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Electron and energy transfer in polymeric and polymerizable systems
Radiat. Phys. Chem. Vol. 49, No. 1, pp. 71-80, 1997 Copyright © 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved P I I : S0969-806X(96)00109-0 0969-806X/97 $17.00 + 0.00
Pergamon
E L E C T R O N A N D E N E R G Y T R A N S F E R IN P O L Y M E R I C A N D P O L Y M E R I Z A B L E SYSTEMS MASAAK] OGASAWARA Center for Research and Development in Higher Education, Hokkaido University, Sapporo 060, Japan Abstract--Studies on electron and energy transfer in polymeric and polymerizable systems are summarized. Electron capture and geminate recombination in solution and matrix containing poly(methyl methacrylate) and poly-(4-vinylbiphenyl)are first described. Later on, intramolecular electron transfer in poly-(4-vinylbiphenyl)in solution is elaborated. Excess electrons produced by ionizing radiation attach to the polymers and then get transferred via the side-group until they are finally trapped by a different kind of side group having higher electron affinity. Further, the electron trapping and the electron transfer in substituted polyacetylene are described. An excess electron gets localized on one or two monomer units depending on the kind of substituent of the polymer. Electron transfer from shallowtrap to deeper one via the alternative double bonds in the main chain is observed in the solution of poly(1-phenyl-1-propyne). Finally, the excitation energy transfer in laser-irradiated crystalline 2,5-distyrylpyrazine is investigated with special reference to the structure of the crystal and the polymerization mechanism. Copyright © 1997 Elsevier Science Ltd
INTRODUCTION
were synthesized. Substituted polyacetylenes such as poly(1-phenyl-l-propyne) were prepared in the laboratory of Professor Higashimura, Kyoto University. 2,5-Distyrylpyrazine was synthezied following the method reported by Hasegawa et al. (1969) and purified by repeated recrystallization. Solvents used were purified by the usual procedures (Ogasawara et al., 1987; Tanaka et al., 1991).
The elementary processes induced by ionizingradiation in condensed phase have been understood fairly well. Following the transfer of energy from radiation to medium, ionization and electronic excitation of constituent molecules occur. Molecules and ions excited to the levels above their dissociation limit undergo self-decomposition. Electrons ejected into the medium are thermalized by transferring their excess energy to the environment, and under appropriate conditions, attach to the solute molecules. The remaining parent cations may also move and react with other molecules. However, in case the molecular weight of the solute is very large, the large body of the solute molecule is affected, especially when the electron and energy transfer processes give rise to different reaction sequences. This article summarizes our studies to unveil the electron and energy migration in polymeric systems. Part of the results has been published elsewhere, but some of the results have been newly obtained. Preliminary results on energy transfer process in photoinduced polymerization in crystalline system are also included for comparison.
Irradiation and measurements Solutions were prepared under vacuum and sealed into quartz cells, with a light path of 10 nun for pulse radiolysis and 1.5 mm for 7-radiolysis. For steadystate measurements, samples were irradiated with y-rays from 6°Co at 77 K in the dark. Optical absorptions were measured at 77 K with a Shimadzu MPS-5000 spectrophotometer. The Hokkaido University 45 MeV electron linear accelerator was used as the source of the electron beam for pulse radiolysis experiments. The half-width of the electron pulse was 50 ns. Time-dependent fluorescence measurements were made on solutions, microcrystalline samples and single crystals, by using Hamamatsu Photonics C4780 picosecond fluorescence lifetime measurement system. Excitation was made by using 337 nm light from a N2-1aser.
EXPERIMENTAL RESULTS AND DISCUSSION
Materials
1. Electron capture, geminate recombination, and intramolecular electron transfer in polymeric systems. What do polymers look like in solution? According to a computer simulation, a molecule of polyethylene, for example, looks like a waste thread floating in water (Guillet, 1985). To express the volume of a
Poly(methyl methacrylate) and substituted poly (methyl methacrylate) from Science Products SP3 were purified by the usuat procedures (Ogasawara et al., 1987). 4-Vinylbiphenyl (Aldrich) was purified by repeated vacuum sublimation. Poly(4-vinylbiphenyl) and poly(4-vinylbiphenyl-co-1-vinylpyrene) RPC 49/1--D
71
72
Masaaki Ogasawara
coiled polymer, the radius of a sphere in which a polymer rotates around a virtual center is being used. The word "polymer domain", which appears in this text, means such a sphere. (a) Electron capture volume o f polymers in matrices. We made one of the first electron attachment experiments in a polymeric system using poly(methyl methacrylate) (PMMA) (Ogasawara et al., 1987). PMMA is one of the most popular and widely-used polymers, but whether it captures electrons or not had been a matter of dispute for a long time. We found that the radical anions of PMMA and its analogues were actually produced by the reaction of solvated electrons with polymers in hexamethylphosphorictriamide. The absorption spectrum of the radical anion has a strong band in the wavelength region < 300 nm and a shoulder at 450 nm. An ab initio MO calculation on a model compound, methyl isobutyrate, suggested that the structure was strongly deformed by the attachment of an excess electron (Tachikawa et al., 1991). The C-C bond of the ether part was found to be 0.15 urn; the bond is extended by 0.04 nm on adding an excess electron to the molecule. The carbonyl group of the radical anion deviates from the Cs symmetry; the excess electron in the polymer is stabilized in the form of deformed anion. This is in contrast to the carbonyl groups of the neutral radical and radical cation, in which the geometry of the carbonyl part is approximately planar. From the orbital phase analysis, the weak absorption in the visible band (400-500 nm) was assigned to the n-n* transition, which was partially allowed owing to the bent configuration of the radical anion. On the basis of the results obtained in these studies, we have investigated the electron capturing reaction of PMMA in 2-methyltetrahydrofuran (MTHF) matrix at low temperature. It is well known that electrons trapped in this matrix at low temperature migrate by tunneling mechanism to react with a solute molecule. Although there are a number of recent electron models (Schiller, 1991, 1992; Jormer et al., 1992), the following conventional treatment of data is still useful. The surviving probability P of the trapped electron decreases exponentially with increasing solute concentration (Miller, 1972). Thus, the capture volume v for the excess electron may be estimated from the gradient of the straight line in the logarithmic plot of the surviving probability against the polymer concentration: -
logP
=
only 38% of the one for a model compound, methyl isobutyrate. In the case of a low-molecular-weight molecule, the solute molecules are distributed homogeneously in the system and the shape of each capture space must be sphere-like. While, in the case of polymers, the chromophores are connected side-by-side to the main chain and their capture spaces must overlap with each other. For this reason the capture space for each polymer cannot be sphere-like anymore. If one assumes a rod-like capture-space for the polymer, the radius of the rod is estimated to be 1.5 nm from our results. This value is comparable to 1.2 nm for methyl isobutyrate. The idea of rod-like capture volume was first proposed by Matsushima et al. (1987) in the study of the poly(4-vinylbiphenyl) (PVB) system. This model is effective only when the overlapping of the capture space is not very extensive. Figure 1 shows the logarithmic plot of the surviving probability of the trapped electron against polymer concentration in the P M M A - M T H F system. When the concentration is higher than 0.2 baseunit mol dm -3, the surviving probability abruptly decreases and then approaches a plateau value. The result suggests a kind of phase transition in the M T H F solution. A simple explanation is that the entangled polymers in the polymer domains spread over the system to form homogeneous, network-like structures at higher concentration; it reduces the overlapping of the capture volume of the chro-
2.5
f
2.0
1.5
/
e~
_= I
1.0
0.5
/
/
/
/f
/
0.2
0.4
i
i
0.6
0.8
Concentration / rrlol . dm-3
vC
where the polymer concentration C is expressed in base-unit. The capture volume means a volume in which all of the excess electrons are "absorbed" by the solute molecule after the 7-irradiation until the time of measurements. The above equation explains the case for PMMA when the polymer concentration is lower than 0.2 base-unit mol dm -3. The capture volume was found to be 7 x 10 -21 cm 3, which was
,-Ph_ Fig, 1. Smwiving probability of the trapped electron against polymer concentration in ~-irradiated M T H F containing poly(methyl methacrylate). The ~-irradiation and measurements were made at 77 K. The polymer concentration is expressed by base-unit.
