciplex in the nonpolar solvent. The kinetic decays observed at 440 nm in the nanosecond transient absorption measurements appear to be monoexponential for 2 and biexponential for 3 (Table 2). If triplet-triplet absorption overlapped with the radical cation of pyrene, biexponential behavior would be observed for all of the polymers. The transient lifetimes are on the order of hundreds of microseconds for all of the compounds, and a longer lifetime is observed in 2a than in 2b. The observed difference in lifetime is probably due to differences in the back-electron-transfer rates. The triblock copolymers 3 show short and long decay components arising from the two different quenching pathways that derive from the two separate acceptor blocks in the polymer. Time-Resolved Emission. Steady-state emission spectra (Figures 1-3), indicate the presence of at least three emitting species, assigned to pyrene fluorescence and emission from CN-pyr exciplex and a DCB-pyr exciplex. These species were resolved by deconvoluting spectra of the block copolymers with the control polymer 2c and the pyrene-terminated homopolymers 2a,b. The relative quantum yields of these species were affected by the polarity of the solvent: the efficiency of emission of DCB-pyr exciplex was increased in benzene as
Pyrene End-Labeled Polynorbornenes
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6389
TABLE 3: Emission Lifetimes,a τ, for 1d, 2, and 3 in THF and Benzene λmax
solvent
%
τ (ns)
χ2 solvent
1d 390 nm benzene 100 200 2a 440 nm benzene 54 20 46 47 2b 390 nm benzene 66 12 34 170 2b 510 nm benzene 9.3 8.6b 91 71c 2c 390 nm benzene 59 39 41 70 3a 390 nm benzene 63 3.2 32 7.8 4.6 54 3b 390 nm benzene 51 4.2 46 10 5.2 46 3c 390 nm benzene 47 4.3 50 11 3.5 44 3d 390 nm benzene 48 4.9 47 14 4.8 39 3a 440 nm benzene 78 5.9 22 43
4.6 THF 1.2 THF
3b 440 nm benzene
87 13
10 36
1.1 THF
3c 440 nm benzene
85 15
12 34
1.2 THF
3d 440 nm benzene
73 27
16 35
1.0 THF
3a 510 nm benzene
25 75 31 69 33 67 33 67
3.2b 69c 3.0b 61c 2.4b 54c 3.1b 47c
1.3 THF
3b 510 nm benzene 3c 510 nm benzene 3d 510 nm benzene
%
τ (ns)
100
340 3.7b 76c
SCHEME 2
χ2
1.2 d 1.5 THF 1.3 THF 1.0 THF 1.0 THF 1.1 THF 1.0 THF 1.2 THF
1.3 THF 1.6 THF 1.5 THF
100 61 36 2.9 59 39 1.9 54 41 4.7 39 47 14 54 42 3.5 28 63 8.6 20 67 13 14 51 35 29 71 33 67 34 66 34 66
3.2b 64c 82 2.3 6.4 43 3.7 10 44 3.2 9.6 31 3.4 9.9 31 1.9 6.1 40 3.3 11 33 4.0 13 34 6.2 18 38 3.1b 37c 2.2b 38d 2.3b 36c 4.0b 40d
1.1 1.1 1.2 1.1 1.3 1.0 1.0 1.1 1.2 2.0 1.9 1.3
a Emission lifetimes determined from degassed solutions using singlephoton-counting techniques with excitation at 350 nm. b Exponential fit of the rise time. c Exponential fit of the emission decay. d Did not measure 390 nm in THF.
