J. MATER. CHEM., 1994, 4(12), 1907-1912 Photoisomerization of lndolinespirobenzopyrant in Anionic Clay Matrices of Layered Double Hydroxides Hideyuki Tagaya,*" Shigemitsu Sato,a Tsuneo Kuwahara,b Jun-ichi Kadokawa: Karasu Masa" and Koji Chiba" " Department of Materials Science and Engineering, Yamagata University, Yonezawa, Yarnagata 992, Japan R&D Center, TDK Corp., Saku, Nagano 389-02 Japan Sulfonated indolinespirobenzopyran (SP-SO3 -) was intercalated into the Mg/AI or Zn/Al layered double hydroxides (LDHs) in the presence of toluene-p-sulfonic acid (PTS). The new intercalates exhibited reversible photoisomerization between SP-S03- and merocyanine (MC). The presence of PTS was critical for the reversible photoisomerization. MC was very stable and the rate constant of decolorization was <9 x s-'.SP interacted with non-polar portions between the layers although MC interacted with the polar interior surface of the LDH. There has been considerable interest in the study of photo- chromic molecules because they are candidates for useful photoresponsive materials.lP3 Spiropyrans (SP) form merocy- anines (MC) reversibly on irradiation and they constitute an important class of photochromic compound^.^ In a non-polar solvent like toluene, SP is stable. When SP is exposed to UV light, SP undergoes ring opening to MC as shown in Scheme l.5,6In a non-polar solvent, MC is fairly unstable and isomerizes to SP immediately. In a polar solvent, the MC form is stable. The thermal reversion of photoinduced MC to the starting SP is influenced by the surrounding Many matrices such as polymer filrn~,~,~~ monolayers,11*12 liquid crystals,' smectite clay^'^-'^ biomolecules'7 and micelles'* are the subjects of intensive research. Many layered solids act as host lattices and react with a variety of guest molecules to give intercalation compounds in which the guest is inserted between the host layer^.'^-'^ When photochromic flugides were supported on a smectite clay in the dark the normal photochromic product was formed thermally.25 Pyridinium SP (Py+-SP) also intercalates between the inter- layers of the clay.26 However, in all of these reports, control of the photochromic reaction was not sufficient.Recently we have achieved co-intercalation of sulfonated SP (SP-S03-) and toluene-p-sulfonic acid (PTS) in the interlayers of an Mg/A1 layered double hydroxide (Mg/Al LDH).27 The intercalates exhibited reversible photoisomerization between IUPAC-recommended name: 1',3',3'-trimethyl-6-nitrospiro[2H-chromene-2,2'-indoline]. SP *.vls~~ uv SPS03-Scheme 2 Structure of SP-S03-SP-S03-and MC with a high stability of the MC forms.The LDHs consist of positively charged metal oxide/hydroxide sheets with intercalated anions, as shown in Fig. 1.28 Their general composition may be represented as [M"1-xM111x(OH)2]X+[A,,,"-rnH,O]"-, where A"- is an exchangeable anion. Fig. 1 shows the typical Mg/AI LDH which intercalates carbonate ions. In the case of the Mg/Al LDH, a variety of inorganic and organic anions have been exchanged into the LDH when x =0.26-0.65.29,30We now report the results of a detailed study of the photochomic properties of SP-SO,-in LDHs.Furthermore, we suggest mechanisms for the high stability of MC forms. Experimental Organic solvents were of analytical grade. If necessary, they were dried and fractionated prior to use. SP, SP-S03-and T c0:-c0:-c0:-c0:-7.8A 1 &$!.0-CH3 0 Oxygen MC Mg or Al Scheme 1 Photoisomerization of SP Fig. 1 Structural model of Mg/A1 LDH all of other materials used were from commercial suppliers and used as received. Preparation of LDHs Mg/AI LDH and the Zn/Al LDH were prepared by the reaction of a mixture of Mg(NO,), or Zn(N0,)2 with Al( .28.29 The reactions were carried out at room tem- perature for 2 h in Na,CO, solution, the pH of which was adjusted to ca.10 with NaOH. The Mg :A1 and Zn :A1 ratios were 0.30 and 0.25, respectively. The products were dried at 80'C for 48 h to give the LDH carbonates as white powders. The LDH carbonates were calcined at 500°C for 3 h to give the Mg/Al or Zn/Al oxides. LDH films were formed by drawing a glass slide out of a 1% suspension of the LDH. The formation of the LDH carbonates and the desorption of carbon dioxide by calcination were confirmed by X-ray diffraction measurements. Intercalation In the intercalation, 50 ml of a PTS (0.25 mmol), SP-SO,- (0.11 mmol) aqueous