Photoresponsive polymers that can alter their optical properties in response to light irradiation have attracted wide interest due to their significant applications as smart optical and biological materials. They have been used in smart surfaces, optical sensors, optical or photoelectric switches, information storage, phase-sensitive optical devices, etc. The utilization of light irradiation as the stimulus enjoys the merits of easy and precise control on the temporal and positional resolution of the response in a non-contact and remote way. The chemical synthesis of photoresponsive polymers are generally achieved by incorporating photoresponsive units into polymer side chains or main chains via different polymerization techniques. Among them, step-growth polymerizations of A2 + B2 play an indispensable role in the synthesis of main-chain photoresponsive polymers. However, these polymerizations require the inevitable use of two difunctional monomers and a strict control on the stoichiometric balance of monomers. Inspired by the rapid progress of C–H activation in organic chemistry, we envision that stepwise polymerizations based on the transition metal-catalyzed C–H activation will be a good choice to transform the abundantly existing but poorly reactive monofunctional monomers into valuable photoresponsive polymers. Meanwhile, the employment of C–H as the reactive functional group may allow the corresponding polymerizations to be more tolerant to the mismatch in functional group stoichiometry than conventional step-growth polymerizations.
Figure 1. Synthesis and applications of photoresponsive spiro-polymers.
By utilizing the C–H bond as a hidden functional group in one monomer, we successfully develop a straightforward stepwise polymerization strategy that can in situ generate photoresponsive polymers from simple monofunctional monomers and meanwhile without the constraint of apparent stoichiometric balance in monomers. In the presence of palladium(II) catalyst, the C–H-activated polyspiroannulations can facilely transform the readily available and inexpensive 2-naphthols and diynes into valuable photoresponsive spiro-polymers with complex structures and multifunctionalities. 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%). Taking advantage of its unique reaction mechanism, the present polyspiroannulation can be applied for the preparation and post-functionalization of spirocyclic telechelic polymers to further expand the libraries of functional spiro-polymers. The obtained spiro-polymers show excellent film-forming ability, high thermal and morphological stability, and interesting photoresponsive optical properties. Well-resolved two-dimensional fluorescent images with both turn-off and turn-on mode can be readily fabricated based on the photobleaching and photoactivation process of different polymer films under strong UV irradiation. Moreover, we utilized the photoresponsive refractive index of these polymer thin films to permanently modify the resonance wavelengths of microring resonators by UV irradiation. The applicability could be further strengthened by developing polymer materials with a larger refractive index change in response to UV irradiation or using polymer substrates to extend the application to flexible photonics.