Switching properties of fluorescent photochromic poly(methyl methacrylate) with spironaphthoxazine and D-π-A type pyran-based fluorescent dye

Spectrochimica Acta Part A 86 (2012) 600–604

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Switching properties of fluorescent photochromic poly(methyl methacrylate) with spironaphthoxazine and D-␲-A type pyran-based fluorescent dye Eun-Mi Lee a , Seon-Young Gwon b , Young-A Son a,∗ , Sung-Hoon Kim b,c,∗∗ a b c

Department of Advanced Organic Materials and Textile System Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea Department of Advanced Organic Materials Science and Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea School of Chemistry Science & Technology, Zhanjiang Normal University, Zhanjiang 524048, PR China

a r t i c l e

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Article history: Received 5 October 2011 Received in revised form 31 October 2011 Accepted 9 November 2011 Keywords: Photochromic switching Fluorescent switching Photoinduced ionic conductivity Photoinduced viscosity switching Spironaphthoxazine Poly(methyl methacrylate) copolymer

a b s t r a c t Fluorescent photochromic poly(methyl methacrylate) (PMMA) with spironaphthoxazine (SPO) and D-␲-A type pyran-base fluorescent dye as a fluorophore was synthesized by typical free radical copolymerization. The poly(MMA-co-SPO-co-fluorophore) in both solution and solid film exhibited excellent photoregulated fluorescence switching behavior and reversible modulation of fluorescence intensity using alternating irradiation with UV and visible light. The poly(MMA-co-SPO-co-fluorophore) also showed viscosity and conductivity switching behaviors along with photoresponse. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Photochromic materials attract much attention, and they now constitute an active research area because of their tremendous importance in biological phenomena and in their potential application for the many new technologies such as data recording, storage, optical switching, displays, and non-linear optics [1–3]. Among various types of photochromic compounds, spironaphthoxazines (SPO) are well-known photochromic compounds that have been attracting much interest from the view-points of both fundamental elucidation of photochemical reactions and their potential applications to optical memories [4–9]. The photochromism of these molecules is due to the photocleavage of the spiro-bond under UV irradiation, creating a deeply colored ring-opened merocyanine form, which has a broad absorption band in the visible region and can be converted back to the closed-ring form by visible-light irradiation or heating [10]. D-␲-A type dyes owing instinct charge transfer properties have gained much attention due to their applications suitable as probes for the determination of solvent polarity, potential applications for

∗ Corresponding author. Tel.: +82 42 821 6620; fax: +82 42 823 3736. ∗∗ Corresponding author at: Department of Advanced Organic Materials Science and Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea. Tel.: +82 53 950 5641; fax: +82 53 950 6617. E-mail addresses: [email protected] (Y.-A. Son), [email protected] (S.-H. Kim). 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.11.023

colorimetric chemosensors [11,12] and the recognition and sensing toward harmful metal ions and so on [13–16]. Poly(methyl methacrylate) (PMMA) is an important polymeric material with outstanding water-clear color, stability of properties upon severe conditions, high surface resistivity, and resistance to weathering and moisture. Due to these superior characteristic, PMMA has been widely used as coating and polishing agents, transparent neutron stopper, optical fiber, high voltage application, and outdoor electrical application [17,18]. A number of attempts to incorporate spironaphthoxazine molecules into polymer systems have been reported [19,20]. In our previous papers, we have reported photoregulated optical and fluorescent switching in carbazole–spironaphthoxazine copolymer [21,22]. And also report has been photoregulated switching properties of poly(N-isopropylacrylamide) with spironaphthoxazine and D-␲-A type dye [23,24]. As an attempt to obtain more various functional polymers with potential photonics application, we have designed first, synthesized and characterized. This paper concerns the preparation, photochromism, and photoregulated fluorescence switching of PMMA with photochormic SPO and D-␲-A type pyran-based fluorescent dye as a fluorophore. The photoresponsive viscosity and photoinduced ionic conductivity response of the copolymer were also studied. 2. Experimental Mass spectra were recorded on HP 6890 and Agilent 5975C MSD spectrometer using electron energy of 70 eV and the direct probe

