A tert-Butyldiphenylsilyl-Containing Polyimide-Based Chemosensor for Sequential Detection of Fluoride Ions and Trace Water in Organic Solvents

A tert-butyldiphenylsilyl-containing polyimide (PI-OSi) has been established as a colorimetric and ratiometric chemosensor for rapid detecting fluoride ions (F−). The UV-vis absorbance ratio value (A322/A288) of PI-OSi in a DMF solution displays a wide linear range change to F− concentrations with a detection limit (DL) value of 2.13 μM. Additionally, adding incremental amounts of F− to a DMF solution of PI-OSi shows an immediate color change to yellow and finally to green from colorless. More interestingly, the resulting PI-OSi plus F− system (PI-OSi·F) could detect trace water in DMF. The A292/A322 value of PI-OSi·F almost linearly increases with low water content, which suggests convenient quantitative sensing of trace water content in DMF. The DL value of PI-OSi·F for sensing water in DMF is determined to be 0.00149% (v/v). The solution color of PI-OSi·F returns to colorless when the water content increases, indicating that PI-OSi·F can conveniently estimate water content in DMF by naked-eye detection. The detection mechanisms confirmed by an 1H NMR study and a DFT calculation involve a F−-induced desilylation reaction of PI-OSi to form phenolate anion followed by protonation with trace water. Finally, PI-OSi film was fabricated for the colorimetric detection of F− and water in CH3CN.


Introduction
As an essential micronutrient, fluoride ions (F − ) can effectively avoid dental caries and contribute to the treatment of osteoporosis in the right amount [1].However, excess F − exposure may damage human health and destroy the ecological environment to a certain extent [2,3].For these reasons, the design of a simple and accurate F − chemosensor has become the focus of current attention, and its technology is becoming more and more mature [4][5][6][7][8][9].At present, colorimetric chemosensors with color change detected by the naked-eye are effectively applied to detect F − [10][11][12][13][14]. Nevertheless, many colorimetric F − chemosensors are based on the sensing mechanism of hydrogen bonding but usually show poor selectivity because they are susceptible to interference by basic anions such as CN − , AcO − , and H 2 PO 4 − [15][16][17][18].Researchers have found that F − has a high affinity for silicon (Si), so F − and Si atoms can form stable Si-F bonds.Based on this reactive mechanism, reactive chemosensors for sensing F − with high selectivity can be designed [19][20][21].In addition, compared with many small organic molecule-based reactive F − chemosensors, polymer-based reactive F − chemosensors have some additional advantages, such as good photo-, thermo-, and chemical stability [22,23].However, due to the relatively slow reaction rate, reactive polymeric F − chemosensors often need excessive F − and have a long response time [24].Moreover, reactive F − chemosensors often can only be used for the one-off detection of F − due to their irreversibility [19,21].It is very meaningful if a reactive F − chemosensor after use to detect F − is able to continue to detect other analytes [20].
Water is very important to our daily life, but water molecules often exist as impurities in organic solvents, so it is necessary to detect trace water in chemical reactions and industry [25,26].Due to the advantages of naked-eye detection, high sensitivity, quick response, and convenience to use in situ, colorimetric water chemosensors are often prepared to detect water [27][28][29][30][31][32][33].However, most colorimetric chemosensors for sensing water are based on small organic molecules, which often show poor photostability [33][34][35][36] and difficulty in sensing trace amounts of water [29].Recently, it has been found that polymer-based water chemosensors can effectively improve photostability and sensitivity [36,37].For example, Wang et al. [38] prepared an aggregation-induced emission (AIE) polymer containing diketopyrrolopyrrole and triphenylamine as a fluorescence sensor for sensing trace water in THF with a detection limit (DL) value of 0.005% (v/v).Zhao et al. [39] developed a ratiometric and colorimetric sensor based on hydroxyl-containing polyimide-fluoride complexes, which show high sensitivity for water detection with extremely low DL values of 0.00084% and 0.0015% in DMSO and DMF.Therefore, it is meaningful to design polymer-based water chemosensors with high sensitivity.
In this work, a tert-butyldiphenylsilyl-containing polyimide (namely, PI-OSi) was easily synthesized by a simple and efficient silicon etherification reaction of a hydroxylcontaining polyimide (namely PI-OH) with a tert-butyldiphenylsilyl chloride (TBDPSCl).The as-obtained PI-OSi contains an O-Si bond, which possesses a strong nucleophilic interaction with F − .As expected, PI-OSi is potentially designed as a reactive chemosensor for the detection of F − with high selectivity in which the specific fluoride-induced desilylation reaction led to the formation of a phenolate anion (Scheme 1).Thus, the intramolecular charge transfer (ICT) in the resulting polymer chains is obviously increased and can be analyzed by observing the changes in some photophysical properties, e.g., UV-vis absorption and fluorescence spectra.Another interesting phenomenon is that the resulting PI-OSi plus F − system (PI-OSi•F) could sensitively detect trace water in a DMF solvent because the trace water is able to protonate the phenolate anion (Scheme 1).The F − recognition behavior of the chemosensor PI-OSi and the water sensing of chemosensor PI-OSi•F were observed and examined by UV-visible spectroscopy and naked-eye detection.