Electron and energy transfer in polymeric and polymerizable systems
73 CH 2 ~
CH -
M T H F solvent ~ S+
Polymer
Domain
Fig. 2. Illustration of the ion-recombination reaction of PVB radical anion (Bp-) with solvent cation (S ÷) in irradiated PVB-MTHF system. Part of the illustration is taken from Kato et al. (1988).
mophores to the minimum and enhances the electron trapping by the polymers. (b) Retarded geminate recombination in polymer domains. When M T H F solution containing PVB is
irradiated by ionizing radiation, ion-pairs of electron and parent solvent-cation are produced anywhere in the system. Some of the electrons undergo reaction with biphenylyl side groups of the solute polymer and the rest may survive for a long time under cryogenic conditions. Different from the P M M A case, the logarithmic plot of the surviving probability of the trapped electron against the polymer concentration was straight up to the limit of the dissolution of the polymer. Matsushima et al. (1987) reported that the tunneling radius was 4 nm, assuming a rod-like shape for the capture volume o f PVB. This result is consistent with the capture volume of biphenyl, obtained assuming a sphere-like shape. We have found two interesting features in electron transfer reactions in the PVB-MTHF system. One is retarded geminate recombination in the polymer domains (Tanaka et al., 1989). In the pulse radiolysis of the liquid phase, the radical anion of PVB decayed following usual second-order kinetics when the polymer concentration was as low as 0.03 baseunit tool d i n - a. The rate constant was about half of that of the homogeneous ion-recombination reaction of monomeric biphenylyl anion. This suggests that the electrons are chemically trapped by the polymers and the produced anions are attacked by solvent-cations coming from the exterior of the polymer domain (see Fig. 2). When the polymer concentration was increased up to 0.3 base-unit mol dm -3, a spike was
found in the time profile of the PVB radical anion. The spike was not affected by dose per pulse and it was ascribed to geminate recombination of the polymer anions with the solvent cations. Geminate recombination between the monomeric biphenylyl radical anion and the solvent cation is very fast at room temperature and cannot be observed in sub-microsecond time scale. At low polymer concentration, the solvent cation can move freely, so that the geminate recombination is not observable by conventional pulse radiolysis technique. But at very high polymer concentration, the ratio of the polymer domain to the total reaction space becomes significant. Part of the geminate pairs are produced in the viscous polymer domains as shown in Fig. 2; the recombination reaction in the interior of the domain is effectively retarded. This may be the reason why the spike was observed in the time-profile. The existence of the polymer domain in this particular system was first suggested by Kato et al. (1988) from the kinetic analysis of the polymer anion formation. Our recent measurements on the effect of nonpolar solvent on the spike intensity confirmed this explanation. The spike was reduced in its intensity by the addition of 3-methylpentane and it was not observable anymore when 3-methylpentane was added up to 30 vol%. The addition of the nonpolar solvent enhances the entanglement of the polymer coil to reduce the size of the polymer domain. Another interesting feature observed is that the sum of the trapped electron yield and the PVB anion yield becomes much smaller with increasing polymer concentration (Fig. 3). The total g-value determined
74
Masaaki Ogasawara
by steady state measurements at 0.3 baseunit mol d m - 3 was only a half of the value obtained at zero polymer concentration. We confirmed that there was no initial fast decay at 77 K and the initial yield determined by pulse radiolysis coincided with the one observed at 77 K by steady state measurement. A plausible explanation is as follows: the polymer domains are crowded with aromatic groups of the polymer, which reduce the local dielectric constant or lower the edge of the conduction band in the polymer domains giving rise to enhanced ion-recombination between the electron and the hole before the stabilization of the excess electron in the matrix. The lower edge of the conduction band is correlated to the energy of the quasi-free electron, V0. The effect of V0 on the free-ion yield has been theoretically explained by Schiller (1992) on the basis of H-like atoms model of the pairs of ions and low-energy electrons.
(c) Intramolecular electron transfer in polymer. A question arises as to whether the electron localized at a biphenylyl side group of PVB really moves in the polymer. As the electron transfer in the polymers to identical chromophores was difficult to observe, we synthesized poly(4-vinylbiphenyl-co-1-vinylpyrene), in which a small number of biphenylyl side groups were substituted by pyrenyl groups. The excess electron produced by pulse radiolysis attaches to one of the biphenylyl groups and migrates to another side-group until it is stably trapped by a pyrenyl group in the polymer. The pyrenyl group was chosen as the indicator because the spectral overlap between anion radicals of biphenyl and pyrene was minimum among possible combinations of aromatic compounds. The experimental results altogether indicated the existence of electron transfer from one side group to another within the life-time of the polymeric radical 3.0 +
I
I
I
I
I [] ei" @ PVB-
2.5
-4 .
2.0
G 1.5
0.5
t
I
I
I
0.05
0.10
0.15
I_ 0.20
I
4
0.25
0,30
Concentration / mol • dm-3 Fig. 3. The yields (in G-value) of trapped electron and PVB radical anion in v-irradiated MTHF containing PVB. The ~,-irradiation and the measurements were made at 77 K.
anion (Tanaka et al., 1991). The electron transfer ceases when the excess electron in the polymer is trapped by a pyrenyl group as its electron affinity is 0.52 eV higher than that of the neighbouring biphenylyl groups. If one assumes one-dimensional well-type potential for the copolymer at the sites where the pyrenyl groups exist, the existence probability of the excess electron in the region between two pyrenyl groups is expressed by a function of time after a pulse and the position from one of the pyrenyl groups. The probability is obtained by solving a simple partial differential equation, assuming an equal probability for the initial electron-trapping by the biphenylyl groups in the region (Tanaka et al., 1991). The transfer coefficient of the excess electrons was obtained by comparing the observed time-profile to the simulated one as 4 × 10 -6 cm 2 s -~. The mean traveling distance in the polymer for 1/~s is estimated as 30 nm. If one takes 0.5 nm for the average distance between the side groups, the rate of the hopping is roughly estimated as 3 x 109 S-1.
