Photochromic mesostructured silica pigments dispersed in latex films

Abstract

We have incorporated the photochromic dyes spiropyran and spirooxazine in surfactant templated mesostructured silica particles. Spherical inorganic–organic hybrid pigments with varying dye content were produced by a one-pot synthesis in an aerosol reactor where the internal mesostructure could be controlled. The mesostructured inorganic–organic hybrid pigments provide a mechanically and chemically rigid framework that protects the dyes and facilitate handling. We show that an organic latex binder can be used to prepare transparent photochromic films of varying thickness. Changing the dye loading in the pigments and the pigment content in the films provides a versatile route for tuning the photochromic response. The pigmented films show both fast and direct photochromism, where the decay time for thermal bleaching is very fast in the case of spirooxazine doped pigments (k_SO = 0.094 s⁻¹), being in the range of the best reported values for solid state composites.

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Introduction

Materials displaying photochromic properties are of interest for applications like protective lenses, optical switches, and optical memories.^1,2 In order to take advantage of the photochromic effect, which refers to the light-induced reversible transformation between two isomers having different absorption spectra, the active molecules must be dissolved in a medium, preferably a solid matrix, since the photochromic compound in its crystalline form usually is inactive.¹ The continuous medium, being a solvent or a solid, can have a significant effect on the absorption band shifts and the kinetics of the transformation.^3,4 Photochromic molecules have been doped into a wide range of solid matrices including polymers like PMMA,^5–7 inorganic materials like sol–gel derived silicates^8,9 and aluminosilicates,¹⁰ and also in hybrid inorganic–organic composites like ormosils and surfactant templated mesostructured materials.^3,11,12 These studies have shown that the response time of the photochromic effect is usually substantially slower for inorganic host materials compared to organic solid matrices like PMMA. In fact, the response time for photochromic switching for organic matrices like PMMA was found to be similar to solutions of the photochromic molecules. Well ordered mesostructured thin films where the inorganic and organic domains were separated on a nanometre scale showed response times for direct photochromism that were similar to the organic matrices.^11,12

Particular attention has been focused on the photochromic families of spiropyran and spirooxazine due to their potential photonic applications.^1,2,7 The closed forms of the spiropyrans and spirooxazines are colourless and undergo photoisomerization to the merocyanine forms by the cleavage of a C–O bond under UV irradiation. The molecular structures of spiropyran and spirooxazine are depicted in Fig. 1. There are two different open forms of the merocyanines, illustrated as the zwitterionic form for spiropyran and as the quinoidal form for the spirooxazine dye. The dominating form depends on the interaction with the environment, and it should be noted that both forms are in principle available for both photochromic molecules.

Fig. 1 The structure of the photochromic dyes spiropyran (SP) and spirooxazine (SO) in both the closed and open forms.

Previous work on photochromic molecules doped into inorganic–organic hybrid materials only involved thin films.¹² However, many applications require the formation of thicker films or bulk samples with the photochromic qualities retained. We will present a novel approach involving the synthesis of photochromic pigments that can be added to various matrices using traditional coating or paint-based technology. Spiropyran and spirooxazine have been introduced into surfactant templated mesostructured particles using a simple one-pot spray synthesis technique, hereinafter these dye-loaded materials are referred to as photochromic pigments.¹³ The photochromic pigments were introduced into a water-based latex dispersion and films with various pigment concentrations and film thicknesses were manufactured using standard laboratory procedures for organic latex-based coatings. The photochromic properties of the pigmented films were characterised and the response kinetics was compared with other solid state matrices.