Synthesis and Characterization of PI-OH and PI-OSi
The hydroxyl-containing polyimide (PI-OH) was prepared via a one-step polymerization of a diamine monomer, DMAFH, and a commercially available

Synthesis and Characterization of PI-OH and PI-OSi
The hydroxyl-containing polyimide (PI-OH) was prepared via a one-step polymerization of a diamine monomer, DMAFH, and a commercially available dianhydride, BPADA, in an NMP solution.The tert-butyldiphenylsilyl-containing polyimide (PI-OSi) was prepared via a simple and efficient silicon etherification reaction of PI-OH and TBDPSCl at room temperature.The chemical structures of PI-OH and PI-OSi are well characterized by 1 H NMR (for the corresponding spectra, see Figure S1 in the Supporting Information (SI)).

Spectral Sensing of F − by PI-OSi
PI-OSi contains an O-Si bond of O-tert-butyldiphenylsilyl (O-TBDPS), which possesses a strong nucleophilic interaction with F − .Therefore, PI-OSi is potentially designed as a reactive chemosensor for the highly selective detection of F − , in which the fluorideinduced desilylation reaction could be analyzed by observing the changes in the spectral properties and solution color.To test this, the UV-vis absorption spectrum measurement was examined first by adding incremental amounts of F − to an anhydrous DMF solution of PI-OSi (Figure 1).The UV-vis absorption spectrum of the free PI-OSi displayed a prominent absorption peak at 288 nm, accompanying a weak shoulder peak at around 322 nm.Adding the incremental amounts of F − to the DMF solution of PI-OSi results in a gradual diminishment in the intensity of the absorption peak at 288 nm, accompanying a gradual increment in the intensity of the absorption peak at 322 nm.And, an isosbestic point at 306 nm was observed, indicating that one stable species at least is present at equilibrium [40].These results originated from the F − -induced desilylation reaction of O-TBDPS in polymer chains to form the phenolate anion, which shows increased electron density compared with O-TBDPS side groups.Therefore, the ICT in the resulting polymer backbone is strongly increased, resulting in a distinct red shift in the UV-vis absorption spectrum.The functional relationship of A322/A288 values of PI-OSi with F − concentration is shown in Figure 1b.The A322/A288 value change with F − concentration (0-0.6 mM) exhibits a good linear relationship (R 2 = 0.991).The DL value of PI-OSi for sensing F − is calculated to be 2.13 μM.These results indicate that chemosensor PI-OSi is capable of serving as a ratiometric chemosensor for the highly sensitive detection of F − , which can be applied to the quantitative analysis of F − concentrations in a DMF solution.
Subsequently, the selectivity of sensor PI-OSi toward 12 different anions (100 eq.), i.e., F − , AcO − , BF4 − , Br − , Cl − , ClO4 − , CN − , H2PO4 − , HSO4 − , I − , NO3 − , PF6 − , and 5 metal cations (100 eq.), i.e., Na + , Al 3+ , Ba 2+ , Co 2+ , and Cs + , was investigated using the UV-vis absorption spectrum.It shows that no significant UV-vis absorption spectra changes were observed with the addition of these anions, except F − (Figure 2a).When 100 eq.F − was added to the DMF solution of chemosensor PI-OSi, a distinct red shift in the resulting UV-vis absorption spectrum was immediately observed.In addition, adding the abovementioned metal cations into the DMF solution of chemosensor PI-OSi could not cause any change in UV-vis absorption spectra (Figure S2 in SI).Furthermore, interference experiments were The functional relationship of A 322 /A 288 values of PI-OSi with F − concentration is shown in Figure 1b.The A 322 /A 288 value change with F − concentration (0-0.6 mM) exhibits a good linear relationship (R 2 = 0.991).The DL value of PI-OSi for sensing F − is calculated to be 2.13 µM.These results indicate that chemosensor PI-OSi is capable of serving as a ratiometric chemosensor for the highly sensitive detection of F − , which can be applied to the quantitative analysis of F − concentrations in a DMF solution.