compared to that obtained in THF. Furthermore, the amount of pyrene quenching was not affected by the solvent polarity, suggesting that the increase in exciplex emission observed in benzene was the result of less solvent separation of the radical ions than would be possible in THF. The kinetics for the emission were determined using single photon counting in benzene and THF (Table 3). The emissions were monitored at the maxima for pyrene fluorescence (λmax at 390 nm) and for emission from CN-pyr exciplex (λmax at 440 nm) and the DCB-pyr exciplex (λmax at 510 nm). The decays at each wavelength were complicated and were generally fit to bi- or triexponential equations. Rise times were observed at the DCB-pyr exciplex maxima at 510 nm. The decay of transients produced by excitation of a benzene solution of the control polymer 2c was found to be biexponential at the maximum for the pyrene fluorescence at 390 nm, and the corresponding decays for benzene solutions of the copolymers 3 were fit to a triexponential equation. The biexponential behavior of 2c is in accordance with photoionization as a primary process, as was determined from the transient absorption measurements. The copolymers possess three components at 390 nm: the shortest component comprises 47-63% of the decay, the middle component comprises 32-50%, and the
longest component is 4-5%. Possible pathways for relaxation of the excited-state pyrene are fluorescence to the ground state, nonradiative decay to the ground state, photoionization to the pyrene radical cation and solvated electron (which presumably would be incorporated by one of the acceptors to result in the respective radical anion), quenching by cyanonaphthalene, and finally quenching by dicyanobenzene (Scheme 2). The pyrene fluorescence has been assigned to the longest lived component because pyrene itself has a very long singlet lifetime: a lifetime of 200 ns was detected for the pyrene monomer. As the length of the cyanonaphthalene block increases, the lifetime of the attached pyrene fluorescence decreases. This suggests that when a larger number of acceptors are present, it is statistically more likely that the excited pyrene will be quenched, possibly because back electron transfer at the block interface is inhibited. The shortest lifetime component in the pyrene fluorescence decay is due to quenching by either dicyanobenzene or cyanonaphthalene. Since an attached dicyanobenzene group appears to be a better pyrene end-group quencher than an attached cyanonaphthalene group (as determined from the steady-state quantum yields in Table 1), a dicyanobenzene group should quench the pyrene fluorescence more efficiently than a cyanonaphthalene group, resulting in a shorter observed lifetime. This assignment was supported by a decrease in fractional contribution of the shortest component consequent to an increase in the length of the cyanonaphthalene block. As the pyrene end group is farther removed from the dicyanobenzene block, the amount of quenching by dicyanobenzene declines. The steady-state quantum yields also showed that the steady-state emission of the DCB-pyr exciplex decreased similarly. The decays at 440 nm observed for transients in benzene correspond to the CN-pyr exciplex, in analogy with the steadystate measurements. Unlike the decay at 390 nm, the decay at 440 nm was biexponential, suggesting that there are fewer pathways for the CN-pyr exciplex than for the excited state of pyrene. The long-lived component has been assigned to the emission of the exciplex, and the short component has been assigned to quenching of the exciplex via charge separation. As the length of the cyanonaphthalene block increases, the lifetime of the CN-pyr exciplex emission decreases, implicating possible radical ion migration through the longer block of cyanonaphthalene repeat units. The decay measured at 510 nm from transients in benzene solution corresponds to the DCB-pyr exciplex emission maxima. The DCB-pyr exciplex emission lifetime became shorter as the cyanonaphthalene block was lengthened in parallel
6390 J. Phys. Chem. B, Vol. 101, No. 33, 1997 with the steady-state observation that as the cyanonaphthalene block was increased, the amount of DCB-pyr exciplex decreased. This maximum also displayed 2-3 ns rise times for the block copolymers corresponding to exciplex formation. The distribution among these components did not vary with the length of the cyanonaphthalene block. The kinetic behavior observed in THF differs from that observed in benzene, as was expected from the significant differences in steady-state quantum yields observed in the two solvents. As in benzene, the decay at 390 nm in THF was triexponential for all of the block copolymers. The spectral assignments for the three components are the same, but the relative contributions differ in THF from those observed in benzene. A more dramatic decrease in the fractional contribution of the short-lived component (quenching due to dicyanobenzene) was observed as the length of the cyanonaphthalene block increased in THF than in benzene. This may be rationalized if THF is a better solvent than benzene, i.e., if the polymer chain is more extended in THF. If so, the contribution of DCB-pyr exciplex to the total emission is reduced as a function of folding of the backbone. The third component, assigned to quenching by cyanonaphthalene, is not appreciably altered by changes in the length of the cyanonaphthalene block. Because THF is a more polar solvent than benzene, radical ion solvation would be greater in THF and therefore a greater preference for charge-separated ions should be observed. If so, this component could arise from direct formation of the pyrene radical cation and cyanonaphthalene radical anion and would be less influenced by the conformation of the polymer. The longest lived species has been assigned to the decay of the pyrene, and its composition increased with the length of the cyanonaphthalene block. The decay at 440 nm in THF solution was triexponential but biexponential in benzene. This suggests that in the more polar solvent, a greater number of pathways are available for the CNpyr exciplex. The longest lived component has been assigned to the decay of the CN-pyr exciplex, and its percent composition increased with an increased cyanonaphthalene block length. The medium-lifetime species corresponds to quenching of the CN-pyr exciplex by charge separation to the pyrene radical cation and the cyanonaphthalene radical anion. As with the longest component, the fractional contribution of this component increased with increasing cyanonaphthalene block length. The shortest lived component is assigned to quenching of the CNpyr exciplex by dicyanobenzene and its composition decreased as the length of cyanonaphthalene units increased. This suggests that with short blocks of cyanonaphthalene, the pyrene radical cation and the dicyanobenzene radical anion are produced. This behavior is not observed in benzene, because the radical ions are less stable in the less polar solvent. The decay at 510 nm in THF corresponds with that observed at 510 nm in benzene. A short rise component is assigned to the formation of DCB-pyr exciplex, and a long-lived component describes the DCB-pyr exciplex decay. The lifetimes are shorter in THF than in benzene because of increased charge separation in THF. The maximum at 440 nm displayed biexponential behavior, but the decays at 510 nm were all single exponential, suggesting that the formation of DCB and pyrene radical ions is fast enough that it does not complicate the decay measurements. From the observation of the emission kinetics, we conclude that the interactions taking place in the block copolymers are complex and are influenced by the length of the acceptor block and the nature of the solvent. A possible description of the
Fossum and Fox
Figure 12. Time-resolved emission spectra of 3a obtained from a picosecond laser pulse in degassed THF with excitation at 350 nm.
