Synthesis of novel silica encapsulated spiropyran-based thermochromic materials

A series of novel spiropyrans were synthesized through the condensation of substituted 3,3-dimethyl-2-methyleneindoline with different nitro-substituted o-hydroxy aromatic aldehydes. Indoles were initially substituted with a variety of alkanes and esters moieties. The substituted 3,3-dimethyl-2-methyleneindoline was then reacted with nitro-substituted o-hydroxy aromatic aldehydes to yield the respective spiropyrans. The synthesized novel spiropyrans were encapsulated in silica nano-shells to protect them from the effect of moisture and pH. The thermochromic behaviour of novel spiropyrans was studied by UV-visible spectroscopy. The thermally induced isomerization of spiropyran derivatives was carried out in a water/ethanol mixture. The thermal isomerization of spiro-heterocyclic (colourless form) to merocyanine (MC) (coloured form) was a discontinuous process and was observed in a temperature range of 5–60°C via UV-visible spectrometer. The absorption process occurs reversibly regardless of the heating/cooling sequence. The spiropyran derivatives, therefore, have a potential application for colorimetric temperature indication.

We also planned the thermochromic behaviour of spiropyran dyes in an aqueous ethanol solution. The silica encapsulation was proposed for the ease of practical application of material over a wide range of surfaces without the effect of moisture and pH on the structure of the dye and henceforth the thermochromic behaviour. The prepared silica-encapsulated spiropyrans have a wide range of applications as temperature monitoring sensors which are used in motors, circuit breakers, heat exchangers and transformers. These can be applied in health indicators and to report food quality. The general applications involve decorative use on cloths, utensils, paper, etc. [36].

Materials
All the reagents were purchased from Sigma Aldrich. The compounds were characterized by their physical constants data and spectro-analytical techniques. Melting points of the synthesized compounds were recorded in open capillaries using Gallenkamp melting point apparatus (MP-D). The chemical structure of synthesized compounds was characterized by infrared (IR), 1 H nuclear magnetic resonance ( 1 H NMR) and 13 C nuclear magnetic resonance ( 13 C NMR) spectroscopic techniques. The IR spectra were recorded on Thermoscientific Nicolet model 6700. 1 H NMR and 13 C NMR spectra were recorded on Bruker AV-300 MHz spectrometer using the desired solvent as indirect reference. UV-Vis spectroscopy was carried out at different temperatures using Shimadzu spectrophotometer, model Pharmaspec UV-1700. Surface morphology was studied by scanning electron microscopy (SEM) (JEOI, JSM-5910), operating at 20 kV with different magnification power.

Synthesis
The general procedure for the synthesis of intermediates and spiropyran derivatives is shown in figure 2.

General procedure for synthesis of substituted indole
First, 0.795 g (5 mmol) of 2,3,3-trimethyl-3H-indole was dissolved in dry distilled acetonitrile (15 ml), and then 2.18 g (20 mmol) of alkyl halide was added to the reaction mixture. The reaction was allowed to reflux for 78 h, and the progress was monitored by thin-layer chromatography (TLC) analysis till the completion of the reaction. The reaction mixture was then cooled to room temperature and concentrated under reduced pressure. The residue was added to n-hexane (15 ml) and dispersed well by ultra-sonication. The insoluble solid was recovered by filtration. Solid KOH (4.0 mmol) was added to water and stirred at room temperature for 10 min. The solution was extracted with diethyl ether, washed with distilled water, brine, dried over anhydrous Na 2 SO 4 , and the excess solvent was removed under reduced pressure.
royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 9: 211385 3 The light orange liquid was obtained with a yield of 80%. TH-1 was used without further purification in the next step.
Dark pink oily product was obtained with a yield of 79%. This product was used in the next step without further purification.
Yellow oily product was obtained with a yield of 86%. This product was used in the next step without further purification.
Pink oily product was obtained with a yield of 95%. This product was used in the next step without further purification.
Brown oily product was obtained with a yield of 91%. This product was used in the next step without further purification.

General procedure for synthesis of spiropyrans
Substituted indole was dissolved in 15 ml of dry, distilled ethanol under a static nitrogen atmosphere. The mixture was set to gentle reflux with stirring followed by the addition of substituted benzaldehyde. The reaction was allowed to stir under reflux for 12 h, and the progress was monitored by TLC until the reaction was complete. The product was recovered by filtration, washed with ethanol and dried in a vacuum. International Union of Pure and Applied Chemistry (IUPAC) names and chemical structures of the synthesized compounds are given in table 1.
Dark brown solid was obtained with a yield of 90%. IR (ῡ max , cm

12
SP-12 The compound (SP-6) was synthesized by the same general procedure as given before, using methyl-3-

General procedure for the encapsulation of synthesized spiropyrans
The synthesized spiropyrans (50 mmol) were dissolved in ethanol (A) (5 ml). This solution (A) (4.6 ml) was added to the ethanol-water mixture (8 : 2 ratio). This reaction mixture was then stirred for 20 min followed by the addition of TEOS (5 ml) and ammonium hydroxide (1 ml). The reaction was stirred for 48 h, and the product was obtained by filtration, washed with ethanol and dried in a vacuum.