Subsequently, the selectivity of sensor PI-OSi toward 12 different anions (100 eq.), i.e., F − , AcO − , BF 4 − , Br − , Cl − , ClO 4 − , CN − , H 2 PO 4 − , HSO 4 − , I − , NO 3 − , PF 6 − , and 5 metal cations (100 eq.), i.e., Na + , Al 3+ , Ba 2+ , Co 2+ , and Cs + , was investigated using the UV-vis absorption spectrum.It shows that no significant UV-vis absorption spectra changes were observed with the addition of these anions, except F − (Figure 2a).When 100 eq.F − was added to the DMF solution of chemosensor PI-OSi, a distinct red shift in the resulting UV-vis absorption spectrum was immediately observed.In addition, adding the abovementioned metal cations into the DMF solution of chemosensor PI-OSi could not cause any change in UV-vis absorption spectra (Figure S2 in SI).Furthermore, interference experiments were conducted in the presence of competitive anions.Adding F − to the solution of chemosensor PI-OSi in the presence of other anions led to a similar trend in UV-vis absorption spectra to that with the addition of F − only (Figure S3 in SI).As shown in Figure S3 in SI, adding F − to the solution of chemosensor PI-OSi in the presence of other anions also caused an obvious red shift in the resulting UV-vis absorption spectrum.Figure 2b shows A 322 /A 288 value changes after adding 100 eq.anions without and with F − .Obviously, the UV-vis absorption spectra response of chemosensor PI-OSi toward F − was not affected by other anions [40,41].
a good linear relationship (R = 0.991).The DL value of PI-OSi for sensing F is calculated to be 2.13 μM.These results indicate that chemosensor PI-OSi is capable of serving as a ratiometric chemosensor for the highly sensitive detection of F − , which can be applied to the quantitative analysis of F − concentrations in a DMF solution.
Subsequently, the selectivity of sensor PI-OSi toward 12 different anions (100 eq.), i.e., F − , AcO − , BF4 − , Br − , Cl − , ClO4 − , CN − , H2PO4 − , HSO4 − , I − , NO3 − , PF6 − , and 5 metal cations (100 eq.), i.e., Na + , Al 3+ , Ba 2+ , Co 2+ , and Cs + , was investigated using the UV-vis absorption spectrum.It shows that no significant UV-vis absorption spectra changes were observed with the addition of these anions, except F − (Figure 2a).When 100 eq.F − was added to the DMF solution of chemosensor PI-OSi, a distinct red shift in the resulting UV-vis absorption spectrum was immediately observed.In addition, adding the abovementioned metal cations into the DMF solution of chemosensor PI-OSi could not cause any change in UV-vis absorption spectra (Figure S2 in SI).Furthermore, interference experiments were conducted in the presence of competitive anions.Adding F − to the solution of chemosensor PI-OSi in the presence of other anions led to a similar trend in UV-vis absorption spectra to that with the addition of F − only (Figure S3 in SI).As shown in Figure S3 in SI, adding F − to the solution of chemosensor PI-OSi in the presence of other anions also caused an obvious red shift in the resulting UV-vis absorption spectrum.Figure 2b shows A322/A288 value changes after adding 100 eq.anions without and with F − .Obviously, the UV-vis absorption spectra response of chemosensor PI-OSi toward F − was not affected by other anions [40,41].

