Photoresponsive spiro-polymers generated in situ by C–H-activated polyspiroannulation

The development of facile and efficient polymerizations toward functional polymers with unique structures and attractive properties is of great academic and industrial significance. Here we develop a straightforward C–H-activated polyspiroannulation route to in situ generate photoresponsive spiro-polymers with complex structures. The palladium(II)-catalyzed stepwise polyspiroannulations of free naphthols and internal diynes proceed efficiently in dimethylsulfoxide at 120 °C without the constraint of apparent stoichiometric balance in monomers. A series of functional polymers with multisubstituted spiro-segments and absolute molecular weights of up to 39,000 are produced in high yields (up to 99%). The obtained spiro-polymers can be readily fabricated into different well-resolved fluorescent photopatterns with both turn-off and turn-on modes based on their photoinduced fluorescence change. Taking advantage of their photoresponsive refractive index, we successfully apply the polymer thin films in integrated silicon photonics techniques and achieve the permanent modification of resonance wavelengths of microring resonators by UV irradiation.

Mercury Arc Lamp at a distance of 25 cm. The incident light intensity was ∼18.5 mW cm −2 and the applied power of the Mercury Arc Lamp was 180 W. The photos of the generated photopatterns were taken under normal room light and 330380 nm UV illumination using a Nikon Eclipse 80i fluorescent microscope.
Monomer synthesis. Monomer 2d was synthesized by the route as shown in Supplementary  Figure 1. 4,4'-(2,2-Diphenylethene-1,1-diyl)bis(bromobenzene) (TPE-2Br) was synthesized according to a previous report. 3 To a 250 mL two-necked round-bottom flask were added Pd(PPh)3Cl2 (700 mg, 1.0 mmol), CuI (381 mg, 2.0 mmol), PPh3 (525 mg, 2.0 mmol), TPE-2Br (4.9 g, 10.0 mmol), and a solvent mixture of THF/triethylamine (30 mL/60 mL) under a nitrogen atmosphere. After the solid substrates were completely dissolved, phenylacetylene (4.39 mL, 40.0 mmol) was then injected into the flask through a syringe under stirring and the reaction mixture was heated to 80 °C. After refluxing overnight, the reaction mixture was cooled down to room temperature. The formed solid was removed by filtration and washed with THF for several times. The filtrate was dried by blowing with condensed air and then extracted with dichloromethane. The organic layers were combined and dried over anhydrous sodium sulfate. After filtration and solvent evaporation, the resulting crude product was purified on a silica gel column chromatography using hexane/ethyl acetate as the eluent. Pure product was obtained as a pale yellow solid; yield: 73.5%. 1

Model reaction.
A 50 mL Schlenk tube equipped with a stirring bar was charged with 2naphthol (1a, 288.3 mg, 2.0 mmol), diphenylacetylene (3, 2.23 g, 5.0 mmol), Pd(OAc)2 (22.5 mg, 0.1 mmol), Cu(OAc)2•H2O (838.5 mg, 4.2 mmol), and K2CO3 (552.8 mg, 4.0 mmol), and then sealed with a rubber stopper. After evacuated under vacuum and purged with dry nitrogen for three times, the tube was injected with 20 mL DMSO. The reaction mixture was heated at 120 °C for 48 h, and then cooled down to room temperature. Water was then added to the mixture, followed by sequential extraction with ethyl acetate and drying over anhydrous MgSO4. After solvent evaporation, the crude product was subjected to a silica-gel column using hexane/ethyl acetate mixture (10:1, v/v) as eluent.

Analysis of the filtrate composition.
To investigate what happened for the excess 1a, the filtrate was purified and analyzed after the filtration treatment process of the polymerization between 2-naphthol (173.0 mg, 1.2 mmol) and internal diyne 2a (141.2 mg, 0.3 mmol). Two pure compounds were isolated from the precipitant mixture by evaporation of the solvents in vacuo and the subsequent column chromatography on silica gel using DCM as eluent. The structures of the isolated products were verified to be 2-naphthol and 1,1-bi-2naphthol, respectively, by HRMS, 1 H NMR and 13 C NMR ( Supplementary Figures 4-11). The recovery yield of 2-naphthol (59.0 mg) was calculated to be 46% and the yield of 1,1-bi-2naphthol (11.7 mg) was determined to be 9%.