Results and discussion
In this work, the synthesis of novel silica-encapsulated spiropyrans is reported. Preparation of silica NPs as well-defined structures requires precise planning and depends upon a number of factors such as selection of proper precursor, molar ratio, temperature, reaction time, stirring rate and rate of addition of precursor to the reaction. All these factors contribute to the final shape and morphology of the silica NP [37]. The type of bonding between the spiropyran and silica is mainly electrostatic interaction, hydrogen bonding and other non-covalent interactions [38]. The target spiropyrans were synthesized through one of the standard methods by the condensation of methylene bases (or their precursors) with o-hydroxy aromatic [39]. The structures of synthesized compounds were characterized by spectroscopic techniques like IR, 1 H NMR and 13 C NMR spectroscopic techniques. All compounds gave satisfactory elemental analysis, and the observed percentage of carbon and hydrogen atoms was found in good agreement with the calculated values.
The IR absorption spectrum of the different spiropyrans showed characteristic bands in the range of 1620-1680, 1310-1360, 1200-1300, 1735-1750, 1150-1250 cm −1 , attributed to C=C (aliphatic), C-N, C-O (ether), C=O (ester) and C-O (ether), respectively [40]. The IR spectroscopic analyses of the encapsulated spiropyrans were also carried out. The characteristic peak for Si-O-Si and Si-O-H bending vibrations appeared in 900-1460 cm −1 range of all the IR spectra, whereas the O-H stretching vibration was observed at an average of 3200 cm −1 [41].
The synthesized mesospheres were also characterized via X-ray diffraction (XRD) technique. The XRD spectra of some of the dye encapsulated mesospheres are shown in figure 3. No characteristic Bragg diffraction peaks at 2θ below 10°are observed, whereas a broad peak with 2θ centred at 21°is found. This peak indicates the disordered nature of the mesosphere [35]. The XRD of simple silica mesospheres without the dye is compared with those having the dye. The observed pattern is almost similar in both cases, which shows that the dye is successfully encapsulated in the silica mesospheres and is not attached to the outer bounderies of the silica shell [42].  royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 9: 211385 SEM micrographs of the encapsulated mesosphere (figure 4) represent that they are spherical and homogeneous with their particle size in the range of 450-500 nm. All the particles are non-porous as no surfactant was used in the synthesis of mesospheres [43]. During synthesis, the reaction mixture was stirred very slowly, so that the mesoparticles are obtained. The reason being the factor that spiropyrans required some space so that they can easily convert thermally from SP to MC form.
The temperature-dependent change in absorption spectra of SP-8 (50 µM) measured in water-MeOH (1 : 1 v/v) is shown (figure 5). Spiropyran shows two different interconvertible absorption peaks in the UVvisible region, each corresponding to the SP and MC forms. At a temperature of 0°C, a peak at 350 nm is observed which corresponds to the SP form, whereas a peak of comparatively smaller intensity is observed in the range of 450-600 nm which corresponds to the MC form. The appearance of this peak is due to the photochromic behaviour of the synthesized spiropyrans. As the temperature was increased after regular intervals, the absorption intensity of the MC form also increased, and this trend conforms to the thermochromic behaviour of the as-synthesized spiropyrans. It is observed that the increase in absorption is very slow up to 25°C. However, the increase in absorption spectra becomes more prominent with a further rise in temperature. The MC form is stabilized in the solution due to the hydrogen bonding. The colour of the solution changes from very light pink to dark pink with a temperature rise [44].
The temperature-dependent transformation of all the synthesized spiropyrans is shown in table 2.

Conclusion
Indoles were substituted with different alkyl and ester moieties by a simple nucleophilic reaction. The substituted indoles were then converted to 12 novel spiropyrans by the condensation of methylene base with nitro-substituted o-hydroxy aromatic aldehydes. All the synthesized spiropyrans were characterized by physical data, 1 H NMR, 13 C NMR and Fourier transform infrared (FTIR) spectroscopy. All the spiropyrans were solids with different colours and show characteristic peaks in FTIR data. The structural novelty of the prepared spiropyrans was confirmed by the 1 H NMR and 13 C NMR. These spiropyrans were encapsulated within the nanospheres of the slice by using tetraethylorthosilicate as a silica precursor. XRD showed that the spiropyrans were fully incorporated within the silica mesospheres. Encapsulation of thermochromic dyes was confirmed by the SEM and FTIR techniques. SEM shows the non-porous mesospheres within the size range of 450-500 nm. The thermochromic properties of these spiropyrans were confirmed by the UV-Vis spectroscopy, in the temperature range of 0-50°C, and discontinuous change in the absorbance spectra of spiropyrans was observed with temperature increase. Overall, we have effectively developed a metal oxide encapsulated spiropyrans as efficient temperature indicator in the temperature range of 0-5°C.