Visual Sensing of F − by PI-OSi
Visual sensing can be implemented without a precise instrument, which is more important for practical application.The sensing behavior of chemosensor PI-OSi for the sensitive and selective detection of F − was further investigated by naked-eye detection.While adding incremental amounts of F − , chemosensor PI-OSi (1 mM in DMF) progressively changed to yellow and finally to green from colorless (Figure 3a).This visible color change occurred almost immediately after adding F − to the DMF solution of chemosensor PI-OSi.Adding 1 eq.F − into the DMF solution of chemosensor PI-OSi caused an obvious color change, and the color change degree of the DMF solution of chemosensor PI-OSi varied with increasing F − concentration.These results indicate that the chemosensor PI-OSi can be conveniently used for the practical estimation of F − concentration by naked-eye detection.Fortunately, adding other anions to the DMF solution of chemosensor PI-OSi does not cause a color change (Figure 3b).The results indicate that PI-OSi is able to serve as a "naked-eye" chemosensor for the highly sensitive and selective detection of F − with a fast response time.
The distinct UV-vis absorption spectrum and color changes in the DMF solution of chemosensor PI-OSi for F − originated from a specific fluoride-induced desilylation reaction of O-TBDPS in polymer chains to form the phenolic hydroxyl anion.The asformed phenolic hydroxyl anion, once encountering trace water, could easily combine with protons to form PI-OH (Scheme 1), which led to the corresponding changes in the UV-vis absorption spectrum and the solution color.Therefore, the PI-OSi plus F − system (PI-OSi•F) may be able to be used as a chemosensor for sensing trace water in DMF.In our case, the DMF solution of chemosensor PI-OSi•F with the trace water caused an obvious blue shift in the UV-vis absorption spectra (Figure S4 in SI) and a color change (Figure S5 in SI).
In addition, adding F − into other organic solutions (such as THF, DMAc, and 1,4-dioxane) of chemosensor PI-OSi caused an obvious red shift in the UV-vis absorption spectrum (Figure S6 in SI).And, subsequently, adding trace water into the abovementioned solution of the PI-OSi plus F − system led to a blue shift in the resulting UV-vis absorption spectrum (Figure S6 in SI).Therefore, these results suggest that PI-OSi is able to sequentially detect fluoride ions and trace water in water-soluble solvents, which can dissolve PI-OSi.
important for practical application.The sensing behavior of chemosensor PI-OSi for the sensitive and selective detection of F − was further investigated by naked-eye detection.While adding incremental amounts of F − , chemosensor PI-OSi (1 mM in DMF) progressively changed to yellow and finally to green from colorless (Figure 3a).This visible color change occurred almost immediately after adding F − to the DMF solution of chemosensor PI-OSi.Adding 1 eq.F − into the DMF solution of chemosensor PI-OSi caused an obvious color change, and the color change degree of the DMF solution of chemosensor PI-OSi varied with increasing F − concentration.These results indicate that the chemosensor PI-OSi can be conveniently used for the practical estimation of F − concentration by naked-eye detection.Fortunately, adding other anions to the DMF solution of chemosensor PI-OSi does not cause a color change (Figure 3b).The results indicate that PI-OSi is able to serve as a "naked-eye" chemosensor for the highly sensitive and selective detection of F − with a fast response time.The distinct UV-vis absorption spectrum and color changes in the DMF solution of chemosensor PI-OSi for F − originated from a specific fluoride-induced desilylation reaction of O-TBDPS in polymer chains to form the phenolic hydroxyl anion.The as-formed phenolic hydroxyl anion, once encountering trace water, could easily combine with protons to form PI-OH (Scheme 1), which led to the corresponding changes in the UV-vis absorption spectrum and the solution color.Therefore, the PI-OSi plus F − system (PI-OSi•F) may be able to be used as a chemosensor for sensing trace water in DMF.In our case, the DMF solution of chemosensor PI-OSi•F with the trace water caused an obvious blue shift in the UV-vis absorption spectra (Figure S4 in SI) and a color change (Figure S5 in SI).In addition, adding F − into other organic solutions (such as THF, DMAc, and 1,4-dioxane) of chemosensor PI-OSi caused an obvious red shift in the UV-vis absorption spectrum (Figure S6 in SI).And, subsequently, adding trace water into the abovementioned solution of the PI-OSi plus F − system led to a blue shift in the resulting UV-vis absorption spectrum (Figure S6 in SI).Therefore, these results suggest that PI-OSi is able to sequentially detect fluoride ions and trace water in water-soluble solvents, which can dissolve PI-OSi.