block copolymer behavior is shown in Scheme 2. In benzene, the pyrene interacts with the first acceptor cyanonaphthalene to form an exciplex that can decay to the ground state or form charge-separated ions. Depending on the length of the cyanonaphthalene block, the pyrene can also form an exciplex with dicyanobenzene, presumably because of kinking or folding of the backbone. This exciplex can also decay to the ground state or form charge-separated ions. Because the kinetics for the decay of the exciplex at 510 nm are single exponential, any charge separation takes place on a time scale that does not affect the monoexponential decay. There are also nonemissive quenching pathways, e.g., direct electron transfer to either the DCB or the CN or nonradiative decay due to vibrational relaxation. Direct electron transfer to produce the radical ions is likely because the pyrene radical cation transient absorption spectrum has been observed (Figure 6). The steady-state fluorescence quantum yields (Table 1) show that the fraction of quenching is greater than the quantum yields for exciplex emission (which could also be a consequence of nonradiative decay of the exciplexes). Thus, the kinetic scheme must include direct electron transfer to either the dicyanobenzene or cyanonaphthalene and possibly some other route for the formation of the pyrene radical cation, likely by photoionization. Time-resolved emission spectra were obtained to deconvolute these very complicated interconversions. A qualitative spectra was generated for 3a by combining the single-photon-counting decays at 20 nm intervals at various time segments (Figure 12). At early times, there is a maximum at 420 nm which corresponds to an overlap of pyrene fluorescence and CN-pyr exciplex. As time passes, a maximum grows in at 520 nm, as the peak at 420 nm decays. There is an isosbestic point at 490 nm between
Pyrene End-Labeled Polynorbornenes
Figure 13. Time-resolved emission spectra of 2c obtained from a nanosecond laser pulse in degassed THF with excitation at 355 nm. Laser pulse power was 6 mJ/pulse.
Figure 14. Time-resolved emission spectra of 2a obtained from a nanosecond laser pulse in degassed THF with excitation at 355 nm. Laser pulse power was 6 mJ/pulse.
time segments 9.8 and 14.4 ns that suggests that the DCB-pyr exciplex originates from either the pyrene or the CN-pyr exciplex. Time-resolved emission spectra of 2c reveal that the maximum for the pyrene fluorescence is at 400 nm (Figure 13) and that the emission from the CN-pyr exciplex grows in as the pyrene fluorescence decays, producing a broad maximum from 440 to 480 nm (Figure 14). In the dicyanobenzene homopolymer 2b, there is no clear growth of the DCB-pyr exciplex as in 2a. However, if the spectrum of 2c is overlapped with that of 2b, the DCB-pyr exciplex is clearly visible. Spectra for the block copolymers were also obtained by this method. Unfortunately the various species could not be resolved. A representative spectrum for 3a is shown in Figure 15 in comparison with the spectra obtained by combining the singlephoton-counting decays shown in Figure 12. The observed spectral features agree with the kinetic description: both require that the two acceptors interact. However, direct spectroscopic evidence for the first charge-separated state decaying into the second charge-separated state is ambiguous. Photophysical Interaction in Viscous and Rigid Media. To reduce the interactions induced by conformational changes in the backbone, steady-state emission was monitored in viscous solutions and as thin films. From the solution phase steadystate emission spectra, we conclude that the block copolymers
J. Phys. Chem. B, Vol. 101, No. 33, 1997 6391
Figure 15. Time-resolved emission spectra of 3a obtained from a nanosecond laser pulse in degassed THF with excitation at 355 nm. Laser pulse power was 6 mJ/pulse.
