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Thermal stability of spectral hole and electron—phonon interaction in donor—acceptor electron transfer system
=
JOURNAL OP
LUMINESCENCE
ELSEVIER
Journal of Luminescence 58 (1994) 191 193
Thermal stability of spectral hole and electron phonon interaction in donor acceptor electron transfer system Mingzhen Tian*, Baozhu Luo, Wenlian Li, Shihua Huang, Jiaqi Yu Changchun Institute of Physics, Chinese Academy of Sciences, Changchun 130021, China
Abstract The thermal properties of spectral holes have been investigated by temperature cycling in the system consisting of an electron donor (zinc-tetraphenylbenzoporphyrin), an acceptor (p-hydroxybenzaldehyde or p-aminoacetophenone) and a polymer host (polymethylmethacrylate or polyvinylbutyral). Special attention was given to thermal hole-filling processes and electron phonon interactions which affect the evolution of the hole area with temperature.
1. Introduction Photon-gated spectral hole-burning (PHB) is most promising for frequency-domain optical storage. Many organic and inorganic material systems have been investigated [1]. However, in most of these, except BaFCl~Br1 ~: Sm2~[2], PHB is restricted to relatively low temperatures. Electron donor acceptor doped polymers undergoing donor acceptor electron transfer are most advantageous PHB systems. However, raising the temperature as while PHB operates is an important problem which needs to be solved. Three factors affect thermal properties of PHB in organic materials: (1) the homogeneous line width of the zero-phonon absorption, (2) thermal hole-filling, (3) the Debye Waller factor. The homogeneous line width of absorption spectra of dye molecules in polymers has been extensively investigated with one-color holeburning. The electron acceptor molecules we used in donor acceptor pairs hardly affect absorption
*
Corresponding author,
spectra of the dye molecules. So we focused our attention on the latter two factors. PHB is also a significant tool of high-resolution spectroscopy. By studying the Debye Waller factor, we can get much information on electron phonon interactions in donor acceptor doped polymers. Some important parameters have been obtained, such as Huang Rhys factors and boson peak frequencies in polymers.
2. Experimental Electron donor, Zinc-tetraphenylbenzoporphyrin (ZnTPBP), acceptor, p-hydroxybenzaldehyde (PHBA) or p-aminoacetophenone (PAAP) were doped in the polymer, polymethylmethacrylate (PMMA) or polyvinylbutyral (PVB). The sample films, ZnTPBP/PHBA/PMMA, ZnTPBP/PAAP/PMMA, ZnTPBP/PHBA/PVB and ZnTPBP/PAAP/PVB were prepared with the method previously reported [3]. The samples were placed in a Helium gas closedcycling cryostat system. He Ne laser (632.8 nm)
0022-2313/94 $07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-231 3(93)E0202-9
192
M. Tian ci a!.
Journal of Luminescence 58 (1994) 191 193
was used as the frequency-selecting light and Nd: YAG (SHG, 532 nm) laser as the gating light. The method of burning and detecting holes was described in Ref. [3]. To study the thermal stability of the hole, we performed temperature cycling experiments. A hole burnt at a relatively low temperature was heated to an annealing temperature, recooled to the burning temperature again, then to a higher temperature for the next cycling, and so on. At each upper and low-temperature point in cycling, the hole area was measured.
3. Thermal hole-filling At first, we observed thermal hole-filling by temperature cycling. For ZnTPBP/ PHBA/ PM MA and ZnTPBP/PAAP/PMMA, holes were burnt at 26 K and annealed at a series of high temperatures. The hole area was detected at 26 K after annealing in each cycling. Figure 1 shows the decrease of hole area with rising temperature. We also noticed that the hole width hardly changed with temperature. These results indicate the existence of thermal hole-filling and the absence of spectral diffusion.
Hole-filling efficiency is different for different donor acceptor pairs. Comparing Fig. 1(a) and (b), we find that the hole was erased by heat more easily for ZnTPBP/PAAP than for ZnTPBP/PHBA. Similar results were obtained for ZnTPBP/ PHBA/PVB and ZnTPBP/PAAP/PVB in which the original PHB temperature was 14K. So we may conclude that holes burnt in ZnTPBP/PHBA have a higher temperature stability than in ZnTPBP/PAAP, an observation which may be attributed to the relative energy between donor and acceptor levels. The polymer matrix also affects the thermal-induced hole-filling process. From this point of view, it seems that holes have a higher thermal stability in PM MA.