Spectral Sensing of Water by PI-OSi•F
The change in the UV-vis absorption spectra of PI-OSi•F with the addition of increasing amounts of water in DMF was systematically investigated (Figure 4).As the water content increased, the absorption peak at 322 nm decreased gradually, accompanying a gradual increase in the absorption peak at 292 nm (Figure 4a).The A 292 /A 322 values show a good linear relationship (R 2 = 0.992) with water content in DMF up to 0.15% (v/v), implying that PI-OSi•F could be used to detect water quantitatively at a trace level (Figure 4b).On the basis of UV-vis absorption titration data, the DL value of chemosensor PI-OSi•F for sensing water in DMF was determined to be 0.00149% (v/v).

Spectral Sensing of Water by PI-OSi•F
The change in the UV-vis absorption spectra of PI-OSi•F with the addition of increasing amounts of water in DMF was systematically investigated (Figure 4).As the water content increased, the absorption peak at 322 nm decreased gradually, accompanying a gradual increase in the absorption peak at 292 nm (Figure 4a).The A292/A322 values show a good linear relationship (R 2 = 0.992) with water content in DMF up to 0.15% (v/v), implying that PI-OSi•F could be used to detect water quantitatively at a trace level (Figure 4b).On the basis of UV-vis absorption titration data, the DL value of chemosensor PI-OSi•F for sensing water in DMF was determined to be 0.00149% (v/v).

Visual Sensing of Water by PI-OSi•F
Changes in the water content in the DMF solution of PI-OSi•F (1 mM PI-OSi + 6 eq.F − ) also triggered significant variational color change (Figure 5).When the water content increased from 0% to 5.0% (v/v), the solution color of chemosensor PI-OSi•F gradually changed from light yellow-green to colorless, and the solution color change was instantaneous.Similarly, the color change degree of the DMF solution of chemosensor PI-

Visual Sensing of Water by PI-OSi•F
Changes in the water content in the DMF solution of PI-OSi•F (1 mM PI-OSi + 6 eq.F − ) also triggered significant variational color change (Figure 5).When the water content increased from 0% to 5.0% (v/v), the solution color of chemosensor PI-OSi•F gradually changed from light yellow-green to colorless, and the solution color change was instantaneous.Similarly, the color change degree of the DMF solution of chemosensor PI-OSi•F varied with higher water content, indicating that chemosensor PI-OSi•F can be conveniently used for the practical estimation of water content by naked-eye detection.These results might pave the way for building a colorimetric chemosensor for the real-time detection of trace water in DMF with high sensitivity.

Visual Sensing of Water by PI-OSi•F
Changes in the water content in the DMF solution of PI-OSi•F (1 mM PI-OSi + 6 eq.F − ) also triggered significant variational color change (Figure 5).When the water content increased from 0% to 5.0% (v/v), the solution color of chemosensor PI-OSi•F gradually changed from light yellow-green to colorless, and the solution color change was instantaneous.Similarly, the color change degree the DMF solution of chemosensor PI-OSi•F varied with higher water content, indicating that chemosensor PI-OSi•F can be conveniently used for the practical estimation of water content by naked-eye detection.These results might pave the way for building a colorimetric chemosensor for the realtime detection of trace water in DMF with high sensitivity.

