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A photochromic memory with a non-destructive read-out property
132
Thin Solid F)'hns, 221 (1992) 132 139
A photochromic memory with a non-destructive read-out property Nobuyuki Tamaoki*, Sawako Yoshimura and Tsuguo Yamaoka Department of Image Science and Technology, Chiba University, 1-33 Yayoi-cho, Chiba 260 (Japan) (Received January 17, 1992; accepted May 26, 1992)
Abstract lsomerizations of [2.2](4,4')-azobenzenophane (1) in polycarbonate films were investigated spectrophotometrically, and the light-intensity dependent efficiency of the photoisomerization of macrocyclic azobenzene dimers, including 1, was analyzed theoretically. It is demonstrated that the efficiency of photoisomerization of 1 is dependent on the light intensity, even in a polymer film. A correlation between the lifetimes of the trans-cis isomers of macrocyclic azobenzene dimers and light intensities, where the efficiency of photoisomerization becomes dependent on light intensity, is also described. A photochromic memory composed of 1 and polycarbonate shows a non-destructive read-out property.
1. Introduction There have been many studies with the aim of applying photochromism to optical memories [1-3]. However, these attempts have encountered various problems. A general and fundamental problem is that memories are destroyed by photoreactions during "read-out" processes. Because the wavelengths 2A and 2B of light used for "writing" and "erasing" memories are the same as those used for "read-out", repetition of "read-out", even with low intensity light, induces the same reaction as "writing" or "erasing" and destroys the memories. Various ideas have been proposed to overcome this problem [4-7]. One promising idea is to synthesize photochromic compounds which show lightintensity dependent photoisomerization. If the quantum yield of photoisomerization of the compounds becomes greater as the light intensity is increased, we can write and erase memories with high intensity light and read out with low intensity light without any serious destruction of memories. We synthesized two macrocyclic azobenzene dimers, 1 and 2, and reported that the photoisomerization of these compounds is dependent on light intensity in solvents [8-9]. These compounds have three isomers, the trans-trans (tt), the trans-cis (tc) and the cis-cis (cc) isomers. Isomerization of these compounds is dependent on light intensity since the photoisomerization from the tt isomer to the cc isomer proceeds via the tc isomer, which is thermally unstable and easily isomer-
*Present address: Research Institute for Polymers and Textiles 1-1-4 Higashi, Tsukuba, lbaraki 305, Japan.
0040-6090/92/$5.00
izes to the tt isomer. In the present paper, the unusual properties of photoisomerization of macrocyclic azobenzene dimers are analyzed theoretically, and the lightintensity dependent isomerization of [2.2](4,4')-azobenzenophane, 1, in polymer films is demonstrated. A photochromic memory system which is composed of ! and polycarbonate shows a non-destructive read-out property.
2. Experimental section
2.1. Materials Spectral grade dichloroethane was used without further purification. Bisphenol A type polycarbonate with Mw = 30 000 was purchased from Teijin Chemical Corporation, Tokyo. 4,4'-Dimethylazobenzene was synthesized by the reduction of p-nitrotoluene with zincNaOH and then purified by recrystallization from ethanol solution [10]. [2.2](4,4')-Azobenzenophane (1) was synthesized according to the method described in an earlier report [8]. 2.2. Preparation of polycarbonate films containing azobenzene derivatives for isomerization measurements [2.2](4,4')-Azobenzenophane (1.861 mg) and bisphenol A polycarbonate (0.1 g).were dissolved in 1.0 ml of 1,2-dichloroethane, and coated on quartz plates with a wire bar (no. 40). After drying for 2 h at room temperature, they were dried completely in an oven at 52 °C under vacuum. Similarly, films containing 4,4'dimethylazobenzene, 3, were obtained by coating quartz plates with a solution of the azo compound (0.01056 g) and polycarbonate (0.6 g) dissolved in 6.0 g
© 1 9 9 2 - Elsevier Sequoia. All rights reserved
133
N. Tamaoki et al. / Photochromic memory with non-destructive read-out
(a)
Writing
(b)
Monitoring
366 nm
~
#
NNNN__~
N%N
366 nm
IIlll
(1)
_
~
.
~
:
,
•
(2)
(Read-out)
-.-IIlll
/_
f
\
/ (c)
Erasing > 450 nm
(3)
Scheme. Structures of compounds i, 2 and 3. of dichloroethane. After drying, the film thickness was 2-3/am, which was calculated from the weight of a given area and the density of polycarbonate, 1.2.
2.3. Preparation of photochromic memory for demonstration of a non-destructive read-out property The same 1,2-dichloroethane solutions as described above were spread on quartz plates ( 5 0 m m × 50 mm × 1.5 mm) using a spin-coater (500 rev rain-l). The plates were dried as described above. After drying the film thickness was 4-5/am.
2.4. Measurement of isomer&ation A high-pressure mercury lamp (Ushio-UM-452, 450 W) equipped with Toshiba glass filters, UV-34 and UV-D2 was used to expose samples to 366 nm light. Kinetic measurements for thermal isomerization of polycarbonate films containing azobenzene derivatives were taken spectrophotometrically on a Hitachi Model 200-20 spectrophotometer equipped with a thermocontrolled cell holder by monitoring changes in absorbance at 328 n m ('~max of the ~rt* transition) in the dark after sufficient irradiation of 366 nm light. Fast thermal isomerization from the tc form to the tt form was monitored immediately after the photoirradiation. Changes in absorbance at 328 nm as a function of time were recorded with a chart speed of 48 cm min- J.
2.5. Measurement of light &tensity dependence Photoirradiation was done at 25 ___2 °C in almost the same manner described above. The light intensity was altered using filters and changing the distance and then
/
lllll
/
Fig. 1. Procedures for writing, monitoring (read-out) and erasing of memories.
measured by Eppley thermopiles (The Eppley Laboratory Ltd.).