Polymerization of telechelic polymer. Into a 10 mL Schlenk tube equipped with a stirring bar was placed with excess 2-naphthol (45.3 mg), telechelic polymer P1a/2a (55 mg, 14.5 μmol), Pd(OAc)2 (3.5 mg, 2.9 μmol), Cu(OAc)2•H2O (65.9 mg, 60.9 μmol), and K2CO3 (43.4 mg, 58.0 μmol) in 1 mL DMSO. The reaction mixture was stirred under nitrogen at 120 °C for 24 h and then cooled to room temperature. The resulting mixture was first dissolved with THF and centrifuged for several times. Then the supernatant solution was passed through a simple column filled with neutral Al2O3 powder and added dropwise to 160-mL hexane/chloroform mixture (7:1 v/v) under vigorous stirring. The precipitates were collected by filtration, and then washed with hexane and dried in vacuum at room temperature to a constant weight. A light yellow solid of extended P1a/2a was obtained in a yield of 95%. Mn = 9,600; Mw = 24,100; Mw/Mn = 2.5 (GPC, polystyrene calibration). Polymer P1ac/2a and P1ad/2a was prepared in similar procedures by using excess 6-methoxyl-2-naphthol (54.7 mg) and 6-benzoyl-2naphthol (78.0 mg) as the co-monomer to react with telechelic polymer P1a/2a (55.0 mg). The characterization data of P1ac/2a and P1ad/2a were given as follows: Reduction reaction of 4. A 100 mL, two-necked, round-bottomed flask fitted with a stirring bar was charged with 10 mL of anhydrous THF and 20 mg LiAlH4 (0.5 mmol) under a nitrogen atmosphere at an ice-water bath. Model compound 4 (200 mg, 0.4 mmol) in anhydrous THF (2 mL) was then added dropwise via a syringe to the cooled suspension of LiAlH4. The mixture was kept stirring at 0 C. The reaction was completed after about 30 min as monitored by the thin-layer chromatography. Afterward, the reaction was quenched by dropwise addition of saturated NH4Cl aqueous solution. The mixture was extracted with DCM three times and the organic layers were combined and dried over anhydrous Na2SO4. Reduction reaction of P1a/2a. A 100 mL, two-necked, round-bottomed flask fitted with a stirring bar was charged with 10 mL of anhydrous THF and 10 mg LiAlH4 (0.26 mmol) under a nitrogen atmosphere at an ice-water bath. P1a/2a (92.7 mg, 0.15 mmol) in anhydrous THF (3 mL) was then added dropwise via a syringe to the cooled suspension of LiAlH4. The mixture was kept stirring at 0 C. The reaction was stopped after about 30 min. Afterward, the reaction was quenched by dropwise addition of saturated NH4Cl aqueous solution. The mixture was extracted with DCM three times and the organic layers were combined and dried over anhydrous Na2SO4. The solvent was then removed in vacuo to give the reduced product P7. Photonic device experiments and simulations. The Si3N4 devices used in this work were fabricated on a silicon chip in the Nanosystem Fabrication Facility (NFF) of The Hong Kong University of Science and Technology. We deposited a Si3N4 film by low-pressure chemicalvapor deposition (LPCVD) in two consecutive runs with a total thickness of ~0.82 μm on an oxide layer of ~4 mm thick. We measured the refractive index of the Si3N4 film to be ~1.96 at 1550 nm wavelength using ellipsometry. We defined the device pattern on the Si3N4 layer by i-line (365 nm) photolithography and inductively coupled plasma (ICP) etching (STS ICP DRIE silicon etcher). The polymer film of P1a/2a was spin-coated on the devices at a spin speed of 1000 r/min for 1 min.
We designed the six racetrack microring resonators under test with the same round-trip lengths of 190π μm but different waveguide-resonator coupling gap widths (D1-D3: 0.4 μm; D4-D6: 0.5 μm) and coupling lengths (D1, D6: 9.4 μm; D2, D5: 6.3 μm; D3, D4: 3.1 μm). The photo-irradiation process was conducted in air at room temperature using UV light from an Oriel Mercury Arc Lamp at a distance of 25 cm. The incident light intensity was ∼18.5 mW cm −2 and the applied power of the Mercury Arc Lamp was 180 W. For transmission spectra measurements, we employed a wavelength-tunable laser (Santec TSL-510) in the 1550 nm wavelength range. The laser was operated at room temperature. We used a lensed polarizationmaintaining single-mode optical fiber to input-couple light by end-firing into the waveguide tapered end-face of ~4 mm width. The optical power from the lensed fiber output before coupling to the chip was ~1 mW. We used a long-working-distance microscope objective lens with a numerical aperture of 0.55 to output-couple light from the chip. Typical insertion loss was 10-15 dB depending on the alignment. The results and analysis for TE polarization are shown in Supplementary Figures 54-57.
We employed finite-element method (FEM) to calculate the effective refractive index of polymer-coated Si3N4 waveguides using the commercial modeling software COMSOL Multiphysics. We adopted the Si3N4 waveguide core and the silica bottom-cladding layer with refractive indices of 1.96 and 1.44, respectively. We adopted the waveguide width and height to be 1.2 μm and 0.82 μm, respectively. We assumed a uniform polymer thickness on the waveguide top and side surfaces for simplicity, although the polymer thickness on the waveguide side surfaces are typically thinner in practice. The resonance wavelength of a microring is determined by the phase-matching condition: neffL = mλm, where L is the roundtrip length of the microring, m is the integral mode number, and λm is the m-th order resonance wavelength in a vacuum. Therefore, the resonance wavelength λres is sensitive to neff.
