Photochemistry in polymer solids-7. Photochromic reaction of spiropyran in polycarbonate film

Fur. Polym. J. Vol. 21, No. 9, pp. 805-810, 1985 Printed in Great Britain. All rights reserved

0014-3057/85 $3.00 + 0.00 Copyright (© 1985 Pergamon Press Ltd

PHOTOCHEMISTRY

IN POLYMER

SOLIDS--7

PHOTOCHROMIC REACTION OF SPIROPYRAN IN POLYCARBONATE FILM K. HORIE, M. TSUKAMOTOand I. M1TA Institute of Interdisciplinary Research, Faculty of Engineering, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153, Japan

(Received 25 February 1985) Abstract--The formation of coloured merocyanine form of spirobenzopyran molecularly dispersed in bisphenol-A polycarbonate by a single laser pulse irradiation at 337 nm and the following rate of decoloration by monitoring light irradiation at 560 nm were measured over the temperature range 80-453 K. The coloration of spiropyran in polycarbonate film proceeds within several nanoseconds and even at temperatures below Ty of the matrix polymer (-120°C). The photo-induced decoloration of merocyanine form in the film proceeds exponentially for T > Tg (150°C), but deviates from single exponential type kinetics for T < T~ probably due to the inhomogeneous distribution of free volume in the matrix polymer. Arrhenius plots of the apparent rate coefficients for the decoloration showed breaks at Tv TB and Tr of the matrix polycarbonate. A brilliant red fluorescence was observed from the merocyanine form of spiropyran.

INTRODUCTION

of free volume distribution in the matrix [7, 8]. The rate of decoloration of spiropyrans in films by visible light irradiation has been less studied and the temperature dependence of the rates has not been reported so far. The rate and mechanism of coloration of free and polymer-bound spiropyrans in solution have been studied recently with laser flash photolysis technique [9, 10]. Measurement of the temperature dependence of these rate processes is very important since the photochemical processes are strongly affected by molecular motion, glass transition and other subglass transitions. In our previous papers, the benzophenone phosphorescence in poly(methyl methacrylate) and other acrylic polymers [11-14], in polystyrene and

The photochemistry of spirobenzopyran and other photochromic compounds or groups in polymer films has become of increasing interest [1, 2] in connection with their potential applicability to erasable photomemory systems. Colourless spiropyran derivatives such as A isomerize to the ring-opened blue violet merocyanine form B by u.v. light irradiation, and the m[rocyanine form B is discoloured to the spiropyran form A thermally or by visible light irradiation. The thermal decoloration of spiropyran derivatives was observed to be much slower in poly(methyl methacrylate) film than in a homogeneous solvent [3] and the molecular size dependence of the decoloration rates of spiropyrans dispersed in or chemically bound to matrix polymers was also reported [4].

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A typical phenomenon encountered in the study of photoprocesses in polymer solids is that reactions, unimolecular in solution, are frequently not first order in polymer films [1, 2]. This phenomenon was also observed for the thermal decoloration of spiropyrans at temperatures below the glass transition temperature, Tg, of the matrix polymer and has been attributed to the existence of several isomeric species of merocyanine form B [5, 6], or to the heterogeneity

polycarbonate [15] has been shown to decay nonexponentially at temperatures between T~ and the temperature corresponding to ester side-group rotation or phenyl group rotation. In the present paper, the formation of the merocyanine form of spirobenzopyran molecularly dispersed in polycarbonate by laser pulse irradiation at 337 nm and the following rate of decoloration by monitor light irradiation at 560 nm are measured in the temperature

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Measurements of absorption and fluorescence spectra The absorption spectra of spiropyran and coloured merocyanine films were measured with a Shimadzu MPS-5000 type spectrophotometer. The fluorescence spectrum of merocyanine form sample film was measured with a Jasco FR-550 type spectrofluorimeter.

range 80--453 K, and the effect of Tg and subglass transitions on the rate of coloration and decoloration is discussed. The red fluorescence from the merocyanine form is also observed. EXPERIMENTAL

Materials A standard sample of poly(oxycarbonyloxy-l,4phenyleneisopropylidene-l,4-phenylene)(bisphenol-A polycarbonate; ~w = 33,800, h~'Jh3, = 2.5) was purchased from Scientific Polymer Product, Inc. The 1,3,3-trimethylindolino-6'-nitrospirobenzopyran (SP; Tokyo Kasei Co.) was recrystallized from ethanol solution.

