Photochromic Spiropyran-and Spirooxazine-Homopolymers in Mesoporous Thin Films by Surface Initiated ROMP

Control of ionic permselectivity in porous films is an interesting aspect in the context of lab-on-chip devices and µ-electronics. Especially visible light triggered ionic permselectivity control is of relevance because control by light can be maintained externally without changing internal system parameters. Additionally, light is a sustainable energy source if sunlight is used. Here, we present the first mesoporous films modified with two different photochromic homopolymers by surface-initiated ring opening metathesis polymerization (SI-ROMP). Spiropyran-and spirooxazine functionaliized norbornene monomers and the corresponding ROMP homopolymers are synthesized in solution and in mesopores and compared concerning their optical properties such as photochromic conversion kinetics, photostability, and ratio of converted molecules. Optical properties are investigated using UV/VIS spectroscopy and 1 H-NMR spectroscopy. Especially spirooxazine, whose surface functionalization has not been studied in detail, shows fast switching properties and higher ratios of photochromically interconverted molecules. After grafting spiropyran-and spirooxazinenorbornene homopolymers into mesopores a slightly faster photochromic interconversion of polymer located inside the mesopores is observed as compared to the solution polymer.


Introduction
For the design of technological innovation nature is a multifaceted source of inspiration. For example wetting on surfaces, 1 release, 2 or complex transport control through pores [3][4][5][6][7] is inspired by nature. In the context of sustainability it is desirable to re-use resources like water or to reduce energy consumption in technological applications. One approach towards more sustainable energy consumption is the use of sunlight. Combining research on transport control through pores and responsivity towards sunlight, the combination of photochromic spiropyran-or spirooxazine polymers as functional component with ceramic mesopores as structural component can be one versatile approach. The photochromic reaction of spiropyran was already known in 1952. 8 Recently, the mechanism of photochromism was investigated with respect to their ultrafast dynamics 9 and merocyanine isomers by using DFT calculations. 10 Since then, two main areas of interest related to spiropyrans developed: One is related to the complexation of ions going along with an ion-specific color change and thus a sensor or release response. 11,12 Secondly, spiropyran is used as surface functionalization to manipulate and switch surface properties such as wetting. The challenge of switching surface wettability is related to the relatively low changes in contact angle which usually are below 15°. [13][14][15] In addition to molecular functionalization many groups are studying spiropyran containing polymer coatings on surfaces for sensor or cell adhesion studies. [15][16][17][18][19] Thereby, changes in surface properties are often limited by the spiropyran content. Using radical polymerizations spiropyran monomers can only be copolymerized with nonspiropyran containing monomers and thus the spiropyran contents reported are mostly in the range of 10-15 mol%. 15,20,21 Only a few studies report on the synthesis of spiropyran homopolymers accessible by ring-opening metathesis polymerization. [22][23][24] These studies report the modification of surfaces with ROMP homopolymers carrying spiropyran in the side chain. Interestingly, nitrospiropyran dominates the literature related to surface functionalization and photochromic behaviour of surface properties. To the best of our knowledge there are no studies related to spirooxazine homopolymer synthesis at surfaces or even in the confined space of pores. Spirooxazine is an interesting photochromic compound because it shows much faster switching kinetics than spiropyran. Especially, in terms of transport control different switching kinetics are of potential interest because they could allow the adjustment of transport kinetics. In this context the functionalization of porous materials with photochromic units is very fascinating to mimic light-gated ion channels. So far mesoporous silica or single pores were functionalized using azobenzenes 25,26 or spiropyran molecules observing a modulation of probe molecule transport upon irradiation. [27][28][29] Additionally, spiropyran copolymers are reported to show an effect on membrane permeability. 30 Transport modulation is ascribed either to electrostatic 27 or to hydrophobic/hydrophilic 29 interaction. Interestingly though, spiropyran homopolymers or even spirooxazine homopolymers have not been used to improve understanding.
Here, the first steps towards photo-responsive pores based on the spiropyran-and spirooxazine homopolymer functionalization of mesoporous silica thin films are reported. A synthesis strategy for spiropyran and spirooxazine monomers for SI-ROMP polymerization is developed and these monomers are applied to graft ROMP homopolymers from silica mesoporous films. The photochromic behavior in terms of the ratio of molecules that respond to light and the interconversion kinetics is investigated by using in situ irradiation NMR 31 in solution and UV/VIS-spectroscopy. By combining both methods extinction coefficients of spiropyran and merocyanine forms are accessible. Furthermore, this is the first study that examines the photochromic behavior of homopolymers using NMR spectroscopy. The presented synthetic approach and the deep understanding of photo-responsive behavior is a strong basis for further investigations towards sustainable transport control by using visible light and sunlight.

