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Dependence of non-photochemical hole-burning efficiency on the packing density of matrix polymers
20 May 1994
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 222 (1994) 325-328
Dependence of non-photochemical hole-burning efficiency on the packing density of matrix polymers Jun-ichi Takahashi, Jun Tsuchiya, Kenji Kawasaki Research
1. Introduction
Among the many problems associated with applications of persistent hole-burning, efficient writing and non-destructive reading are the most important. To solve them, two-photon hole-burning of the electron transfer type using porphyrin/acceptor systems has attracted much interest [ l-31. However, because the hole-burning efficiency cannot exceed unity, it is important not only to increase the two-photon hole-burning efficiency but also to suppress onephoton hole-burning. In this regard, the use of porphyrin homologues as chromophores for hole-buming is difficult because hole-burning occurs by many mechanisms such as photo-tautomerization [ 41 and non-photochemical mechanisms [ 5 ] other than electron transfer. We have investigated one-photon and two-photon hole-burning for Zn tetrabenzoporphyrine (TZT) with various accepters [ 61 and the results showed that efficient one-photon and two-photon hole-burning due to electron transfer were possible in exothermic regions. Furthermore, even in the endothermic regions, one-photon hole-burning was still possible by non-photochemical hole-burning
(NPHB). However, there are few reports on the quantitative relation between hole-burning efficiencies and matrix properties other than by NPHB, and the burning mechanism is essentially a reconnection of the hydrogen-bonding network of the matrix [ 7,8]. Because TZT has little polarity, the mechanism of NPHB for TZT doped in non-polar polymers such as polystyrene is thought to be due to changes in the local environment of the impurities. If the matrix was rigid, changing the environment, and thus hole-buming, would become difficult. We have investigated the thermal stability of a photochemically burned spectral hole. The rates of thermally induced spectral diffusion (TISD) were influenced by the packing density of the matrix polymers which were calculated by Bondi [ 9 1. The higher the packing density, the higher the thermal stability of a spectral hole (i.e. the more stable the local environment ) [ 10 1. Thus, hole-buming is thought to be difficult in polymers with a high packing density. In this report, NPHB of TZT in various polymers is reported.
0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved ,WDIOOO9-2614(94)00351-P
J. Takahashi et al. /Chemical Physics Letters 222 (1994) 325-328
326
2. Experimental
TZT was doped Petrochemical Co. MW = 203000), atactic poly (methyhnethacrylate) (PMMA) (Wako Pure Chemical Industries, Ltd. MW= lOOOOO),high-density polyethylene (PE) (Idemitsu Petrochemical Co. MW= 122000, densityz0.954 g/cm’) and polyisobutylene (PIB) (Aldrich Chemical Company, Inc. MW = 84000). The packing density p* has been defined by Bondi [ 91 as P* =P c
VW/M 9
(1)
where p is the density, VW is the van der Waals volume of the atom groups and A4 is the molecular weight. The polymers were used after reprecipitation. The concentrations of TZT used were 1O-3 mol/ J?.TZT was doped in polymers by dissolving them with suitable solvents (dichloromethane for PS and PMMA, xylene for PE and toluene for PIB). The mixed solutions of PS or PMMA were cast in air while PE or PIB were heated in vacua because of the high boiling point of the solvents. After drying, the samples were put on a glass plate on a hot plate. They were heated above the melting temperature to evaporate the residual solvents. After bubbling ended, they were covered with another glass plate and pressed to a thickness of several hundred microns, then they were left to cool at room temperature. The films were used after being stripped from the glass plates. The samples were irradiated in a gas flow type cryostat CF1204 (Oxford Instruments) at 6 K. A pulsed dye laser FL2002 (Lambda Physik) excited by a XeCl excimer laser EMGSO (Lambda Physik) was used for burning. The repetition rate was 20 Hz. The average intensities were 7 to 40 uW/mm2. Hole spectra were monitored as transmission intensities. The light source used was the light from a 500 W Xe lamp monochromated by a 0.85 m double monochromator SPEX 1403. The holes were fitted by Lorentzians and the values of width and area were obtained from them.