4. Debye Wailer factors When the temperature rises, the hole area is reduced by not only the thermal filling but also the decreasing of zero-phonon line in the homogeneous absorption band. The Debye Waller (D W) factor is given by ~(T)
S0(T)/[S0(T) + S~(T)], (1) where S0(T) and S~(T)are the integrated intensity of the zero-phonon line and the phonon side band at temperature TK, respectively. In each cycling the whole area, S0(T) + S~(T),can be considered as a constant after having excluded the affection of thermal filling. Thus, we can express the relative D Wfactoras
__________________________
1
• 0.5
)
(
(d)
C
__________________
C
30
60 =
A(T)~A(T0),
(2)
1
where A(T) is the hole area, T0 is hole-burning temperature, also the low-temperature point in
o.
each cycling and T is annealing temperature. According to the theory on linear electron phonon interaction, and assuming a single-phonon
_______________________
(a)
(b) -
0
~‘
___________________________ 30 60 Temperature(k)
Fig. 1 Hole area dependence on annealing temperature in SampIes ZnTPBP PHBA PMMA (a) and ZnTPBP/PAAP PMMA (b) detected at 26 K, in ZnTPBP/PHBA PVB (c) and ZnTPBP PAAP PVB (d) detected at 14 K.
mode approximation at low temperature, D W factor can be simplified as ~(T) = exp[ Scoth(hv/2kT)].
(3)
.
Using Eq. (3) to fit the experimental data, we can get the Huang Rhys factor and the frequency of the low-vibration mode, the so-called boson peak.
M. Tian ci a!.
Journal of Luminescence 58 (1994) 191 193
range. The deviation (in Fig. 2(b) (d)) is due to not considering multi-phonon vibration modes at higher temperatures. Table 1 gives Huang Rhys factors and boson
fl~
0. __________________________________
20
0.
~
40
20
60
40
02
60
40
60
peak frequencies obtained in four samples by numerically fitting. Huang Rhys factors hardly change in the different samples. We may consider that the four donor acceptor-polymer systems have the same electron phonon coupling strength. The boson peak frequency observed in the dye-
80
80
doped polymer corresponds to the polymer material because of the small dye concentration. Saikan [4] has presented this result using one-color holeburning and femtosecond photon-echos and obtamed the low-frequency vibration mode at 14 and 18cm 1 for dye-doped PMMA and PVB, respectively. In our samples we obtained different results
80
__________________________
because of the presence of electron acceptors which occupy about 1/5 inthe weight of avibration sample and which may strongly affect phonon modes in
0. ~a)
40
60
80
Temperature (K)
Fig. 2 Relative Debye WaIler factor dependence on annealing temperature in samples ZnTPBP/PHBA PMMA (a), ZnTPBP PAAP/PMMA, (b), ZnTPBP PHBA/PVB (c) and ZnTPBP/ PAAP/PVB (d). points: the experimental data, solid lines: the computed curves with linear electron phonon interaction theory. Table 1 Huang Rhys factors and boson peak frequencies in DA ET systems Sample
S
v(cm
ZnTPBP/PHBA/PMMA ZnTPBP/PAAP PMMA ZnTPBP/PHBA PVB ZnTPBP/PAAP PVB
0.19 0.19 0.20 0.20
18 15 25 23
193
the polymers. Boson peak frequencies are larger in PYB than PM MA, consistent with the tendency observed in the dye-doped polymers in Ref. [4]. However, the frequencies are enhanced by acceptors in our four donor acceptor-polymer systems. This may be caused by the hydrogen bonds in the systems introduced by the acceptors resulting in higher boson peak frequencies.
I)
Figure 2 shows that the experimental results follow the theoretical curve well in the low-temperature
References [1] W.E. Moerner, Persistent Spectral Holeburning: Science and Application (Springer, Berlin, 1988). [2] C. Wei, S. Huang and J. Yu, J. Lumin. 43(1989)161. [3] M. Tian, B. Luo, W. Li, S. Huang and J. Yu, Jpn. J. AppI. Phys. 6 (1991) 366. [4] S. Saikan, J. Lumin. 53 (1992) 147.