Experimental
The synthesis of all applied compounds ( Figure 1) including their characterization is described in detail within the supporting information. Infrared (IR) Spectroscopy. IR measurements are performed on a Spectrum One (PerkinElmer) instrument in attenuated total reflection (ATR) mode. Mesoporous films are scratched from the substrate to record IR spectra in a range from 4000 to 600 cm −1 . The measured spectra are background corrected and normalized to the Si−O−Si band at 1080 cm −1 .

UV/VIS-spectroscopy.
UV/VIS absorption and kinetic measurements of spiropyran and spirooxazin containing compounds are carried out with a Cary 60 UV/VIS-spectrometer (Agilent). The recording of measured data is done with the software package Agilent Cary WinUV-Software. All data are background corrected prior to each measurement. The used solvent is dried carefully and stored above molecular sieve. UV and VIS irradiation is performed by using a Modell Superlite 410 with different cut-off filters (λ UVA = 320 bis 400 nm(I=11.8 mW/cm²) and λ green = 550 nm (I=30.6 mW/cm²)) (LUMATEC GmbH Deisenhofen, Germany). Samples are irradiated with a distance of 3.0 cm in an angle of 45° to the quartz glass (d = 1.0 cm) cuvette. NMR. All employed compounds and precursors for the attachment on mesoporous silica films are characterized by NMR-spectroscopy with a Bruker DRX 500 or a Bruker AC 300 spectrometer. Sample concentrations of 10 mg substance per 1 mL deuterated solvent are used and 1 H-NMR and 13

Synthesis of spiropyran and spirooxazine monomers.
To ensure a relatively high functional density, spiropyran-and spirooxazine homopolymers are used to prepare photo-responsive mesopores. Spiropyran-norbornene (SP-Nb) (5) and spirooxazine-norbornene (SPO-Nb) (7) monomers are synthesized in a four step procedure summarized in Figure 1 (details can be found in the supporting information). The synthesis concept for SP-OH (4) and the active ester mediated coupling of the norbornene polymerizable unit (5) to the SP-OH (4) is based on previously reported studies. [33][34][35] The NMR analysis shows a pure SP-Nb monomer (6) with a total yield of around 20 % after four reaction steps, which is close to the literature yield of 25 %. 33 This synthetic approach is successfully transferred to SPO-Nb (7) with a comparable total yield. Polymerization of spiropyran-and spirooxazine norbornene. Ring opening metathesis polymerization of the monomers SP-Nb (5) and SPO-Nb (7) are performed in the presence of catalyst-modified 36 mesoporous allylsilica films resulting in the formation of photochromic homopolymers PSP (8) and PSPO (9) in solution and covalently attached to the mesoporous films (see below). The formation of PSP (8) and PSPO (9) homopolymer in solution is probably due to the gentle purification procedure, used in the grafting approach (see below) resulting in non-bound catalyst being still present which then leads to homopolymer generation in solution. After precipitation in methanol, solution homopolymers with a molecular weight in the range of 6.6·10 4 g/mol for PSP (8) and 1.4·10 4 g/mol for PSPO (9) (supporting information) and a molar mass distribution with a PDI between 1.6 and 1.2 were obtained, respectively.

Photochromism of SP-Nb, SPO-Nb, PSP and PSPO.
