As we know, the isomerization of photochromic molecule between SP and MC forms varies from different solvents, and the effect of solvents polarity on the photo-responsive behaviors of P(SPMA-co-MAA) is investigated by the UVeVIS absorption spectra. Fig. 2 shows the UVeVIS spectra of P(SPMA-co-MAA) in several solvents including toluene, THF, ethanol, acetonitrile and water (pH 1⁄4 11) with an increasing polarity. As can be seen, a strong absorption at 562 nm in toluene is due to the formation of the planar zwitterionic. It can be found that the lmax decreases with the increasing polarity of solvents (562 nm in toluene (spp: 0.655), 551 nm in THF (spp: 0.838), 544 nm in ethanol (spp: 0.853), 540 nm in acetonitrile (spp: 0.895) and 526 nm in water (pH 1⁄4 11) (spp: 0.962)). The negative solvatochromic effect associates with the stabilization of MC in different solvents. In more polar solvents, the ground state of the bipolar MC form is more stabilized compared to the excited state (mground > mexcited). It results in a greater energy gap between the two MC states and a hypsochromic shift of the ab- sorption band [26]. The negative solvatochromic of light- responsive copolymers P(SPMA-co-MAA) is also visually observed, and color changes from purple in THF to light pink in water (pH 1⁄4 11) (see Fig. 2 inset).
Except for the solvents, the comonomers in the copolymer also exert effect on the photochromic behavior of SPMA unit, so we compare the UVeVIS spectra of P(SPMA-co-tBMA) before and after hydrolysis in THF. As clearly shown in Fig. 3, The lmax of P(SPMA-co- tBMA) and P(SPMA-co-MAA) in THF are 588 and 551 nm, respec- tively. We can also observe the color change from blueviolet to amaranth. The reason of this phenomenon can be explained by the polarity of comonomers. Since the polar monomer can lead to hypsochromic shift in the absorption band, lower lmax is found in the UVeVIS curve of the copolymer with more polar carboxylic group.