2.6. Procedures of writ&g, reading and eras&g memory The photochromic films on quartz plates were exposed to 366 nm light (5 mW cm -2) from a super high pressure mercury lamp with appropriate filters through a mask in contact (Fig. l(a)). Visualization (read-out) of the patterns was performed by exposure of the plates to 366 nm light (20 laW cm -2) on a fluorescent sheet. During the visualization process the surface of the photochromic film was in contact with the fluorescent sheet (Fig. l(b)). The fluorescent sheet used absorbs UV light and emits blue light. The memory was erased with light of wavelength more than 450 nm from a super high pressure mercury lamp with a sharp cut filter.
3. Results and discussion
3.1. The cis- trans isomer&ation in polycarbonate film To apply the dependence on light intensity of photoisomerization from the tt form to the cc form of [2.2](4,4')-azobenzenophane in solution to functional materials, it is important to know whether this phenomenon can also be seen in a polymer matrix. Therefore, the photoisomerization and thermal isomerization properties of [2.2](4,4')-azobenzenophane were investigated in a polycarbonate film, which js an important polymer for use as a substrate for optical memories because of its transparency and thermal stability.
134
N. Tamaoki et al. / Photochromic' memory with non-destructive read-out
0.08
041 ~ ,-, 0.3 o
a
b
a
004 I.OZ
0.1
°zI 0
006
'
300
400
,
I 5OO
Wavelength (nm) Fig. 2. Changes in absorption spectra of 1 in a polycarbonate film under 366 nm light: a, before irradiation; b, photostationary state; c, after arbitrary extent of irradiation; d, after rapid thermal change from c.
Figure 2 shows spectral changes of 1 in polycarbonate upon 366 nm irradiation. Before irradiation, 1 in a polycarbonate film shows an absorption spectrum similar to that in benzene. The spectrum has two peaks at 328 nm and around 460 nm (curve a). These spectral characteristics are the same as that of trans-4,4'dimethylazobenzene, a strong rc~* transition peak at 330 nm and a weak n~* transition peak around 440 nm. These facts indicate that 1 exists as the t r a n s - t r a n s form in polycarbonate film. U p o n 366 nm irradiation,
the intensities of the peaks at 328 nm and around 4 6 0 n m decrease and a new peak appears around 440 nm and its intensity increases. A photostationary state is then attained (curve b). The characteristics of this spectrum at the photostationary state are similar to that of the cis form of the parent 4,4'-dimethylazobenzene in which just a weak nrt* transition peak is observed in the region above 300 nm. These facts suggest that 1 exists mainly as the cis-cis form. F r o m this photostationary state the initial spectrum is recovered by exposure to light above 420 nm. These spectral changes are similar to those observed in benzene. Immediately after irradiation with 366 nm light to an arbitrary extent (curve c), a small amount of rapid thermal recovery of the spectrum is observed for 1 in a polycarbonate film (from curve c to curve d). This kind of spectral change is only observed for azobenzenophanes with short connecting chains. Although it is not possible to isolate the pure tc isomer of 1 because of its instability, it is assumed that the spectrum of the tc isomer resembles the sum of spectra of trans- and cis-4,4'-dimethylazobenzene. Based on this assumption, the small amount of rapid thermal change of the spectrum, where the main change is seen for the peak at 328 nm, can be assigned to thermal isomerization from the tc isomer to the tt isomer. The assumption mentioned above is well supported by a theoretical analysis described in the next section. The spectrally determined lifetime of the tc isomer at 298 K was about 25 s. This value is almost the same as that in benzene, 22 s [8].
0.3
0.2
? I
~
0
100
200 Time ( m i n . )
300
Fig. 3. Thermalisomerization of l in polycarbonate films: A, 38°C; D, 33°C; O,.25°C.
N. Tamaoki et al. / Photochromic memory with non-destructive read-out
Figure 3 shows a change in absorption at several temperatures over long periods of time. This absorption change includes both the thermal steps from the cc isomer to the tc isomer and from the tc isomer to the tt isomer. The thermal change from the tc isomer to the tt isomer is very fast as described above, so this slow absorption change should correspond to the other isomerization step, i.e. thermal isomerization from the cc isomer to the tc isomer. In other words, the isomerization step from the cc isomer to the tc isomer is the rate-determining step. Plots of ln(A-A0) at 328nm against time are slightly curved for 1 in polycarbonate films. This means that the isomerization of 1 from the cc isomer to the tc isomer in polycarbonate films does not follow a first-order equation. Similar behavior has
135
been observed for non-cyclic azobenzene .derivatives embedded in polymers [11] or non-cyclic azobenzene units substituted in a polymer backbone or as polymer side chains [12]. In Figs. 4 and 5, the degrees of photoisomerization in polycarbonate films at two light intensities are plotted against exposure energy for [2.2](4,4')-azobenzenophane and 4,4'-dimethylazobenzene respectively. The light intensities used for the experiments were 2.64 and 0.269 mW cm -2. The photoisomerization of 4,4'dimethylazobenzene does not depend on the light intensity, but only on the exposure energy, as is known already. In contrast, the photoisomerization of [2.2](4,4')-azobenzenophane is more efficient under higher light intensity. This behavior in polycarbonate
1.13
0 0
,
,
Exposure
!
,
,
,
energy
(mJ/cm 2 )
500
Fig. 4. Changes in absorbance at 328 n m of I in polycarbonate films with exposure energy. Intensitiesof fight used are 2.64 (©) and 0.269 (Q) m W cm-=.
1.0
0 o
500
Exposure
energy
(mJ/cm
~ )
Fig. 5. Changes in absorbance at 339 nm o f 4,4'-dimethylazobenzene in polycarbonate films with exposure energy. Intensities of light used are 2.64 ( © ) and 0.269 ( 0 ) mW cm -2.
136
N. Tamaoki et al. [ Photochromic memory with non-destructive read-out
films coincides with that in benzene [8]. That is, as expected from the fact that the lifetime of the tc form of [2.2](4,4')-azobenzenophane in a polycarbonate film does not change much from the value in benzene, the efficiency of photoisomerization from the tt isomer to the cc isomer of the macrocycle is seemingly dependent on light intensity, even in a polycarbonate film.