3. Results and discussion For the matrices PS and PMMA, distinct holes were observed after a few minutes of irradiation (Figs. 1a and lb). Much more time was necessary to bum a
-4.0
-2.0
0.0
2.0
4.0
6.0
A(1/h) / cm-’ Fig. 1. (a) A hole spectrum forTZT/PS, wavelength=6503.6 A, laser power= 17 pW/mmr and irradiation time=300 s. (b) A hole spectrum for TZT/PMMA, wavelength=6510.4 A, laser power= 14 pW/mm2 and irradiation time=300 s. (c) A hole spectrum for TZT/PE, wavelength=6505.3 A, laser power= 14 pW/mm’ and irradiation time=30 min. (d) No hole was observed for TZT/PIB after irradiation for one hour, wavelength= 6485.3 A, laser power= 14 pW/mm’. Each base line was shifted for clarity. Table 1 Onephoton hole-bunting efficiencies of TZT and the packing density calculated by Bondi for various polymers. The efficiency for PIB was the upper bound estimated from the experimental accuracy, because no hole was observed in PIB Polymer
Burning efficiency
PS PMMA PE PIB
1x10-3 3x lo-’ 2x 10-S <3x lo-6
Packing density 0.636 0.660 0.710 0.715
hole in the PE matrix (Figs. lc). No hole could be burned in the PIB matrix even after the sample had been irradiated for an hour (Figs. Id). Table 1 shows the one-photon hole-burning efficiency and the packing density of the matrix polymers calculated by Bondi [ 9 1. The hole-burning efficiencies were determined from the approximation
N&(t) ‘,‘= 2.3(Pt/hv)A&,
’
(2)
321
J. Takahashi et al. /Chemical Physics Letters 222 (1994) 325-328
where No is the concentration of TZT, P is the buming power, t is the burning time, h v is the photon energy of the burning beam, I is the optical pass length, A is the value of the optical density at the burning wavelength, S(t) is the hole area for burning time t and S,, is the integrated area of the Q band. The efficiencies were calculated from the initial slope of the lines of S( t ) . The efficiency for PIB was too small to be determined but it was estimated to be less than 3 x 10m6from our experimental accuracy. Our results showed that the higher packing densities were associated with the lower hole-burning efficiencies. It is difficult to determine the mechanism of nonphotochemical hole-burning. However, the mechanism of NPHB for TZT in this work is thought to be the change of the strain field of the local environment because: ( 1) TZT is photochemically so stable that photochemical change of the impurity is difficult, (2) the polarity of both TZT and the matrix (especially for PS) are so weak that a hydrogen-bonding network cannot be formed, (3) the electron accepting power of the matrices are so weak that photoinduced electron transfer cannot occur (at least, the electron accepting power of the phenyl group of PS cannot be stronger than that of the carboxyl group of PMMA). Shu and Small discussed qualitatively the dynamics of the NPHB of such a type using a hierarchical model of a two-level system (TLS) [ 111. They noted the importance of the time-dependent change of the free volume. They argued that tunneling along the intrinsic TLS coordinate triggered free volume expansion, the free volume expansion led to the symmetrization of the extrinsic TLS and then site conversion occurred by tunneling along the extrinsic TLS coordinate. The dynamics of the free volume were first theoretically formulated by Cohen and Grest to explain glass transitions [ 121, and they later discussed the relation between free volume and TLS [ 13 1. On the other hand, Kahler and Friedrich applied the tunneling model of Black and Halperin to TISD [ 14 1. According to their model, the rate of TISD is proportional to the number of TLS. We investigated TISD of phototautomerization systems in polymers, the packing densities of which had been calculated by Bondi [ 91. The higher the packing density, the smaller the rate of TISD. This means that higher packing densities are associated with smaller TLS. We applied a free volume model to TISD by extending
the Cohen and Grest model and obtained a Doolittle type relation between the rate of TISD and the packing density as