E.-M. Lee et al. / Spectrochimica Acta Part A 86 (2012) 600–604

2.1. Materials Methyl methacrylate (MMA) purchased from Aldrich was washed with an aqueous solution of NaHSO3 and water and dried with anhydrous CaCl3 , followed by distillation in nitrogen atmosphere under a reduced pressure. 2,2 -Azobis-(isobutyronitrile) (AIBN) was recrystallized from methanol. The other chemicals were of the highest grad available and were used without further purification. All employed solvents are analytically pure and were employed without any further drying or purification. The D␲-A type pyran-base fluorescent dye monomer 1 and SPO 2 were prepared by previously described procedures, respectively [25–27]. 2.2. Synthesis of poly(MMA-co-SPO-co-fluorophore) The switchable fluorescent photochromic polymer was prepared by typical free radical copolymerization. MMA monomer 3 (5.01 g, 50 mmol), fluorophore monomer 1 (0.37 g, 0.5 mmol), SPO monomer 2 (0.21 g, 0.5 mmol), and AIBN (0.08 g, 0.5 mmol) were dissolved in anhydrous THF (40 ml) under dry nitrogen. After heating for 3 days at 70 ◦ C, the resultant mixture was precipitated from ether. The resulting polymer was refined by Soxhlet extraction and dried in vacuum to give satisfactory yield as an orange-colored powder. Yield: 45%, gel permeation chromatography (GPC): Mn : 15,500; Mw : 19,200; Mw /Mn : 1.24. 3. Results and discussion The synthetic route of the poly(MMA-co-SPO-co-fluorophore) is depicted in Scheme 1. We took the following point into consideration in our design and synthetic strategy: incorporation of fluorophore and photochromic SPO units in the polymer system could lead to fluorescent and photochromic switching units. We have designed a new D-␲-A charge transfer dye, the fluorophore monomer 1, containing a polymerizable functional allyl group. Reaction of a fluorophore 1, SPO 2, and MMA 3 in anhydrous THF affords an orange-colored powder of poly(MMA-co-SPO-cofluorophore). As shown in Scheme 1, the synthetic route was very simple. The absorption spectra change of poly(MMA-co-SPO-cofluorophore) in DMF solution upon UV and visible light irradiation

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EI method. Melting points were determined on an Electrothermal IA 9000 series melting point apparatus and are uncorrected. 1 H NMR spectra were obtained using a Varian Inova 400 MHz FTNMR employing TMS as an internal standard. The weight–average molecular weight (Mn ) and polydispersity (Mw /Mn ) of the polymer was measured on an Alliance e2695 chromatograph at 25 ◦ C using tetrahydrofuran (THF) as the eluent and standard polystyrene as the reference. Fluorescence spectra were measured on a Shimadzu RF5301PC fluorescence spectrophotometer; The UV–vis spectra were obtained on an Agilent 8457 UV-vis spectrophotometer. The polymer film is spin-cast from N,N -dimethyl formamide (DMF) solution (0.2 g/ml) approximately 10–25 nm thick for UV–vis and fluorescence measurements. For the measurement of photoinduced ionic conductivity, the device comprised with two ITO glasses (2 cm × 2 cm) separated by a 0.1 mm thickness spacer of photoinduced electron transfer (PET) film was prepared and placed so as to face each other on the inside of the cell, and the edges of the cell were sealed with insulating epoxide resin. Prior to the final sealing, the space between the electrodes was filled with poly(MMA-coSPO-co-fluorophore) in DMF solution (0.001 g/ml) and 1 mmol of tetra-n-butylammonium perchlorate, [CH3 (CH2 )3 ]4 NClO4 , as electrolyte. A high-pressure mercury lamp (Ushio, SP3-250D) was used as the UV radiation source and calibrated with a monochromator at 366 nm.