2. Electron trapping and transfer in aromatic substitutedpolyacetylene. Polyacetylenes have drawn much attention because of their potential utility as one-dimensional conductor. This expectation inherently assumes an excess electron or a hold being delocalized over the polymer chain. We questioned whether this assumption was valid or not. In imaginary, metallic, equidistant conjugated polymers, the LUMO band and HOMO band overlap each other and the electron in the HOMO band moves freely in the molecule to show a metallic character. In the polymers containing alternative double bonds, however, the C-C distances of the neighbouring single bond and double bond are different. When a double bond shifts to the next C-C bond, the system sees a potential barrier. This barrier is important when one thinks about the electric conductivity of the polymer. Actually in trans-polyacetylene, for example, Peierls distortion opens up a gap at Fermi level because of the high degree of bond alternation. The electric conductivity of undoped trans-polyacetylene is in the domain of a typical semiconductor. The polymer may be changed co a conductor when it is highly doped with lithium, for instance, to fill the Peierls gap by impurity levels. A pertinent question is: what will happen if an excess electron is introduced into the system? The band structure will never change, because only one excess electron is introduced into the sea of the valence electrons. The electron may be put into the lower edge of the conduction band and the electronic spectrum of the polymer radical anion must be spread over a wide wavelength range from the visible to the IR region. Thus, the band model predicts the delocalization of the excess electron over the main chain of the polymer. Whatever the prediction is, the excess electron introduced to the polymers can be
Electron and energy transfer in polymeric and polymerizable systems used as a probe for the electronic and geometric structure of the polymers. So we studied the irradiated matrices and solutions containing poly (1-phenyl-l-propyne) (PPPr) and poly(phenylacetylene) (PPA) (Ogasawara et al., 1992) Experiments were made mainly on PPPr, because the solution of this polymer was transparent in the visible region, which was a necessary condition for optical absorption measurements. M T H F containing PPPr was frozen at 77 K and subjected to y-irradiation and subsequent spectral measurements at 77 K. The absorption spectra observed from a y-irradiated sample had maxima at 390, 560 and 1200 nm, and a shoulder at 800 nm. These bands, except for the 1200 nm band, are attributed to the polymer radical anion produced by the electron attachment to the solute polymer. This assignment is confirmed by ESR measurement, as will be explained later. The spectrum after the subtraction of the trapped electron band shows a prominent band at 560 nm with a shoulder at about 800 nm and part of a band peaking at 390 nm. Thus, the absorption spectrum of the radical anion of PPPr (PPPr-) was found to range over a wide wavelength region extending from the UV to the near IR region. The difference spectrum between the spectra obtained immediately after y-irradiation and after warming the irradiated sample is analyzed by a two-state model: the original spectrum peaking at 390 and 560 nm is changed to a new one having absorption maxima at 450 and 700 nm. We call the species correlated to the original spectrum Radical Anion I and the spectrum after warming as that of Radical Anion II. The ESR spectrum observed at 77 K from y-irradiated M T H F containing PPPr showed a sharp singlet with a linewidth of 4.25 gauss and an underlying septet [Fig. 4(a)]. The spectrum was, as a whole, similar to that observed in irradiated neat MTHF. The singlet with g-value of 2.002 is mostly due to the trapped electron in MTHF. Photobleaching with the light longer than 1000 nm reduced the intensity of the singlet signal to one third of its original intensity and increased the linewidth up to 7.00 gauss. On warming the sample, the background signal due to M T H F radical disappeared but the singlet remained with its original linewidth kept intact. Table 1 summarizes the linewidths and the g-values of the ESR spectra. The singlet with a wider linewidth is assigned to Radical Anion I and correlated to the absorption spectrum peaking at 390 and 560 urn. The singlet observed after warming is correlated to the Radical Anion II. The spectrum is definitely not due to the trapped electrons in the matrix, because they cannot survive in the warmed sample where the solvent radicals decay completely. The radical anion of fl-methylstyrene was studied as a model of P P P r - . The solid line in Fig. 5 shows a steady-state spectrum obtained from the y-irradiated M T H F containing trans-fl-methylstyrene corn-
a
b
75
A
I 10G t
i
~~'-~
x 2.6
Fig. 4. ESR spectra observed from ),-irradiated MTHF containing PPPr: (a) immediately after ),-irradiation; (b) after photobleaching by the light of 2 > 1000 nm; (c) after warming. The measurements were made at 77 K.
prising a narrow band at 400 nm and a broad one at ca. 600 nm. The sample containing a mixture of trans and cis-fl-methylstyrene gave the same spectra. These two bands in the UV and the visible region are commonly observed in most of the radical anions of aromatic olefins. A computer simulation of the model compound by the MNDOC MO method suggested that the excess charge was delocalized over the olefin and the benzene part. It is striking that the absorption spectra of P P P r - , either Radical Anion I or Radical Anion II, are similar to that of radical anion of monomeric aromatic olefins as shown in Fig. 5. This fact leads us to conclude that the excess electron in the polymer is confined to a relatively narrow region, probably one monomer unit, of the polymer. Now, what about another polymer, PPA? The absorption spectrum showed a charge-resonance band in the IR region which suggested the delocalized nature of the excess electron in PPA radical anion. The trans structure of this polymer favour the overlapping of the two adjacent phenyl rings. Such an overlapping of the phenyl rings may be difficult to be attained in PPPr, because they are separated from each other due to the twisted structure of the main chain, as explained in the following paragraph.
Table 1. Linewidthand g-factor of the ESR spectra obtainedfrom v-irradiatedMTHF containingPPPr Sample Linewidth/gauss g-factor v-irradiated 4.25 2.002 Photobleached 7.00 2.002 Warmed 7.00 2.002
Masaaki Ogasawara
76
Wavelength / nm 500
1000 eCH 3
400 nm 0.15
_
A Y
O
o.lo
-
.J
c "°'''''''`4",
-
b
0.05
....
[ 30,000
25,000
"*'~,,
[
I
20,000
15,000
a
..... -- ...............4........ 10,000
Wavenumber / cm-1 Fig. 5. Absoption spectra of fl-methylstyrene radical anion and PPPr radical anion in MTHF matrices measured at 77 K: (a) fl-methylstyrene radical anion; (b) Radical Anion I of PPPr; (c) Radical Anion II of PPPr.
The twisted nature of PPPr was confirmed by the MNDOC MO calculations on a model compound, di-fl-methylstyrene. The torsion angle between neighbouring double bonds is 91 ° in the neutral molecule and 84 ° in the negatively charged molecule. The benzene ring of the dimer may rotate to some extent after capturing the excess electron, but the main chain remains twisted as before. Briefly, the results of the MO calculation indicate that PPPr should hardly show any electric conductivity. It contrasts to PPA which shows metallic conductivity on doping with iodine. The difference comes from the low conjugation of the alternative double bonds in PPPr due to the twisted structure of the main chain. The confinement of the excess electron in PPPr polymer to approximately one monomer unit is thus attributed to the twisted structure of PPPr arising from the repulsive interactions between the bulky phenyl groups and the methyl groups. The structural relaxation of the PPPr after electron attachment is nominal. The main chain of the polymer is originally twisted and the addition of the excess charge does not induce further modification of the polymer structure. Consequently, the spectral change cannot be explained by the geometric relaxation of the polymer configuration. This conclusion is supported by the fact that the addition of a nonpolar solvent, 3-methylpentane, has effect neither on the spectrum of Radical Anion I nor on that of Radical Anion II. The spectral change observed on warming the
sample is ascribed to the electron transfer from one site to another in the polymer which gives rise to a different electronic state of the excess electron; namely, from the state of Radical Anion I to the state of Radical Anion II. The efficient conversion from Radical Anion I to Radical Anion II suggests a higher electron affinity of the site which gives Radical Anion II. A plausible mechanism is site-to-site hopping of the electron via the main chain, which leads to the ultimate stabilization of the electron in a deeper trap. Possible structures of the diad in the PPPr polymer chain are listed in Fig. 6. One can categorize these structures into three types in terms of the combination of the side groups. Type I is similar to fl-methylstyrene and type II is similar to stilbene; the existence of these two structures in the polymer was confirmed by UV spectra of the polymer M T H F as shown in Fig. 7. Type I is the most abundant in the polymer, so that most of the electrons produced by ionizing radiation are trapped by this structure in the polymer. Type III may not trap the electron efficiently. Type II is rarely seen in the polymer because of the steric hindrance, b u t its electron affinity is the highest among the three types, In conclusion, the excess electron is first trapped by the type I structure and then migrates via the main chain to be ultimately trapped by the type II structure in the polymer (Fig. 8). The latter structure is supported by the absorption spectrum of Radical
Electron and energy transfer in polymeric and polymerizable systems Anion II which is similar to that of the stilbene radical anion.