~ilo,e =tCkew dTexc
-
m
0
x sech’x cash x sinh x
’
(3)
where r,,,, is the hole width, T,,is the excursion temperature, C is the coupling strength, k is the Boltzmann constant, v. is the total volume, v* is a critical value, < is some numeric factor and p* is the packing density calculated by Bondi [ 15 1. The value of
328
J. Takuhashi et al. /Chemical Physics Letters 222 (1994) 325-328
would reside in the amorphous region. Because the packing densities of the crystalline and the amorphous regions are different from each other, the packing density around TZT must be different from the value calculated by Bondi. Our model is essentially similar to the Shu and Small model of NPHB. However, in their model, the free volume is thought to mediate between the intrinsic TLS and the extrinsic TLS whereas we believe that the free volume itself is transferred by tunneling along the intrinsic TLS coordinate. We also noted the coupling between the TLS and the impurities. It is known that the doping of impurities changes the dynamics around the impurities [ 16,17 1. It is of concern that the coupling would mask the contribution from the transfer dynamics of the free volume in matrices. Pack et al. investigated the coupling dynamics between TLS and the impurity. They renormalized the contribution of the coupling to the distance-dependent density of active TLS. They termed the strong coupled region localized around the impurity the ‘solvent shell’ and the outer region the ‘outer shell’. The difference between them is the density of active TLS. They estimated the ratio of the densities from photon echo and hole-burning, and reported that the local dynamics were changed by at most 30% in density even in a polar matrix doped with ionic dye [ 17 1. Because we used weakly polar molecules as the impurity and the matrices, the contribution of the change in the local environment due to the doping would be much smaller. Therefore, even if the local structure was changed by the coupling, its contribution to the
burning efficiency would be smaller than the transfer dynamics of the free volume.
References [ 1] T.P. Carter, C. Briiuchle, V.Y. Lee, M. Manavi and W.E. Moemer, J. Phys. Chem. 91 (1987) 3998. [2] H. Suzuki, T. Shimada and H. Hiratsuka, Chem. Phys. Letters 183 (1991) 570. [ 31 K Horie, S. Machida and T. Yamashita, in: Technical Digest Series, Vol. 16. Persistent spectral hole bunting: science and applications topical meeting, Monterey ( 199 1) p. 154. [4] A.A. Gorokhovskii and R.K. Kaarli, L.A. Rebane, JETP Letters 20 (1974) 216. [5]S.G. Boxer, D.S. Gottfried, D.J. Lockhart and T.R. Middendorf, J. Chem. Phys. 86 (1987) 2439. [6] J. Takahashi, J. Tsuchiya and K. Kawasaki, Chem. Phys. Letters 209 (1993) 479. [ 71 H. Tanaka, F. Kokai, J.I. Brauman and M.D. Fayer, Chem. Phys. Letters 142 (1987) 371. [ 8 ] F. Kokai, H. Tanaka, J.I. Brauman and M.D. Fayer, Chem. Phys. letters 143 (1988) 1. [9] J. Tsuchiya, J. Takahashi and H. Tanaka, in: Technical Digest Series, Vol. 16. Persistent spectral hole burning: science and applications topical meeting, Monterey ( 199 1) p. 129. [ lo] A. Bondi, Physical properties of molecular crystals, liquids, and glasses (Wiley, New York, 1968 ) . [ 111 L. Shu and G.J. Small, Chem. Phys. 141 (1990) 447. [ 121 M.H. Cohen and G.S. Grest, Phys. Rev. B 20 (1979) 1077. [ 131 M.H. Cohen and G.S. Crest, Phys. Rev. Letters 45 (1980) 1271. [ 141 W. Kiibler and J. Friedrich, Europhys. Letters 7 ( 1988) 5 17. [ 151 J. Takahashi, J. Tsuchiya, H. Tanaka and K. Kawasaki, in press. [ 161 B.L. Fearey, T.P. Carter and G.J. Small, Chem. Phys. 101 (1986) 279. [ 171 D.W. Pack, L.R. Narasimhan and M.D. Fayer, J. Chem. Phys. 92 (1990) 4125.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 222 (1994) 325-328
Dependence of non-photochemical hole-burning efficiency on the packing density of matrix polymers Jun-ichi Takahashi, Jun Tsuchiya, Kenji Kawasaki Research
1. Introduction
