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The effect of substituents and polymer media on photochromism kinetics of indolinospironaphthoxazine* LIU Ping ( f l
) , MING Yangfu ( 4
4s )
and FAN Meigong ( % %& ) " *
(Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing 100101, China) Received January 25, 1999
Abstract
The effect of substituents in indoline moiety and polymer media on photochromism and thermal decay processes of spirooxazine (ASP) was investigated. The thermal decoloration rate was decreased with increasing steric hindrance of substituents in I-position of indoline moiety. The stability of the colored forms was improved when the hydrogen in the 5-position of indoline moiety was replaced with electron-donating groups and was decreased when substituted by electron-withdrawing groups. In addition, the stability of the colored forms was related to properties of polymer media. The thermal decay rate decreased with an increase in the polarity and rigidity of polymer media. The themal decay kinetics of the colored forms obeyed biexponential decay law.
Keywords :
spirooxazine , photochromism , thermal decay kinetics.
Spirooxazines are an important class of photochromic compounds developed on the basis of spiropyran. Owing to their excellent fatigue resistance and good photoresponsive sensitivity, spirooxazines have extensive applications in image display, optical information storage, manufacture of variable optical density light-filtered elements, photographic materials and light control switch and so on. However, as it is generally not easy to produce devices using common organic compohds, photochromic polymers have found much more extensive applications in practice['-31. Therefore, photochromic polymers have become a frontier field of research of functional materials such as in technologies related to laserF4]. According to the way photochromic chromophores incorporate with polymers, photochromic polymers can be divided into two types. One is the polymer proportionally doped with photochromic compounds. The other is the polymer with photochromic chromophore incorporated in the polymer chain or attached to the main chain as side groups. The research results of the first kind of photochromic polymers will be reported in this paper. The photocoloration and thermal decay curves of indolinospironaphthoxazine doped in different polymers were measured, the effects of substituents in the 1-and 5-position of indoline moiety and polymer media on photocoloration and thermal decay processes of spirooxazine were studied in detail.
1 Experimental 1.1
Apparatus A Varian DMS-300 UV-visible spectrophotometer with a strong-light monochromator was used for kinetic measurement. In kinetic measurement, the light source, a 450 W middle-pressure mercury lamp, was made by Institute of Beijing Electronic Lamp Resources, the strong-light monochromator
* Project supported by the * * Corresponding author.
National Natural Science Foundation of China (Grant No. 597730117).
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was designed and manufactured by Beijing Optical Instrument Factory.
1 . 2 Materials 1 . 2 . 1 Poly ( methyl methacrylate ) ( PMMA ) , vinyl chloride-vinyl acetate copolymer ( VCVAP) , polystyrene ( PS ) and poly ( vinyl butyral ) ( PVB ) are commercial reagents. 1 , 2-dicholoroethane is reagent purchased from Beijing Chemicals Factory. 1.2.2
9 ' - [4- ( allyloxy) benzoxy] indolinospironaphthoxazine(AS~a-e) will be reported in another pa-
per. Their structures are as follows :
ASP
R'
1.3
I
a
H
b H
c
d
e
H
C1
cH3
Measurement of photocoloration and thermal decoloration kinetics curves
1 . 3 . 1 Preparation of photochromic films (sample) . Polymer (100 mg) was dissolved in 1 . 7 mL of 1,2- dichloroethane, to which 10 mg indolinespirooxazine ( ASPa-e) was added. Then the solution was dropped onto glass plates and these plates were placed in the dark. The solvent was evaporated spontaneously; as a result the polymer film was formed. 1 . 3 . 2 Measurement. The sample was placed in the DMS-300 UV-visible spectrophotometer and exposed to the filtered irradiation ( A = 365 nm) from a 450 W high-pressure mercury lamp at room temperature. Then photocoloration curves at 600 nm were recorded. Thermal decoloration curves were measured after irradiation. The equipment for kinetic measurement is illustrated in scheme 1 .
Scheme 1 .
Schematic diagram of the apparatus used for the measurement of photocoloration and thermal decay kinetics
curves. 1 , DMS-300 wpetrophotometer; 2, lamp; 3 , reference light; 4 , measured light; 5 , measurement chamber; 6 , sample plate ; 7 , PM ; 8 , monochromator; 9 , lens ; 10, 450 W Hg lamp ; 11 , fans.