3.2. Theoretical analysis of the dependence on light intensity The light-intensity dependence of the efficiency of photoisomerization is observed for compounds in which photoisomerization proceeds via a metastable state such as the trans-cis isomer for macrocyclic azobenzene dimers. The dependence on light intensity for such compounds is detected in the range of higher light intensities as the lifetime of the metastable state becomes shorter. When the dependence is used for non-destructive read-out, it is important to know the relation between the range where the efficiency is lightintensity dependent, and the lifetime of the metastable states. In order to clarify the relation, calculations are conducted. Based on the reaction scheme for [2.2](4,4')azobenzenophane shown below hv
tt.
_
d[tc] dt
hv
1000 ( 1 ) ~;tt[tt]L L I° I - - 1 ~ O ~--~btt~tc+ktc-tt[tc] I000 ( L I° 1
1 )e.[tt]L 10o ] D ~tt ~tc
lOO( 1)o.,,Icl,o
- k,c~,[tc] + k~¢~,c[cc]
hv',A
the following three differential equations are proposed for the three isomers. Each term in the equations corresponds to one route of the photoisomerization or thermal isomerization. _
d[tt] d~--
,'. tc.::." cc
hv',A
d[tt] dt
tion of the cc isomer, I 0 is the intensity of light, L is the path length of samples, ~btt~tc is the quantum yield from the tt isomer to the tc isomer, qS,~.c~ is the quantum yield from the tc isomer to the cc isomer, ett is the molar absorption coefficient of the tt isomer, et~ is the molar absorption coefficient of the tc isomer, ec~ is the molar absorption coefficient of the cc isomer, kcc~t~ is the rate constant of thermal isomerization from the cc isomer to the tc isomer, and kt¢_tt is the rate constant of thermal isomerization from the tc isomer to the tt isomer. The rate of photoisomerization of the cis units at 366 nm is quite low in comparison with the trans units because the molecular extinction coefficient of the cis units is small. Therefore, the photoisomerizations c c ~ t c and t c ~ t t are neglected, and et~ =0.5~u is assumed. The equations are then rewritten as follows:
1
0 lo
1 - 1~
D
-,-t,
~
tc
1000 ( 1 "]tt¢[tc]L + - - ~ I0 1 - 1-~ ] ~ ~bt~tt + kt¢.tt[tc] d[tc] 1000 ( 1 )ett[tt]L dt - L Io 1 - ~ Tt~tt~tc
1000 ( L
1 ) ,o[tclC
Io 1 - 1 ~ j - - - - D ~ q S t ~ t t - k t ¢ ~ t t [ t c ]
1000LIo ( 1 - 1 -16 5 )/ - -e,~[tc]L -g--
d[cc] 1000 ( 1 )et¢[tclL dt - L I° 1 - 1 ~ T ~t . . . . L
1000 (io ~-~1)0.5Err[telL L 1 D ~bt. . . . ~ kCC~ tc[CC ]
With these equations, changes in concentrations of the three isomers under 366 nm light can be calculated by a computer. Using the following data for gtt, thickness of the sample, kcc~tc and initial absorbance at 328 nm Ao for [2.2](4,4')-azobenzenophane, and assuming quantum yields for tt o tc and tc ~ c c as follows, the change in absorbance at 328 nm obtained by the calculations was plotted against exposure energy. gtt = 13 600(366 nm)
~tt~tc =
0.25
ett = 41 300(328 nm)
qS,. . . . = 0.25
L = 0.0003 cm kcc~tc = 5.14 x 10 -6 s -I
....
1000 ( 1 ~ ~[cc]L + ---L-- I o 1 - 1 - ~ j - - - - D ~ ~b~ ~ tc + k~ ~t~[cc]
looo (
d[cc] dt
1 "]~dcc]L
I0 1 - 1 - ' - 0 - ~ j ~ q S c ~ t c - k ~ . t ~ [ c c ]
where [tt] is the concentration of the tt isomer, [tc] is the concentration of the tc isomer, [cc] is the concentra-
Ao = 0.55 The quantum yields were selected to fit a calculated curve to the observed result under a light intensity of 2.64 mW cm -2 shown in Fig. 4. Figures 6, 7, 8 and 9 show the results of the calculations in which the lifetimes of the tc isomer were 10, 25, 100 and 1000s respectively. Four calculations assuming four different intensities of light, 0.1, 1.0, 10 and 100 mW cm -2, for each lifetime of tc isomer were undertaken. Based on these figures, relative initial isomerization rates are plotted against the intensity of light for several lifetimes of
137
N. Tamaoki et al. / Photochromic memory with non-destructive read-out
1.0
1.0
'o 0.5
<
0
100
200
300
400
5 0
600
700
0
Exposure energy / mJ cm -2
i
i
i
100
200
300
L
400
i
t
500
600
700
Exposure energy / mJ ern -2
Fig. 6. Theoretical curves of changes in absorbance at the nn* transition band of macrocyclic azobenzene dimers under 366 nm with exposure energy. It is assumed that the lifetime of the tc isomer is 10s. Light intensities are 100, 10, 1, 0.1 m W c m -2 from top to bottom.
1.0,
Fig. 9. Theoretical curves of changes in absorbance at the =~z* transition band of macrocyclic azobenzene dimers under 366 nm with exposure energy. It is assumed that the lifetime of the tc isomer is 1000s. Light intensities are 100, 10, 1, 0.1 m W c m -2 from top to bottom.
o~
1.0
O
0.8
0.5
'~
0.6 0.4
i
i
i
i
i
J
100
200
300
400
500
600
5 0[ 0.2
700
0
Exposure energy / mJ cm -2
......
~ 10-210-1100
Fig. 7. Theoretical curves of changes in absorbance at the ~ * transition band of macrocyclic azobenzene dimers under 366 nm with exposure energy. It is assumed that the lifetime of the tc isomer is 25s. Light intensities are 100, 10, 1, 0.1 m W c m -2 from top to bottom.
a
......
.I
.....
I
......