Experimental

Synthesis of pigments and latex films

Mesostructured photochromic colloidal silica pigments were synthesized in a spray-drying reactor from an inorganic siliceous precursor solution which contained the inorganic precursors, amphiphilic molecules and the photochromic dye. The solutions were sprayed together with a carrier gas through a two-flow spray-nozzle into an evaporation chamber at room temperature where the volatile components (mainly water and ethanol) were evaporated, and the internal liquid crystalline mesostructure was formed by the cooperative self assembly of the amphiphilic molecules, hydrolysed TEOS (silicic acid) and the dyes.¹⁴ The structured, yet soft, aerosol particles were then transported by the carrier gas through a heating stage of the reactor (200 °C for 10 seconds) where the hydrolysed silica cross linked in order to “freeze in” the mesostructure templated by the amphiphilic molecules. The aerosol was then cooled at the outlet of the heating stage and the mesostructured silica pigments were collected on a Teflon tube filter. The precursor solution was prepared by prehydrolysing tetraethoxysilane, TEOS, in ethanol and dilute hydrochloric acid (pH = 2) under vigorous stirring at room temperature for 20 minutes. The photochromic dye was dissolved in 16 g of ethanol (99.7%) together with 4.4 g of the organic template, a block copolymer P104. The block copolymer is a so-called Pluronic which is a triblock copolymer consisting of ethylene oxide (EO), propylene oxide (PO) and ethylene oxide blocks; the Pluronic that was used in this study, P104, had the composition EO₂₇PO₆₁EO₂₇. Before spraying, the two solutions were mixed together. For all experiments, 20.8 g TEOS (Purum > 98%) was pre-hydrolyzed in a solution consisting of 24 g ethanol (99.7%) and 10.8 g diluted hydrochloric acid (pH 2).

Two different photochromic dyes were used, 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3′-[3H]naphth[2,1-b][1,4]oxazine] (Aldrich) and 1′3-dihydro-1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-2(H)-indole] (Aldrich), hereafter referred to as spirooxazine (SO) and spiropyran (SP), respectively. The photochromic pigments were dispersed in an aqueous latex dispersion and ultrasonicated for 20 s with an ultrasonic horn. Film applicators were used to make 90, 120 and 150 µm thick wet films from the pigmented latex dispersions onto glass substrates (Fig. 2). Thicker films were made by a cast procedure. The films were dried in a humidity chamber at 25 °C and 60% relative humidity. The concentration of pigments in the dry films was varied between 1, 5 and 10 wt% (Table 1). The commercially available latex (Vinamul polymers) was vinyl acetate ethylene, VAE, supplied as an aqueous dispersion stabilized by anionic surfactants. The dry weight for the latex is 54 wt% and the pH of the dispersion was 4.5. The glass transition temperature of the latex was around 9 °C.

Fig. 2 Schematic illustration of the preparation of pigment-containing latex dispersions and the production of films. The pigment dispersion was applied on a glass substrate with a film applicator and allowed to dry.

Table 1 Contents and the two different photochromic dyes in the inorganic–organic hybrid pigments

Pigment	SO/g	SP/g	Dye (wt%)
SO 1	0.05		0.48
SO 2	0.1		0.95
SO 3	0.2		1.9
SP 1		0.05	0.48
SP 2		0.1	0.95
SP 3		0.2	1.9

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Characterization

The internal mesostructure in the pigments was investigated with a transmission electron microscope, TEM (JEOL 2000 FX), operated at an accelerating voltage of 200 kV. The particle size and morphology of the colloids were studied with a scanning electron microscope, SEM (FEI XL 30). The surface roughness of the prepared latex films was evaluated using a white light profilometer, New View 5010, Zygo.

Optical characterization

The films were characterized with reference to their transient absorption properties by two methods.

(1) The UV irradiation of the samples was performed using a UV lamp at 254 nm, Model UVGL-58 with mineralight lamp, multiband UV-254/365, at a distance of 5 cm from the films. The absorption spectra of the coloured films, as well as the rate of the bleaching process, were measured using a UV/VIS spectrometer, Perkin Elmer Lambda 18. The instrument arrangement during measurements is shown in Fig. 3a. The bleaching of the photochromic effect was monitored as a function of time at the wavelength of the maximum absorption of each sample (595 and 567 nm for the SP and SO photochromic dyes, respectively). Thermal bleaching was recorded in the dark and the photobleaching at 30 lux. The kinetic constant, k, was calculated from the thermal bleaching curves using a mono-exponential curve fit. The transmission of the colourless latex films with dispersed pigments was also recorded on the same UV/VIS instrument.

Fig. 3 Illustration of the two arrangements for the optical characterization of the pigment containing films: a) a UV/VIS spectrometer with the UV radiation from an external UV lamp; b) UV radiation originating from a laser system where the white light transmittance is measured (F = Filter).