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Wavelength (nm) Fig. 1. UV–vis absorption spectral change of (a) poly(MMA-co-SPO-co-fluorophore) in DMF solution (0.01 g/ml) upon irradiation with UV and visible light. The inset figure shows the cyclic absorbance changes at 605 nm of poly(MMA-co-SPO-cofluorophore) in DMF solution by alternative irradiation with UV and visible light, (b) poly(MMA-co-SPO-co-fluorophore) in film upon irradiation with UV and visible light. The inset figure shows the cyclic absorbance changes by alternative irradiation with UV and visible light at 605 nm of poly(MMA-co-SPO-co-fluorophore) in film.

is shown in Fig. 1(a). By UV irradiation, the visible range absorbance intensity increased gradually, corresponding to the generation of the opened merocyanine form from the closed spiro form in SPO units of poly(MMA-co-SPO-co-fluorophore). When the sample of poly(MMA-co-SPO-co-fluorophore) was left in the dark at room temperature or irradiated visible light after UV irradiation, the absorbance was decreased again. The generated opened merocyanine form was thermally unstable and underwent thermal bleaching to the closed spiro form. The inset figure shows the cyclic absorbance changes at 605 nm of poly(MMA-co-SPO-cofluorophore) in DMF solution by consecutive irradiation with UV and visible light. This cyclic absorbance changes could be repeated many times. The poly(MMA-co-SPO-co-fluorophore) was soluble in DMF and can be cast into transparent, uniform thin films from solutions by spin-coating onto the glass substrate. Fig. 1(b) shows the absorption spectra changes of poly(MMA-co-SPO-co-fluorophore) in film. The absorbance of film was investigated immediately after 20 s irradiation with UV light. The absorption band appeared around 605 nm, which demonstrated that the polymer film exhibited photochromic properties. The decoloration process of film was similar to that of DMF solution. The cyclic absorbance changes at 605 nm of poly(MMA-co-SPO-co-fluorophore) in film by alternative irradiation with UV and visible light are shown in Fig. 1(b) inset figure.

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Scheme 1. Synthesis of poly(MMA-co-SPO-co-fluorophore).

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fluorescence intensity reversibly changed at 575 nm (excitation: 430 nm) by alternate irradiation with UV and visible light, and this cycle could be repeated many times, which is an excellent photochromic fluorescence switch. The fluorescence quenching in the poly(MMA-co-SPO-cofluorophore) is attributed to the energy transfer from the excited fluorophore dye unit to the opended merocyanine form, because the specral overlap in the range of 500–700 nm between the enhanced absorption band of the opened merocyanine form and

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This cycle could be repeated many times, which is excellent photochromic switch. The decoloration plots of poly(MMA-co-SPO-co-fluorophore) in DMF solution and film are described in Fig. 2. The decoloration rate can be estimated from the A0 − A/A0 where Ao is the absorbance at max right after UV irradiation and A is the absorbance at any time after UV irradiation. As shown in Fig. 2, the thermal decoloration of poly(MMA-co-SPO-co-fluorophore) film was markedly slower than that of in DMF solution, which indicate that steric effects play an essential role in film [2,3]. Fig. 3(a) shows fluorescence intensity changes of poly(MMAco-SPO-co-fluorophore) in DMF solution upon irradiation with UV and visible light at 0 ◦ C (excitation: 450 nm). The fluorescence intensity change was regulated by the photochromic reaction. The fluorescence intensity of poly(MMA-co-SPO-co-fluorophore) was decreased with changing from the closed spiro form to the opened merocyanine form as a results of irradiation with UV light, and the original emission spectra were recovered by irradiation with visible light. The inset figure shows fluorescence switching of poly(MMAco-SPO-co-fluorophore) in DMF solution. Fluorescence intensity at 590 nm of poly(MMA-co-SPO-co-fluorophore) in DMF solution under 450 nm excitation is switched on and off upon continuous UV and visible irradiation. The fluorescence emission spectral change of poly(MMA-coSPO-co-fluorophore) in film with excitation wavelength at 430 nm is shown in Fig. 3(b). The fluorescence intensity change of poly(MMA-co-SPO-co-fluorophore) in film was also regulated by the photochromic reaction. Upon irradiation with UV light, the fluorescent intensity of poly(MMA-co-SPO-co-fluorophore) gradually decreased and almost quenched at room temperature. After irradiation with visible light, the original emission spectra were regenerated. From what can be seen in Fig. 3(b) inset figure, the

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Time (sec) Fig. 2. The decoloration plot of poly(MMA-co-SPO-co-fluorophore) in DMF solution (0.01 g/ml) and film. (䊉), poly(MMA-co-SPO-co-fluorophore) in DMF solution (0.01 g/ml); (), film.