10 1 10 2 10 3 10 4 10 5
Light-intensity / mW crri2 Fig. 10. Theoretical curves of initial photoisomerization rates of macrocyclic azobenzene dimers with different lifetimes of tc isomers against light intensity. The lifetimes are 10 000, I000, 100, 25, 10, 1, 0.1 s from left to right.
1.0
the tc isomer is 25 s. These results of the calculations agree well with the results obtained for [2.2](4,4')azobenzenophane in films or in solution.
S 0.5
<
0
i
~
i
i
i
i
100
200
300
400
500
600
700
Exposure energy / mJ cm -2 Fig. 8. Theoretical curves of changes in absorbance at the ~z~z* transition hand of macrocyclic azobenzene dimers under 366 nm with exposure energy. It is assumed that the lifetime of the tc isomer is 100s. Light intensities are 100, 10, 1, 0.1 m W c m -2 from top to bottom.
the tc isomer in Fig. 10. This figure shows that the light-intensity dependence would occur in the region of higher light intensities as the lifetime of the tc isomer decreases. This figure also predicts that the light-intensity dependence should be observed in the range 0.110 mW cm -2 for compounds of which the lifetime of
3.3. Tests of a non-destructive read-out property The absorbance was 0.8-1.0 at 2max of the ~r~* transition of azobenzene derivatives contained in samples for tests of a non-destructive read-out property. After exposure to 366 nm light (5 mW cm -2) through a mask (writing process), azobenzene derivatives in the exposed area are changed from the tt (or trans in the case of 3) form to the cc (or cis in the case of 3) form while those in unexposed areas remained unchanged. The patterns were not visible because the difference in absorbance of visible light between the tt (or ,trans-) and the cc (or cis-) azobenzene derivatives used was small. Visualization (read-out) of the patterns was accomplished by exposure of samples to 366 nm light (20 pW cm -2) on a fluorescent sheet, the surface of the fluoroescent sheet being in contact with the surface of the photochromic films. In the area of the tt (or trans)
138
(a)
(c)
(e)
N. Tamaoki et al. ] Photochromic memory with non-destructive read-out
(b)
(d)
(f) Conditions of exposure
Writing Erasure Monitoring (read-out)
(g)
Wavelength (nm)
Intensity (mW cm -~)
Duration (min)
366 > 450 366
5
2 2
0.02
Fig. 11. Visualized patterns before and after exposure to light for visualization, and one visualized pattern after re-writing: (a), (c), (e), (g) [2.2](4,4')-azobenzenophane; (b), (d), (f) 4,4'-dimethylazobenzene (a), (b) before exposure to a monitor light; (c), (d) after exposure to a monitor light for 1 h; (e), (f) after exposure to a monitor light for 2 h; (g) Japanese characters written after erasure of alphabets.
N. Tamaoki et al. [ Photochromic memory with non-destructive read-out
form, 366 nm light does not reach the fluoroescent sheet while in the area of the cc (or cis) form, 366 nm light reaches the sheet and blue light is emitted from the sheet. Figures l l(a) and (b) show the visualized patterns just after the writing process for photochromic films containing 1 and 3 respectively. These figures demonstrate that the patterns are successfully visualized with rather high contrast by the visualization procedure described above. Figures l l(c), (d), (e) and (f) show visualized patterns for the written photochromic films after exposure to a monitor light (20 ~Wcm-2). The pattern of the photochromic film containing 3 almost disappeared after 2 h exposure to the monitor light (Figure l l(f)). However, the pattern of the photochromic film containing the macrocyclic azobenzene derivative 1 was not changed even after 2 h monitoring. These tests apparently demonstrate that the photochromic memory composed of the macrocyclic azobenzene derivative 1 and polycarbonate have a nondestructive read-out property. Figure l l(g) shows Japanese characters which were written by exposure to 366 nm light (5 mWcm -2) through a new mask after initialization of the sample in Fig. l l(e) with light of wavelength greater than 450 nm. This test demonstrates the re-writability of the photochromic memory.
4. Conclusion A non-destructive read-out property was demonstrated for a newly developed photochromic memory consisting of azobenzenophane 1 and polycarbonate. The memory was written by exposure to 366 nm light at 5 mW cm -~, and the written memory was non-destruc-
139
tively visualized by exposure to 366nm light at 20 ~tW cm -2. It was verified experimentally and theoretically that the non-destructive read-out property originates in the novel isomerization pathway of the tt isomer to the cc isomer through the sterically destabilized tc isomer.
Acknowledgment We thank Mr. Toshinao Takahashi for taking photographs of visualized patterns.
References 1 H. Diirr and H. Bouas-Laurent (eds.), Photochromism, Elsevier, Amsterdam, 1990. 2 H. Diirr, Angew. Chem. Int. Ed. EngL, 28(1989) 413. 3 M. Irie and M. Mohri, J. Org. Chem., 53 (1988) 803. 4 K. Ichimura, Y. Suzuki, T. Seki, A. Hosoki and K. Aoki, Langmuir, 4 (1988) 1214. 5 Y. Suzuki, K. Ozawa, A. Hosoki and K. Ichimura, Polym. Bull., 17 (1987) 285. 6 E. Ando, J. Miyazaki, K. Morimoto, H. Nakahara and K. Fukuda, Thin Solid Films, 133 (1985) 21. 7 Z. F. Liu, K. Hashimoto and A. Fujishima, Nature, 347 (1990) 658. 8 N. Tamaoki, K. Koseki and T. Yamaoka, Angew. Chem. Int. Ed. Engl., 29 (1990) 105. N. Tamaoki, K. Ogata, K. Koseki and T. Yamaoka, Tetrahedron, 46 (1990) 5931. 9 N. Tamaoki and T. Yamaoka, J. Chem. Soc., Perkin Trans. 2, (1991) 873. 10 H. E. Bigelow and D. B. Robinson, Org. Synth. Coll., 3 (1955) 103. 11 W. Priest and M. M. Sifain, J. Polym. Sci. A-l, 9(1971) 3161. 12 C. D. Eisenbach, Makromol. Chem., 179 (1978) 2489.