The bulk homopolymers PSP and PSPO as well as the SP-Nb (5) and SPO-Nb (7) monomers are characterized concerning their photochromic behavior. Molecular spiropyran as well as molecular spirooxazine is known to interconvert from a closed form into an open merocyanine (MC) form upon irradiation with UV-light ( Figure 2). The merocyanine form shows a characteristic absorption at 568 nm in case of SP-Nb and PSP (Figure 2 a, c) and 608 nm in case of SPO-Nb and PSPO (Figure 2 b, d). Merocyanine can be described by various mesomeric structures, only one of which is zwitterionically charged. We included this fact as a zwitterionic and quinoidal form into the scheme in Figure 2. The extent to which charge separation is actually present depends on the solvent polarity. 10,37 It is reported in the literature that in polar solvents the uncharged mesomeric forms dominate. 10,37,38 Heating or visible light irradiation results in relaxation of the merocyanine into the closed SP or SPO state. Additionally to photochromic behavior, SP and SPO do show acidochromic 39 or even mechanochromic 40,41 properties ( Figure S1) resulting in proton induced ring opening reactions. Therefore, absorption can be affected by the solution pH. 42 In this study all experiments related to photochromism have been performed in DMF (NMR: DMF-d 7 ) because precipitation during irradiation is prevented under these conditions. The shift in the absorption wavelength of spiropyrans and the corresponding merocyanines is caused by a severe change in the electronic structure upon ring opening. This difference in the electronic structure likewise induces a severe difference in chemical shifts of spiropyrans and merocyanines. 43 This can be exploited to examine molar ratios of spiropyran and merocyanine during irradiation with UV light or in photostationary states (PSS). For spiropyrans the signals of the two geminal methyl groups can be used to determine ratios of spiropyran and merocyanine forms, as in case of the closed spiro form both methyl groups are anisochronous, while they are isochronous for the merocyaninic form. A significant chemical shift difference between spiropyran and merocyanine is observed allowing for individual integration. For a detailed investigation of the photochromic process, absorption spectra of the monomer, polymer and the mesoporous films are recorded using UV/VIS spectroscopy. Figure 2a-d shows the UV/VIS spectra of the monomers and bulk polymers before and after UV irradiation. The absorption maxima of the SP-Nb (5) and SPO-Nb (7) in DMF are at 345 and 348 nm (Figure 2 a black, b blue).
The absorption maxima of the corresponding merocyanine are located at 568 nm for MC SP-Nb and 608 nm for MC SPO-Nb . Comparing the UV-VIS absorption spectra for SP-Nb and PSP (Figure 2a, c) and between SPO-Nb (7) and PSPO (9) (Figure 2b, d) before and after UV-irradiation, no significant difference can be observed between the monomer and the corresponding homopolymer. Comparing SP-Nb (5) and SPO-Nb (7) or PSP (8) and PSPO (9) the expected significant difference in time dependence is confirmed by comparing the absorption spectra directly after irradiation and one second later. This trend is not affected by generating a PSPO homopolymer. This observation of much faster thermal relaxation kinetics for spirooxazine compounds is supported by kinetic UV/VIS measurements ( Figure 3). Under UV-irradiation with an energy density of 106.2 mJ/cm 2 and a wavelength range of 320-400 nm the intensity of the merocyanine absorbance in DMF at 608 nm increases as the irradiation time increases until a photostationary state (PSS) is reached after less than 3 s in the case of spirooxazine monomer (SPO-Nb) and polymer (PSPO). The spiropyran polymer (PSP) reaches photostationary state after approximately 18 s whereas it takes monomer (SP-Nb) approximately 30 s of UVirradiation. After excitation with UV-light, thermal relaxation and induced relaxation by irradiation with visible light have been investigated. Both, excitation with UV and induced relaxation with visible light irradiation, appear to follow first order kinetics for the monomer as well as for the homopolymers in solution. This is consistent with reported observations for spiropyran-based molecules and co-polymers. 15,44 Fitting the time dependent merocyanine absorption ( Figure 3a) using first order rate laws, assuming relatively low concentrations of absorbing species, yields photon-flux-dependent rate constants. Small deviations of measured data from first order rate laws are observed, the reason for which is not yet understood. More detailed kinetic investigations will be performed in the future. These k UV are observed in the following order: SPO-Nb > PSPO >> PSP > SP-Nb. All determined rate constants are summarized in Table 2.  value obtained by UV/VIS spectroscopy under NMR irradiation conditions Subsequently irradiating the same solution with visible light (Figure 3b), the merocyanine absorbance decreases monoexponentially. The decrease of merocyanine absorption in case of SPO-Nb and PSPO at room temperature is so fast ( Table 2), that an induced relaxation with VIS irradiation (λ green = 550 nm, I=30.6 mW/cm²) under the applied experimental conditions is only possible for SP-Nb and PSP. Zero absorption is again reached after 18 s for the SP-Nb monomer and after 12 s for the PSP polymer. The determined rate constants are summarized in Table 2. For thermal relaxation into the closed SP form at ambient conditions a much longer time of approximately 30 minutes is needed to reach zero merocyanine absorption for SP-Nb and SPO-Nb (Table 2). In contrast to SP-Nb and PSP, thermal relaxation back into the closed SPO form is very fast for