Reaction Mechanism
To gain insight into the possible sensing mechanisms shown in Scheme 1, the 1 H NMR experiments were first studied, and the chemical shift values of key protons are listed in Table 1.At first, when 0.2 eq.F − was added to PI-OSi (0.5 mL, 10 mM in DMF-d7),

Reaction Mechanism
To gain insight into the possible sensing mechanisms shown in Scheme 1, the 1 H NMR experiments were first studied, and the chemical shift values of key protons are listed in Table 1.At first, when 0.2 eq.F − was added to PI-OSi (0.5 mL, 10 mM in DMF-d 7 ), the signal peaks belonging to the Si-C(CH 3 ) 3 (H l5 , see inset in Figure 6a and Table 1) of PI-OSi at 1.067 ppm moved to the upfield region (1.043 ppm, H 15 , see inset in Figure 6b and Table 1).Moreover, a multiple peak at 7.843-7.799ppm attributed to Ar-H of tertbutyldiphenylfluorosilane (H 12,13,14 , Figure 6b) appeared.Compared with the 1 H NMR spectrum of pure PI-OH (Figure 6d), the chemical shift values of aromatic protons in the PI-OSI with F − system are almost the same except for the aromatic protons H 1,2,3 being adjacent to O-TBDPS.The signal peaks of H 1,2,3 shifted to the upfield region (Figure 6b and Table 1).This phenomenon confirms the F − -induced desilylation reaction of O-TBDPS in polymer chains to form the phenolic hydroxyl anion, causing an increase in electron density compared with PI-OH.After adding trace heavy water to the PI-OSi with F − system, the signal peaks of H 1,2,3 shifted downfield (Figure 6c and Table 1).Although there were still little differences with those of pure PI-OH, the signal peaks of H 2,3 were consistent with the 1 H NMR spectrum of PI-OH with heavy water (Figure 6e and Table 1).This suggests that the phenolic hydroxyl anion was protonated to the generation of PI-OH in the presence of trace water.The signal peak of the hydroxyl group in the 1 H NMR spectrum of PI-OSi•F with water (Figure 6c) did not appear because of the rapid chemical exchange with D 2 O, which agrees with the 1 H NMR spectrum of pure PI-OH with D 2 O (Figure 6e).The above 1 H NMR experiments can be explained by the chemical structure of the sensor systems changing when the F − and subsequent water were added.To further understand the mechanism of the UV-vis absorption spectra response, a DFT calculation optimized at the B3LYP/6-31G theoretical level was performed.The simplified unit structures of polymers as the model compounds were used to optimize structure geometries (Figure S7 in SI).The calculated energy of the highest occupied molecular orbital (E HOMO ) and the lowest unoccupied molecular orbital (E LUMO ) and their energy gaps (∆E) of MOSi, MO − , and MOH are shown in Figure 7.The E HOMO value of MO − (1.594 eV) is much higher than that of MOSi (−5.043 eV), indicating that the fluoride-induced desilylation reaction of O-TBDPS in polymer chains that forms the phenolic hydroxyl anion enhances the electron-donating ability of diamine fragments.In addition, the ∆E values of MO − (0.273 eV) are lower than those of MOSi (2.903 eV), meaning that the ICT in MO − is significantly increased compared with that in MOSi.This phenomenon can explain how adding F − to PI-OSi led to the red shift of the resulting UV-vis absorption spectrum.For MOH, the E HOMO and ∆E values are about the same as those for MOSi, indicating that the degree of ICT in MOH is almost equal to that in MOSi and much lower than in MO − .This phenomenon can explain the UV-vis absorption spectrum of chemosensor PI-OSi•F after the addition of water being blue shifted to a similar state to that of PI-OSi.