Thin Solid F)'hns, 221 (1992) 132 139
A photochromic memory with a non-destructive read-out property Nobuyuki Tamaoki*, Sawako Yoshimura and Tsuguo Yamaoka Department of Image Science and Technology, Chiba University, 1-33 Yayoi-cho, Chiba 260 (Japan) (Received January 17, 1992; accepted May 26, 1992)
Abstract lsomerizations of [2.2](4,4')-azobenzenophane (1) in polycarbonate films were investigated spectrophotometrically, and the light-intensity dependent efficiency of the photoisomerization of macrocyclic azobenzene dimers, including 1, was analyzed theoretically. It is demonstrated that the efficiency of photoisomerization of 1 is dependent on the light intensity, even in a polymer film. A correlation between the lifetimes of the trans-cis isomers of macrocyclic azobenzene dimers and light intensities, where the efficiency of photoisomerization becomes dependent on light intensity, is also described. A photochromic memory composed of 1 and polycarbonate shows a non-destructive read-out property.
1. Introduction There have been many studies with the aim of applying photochromism to optical memories [1-3]. However, these attempts have encountered various problems. A general and fundamental problem is that memories are destroyed by photoreactions during "read-out" processes. Because the wavelengths 2A and 2B of light used for "writing" and "erasing" memories are the same as those used for "read-out", repetition of "read-out", even with low intensity light, induces the same reaction as "writing" or "erasing" and destroys the memories. Various ideas have been proposed to overcome this problem [4-7]. One promising idea is to synthesize photochromic compounds which show lightintensity dependent photoisomerization. If the quantum yield of photoisomerization of the compounds becomes greater as the light intensity is increased, we can write and erase memories with high intensity light and read out with low intensity light without any serious destruction of memories. We synthesized two macrocyclic azobenzene dimers, 1 and 2, and reported that the photoisomerization of these compounds is dependent on light intensity in solvents [8-9]. These compounds have three isomers, the trans-trans (tt), the trans-cis (tc) and the cis-cis (cc) isomers. Isomerization of these compounds is dependent on light intensity since the photoisomerization from the tt isomer to the cc isomer proceeds via the tc isomer, which is thermally unstable and easily isomer-
*Present address: Research Institute for Polymers and Textiles 1-1-4 Higashi, Tsukuba, lbaraki 305, Japan.
0040-6090/92/$5.00
izes to the tt isomer. In the present paper, the unusual properties of photoisomerization of macrocyclic azobenzene dimers are analyzed theoretically, and the lightintensity dependent isomerization of [2.2](4,4')-azobenzenophane, 1, in polymer films is demonstrated. A photochromic memory system which is composed of ! and polycarbonate shows a non-destructive read-out property.
2. Experimental section
2.1. Materials Spectral grade dichloroethane was used without further purification. Bisphenol A type polycarbonate with Mw = 30 000 was purchased from Teijin Chemical Corporation, Tokyo. 4,4'-Dimethylazobenzene was synthesized by the reduction of p-nitrotoluene with zincNaOH and then purified by recrystallization from ethanol solution [10]. [2.2](4,4')-Azobenzenophane (1) was synthesized according to the method described in an earlier report [8]. 2.2. Preparation of polycarbonate films containing azobenzene derivatives for isomerization measurements [2.2](4,4')-Azobenzenophane (1.861 mg) and bisphenol A polycarbonate (0.1 g).were dissolved in 1.0 ml of 1,2-dichloroethane, and coated on quartz plates with a wire bar (no. 40). After drying for 2 h at room temperature, they were dried completely in an oven at 52 °C under vacuum. Similarly, films containing 4,4'dimethylazobenzene, 3, were obtained by coating quartz plates with a solution of the azo compound (0.01056 g) and polycarbonate (0.6 g) dissolved in 6.0 g
© 1 9 9 2 - Elsevier Sequoia. All rights reserved
133
N. Tamaoki et al. / Photochromic memory with non-destructive read-out
(a)
Writing
(b)
Monitoring
366 nm
~
#
NNNN__~
N%N
366 nm
IIlll
(1)
_
~
.
~
:
,
•
(2)
(Read-out)
-.-IIlll
/_
f
\
/ (c)
Erasing > 450 nm
(3)
Scheme. Structures of compounds i, 2 and 3. of dichloroethane. After drying, the film thickness was 2-3/am, which was calculated from the weight of a given area and the density of polycarbonate, 1.2.
2.3. Preparation of photochromic memory for demonstration of a non-destructive read-out property The same 1,2-dichloroethane solutions as described above were spread on quartz plates ( 5 0 m m × 50 mm × 1.5 mm) using a spin-coater (500 rev rain-l). The plates were dried as described above. After drying the film thickness was 4-5/am.
2.4. Measurement of isomer&ation A high-pressure mercury lamp (Ushio-UM-452, 450 W) equipped with Toshiba glass filters, UV-34 and UV-D2 was used to expose samples to 366 nm light. Kinetic measurements for thermal isomerization of polycarbonate films containing azobenzene derivatives were taken spectrophotometrically on a Hitachi Model 200-20 spectrophotometer equipped with a thermocontrolled cell holder by monitoring changes in absorbance at 328 n m ('~max of the ~rt* transition) in the dark after sufficient irradiation of 366 nm light. Fast thermal isomerization from the tc form to the tt form was monitored immediately after the photoirradiation. Changes in absorbance at 328 nm as a function of time were recorded with a chart speed of 48 cm min- J.
2.5. Measurement of light &tensity dependence Photoirradiation was done at 25 ___2 °C in almost the same manner described above. The light intensity was altered using filters and changing the distance and then
/
lllll
/
Fig. 1. Procedures for writing, monitoring (read-out) and erasing of memories.
measured by Eppley thermopiles (The Eppley Laboratory Ltd.).