may adopt a folded or kinked conformation, producing two different exciplexes, namely those between pyrene and cyanonaphthalene and between pyrene and dicyanobenzene. To ascertain whether these exciplexes were due to dynamic folding of the backbone, the emission was studied in viscous solutions and thin films. As mentioned in the discussion of the steady-state emission, frozen glasses of 2 and 3 did not reveal any exciplex, suggesting that the chromophores require some amount of organization to obtain sandwich type complexes (exciplexes). In solutions of increasing viscosity, the amount of exciplex should decrease if the exciplex requires polymer folding. Solutions of the donoracceptor substituted polynorbornenes (10-5 M in THF) were blended with filler polymers poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) in concentrations of 5, 10, 20, and 25 wt % in THF. The steady-state fluorescence revealed that in the presence of PVC, pyrene fluorescence, CN-pyr exciplex and DCB-pyr exciplex were all present. In fact, the resulting spectra in these viscous solutions were very similar to the spectra obtained in THF alone, albeit not as intense. Increasing the concentration of PVC did not substantially vary the composition of the different components, suggesting that exciplex formation was not affected by increased viscosity. When PMMA was used as a filler polymer, pyrene fluorescence and CN-pyr and DCB-pyr exciplex emission were detected. However, the intensity of the emission decreased as a function of increasing viscosity. The same quenching behavior was found for the control polymer as the block copolymer in a PMMA mixture, suggesting that the decrease in emission in the presence of PMMA is due to a quenching of the excited state. The quenching induced here could be caused by an impurity in the polymer or by the methacrylate repeat units. PMMA has been known to act as an electron acceptor for pyrene and perylene in thin films of PMMA because the ester can stabilize the radical anion.23 From the PVC solution results, we conclude that intramolecular exciplex formation does not require large dynamic changes in the conformation of the polymer backbone. Another possible explanation for the observation that the exciplexes cannot be disrupted is that the polynorbornene is not well “solvated” by the PVC and that the polynorbornene exists in pockets or islands in the PVC solution. These viscous solution emission results contrast with the emission observed in rigid MTHF glasses at 77 K. The absence of any exciplex emission in rigid glasses must arise because rotation of the chromophores
6392 J. Phys. Chem. B, Vol. 101, No. 33, 1997 (at the acetal/ketal carbon) is completely frozen out, and this rotation is required to allow the chromophores to orient themselves into exciplexes. Spectra in Thin Films. Given that no change was observed in the emission of the pyrene-terminated polymers in viscous solutions, the interactions of these polymers in thin films were studied. Since polynorbornene itself has been shown to undergo strain-induced crystallization, thin films of the pyrene-terminated polymers alone were expected to be brittle and heterogeneous because of crystallization defects.24 Spin-coated films were attempted to be prepared from several solvents (THF, toluene, p-xylenes, 2-methoxyethyl ether, and chlorobenzene) at various concentrations, but the films remained brittle with many defects due to cracking and crystallization (determined by visualization under a microscope). In all of the polynorbornene films, pyrene excimer dominated the fluorescence spectrum. In fact, no pyrene fluorescence was observed. Complete formation of excimer is not surprising, but it is disastrous for applications requiring efficient electron transfer to the acceptors. Pyrene has been known to self-absorb on glassy electrodes.13,25,26 Thus, during film formation the polymer chains may orient themselves such that the pyrenes form sandwich structures that favor excimer emission. It is possible that the polymer chains align so that the pyrenes undergo extensive π-stacking, but a detailed study of polymer packing is beyond the scope of this work. Because the excimer is expected to hinder any intramolecular processes, films of 3 were prepared as blends with filler polymers. Similarly to the viscosity measurements described above, solutions of the homo- and block copolymers were prepared and mixed with PVC, polystyrene (PS), poly(vinylbenzyl chloride) (PVBC), Formvar, polypropylene, and poly(ethylene oxide). These mixed films showed pyrene fluorescence and emission from the pyrene excimer and from the CNpyr and DCB-pyr exciplexes. Polypropylene used as a filler polymer did not disrupt the pyrene excimer very well. In mixed poly(ethylene oxide) films, the polymer had crystallized out, as established by visualization under the microscope. Therefore, films with PVC, PS, and PVBC were used to study the photophysics in the solid state. The emission of films of 3 blended with PVC are shown in Figure 16A. It was difficult to quantify the fractional contribution of emission from the exciplex because of overlap. The corrected spectra of the films of 3 using PVC as a filler polymer are shown in Figure 16B. From the difference spectra, the CNpyr exciplex is present, but the DCB-pyr exciplex is not. This result suggests that the pyrene and cyanonaphthalene chromophores can interact either intra- or intermolecularly, but since DCB-pyr exciplex is not observed, the polymer cannot fold back on itself, as was observed in solution.