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Results and discussion
2 . 1 The effect of substituents in the 1-position of indoline moiety on photocoloration and thermal decay processes of spirooxazine The photochromism of spironaphthoxazine is as follows:
W
-
A or vis
ASP
PMC
The spirocarbon-oxygen bond in oxazine ring of the closed form (ASP) is cleaved with ultraviolet irradiation and the closed form is transformed into blue colored form ( PMC) . In the dark, PMC will quickly reverse to ASP. The colored form (PMC) has many isomers due to the cis-tram isomerization of three bonds between indoline and naphthalene moieties[5761. In photocoloration processes, the optical density of the colored forms of ASPa and ASPb in photoequilibrium state was slightly different in PMMA and VCVAP media when the methyl in the l-position of indoline moiety was replaced by n-ocytyl. However, the optical density of the colored forms of ASPc in photoequilibrium state, compared with ASPa, was markedly decreased in both media when the methyl was substituted by benzyl. The result on PMMA medium is shown in fig. 1. This may be explained by the fact that the steric hindrance of benzyl is larger than that of alkyl and hinders the ring-open reaction of spirooxazine . The thermal decay rate of the colored forms was decreased in PMMA and VCVAP media when the methyl was replaced by n-octyl or by benzyl. Fig. 2 shows the result on VCVAP medium-the 0.020 r
Fig. 1. Photocoloration curves of spirooxazines (ASPac) in PMMA medium. Curves a , b , c are photocoloration curves of ASP ( a , b ,c ) , respectively.
1.0 r
Fig. 2 .
Thermal decay curves of the colored forms of
spirooxazines ( ASPa-c)
in VCVAP medium. (Curves
a , b, c are thermal decay c w e s of the colored forma of ASP ( a , b, c )
, respectively)
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photocoloration rate of ASPc is low and so is its thermal decay rate. This further indicated that the steric hindrance of substituents in the 1-position of indoline moiety not only affected the photocoloration process but also hindered the ring-close reaction.
2 . 2 The effect of substituents in the 5-position of indoline moiety on photocoloration and thermal decay processes of spirooxazine In photocoloration processes, the optical density of the colored forms of ASPe in photoequilibrium state was greatly increased in comparison with ASPa when the hydrogen at the 5-position of indoline moiety was substituted by methyl, whether in PMMA , VCVAP or PS . Nevertheless, the optical density of the colored forms of ASPd in photoequilibrium state was very slow in any of the three media when the hydrogen at the 5-position was replaced with chloro. The result on PMMA medium is shown in figure 3 . The thermal decay rate of the colored forms of ASPe was decreased compared with ASPa when methyl took the place of the hydrogen in the 5-position, whether in PMMA, VCVAP or polystyrene. However, when chloro was in the place of the hydrogen in the 5-position, the thermal decoloration rate of the colored forms of ASPd was related to the nature of polymer medium. Its thermal decay rate in PMMA and VCVAP media was lower than that of ASPa. The result on PMMA media is shown in fig. 4 . The thermal decay rates of the three compounds were all faster in PS.
Fig. 3
.
Photocoloration curves of spirooxazines ( ASPa ,
Fig. 4 .
Thermal decay curves of the colored forms of
d , e ) in PMMA medium. Curves a , d ,e are photocoloration
spirooxazines ( ASPa, d , e ) in PMMA medium. Curves a ,
curves of ASP ( a , d ,e ) , respectively.
d , e are thermal decay curves of the colored forms of ( a , d ,e )
ASP
, respectively.
The effect of substituents in the 5-position of indoline moiety on photocoloration and thermal decay processes of spirooxazine results from different electronic effects of substituents. The methyl at the 5-position of ASPe, which belongs to electron-donating groups, results in an increase in the electron density of the nitrogen in indoline moiety. The positive charge density of the nitrogen in indoline moiety decreases, the colored forms are stabler. Consequently, the optical density was the highest, the thermal decay rate was the lowest and the lifetime was the longest (table 1) . The chloro substituent (--el) of ASPd, whose electron-withdrawing inductive effect is stronger than electron-donating conjugative effect, gives rise to a decrease in the electron density of the nitrogen in indoline moiety, compared with ASPa. Therefore, the positive charge density of the nitrogen in indoline moiety becomes stronger. This is unfavorable for the existence of the colored form, but makes the colored form (PMC)
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easily reverse to ASP. Then the optical density of the colored forms of ASPd was the lowest. But its thermal decay rate in media with stronger polarity was lower than that of ASPa. This may be contributed to the increased positive charge density of the nitrogen in indoline moiety, which strengthens the interaction between the colored forms and polar polymers, the colored forms themselves, and the interaction between the colored forms and the closed forms because of the strong polarity of ~~(4-(alloxy) benzoyl) at the 9'-position. So the ring-close reaction was hindered and the thermal decay rate became slower. Table 1 Polymer
PIMMA
The lifetime of thermal decoloration ( t ) , efficiency A and chisqr
Spirooxazine (ASP)
t,/min
A,
t2/min
A2
chisqr
a b c d a
0.19 0.24 0.23 0.36 0.28 0.28 0.25 0.23 0.78 0.12 0.066 0.17