Among the many problems associated with applications of persistent hole-burning, efficient writing and non-destructive reading are the most important. To solve them, two-photon hole-burning of the electron transfer type using porphyrin/acceptor systems has attracted much interest [ l-31. However, because the hole-burning efficiency cannot exceed unity, it is important not only to increase the two-photon hole-burning efficiency but also to suppress onephoton hole-burning. In this regard, the use of porphyrin homologues as chromophores for hole-buming is difficult because hole-burning occurs by many mechanisms such as photo-tautomerization [ 41 and non-photochemical mechanisms [ 5 ] other than electron transfer. We have investigated one-photon and two-photon hole-burning for Zn tetrabenzoporphyrine (TZT) with various accepters [ 61 and the results showed that efficient one-photon and two-photon hole-burning due to electron transfer were possible in exothermic regions. Furthermore, even in the endothermic regions, one-photon hole-burning was still possible by non-photochemical hole-burning
(NPHB). However, there are few reports on the quantitative relation between hole-burning efficiencies and matrix properties other than by NPHB, and the burning mechanism is essentially a reconnection of the hydrogen-bonding network of the matrix [ 7,8]. Because TZT has little polarity, the mechanism of NPHB for TZT doped in non-polar polymers such as polystyrene is thought to be due to changes in the local environment of the impurities. If the matrix was rigid, changing the environment, and thus hole-buming, would become difficult. We have investigated the thermal stability of a photochemically burned spectral hole. The rates of thermally induced spectral diffusion (TISD) were influenced by the packing density of the matrix polymers which were calculated by Bondi [ 9 1. The higher the packing density, the higher the thermal stability of a spectral hole (i.e. the more stable the local environment ) [ 10 1. Thus, hole-buming is thought to be difficult in polymers with a high packing density. In this report, NPHB of TZT in various polymers is reported.
0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved ,WDIOOO9-2614(94)00351-P
J. Takahashi et al. /Chemical Physics Letters 222 (1994) 325-328
326
2. Experimental
TZT was doped Petrochemical Co. MW = 203000), atactic poly (methyhnethacrylate) (PMMA) (Wako Pure Chemical Industries, Ltd. MW= lOOOOO),high-density polyethylene (PE) (Idemitsu Petrochemical Co. MW= 122000, densityz0.954 g/cm’) and polyisobutylene (PIB) (Aldrich Chemical Company, Inc. MW = 84000). The packing density p* has been defined by Bondi [ 91 as P* =P c
VW/M 9
(1)
where p is the density, VW is the van der Waals volume of the atom groups and A4 is the molecular weight. The polymers were used after reprecipitation. The concentrations of TZT used were 1O-3 mol/ J?.TZT was doped in polymers by dissolving them with suitable solvents (dichloromethane for PS and PMMA, xylene for PE and toluene for PIB). The mixed solutions of PS or PMMA were cast in air while PE or PIB were heated in vacua because of the high boiling point of the solvents. After drying, the samples were put on a glass plate on a hot plate. They were heated above the melting temperature to evaporate the residual solvents. After bubbling ended, they were covered with another glass plate and pressed to a thickness of several hundred microns, then they were left to cool at room temperature. The films were used after being stripped from the glass plates. The samples were irradiated in a gas flow type cryostat CF1204 (Oxford Instruments) at 6 K. A pulsed dye laser FL2002 (Lambda Physik) excited by a XeCl excimer laser EMGSO (Lambda Physik) was used for burning. The repetition rate was 20 Hz. The average intensities were 7 to 40 uW/mm2. Hole spectra were monitored as transmission intensities. The light source used was the light from a 500 W Xe lamp monochromated by a 0.85 m double monochromator SPEX 1403. The holes were fitted by Lorentzians and the values of width and area were obtained from them.