RESULTS AND DISCUSSION

Fluorescence o f merocyanine form Absorption spectra of the spiropyran in polycarbonate and its merocyanine form after 10min irradiation of weak high-pressure Hg lamp are shown in Fig. 2. The absorption peaks were observed at 342 nm for spiropyran form and at 595 nm as well as 375 nm for merocyanine form in polycarbonate. During experiments with laser excitation, we noticed that the sample film emits brilliant red light during 337 nm irradiation attributed to a fluorescence from the excited state of merocyanine form produced by the coloration of spirobenzopyran. This was ascertained from the fluorescence and fluorescence-excitation spectra shown in Fig. 3. The fluorescence excitation spectrum monitored at 670 nm corresponds to the absorption spectrum of merocyanine form. It is noteworthy that the intensity of fluorescence increases during the measurements with spectrofluorimeter due to the formation of merocyanine form by the monitoring light irradiation of the spectrofluorometer. The lifetime of the red fluorescence is supposed to be shorter than a few nanoseconds, since the same emission rise and decay profile as the profile of excitation pulse was observed by laser pulse excitation of both coloured and transparent sample films (Fig. 4). The intensity of the emission peak has a wavelength dependence similar to the merocyanine fluorescence (Fig. 4, insert). The fact that the emission rise and decay profile of the merocyanine fluorescence for transparent spiropyran form film was the same as that for coloured merocyanine form film suggests that the isomerization from spiropyran to

Sample preparation Bisphenol-A polycarbonate film containing 5.3 × 10-2mol/l of the spiropyran (40#m thickness) was solvent-cast on to a quartz plate from 10% polycarbonate solution with spiropyran in dichloromethane. The film was evacuated at room temperature for 2 days and then heated under vacuum at 100°C for 4 hr to eliminate the residual solvent. The concentration of spiropyran was determined by u.v. absorption measurements (e - 8.4 × 103 M -~ cm -~ at 337 nm). Measurements of rates of coloration and decoloration The change in the transmittance of 560 nm light for the sample film after the 337nm laser pulse irradiation at 80--453 K was followed by using a laser fash photolysis apparatus previously prepared for the triplet probe study of intermacromolecular reactions [16] and phosphorescence decay study in polymer solids [11-15] and modified for the present purpose. The apparatus is represented in Fig. 1. The coloration of spiropyran film set in a cryostat (Oxford DN704) was carried out by single pulse irradiation of 337 nm light (2.5 mJ, 10 nsec) from a pulsed nitrogen laser (Avco C950B). The monitoring light of 560 nm from 500 W Xenon lamp (Ushio UXL 500) with a combination of interference filter (KL56) and colour-giass filter (VO54) was transmitted through the film and the change in its intensity was detected by a photomultiplier (HTV R1464), and recorded by a X-T type recorder directly, by transient-time converter (Riken Denshi TCG8000), or by a digitizing storagescope (Iwatsu TS8123) depending on the time scale of the measurements.

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Rates of decoloration under visible light irradiation Typical examples for the changes in the intensity of monitoring light, U, at 560 nm transmitted by the