0.327 0.306 0.249 0.292 0.248 0.640 0.416 0.567 0.408 0.561 0.525 0.598
1.48 1.74 1.56 1.87 1.15 1.41 1.53 1.27 1.72 0.59 0.61 0.89
0.508 0.506 0.579 0.536 0.539 0.336 0.537 0.364 0.457 0.348 0.437 0.286
3 . 0 x lo-' 7 . 0 x lo-' 2 . 0 lo-' 1 . 2 x lo-' 2 . 2 x lo-' 4 . 0 x lo-' 2 . 0 x lo-' 6 . 0 x lo-' 8 . 0 l~o - 5 1 . O Xl o - 4 1.6 x 1.4 x 1 0 ' ~
b VCVAP
c d e a
PS
b c
The influence of polymer media on photocoloration and thermal decay processes of spirooxazine The optical density of the colored forms of spirooxazine in polymer media with relatively strong polarity was higher than that in polymer with relatively weak polarity. For example, the optical density of the colored forms of ASPe in PMMA(1) and VCVAP ( 2 ) media, as shown in fig. 5 , was relatively high whereas it was very low (about 0.005) in PS ( 3 ) . The thermal decay rate of the colored forms of most spirooxazines in polymer media with relatively strong polarity is slower than that in those with relatively weak polarity. For instance (fig. 6 ) , the thermal decay rate of the colored forms of ASPd in PS was the fastest and its thermal decay lifetime
2.3
t/min Fig. 6 .
Thermal decay curves of the colored forms of
Photocoloration curves of the colored forms of
spimoxazines ( ASPd) in different media. Curves 1 , 2 , 3
spirooxazines ( ASPe) in different media. Curves 1 , 2 , 3
are thermal decay curves of the colored forms of ASPd in
are photocoloration curves of the colored forms of ASPe in
PMMA , VCVAP and
Fig. 5 .
PMMA , VCVAP and PS , respectively.
PS , respectively.
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was the shortest. The thermal decay lifetimes of the colored forms in PMMA, VCVAP and PS were 1.62, 1 .27 and 0 .61 min , respectively (table 1 ) . The polarity of the colored forms of spirooxazine is stronger than its precursor (ASP) , which promotes the interaction between the colored forms and polymers with stronger polarity. Accordingly, the ring-closed reaction rate of the colored forms was relatively slow in polymer media with stronger polarity, i .e . the colored forms were stabler and have higher optical density with slower thermal decay rate. The ring-close reaction rate of the colored forms was fast in poIymer media with weaker polarity. The stability of the colored forms was weaker. Consequently, the optical density in photoequilibrium state was lower and the thermal decay rate was faster"] . Besides, the optical density and thermal decay rate were also related to the rigidity of polymers", 91 . The polarity of PVB is obviously stronger than that of PS. But the optical densities of the colored forms of spirooxazines ( ASPa , d , e ) were very slow, all around 0.002 in PVB . Their thermal decay rates were very fast. This is due to the weaker rigidity of PVB which has longer branches and larger free volumes between molecules. The weaker the rigidity of polymers, the larger the free volume between molecules, the faster the ring-close reaction, and the less stable the colored forms in polymer media. Therefore, the optical density of the colored forms was lower and the thermal decay rate was faster. Although the polarity of VCVAP is a bit stronger than that of PMMA, the thermal decay rate of the colored forms was fast in VCVAP. This may be also due to the weaker regidity of VCVAP compared with PMMA. 2 .4
Macroscopic thermal decay kinetics This paper only discusses macroscopic kinetics. The photocoloration process was the result of mutual equilibration between photocoloration and thermal decoloration reactions with constant ultraviolet irradiation. It is different from pulse laser photolysis and cannot be dealt with by simple kinetics laws. However, the macroscopic result, especially the optical density at photoequilibrium state, has important application values. The thermal decoloration process occurred after light source was cut off. It is essentially the same as the result of pulse laser photolysis. Both of them are processes in which the colored forms at the ground state are thermally transformed into spirooxazine in the dark. It has nothing to do with any reaction of excited states. The thermal decoloration kinetics was investigated by the aid of computer software-Origin . The result showed that the thermal decoloration obeyed biexponential decay law as follows : R ( t ) = Alexp(- t / t l ) + A2exp(- t / t z ) + C. One process was fast and the other slow[101. This may be explained by the fact that the colored forms have different isomers, making thermal decay kinetics of the colored forms deviate from simple firstorder law but obey the biexponential decay . According to Schneider' s research result with laser Raman spectrum, the intermediate X , whose confirmation is basically the same as that of spirooxazine, results from the heterolytic cleavage of the spirocarbon-oxygen bond of the oxazine ring. Then the rotation of the three bonds between indoline and naphthalene moiety makes X form four different planar conformational isomers of PMC (scheme 2 ) . On the basis of the result of approximate calculation of quantum chemistry PPP, isomers 1 and 2 are less stable than isomers 3 and 4 because the molecules of 1 and 2 have intramolecular H-H repulsive forces, while the molecules of 3 and 4
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have weak intramolecular hydrogen bond. Therefore, the thermal decoloration rate of 1 and 2 was faster than that of 3 and 4 . This may be the main reason why the thermal decoloration abides by biexponential decay.
