Novel concept for non-destructive readout method for photochromic rewriteable memory devices

26 September 1997

ELSEVIER

CHEMICAL PHYSICS LETTERS Chemical Physics Letters 276 (1997) 450-454

Novel concept for non-destructive readout method for photochromic rewriteable memory devices Jiro Abe a, Nobukatsu Nemoto b, Yu Nagase b, Yasuo Shirai a a Department of Photo-Optical Engineering, Faculty of Engineering, Tokyo Institute of Polytechnics, liyama 1583, Atsugi, Kanagawa 243-02, Japan b Sagami Chemical Research Center, 4-4-1 Nishi-Ohnuma, Sagamihara, Kanagawa 229, Japan Received 19 February 1997; in final form 26 June 1997

Abstract

A new concept for a non-destructive readout method for a photochromic rewriteable device is proposed based on a solvatochromic dye-linked photochromic dye system. From the theoretical molecular orbital calculations for the model compound, it was shown that the UV-Vis absorption band of the solvatochromic dye unit is sensitive to the conformation of the nearby photochromic unit. These calculations indicate that light corresponding to the UV-Vis absorption band of the solvatochromic dye unit can be used as the non-destructive readout light, since light in this region is found inactive to irradiation for achieving the photochromic reaction of the photochromic dye unit. © 1997 Elsevier Science B.V.

Recently, progress has been achieved in the synthesis of thermally stable and fatigue-resistant photochromic compounds [1-5]. Among the many requirements of the erasable recording medium, readout unstability is one of the remaining problems to be solved for practical applications in optical memory devices. Some possible solutions have been proposed, but the development of a non-destructive readout method remains a challenging problem [613]. In this Letter, we propose a novel concept for a non-destructive readout method based on a solvatochromic dye-linked photochromic dye system. A photochromic reaction is the reversible transformation of a molecule between two isomers (A, B) having two different UV-Vis absorption bands AI

A~B A2

Let us suppose that the absorption maximum of A is

At and that of B is A2. A is then converted to B by irradiation with At light and the reverse reaction is caused by irradiation with A2 light. In general photochromic memory media, Al light is used as writing light and A2 light is used as reading light, since A has no absorption band at A2. The transmittance difference at A2 between A and B can be used to distinguish these isomers. However, during the reading process, A2 light causes the photoreaction of the photochromic molecules from B to A and the recorded information will be destroyed after many readout processes. This type of readout is called a destructive readout. Even weak light can induce the reaction, proportional to the number of photons absorbed by the media [11]. This is an important and fundamental problem for practical applications in optical memory devices. Up to now, no solutions for a non-destructive readout method by a simple procedure have been reported.

0009-2614/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 ( 9 7 ) 0 0 8 2 8 - 2

J. Abe et al. / Chemical Physics Letters 276 (1997) 450-454

We have considered that a simple non-destructive readout method may be possible by introducing a solvatochromic dye unit into a photochromic molecule. Solvatochromism is the solvent dependence of the UV-Vis absorption (or emission) spectrum of a molecule. Among the many solvatochromic molecules, betaine compounds are known to be highly negatively solvatochromic; polar solvents shift their longest wavelength UV-Vis absorption band to shorter wavelengths, and nonpolar solvents shift the band to longer wavelengths [14]. The betaine dye 2,6-diphenyl-4-(2,4,6-triphenyl-N-pyridinio)phenolate (Reichardt's dye) is one of the most highly solvatochromic compounds known [15]. The absorption maximum of Reichardt's dye shifts from Areax = 453 nm in water to Areax = 810 nm in diphenyl ether [14]. Because of this pronounced solvatochromism, these betaine dyes have been used for the empirical characterization of solvents [15,16]. These results are indicative of the sensitivity of such betaine dyes to environmental changes in the surrounding medium. Therefore, if these solvatochromic dyes are incorporated into a photochromic molecule, one can expect to induce a wavelength shift of the

solvatochromic dye unit with the photochromic reaction, since the molecular dipole moment of the photochromic dye unit will vary with the photoisomerization (Scheme 1). That is, the essential feature of our non-destructive readout concept lies in using the transmittance difference at A3, the absorption band assigned to the solvatochromic dye unit, between the two isomers. We have investigated the model compound shown in Fig. 1, which consists of a heterocyclic pyridinium betaine [17-21] as solvatochromic dye unit and an indolylfulgide as the photochromic dye unit, by means of molecular orbital (MO) calculations. By irradiation with UV light, the colorless fulgide derivatives are known to give colored isomers (closed-ring form) by a ring-closing reaction and the colored isomers are converted to the colorless isomers (open-ring form) by a ring-opening reaction upon irradiation with visible light [1,22-24]. The purpose of this study is to confirm the wavelength shift in the UV-Vis absorption maximum of the solvatochromic dye unit with the photochromic reaction using the model compound. The geometries both of the open-ring form and the closed-ring form of the

soivatochromicdye unit /

readout /

readout

photochromicdye unit

required condition for non-destructive readout

.:"... 0 0 C m

i JD m ...."

short

4

451

wavelength

-

long

Scheme I. The conceptfor a non-destructivereadoutmethodbasedon a soivatochromicdye-linkedphotochromicdye system.

