Cyclic siloxanes conjugated with fluorescent aromatic compounds as fluoride sensors

When cyclic siloxanes were exposed to fluoride ions, a blue-shift and enhanced fluorescence emissions appeared in most organic solvents.


Introduction
Polysiloxane (R 2 SiO) n can be found as major components in commercial products in our daily life such as scented hydrogels, silicon ware, household products, personal care products, electrical devices and cookware. 1,2 They are described as organosilicon compounds that include silicon-oxygen backbones with organic side chains attached to each silicon atom. [3][4][5] Flexible siloxane polymers or silicones have been extensively used due to its high thermal stability, chemical resistance, low tension, smooth texture, and non-toxicity and biocompatibility of compounds depending on functionalization. [6][7][8][9] Siloxanes have two general forms: cyclic and linear. 10 For cyclic siloxanes, silicon and oxygen atoms bond and connect at each end to form a ring structure. 4,11 Cyclic siloxanes are used in a wide range of applications due to the hydrocarbon side chain of siloxane, which is beneficial for side-chain functionalization. In terms of synthesis, cyclic siloxanes are frequently used as precursors for the synthesis of high performance polysiloxane elastomers, 12,13 and as cross-linking agents to strengthen materials. [14][15][16] Recently, polysiloxanes and cyclic siloxanes functionalized with fluorophores have exhibited excellent sensing ability to detect specific chemical species. For example, Ren et al. reported tetraphenylethene units modified into polysiloxane chains possessing fluorescence ability and capable of detecting the vapor-phase of explosive compounds. 17 Lang et al. successfully synthesized a siloxane polymer containing fluoranthene groups that showed high thermal stability and strong fluorescence emission that can be quenched by picric acid detection. 18 Polydimethylsiloxane bearing spirobenzopyran, which acted as a colorimetric probe for pH determination and Ag + and Fe 3+ ion detection, was reported by Li and co-workers. 19 Recently, Lin et al. studied pyrenyl-functionalized cyclic siloxane and polysiloxane for nitroaromatic compound detection. 20 Along this line, the development of anion sensors from cyclic siloxanes would be very intriguing and challenging. In the meantime, a lot of molecular interactions were proposed for designing probes, including hydrogen bonding, 21,22 electrostatic interaction, 23 deprotonation, 24,25 nucleophilic substitution 26 and Si-O bond cleavage. 27,28 In the last two decades, the usage of organosilica-based cage materials has expanded to cover many fields of applications such as catalytic supports, polymers, biocompatible materials and sensors. 29 Previously, we have reported fluorescent organosilica-based cage materials for both efficient fluoride detection and adsorption. [30][31][32] The principle of electrostatic interaction between fluoride ions and the potential surface of organosilica cages, which plays a role in the attraction of positive and negative parts to each other, in those materials has been proposed. 33 Those responsive changes in fluorescence emission involve the conversion of an exciplex signal to a monomer signal due to F À induced conformational changes of the cage. 34 Intriguingly, the less rigid cyclic siloxanes, in comparison with the cage siloxanes, are worth exploring as they are more common starting materials for silicone and gel materials. Herein, two cyclic siloxanes conjugated with anthracene (D 4 An) and pyrene (D 4 Py) were successfully synthesized with high fluorescence emission. UV-vis absorption and fluorescence spectroscopy were used to investigate their absorption and emission. Furthermore, the interactions of fluoride and cyclic siloxanes were supported by computations as well as FTIR and 19 F NMR spectroscopy. Quantum calculations were performed to propose the optimized geometry as well as changes in the conformation of the fluorophores. This work aims to demonstrate the application of cyclic siloxanes as an anchoring platform for anion sensors.

Results and discussion
Synthesis and solvent effects of cyclic siloxanes (D 4 An and D 4 Py) Compounds of D 4 An and D 4 Py were synthesized via Heck coupling reaction from 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane and 9-bromoanthracene or 1-bromopyrene as precursors (Scheme 1). A dark-orange powder of D 4 An and yellow powder of D 4 Py were obtained in 72% and 87% yields, respectively (Fig. S1, ESI †). The solid state 1 H MAS NMR spectrum of D 4 An shows a broad peak at 6.28 ppm and sharp peaks in the range of 0.02-1.29 ppm, which indicate the presence of aromatic and methyl groups, respectively. For D 4 Py, a broad peak of pyrene appeared at 5.97 ppm while a sharp peak of the methyl group was found at 0.03 ppm (Fig. S2, ESI †). The solid state 29 Si MAS NMR spectra of D 4 An and D 4 Py only showed a singlet peak at À32 and À30 ppm, respectively ( It is well-known that anthracene and pyrene monomers possess solvent-dependent properties where solvent polarity could induce different ratios of monomer and excimer emissions. [35][36][37][38] The fluorescence intensity of both 9-bromoanthracene and 1-bromopyrene slightly increased with the increase of solvent polarity while their fluorescence intensity showed two or three sharp peaks ( Fig. 1a and b), confirming the presence of monomer emission. However, small broad peaks at longer wavelengths were observed, attributed to intermolecular excimer formation. 39,40 Confirming the successful synthesis of cyclic siloxanes conjugated with fluorescent aromatic compounds, the spectra of both cyclic siloxanes in organic solvents appeared broader and showed a red-shift of fluorescence emissions at longer wavelengths (450-600 nm) ( Fig. 1c and d). This result indicated significant intramolecular exciplex formation among the fluorophore units within the cyclic siloxanes; 41 at the same time, the largest Stokes shift (Dl = 146 nm) from the absorption band at 350 nm to the excimer emission band at 496 nm of D 4 Py was observed in DMSO as a representative of highly polar solvents (Fig. 1d). In slightly polar solvents, the exciplex emissions of D 4 Py showed a shorter Stokes shift (126 nm for toluene and THF). Therefore, intramolecular exciplex formation within cyclic siloxanes of both D 4 An and D 4 Py is favourable in polar organic solvents. In comparison with D 4 Py, all D 4 An solutions gave bright blue emission and a shorter Stokes shift of 68-73 nm, measured using the differences between an absorption peak at 376-379 nm and a peak of excimer emission at 446-452 nm (Fig. 1c). It was suggested that p-p interactions in anthracene units within cyclic siloxanes are weaker than those within pyrenyl siloxanes. This suggestion can also be confirmed by the solid-state fluorescence spectra of D 4 An and D 4 Py that showed broad spectra with only single peaks at 477 and 532 nm, respectively ( Fig. 1e and f).