suspension and 0.1 g of the powdered calcined LDH were placed in a 100ml conical flask and stirred at 60 "C for 1 h under a nitrogen atmosphere.Reaction products were filtered and washed with acetone. For the LDH film, the film on the glass was soaked in an aqueous solution containing the guest molecules (SP-SO,- : 0.005-0.04 mmol per 50 ml, PTS: 0.3-1.2 mmol per 50 ml). After the reaction the film was washed with distilled water. Characterization of the Intercalates Powder X-ray diffraction spectra were recorded on a Rigaku powder diffractometer unit using Cu-Ka (filtered) radiation at 40 kV and 20 mA. Thermal analyses [thermogravimetry/ differential thermal analysis (TG/DTA)] were performed on a Seiko SSC5000 thermal analysis system using a heating rate of 10°C min-'. IR measurements were performed using a Horiba FTIR spectrometer. Photochromic Properties of Intercalates UV irradiation was carried out using a 400 W high-pressure mercury lamp.Visible light irradiation was carried out using a 500 W xenon lamp. Cut-off filters were used, if necessary. The power densities of the radiation were not measured. Absorption and fluorescence spectra were recorded using a Shimadzu UV-2200A spectrophotometer and a Hitachi spec- trofluorophotometer, respectively. Results and Discussion Intercalationof SP-S03-An XRD pattern of the Mg :A1 (0.70:0.30) LDH carbonate is shown in Fig. 2(a): The layer distance of the Mg/Al LDH carbonate is ca. 7.8 A. Ion exchange of carbonate ions with organic anions in the LDH is not easy. Therefore, deintercal- ation of carbonate ions was carried out by thermal treatment at 500 "C.The XRD pattern of calcined Mg/A1 LDH is shown in Fig. 2(h). There are no clear peaks because the calcined LDH was amorphous. Thermal analyses indicated that weight losses at 200-500 'C corresponded to evolution of water and carbon dioxide [Fig. 3(b)].28No large weight loss was observed for the calcined LDH [Fig. 3(u)J, confirming that the carbonate ion had been deintercalated. It is well known that smectite clays adsorb coloured organic J. MATER. CHEM.. 1994, VOL. 4 )25.5A IIu 5 10 15 20 25 2Bldegrees Fig. 2 XRD patterns of (a) the Mg/Al LDH, (b)the calcined Mg/A1 LDH, (c) the PTS intercalate and (d)the PTS/SP-SO, -co-intercalate r I J 100 200 300 400 500 600 T1°C Fig.3 Thermal analysis of (a)the calcined Mg/Al LDH, (h)the Mg/A1 LDH and (c) the PTS/SP-S03 co-intercalate~ compounds. SP in ethanol reacted with the calcined Mg/Al LDH; however, layer expansion was not observed by XRD measurements. Sulfonated SP was allowed to react with the calcined Mg/AI LDH in an aqueous !elution at 60 'C for 1 h. The layer distance expanded to 7.8 A. When the amount of intercal$ed anion was small, the interlayer spacing expanded to 7.8 A which indicates that the planes of the guests are parallel to the plane of the host layers.29 We have already confirmed that in such cases the interlayer spacing was independent of the size of guests.31 The size of the expansion indicated that the amount of intercalated SP-SO,- was small.The colour of the intercalate was red. The MC form was fairly stable and no isomerization to SP occurred even when the intercalate was irradiated by visible light. J. MATER. CHEM., 1994, VOL. 4 Co-intercalation of PTS and SP-SO,- As mentioned above, an intercalate containing the stable MC form was obtained. The LDH surface is covered by hydroxy groups and it has a high polarity.32 For smectite clay, there are two different kinds of microscopic environment, e.g. non-polar and polar regions.26 As already described, SP is stable in non-polar solvents and MC is stable in polar solvents. Therefore, we considered that to attain reversible photochro- mism, the creation of non-polar regions between LDH layers was important.We tried to co-intercalate PTS and SP-SO,-. The methylphenyl portion of PTS was expected to provide a non-polar region. PTS alone reacted with the calcined Mg/A1 LDH. In the intercalation in $n alkaline solution, the interlayer distance expanded to 7.8 A, indicating that the amount of intercalated PTS was small. In the int5rcalation in a weak acid solution the layer expande: to 17.7 A, as shown in Fig. 2(c). The length of PTS is ca. 8.5 A. Thc thickness of the brucite layer of t!e Mg/A1 LDH was 4.77A.33 The layer expansion was 12.9A, significantly larger than the length of the PTS anion. These indicated that the plane of PTS in the LDH layers was perpendicular to the plane of the host layers, as proposed by Dre~dzon.