both, SPO-Nb and PSPO. Monomer and homopolymer are converted into the colorless closed form within less than 3s (Table 2). NMR experiments. Additionally, NMR measurements with in situ irradiation are performed to determine molar ratios of the spiropyran and merocyanine forms during irradiation, photostationary states (PSS) and relaxation for the SP-Nb, SPO-Nb systems and the corresponding polymers. Proton NMR spectra of SP-Nb as well as for the PSP polymer prior and during irradiation with UV light (λ = 365 nm) are shown in Figure 4a, b. Molar ratios of the SP-Nb and MC SP-Nb during irradiation are calculated from the integrals of the methyl groups (1.38 ppm and 2.92 ppm) obtained in series of 1 H-NMR spectra. In comparison with the previously shown UV/VIS data a slower saturation, and thermal relaxation on the minute time scale, is observed (Fig.4c). This can be explained by the reduced photon flux, and thus the reduced excitation probability, which is caused by the in situ irradiation setup used as compared to the direct irradiation of the sample in the UV/VIS measurement. Additionally, a higher concentration of SP-Nb is necessary for the NMR experiments and thereby quenching effects could play a more important role. Nevertheless, from 320 K down to 240 K excitation of MC SP-Nb , PSSs, thermal and induced relaxation can be monitored. In PSSs fractions of MC SP-Nb of 4.5 % at 320 K, 13% at 300 K 14 % at 260 K and 17% at 240 K are observed. As in the UV/VIS measurements monoexponential excitation, thermal and induced relaxation behavior is observed (Table 2). For the thermal relaxation a comparable rate constant k therm (SP-Nb and PSP) can be obtained for NMR and UV/VIS measurements. In contrast to UV/VIS measurements in NMR experiments it becomes clear that irradiation with visible light does not give the thermal equilibrium composition, in fact the fraction of MC SP -Nb is reduced below the equilibrium value. This has already been observed for other spiropyrans 46 and it shows that MC SP-Nb is forced into the closed SP form under visible light irradiation. For the polymers much broader NMR signals are observed in 1 H-NMR spectra in comparison to the SP-Nb and SPO-Nb monomers ( Fig. 4b, 5c). Nevertheless, molar fractions of SP and MC SP moieties within the polymers can be estimated by integrating methyl group resonances at 1.35 (SP-PSP) and 1.85 ppm (MC-PSP) for the PSP and 1.25 (SP-PSPO) and 1.85 ppm (MC-PSPO) for the PSPO. In agreement with UV/VIS spectroscopy a mono exponential excitation and relaxation behavior is observed for the PSP polymer.
In contrast to the SP-Nb monomer at 300 K, the total amount of MC PSP in PSP is only 6 % under the applied experimental conditions. This is a smaller ratio than observed for the monomer under identical conditions which might indicate a hindrance of interconversion into the open merocyanine form in case of confining SP-Nb into a polymeric structure. Unfortunately, for the SPO-Nb the geminal methyl groups are not suitable for integration due to a severe signal crowding caused by the signals of the aliphatic sidechain (Fig. 5a). Nevertheless, extraction of molar fractions is possible based on the well separated signals in the aromatic region (Fig. 5b). At room temperature in situ irradiation of the SPO-Nb does not give an observable fraction of MC SPO-Nb with the described irradiation setup, sample concentration and NMR-parameters, as the fraction of MC SPO-Nb is below the limit of detection. (Fig.5d). By stepwise reducing the temperature down to 240 K increasing fractions of MC SPO-Nb up to 60 % can be obtained (Fig. 5c). At 280 K only a fraction of 2 % MC SPO-Nb can be observed. This temperature dependence of SP/MC photochromism is more pronounced for SPO-Nb as compared to SP-Nb. A possible explanation can be seen in the different electronic transitions involved in the photochromic transition which passes through a triplet state for SP but not for SPO. 9,38,45,47 Furthermore, the temperature dependent first order rate constants for the thermal relaxation of SPO-Nb show a typical Arrhenius-type behavior ( Figure  S5). Irradiating SPO-Nb with UV light leads to a PSS within one minute and a rapidly occurring thermal relaxation can be observed after switching off of UV light. Those observations match those obtained from UV/VIS spectroscopy. For the PSPO polymer (Fig. 5c) line broadening was even more pronounced as for the PSP polymer (Fig. 4b). Consequently, signals in the aromatic region are not suitable for integration and molar fraction determination. In contrast to the SP-Nb and PSP, no signal separation for the methyl groups can be observed at all. Nevertheless, a rough approximation of the molar ratios can be made based on the integration of the methylene group signals of PSPO and MC PSPO moieties (Fig. 5c). For the PSPO polymer a MC PSPO fraction of about 17 % is estimated. This is a reduction of a factor 3 as compared to the SPO-Nb monomer and is in agreement with the observed MC fraction of PSP in comparison to SP-Nb. Thus, the MC fraction is smaller for PSP and PSPO homopolymers as compared to the corresponding monomers in DMF and the excitation and thermal relaxation are slower for the polymer than for the monomer as already observed by UV/VIS measurements at ambient temperature. While the reason for this observation remains unclear, one could speculate that the molecular rearrangement in the homopolymers as compared to the monomers in solution is restricted or that quenching effects due to the spatial proximity of photochromic units in the homopolymer are effective. The different ratios between UV/VIS and NMR experiments result from the different irradiation intensities between the two setups.