3. Results and discussion For the matrices PS and PMMA, distinct holes were observed after a few minutes of irradiation (Figs. 1a and lb). Much more time was necessary to bum a
-4.0
-2.0
0.0
2.0
4.0
6.0
A(1/h) / cm-’ Fig. 1. (a) A hole spectrum forTZT/PS, wavelength=6503.6 A, laser power= 17 pW/mmr and irradiation time=300 s. (b) A hole spectrum for TZT/PMMA, wavelength=6510.4 A, laser power= 14 pW/mm2 and irradiation time=300 s. (c) A hole spectrum for TZT/PE, wavelength=6505.3 A, laser power= 14 pW/mm’ and irradiation time=30 min. (d) No hole was observed for TZT/PIB after irradiation for one hour, wavelength= 6485.3 A, laser power= 14 pW/mm’. Each base line was shifted for clarity. Table 1 Onephoton hole-bunting efficiencies of TZT and the packing density calculated by Bondi for various polymers. The efficiency for PIB was the upper bound estimated from the experimental accuracy, because no hole was observed in PIB Polymer
Burning efficiency
PS PMMA PE PIB
1x10-3 3x lo-’ 2x 10-S <3x lo-6
Packing density 0.636 0.660 0.710 0.715
hole in the PE matrix (Figs. lc). No hole could be burned in the PIB matrix even after the sample had been irradiated for an hour (Figs. Id). Table 1 shows the one-photon hole-burning efficiency and the packing density of the matrix polymers calculated by Bondi [ 9 1. The hole-burning efficiencies were determined from the approximation
N&(t) ‘,‘= 2.3(Pt/hv)A&,
’
(2)
321
J. Takahashi et al. /Chemical Physics Letters 222 (1994) 325-328
where No is the concentration of TZT, P is the buming power, t is the burning time, h v is the photon energy of the burning beam, I is the optical pass length, A is the value of the optical density at the burning wavelength, S(t) is the hole area for burning time t and S,, is the integrated area of the Q band. The efficiencies were calculated from the initial slope of the lines of S( t ) . The efficiency for PIB was too small to be determined but it was estimated to be less than 3 x 10m6from our experimental accuracy. Our results showed that the higher packing densities were associated with the lower hole-burning efficiencies. It is difficult to determine the mechanism of nonphotochemical hole-burning. However, the mechanism of NPHB for TZT in this work is thought to be the change of the strain field of the local environment because: ( 1) TZT is photochemically so stable that photochemical change of the impurity is difficult, (2) the polarity of both TZT and the matrix (especially for PS) are so weak that a hydrogen-bonding network cannot be formed, (3) the electron accepting power of the matrices are so weak that photoinduced electron transfer cannot occur (at least, the electron accepting power of the phenyl group of PS cannot be stronger than that of the carboxyl group of PMMA). Shu and Small discussed qualitatively the dynamics of the NPHB of such a type using a hierarchical model of a two-level system (TLS) [ 111. They noted the importance of the time-dependent change of the free volume. They argued that tunneling along the intrinsic TLS coordinate triggered free volume expansion, the free volume expansion led to the symmetrization of the extrinsic TLS and then site conversion occurred by tunneling along the extrinsic TLS coordinate. The dynamics of the free volume were first theoretically formulated by Cohen and Grest to explain glass transitions [ 121, and they later discussed the relation between free volume and TLS [ 13 1. On the other hand, Kahler and Friedrich applied the tunneling model of Black and Halperin to TISD [ 14 1. According to their model, the rate of TISD is proportional to the number of TLS. We investigated TISD of phototautomerization systems in polymers, the packing densities of which had been calculated by Bondi [ 91. The higher the packing density, the smaller the rate of TISD. This means that higher packing densities are associated with smaller TLS. We applied a free volume model to TISD by extending