~~ In the reaction of the mixture of PTS and SP-SO,-with the calcined Mg/Al LDH, a yellow compound was obtained.The layer dist$nce of the Mg/A1 LDH expanded to between 7.8 and 25.5 A depending on the reaction conditions and whether the Mg/Al LDH was a powder or a film. Thermal analysis of the PTS/SP-S03- intercalate showed a larger weight loss than that of the Mg/Al LDH carbonate [Fig. 3(c)]. We have already reported that an organic anion in LDH interacts moderately with the positive charge of the layers2* However, the interaction was stronger than that of an anion with a proton and weaker than that of an anion with a sodium cation. Therefore, the evolution temperature of an organic anion in the Mg/AI LDH layer was higher than that of an acid, and lower than that of sodium salts.Weight losses continued above 600 "C, indicating the strong inter- actions between the Mg/A1 LDH and the PTS and SP-S03-guests. PTS is a strong acid. The interlayer spacing of the PTS intercalates was larger than with the SP-S03 -intercalate. The IR spectra of the PTS intercalate and PTS/SP-S03- co-intercalate were similar [Fig. 4(b) and (41,indicating that the amount of intercalated PTS was larger than that of SP-so3 -. Photochromic Properties of Intercalation Compounds The fluorescence spectrum of powdered PTS/SP-SO,-co-intercalate in the Mg/A1 LDH is shown in Fig. 5. In a non-polar solvent, toluene, the fluorescence maximum of the SP was 527 nm. On the other hand in a polar solvent, ethanol, the solution was red, indicating that the MC form was stable in ethanol.The fluorescence maximum of the MC form was 620nm. The fluorescence maximum of the yellow powdered co-intercalates was 574 nm as shown in Fig. 5(b). This maxi- mum was between that of SP (527 nm) and MC (620 nm). When the sample was irradiated with UV the co-intercalate changed to red. At the same time the fluorescence maximum shifted to 610 nm as shown in Fig. 5(4. The colour of the intercalate changed from red to yellow again on irradiation with visible light. At the same time the fluorescence maximum returned to 574nm. To confirm the reversibility between the SP and MC forms, repeated alternate irradiation with UV and visible light was carried out. As shown in Fig.6, the fluorescence intensity at 610 nm increased 3000 2000 1000 wavenum berkm-' Fig. 4 IR spectra of (a) the calcined Mg/A1 LDH, (b) the PTS intercalate, (cj the SP-SO,-intercalate and (dj the PTS/SP-S03-co-intercalate 500 600 700 wavelengthhm Fig. 5 Fluorescence spectra of (a) SP in toluene, (b)SP in ethanol, (c) the PTS/SP-S03-co-intercalate as reacted and (dj the PTS/S P-SO,-co-intercalate after UV irradiation on UV irradiation and decreased on visible light irradiation during each cycle. The same results were observed with the powdered Zn/Al LDH. The same photoisomerization was also observed on photoirradiation of the Mg/A1 LDH film. Fig. 7 shows the absorbance of the Mg/A1 LDH film with PTS/SP-SO, -co-intercalate.The absorption maximum was at 535 nm, i.e. close to the maximum of MC, 526nm, in methanol. There are, so-called, H and J aggregates for SP the absorption maxima of which are near 480 and 615 nm, re~pective1y.l~ The absorption maximum of the PTS/SP-S03-co-intercalates after UV irradiation was J. MATER. CHEM., 1994, VOL. 4 U 3 I Fig. 6 Reversible photochromism of the PTS/SP-SO,-co-intercalate in (a) the Mg/Al LDH and (b)the Zn/Al LDH \ (a., 300 400 500 600 700 wavelengthhm Fig. 7 Absorption spectra of (a) SP in toluene, (b)SP in toluene after UV irradiation, (c) SP-SO,-in methanol and (d) the PTS/SP-SO,- co-intercalate after UV irradiation different from those of the H and J aggregates. It was therefore concluded that the MC in the LDH layers did not aggregate.The absorbance at 535nm decreased with time after UV irradiation (Fig. 8). The half-life of photoinduced MC in toluene solution is only a few seconds at room temperature. To compare with solution, SP was trapped in a polyolefin polymer matrix. The half-life of the photoinduced MC form in the polyolefin was 3 min at 25 "C. Compared to these short lifetimes, the photoinduced MC in the Mg/A1 LDH was stable. When PTS (0.6mmol per 50ml) and SP-SO,-(0.01mmol) had reacted for 2 h, the amount of SP-SO,-intercalated into the Mg/A1 