In order to better correlate the observed blue-shift (∆λ) of resonance wavelength to the UVinduced polymer refractive index change (∆n), we prepared several polymer films coated on unpatterned silicon chips using the same spin-coating conditions as those coated on the microring devices. Then we measured the refractive index of these polymer films on unpatterned silicon chips upon different UV exposure durations. The corresponding change in the n value at 1550 nm is shown in Figure 7g.
The measured |∆λ/λres| was compared with the FEM-simulated results, where we use the measured n and ∆n of the polymer film to calculate the neff and ∆neff and assume that ∆λ/λres = ∆neff/neff by following the phase-matching condition. The polymer thickness coated on the devices was estimated by finding the closest match between the experimental data of |∆λ/λres| of the six devices and the simulation results with different polymer thickness. For each device, two estimated polymer thickness values are obtained based on TM and TE polarizations. The estimation based on TM and TE polarization is sensitive to the polymer thickness on the waveguide top surface and the waveguide sidewall, respectively.

Supplementary Tables
Supplementary Table 1   a The polymer thickness coated on the devices was estimated by finding the closest match between the experimental data of the six devices and the FEM-simulated results with different polymer thickness. The measured |∆λ/λres| is compared with the simulation results, where the measured n and ∆n of the polymer film is used to calculate the neff and ∆neff and ∆λ/λres = ∆neff/neff is assumed according to the phase-matching condition. For each device, the estimated value from TM and TE polarization is set as the upper and lower limit, respectively. The results indicate that the uncertainty for the estimated polymer thickness is 30 nm.          Table 1, entry 2) in CD2Cl2.

Supplementary Discussion
Mechanistic study on the UV-responsive refractive index. It is known that refractive index (RI) can be influenced by the polarizability of the components composing the polymer repeating unit and the presence of highly polarizable π-conjugated functionalities are beneficial for increasing the refractive indices of polymers. 5 Therefore, the decrease in electronic conjugation could result in the decrease in RI. For example, the RI of P7 is obviously lower than that of P1a/2a (Supplementary Table 6 and Supplementary Figure 41), which indicated that the conversion of the unsaturated C=C to saturated C-C can lead to a decrease in RI due to the decreased electronic conjugation.
There are multiple reactive sites in the multisubstituted spirocyclic polymer structures that could be photo-oxidized under strong UV irradiation, which will reduce the electronic conjugation and thus lead to the decrease in RI. The absorption spectrum of P1a/2a were then measured before and after UV irradiation. Supplementary Figure 42A suggested that the strong UV irradiation could indeed lead to a decrease in the electronic conjugation of P1a/2a. Due to the intrinsic complexity and polydispersity of polymers, the UV responsiveness of model compound 4 was also investigated to assist the mechanistic understanding. The absorption spectra of its thin films onto quartz plates were measured before and after UV irradiation. As shown in Supplementary Figure 42B, the absorption spectrum significantly changed after UV irradiation. The absorption peak at 318 nm disappeared after UV irradiation, which is also indicative of the decreased electronic conjugation. Therefore, the photo-induced RI decrease of P1a/2a is possibly due to the decrease in its electronic conjugation under the irradiation of strong UV light.
To gain insight into the structural change, the films of 4 were exposed to UV light for different time and then washed by CD2Cl2 to do the 1 H NMR characterization. The obtained 1 H NMR spectra shown in Supplementary Figure 43 implied that the intense UV irradiation indeed changed the chemical structure of 4 because some new peaks were observed in the aromatic proton region of the 1 H NMR spectra of the irradiated samples. Furthermore, these new resonance peaks became more and more obvious with the increase in UV irradiation time. We have tried very hard to analyze the photoreaction products, but the photoreaction of 4 seems to be very complicated under such a strong UV light source and the exact structures of the photoreaction products are still difficult to be determined currently.
Mechanistic study on the UV-responsive fluorescence. A radical marker 2,2,6,6tetramethylpiperidin-1-yl)oxyl (TEMPO) was employed to test whether the photochemical reaction of the present system involved free radical intermediates. The toluene solution of 4 (5 mg/3 mL) and the toluene solution (3 mL) of model compound 4 (5 mg) together with 20 equivalents of TEMPO (31.3 mg) in quartz cells were simultaneously irradiated by UV light for 3 h. Afterward, the toluene solvent was removed and the solid was dissolved to measure the 1 H NMR. As summarized in Supplementary Figure 48, the 1 H NMR spectrum of the sample with the addition of TEMPO did not show the aforementioned new peaks in the aromatic proton region after UV irradiation, indicating that the photochemical reaction of this carbonylcontaining spirocyclic system may involve free radical intermediates. The generated free radicals can be immediately trapped by TEMPO and thus prevent the occurrence of further reaction.