452

J. Abe et aL / Chemical Physics Letters 276 (1997) 450-454

•

~,~

N

•

),1

open-ring form

closed-ring form

Fig. 1. Molecular structures and photochromic reaction of the model compound investigated in this Letter.

model compound were fully optimized at the ab initio SCF level of theory using a 6-31G double ~" quality basis set. The total energy of the closed-ring form was calculated to be 4.56 kcal/mol higher than that of the open-ring form. On the basis of these geometries, the transition energies and the oscillator strengths were calculated by means of the semiempirieal INDO/S method with configuration interaction (CI) wave functions, where all electronic states were included which are generated by singly exciting all electrons in the 20 highest occupied MOs to the 20 lowest unoccupied MOs with respect to the ground state [25,26]. This level of CI calculation showed that the absorption maximum of unsubstituted indolylfulgide shifts from 356 nm to 422 nm by the ring-closing reaction. The experimental values in PMMA film arc reported to be 392 nm for the open-ring form and 589 nm for the closed-ring form [22]. It is considered that the semiempirical INDO/S CI method can be employed to withdraw a qualitative tendency in spectral changes for the photochromic reaction of fulgide derivatives, though the method would give shorter wavelengths when compared to the experimental values. The calculated UV-Vis gas phase absorption spectra for the model compound are shown in Fig. 2. The open-ring form has two characteristic absorption maximums at Areax = 330 and 560 rim. On the other hand, the spectrum of the closed-ring form are char-

acterized by four absorption maximums at Amax-315, 420, 490 and 620 nm. In order to assign these absorption bands, we calculated the difference electron density between the electronic ground state and the excited states as shown in Fig. 3 [27]. The dark-colored region indicates the region in which electron density increases upon transition to the excited state, and the light-colored region indicates the region in which electron density decreases upon the transition. One can find that the S] state of the closed-ring form is localized at the betaine unit and the S 3 state is characterized by a local excitation in the fulgide unit. The S 2 state was also a localized excited state at the betaine unit. Thus, the strong 1.2 ..... o p e n - d n g form

1.(]

~

So*S,

clolad-rlng form

l

(rim)

Fig. 2. Calculated UV-Vis absorption spectra of the closed-ring

formand the open-ringform of the rno~l compound.

J. Abe et aL / Chemical Physics Letters 276 (1997) 450-454

Fig. 3. The difference electron density between the electronic ground state and (a) the S1 state of the closed-ringform, (b) the S3 state of the closed-ring form, (c) the S2 state of the open-ring form, and (d) the S5 state of the open-ring form. All of the calculations were carried out through the INDO/S-SCI method. The dark-colored region indicates the region in which electron density increases upon transition to the excited state, and the light-colored region indicates the region in which electron density decreases upon the transition.

absorption band at 620 nm can be assigned to excitation at the solvatochromic dye unit and the band at 420 nm can be assigned to the photochromic dye unit. The absorption bands for the open-ring form were also assigned by the same procedure. As can be seen from Fig. 3(c) and (d), the S 2 state of the open-ring form is localized at the betaine unit and the S 5 state is localized at the fulgide unit. The S 1 state was also confirmed to be a localized excited state at the betaine unit. Of special interest is the fact that a large wavelength shift (about 60 nm) of the absorption band assigned to the solvatochromic dye unit can be induced with the photochromic reaction at the fulgide unit. From these assignments for the absorption bands, we can expect the photochromic reaction of the model compound to be as follows. By irradiation with UV light, the open-ring form will

453

isomerize to the closed-ring form and the closed-ring form will return to the open-ring form by irradiation with visible light around 420 rim. Moreover, the band of the open-ring form at 560 nm and that of the closed-ring form at 620 nm are inactive to irradiation for achieving the photochromic reaction of the fulgide unit, since these absorption bands are assigned to the solvatochromic dye unit. Therefore, light in the 6 0 0 700 nm region can be used as the non-destructive readout light. In conclusion, we herein propose a novel concept for a non-destructive readout method based on a solvatochromic dye-linked photochromic dye system. For this system, it is shown by theoretical investigations that a wavelength shift of the absorption band assigned to the solvatochromic dye unit will be induced by the photochromic reaction of the photochromic dye unit. This shift of the absorption band assigned to the solvatochromic dye unit is considered to originate in the dipole moment variation at the photochromic dye unit with the ring-closing or ringopening photochromic reactions. Our new concept for a non-destructive readout method is remarkable in that the recorded information is not read by the transmittance difference at A2 between A and B but read by the transmittance difference at A3 between A and B. Of course, we do not think that the model compound studied here is the optimum to realize our concept, and moreover, the medium effects in solid polymers should be considered. However, it is possible to shift the UV-Vis absorption band of the solvatochromic dye unit to longer wavelengths by optimizing the solvatochromic chromophore. We have considered that, in principle, this solvatochromic dye-linked photochromic dye system will also be applicable to other photochromic systems such as 1,2-diarylethenes.

