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

Abstract

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.

Fig. 1

Developing the spiroannulation reaction into a polyspiroannulation strategy. a Palladium(II)-catalyzed oxidative spiroannulations of naphthols and diarylacetylenes and the associated mechanism. b Palladium(II)-catalyzed polyspiroannulations of free naphthols and internal diynes for the construction of complex spiro-polymers in this work. [1]:[2] = molar feed ratio of monomer 1 and 2.

Fig. 2

Structural characterization and analysis of the polymeric products. a Synthetic route to model compound 4. b–e ¹H NMR spectra of b monomer 1a, c monomer 2a, d model compound 4, and e P1a/2a in CD₂Cl₂. f–i ¹³C NMR spectra of f monomer 1a, g monomer 2a, h model compound 4, and i P1a/2a in CD₂Cl₂. j ¹³C NMR spectra in CD₂Cl₂ (left) and IR spectra (right) of monomer 2a and P1a/2a generated from polymerizations at different monomer ratios (sample taken from Table 1, entries 1–5, respectively).

Fig. 3

Preparation and post-polymerization of telechelic polymer. The telechelic polymer P1a/2a is prepared at a monomer feed ratio of 1:1, and the polymerizations of telechelic P1a/2a with 1a (route I), 1c (route II), and 1d (route III) are conducted based on the apparent monomer-nonstoichiometry-promoted effect.

Fig. 4

Photophysical properties of model compounds and polymers. a Reduction of model compound 4. b Reduction of P1a/2a. Inset: fluorescent photographs of THF solutions and powder of 4 and P1a/2a (left side) and 7 and P7 (right side). c Molecular orbitals of 4 and 7 in the ground state calculated by B3LYP/6-31G(d,p). d Schematic illustration of the modulation of fluorescence properties by the photoinduced electron transfer (PET) process. e Photoluminescence (PL) spectra of 7 in THF and THF/water mixtures with different water fractions (f_w). f Plot of the relative PL intensity (I/I₀) versus the composition of the aqueous mixtures of 7, P7, P1a/2d, and P1a/2e. α_AIE = I/I₀, where I₀ = intensity at f_w = 0%. Solution concentration: 10 μM. g Normalized PL spectra of the powder of 7, P7, P1a/2d, and P1a/2e and their associated fluorescent photographs. Excitation wavelength: 320 nm (for 7 and P7); 350 nm (for P1a/2d and P1a/2e). All fluorescent photographs were taken under UV irradiation at 365 nm.

Fig. 5

Refractive index and chromatic dispersion of polymer thin films. a Wavelength dependence of refractive indices of thin films of P1a–b/2a–e. b Change in the refractive index of a thin film of P1a/2a by UV irradiation for different durations. Abbreviation: n = refractive index; D = chromatic dispersion in the visible region = (n_F − n_C)/(n_D − 1), where n_D, n_F, and n_C are the n values at wavelengths of Fraunhofer D, F, and C spectral lines of 589.2, 486.1, and 656.3 nm, respectively; D’ = chromatic dispersion in the IR region = (n₁₀₆₄ − n₁₅₅₀)/(n₁₃₁₉ − 1), where n₁₀₆₄, n₁₃₁₉, and n₁₅₅₀ are the n values at 1064, 1319, and 1550 nm, respectively.

Fig. 6

Fabrication of photopatterns using polymer thin films and the obtained images. a Schematic illustration of the fabrication of photopatterns: two-dimensional photopatterns were generated by the photo-masked UV irradiation of polymer thin films on silicon wafers. b Fluorescent images of the turn-off-type photopatterns (left) and the grayscale intensity profile of the arrowed area in the patterned P7 film. c Fluorescent images of the turn-on-type photopatterns (left) and the grayscale intensity profile of the arrowed area in the patterned P1b/2a film. The fluorescent images in (b) and (c) are taken under 330–380 nm UV illumination using a fluorescent microscope. d Photographs of the photopatterns taken under normal room light using an optical microscope. All images in (b), (c), and (d) share the same scale bar = 100 μm. e A fluorescent flower-like photopattern of P7 taken under UV irradiation at 365 nm using a camera.

Fig. 7

Applications of photoresponsive polymer films in integrated silicon photonics. a Schematic of a silicon-based microring resonator evanescently coupled with a bus waveguide. b Schematic of the cross section of a polymer-coated Si₃N₄ waveguide. c, d FEM-simulated mode-field amplitude distributions with c TM polarization and d TE polarization. e Measured transmission spectra of a microring near 1550 nm wavelength in TM polarization with UV exposure duration varying from 0 to 40 min at an interval of 10 min. f Extracted |∆λ/λ_res| of all the six measured devices in TM polarization. g Comparison between the extracted |∆λ/λ_res| of the microrings in TM polarization and the extracted |∆n/n| of the polymer films as a function of the UV exposure duration. The error bars indicate the standard deviations. h Comparison of |∆λ/λ_res| between the six measured devices and the simulations in TM polarization with the polymer thickness varying from 60 to 110 nm at an interval of 10 nm. i Estimated polymer thickness of the six devices based on both TM and TE polarizations. The upper and lower limits of the error bars indicate the estimated thickness values from TM and TE polarizations, respectively. The height of each colored bar is defined as the average value of the upper and lower limit.