3. Energy transfer in polymerizable compounds: 2 + 2 cycloaddition of 2,5-distyrylpyrazine in the crystalline phase. It is known that photo-illumination on single crystals of 2,5-distyrylpyrazine (DSP) induces a 2 + 2 cycloaddition in the solid state as shown in Fig. 9 (Hasegawa et al., 1969). The reaction is a typical topological polymerization, in which ordered molecules undergo a geometrically restricted reaction in a well-defined environment. One can imagine that the energy transfer process may play a dominant role in this reaction. In the last part of this article the author touches upon the recent experiments on this system, aimed at obtaining basic data for application of ionizing radiation to this system. An interesting feature of photoinduced reaction in DSP in crystalline phase is that the illumination by the light of 2 < 400 nm leads only to the oligomer formation (Tamaki et al., 1972). To clarify the reason for such a difference, we measured the steady-state and the time-dependent emission spectra of the photo-illuminated DSP in solution, microcrystalline state, and single crystalline state.
In dilute solution the DSP molecule is supposed to exist in isolated state, so the fluorescence spectrum obtained in the solution is mostly due to the isolated DSP monomers. In crystalline state, the spectrum was shifted as a whole to the lower energy side and new shoulders were observed at both sides of the main peak. The intensity of the shoulder at 470 nm was changed by changing the angle of the single crystal. Figure 10 shows the polarized fluorescence spectrum obtained by time-resolved emission spectroscopy of the single crystal of DSP. For comparison, the absorption spectrum obtained by Peachey and Eckhardt (1993) by steady-state reflectance spectroscopy is also shown. It turned out that the structure shown in the fluorescence spectrum was not exclusively due to the vibrational modes of DSP. The bands around 444 nm had the same lifetime as that of the isolated DSP in T H F solution. However, the lifetimes of the 495 nm band and those of the bands in the lower energy side were distinctly different from the lifetime of isolated DSP monomer. From a careful examination of the spectra, we concluded that the 0-0 band of the isolated DSP monomer was at about 444 nm and the bands
c.3
CH. CH3
77
CH3
TypeIII
Type I
CH3 ~CH 3 I
~ C H CH3~
3
TypeII
~
CH3
CH3 H
-J@
C
3
H3C~ ~ ' ~ 1 1 f~,CH3
Fig. 6. Possible structures of diad in PPPr polymer chain.
?
Masaaki Ogasawara
78 -I
I
I
I
-
CH 3
0.5 mM
/
"~"~,.~,
..........0,25 m M
©
--
. . . . . . 0.1 m M _ _ .0,05 mM
I,
280
240
320 Wavelength
360
400
/ nm
Fig. 7. The UV spectra of PPPr in MTHF at room temperature. appearing in the lower energy side of the 495 nm were due to different excimers, i.e. dimer and trimer produced in the crystals. The last conclusion is consistent with the previous assignment (Ebeid and Bridge, 1984). We think that the long tail toward lower energy side ( > 444 rim) of the absorption spectrum is due to the transition to the predestined excimer states; contrary to the situation in the gas phase and the liquid phase, two or more molecules are forced to exist side by side because of the lattice structure of the crystal, resulting in the transition from the ground state to the excimer state being observed.
An important conclusion drawn from the above arguments is that the "single crystal" of DSP is not really homogeneous in nature but it includes variety of sites comprising of monomer, dimer, trimer, etc. These sites may be a kind of defect of the crystal produced during the preparation or photo-illumination. The energy absorbed by the crystal is first delocalized over the crystal in the form of excitation and then localized at one of the heterogeneous parts of the crystal; the excitation energy is ultimately lost by radiative and non-radiative transitions. Since a similar emission spectrum was obtained from pulse radiolysis measurements, we expect more or less
e-
•
Type I
kma x
-- ~j"390 nm - - 5 6 0 nm L
Type
°
II
kmax = ~ 450 nm I . 7 0 0 nm
Fig. 8. Site-to-site electron transfer in PPP~-.
Electron and energy transfer in polymeric and polymerizable systems
79
Fig. 9. Cycloaddition of 2,5-styrylpyrazine (open and closed circles indicate r~-orbitals). similar energy transfer processes to occur in the electron-irradiated crystal. The peculiar reaction of the DSP crystal depending on the wavelength of the incident light is tentatively interpreted in terms of the extent of the n-conjugation of the DSP molecule. The emission band due to the isolated DSP monomer in the oligomerized crystal is shifted to higher energy side in comparison
495
nm
to that of the monomer crystal (Peachey and Eckhardt, 1994). This is due to the breaking of the n-conjugation of the DSP monomer. As shown in Fig. 9, when the 2 + 2 cycloaddition takes place at one of the double-bond pairs in the crystal, the n-conjugation through the double bond is broken and the styrylpyrazine part of the other side of the molecule is separated from the n-conjugation of the
435 nm
,
!i
-
60
-
50 o
0
-
40
-
30
-
20
m
10
O
E~
< >
_..I 15,000
...............
I-........... ""' 20.000
"~
I 25,000 Wavenumber
30,000
35,000
/ c m -1
Fig. 10. The polarized, low temperature absorption spectra of DSP single crystal. The absorption spectrum was taken from Peachey and Eckhardt (1993).
Masaaki Ogasawara
80
benzene ring of this side. The energy o f light of 2 > 400 nm is not enough to excite this isolated part and further reactions may not be possible. But, when the oligomerized crystal is illuminated by light of 2 < 400 nm, the isolated styrylpyrazine part is excited to induce the cycloaddition reaction with another isolated styrylpyrazine part. Acknowledgements--The author would like to express his sincere thanks to Professor Hiroshi Yoshida (Hokkaido University), Professor Hitoshi Yamaoka (Kyoto University), Dr Hirotomo Hase (Kyoto University), Dr Yasuyuki Takenaka (Hokkaido University of Education), Dr Yoko Miyatake (Osaka University), Mr Takeshi Suganuma, Mr Kuniyuki Hayashi and Mr Wataru Ishida for their help in completing this work. The author also greatly appreciates the financial support from the Ministry of Education, Sports and Culture (Grant-in-Aid for Scientific Research No. 06453013). REFERENCES
Ebeid E.-Z. M. and Bridge N. J. (1984). J. Cheml Soc. Faraday Trans. 1 80, 1113. Guillet T. (1985) In Polymer Photophysics and Photochemistry, p. 35. Cambridge University Press, Cambridge.
Hasegawa M., Suzuki Y., Suzuki F. and Nakanishi H. (1969). J. Polym. Sci. A1 7, 743. Jortner J., Bixon M., Heitele H. and Michael-Beyerle M. E. (1992): Chem. Phys. Lett. 197, 131. Kato N., Miyazaki T., Fueki K. and Saito A. (1988). Int. J. Chem. Kinet. 20, 877. Matsushima M., Kato N., Miyazaki T. and Fueki K. (1987). Radiat. Phys. Chem. 29, 231. Miller J. (1972). J. Chem. Phys. 56, 5173. Ogasawara M., Suganuma T., Junke N., Yamaoka H. and Yoshida H. (1992)~ Radiat. Phys. Chem. 40, 111. Ogasawara M., Tanaka M. and Yoshida H. (1987). J. Phys. Chem. 91, 937. Peachey N. M. and Eckhardt C. J. (1993). J. Am. Chem. Soc. 115, 3519. Peachey N. M. and Eckhardt C. J. (1994). J. Am. Chem. Soc. 98, 685. Schiller R. (1991) In Excess Electron in Dielectric Media (Edited by Ferradini C. and Jay-Gerin J.P.), p. 105. Schiller R. (1992). J. Chem. Phys. 96, 6531. Tachikawa H., Yoshida H. and Ogasawara M. (1991). Radiat. Phys. Chem. 37, 107. Tamaki T., Suzuki Y. and Hasegawa M. (1972). Bull. Chem. Soc. Japan 45, 1988. Tanaka M., Yoshida H. and Ogasawara M. (1989). Radiat. Phys. Chem. 34, 591. Tanaka M., Yoshida H. and Ogasawara M. (1991). J. Phys. Chem. 95, 956.