Acknowledgements We thank Makoto Ogura and Atsuya Takahashi of Fujitsu Ltd. for the use of the latest version of the MOS-F package and the ANCHOR II system. We also thank Azuma Matsuura and Tomoaki Hayano of Fujitsu Laboratories Ltd. for the helpful discussions of the semiempirical MO method.

454

J. Abe et al. / Cheraical Physics Letters 276 (1997) 450-454

References [1] P.J. Darcy, H.G. Heller, P.J. Strydom, J. Whittall, J. Chem. Soc., Perkin Trans. 1 (1981) 202. [2] M. Irie, M. Mohri, J. Org. Chem. 53 (1988) 803. [3] F. Buchholz, K. Zelichenok, V. Krongauz, Macromolecules 26 (1993) 906. [4] M. Hanazawa, R. Sumiya, Y. Horikawa, M. Irie, J. Chem. Soc., Chem. Commun. (1992) 206. [5] M. Irie, Mol. Cryst. Liq. Cryst. 227 (1993) 263. [6] G.M. Tsivgoulis, J.-M. Lehn, Angew. Chem. 107 (1995) 1188; Angew. Chem., Int. Ed. Engl. 34 (1995) 1119. [7] S.H. Kawai, S.L. Gilat, R. Ponsinet, J.-M. Lehn, Chem. Eur. J. 1 (1995) 285. [8] S.H. Kawai, S.L. Gilat, J.-M. Lehn, J. Chem. Soc., Chem. Commun. (1994) 1011. [9] Y. Yokoyama, T. Yamane, K. Kurita, J. Chem. Soc., Chem. Commun. (1991) 1722. [10] M. Irie, O. Miyatake, K. Uchida, T. Eriguchi, J. Am. Chem. Soc. 116 (1994) 9894. [11] F. Tatezono, T. Harada, Y. Shimizu, M. Ohara, M. Irie, Jpn. J. Appl. Phys. 32 (1993) 3987. [12] M. Irie, O. Miyatake, K. Uchida, J. Am. Chem. Soc. 114 (1992) 8715. [13] M. Irie, K. Sayo, J. Phys. Chem. 96 (1992) 7671.

[14] M.S. Paley, J.M. Harris, J. Org. Chem. 56 (1991) 568. [15] (a) C. Reichardt, Angew. Chem. 77 (1965) 30; Angew. Chem., Int. Ed. Engl. 4 (1965) 29; (b) C. Reichardt, Chem. Rev. 94 (1994) 2319. [16] M.J. Kamlet, R.M. Doherty, J.-L.M. Abboud, M.H. Abraham, R.W. Taft, Chem. Tech. (1988) 566. [17] E. Alcalde, Adv. Heterocyel. Chem. 60 (1994) 197. [18] J. Abe, Y. Shirai, J. Am. Chem. Soc. 118 (1996)4705. [19] N. Nemoto, J. Abe, F. Miyata, M. Hasegawa, Y. Shirai, Y. Nagase, Chem. Lett. (1996) 851. [20] J. Abe, N. Nemoto, Y. Nagase, Y. Shirai, Chem. Phys. Left. 261 (1996) 18. [21] J. Abe, Y. Shirai, N. Nemoto, F. Miyata, Y. Nagase, J. Phys. Chem. B 101 (1997) 576. [22] A. Kaneko, A. Tomoda, M. Ishizuka, R. Matsushima, Bull. Chem. Soc. Jpn. 61 (1988) 3569. [23] Y. Yokoyama, T. Iwal, Y. Yokoyama, Y. Kurita, Chem. Lett. (1994) 225. [24] Y. Yokoyama, Y. Shimizu, S. Uchida, Y. Yokoyama, J. Chem. Soc., Chem. Commun. (1995) 785. [25] J.E. Ridley, M. Zerner, Theor. Chim. Acta (Berl.) 32 (1973) 111. [26] MOS-F V3L1, Fujitsu, Ltd., Tokyo, Japan, 1995. [27] ANCHOR 11 V1L5, Fujitsu, Ltd., and Kureha Chemical Industry, Co. Ltd., Tokyo, Japan, 1995.