Mesostructured self-assembled silica films with reversible thermo-photochromic properties

Microporous and Mesoporous Materials 120 (2009) 375–380

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Mesostructured self-assembled silica films with reversible thermo-photochromic properties Luca Malfatti a, Stefano Costacurta a, Tongjit Kidchob a, Plinio Innocenzi a,*, Maria Casula b, Heinz Amenitsch c, Davide Dattilo d, Michele Maggini d a

Laboratorio di Scienza dei Materiali e Nanotecnologie, D.A.P., Università di Sassari, CR-INSTM, Palazzo del Pou Salid, Piazza Duomo 6, 07041 Alghero (Sassari), Italy Dipartimento di Scienze Chimiche and INSTM, Università di Cagliari, 09042 Cagliari, Italy c Institute of Biophysics and Nanosystems Structure Research, Austrian Academy of Sciences, Schmiedlstraße 6, A-8042 Graz, Austria d Dipartimento di Scienze Chimiche, Università di Padova, Via Marzolo 1, 35131 Padova, Italy b

a r t i c l e

i n f o

Article history: Received 1 October 2008 Received in revised form 1 December 2008 Accepted 2 December 2008 Available online 13 December 2008 Keywords: Self-assembly Mesostructured films Nanocomposites Spiropyrans Photochromism

a b s t r a c t A spiropyran molecule has been introduced via one-pot synthesis into mesostructured silica films, which have been obtained through a micelle templating self-assembling process. The mesostructure has been characterized by transmission electron microscopy and small-angle X-ray scattering performed with synchrotron light. The final material exhibited a p6mm 2d-hexagonal organization and the mesostructure has not been affected by the introduction of spiropyrans in the precursor solution. A comparative characterization of the optical properties of the spiropyrans dissolved in different solvents and after incorporation in the mesostructured films has been done by UV–Vis absorption spectroscopy, fluorescence spectroscopy and ellipsometric spectroscopy. The spiropyran-doped films appeared transparent before external stimuli had applied, a colour change to yellow (thermally induced) or to red (light induced) has been observed. Both the colour changes have been observed to be reversible under room temperature or visible light exposure; the colour transitions have been attributed to different equilibria among the various forms of merocyanine of spiropyrans. Ó 2008 Elsevier Inc. All rights reserved.

1. Introduction Spiropyrans are an important example of chemical compounds that may exist in two thermodynamically stable states and are capable of interconversion under the action of different external sources [1]. Reversible rearrangements of the molecules between two forms can be induced by light (photochromism [1]) or heat (thermochromism [2]) (Scheme 1). Molecular systems that act as on–off bistable systems have raised a high interest because of the different potential applications in molecular electronics, photonics, computing and chemical sensing [3]. Upon incorporation of spiropyrans into a solid state matrix, stimuli-responsive materials such as heat and pH-sensitive materials can be fabricated. Photochromic materials, in particular, require that either the nanostructure or the molecular structure respond to an external stimulus, in this case, light. Organic functional groups with stimuli-responsive photochromic units, therefore, have been widely applied to the fabrication of photoswitchable materials such as organic polymer thin films and organic–inorganic hybrid materials [4–6]. Recently the reversible photochemical switching of the relatively hydrophobic spiropyran into the more polar merocyanine * Corresponding author. Tel.: +39 07998630; fax: +39 0799720420. E-mail address: [email protected] (P. Innocenzi). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.12.004

form has been used to produce drug delivery systems, microfluidic actuators, or mass transporters [7–9] (see Scheme 1). Sol–gel materials, which are prepared through a soft-chemistry, are a popular matrix to incorporate optically active organic molecules such as spiropyrans [10]. Mesoporous-mesostructured materials, on the other hand, because of their peculiar porous topology [11] represent an ideal box for developing guest-host functional materials [12] and some examples of incorporation of spiropyrans have been also reported so far [13]. The high interest in porous materials containing spiropyrans relies in the possibility of fine tuning of the optical properties: antireflective [14], photochromic and thermochromic properties can be modulated through the control of material processing. The porous environment in mesostructured materials is, however, a quite complex system because of the restriction in the dimension, the pore surface coverage and the presence of surfactant or other guest molecules [15]. A careful engineering of the system is, therefore, required and detailed studies on the effect of the different parameters on the material performance have to be carried out. The optical response and the stability of the material performances are, in fact, strongly dependent on a set of different parameters, this is especially true in the case of mesostructured materials that contain organic templates. In the present work, we have fabricated silica mesostructured thin films containing spiropyran 1 (Scheme 2) through one-pot synthesis.