Mesoporous films functionalized with photochromic polymers.
To graft a PSP and PSPO ROMP homopolymer to a mesoporous surface allyltriethoxysilane was co-condensed into the mesoporous silica walls (Figure 6a). Mesoporous films containing 20 mol% allyltriethoxysilane with a porosity of approximately 20 vol% according to ellipsometry and effective medium theory 48 are obtained. TEM images (Figure 6b) support the presence of a porous structure with pore sizes smaller than 6 nm corresponding to the Pluronic® F127 template. In a consecutive step the ROMP catalyst is attached to the double bond located at the mesopore wall. 22 Catalyst binding is indirectly proven by stable polymer attachment and reference experiments that did not show stable polymer attachment without the presence of catalyst or surface attached allyl groups. The activated catalyst is then able to perform the ROMP polymerization of SP-Nb and SPO-Nb resulting in surface grafted PSP and PSPO. To distinguish between polymer grafted inside the porous matrix and the one on the outer surface we used a CO 2 -based plasma treatment before catalyst binding. This CO 2based plasma destroys the double bonds at the external surface (supporting information). Consequently, non-plasma treated samples are functionalized within the mesopores and on the  The time-dependent absorption increases, and decreases mono exponentially (Figure 7e) upon UV-irradiation and thermal relaxation (Figure 7g, h). As observed for monomer and polymer in solution, the grafted PSPO interconverts into the closed from within a few seconds at room temperature. Thus, the visible-light induced relaxation into the closed spiro form can be measured for surface grafted PSP under the applied experimental conditions (Figure 7f) but not for grafted PSPO. Comparing the rate constants between surface grafted PSP and PSPO with and without plasma treatment it seems that the polymer inside the mesopores (plasma treated samples) interconverts slightly faster into the open merocyanine form upon UV-irradiation as compared to the polymer on the external surface (non-plasma treated samples) (Figure 7e). This might be due to lower molecular weight inside the pores as observed for RAFT polymerizations 49 or due to interaction with the silica pore walls. The determined rate constants (k UV ) for grafted PSP and PSPO at plasma treated mesoporous surfaces and nonplasma treated mesoporous surfaces are summarized in Table 2 Without plasma treatment, and thus with PSPO grafted also at the external surface, k therm is slightly slower as compared to the plasma treated films. Thus, both, k UV and k therm, seem to be faster within the mesopores as compared to the external surface. Comparing the thermal relaxation from the merocyanine into the closed spiro form a slower interconversion for surface grafted polymers as for solution polymer is observed under the applied experimental conditions. This gives a first insight into potential effects of spatial confinement in mesopores on photochromic properties of PSP and PSPO homopolymer.
To systematically correlate switching constants to spatial confinement in pores or at planar surfaces molecular weight, polymer density and surface chemistry would have to be comparable between solution, porous and planar surfaces. This will be part of our future studies. Nevertheless, it is clear that SP and SPO species in the form of monomers or polymers in solution or grafted to surfaces show different switching kinetics which might be beneficial for transport modulation in porous devices for example if transport velocity should be adjusted.

Photostability of PSP, PSPO, SP-Nb, SPO-Nb.
Additionally to switching kinetics, switching fatigue and thus the behavior over several switching cycles is important for later potential transport control in pores. The photochromic interconversion process as well as the stability of the monomers and polymers during irradiation, the time-dependent absorption in several switching cycles for monomers and polymers in DMF as well as the surface grafted polymers in contact with DMF have been analyzed (Figure 8).