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spiropyran in polycarbonate film by a single pulse irradiation of nitrogen laser (337 nm) are illustrated in Fig. 5. The rate of coloration is very high and the minimum transmittance at 560 nm is achieved immediately after the laser pulse excitation (Fig. 6). Figure 6 together with the results on the emission of merocyanine fluorescence suggests the formation of coloured merocyanine form in polycarbonate film within several nanoseconds, compared with the value in solution by laser photolysis (kB > 4 × 109 sec-') [10] and the value in poly(methyl methacrylate) by pulse radiolysis (ks > 5 × l0 s sec - ' ) [17]. The extent of coloration by a single pulse irradiation evaluated as the change in transmittance at 560 nm is about 10-20%; this point will be discussed later. The decoloration of merocyanine Iorm by 560 nm monitoring light proceeds exponentially for temperatures above Tg (150°C) of the matrix polycarbonate. But, for T < Tg, the decoloration curves deviate from the single exponential type and can be divided into slow decoloration and fast decoloration. The early stage of the decoloration curves in Fig. 5 (a) and (c) corresponding to the fast decoloration are expanded in Fig. 5 (b) and (d), respectively, by the simultaneous measurements with transient time converter. The semilogarithmic plots of the change in absorbance, E = log(Uo/U), for decoloration curves at various temperatures are shown in Fig. 7. The rate constant, k~, for slow decoloration was calculated from the slope of the linear part in the later stage of decoloration. The replot of E subtracted by the extrapolated value of the linear part in semilogarithmic scale gave a nearly straight line, with slope giving the rate constant, k2, for fast decoloration. The rate constant for slow decoloration, kt, was proportional to the intensity of monitoring light at 560 nm, varying in the range of 0.01 to 1.0 by using neutral density filters. The rate of thermal decoloration of the same film in the dark at room temperature was about 3 x 10 -6 sec ' and is more than 103 times smaller than k~ under visible light irradiation in the present experiments. The Arrhenius plots of k, and k2 in Fig. 8 show breaks at 7-transition temperature corresponding to the onset of phenyl group rotation in polycarbonate (T~ = - 1 2 0 ° C ) , at fl-transition temperature due to co-operative local mode relaxation of a few monomeric units (Ta = 20°C) and at Tg --- 150°C. The k2 disappears for T > Tg. The difference of Tr from the usual literature value ( - 1 0 0 ° C ) [14, 18, 19] may be due to the very slow time scale of the present measurements. The activation energies for k, are 7.9 kJ/mol at Tr < T < Ta, 64 kJ/mol at T~ < T < Tg and 149 kJ/mol at T > Tg. The results suggests that the rate of decoloration is controlled by the molecular motion of the matrix polymer and indicates the importance of subglass transition such as T~ and T7 for the active control of intramolecular isomerization in polymer matrices. The extent of coloration by a single pulse irradiation, q~, was evaluated as the change in transmittance of 560 nm light at the moment of pulse irradiation (~b = 100 x (U0 - U1)/Uo), and is shown in Fig. 9, together with the extent of coloration of the

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component which contributes to the slow decoloration, @,. The #action of @, in @ is about a half and is independent of the temperature for the wide range of T < Ts. The value of @ is almost constant for T < T~ and then increases with increasing temperature for TB < T < .Ts. The formation of t r a n s type isomer among various isomers of merocyanine form in polycarbonate film by a single pulse irradiation at temperature below T~ is supposed to be difficult, since the phenyl group rotation is suppressed at the temperature range. The ion dissociation of spiropyran form in polycarbonate was ascertained to proceed even for T < Ty with @ g 10% and very rapidly (within nanoseconds) and therefore the structure of the coloured species for T < T~ is supposed to have a conformation X which needs no internal rotation after the bond dissociation of the spiropyran form,

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Acknowledgement--This work was supported in part by the Grant-in-Aid on Special Project Research for "Organic Thin Films for Information Conversion" from the Ministry of Education of Japan.

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REFERENCES

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Thus, the existence of slow and fast decoloration cannot be attributed to the formation of two types of isomers of the merocyanine form, but can be probably related to the inhomogeneous distribution of free volume [2, 8] in the matrix polymer. The annealing of polymer films at temperatures just below Tg improved the packing of molecules and the distribution of free volume, resulting in the exponential progress of isomerization of azobenzene group in polyurethane films even for T < Tg [19]. The effect of annealing of sample films on the rates of decoloration in polymer films is the subject of further investigation. It should be noted that the sample films showed no irreversible coloration throughout the present work with more than 100 times of laser pulse irradiation and decoloration. 20

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