2.6. Procedures of writ&g, reading and eras&g memory The photochromic films on quartz plates were exposed to 366 nm light (5 mW cm -2) from a super high pressure mercury lamp with appropriate filters through a mask in contact (Fig. l(a)). Visualization (read-out) of the patterns was performed by exposure of the plates to 366 nm light (20 laW cm -2) on a fluorescent sheet. During the visualization process the surface of the photochromic film was in contact with the fluorescent sheet (Fig. l(b)). The fluorescent sheet used absorbs UV light and emits blue light. The memory was erased with light of wavelength more than 450 nm from a super high pressure mercury lamp with a sharp cut filter.
3. Results and discussion
3.1. The cis- trans isomer&ation in polycarbonate film To apply the dependence on light intensity of photoisomerization from the tt form to the cc form of [2.2](4,4')-azobenzenophane in solution to functional materials, it is important to know whether this phenomenon can also be seen in a polymer matrix. Therefore, the photoisomerization and thermal isomerization properties of [2.2](4,4')-azobenzenophane were investigated in a polycarbonate film, which js an important polymer for use as a substrate for optical memories because of its transparency and thermal stability.
134
N. Tamaoki et al. / Photochromic' memory with non-destructive read-out
0.08
041 ~ ,-, 0.3 o
a
b
a
004 I.OZ
0.1
°zI 0
006
'
300
400
,
I 5OO
Wavelength (nm) Fig. 2. Changes in absorption spectra of 1 in a polycarbonate film under 366 nm light: a, before irradiation; b, photostationary state; c, after arbitrary extent of irradiation; d, after rapid thermal change from c.
Figure 2 shows spectral changes of 1 in polycarbonate upon 366 nm irradiation. Before irradiation, 1 in a polycarbonate film shows an absorption spectrum similar to that in benzene. The spectrum has two peaks at 328 nm and around 460 nm (curve a). These spectral characteristics are the same as that of trans-4,4'dimethylazobenzene, a strong rc~* transition peak at 330 nm and a weak n~* transition peak around 440 nm. These facts indicate that 1 exists as the t r a n s - t r a n s form in polycarbonate film. U p o n 366 nm irradiation,
the intensities of the peaks at 328 nm and around 4 6 0 n m decrease and a new peak appears around 440 nm and its intensity increases. A photostationary state is then attained (curve b). The characteristics of this spectrum at the photostationary state are similar to that of the cis form of the parent 4,4'-dimethylazobenzene in which just a weak nrt* transition peak is observed in the region above 300 nm. These facts suggest that 1 exists mainly as the cis-cis form. F r o m this photostationary state the initial spectrum is recovered by exposure to light above 420 nm. These spectral changes are similar to those observed in benzene. Immediately after irradiation with 366 nm light to an arbitrary extent (curve c), a small amount of rapid thermal recovery of the spectrum is observed for 1 in a polycarbonate film (from curve c to curve d). This kind of spectral change is only observed for azobenzenophanes with short connecting chains. Although it is not possible to isolate the pure tc isomer of 1 because of its instability, it is assumed that the spectrum of the tc isomer resembles the sum of spectra of trans- and cis-4,4'-dimethylazobenzene. Based on this assumption, the small amount of rapid thermal change of the spectrum, where the main change is seen for the peak at 328 nm, can be assigned to thermal isomerization from the tc isomer to the tt isomer. The assumption mentioned above is well supported by a theoretical analysis described in the next section. The spectrally determined lifetime of the tc isomer at 298 K was about 25 s. This value is almost the same as that in benzene, 22 s [8].
0.3
0.2
? I
~
0
100
200 Time ( m i n . )
300
Fig. 3. Thermalisomerization of l in polycarbonate films: A, 38°C; D, 33°C; O,.25°C.
N. Tamaoki et al. / Photochromic memory with non-destructive read-out
Figure 3 shows a change in absorption at several temperatures over long periods of time. This absorption change includes both the thermal steps from the cc isomer to the tc isomer and from the tc isomer to the tt isomer. The thermal change from the tc isomer to the tt isomer is very fast as described above, so this slow absorption change should correspond to the other isomerization step, i.e. thermal isomerization from the cc isomer to the tc isomer. In other words, the isomerization step from the cc isomer to the tc isomer is the rate-determining step. Plots of ln(A-A0) at 328nm against time are slightly curved for 1 in polycarbonate films. This means that the isomerization of 1 from the cc isomer to the tc isomer in polycarbonate films does not follow a first-order equation. Similar behavior has
135
been observed for non-cyclic azobenzene .derivatives embedded in polymers [11] or non-cyclic azobenzene units substituted in a polymer backbone or as polymer side chains [12]. In Figs. 4 and 5, the degrees of photoisomerization in polycarbonate films at two light intensities are plotted against exposure energy for [2.2](4,4')-azobenzenophane and 4,4'-dimethylazobenzene respectively. The light intensities used for the experiments were 2.64 and 0.269 mW cm -2. The photoisomerization of 4,4'dimethylazobenzene does not depend on the light intensity, but only on the exposure energy, as is known already. In contrast, the photoisomerization of [2.2](4,4')-azobenzenophane is more efficient under higher light intensity. This behavior in polycarbonate
1.13
0 0
,
,
Exposure
!
,
,
,
energy
(mJ/cm 2 )
500
Fig. 4. Changes in absorbance at 328 n m of I in polycarbonate films with exposure energy. Intensitiesof fight used are 2.64 (©) and 0.269 (Q) m W cm-=.
1.0
0 o
500
Exposure
energy
(mJ/cm
~ )
Fig. 5. Changes in absorbance at 339 nm o f 4,4'-dimethylazobenzene in polycarbonate films with exposure energy. Intensities of light used are 2.64 ( © ) and 0.269 ( 0 ) mW cm -2.
136
N. Tamaoki et al. [ Photochromic memory with non-destructive read-out
films coincides with that in benzene [8]. That is, as expected from the fact that the lifetime of the tc form of [2.2](4,4')-azobenzenophane in a polycarbonate film does not change much from the value in benzene, the efficiency of photoisomerization from the tt isomer to the cc isomer of the macrocycle is seemingly dependent on light intensity, even in a polycarbonate film.