Fossum and Fox
Figure 16. Steady-state emission spectra of films of 3 blended with PVC (A), and of 3 blended with PVC corrected for pyrene excimer (B). Excitation at 355 nm.
The transient absorption lifetimes of the charge separated species are very similar for 2 and 3 but are longer when dicyanobenzene (rather than cyanonaphthalene) is the acceptor. Transient absorption spectra on the nanosecond regime reveal a charge-transfer absorption that decays into the pyrene radical cation. Similar behavior is obtained in both THF and benzene. The emission lifetimes reveal many pathways available to the excited-state pyrene and suggest that both dicyanobenzene and cyanonaphthalene groups can accept an electron from pyrene. The electron transfer is not affected by increased viscosity, as evidenced by spectra of 2 and 3 in solutions with 5-25% PVC. Thin films of 3 show complete pyrene excimer formation which can be disrupted in the solid state by preparing films from blends with filler polymers. Thin films from blends also reveal the complete suppression of the DCB-pyr exciplex, confirming that this second exciplex originates from polymer folding. Experimental Section
Conclusions Our results indicate that electron transfer does occur in pyrene-terminated block polynorbornenes substituted with 1-cyanonaphthalene and 1,4-dicyanobenzene groups. Dual intramolecular exciplex formation in the block copolymers is evidenced by steady-state and time-resolved emission spectra, suggesting that successive electron transfers (between pyrene and cyanonaphthalene, followed by electron transfer between cyanonaphthalene and dicyanobenzene) take place, although the data do not unambiguously require this assignment. The observed photophysics can be rationalized by interactions between adjacent repeat units or by remote units brought into proximity by folding of the polymer backbone. The behavior observed is probably a convolution of extended and folded conformations.
Kinetic Methods. Absorption spectra were obtained on a Hewlett-Packard 8451A diode array spectrometer. Fluorescence and phosphorescence spectra were measured on a SLM Aminco SPF 500 fluorometer. A phosphoroscope attachment equipped with light baffles and a variable-speed chopper (0-10 000 rpm) was used to differentiate short- and long-lived excited species. Fluorescence quantum yields were determined according to standard procedures.14 Transient absorption spectra were obtained with a Q-switched, frequency-tripled (λ ) 355 nm, 8 mJ/pulse) Quantel YG481 Nd:YAG laser and a pulsed highintensity Xe arc lamp in a 1 cm cell containing a solution with OD ) 0.2-0.4 AU. The decay measurements reported were averages of 20 laser pulses and transient spectra were obtained as the average of three laser pulses at 10 nm intervals.
Pyrene End-Labeled Polynorbornenes Transient emission lifetimes were obtained at 77 K with a Q-switched, frequency-tripled (λ ) 355 nm, 8 mJ/pulse) Quantel YG481 Nd:YAG laser. Transient emission spectra were obtained using a Continuum Surelite Q-switched Nd:YAG laser (6 ns pulse width, 10-20 mJ/pulse). Decay measurements and spectra were obtained as the average of 50 laser pulses at 10 nm intervals. Structural Analysis of Polymers. Gel permeation chromatography (GPC) measurements were made in CH2Cl2 at an elution rate of 1 mL/min using a Waters 6000A solvent delivery system through 7.8 × 300 mm Styrogel columns (104 Å, 103 Å, 500 Å) with a Rainin Dynamax UV detector and a Waters 410 differential refractometer. Preparation of Thin Films. Films of polymers 2 and 3 were prepared by spin coating 2-5 wt % solutions of the polymer in THF onto indium tin oxide (ITO) coated glass (100 Ω) using a Specialty Coating Systems P-6000 spin coater. The slides were spun at 4000 rpm for 30 s. Films from blends of a 2-5 wt % solution of 2 or 3 and a 20-30 wt % solution of a filler polymer in a ratio of 1 to 10 were prepared in the same manner. Acknowledgment. This work was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy. The authors wish to thank Dr. T. A. Rhodes for assistance with the single-photon-counting measurements which were conducted at the Center for Photoinduced Charge Transfer at the University of Rochester, Rochester, NY. The authors also wish to thank Dr. D. O’Connor for assistance with the transient absorption measurements which were conducted at the Center for Fast Kinetics Research at the University of Texas at Austin. Supporting Information Available: Experimental details and characterization of compounds 1-6 (7 pages). Ordering information is given on any current masthead page. References and Notes (1) Willemse, R. J.; Verhoeven, J. W.; Brouwer, A. M. J. Phys. Chem. 1995, 99, 5753.
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