The high-resolution X-ray photoelectron spectroscopy (XPS) results of P1a/2a film clearly revealed the change in the surface of the sample after UV irradiation (Supplementary Figure  49). The C 1s spectra showed that the C-O and C=O band become higher after UV irradiation and meanwhile the proportion of the C-C/C=C band decreased. Therefore, photooxidation reactions possibly occurred after irradiating the P1a/2a film under strong UV light. The relative content of C=O and C-O signal in the C 1s and O 1s spectra indicated that the photooxidation process may generate both the C=O and C-O groups in the product structures.
The effect of oxygen on the photochemical reactions was then investigated by conducting the photoirradiation process of the film of 4 under air and under N2, respectively. The 1 H NMR spectra shown in Supplementary Figure 50 suggested that the presence of oxygen is favorable for the proceeding of photoreactions. The resonance signal of the carbonyl-activated alkene proton of 4 (at ~δ 6.16) was obviously weakened after UV irradiation under air, and meanwhile some new resonance peaks were detected in the aromatic proton region. Therefore, the polymers are possible to undergo photooxidation reactions with the participation of the carbonyl-activated C=C double bonds under the intense UV irradiation.
Supplementary Figure 51 suggested that model compound 7 (without the carbonylactivated C=C double bond) can also undergo photochemical reactions under intense UV irradiation. Some new resonance peaks were detected in the aromatic proton region of the 1 H NMR spectrum of the UV-irradiated 7 although the new signals seem to be less than those of 4. These results suggested that besides the carbonyl-activated C=C double bond, the other reactive sites of this unique multisubstituted spirocyclic structure may also be reacted under the irradiation of strong UV light.
To further testify the possible photooxidation mechanism, we then tried to oxidize the film of 4 using ozone as the oxidant and investigate the change in its photophysical properties. The thin film was fabricated by spin-coating the 1,2-dichloroethane solution of 4 onto the silicon wafers. As shown in Supplementary Figure 52A, the non-emissive film of 4 became fluorescent after the film was exposed to ozone (generated from an ozonator) for about 1 h. The changes in the UV and 1 H NMR spectrum of 4 after O3 treatment (Supplementary Figure 52B and C) were similar to the results obtained after UV irradiation (Supplementary Figures 42, 43 and 50). Therefore, the UV-activated fluorescence of P1a/2a and P1b/2a is very likely due to the generation of luminescent species from their photooxidation reactions.
The absorption spectrum of P1a/2e and P7 before and after UV irradiation suggested that UV irradiation could lead to a decrease in the electronic conjugation of the tetraphenylethylene (TPE)-containing polymers (P1a/2e) and the reduced polymer (P7) (see Supplementary Figure  53). There are many reactive sites in these polymer structures that could be photo-oxidized to result in the decreased electronic conjugation. Therefore, these polymers are possible to undergo photooxidation reactions under the irradiation of strong UV light.
The opposite photoresponsive behavior between P1a/2d-e and P1a/2a suggested that the TPE moiety may play a crucial role in the UV-quenched fluorescence of P1a/2d-e. It has been reported that the C=C group of TPE is photosensitive and can be photo-oxidized under intense UV irradiation. 6 Various TPE-containing polymers have been reported to show photo-oxidative bleaching. 7,8 We also tried to oxidize the TPE film using ozone (generated from an ozonator) as the oxidant to study the oxidation-induced structure change in their luminescent properties. After exposure to ozone for about 1 h, the strong fluorescence of TPE film was found to be almost completely quenched. By contrast, the tetraphenyl-substituted spirocyclic compound (4) was found to show a phenomenon of UV-activated fluorescence (Supplementary Figure 47A) or oxidation-activated fluorescence (Supplementary Figure 52A). These results indicated that the photooxidation of the TPE moiety could play a leading role in the photobleaching behavior of the TPE-containing spiropolymers (P1a/2d and P1a/2e).
The investigation on the photoresponsiveness of model compound 7 ( Supplementary  Figures 47B and 51) suggested that the photobleaching behavior of P7 is also related to the occurrence of photochemical reactions that can lead to a decrease in its electronic conjugation. Because 1,3-cyclopentadiene has been widely reported to be a photosensitive unit that can undergo photooxidation reactions to afford oxidized products with less conjugated structures, 9,10 it is reasonable to propose that the photobleaching behavior of P7 may mainly result from the photooxidation of its AIE-active 1,2,3,4-tetraphenyl-1,3-cyclopentadiene (TPC) moiety.