3.2. Theoretical analysis of the dependence on light intensity The light-intensity dependence of the efficiency of photoisomerization is observed for compounds in which photoisomerization proceeds via a metastable state such as the trans-cis isomer for macrocyclic azobenzene dimers. The dependence on light intensity for such compounds is detected in the range of higher light intensities as the lifetime of the metastable state becomes shorter. When the dependence is used for non-destructive read-out, it is important to know the relation between the range where the efficiency is lightintensity dependent, and the lifetime of the metastable states. In order to clarify the relation, calculations are conducted. Based on the reaction scheme for [2.2](4,4')azobenzenophane shown below hv
tt.
_
d[tc] dt
hv
1000 ( 1 ) ~;tt[tt]L L I° I - - 1 ~ O ~--~btt~tc+ktc-tt[tc] I000 ( L I° 1
1 )e.[tt]L 10o ] D ~tt ~tc
lOO( 1)o.,,Icl,o
- k,c~,[tc] + k~¢~,c[cc]
hv',A
the following three differential equations are proposed for the three isomers. Each term in the equations corresponds to one route of the photoisomerization or thermal isomerization. _
d[tt] d~--
,'. tc.::." cc
hv',A
d[tt] dt
tion of the cc isomer, I 0 is the intensity of light, L is the path length of samples, ~btt~tc is the quantum yield from the tt isomer to the tc isomer, qS,~.c~ is the quantum yield from the tc isomer to the cc isomer, ett is the molar absorption coefficient of the tt isomer, et~ is the molar absorption coefficient of the tc isomer, ec~ is the molar absorption coefficient of the cc isomer, kcc~t~ is the rate constant of thermal isomerization from the cc isomer to the tc isomer, and kt¢_tt is the rate constant of thermal isomerization from the tc isomer to the tt isomer. The rate of photoisomerization of the cis units at 366 nm is quite low in comparison with the trans units because the molecular extinction coefficient of the cis units is small. Therefore, the photoisomerizations c c ~ t c and t c ~ t t are neglected, and et~ =0.5~u is assumed. The equations are then rewritten as follows:
1
0 lo
1 - 1~
D
-,-t,
~
tc
1000 ( 1 "]tt¢[tc]L + - - ~ I0 1 - 1-~ ] ~ ~bt~tt + kt¢.tt[tc] d[tc] 1000 ( 1 )ett[tt]L dt - L Io 1 - ~ Tt~tt~tc
1000 ( L
1 ) ,o[tclC
Io 1 - 1 ~ j - - - - D ~ q S t ~ t t - k t ¢ ~ t t [ t c ]
1000LIo ( 1 - 1 -16 5 )/ - -e,~[tc]L -g--
d[cc] 1000 ( 1 )et¢[tclL dt - L I° 1 - 1 ~ T ~t . . . . L
1000 (io ~-~1)0.5Err[telL L 1 D ~bt. . . . ~ kCC~ tc[CC ]
With these equations, changes in concentrations of the three isomers under 366 nm light can be calculated by a computer. Using the following data for gtt, thickness of the sample, kcc~tc and initial absorbance at 328 nm Ao for [2.2](4,4')-azobenzenophane, and assuming quantum yields for tt o tc and tc ~ c c as follows, the change in absorbance at 328 nm obtained by the calculations was plotted against exposure energy. gtt = 13 600(366 nm)
~tt~tc =
0.25
ett = 41 300(328 nm)
qS,. . . . = 0.25
L = 0.0003 cm kcc~tc = 5.14 x 10 -6 s -I
....
1000 ( 1 ~ ~[cc]L + ---L-- I o 1 - 1 - ~ j - - - - D ~ ~b~ ~ tc + k~ ~t~[cc]
looo (
d[cc] dt
1 "]~dcc]L
I0 1 - 1 - ' - 0 - ~ j ~ q S c ~ t c - k ~ . t ~ [ c c ]
where [tt] is the concentration of the tt isomer, [tc] is the concentration of the tc isomer, [cc] is the concentra-
Ao = 0.55 The quantum yields were selected to fit a calculated curve to the observed result under a light intensity of 2.64 mW cm -2 shown in Fig. 4. Figures 6, 7, 8 and 9 show the results of the calculations in which the lifetimes of the tc isomer were 10, 25, 100 and 1000s respectively. Four calculations assuming four different intensities of light, 0.1, 1.0, 10 and 100 mW cm -2, for each lifetime of tc isomer were undertaken. Based on these figures, relative initial isomerization rates are plotted against the intensity of light for several lifetimes of
137
N. Tamaoki et al. / Photochromic memory with non-destructive read-out
1.0
1.0
'o 0.5
<
0
100
200
300
400
5 0
600
700
0
Exposure energy / mJ cm -2
i
i
i
100
200
300
L
400
i
t
500
600
700
Exposure energy / mJ ern -2
Fig. 6. Theoretical curves of changes in absorbance at the nn* transition band of macrocyclic azobenzene dimers under 366 nm with exposure energy. It is assumed that the lifetime of the tc isomer is 10s. Light intensities are 100, 10, 1, 0.1 m W c m -2 from top to bottom.
1.0,
Fig. 9. Theoretical curves of changes in absorbance at the =~z* transition band of macrocyclic azobenzene dimers under 366 nm with exposure energy. It is assumed that the lifetime of the tc isomer is 1000s. Light intensities are 100, 10, 1, 0.1 m W c m -2 from top to bottom.
o~
1.0
O
0.8
0.5
'~
0.6 0.4
i
i
i
i
i
J
100
200
300
400
500
600
5 0[ 0.2
700
0
Exposure energy / mJ cm -2
......
~ 10-210-1100
Fig. 7. Theoretical curves of changes in absorbance at the ~ * transition band of macrocyclic azobenzene dimers under 366 nm with exposure energy. It is assumed that the lifetime of the tc isomer is 25s. Light intensities are 100, 10, 1, 0.1 m W c m -2 from top to bottom.
a
......
.I
.....
I
......