LDH was 3.2%. The half-life of thermal decolorization for the photoinduced MC in the Mg/A1 LDH was >200 h. This indicated that the rate constant of decoloration was t9x lo-' s-'.We believe that this rate constant is the smallest of those reported for non-H and -J aggregates. To make clear the relationship between reaction conditions and the stability of the photoinduced MC, we prepared PTS/SP-SO, -co-intercalates under various system- atic reaction conditions. As shown in Table 1, the amounts of intercalated SP-SO,- were almost the same (3.2-3.5%) at reaction times >0.5 h. However, the stability of the photoind- uced MC (half-life) increased with the reaction time (nos. 2-5). This suggests that rearrangement of the intercalated SP-SO, -to a stable conformation might proceed with longer reaction i400 500 600 700 waveleng t hlnm Fig. 8 Typical decoloration of the PTS/SP-SO, -intercalated Mg/AI LDH film after UV irradiation.Absorption spectra at (u) r=O and (b)t=m. times. The amount of SP-S03-intercalated increased from 3.5 to 7.0% (nos. 6-8) with increasing concentration of SP-SO,-in the reactant. The absorbance after UV irradiation increased with higher concentrations of SP -SO3-in the reactant; however, the stability of the SP-S03 form did not change. A high concentration of PTS in the reactants sup- pressed the amount of intercalated SP-SO,- (nos. 9, 10).The half-life of the photoinduced MC decreased slightly at higher concentrations of PTS. Mechanism of High Reversibility and High Stability of Photoinduced MC As already mentioned, the LDH surface has a high polarity. Therefore, the intercalate containing SP-SO, gave only the MC form and did not isomerize to the SP form thermally or photochemically.The colour of the PTS/SP-SO, -co-intercalate was yellow. This indicated that within the LDH layers, SP-S03- does not interact strongly with the LDH surface and might exist near a non-polar region such as the methylphenyl portion of PTS, as shown in Fig. 9. UV irradiation of PTS/SP-SO,- co-intercalates produced ring opening to the high-polarity MC form. This MC form might interact with the LDH surface and remain near the LDH surface. Interaction of the MC with the LDH surface was moderately strong; therefore, a high stability of the photoin- duced MC form was attained. To help confirm the mechanism, a PTS intercalate without SP-S03-was soaked with a toluene solution containing SP.The colour of the Mg/Al LDH surface was u hite, indicating that SP did not interact with the Mg/Al LDH surface. When this was UV-irradiated, the Mg/Al LDH surface changed to pink. This indicated that the SP in toluene underwent ring opening and the resulting high-polarity MC interacted with the Mg/A1 LDH surface. When this was irradiated with visible light the pink colour disappeared. Repeated cycling of the colour by UV and visible light irradiation was attained. These results suggest the importance of both the non-polar portions, toluene, and the polar portion, the LDH surface. When the toluene was removed by evaporation, the LDH surface changed to pink, and even when the surface was irradiated with visible light, the pink colour did not disappear because of the absence of non-polar regions.Conclusions New intercalates of SP-S03-and PTS into Mg/Al or Zn/Al LDHs exhibited reversible photoisomerization between J. MATER. CHEM., 1994, VOL. 4 191 1 Table 1 Photochromic properties of PTS/SP-S03 -intercalates reactant (mmol per 50 ml) reaction absorbance (arb. units) thermal weight of time/ decolouration, intercalated no. SP-SO3-PTS h t=O t=22 h t=86 h t,:,/h SP (Yo) 1 0.005 0.3 1 -----2 0.0 1 0.6 0.25 0.36 0.08 0.07 10 2.3 3 0.01 0.6 0.5 0.51 0.33 0.25 80 3.4 4 0.01 0.6 1 0.45 0.35 0.30 >200 3.5 5 0.01 0.6 2 0.33 0.30 0.27 >200 3.2 6 0.01 0.6 2 0.29 0.26 0.23 >200 3.5 7 0.02 0.6 2 0.63 0.54 0.50 >200 5.5 8 0.04 0.6 2 0.77 0.64 0.6 1 >200 7.0 9 0.01 1.2 2 0.14 0.12 0.10 >200 1.3 10 0.02 1.2 1 0.43 0.37 0.3 1 >200 3.8 SO3-SO3-SOB-SO3-SO3-I I I I IQ Q Q Q CH3 CH3 CH3 CH3 SO, SO,-SO, /////////////////////////// Fig.9 Possible mechanism for the reversibl le photoisomerization and high stability of MC SP-SO,-and MC. The high stability of the MC indicated the presence of a very suitable environment for MC in the layered solids in which MC interacted with the polar interior surface of the LDH. 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