Pergamon
E L E C T R O N A N D E N E R G Y T R A N S F E R IN P O L Y M E R I C A N D P O L Y M E R I Z A B L E SYSTEMS MASAAK] OGASAWARA Center for Research and Development in Higher Education, Hokkaido University, Sapporo 060, Japan Abstract--Studies on electron and energy transfer in polymeric and polymerizable systems are summarized. Electron capture and geminate recombination in solution and matrix containing poly(methyl methacrylate) and poly-(4-vinylbiphenyl)are first described. Later on, intramolecular electron transfer in poly-(4-vinylbiphenyl)in solution is elaborated. Excess electrons produced by ionizing radiation attach to the polymers and then get transferred via the side-group until they are finally trapped by a different kind of side group having higher electron affinity. Further, the electron trapping and the electron transfer in substituted polyacetylene are described. An excess electron gets localized on one or two monomer units depending on the kind of substituent of the polymer. Electron transfer from shallowtrap to deeper one via the alternative double bonds in the main chain is observed in the solution of poly(1-phenyl-1-propyne). Finally, the excitation energy transfer in laser-irradiated crystalline 2,5-distyrylpyrazine is investigated with special reference to the structure of the crystal and the polymerization mechanism. Copyright © 1997 Elsevier Science Ltd
INTRODUCTION
were synthesized. Substituted polyacetylenes such as poly(1-phenyl-l-propyne) were prepared in the laboratory of Professor Higashimura, Kyoto University. 2,5-Distyrylpyrazine was synthezied following the method reported by Hasegawa et al. (1969) and purified by repeated recrystallization. Solvents used were purified by the usual procedures (Ogasawara et al., 1987; Tanaka et al., 1991).
The elementary processes induced by ionizingradiation in condensed phase have been understood fairly well. Following the transfer of energy from radiation to medium, ionization and electronic excitation of constituent molecules occur. Molecules and ions excited to the levels above their dissociation limit undergo self-decomposition. Electrons ejected into the medium are thermalized by transferring their excess energy to the environment, and under appropriate conditions, attach to the solute molecules. The remaining parent cations may also move and react with other molecules. However, in case the molecular weight of the solute is very large, the large body of the solute molecule is affected, especially when the electron and energy transfer processes give rise to different reaction sequences. This article summarizes our studies to unveil the electron and energy migration in polymeric systems. Part of the results has been published elsewhere, but some of the results have been newly obtained. Preliminary results on energy transfer process in photoinduced polymerization in crystalline system are also included for comparison.
Irradiation and measurements Solutions were prepared under vacuum and sealed into quartz cells, with a light path of 10 nun for pulse radiolysis and 1.5 mm for 7-radiolysis. For steadystate measurements, samples were irradiated with y-rays from 6°Co at 77 K in the dark. Optical absorptions were measured at 77 K with a Shimadzu MPS-5000 spectrophotometer. The Hokkaido University 45 MeV electron linear accelerator was used as the source of the electron beam for pulse radiolysis experiments. The half-width of the electron pulse was 50 ns. Time-dependent fluorescence measurements were made on solutions, microcrystalline samples and single crystals, by using Hamamatsu Photonics C4780 picosecond fluorescence lifetime measurement system. Excitation was made by using 337 nm light from a N2-1aser.
EXPERIMENTAL RESULTS AND DISCUSSION
Materials
1. Electron capture, geminate recombination, and intramolecular electron transfer in polymeric systems. What do polymers look like in solution? According to a computer simulation, a molecule of polyethylene, for example, looks like a waste thread floating in water (Guillet, 1985). To express the volume of a
Poly(methyl methacrylate) and substituted poly (methyl methacrylate) from Science Products SP3 were purified by the usuat procedures (Ogasawara et al., 1987). 4-Vinylbiphenyl (Aldrich) was purified by repeated vacuum sublimation. Poly(4-vinylbiphenyl) and poly(4-vinylbiphenyl-co-1-vinylpyrene) RPC 49/1--D
71
72
Masaaki Ogasawara
coiled polymer, the radius of a sphere in which a polymer rotates around a virtual center is being used. The word "polymer domain", which appears in this text, means such a sphere. (a) Electron capture volume o f polymers in matrices. We made one of the first electron attachment experiments in a polymeric system using poly(methyl methacrylate) (PMMA) (Ogasawara et al., 1987). PMMA is one of the most popular and widely-used polymers, but whether it captures electrons or not had been a matter of dispute for a long time. We found that the radical anions of PMMA and its analogues were actually produced by the reaction of solvated electrons with polymers in hexamethylphosphorictriamide. The absorption spectrum of the radical anion has a strong band in the wavelength region < 300 nm and a shoulder at 450 nm. An ab initio MO calculation on a model compound, methyl isobutyrate, suggested that the structure was strongly deformed by the attachment of an excess electron (Tachikawa et al., 1991). The C-C bond of the ether part was found to be 0.15 urn; the bond is extended by 0.04 nm on adding an excess electron to the molecule. The carbonyl group of the radical anion deviates from the Cs symmetry; the excess electron in the polymer is stabilized in the form of deformed anion. This is in contrast to the carbonyl groups of the neutral radical and radical cation, in which the geometry of the carbonyl part is approximately planar. From the orbital phase analysis, the weak absorption in the visible band (400-500 nm) was assigned to the n-n* transition, which was partially allowed owing to the bent configuration of the radical anion. On the basis of the results obtained in these studies, we have investigated the electron capturing reaction of PMMA in 2-methyltetrahydrofuran (MTHF) matrix at low temperature. It is well known that electrons trapped in this matrix at low temperature migrate by tunneling mechanism to react with a solute molecule. Although there are a number of recent electron models (Schiller, 1991, 1992; Jormer et al., 1992), the following conventional treatment of data is still useful. The surviving probability P of the trapped electron decreases exponentially with increasing solute concentration (Miller, 1972). Thus, the capture volume v for the excess electron may be estimated from the gradient of the straight line in the logarithmic plot of the surviving probability against the polymer concentration: -
logP
=
only 38% of the one for a model compound, methyl isobutyrate. In the case of a low-molecular-weight molecule, the solute molecules are distributed homogeneously in the system and the shape of each capture space must be sphere-like. While, in the case of polymers, the chromophores are connected side-by-side to the main chain and their capture spaces must overlap with each other. For this reason the capture space for each polymer cannot be sphere-like anymore. If one assumes a rod-like capture-space for the polymer, the radius of the rod is estimated to be 1.5 nm from our results. This value is comparable to 1.2 nm for methyl isobutyrate. The idea of rod-like capture volume was first proposed by Matsushima et al. (1987) in the study of the poly(4-vinylbiphenyl) (PVB) system. This model is effective only when the overlapping of the capture space is not very extensive. Figure 1 shows the logarithmic plot of the surviving probability of the trapped electron against polymer concentration in the P M M A - M T H F system. When the concentration is higher than 0.2 baseunit mol dm -3, the surviving probability abruptly decreases and then approaches a plateau value. The result suggests a kind of phase transition in the M T H F solution. A simple explanation is that the entangled polymers in the polymer domains spread over the system to form homogeneous, network-like structures at higher concentration; it reduces the overlapping of the capture volume of the chro-
2.5
f
2.0
1.5
/
e~
_= I
1.0
0.5
/
/
/
/f
/
0.2
0.4
i
i
0.6
0.8
Concentration / rrlol . dm-3
vC
where the polymer concentration C is expressed in base-unit. The capture volume means a volume in which all of the excess electrons are "absorbed" by the solute molecule after the 7-irradiation until the time of measurements. The above equation explains the case for PMMA when the polymer concentration is lower than 0.2 base-unit mol dm -3. The capture volume was found to be 7 x 10 -21 cm 3, which was
,-Ph_ Fig, 1. Smwiving probability of the trapped electron against polymer concentration in ~-irradiated M T H F containing poly(methyl methacrylate). The ~-irradiation and measurements were made at 77 K. The polymer concentration is expressed by base-unit.