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NO2

UV, Δ N O R

NO 2

Vis, Δ

N

O

R Scheme 1. Structures of spiropyran (closed), left, and merocyanine, right. The isomerization can also occur thermally.

b

a N

N

N

O

NO2

2 OH

OH

1 Scheme 2. Synthesis of spiropyran 1. Conditions: (a) 2-iodoethanol, CH3CN, reflux, 1 day. (b) 2-Hydroxy-5-nitrobenzaldehyde, EtOH, reflux, 4 h.

The photophysical characterization has shown that the materials have a reversible photo and thermal response. 2. Experimental All reagents were purchased from Aldrich and used without further purification. Spiropyran 1 was prepared as previously reported [16]. Hybrid mesostructured silica films were prepared adding different amounts of 1 to a solution of tetraethoxysilane (TEOS), ethanol (EtOH), water, Pluronic F127 (EO106–PO70–EO106) and HCl. A precursor sol containing the silica source was prepared by adding in the following order: 3.08 cm3 of ethanol (EtOH), 4.26 cm3 of TEOS, 0.355 cm3 of HCl aqueous solution (0.077 M); this sol was stirred for 1 h at room temperature. Templating solutions were prepared dissolving 1.3 g of Pluronic F127 in a mixture of EtOH (10 cm3) and HCl (0.006 M) aqueous solution (1.5 cm3). For UV–Vis and fluorescence measurements on films, the final precursors sol was obtained by introducing 48 mg of 1 (1.43
10 4 mol) and stirring for 1 h; the solutions were protected from light during and after preparation. The final molar ratios of the sol was TEOS:EtOH:H2O:F127:spiropyran 1 = 1:40:20:5
10 3:7.5
10 3. For SAXS measurements, the final precursor’s sols were obtained by introducing increasing amounts of 1 and stirring for 1 h; also in this case, the solutions were protected from light during and after the preparation of the samples. The final molar ratios of the precursor sols were TEOS:EtOH:H2O:F127:Spiropyran 1 = 1:40:20:5
10 3:X with X ranging from 1.5
10 3 to 7.5
10 3. Silicon wafers (thickness 500 lm, test grade) or silica slides (UV grade) were used as the substrates that were dip-coated with a pulling rate of 5 cm
min 1 at the relative humidity (RH) of (27 ± 3)%. The substrates, previously cleaned with detergent solution, EtOH, and acetone, were dip-coated into the sol. The films, after deposition, were dried at 60 °C for 24 h. Fluorescence spectra were recorded by a FluoroMax-3 Horiba Jobin-Yvon spectrofluorometer. Measurements in solution were done using a fused-silica cuvette, 1 mg of 1 was dissolved in 40 ml of the solvent for the measure; ethanol, water and tetrahydrofuran (THF) were used as solvents. Emission spectra were collected between 550 and 800 nm, using an excitation wavelength of 365 nm with an integration time of 0.1 s. Measurements on solids were conducted on thin films using a particular orientation of the samples. The probing beam was set to impinge on one side of the sample (incidence angle of 2°–3°) so that the sample acted as a waveguide for the incident light wave, while the luminescence