Portable Test Polyimide Film-Based Chemosensor Device
Polymeric film materials can directly act as a portable chemosensor device based on colorimetric detection with many advantages, such as its handy, durable, easily operated, and eco-friendly nature [39,40].Polyimides (PIs) possess superior film-forming performance.PI films, with their excellent integrative properties, are effectively employed even in extreme conditions.Therefore, PI materials are excellent candidates for the design and fabrication of polymer film-based chemosensor devices.PI-OSi film is insoluble in some water-miscible solvents, such as CH 3 CN, which provides potential for using PI-OSi films to achieve F − and water detection in CH 3 CN.Thus, the possibilities of using a PI-OSi filmbased chemosensor device for detecting F − and water in CH 3 CN were explored.As shown in Figure 8, the UV-vis absorbance intensity of PI-OSi film after immersion in a dry CH 3 CN solution containing 10 −2 M of F − was dramatically increased at visible wavelengths.And, the color of PI-OSi film changed accordingly to yellow-green from pale yellow (see inset in Figure 8).However, almost no UV-vis absorbance intensity or color changes were observed after immersing PI-OSi film into a wet CH 3 CN solution containing 10 −2 M of F − .These results suggest that water can affect the interaction between PI-OSi film and F − .Therefore, a colorimetric detection of F − and water in CH 3 CN by PI-OSi film may be achievable.
sensor systems changing when the F − and subsequent water were added.To further understand the mechanism of the UV-vis absorption spectra response, a DFT calculation optimized at the B3LYP/6-31G theoretical level was performed.The simplified unit structures of polymers as the model compounds were used to optimize structure geometries (Figure S7 in SI).The calculated energy of the highest occupied molecular orbital (EHOMO) and the lowest unoccupied molecular orbital (ELUMO) and their energy gaps (ΔE) of MOSi, MO − , and MOH are shown in Figure 7.The EHOMO value of MO − (1.594 eV) is much higher than that of MOSi (−5.043 eV), indicating that the fluoride-induced desilylation reaction of O-TBDPS in polymer chains that forms the phenolic hydroxyl anion enhances the electron-donating ability of diamine fragments.In addition, the ΔE values of MO − (0.273 eV) are lower than those of MOSi (2.903 eV), meaning that the ICT in MO − is significantly increased compared with that in MOSi.This phenomenon can explain how adding F − to PI-OSi led to the red shift of the resulting UV-vis absorption spectrum.For MOH, the EHOMO and ΔE values are about the same as those for MOSi, indicating that the degree of ICT in MOH is almost equal to that in MOSi and much lower than in MO − .This phenomenon can explain the UV-vis absorption spectrum of chemosensor PI-OSi•F after the addition of water being blue shifted to a similar state to that of PI-OSi.

Portable Test Polyimide Film-Based Chemosensor Device
Polymeric film materials can directly act as a portable chemosensor device based on colorimetric detection with many advantages, such as its handy, durable, easily operated, and eco-friendly nature [39,40].Polyimides (PIs) possess superior film-forming performance.PI films, with their excellent integrative properties, are effectively employed even in extreme conditions.Therefore, PI materials are excellent candidates for the design and fabrication of polymer film-based chemosensor devices.PI-OSi film is insoluble in some water-miscible solvents, such as CH3CN, which provides potential for using PI-OSi films to achieve F − and water detection in CH3CN.Thus, the possibilities of using a PI-OSi film-based chemosensor device for detecting F − and water in CH3CN were explored.As shown in Figure 8, the UV-vis absorbance intensity of PI-OSi film after immersion in a dry And, the color of PI-OSi film changed accordingly to yellow-green from pale yellow (see inset in Figure 8).However, almost no UV-vis absorbance intensity or color changes were observed after immersing PI-OSi film into a wet CH3CN solution containing 10 −2 M of F − .These results suggest that water can affect the interaction between PI-OSi film and F − .Therefore, a colorimetric detection of F − and water in CH3CN by PI-OSi film may be achievable.
The UV-vis absorption experiments were measured on a Shimadzu UV-2700 spectrophotometer (Kyoto, Japan). 1 H-NMR experiments were carried out on a Bruker Avance DRX500 NMR Spectrometer in DMF-d 7 (Billerica, MA, USA), as the solvent and its residual peak were used as an internal reference.