Fig. 3. Fluorescence emission spectral changes of (a) poly(MMA-co-SPO-cofluorophore) in DMF solution (0.01 g/ml) upon irradiation with UV and visible light at 0 ◦ C. The inset figure shows fluorescence switching at 590 nm by alternating irradiation with UV and visible light (excited at 450 nm), (b) poly(MMA-co-SPO-cofluorophore) in film upon irradiation with UV and visible light at room temperature. The inset figure shows fluorescence switching at 575 nm by alternating irradiation with UV and visible light (excited at 430 nm).

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Scheme 2. Fluorescence modulation of poly(MMA-co-SPO-co-fluorophore) by photochromic swtiching.

the polymer pendant group or backbone group. The viscosity during UV irradiation returns to the initial value in 5 min at −5 ◦ C after the UV light is removed. The recovery cycles of the viscosity could be repeated many times. The photoinduced conductivity response was investigated at room temperature. The photoinduced conductivity can be estimated from the (1/Rt )/(1/R0 ) where Ro and Rt are the resistance before and after UV irradiation. Fig. 5(b) exhibits the photoinduced conductivity response of poly(MMA-co-SPO-cofluorophore). Upon UV irradiation the conductivity of poly(MMAco-SPO-co-fluorophore) increased, which led to the generation of zwitterions form, opended merocyanine, interaction with fluorophore dye and methyl methacrylate, and subsequently decreased with visible light, which led to the generation of closed spiro form (Scheme 3).

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the emission band of fluorophore dye unit. In other words, the opened merocyanine absorbs the emitted light of the fluorophore dye unit due to the energy transfer so that the fluorescence of the fluorophore dye unit can be quenched by the merocyanine. A realizable mechanism for energy transfer and fluorescence switching is shown in Scheme 2. Fig. 4 indicates the fluorescence regeneration plot of poly(MMAco-SPO-co-fluorophore) in DMF solution and film. The fluorescence regeneration rate can be estimated from the F0 − F/F0 where Fo is the fluorescent intensity at max right after UV irradiation and F is the fluorescent intensity at max at any time after UV irradiation. As shown in Fig. 4, the thermal regeneration of poly(MMA-co-SPOco-fluorophore) film was markedly slower than that of in solution, which indicate that steric effects play an essential role in film. The tendency of this fluorescence regeneration rate is also similar to the decoloration plots of poly(MMA-co-SPO-co-fluorophore) in DMF solution and film. We have reported the viscosity change by UV irradiation of carbazole–spironaphthoxazine copolymer [21,22]. A viscosity change by UV irradiation of spiropyran system of poly(methyl methacrylate) with spiropyran pendant groups was studied for the first time by Irie et al. [28]. Fig. 5(a) shows the viscosity changes of poly(MMA-co-SPO-co-fluorophore) in DMF solution before and after UV irradiation. The relative viscosity of poly(MMA-co-SPO-cofluorophore) in DMF solution after UV irradiation is 10% lower than that of before UV irradiation. The decrease in viscosity implies that light energy is utilized in changing the conformation of the polymer chain [28]. In other words, a change in dipole moment caused by isomerization from closed spiro form to the opened merocyanine form would be expected to alter intramolecular interaction of polymer chain when the spironaphoxazines are incorporated into

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Scheme 3. Photoswitchable process of poly(MMA-co-SPO-co-fluorophore) by alternative irradiation with UV and visible light.

4. Conclusions In conclusion, fluorescent photochromic poly(merhyl methacrylate) with spironaphthoxazine and D-␲-A type pyranbase fluorescent dye was synthesized, and this copolymer in both solution and film exhibited photoregulated fluorescence switching behavior and reversible modulation of fluorescence intensity by photochromic switching between spiro and merocyanine using alternating irradiation with UV and visible light. The concept presented in this paper, which controls the photochromic and fluorescence switching by UV and visible light, may contribute to the development of photoregulated fluorescent molecular switching. Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation (NRF) grant funded by the Korea Government (MEST) (No. 2011-0001084). This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. References [1] S. Arakami, G.H. Atkinson, J. Am. Chem. Soc. 114 (1992) 438–444. [2] S.H. Kim, C.H. Ahn, S.R. Keum, K. Koh, Dyes Pigments 65 (2005) 179–182. [3] H. Durr, H. Bouas-Laurent, Photochromism: Molecules and Systems, Elsevier, Amsterdam, Netherlands, 1990.

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