was collected at 90° with respect to the incident beam. This configuration enhanced the signal-to-noise ratio and limited the reflection effects. Emissions were collected between 420 and 600 nm, using an excitation wavelength of 365 nm. Integration time and slit widths were optimized to maximize signal-to-noise ratio and to avoid saturation of the detector. Absorption spectra were measured in the 190–900 nm wavelength range using a UV–Vis Nicolet Evolution 300 spectrometer, at 500 nm
min 1 scan rate. Each acquisition is the average of three different scans collected with a bandwidth of 1.5 nm. Transmission electron microscopy (TEM) images were obtained on a JEOL 200CX microscope equipped with a tungsten cathode operating at 200 kV. Finely ground films scratched from the silicon substrate were dispersed in n-octane by sonication, then they were dropped on a carbon-coated copper grid and dried for TEM observations. The mesostructure of the films was investigated by two-dimensional grazing incidence small-angle X-ray scattering (GISAXS) at the Austrian SAXS beamline of ELETTRA synchrotron (Trieste, Italy) [17]. The incident energy was set to 8 keV (wavelength 1.54 Å). The instrumental glancing angle between the incident radiation and the sample was set slightly above the critical angle (grazing incidence). A two-dimensional CCD detector (Photonic Science, UK) was used to acquire the scattering patterns; each measurement consisted typically in the average of 10 acquisitions with integration time of 500 ms. Thickness and refractive index of films deposited on silicon substrates were measured by an Alpha-Spectroscopic Ellipsometry (aSE ) instrument (J.A. Woollam, USA). The data were modelled using a Cauchy film model for films not exposed to UV light and a splineabsorbing film model for exposed films. Self-assembled films were irradiated with a UV lamp to study the photochromism behaviour. The UV light source was a MODEL ENF-280C/FE, Spectroline lamp with a fluorescence tube at the wavelength of k = 365 nm. This generates a nominal power density of 470 mW
cm 2 at a distance of 15 cm. The samples were located around 5 cm under the lamp and irradiation times were ranged from 3 to 15 min. TM

3. Results and discussion Spiropyran 1 has been prepared by reacting commercially available 2,3,3-trimethylindolenine 2, 2-iodoethanol and 2-hydroxy-5nitrobenzaldehyde, as reported earlier [13] (Scheme 2). Previous

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b

a

40 nm

40 nm

Fig. 1. Representative TEM bright-field images of mesostructured self-assembled silica nanocomposite films (a and b); black dots indicate the 2d-hexagonal mesostructure (a), and black dot lines indicate the planes (b).

Sz / Å-1

0.012

01

0.008

_ 10

10

0.004

-0.008 -

-0.004 -

0

0.004

0.008

5x106 EtOH THF

4x106

Intensity

reports have shown that by the one-pot synthesis the spiropyrans preferentially are located within the hydrophobic core of the templating micelle [13a]. We have, therefore, at first investigated the mesostructure that is obtained via one-pot processing of thin films, to verify if the doping dye has affected the mesophase formation. We have used GISAXS and TEM analysis on 1-doped films to identify the mesostructures obtained by evaporation induced selfassembly (EISA). TEM picture in Fig. 1a shows the front view of an ordered array of mesopores having a 6-fold rotational axis typical of a hexagonal symmetry; this attribution is confirmed by the observation of stacks of well-aligned tubular micelles (Fig. 1b). These results are consistent with previous studies on self-assembly mesoporous silica films templated by Pluronic F127, where a mesophase with two-dimensional hexagonal p6mm symmetry was obtained [18]. From TEM pictures, we have evaluated the average pore diameter and pore walls thickness. The average centre-tocentre distance between two adjacent cylindrical pores is 6.2 ± 1.0 nm in the direction perpendicular to their axes; the pores diameter is 3.5 ± 1.0 nm and the inorganic pore wall thickness is 4.6 ± 1.0 nm. To evaluate local order of the mesophase as well as cell parameters, GISAXS measurements have been done on mesostructured films; Fig. 2 shows a typical GISAXS pattern that can be indexed as 2d-hexagonal p6mm. Following this attribution the cell param-

H 2O

3x106 2x106 1x106 0 550

600

650

700

750

800

Wavelength / nm Fig. 3. Fluorescence emission spectra of spiropyrans in ethanol (black line), tetrahydrofuran (red line) and water (green line). The spectra were obtained by excitation at 365 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

eter has been calculated as 6.1 ± 0.9 and 6.5 ± 0.9 nm in the inplane (x) and out-of-plane (z) directions, respectively. The presence of a distorted ring intersecting the spots shows the coexistence in the film of ordered domains together with some disordered or wormlike regions. It is important to stress that even if measurements have been done on samples prepared with increasing amounts of 1, the compositional variation did not affect neither the cell parameters nor the degree of order in the mesophase. The organic molecules, at least in the concentration range we have used, do not influence the formation of the interface between the micelles and the inorganic network. To study the photochemical properties of the molecules, we have prepared several solutions dissolving 1 in solvents of different polarity; Fig. 3 shows the photoluminescence emission spectra of 1 in the 550–800 nm range at an excitation wavelength of 365 nm. Equimolar amounts of 1 were added to water, THF and ethanol to obtain the emission spectra; 1 in water has the lowest intensity with an emission band peaking around 670 nm (green line),1 the emission band of 1 in THF (red line) increases in intensity with respect to 1 in water and shifts to lower wavelengths. The band of 1 in THF appears the sum of two contributions: one centred around

Sx / Å-1 Fig. 2. GISAXS pattern of the as-deposited spiropyran-doped mesostructured silica films. The diffraction peaks are labelled and indicated by a dot line circle.