General Procedure for UV-vis Absorption Spectra Measurements
PI-OSi was dissolved in extra-dry DMF to acquire 1 mM (calculated in terms of the monomeric units) stock solution, which was further diluted to 10 µM for the F − sensing measurements.The solutions of TBA + salts (1 mM) were prepared by dissolving them in extra-dry DMF.
Deionized water was utilized for the water titration study.The increasing water content was added to the solutions of the PI-OSi plus F − system in extra-dry DMF to create the DMF aqueous solutions with the required water content.
All of the above test solutions (2 mL) were placed in a quartz cell for the UV-vis absorption spectra measurements.

Calculation Method for Detection Limit (DL)
On the basis of UV-vis absorption titration data, the DL value was calculated by using the formula DL = 3σ/k [43].Herein, σ is the standard deviation of a blank, and k is the slope of the calibration line.

Synthesis of PI-OSi
A round-bottom flask was charged with PI-OH resin (1.842 g; 2 mmol), TBDPSCl (0.904 g; 6 mmol), and imidazole (0.408 g; 6 mmol) in 20 mL of DMF.The reaction mixture was stirred at room temperature under N 2 for 24 h.After the reaction, the homogeneous solution was slowly poured into stirred ethanol (400 mL) to afford a fibrous precipitate, which was washed thoroughly with ethanol and dried in a vacuum oven at 80 • C. Finally, the crude PI-OSi precipitate was reprecipitated by CHCl 3 -ethanol to give pure PI-OSi precipitate.

Preparation of PI-OSi Film
The DMAc solution was made by dissolving PI-OSi with the solid content of 15 wt.%.The abovementioned polymer solution was poured into the glass substrate, which was heated in vacuo at 50 • C for 12 h to remove most of the solvent; then, the semi-dried film was further dried at 100 • C for 3 h, at 150 • C for 3 h, and at 200 • C for 2 h to thoroughly remove the residual DMAc solvent.The obtained PI-OSi film was about 50 µm thick.

Conclusions
In this work, we have conveniently synthesized a tert-butyldiphenylsilyl-containing polyimide (PI-OSi) as a colorimetric and ratiometric chemosensor for rapid and highly sensitive and selective detection of F − by employing fluoride-induced cleavage of O-Si bonds.The chemosensor PI-OSi shows a highly selective response toward F − over other tested anions via a red shift of the UV-vis absorption spectrum and solution color changes in DMF with a detection limit (DL) value of 2.13 µM.Intriguingly, the prepared PI-OSi plus F − system (PI-OSi•F) effectively detected the water content in the DMF solution, exhibiting a blue shift of the UV-vis absorption spectrum and a solution color change.The DL value of chemosensor PI-OSi•F for sensing trace water in DMF is as low as 0.00149% (v/v).The water-sensing mechanism was the F − -induced desilylation reaction of PI-OSi that formed the phenolic hydroxyl anion being protonated to generate PI-OH in the presence of trace water.In addition, PI-OSi film is directly used to visually detect F − and water in CH 3 CN, which can serve as a handy polymer film-based chemosensor device for the detection of F − and water in CH 3 CN.This work provides a novel design strategy to construct a polymerbased chemosensor for the rapid, highly selective and sensitive, and visible detection of F − as well as trace water.

Molecules 2023 , 13 Scheme 1 .
Scheme 1.The structures and sensing mechanisms of PI-OSi for F − and PI-OSi•F for trace water in DMF.

Scheme 1 .
Scheme 1.The structures and sensing mechanisms of PI-OSi for F − and PI-OSi•F for trace water in DMF.

Figure 8 .
Figure 8. UV-vis absorption spectral changes in PI-OSi film upon immersion in dry and wet CH3CN solutions containing 10 −2 M of F − .(Inset: color changes in PI-OSi film.)

Figure 8 .
Figure 8. UV-vis absorption spectral changes in PI-OSi film upon immersion in dry and wet CH 3 CN solutions containing 10 −2 M of F − .(Inset: color changes in PI-OSi film).

Table 1 .
Summary of the chemical shift values of key protons a .
a All spectra were acquired on a 600 MHz NMR; solvent: DMF-d 7.a All spectra were acquired on a 600 MHz NMR; solvent: DMF-d7.