Selectivity test
The fluorescence emission patterns of D 4 An and D 4 Py before and after the addition of anions were investigated in four solvents (e.g. toluene, THF, DMF and DMSO). When 1 equiv. of tetrabutylammonium salts (TBA-X: X = F À , Cl À , Br À , CN À , ClO 4 À , NO 3 À and PO 4

3À
) was added into both D 4 An and D 4 Py solutions except in DMSO where 10 equiv. was added, only F À can significantly enhance the fluorescence intensity of the monomer emission ( Fig. 2) with a deep blue color in all solvents (Fig. 3a). For example, upon addition of F À into D 4 Py solutions, the monomer emissions (l max B 395 nm) increased while the intramolecular exciplex emissions (l max B 480 nm) decreased (Fig. 2). This result confirmed that F À was involved in the reorganization and orientation of the pyrene units. The green emission of the D 4 Py solution changed to deep blue emission after adding fluoride, as shown in Fig. 3a, which also supported the separation of the intramolecular pyrene-pyrene excimer into a single monomer. Moreover, after the addition of excess anions (200 equiv.) into both cyclic siloxanes in all solvents, these compounds additionally responded to CN À and PO 4 3À ( Fig. S4 and S5, ESI †). The fluorescence emissions of D 4 An and D 4 Py could also be intensified in highly polar solvents (DMF and DMSO) after the addition of F À , CN À and PO 4 3À . Nonetheless, the fluorescence emissions of D 4 Py in THF with a large excess of F À were completely quenched (Fig. S5, ESI †). Furthermore, only F À (200 equiv.) could induce naked-eye color changes of D 4 An and D 4 Py solutions in THF from colorless to pink and orange, respectively (Fig. 3b). These results suggested charge-transfer (CT) complexation between F À and the fluorophores via anion-p interaction. 31 Notably, single fluorophore compounds such as 1-bromopyrene and 9bromoanthracene did not show any color change suggesting that the anchoring of multiple fluorophores within the same molecule was necessary to promote this interaction.

Kinetic study
The fluorescence responses of D 4 An and D 4 Py after adding 200 equiv. of F À , CN À , and PO 4 3À were measured at a fixed excitation wavelength for 30 minutes. For F À , the change in fluorescence signal occurred and completed almost instantly in DMF. In contrast, the response was slower in slightly polar solvents like toluene, as shown in Fig. S6 (ESI †). For CN À , D 4 An responses were slower in all solvents, but a faster response was seen in slightly polar solvents for D 4 Py. In addition, PO 4 3À caused a fast kinetic response of D 4 Py in highly polar solvents (DMSO and DMF), but a similar rate in all solvents for D 4 An.
The pseudo-first order kinetic rate constants of the three ions were calculated and displayed in Table S1 (ESI †).

Quantitative analyses of fluoride
An optimized concentration of D 4 An at 2 Â 10 À5 M was used in fluorescence titration for quantitative analysis of F À . The fluorescence emission of D 4 An in THF displayed a peak maximum at 427 nm; after the addition of F À into the D 4 An solution, the peak shifted to 420 nm, and it then increased when the concentration of F À was increased (Fig. 4a). The same trend of emission spectra was observed in toluene, DMF and DMSO, as shown in Fig. S7 (ESI †). The association constant (K a ) was calculated based on Benesi-Hildebrand plots from titration spectra. The K a values were 4.9, 11.8, 2.  Table S2 (ESI †).