1 For interpretation of color in Figs. 3–5, 7 and 8, the reader is referred to the web version of this article.

L. Malfatti et al. / Microporous and Mesoporous Materials 120 (2009) 375–380

6

6x10

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Wavelength / nm Fig. 5. Real (n, black line, left side scale) and imaginary part (k, red line, right side scale) of the refractive index of a spiropyran-doped mesostructured silica film as a function of the wavelength. The measurements were done on the as-deposited film (a) and on the film after 15 min of UV exposure at 365 nm (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

ple) films; it is important to note that both colour changes are reversible under room temperature or visible light exposure. Moreover, it was impossible to turn the colour of the film from transparent to yellow using UV light and from transparent to purple using thermal treatment. These different coloured states of the films can be assigned to different equilibria among the various forms of merocyanine. The colours of a spiropyran-based system are due to an equilibrium mixture of a number of possible stereoisomers between cis-merocyanine and more stable s-trans-merocyanine [10]. Despite many authors reported photochromism response of

b Intensity

Intensity

7x10

1.510

Extinction coefficient / k

a

a

Extinction coefficient / k

660 nm and the other around 640 nm. These data are consistent with previous studies reporting that neither spiro nor meroforms of the dye show an intense fluorescence in water, although the meroform has a higher fluorescence in polar organic solvents and within self-assembled films [19,20]. The maximum of fluorescence intensity is reached when 1 is dissolved in ethanol: the fluorescence spectra (Fig. 3, black line) show a strong emission band centred around 650 nm. This emission is attributed to the merocyanine form even if the interpretation is not straightforward because this form has several geometrical isomers. Each isomer has different fluorescence quantum yields and lifetimes that interconvert on the same time scale as fluorescence decay [18]. To confirm this hypothesis, we have done a comparative measurement of spiropyran in ethanol before and after exposure to UV light for a short time; the UV irradiation should in fact increase the concentration of the merocyanine form in solution. Fig. 4a shows the emission spectra in the 550–800 nm range of 1 in ethanol before (black line) and after irradiation (red line) with a UV lamp at 365 nm for 60 s. After irradiation, the fluorescence intensity increases confirming that the fluorescence response can be ascribed to the open form of spiropyrans. The same trend can be found in the emission of as-deposited mesostructured films (Fig. 4b). In this case, the incorporation of 1 in the mesostructured films slows down the kinetic of isomerization from the open to the close form; this is reflected in the difference between photoluminescence spectra of film before (black line) and after UV irradiation (red line). The same film was also analysed by spectroscopic ellipsometry to measure thickness and refractive index. Before irradiation, the film appears transparent and consequently the polarization changes of the incident wavelength measured by the spectroscopic ellipsometry can be fitted using a non-absorbing Cauchy model [21]. Fig. 5a shows the dispersion curves as a function of wavelength of imaginary, k, and real part, n, of the refractive index in as-deposited spiropyran-doped films. The real refractive index n at 632.8 nm was 1.489 while the extinction coefficient k remained constant to zero on the basis of the assumption of non-absorbing film. After irradiation, the film absorbs in the visible range and we have applied, therefore, a spline model to fit an absorbing film on a transparent substrate. The extinction coefficient, k, (Fig. 5b, right scale, red line) shows a broad absorption band centred at 530 nm, which is in agreement with the UV–Vis spectra measured on films after UV irradiation (vide infra). When 1 is embedded in a mesostructured matrix, the local environment affects the photo- and the thermochromic properties of the molecules. Fig. 6 shows representative pictures of as-deposited (transparent), thermally treated (yellow) and UV irradiated (pur-

Refractive index / n

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6.0x10

5

4.0x10

5

2.0x10

5

0.0 550

before irradiation after irradiation

600

650

700

750

800

Wavelength / nm

Fig. 4. (a) Photoluminescence emission spectra of spiropyrans dissolved in ethanol before (black line) and after irradiation (red line) at 365 nm for 60 s and (b) photoluminescence emission spectra of spiropyran-doped mesostructured silica films before (black line) and after irradiation (red line) at 365 nm for 300 s. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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a