(2) The rate of thermal bleaching of the pigmented photochromic films was studied in detail by recording the white light transmittance through the film during and after UV excitation of the samples with laser light. UV irradiation was accomplished using a MIRA model 900 (Coherent) mode-locked titanium : sapphire laser pumped with a 10 W Verdi™ laser. The outcoming light was directed through a pulse picker, model 9200 (Coherent), and an ultrafast harmonic generation system, model 5-050 (Inrad), to create frequency-doubled light at 375 nm with a light power of 5.5 mW. The photochromic films were illuminated with ∼5.5 mW (SO samples) and ∼0.55 W (SP samples; use of ND filter) at an angle of incidence of 45°. The films were probed by white light of low intensity of a xenon flashlamp (model 5000XeF, Horiba Jobin Yvon) at an angle of incidence of 45°. The probe light transmitted through the film was detected by a photonic multi-channel analyzer (Model C7473, Hamamatsu) placed at a distance from the film where no fluorescent light could be detected. The experimental setup is depicted in Fig. 3b. The measurements were performed in darkness. It was assured that the probe light did not affect the photochromic films; in the case of SO, a ten-fold increase in probe light intensity did not, within experimental accuracy, affect the thermal bleaching rate, and in the case of SP, the probe light was incident on the film only during the few seconds that each measurement lasted. The absorbance of the film was calculated from the measurement data under the assumption that the film was transparent before excitation by laser light. Analysis was made on wavelength intervals of 540–640 nm (SO) and 550–600 nm (SP). The time dependence of the thermal fading of the absorbance (A) could be described by fitting the kinetic constant k of the mono-exponential function A = A₀exp(−kt) + C to measurement data.

Results and discussion

Pigments and pigmented latex films

Photochromic dye-containing spherical hybrid inorganic–organic mesostructured pigments were prepared through a simple aerosol assisted process. This simple, one-pot synthesis approach¹⁴ produces spherical particles that have a very well defined internal mesostructure. Our previous work that involved a combination of transmission electron microscopy and small angle X-ray scattering data, SAXS, showed that the internal mesostructure can be controlled by the ratio between the tetraethoxysilane, TEOS, and the block copolymer.^14–16 We used a ratio of 4.7 g TEOS per 1 g of the templating triblock copolymer (EO₂₇PO₆₁EO₂₇) generating an intermediate liquid crystalline structure, which results in a 2D hexagonal internal mesostructure, as shown in Fig. 4. The spherical shape of the pigments is a result of the surface tension driven minimisation of the surface area of the fluid droplets. It is possible to vary the particle size by the gas pressure, nozzle size and solvent content in the precursor solution. The use of an exit orifice with a diameter of 1 mm, a gas flow of 20 l (STP) min⁻¹ together with a liquid flow of 3 cm³ min⁻¹ resulted in a particle size distribution that is centred around 3–5 µm (Fig. 4). The photochromic dyes were incorporated into the mesostructured hybrid particles by simply adding the soluble dyes to the block copolymer solution.

Fig. 4 (a) TEM image of an inorganic–organic hybrid pigment particle displaying a well-ordered internal hexagonal mesostructure and (b) SEM image of the particles.

The inorganic–organic hybrid silica particles that do not contain any photochromic dye are white, which show that neither the hydrolysed silica nor the block copolymer display any significant absorption in the visible range. The mesostructured pigments doped with spirooxazine were also white while the pigments doped with spiropyran were slightly pink. This suggests that SO is in the uncoloured, closed form in the mesostructured pigments while some fraction of the SP dye also exists in the open, coloured form. It is possible that the silanol groups present at the surface of the silica walls in the mesostructured particles could stabilise the open zwitterionic coloured forms for SP; previous work has shown that this molecular configuration is stabilized by hydrogen bonding.^17,18 The pink colour disappears when the powder is exposed to light in the visible range, which shows that the dye displays reverse photochromism.^3,11 It should be noted that the coloration of the as-prepared SP pigments is much less than the colour obtained under UV irradiation; hence, only a minor fraction of the SP molecules are stabilised in their opened forms.

Transparent films containing the photochromic pigments could be produced by dispersing the particles together with an organic latex binder in aqueous media and then applying and drawing thin films of the dispersion onto a substrate. We found that the inorganic–organic hybrid pigments could be well dispersed by treating the aqueous suspension with an ultrasonic horn for a few seconds without any addition of dispersing aids. The spherical shape and the relatively large particle size make it relatively easy to deagglomerate the pigments and the negative charge of the silica dominated surface stabilises the pigments in a latex-based aqueous system.