the Cohen and Grest model and obtained a Doolittle type relation between the rate of TISD and the packing density as
~ilo,e =tCkew dTexc
-
m
0
x sech’x cash x sinh x
’
(3)
where r,,,, is the hole width, T,,is the excursion temperature, C is the coupling strength, k is the Boltzmann constant, v. is the total volume, v* is a critical value, < is some numeric factor and p* is the packing density calculated by Bondi [ 15 1. The value of
328
J. Takuhashi et al. /Chemical Physics Letters 222 (1994) 325-328
would reside in the amorphous region. Because the packing densities of the crystalline and the amorphous regions are different from each other, the packing density around TZT must be different from the value calculated by Bondi. Our model is essentially similar to the Shu and Small model of NPHB. However, in their model, the free volume is thought to mediate between the intrinsic TLS and the extrinsic TLS whereas we believe that the free volume itself is transferred by tunneling along the intrinsic TLS coordinate. We also noted the coupling between the TLS and the impurities. It is known that the doping of impurities changes the dynamics around the impurities [ 16,17 1. It is of concern that the coupling would mask the contribution from the transfer dynamics of the free volume in matrices. Pack et al. investigated the coupling dynamics between TLS and the impurity. They renormalized the contribution of the coupling to the distance-dependent density of active TLS. They termed the strong coupled region localized around the impurity the ‘solvent shell’ and the outer region the ‘outer shell’. The difference between them is the density of active TLS. They estimated the ratio of the densities from photon echo and hole-burning, and reported that the local dynamics were changed by at most 30% in density even in a polar matrix doped with ionic dye [ 17 1. Because we used weakly polar molecules as the impurity and the matrices, the contribution of the change in the local environment due to the doping would be much smaller. Therefore, even if the local structure was changed by the coupling, its contribution to the
burning efficiency would be smaller than the transfer dynamics of the free volume.
References [ 1] T.P. Carter, C. Briiuchle, V.Y. Lee, M. Manavi and W.E. Moemer, J. Phys. Chem. 91 (1987) 3998. [2] H. Suzuki, T. Shimada and H. Hiratsuka, Chem. Phys. Letters 183 (1991) 570. [ 31 K Horie, S. Machida and T. Yamashita, in: Technical Digest Series, Vol. 16. Persistent spectral hole bunting: science and applications topical meeting, Monterey ( 199 1) p. 154. [4] A.A. Gorokhovskii and R.K. Kaarli, L.A. Rebane, JETP Letters 20 (1974) 216. [5]S.G. Boxer, D.S. Gottfried, D.J. Lockhart and T.R. Middendorf, J. Chem. Phys. 86 (1987) 2439. [6] J. Takahashi, J. Tsuchiya and K. Kawasaki, Chem. Phys. Letters 209 (1993) 479. [ 71 H. Tanaka, F. Kokai, J.I. Brauman and M.D. Fayer, Chem. Phys. Letters 142 (1987) 371. [ 8 ] F. Kokai, H. Tanaka, J.I. Brauman and M.D. Fayer, Chem. Phys. letters 143 (1988) 1. [9] J. Tsuchiya, J. Takahashi and H. Tanaka, in: Technical Digest Series, Vol. 16. Persistent spectral hole burning: science and applications topical meeting, Monterey ( 199 1) p. 129. [ lo] A. Bondi, Physical properties of molecular crystals, liquids, and glasses (Wiley, New York, 1968 ) . [ 111 L. Shu and G.J. Small, Chem. Phys. 141 (1990) 447. [ 121 M.H. Cohen and G.S. Grest, Phys. Rev. B 20 (1979) 1077. [ 131 M.H. Cohen and G.S. Crest, Phys. Rev. Letters 45 (1980) 1271. [ 141 W. Kiibler and J. Friedrich, Europhys. Letters 7 ( 1988) 5 17. [ 151 J. Takahashi, J. Tsuchiya, H. Tanaka and K. Kawasaki, in press. [ 161 B.L. Fearey, T.P. Carter and G.J. Small, Chem. Phys. 101 (1986) 279. [ 171 D.W. Pack, L.R. Narasimhan and M.D. Fayer, J. Chem. Phys. 92 (1990) 4125.