As-deposited

c

b

UV exposure

Thermal treatment

Fig. 6. Pictures of spiropyran-doped mesostructured silica films as-deposited (a); after thermal treatment (b) and after illumination by UV light at 365 nm for 300 s (c).

a

0.150

spiropyran-functionalized materials [22–24] to the best of our knowledge this is the first time that nanocomposites self-assembled films show a different colour change depending on thermal or photo stimuli. During the experiments we observed a fast colour transition in the films after exposure to UV light; to follow the time-dependent colour transition from purple to transparent we have set-up a specific protocol of measurement. We have exposed the films to a UV lamp (365 nm) for 5 min; after this step we have recorded the UV absorption spectra every 15 min to obtain the intensity decay and the samples have been exposed to ambient light between each measurement. Fig. 7a shows the different absorption curves obtained at the various exposure times: 0 min, red line; 15 min, green line; 30 min, blue line; 60 min, light blue. The UV–Vis spectra show a complex feature with a band peaking around 520 nm, which is assigned to the ring-opened photoisomer [15]; this band decreases in intensity with the exposure time and after 60 min is very weakly detectable. The decolouration effect is caused by the closing of the ring, which simulates a nano-valve effect within the mesopores. The photochromic decay of the absorption intensity at 550 nm is reported in Fig. 7b (blue squares), the points are well fitted by a first order exponential decay curve. We compared these results to the colour change induced by thermal treatments at 100 °C for increasing times, Fig. 8 shows the UV–Vis absorption spectra of

b

0.075 0.070

0.100

Absorbance

Absorbance

0.125 0 min 15 min 30 min 60 min

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absorbance at 550 nm exponential decay fit

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300

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Time / min

Fig. 7. (a) UV–Vis absorption spectra of a spiropyran-doped mesostructured silica film after UV exposure at k = 365 nm for different times (0 min, red line; 15 min, green line; 30 min, blue line; 60 min, light blue line) and (b) variation of absorbance at 550 nm as a function of time exposure (blue squares, the blue line is a guide for eyes) and exponential decay fit of the direct photochromic behaviour (red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

0.15

b 0 min 30 min 60 min 90 min 150 min 210 min

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Absorbance at 550 nm Exponential growthfit 0.00

0.000 400

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Wavelength / nm

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60

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Time / min

Fig. 8. (a) UV–Vis absorption spectra of a spiropyran-doped mesostructured silica film as a function of time of thermal treatment at 100 °C and (b) variation of absorbance at 550 nm as a function of exposure time (blue squares, the blue line is a guide for eyes) and exponential growth fit of the direct photochromic behaviour (red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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the film as a function of the time of thermal treatment at 100 °C, from 0 to 210 min, also in this case the points can be fitted by a first order exponential growth curve. When the as-deposited films are thermally treated, the response of the material appears different with respect to the films exposed to UV light. Even in this case, the film exhibits a direct photochromism but the absorption maxima resulted shifted to shorter wavelength. This absorption, results, in turn, into a final yellowish colour. At the beginning, the samples show an absorption band at 410 nm that is gradually bleached as the system reaches the thermal equilibrium between open and closed forms of spiropyrans. At the end, a general absorption increase in the 500 nm range causes the final appearance of the film (Fig. 6b). 4. Conclusions Mesostructured silica films have been used as a host matrix to incorporate spiropyran molecules via one-pot synthesis. The mesostructure of the films has not been changed by the introduction of the molecules and the films appeared optically transparent immediately after the deposition. The material showed to be responsive to external stimuli that induced changes into the equilibria of the merocyanine forms. A colour transition from transparent to yellow or red has been induced by heat or light, respectively, both colour changes are reversible under room temperature or visible light exposure. The colour transition has shown to be specific of the source that has been used, it has been, in fact, impossible to turn the colour of the films from transparent to yellow using UV light and from transparent to purple using a thermal treatment. Acknowledgment This research was supported by the Italian Ministero dell’Università e della Ricerca (MiUR) through FIRB2003 (RBNE033KMA) Grant. References [1] V.I. Minkin, Chem. Rev. 104 (2004) 2751. [2] J.H. Day, Chem. Rev. 63 (1963) 65.

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