Scheme 2. Possible impacts of the ester group substituted at one of the four possible positions on the chromene moiety (o-, m-, p- and m′-, with respect to the benzopyran oxygen) on the stability of the opened-form of spiropyran, impacting the photo- and halochromism. We hypothesize that the resulting photo- and halochromism of the four polymers in DCM solutions originate from a complex combination of electronic effects (induction and resonance), steric hindrance and hydrogen-bonding capability caused by the regio-substitution of the ester (–OCOR) group. The relative orientation of the phenolate (O−)/phenolic (OH) oxygen, the ethene bridge and the ester group (which are all electron-donating groups (EDGs)) may influence the electron densities on these groups. In the case of PSP1 and PSP3, for instance, both the ethene bridge and the ester groups are at either ortho-/ortho- or ortho-/para-positions with respect to the phenolate oxygen and vice versa. Thus, the former groups can synergistically direct electrons to the phenolate oxygen. There exists also a possibility for the phenolate oxygen (which is a much stronger EDG) to redirect electron density to the other two groups. For PSP1, the steric contribution from the ester group and the possible electronic repulsion between the phenolate oxygen (O−) and the carbonyl oxygen in the ester group may further reduce the photochromic response of the polymer compared to PSP3. However, upon protonation, a possible hydrogen bonding between these two groups can be established, co-contributing to the strong halochromism of PSP1. In the case of PSP2 and PSP4, the phenolate oxygen is meta-substituted with respect to the ester group. Thus, these two EDGs can cooperatively push electron density to the ethene bridge, leading to lower electron densities on the phenolate oxygen (which means stronger photochromism). With regard to the observed halochromism, PSP2 and PSP4 are expected to be inactive or significantly less responsive compared to PSP1 and PSP3. However, while the prediction holds true for PSP4, PSP2 responds fastest to acid-addition. Upon careful structural analysis, we submit that the possible intermolecular interaction with the solvent (DCM), i.e., hydrogen-bonding, and the geometry of the ester group (which also functions as the connecting arm) beside the electronic contribution, lead to the unexpected performance of PSP2. Within all four polymers, the ester group in PSP2 is at the para-position with respect to the ethene bridge, where E/Z-isomerization takes place upon ring-opening. In terms of steric hindrance, such a substitution site is the most ideal and thus does not interfere with the E/Z-isomerization at the ethene bridge. The term “ortho-directing” refers to the electron-donation from the substituent under consideration to its ortho-position.