JOURNAL OP
LUMINESCENCE
ELSEVIER
Journal of Luminescence 58 (1994) 191 193
Thermal stability of spectral hole and electron phonon interaction in donor acceptor electron transfer system Mingzhen Tian*, Baozhu Luo, Wenlian Li, Shihua Huang, Jiaqi Yu Changchun Institute of Physics, Chinese Academy of Sciences, Changchun 130021, China
Abstract The thermal properties of spectral holes have been investigated by temperature cycling in the system consisting of an electron donor (zinc-tetraphenylbenzoporphyrin), an acceptor (p-hydroxybenzaldehyde or p-aminoacetophenone) and a polymer host (polymethylmethacrylate or polyvinylbutyral). Special attention was given to thermal hole-filling processes and electron phonon interactions which affect the evolution of the hole area with temperature.
1. Introduction Photon-gated spectral hole-burning (PHB) is most promising for frequency-domain optical storage. Many organic and inorganic material systems have been investigated [1]. However, in most of these, except BaFCl~Br1 ~: Sm2~[2], PHB is restricted to relatively low temperatures. Electron donor acceptor doped polymers undergoing donor acceptor electron transfer are most advantageous PHB systems. However, raising the temperature as while PHB operates is an important problem which needs to be solved. Three factors affect thermal properties of PHB in organic materials: (1) the homogeneous line width of the zero-phonon absorption, (2) thermal hole-filling, (3) the Debye Waller factor. The homogeneous line width of absorption spectra of dye molecules in polymers has been extensively investigated with one-color holeburning. The electron acceptor molecules we used in donor acceptor pairs hardly affect absorption
*
Corresponding author,
spectra of the dye molecules. So we focused our attention on the latter two factors. PHB is also a significant tool of high-resolution spectroscopy. By studying the Debye Waller factor, we can get much information on electron phonon interactions in donor acceptor doped polymers. Some important parameters have been obtained, such as Huang Rhys factors and boson peak frequencies in polymers.
2. Experimental Electron donor, Zinc-tetraphenylbenzoporphyrin (ZnTPBP), acceptor, p-hydroxybenzaldehyde (PHBA) or p-aminoacetophenone (PAAP) were doped in the polymer, polymethylmethacrylate (PMMA) or polyvinylbutyral (PVB). The sample films, ZnTPBP/PHBA/PMMA, ZnTPBP/PAAP/PMMA, ZnTPBP/PHBA/PVB and ZnTPBP/PAAP/PVB were prepared with the method previously reported [3]. The samples were placed in a Helium gas closedcycling cryostat system. He Ne laser (632.8 nm)
0022-2313/94 $07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-231 3(93)E0202-9
192
M. Tian ci a!.
Journal of Luminescence 58 (1994) 191 193
was used as the frequency-selecting light and Nd: YAG (SHG, 532 nm) laser as the gating light. The method of burning and detecting holes was described in Ref. [3]. To study the thermal stability of the hole, we performed temperature cycling experiments. A hole burnt at a relatively low temperature was heated to an annealing temperature, recooled to the burning temperature again, then to a higher temperature for the next cycling, and so on. At each upper and low-temperature point in cycling, the hole area was measured.
3. Thermal hole-filling At first, we observed thermal hole-filling by temperature cycling. For ZnTPBP/ PHBA/ PM MA and ZnTPBP/PAAP/PMMA, holes were burnt at 26 K and annealed at a series of high temperatures. The hole area was detected at 26 K after annealing in each cycling. Figure 1 shows the decrease of hole area with rising temperature. We also noticed that the hole width hardly changed with temperature. These results indicate the existence of thermal hole-filling and the absence of spectral diffusion.
Hole-filling efficiency is different for different donor acceptor pairs. Comparing Fig. 1(a) and (b), we find that the hole was erased by heat more easily for ZnTPBP/PAAP than for ZnTPBP/PHBA. Similar results were obtained for ZnTPBP/ PHBA/PVB and ZnTPBP/PAAP/PVB in which the original PHB temperature was 14K. So we may conclude that holes burnt in ZnTPBP/PHBA have a higher temperature stability than in ZnTPBP/PAAP, an observation which may be attributed to the relative energy between donor and acceptor levels. The polymer matrix also affects the thermal-induced hole-filling process. From this point of view, it seems that holes have a higher thermal stability in PM MA.