The pigment containing latex dispersions were deposited on glass substrate and the film thickness could be varied over a range of 50 to 200 µm by the choice of film applicator. The photochromic responses were generally evaluated for 70 µm thick films. Even thicker films with thicknesses up to 1 mm had to be cast due to the low viscosity of the dispersion resulting in an uneven thickness of drawn films. We found that it was important to control the drying process of the latex-based coating to obtain smooth, defect-free films of good optical quality. Films of the best optical quality could be obtained when the drawn suspensions were dried at 25 °C and 60% relative humidity. The latex films were transparent with transmission of 97–100% and colourless, indicating that the pigments are well dispersed in the latex-based film and that the photochromic dyes in the pigments are mainly present in their closed forms. The surface roughness was found to depend on the pigment contents in the films, where a higher pigment content resulted in an increased surface roughness. However, the surfaces of the films were quite smooth with a maximum root-mean-square value of 0.073 µm.

The optical response of the pigmented latex-based films could be easily controlled by varying the film thickness, the pigment concentration or the dye content in the pigments. The linear increase of the absorbance with increased SP content in the pigments (Fig. 5) shows that the mesostructured photochromic response is not compromised within the investigated concentration range. This suggests that the spiropyrane dye is organised in the inorganic–organic hybrid mesostructured material similar to a solution; hence, the conformational transformation of the dyes is unconstrained and the aggregation and precipitation of the dye molecules are insignificant. Fig. 6 shows that the absorbance increased linearly with increasing pigment content in the UV illuminated films, which clearly illustrates that the pigments are well dispersed in the latex-based film.

Fig. 5 Absorbance spectra displaying UV-irradiated 70 µm thick latex films containing 10 wt% of hybrid pigments that contain various amounts of spiropyran (0.48, 0.95 and 1.9 wt%). The absorbance varies linearly with the dye content in the pigments, as shown in the inset, where the absorbance values at 567 nm are plotted versus the dye concentration.

Fig. 6 Absorbance spectra of UV-irradiated 70 µm thick latex films containing 1, 5, and 10 wt%, respectively of the spiropyran containing hybrid pigment SP 3. The absorbance varies linearly with the pigment content in the films, as shown in the inset, where the absorbance values at 567 nm are plotted versus the pigment concentration.

Photochromism

The films show rapid coloration upon exposure to UV light (254 or 375 nm), acquiring a red colour for the films containing SP pigments and a blue colour when the films contain SO pigments. The absorbance spectra of the pigment-containing latex films are shown in Fig. 7, for the open form of the photochromic dyes. For the SP pigmented films the absorption maximum is near 567 nm and for the SO pigmented films two maxima were found; one near 595 nm and a weaker second maximum near 569 nm. Measurements on pure block copolymer films with the dyes incorporated gave absorbance maxima at the same wavelengths. It is well known that the position of the absorption maximum varies with the environment and we find that the measured peaks for both SP and SO are close to the reported values for the dyes dissolved in acetone, 565 nm and 600 nm respectively,¹⁹ while less polar solvents like benzene or toluene result in a shift of the positions of the band maxima to longer wavelengths. Hence, our measurements clearly show that the SO and SP dyes are mainly incorporated into the organic phase in the inorganic–organic hybrid pigments. This conclusion is corroborated by a previous study on spiropyran in a mesostructured thin film with the block copolymer P123 as the organic nanophase.¹²

Fig. 7 Absorption spectra for the UV irradiated pigmented films (at a wavelength of 254 nm) containing a) SP and b) SO. The absorption maxima correspond to the opened forms of the SP and SO dyes in the inorganic–organic hybrid pigments.

The photo-coloration upon UV exposure and the thermal fading of the pigmented latex films were studied using the laser system shown in Fig. 3b. The rapid coloration of the SO-doped dyes in pigmented latex films upon UV irradiation is depicted in Fig. 8. The SO-doped film undergoes a fast shift from transparent to blue upon exposure to UV light, and a rapid thermal fading to colourless as the UV light is blocked. Also the SP-doped films exhibited a rapid photochromic response upon UV exposure but the rate of thermal fading of the pigmented latex films was different compared to the SO-doped films.

Fig. 8 Time-dependent photo-coloration by UV irradiation at 375 nm and the subsequent thermal fading when the irradiation is turned off of mesostructured SO-doped silica pigments dispersed in latex film. The probe wavelength is 600 nm.