Scheme 2 .
Four different conformational isomers of the colored forms of spimoxazine.
The lifetime of the colored forms of spirooxazine was very short and in the order of seconds in solution. But it is longer in polymer than in fluid solvent. The lifetimes of the fast process ( t l ) and of the slow process ( t 2 ) , A , , A2 and the chisqr are shown in table 1. As is evident in table 1 , the lifetime of ASPa was remarkably different from that of ASPe whether in the fast process or in the slow process. The former was shorter than the latter in any polymer medium,suggesting that thermal stability or decay rate is mainly determined by the molecular structure. Hwwever, the thermal kinetics is also influenced by polymer medium to some extent, as shown in the above discussion. With respect to ASP(b, c , d ) , their lifetimes only have slight difference. The reason is that the electronic effects of substituents at l-position of indoline moiety are close to each other. On the other hand, the action direction of the electron-withdrawing inductive effect is opposite to that of electron-donating conjugative effect of the chloro substituent. The interaction of the two effects reduces the influence of chloro substituent.
3
Conclusion
The effect of substituents in indoline moiety and polymer media on photochromism and thermal decay processes of spirooxazine (ASP) was studied. When the methyl in the 1-position of indoline moiety was replaced by octyl or by benzyl, the thermal decoloration rate was decreased with an in-
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crease in steric hindrance of substituents ; furthermore, replacing methyl with benzyl lowers the optical density of the colored forms at photoequilibration state. When the hydrogen in the 5-position of indoline moiety was substituted by electron-donating groups (-CH3) , the stability of the colored forms was improved, and the thermal decay lifetime was prolonged. When the hydrogen in the 5-position of indoline moiety was substituted by chloro, the stability of the colored forms was decreased, and the optical density of the colored forms was reduced ; however, its thermal decay rate was slower than that of ASPa in polymer medium with relatively strong polarity. In addition, the stability of the colored forms was related to the properties of polymer media. The thermal decay rate decreased with an increase in the polarity and rigidity of polymer media. The thermal decay kinetics of the clored forms obeys the biexponential decay law because of different isomers of the colored forms.
References , The structure characteristics of photoproducts of indolinospimxazines, Res . 21 : 17-24. Dorion, G. H . , Wiehe , A . F . , Photochromism : Optical and Photographic Application, London: Focal Press, 1970. Wu Guosheng, LIN Guangcan, Photochromic spiropyran, Kexue Tongbm , 1989, 34( 1) : 14.
Zhang, X . Y. , Jin, S. , Ming, Y . F. et al. Chem. Intenned .
, 1995,
Krongauz, V . , Applied Photochromic Polymer System, New York: Chapman and Hall Inc . , 1992, Chapt . 4 . Schneider , S . , Mind1 , A .
, Effinger,
G. et a1 . , Photochromism of spirooxazines I : Investigation of the primary process in the
ring-opening reaction by picosecond time-resolved absorption and emission spectroscopy, Ber . Bunsenges . Phys . Chem.
,
1987, 91:l 222. Schneider , S . , Baumann , F. , Kluter, U . et al . , Photochromism of spirooxazines 11: CARS-investigation of solvent effects in the isomeric distribution, Ber . Bunsenges . Phys . Chem . , 1987, 91 : 1 225. Hu , A . T . , Wang , W . H . , Lee, H . J . , Photochromism of spiroaxazine doped or bonded in polymer matrices, J M S-Pure Appl. Chem. , 1996, A33(6) : 803. Yitzchaik, S . , Ratner, J . , Krongauz, V . et a1 . , Photochromism of side-chain liquid crystal polymers containing spironaphthoxazines , Liquid Crystals , 1990, 8(5) : 677 . Crano, J. C . , Kwak, W. S . , Welch, C. N . , Applied Photochromic Polymer System, Chap. 2 , New York: Chapman and Hall, Inc . , 1992. Li Qun, Bi Dongying, Photochromic kinetics of azo group attached to a polymer main chain, Acta Photonics Sinica ( i n Chinese), 1997, 26(5) : 403. Schneider, S .
,
Investigation of the photochromic effect of spin, [indolino-naphthoxazine] derivatives by time-resolved spec-
troscopy. Zeitschn3 f i r physikalische Chemie , Neue Folge , 1987, 154: 91 .