Electron and energy transfer in polymeric and polymerizable systems
73 CH 2 ~
CH -
M T H F solvent ~ S+
Polymer
Domain
Fig. 2. Illustration of the ion-recombination reaction of PVB radical anion (Bp-) with solvent cation (S ÷) in irradiated PVB-MTHF system. Part of the illustration is taken from Kato et al. (1988).
mophores to the minimum and enhances the electron trapping by the polymers. (b) Retarded geminate recombination in polymer domains. When M T H F solution containing PVB is
irradiated by ionizing radiation, ion-pairs of electron and parent solvent-cation are produced anywhere in the system. Some of the electrons undergo reaction with biphenylyl side groups of the solute polymer and the rest may survive for a long time under cryogenic conditions. Different from the P M M A case, the logarithmic plot of the surviving probability of the trapped electron against the polymer concentration was straight up to the limit of the dissolution of the polymer. Matsushima et al. (1987) reported that the tunneling radius was 4 nm, assuming a rod-like shape for the capture volume o f PVB. This result is consistent with the capture volume of biphenyl, obtained assuming a sphere-like shape. We have found two interesting features in electron transfer reactions in the PVB-MTHF system. One is retarded geminate recombination in the polymer domains (Tanaka et al., 1989). In the pulse radiolysis of the liquid phase, the radical anion of PVB decayed following usual second-order kinetics when the polymer concentration was as low as 0.03 baseunit tool d i n - a. The rate constant was about half of that of the homogeneous ion-recombination reaction of monomeric biphenylyl anion. This suggests that the electrons are chemically trapped by the polymers and the produced anions are attacked by solvent-cations coming from the exterior of the polymer domain (see Fig. 2). When the polymer concentration was increased up to 0.3 base-unit mol dm -3, a spike was
found in the time profile of the PVB radical anion. The spike was not affected by dose per pulse and it was ascribed to geminate recombination of the polymer anions with the solvent cations. Geminate recombination between the monomeric biphenylyl radical anion and the solvent cation is very fast at room temperature and cannot be observed in sub-microsecond time scale. At low polymer concentration, the solvent cation can move freely, so that the geminate recombination is not observable by conventional pulse radiolysis technique. But at very high polymer concentration, the ratio of the polymer domain to the total reaction space becomes significant. Part of the geminate pairs are produced in the viscous polymer domains as shown in Fig. 2; the recombination reaction in the interior of the domain is effectively retarded. This may be the reason why the spike was observed in the time-profile. The existence of the polymer domain in this particular system was first suggested by Kato et al. (1988) from the kinetic analysis of the polymer anion formation. Our recent measurements on the effect of nonpolar solvent on the spike intensity confirmed this explanation. The spike was reduced in its intensity by the addition of 3-methylpentane and it was not observable anymore when 3-methylpentane was added up to 30 vol%. The addition of the nonpolar solvent enhances the entanglement of the polymer coil to reduce the size of the polymer domain. Another interesting feature observed is that the sum of the trapped electron yield and the PVB anion yield becomes much smaller with increasing polymer concentration (Fig. 3). The total g-value determined
74
Masaaki Ogasawara
by steady state measurements at 0.3 baseunit mol d m - 3 was only a half of the value obtained at zero polymer concentration. We confirmed that there was no initial fast decay at 77 K and the initial yield determined by pulse radiolysis coincided with the one observed at 77 K by steady state measurement. A plausible explanation is as follows: the polymer domains are crowded with aromatic groups of the polymer, which reduce the local dielectric constant or lower the edge of the conduction band in the polymer domains giving rise to enhanced ion-recombination between the electron and the hole before the stabilization of the excess electron in the matrix. The lower edge of the conduction band is correlated to the energy of the quasi-free electron, V0. The effect of V0 on the free-ion yield has been theoretically explained by Schiller (1992) on the basis of H-like atoms model of the pairs of ions and low-energy electrons.
(c) Intramolecular electron transfer in polymer. A question arises as to whether the electron localized at a biphenylyl side group of PVB really moves in the polymer. As the electron transfer in the polymers to identical chromophores was difficult to observe, we synthesized poly(4-vinylbiphenyl-co-1-vinylpyrene), in which a small number of biphenylyl side groups were substituted by pyrenyl groups. The excess electron produced by pulse radiolysis attaches to one of the biphenylyl groups and migrates to another side-group until it is stably trapped by a pyrenyl group in the polymer. The pyrenyl group was chosen as the indicator because the spectral overlap between anion radicals of biphenyl and pyrene was minimum among possible combinations of aromatic compounds. The experimental results altogether indicated the existence of electron transfer from one side group to another within the life-time of the polymeric radical 3.0 +
I
I
I
I
I [] ei" @ PVB-
2.5
-4 .
2.0
G 1.5
0.5
t
I
I
I
0.05
0.10
0.15
I_ 0.20
I
4
0.25
0,30
Concentration / mol • dm-3 Fig. 3. The yields (in G-value) of trapped electron and PVB radical anion in v-irradiated MTHF containing PVB. The ~,-irradiation and the measurements were made at 77 K.
anion (Tanaka et al., 1991). The electron transfer ceases when the excess electron in the polymer is trapped by a pyrenyl group as its electron affinity is 0.52 eV higher than that of the neighbouring biphenylyl groups. If one assumes one-dimensional well-type potential for the copolymer at the sites where the pyrenyl groups exist, the existence probability of the excess electron in the region between two pyrenyl groups is expressed by a function of time after a pulse and the position from one of the pyrenyl groups. The probability is obtained by solving a simple partial differential equation, assuming an equal probability for the initial electron-trapping by the biphenylyl groups in the region (Tanaka et al., 1991). The transfer coefficient of the excess electrons was obtained by comparing the observed time-profile to the simulated one as 4 × 10 -6 cm 2 s -~. The mean traveling distance in the polymer for 1/~s is estimated as 30 nm. If one takes 0.5 nm for the average distance between the side groups, the rate of the hopping is roughly estimated as 3 x 109 S-1.