After UV irradiation, the SO pigmented film rapidly lost its colour (Fig. 9), whereas the SP pigmented film exhibited a much slower fading; the thermal fading time was around an hour in a lighted room, and between six to seven hours in darkness (Fig. 10). The thermal rate constant, k, which corresponds to the ring closure of the photochromic molecules, was determined as k_SO = 0.094 s⁻¹ and k_SP = 1.3 × 10⁻⁴ s⁻¹ for the spirooxazine and spiropyran pigmented latex films, respectively. The values of the kinetic constants in this work are comparable to previously reported values for the dyes in ethanol (k_SO = 0.2 s⁻¹, k_SP = 3.7 × 10⁻⁴ s⁻¹),²⁰ in mesostructured thin films with the dyes embedded in a poly(ethylene oxide)-poly(propylene oxide) copolymer (k_SO = 0.15 s⁻¹, k_SP = 2.0 × 10⁻⁴ s⁻¹),¹² and in organically modified sol–gel materials with the dyes embedded in copolymerized methyldiethoxysilane and triethoxysilane (k_SO = 0.2 s⁻¹, k_SP = 5 × 10⁻³ s⁻¹).¹¹ We find that the rate constants are generally much lower in purely inorganic materials and also for certain xerogels with the dyes embedded in alkoxysilane-polydimethylsiloxane copolymer (k_SP = 6.2 × 10⁻⁵ s⁻¹)²¹ or alkoxide gels (biexponential fit: k_SO,1 = 5 × 10⁻² s⁻¹, k_SO,2 = 3 × 10⁻³ s⁻¹).³ It is clear that the kinetics of the thermal fading process depends on the environment of the dyes. The relatively fast rate constants for the thermal fading process of our photocromic dye-containing pigments corroborate the previous notion that the dyes are predominantly incorporated in the organic nanophase of the mesostructured inorganic–organic hybrid composite.

Fig. 9 Thermal fading of the spirooxazine containing pigment SO 3, dispersed in a latex film. Excitation of the photochromic film is accomplished by UV laser light at 375 nm. The absorbance decreases as the laser excitation is blocked. The absorbance was measured in the wavelength interval 540–640 nm. The thermal fading follows a monoexponential decay with k = 0.094 s⁻¹.

Fig. 10 Thermal fading of the spiropyran containing pigment SP 3, dispersed in a latex film, after excitation by UV laser light at 375 nm. The absorbance was measured in the wavelength interval 550–600 nm. The thermal fading follows a monoexponential decay with k = 1.3 × 10⁻⁴ s⁻¹ (or 7.8 × 10⁻³ min⁻¹).

Conclusions

We have incorporated the photochromic dyes spiropyran (SP) and spirooxazine (SO) in mesostructured organic–inorganic hybrid pigments using a simple aerosol assisted one-pot synthesis approach. This material combines the mechanically and chemically rigid silica matrix with an organic phase where the dyes have a high conformational degree of freedom. The pigments have a spherical morphology and a mean diameter around 3–5 µm. The SO photochromic dye is stabilised in the closed, uncoloured form in the pigment while a faint pink colour reveals that a small fraction of the SP dye is stabilised in the open, coloured configuration, probably due to a weak dye–silica matrix interaction. The versatility of the photochromic pigments was demonstrated by the preparation of latex-based films of good optical quality using simple coating techniques. This pigment-based approach opens up new possibilities to produce photochromic objects of various shapes and materials.

In good agreement with a previous study on mesostructured thin films doped with SP and SO dyes,¹² we find that both the SP and SO doped pigments in latex films show direct photochromism, becoming coloured upon UV illumination and bleaching thermally back to their colourless closed forms. The rate constants for the thermal bleaching are fast, especially for SO, being in the range of the best reported so far for solid state composites. The fast rate constants and the absorption peaks measured strongly indicate that the dyes are dispersed in the organic nanophase of the mesostructured composite. The produced pigments have good photochromic quality and they are potentially usable for production of photochromic components such as photoactivated optical limiting devices or light illumination devices.

Acknowledgements

This work was funded by a grant from the Swedish Reseach Council (VR). Mikael Lindgren and Jonas Örtegren also acknowledge funding by the Norwegian Research Council within the NanoMat program (Contract #153529/s10) (M. L. and J. Ö.), as well as from the Nanotechnology program (www.nanotek.se) supported and managed by Swedish Defence Research Agencies (M. L.).

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