10 1 10 2 10 3 10 4 10 5
Light-intensity / mW crri2 Fig. 10. Theoretical curves of initial photoisomerization rates of macrocyclic azobenzene dimers with different lifetimes of tc isomers against light intensity. The lifetimes are 10 000, I000, 100, 25, 10, 1, 0.1 s from left to right.
1.0
the tc isomer is 25 s. These results of the calculations agree well with the results obtained for [2.2](4,4')azobenzenophane in films or in solution.
S 0.5
<
0
i
~
i
i
i
i
100
200
300
400
500
600
700
Exposure energy / mJ cm -2 Fig. 8. Theoretical curves of changes in absorbance at the ~z~z* transition hand of macrocyclic azobenzene dimers under 366 nm with exposure energy. It is assumed that the lifetime of the tc isomer is 100s. Light intensities are 100, 10, 1, 0.1 m W c m -2 from top to bottom.
the tc isomer in Fig. 10. This figure shows that the light-intensity dependence would occur in the region of higher light intensities as the lifetime of the tc isomer decreases. This figure also predicts that the light-intensity dependence should be observed in the range 0.110 mW cm -2 for compounds of which the lifetime of
3.3. Tests of a non-destructive read-out property The absorbance was 0.8-1.0 at 2max of the ~r~* transition of azobenzene derivatives contained in samples for tests of a non-destructive read-out property. After exposure to 366 nm light (5 mW cm -2) through a mask (writing process), azobenzene derivatives in the exposed area are changed from the tt (or trans in the case of 3) form to the cc (or cis in the case of 3) form while those in unexposed areas remained unchanged. The patterns were not visible because the difference in absorbance of visible light between the tt (or ,trans-) and the cc (or cis-) azobenzene derivatives used was small. Visualization (read-out) of the patterns was accomplished by exposure of samples to 366 nm light (20 pW cm -2) on a fluorescent sheet, the surface of the fluoroescent sheet being in contact with the surface of the photochromic films. In the area of the tt (or trans)
138
(a)
(c)
(e)
N. Tamaoki et al. ] Photochromic memory with non-destructive read-out
(b)
(d)
(f) Conditions of exposure
Writing Erasure Monitoring (read-out)
(g)
Wavelength (nm)
Intensity (mW cm -~)
Duration (min)
366 > 450 366
5
2 2
0.02
Fig. 11. Visualized patterns before and after exposure to light for visualization, and one visualized pattern after re-writing: (a), (c), (e), (g) [2.2](4,4')-azobenzenophane; (b), (d), (f) 4,4'-dimethylazobenzene (a), (b) before exposure to a monitor light; (c), (d) after exposure to a monitor light for 1 h; (e), (f) after exposure to a monitor light for 2 h; (g) Japanese characters written after erasure of alphabets.
N. Tamaoki et al. [ Photochromic memory with non-destructive read-out
form, 366 nm light does not reach the fluoroescent sheet while in the area of the cc (or cis) form, 366 nm light reaches the sheet and blue light is emitted from the sheet. Figures l l(a) and (b) show the visualized patterns just after the writing process for photochromic films containing 1 and 3 respectively. These figures demonstrate that the patterns are successfully visualized with rather high contrast by the visualization procedure described above. Figures l l(c), (d), (e) and (f) show visualized patterns for the written photochromic films after exposure to a monitor light (20 ~Wcm-2). The pattern of the photochromic film containing 3 almost disappeared after 2 h exposure to the monitor light (Figure l l(f)). However, the pattern of the photochromic film containing the macrocyclic azobenzene derivative 1 was not changed even after 2 h monitoring. These tests apparently demonstrate that the photochromic memory composed of the macrocyclic azobenzene derivative 1 and polycarbonate have a nondestructive read-out property. Figure l l(g) shows Japanese characters which were written by exposure to 366 nm light (5 mWcm -2) through a new mask after initialization of the sample in Fig. l l(e) with light of wavelength greater than 450 nm. This test demonstrates the re-writability of the photochromic memory.
4. Conclusion A non-destructive read-out property was demonstrated for a newly developed photochromic memory consisting of azobenzenophane 1 and polycarbonate. The memory was written by exposure to 366 nm light at 5 mW cm -~, and the written memory was non-destruc-
139
tively visualized by exposure to 366nm light at 20 ~tW cm -2. It was verified experimentally and theoretically that the non-destructive read-out property originates in the novel isomerization pathway of the tt isomer to the cc isomer through the sterically destabilized tc isomer.
Acknowledgment We thank Mr. Toshinao Takahashi for taking photographs of visualized patterns.
References 1 H. Diirr and H. Bouas-Laurent (eds.), Photochromism, Elsevier, Amsterdam, 1990. 2 H. Diirr, Angew. Chem. Int. Ed. EngL, 28(1989) 413. 3 M. Irie and M. Mohri, J. Org. Chem., 53 (1988) 803. 4 K. Ichimura, Y. Suzuki, T. Seki, A. Hosoki and K. Aoki, Langmuir, 4 (1988) 1214. 5 Y. Suzuki, K. Ozawa, A. Hosoki and K. Ichimura, Polym. Bull., 17 (1987) 285. 6 E. Ando, J. Miyazaki, K. Morimoto, H. Nakahara and K. Fukuda, Thin Solid Films, 133 (1985) 21. 7 Z. F. Liu, K. Hashimoto and A. Fujishima, Nature, 347 (1990) 658. 8 N. Tamaoki, K. Koseki and T. Yamaoka, Angew. Chem. Int. Ed. Engl., 29 (1990) 105. N. Tamaoki, K. Ogata, K. Koseki and T. Yamaoka, Tetrahedron, 46 (1990) 5931. 9 N. Tamaoki and T. Yamaoka, J. Chem. Soc., Perkin Trans. 2, (1991) 873. 10 H. E. Bigelow and D. B. Robinson, Org. Synth. Coll., 3 (1955) 103. 11 W. Priest and M. M. Sifain, J. Polym. Sci. A-l, 9(1971) 3161. 12 C. D. Eisenbach, Makromol. Chem., 179 (1978) 2489.