2. Electron trapping and transfer in aromatic substitutedpolyacetylene. Polyacetylenes have drawn much attention because of their potential utility as one-dimensional conductor. This expectation inherently assumes an excess electron or a hold being delocalized over the polymer chain. We questioned whether this assumption was valid or not. In imaginary, metallic, equidistant conjugated polymers, the LUMO band and HOMO band overlap each other and the electron in the HOMO band moves freely in the molecule to show a metallic character. In the polymers containing alternative double bonds, however, the C-C distances of the neighbouring single bond and double bond are different. When a double bond shifts to the next C-C bond, the system sees a potential barrier. This barrier is important when one thinks about the electric conductivity of the polymer. Actually in trans-polyacetylene, for example, Peierls distortion opens up a gap at Fermi level because of the high degree of bond alternation. The electric conductivity of undoped trans-polyacetylene is in the domain of a typical semiconductor. The polymer may be changed co a conductor when it is highly doped with lithium, for instance, to fill the Peierls gap by impurity levels. A pertinent question is: what will happen if an excess electron is introduced into the system? The band structure will never change, because only one excess electron is introduced into the sea of the valence electrons. The electron may be put into the lower edge of the conduction band and the electronic spectrum of the polymer radical anion must be spread over a wide wavelength range from the visible to the IR region. Thus, the band model predicts the delocalization of the excess electron over the main chain of the polymer. Whatever the prediction is, the excess electron introduced to the polymers can be
Electron and energy transfer in polymeric and polymerizable systems used as a probe for the electronic and geometric structure of the polymers. So we studied the irradiated matrices and solutions containing poly (1-phenyl-l-propyne) (PPPr) and poly(phenylacetylene) (PPA) (Ogasawara et al., 1992) Experiments were made mainly on PPPr, because the solution of this polymer was transparent in the visible region, which was a necessary condition for optical absorption measurements. M T H F containing PPPr was frozen at 77 K and subjected to y-irradiation and subsequent spectral measurements at 77 K. The absorption spectra observed from a y-irradiated sample had maxima at 390, 560 and 1200 nm, and a shoulder at 800 nm. These bands, except for the 1200 nm band, are attributed to the polymer radical anion produced by the electron attachment to the solute polymer. This assignment is confirmed by ESR measurement, as will be explained later. The spectrum after the subtraction of the trapped electron band shows a prominent band at 560 nm with a shoulder at about 800 nm and part of a band peaking at 390 nm. Thus, the absorption spectrum of the radical anion of PPPr (PPPr-) was found to range over a wide wavelength region extending from the UV to the near IR region. The difference spectrum between the spectra obtained immediately after y-irradiation and after warming the irradiated sample is analyzed by a two-state model: the original spectrum peaking at 390 and 560 nm is changed to a new one having absorption maxima at 450 and 700 nm. We call the species correlated to the original spectrum Radical Anion I and the spectrum after warming as that of Radical Anion II. The ESR spectrum observed at 77 K from y-irradiated M T H F containing PPPr showed a sharp singlet with a linewidth of 4.25 gauss and an underlying septet [Fig. 4(a)]. The spectrum was, as a whole, similar to that observed in irradiated neat MTHF. The singlet with g-value of 2.002 is mostly due to the trapped electron in MTHF. Photobleaching with the light longer than 1000 nm reduced the intensity of the singlet signal to one third of its original intensity and increased the linewidth up to 7.00 gauss. On warming the sample, the background signal due to M T H F radical disappeared but the singlet remained with its original linewidth kept intact. Table 1 summarizes the linewidths and the g-values of the ESR spectra. The singlet with a wider linewidth is assigned to Radical Anion I and correlated to the absorption spectrum peaking at 390 and 560 urn. The singlet observed after warming is correlated to the Radical Anion II. The spectrum is definitely not due to the trapped electrons in the matrix, because they cannot survive in the warmed sample where the solvent radicals decay completely. The radical anion of fl-methylstyrene was studied as a model of P P P r - . The solid line in Fig. 5 shows a steady-state spectrum obtained from the y-irradiated M T H F containing trans-fl-methylstyrene corn-
a
b
75
A
I 10G t
i
~~'-~
x 2.6
Fig. 4. ESR spectra observed from ),-irradiated MTHF containing PPPr: (a) immediately after ),-irradiation; (b) after photobleaching by the light of 2 > 1000 nm; (c) after warming. The measurements were made at 77 K.
prising a narrow band at 400 nm and a broad one at ca. 600 nm. The sample containing a mixture of trans and cis-fl-methylstyrene gave the same spectra. These two bands in the UV and the visible region are commonly observed in most of the radical anions of aromatic olefins. A computer simulation of the model compound by the MNDOC MO method suggested that the excess charge was delocalized over the olefin and the benzene part. It is striking that the absorption spectra of P P P r - , either Radical Anion I or Radical Anion II, are similar to that of radical anion of monomeric aromatic olefins as shown in Fig. 5. This fact leads us to conclude that the excess electron in the polymer is confined to a relatively narrow region, probably one monomer unit, of the polymer. Now, what about another polymer, PPA? The absorption spectrum showed a charge-resonance band in the IR region which suggested the delocalized nature of the excess electron in PPA radical anion. The trans structure of this polymer favour the overlapping of the two adjacent phenyl rings. Such an overlapping of the phenyl rings may be difficult to be attained in PPPr, because they are separated from each other due to the twisted structure of the main chain, as explained in the following paragraph.
Table 1. Linewidthand g-factor of the ESR spectra obtainedfrom v-irradiatedMTHF containingPPPr Sample Linewidth/gauss g-factor v-irradiated 4.25 2.002 Photobleached 7.00 2.002 Warmed 7.00 2.002
Masaaki Ogasawara
76
Wavelength / nm 500
1000 eCH 3
400 nm 0.15
_
A Y
O
o.lo
-
.J
c "°'''''''`4",
-
b
0.05
....
[ 30,000
25,000
"*'~,,
[
I
20,000
15,000
a
..... -- ...............4........ 10,000
Wavenumber / cm-1 Fig. 5. Absoption spectra of fl-methylstyrene radical anion and PPPr radical anion in MTHF matrices measured at 77 K: (a) fl-methylstyrene radical anion; (b) Radical Anion I of PPPr; (c) Radical Anion II of PPPr.
The twisted nature of PPPr was confirmed by the MNDOC MO calculations on a model compound, di-fl-methylstyrene. The torsion angle between neighbouring double bonds is 91 ° in the neutral molecule and 84 ° in the negatively charged molecule. The benzene ring of the dimer may rotate to some extent after capturing the excess electron, but the main chain remains twisted as before. Briefly, the results of the MO calculation indicate that PPPr should hardly show any electric conductivity. It contrasts to PPA which shows metallic conductivity on doping with iodine. The difference comes from the low conjugation of the alternative double bonds in PPPr due to the twisted structure of the main chain. The confinement of the excess electron in PPPr polymer to approximately one monomer unit is thus attributed to the twisted structure of PPPr arising from the repulsive interactions between the bulky phenyl groups and the methyl groups. The structural relaxation of the PPPr after electron attachment is nominal. The main chain of the polymer is originally twisted and the addition of the excess charge does not induce further modification of the polymer structure. Consequently, the spectral change cannot be explained by the geometric relaxation of the polymer configuration. This conclusion is supported by the fact that the addition of a nonpolar solvent, 3-methylpentane, has effect neither on the spectrum of Radical Anion I nor on that of Radical Anion II. The spectral change observed on warming the
sample is ascribed to the electron transfer from one site to another in the polymer which gives rise to a different electronic state of the excess electron; namely, from the state of Radical Anion I to the state of Radical Anion II. The efficient conversion from Radical Anion I to Radical Anion II suggests a higher electron affinity of the site which gives Radical Anion II. A plausible mechanism is site-to-site hopping of the electron via the main chain, which leads to the ultimate stabilization of the electron in a deeper trap. Possible structures of the diad in the PPPr polymer chain are listed in Fig. 6. One can categorize these structures into three types in terms of the combination of the side groups. Type I is similar to fl-methylstyrene and type II is similar to stilbene; the existence of these two structures in the polymer was confirmed by UV spectra of the polymer M T H F as shown in Fig. 7. Type I is the most abundant in the polymer, so that most of the electrons produced by ionizing radiation are trapped by this structure in the polymer. Type III may not trap the electron efficiently. Type II is rarely seen in the polymer because of the steric hindrance, b u t its electron affinity is the highest among the three types, In conclusion, the excess electron is first trapped by the type I structure and then migrates via the main chain to be ultimately trapped by the type II structure in the polymer (Fig. 8). The latter structure is supported by the absorption spectrum of Radical
Electron and energy transfer in polymeric and polymerizable systems Anion II which is similar to that of the stilbene radical anion.