4. Debye Wailer factors When the temperature rises, the hole area is reduced by not only the thermal filling but also the decreasing of zero-phonon line in the homogeneous absorption band. The Debye Waller (D W) factor is given by ~(T)
S0(T)/[S0(T) + S~(T)], (1) where S0(T) and S~(T)are the integrated intensity of the zero-phonon line and the phonon side band at temperature TK, respectively. In each cycling the whole area, S0(T) + S~(T),can be considered as a constant after having excluded the affection of thermal filling. Thus, we can express the relative D Wfactoras
__________________________
1
• 0.5
)
(
(d)
C
__________________
C
30
60 =
A(T)~A(T0),
(2)
1
where A(T) is the hole area, T0 is hole-burning temperature, also the low-temperature point in
o.
each cycling and T is annealing temperature. According to the theory on linear electron phonon interaction, and assuming a single-phonon
_______________________
(a)
(b) -
0
~‘
___________________________ 30 60 Temperature(k)
Fig. 1 Hole area dependence on annealing temperature in SampIes ZnTPBP PHBA PMMA (a) and ZnTPBP/PAAP PMMA (b) detected at 26 K, in ZnTPBP/PHBA PVB (c) and ZnTPBP PAAP PVB (d) detected at 14 K.
mode approximation at low temperature, D W factor can be simplified as ~(T) = exp[ Scoth(hv/2kT)].
(3)
.
Using Eq. (3) to fit the experimental data, we can get the Huang Rhys factor and the frequency of the low-vibration mode, the so-called boson peak.
M. Tian ci a!.
Journal of Luminescence 58 (1994) 191 193
range. The deviation (in Fig. 2(b) (d)) is due to not considering multi-phonon vibration modes at higher temperatures. Table 1 gives Huang Rhys factors and boson
fl~
0. __________________________________
20
0.
~
40
20
60
40
02
60
40
60
peak frequencies obtained in four samples by numerically fitting. Huang Rhys factors hardly change in the different samples. We may consider that the four donor acceptor-polymer systems have the same electron phonon coupling strength. The boson peak frequency observed in the dye-
80
80
doped polymer corresponds to the polymer material because of the small dye concentration. Saikan [4] has presented this result using one-color holeburning and femtosecond photon-echos and obtamed the low-frequency vibration mode at 14 and 18cm 1 for dye-doped PMMA and PVB, respectively. In our samples we obtained different results
80
__________________________
because of the presence of electron acceptors which occupy about 1/5 inthe weight of avibration sample and which may strongly affect phonon modes in
0. ~a)
40
60
80
Temperature (K)
Fig. 2 Relative Debye WaIler factor dependence on annealing temperature in samples ZnTPBP/PHBA PMMA (a), ZnTPBP PAAP/PMMA, (b), ZnTPBP PHBA/PVB (c) and ZnTPBP/ PAAP/PVB (d). points: the experimental data, solid lines: the computed curves with linear electron phonon interaction theory. Table 1 Huang Rhys factors and boson peak frequencies in DA ET systems Sample
S
v(cm
ZnTPBP/PHBA/PMMA ZnTPBP/PAAP PMMA ZnTPBP/PHBA PVB ZnTPBP/PAAP PVB
0.19 0.19 0.20 0.20
18 15 25 23
193
the polymers. Boson peak frequencies are larger in PYB than PM MA, consistent with the tendency observed in the dye-doped polymers in Ref. [4]. However, the frequencies are enhanced by acceptors in our four donor acceptor-polymer systems. This may be caused by the hydrogen bonds in the systems introduced by the acceptors resulting in higher boson peak frequencies.
I)
Figure 2 shows that the experimental results follow the theoretical curve well in the low-temperature
References [1] W.E. Moerner, Persistent Spectral Holeburning: Science and Application (Springer, Berlin, 1988). [2] C. Wei, S. Huang and J. Yu, J. Lumin. 43(1989)161. [3] M. Tian, B. Luo, W. Li, S. Huang and J. Yu, Jpn. J. AppI. Phys. 6 (1991) 366. [4] S. Saikan, J. Lumin. 53 (1992) 147.