Proposed mechanism of fluoride interaction
Since D 4 An and D 4 Py are structures of cyclic siloxanes conjugated with a polyaromatic group, both compounds exhibited a positive contour within the cyclic siloxanes, which provides good possibilities for attracting F À . 33 After the addition of F À in the system, the Si-F bond can change the conformation of fluorophores on cyclic siloxanes, the destruction of excimers was observed from the fluorescence emission spectra (Fig. 4a and c) and then the cyclic siloxanes were cleaved. The FTIR results showed that D 4 An and D 4 Py exhibited the asymmetric stretching vibration of Si-O-Si at 1025 cm À1 . Nevertheless, a broad peak at around 3378 cm À1 (-OH) and a sharp peak at 882 cm À1 (Si-OH) appeared after the addition of fluoride (1 equiv.) into both D 4 An and D 4 Py (Fig. S8, ESI †). These results confirmed that the cyclic siloxane was cleaved by fluoride ions. Moreover, the 19 F NMR titration experiment in CDCl 3 showed free TBAF at À112 ppm. The signal appeared at À124 ppm represented the free F À on a glass tube (SiF 6 2À ). 42,43 Upon the addition of fluoride into D 4 An and D 4 Py solutions, new peaks appeared at À129 ppm, indicating the interaction between the cyclic siloxane and F À (Fig. S9 and S10, ESI †). Interestingly, their solid state 29 Si MAS NMR spectra after the addition of F À showed new smaller peaks at À55 and À64 ppm for D 4 An, while the peak of D 4 Py almost completely shifted from À30 to À66 ppm. These NMR results suggested that the Si-O linkages of cyclic siloxanes were not only cleaved using F À , but some Si-C bonds were also broken and rearranged to form silsesquioxanes (T unit) of silicon (Fig. 5). 44,45 Furthermore, the ESI-MS results of D 4 An and D 4 Py with fluoride also showed small fractions of cyclic siloxane caused by fluoride ions (Fig. S11 and S12, ESI †). These results also validated the fact

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UV-vis spectroscopy
In the UV-vis spectra (Fig. S13, ESI †), the absorption spectrum of D 4 An showed three major peaks at 360, 379 and 391 nm while that of D 4 Py also showed three peaks at 337, 350 and 370 nm. The absorption spectra of D 4 An and D 4 Py in the presence of various anions were studied in an excessive amount (200 equiv.) in THF solvent, as shown in Fig. S14 (ESI †). The result suggested that D 4 An exhibited a new absorption peak at 510 nm while peaks at 379 and 391 nm dropped in the presence of F À , and a clear isosbestic point at 415 nm was observed. The absorption spectra of D 4 Py at 350 and 368 nm plummeted whereas a new peak at 493 nm increased with the isosbestic point at 382 nm after the addition of F À (Fig. 4b); however, the other anions did not significantly cause a change to the spectral pattern (Fig. S14, ESI †). The new red-shift band along with the isosbestic point indicated a conversion of the fluorophore to its CT complex with F À . Moreover, the association constant (K a ) values for the CT complex formation of D 4 Py and D 4 An were 1.2 Â 10 3 and 394 M À1 , respectively. The detection limits of F À obtained via UV-vis absorption titration for D 4 An and D 4 Py were 3.6 Â 10 À5 and 1.6 Â 10 À5 M, respectively. The association constant (K a ), LOD and LOQ of D 4 An and D 4 Py with fluoride in THF are summarized in Table S2 (ESI †). These results indicated that D 4 Py is more sensitive for F À detection in comparison with D 4 An, which is consistent with the fluorescence titration data described earlier.

Computational study
Quantum calculations were performed to investigate the changes in fluorescence emission upon the formation of the D 4 An-F À model, which has been speculated as an intermediate before the cleavage of the D 4 ring. D 4 isomerism basically has four major isomers, which are all-cis, all-trans, cis-trans-cis and cis-cis-trans (Fig. S21, ESI †). 46 After geometrical optimization, all-cis and alltrans isomers were the most stable species due to their lower energy compared to the rest (Table S3, ESI †); therefore, all-cis and all-trans isomers of D 4 An were chosen for further study. The photon absorption and fluorescence of all-cis and all-trans D 4 An are shown in Fig. 6, and the obtained results were in agreement with our previous report. 31 A blue shift of fluorescence wavelength was found in both structures evidently from the different conformations of fluorophores caused by F À . 47 The pristine formation of anthracene was dominated by graphite-like stacking. When D 4 interacted with fluoride, the formation of stacking anthracene changed to slipped-parallel and single monomers in the all-cis and all-trans isomers of D 4 An, respectively. Therefore, the quantum calculations can well explain the fluorescence wavelength changes after fluoride addition. bonds could also be broken and rearranged to form the silsesquioxane (T unit) network of silicon. The host-guest interaction was also studied through computational modelling; intramolecular excimer formation within cyclic siloxanes can be interrupted by fluoride ions leading to monomer formation causing a blue-shift of fluorescence emissions. Therefore, cyclic siloxanes can be one of the promising material candidates for F À sensors in organic solvents.

Fluorescence and absorption studies of D 4 An and D 4 Py with anions
The stock solutions of anions (e.g. F À , Cl À , Br À , CN À , ClO 4

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Open Access Article. Published on 11 November 2020. Downloaded on 4/28/2021 6:09:01 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.