3. Energy transfer in polymerizable compounds: 2 + 2 cycloaddition of 2,5-distyrylpyrazine in the crystalline phase. It is known that photo-illumination on single crystals of 2,5-distyrylpyrazine (DSP) induces a 2 + 2 cycloaddition in the solid state as shown in Fig. 9 (Hasegawa et al., 1969). The reaction is a typical topological polymerization, in which ordered molecules undergo a geometrically restricted reaction in a well-defined environment. One can imagine that the energy transfer process may play a dominant role in this reaction. In the last part of this article the author touches upon the recent experiments on this system, aimed at obtaining basic data for application of ionizing radiation to this system. An interesting feature of photoinduced reaction in DSP in crystalline phase is that the illumination by the light of 2 < 400 nm leads only to the oligomer formation (Tamaki et al., 1972). To clarify the reason for such a difference, we measured the steady-state and the time-dependent emission spectra of the photo-illuminated DSP in solution, microcrystalline state, and single crystalline state.
In dilute solution the DSP molecule is supposed to exist in isolated state, so the fluorescence spectrum obtained in the solution is mostly due to the isolated DSP monomers. In crystalline state, the spectrum was shifted as a whole to the lower energy side and new shoulders were observed at both sides of the main peak. The intensity of the shoulder at 470 nm was changed by changing the angle of the single crystal. Figure 10 shows the polarized fluorescence spectrum obtained by time-resolved emission spectroscopy of the single crystal of DSP. For comparison, the absorption spectrum obtained by Peachey and Eckhardt (1993) by steady-state reflectance spectroscopy is also shown. It turned out that the structure shown in the fluorescence spectrum was not exclusively due to the vibrational modes of DSP. The bands around 444 nm had the same lifetime as that of the isolated DSP in T H F solution. However, the lifetimes of the 495 nm band and those of the bands in the lower energy side were distinctly different from the lifetime of isolated DSP monomer. From a careful examination of the spectra, we concluded that the 0-0 band of the isolated DSP monomer was at about 444 nm and the bands
c.3
CH. CH3
77
CH3
TypeIII
Type I
CH3 ~CH 3 I
~ C H CH3~
3
TypeII
~
CH3
CH3 H
-J@
C
3
H3C~ ~ ' ~ 1 1 f~,CH3
Fig. 6. Possible structures of diad in PPPr polymer chain.
?
Masaaki Ogasawara
78 -I
I
I
I
-
CH 3
0.5 mM
/
"~"~,.~,
..........0,25 m M
©
--
. . . . . . 0.1 m M _ _ .0,05 mM
I,
280
240
320 Wavelength
360
400
/ nm
Fig. 7. The UV spectra of PPPr in MTHF at room temperature. appearing in the lower energy side of the 495 nm were due to different excimers, i.e. dimer and trimer produced in the crystals. The last conclusion is consistent with the previous assignment (Ebeid and Bridge, 1984). We think that the long tail toward lower energy side ( > 444 rim) of the absorption spectrum is due to the transition to the predestined excimer states; contrary to the situation in the gas phase and the liquid phase, two or more molecules are forced to exist side by side because of the lattice structure of the crystal, resulting in the transition from the ground state to the excimer state being observed.
An important conclusion drawn from the above arguments is that the "single crystal" of DSP is not really homogeneous in nature but it includes variety of sites comprising of monomer, dimer, trimer, etc. These sites may be a kind of defect of the crystal produced during the preparation or photo-illumination. The energy absorbed by the crystal is first delocalized over the crystal in the form of excitation and then localized at one of the heterogeneous parts of the crystal; the excitation energy is ultimately lost by radiative and non-radiative transitions. Since a similar emission spectrum was obtained from pulse radiolysis measurements, we expect more or less
e-
•
Type I
kma x
-- ~j"390 nm - - 5 6 0 nm L
Type
°
II
kmax = ~ 450 nm I . 7 0 0 nm
Fig. 8. Site-to-site electron transfer in PPP~-.
Electron and energy transfer in polymeric and polymerizable systems
79
Fig. 9. Cycloaddition of 2,5-styrylpyrazine (open and closed circles indicate r~-orbitals). similar energy transfer processes to occur in the electron-irradiated crystal. The peculiar reaction of the DSP crystal depending on the wavelength of the incident light is tentatively interpreted in terms of the extent of the n-conjugation of the DSP molecule. The emission band due to the isolated DSP monomer in the oligomerized crystal is shifted to higher energy side in comparison
495
nm
to that of the monomer crystal (Peachey and Eckhardt, 1994). This is due to the breaking of the n-conjugation of the DSP monomer. As shown in Fig. 9, when the 2 + 2 cycloaddition takes place at one of the double-bond pairs in the crystal, the n-conjugation through the double bond is broken and the styrylpyrazine part of the other side of the molecule is separated from the n-conjugation of the
435 nm
,
!i
-
60
-
50 o
0
-
40
-
30
-
20
m
10
O
E~
< >
_..I 15,000
...............
I-........... ""' 20.000
"~
I 25,000 Wavenumber
30,000
35,000
/ c m -1
Fig. 10. The polarized, low temperature absorption spectra of DSP single crystal. The absorption spectrum was taken from Peachey and Eckhardt (1993).
Masaaki Ogasawara
80
benzene ring of this side. The energy o f light of 2 > 400 nm is not enough to excite this isolated part and further reactions may not be possible. But, when the oligomerized crystal is illuminated by light of 2 < 400 nm, the isolated styrylpyrazine part is excited to induce the cycloaddition reaction with another isolated styrylpyrazine part. Acknowledgements--The author would like to express his sincere thanks to Professor Hiroshi Yoshida (Hokkaido University), Professor Hitoshi Yamaoka (Kyoto University), Dr Hirotomo Hase (Kyoto University), Dr Yasuyuki Takenaka (Hokkaido University of Education), Dr Yoko Miyatake (Osaka University), Mr Takeshi Suganuma, Mr Kuniyuki Hayashi and Mr Wataru Ishida for their help in completing this work. The author also greatly appreciates the financial support from the Ministry of Education, Sports and Culture (Grant-in-Aid for Scientific Research No. 06453013). REFERENCES
Ebeid E.-Z. M. and Bridge N. J. (1984). J. Cheml Soc. Faraday Trans. 1 80, 1113. Guillet T. (1985) In Polymer Photophysics and Photochemistry, p. 35. Cambridge University Press, Cambridge.
Hasegawa M., Suzuki Y., Suzuki F. and Nakanishi H. (1969). J. Polym. Sci. A1 7, 743. Jortner J., Bixon M., Heitele H. and Michael-Beyerle M. E. (1992): Chem. Phys. Lett. 197, 131. Kato N., Miyazaki T., Fueki K. and Saito A. (1988). Int. J. Chem. Kinet. 20, 877. Matsushima M., Kato N., Miyazaki T. and Fueki K. (1987). Radiat. Phys. Chem. 29, 231. Miller J. (1972). J. Chem. Phys. 56, 5173. Ogasawara M., Suganuma T., Junke N., Yamaoka H. and Yoshida H. (1992)~ Radiat. Phys. Chem. 40, 111. Ogasawara M., Tanaka M. and Yoshida H. (1987). J. Phys. Chem. 91, 937. Peachey N. M. and Eckhardt C. J. (1993). J. Am. Chem. Soc. 115, 3519. Peachey N. M. and Eckhardt C. J. (1994). J. Am. Chem. Soc. 98, 685. Schiller R. (1991) In Excess Electron in Dielectric Media (Edited by Ferradini C. and Jay-Gerin J.P.), p. 105. Schiller R. (1992). J. Chem. Phys. 96, 6531. Tachikawa H., Yoshida H. and Ogasawara M. (1991). Radiat. Phys. Chem. 37, 107. Tamaki T., Suzuki Y. and Hasegawa M. (1972). Bull. Chem. Soc. Japan 45, 1988. Tanaka M., Yoshida H. and Ogasawara M. (1989). Radiat. Phys. Chem. 34, 591. Tanaka M., Yoshida H. and Ogasawara M. (1991). J. Phys. Chem. 95, 956.