value obtained by UV/VIS spectroscopy under NMR irradiation conditions
The photochromic behavior is reversible for multiple cycles for SP-Nb, PSP, SPO-Nb and PSPO. However, the number of possible cycles is limited by fatigue as a result of the intense UV irradiation. In accordance with reported studies a constant exposure to UV-light leads to decomposition and thus to a decrease in absorption (data not shown). 38,50 Upon alternating UV-and visible irradiation or UVand thermal influence, three to five UV-VIS irradiation cycles were measured for SP-Nb, PSP and surface attached PSP. Because of the fast thermal relaxation of SPO-Nb and PSPO from the merocyanine into the closed spiro form at ambient temperature, two to ten UVthermal cycles were analyzed for SPO-Nb and PSPO. It can be observed, that PSPO in contrast to SPO-Nb does not interconvert completely into the colorless closed form, visible by the merocyanine absorption that decreases to 0.02 a.u. (2 %) but not to zero. Because of this a VIS irradiation step was included before subsequently irradiating with UV-light and starting the next switching cycle (Fig. 8 c). All samples were irradiated until the merocyanine absorption reaches an equilibrium value (PSS). After in total 10.  (Table S2).
It is reported that spiropyrans can undergo side reactions (e.g., oxidation reactions or reactions driven by radical formation) and that photofatigue is facilitated by dimer formation. 15,38,51,52 In contrast to SP, SPO does not pass through a triplet state, which is susceptible to side reactions, but only through a singlet state upon UV-excitation. 9,38 This might explain the observed better photostability of SPO-Nb and PSPO as compared to SP-Nb and PSP. Besides the difference in switching kinetics this better photostability in subsequent irradiation cycles favors the use of spirooxazine derivatives instead of spiropyran derivatives in photochromic devices.
Comparing the photo-fatigue upon photochromic conversion for surface grafted polymer (Figure 8e, f) with the bulk polymer ( Figure  8c, d) no significant differences can be observed for up to three switching cycles corresponding to a total irradiation time of 3 minutes for grafted PSPO and grafted PSP. Comparing the plasma and non-plasma treated mesoporous films it seems that the grafted polymer inside the mesopores in case of PSP (Figure 8h) as well as for PSPO (Figure 8g) seems to show a slightly stronger absorption decrease within 3 switching cycles than the non-plasma treated samples (Figure 8 e, f). This could be related to the proximity of photoswitchable units in the spatial confinement of mesopores which might facilitate aggregate formation and thus photofatigue.

Conclusions
Based on a synthetic strategy for spiropyran-functionalized norbornene monomers and the derived ROMP homopolymers we have developed a synthetic strategy for spirooxazine-functionalized norbornene-based monomers and the corresponding SI-ROMP homopolymers. Spiropyran and spirooxazine substituted polynorbornenes were prepared in solution or grafted from mesoporous silica thin films using surface initiated ROMP. Based on UV/VIS-and NMR spectroscopic data spiropyran-and spirooxazine homopolymers show very different photochromic behavior and switching kinetics than the related molecular species. Different kinetics are expected to be useful for subsequent photochromic control of ion transport through pores. Based on NMR spectroscopy the molar attenuated extinction coefficient (ε) was determined as well as the relative amount of species undergoing photochromic transition upon irradiation. This ratio varies between 6 % and 60 % under identical irradiation conditions and is significantly lower for homopolymers as compared to the behavior of the respective monomer solutions. Interestingly, spiropyran-and spirooxazine homopolymers show slightly faster responses to irradiation when located inside the mesoporous matrix rather than in solution or on the external surface of a mesoporous silica film. This might be attributed to the influence of the spatial confinement on the photochromic behavior itself or on a lower molecular weight of polymer inside the pores. The latter was observed for example for iniferter-initiated polymerizations in our group and is attributed to the potential influence of spatial confinement on the polymerization. This is a fascinating aspect of our current research. Such a tuning of photochromic rate constants should offer interesting possibilities in the sensing of confinement effects, control of release or ion transport characteristics. Consequently, our synthetic strategy and the understanding of switching properties in solution and inside mesopores offers a good basis towards further investigation of photochromically controlled ionic transport through mesopores. To achieve relevant transport rates the photochromic behavior of the functional polymers inside the pores and the wetting properties need to be optimized. This will constitute the next step towards light-controlled ionic transport through mesopores.