As described in literature, colorless spiropyran form undergoes a reversible photocleavage of the spiro CeO bond, generating colorful merocyanine form.
The merocyanine exists as a resonance hybrid between the charged zwitterion and neutral quinoidal form.
Divalent Metal ions can complex with the merocyanine isomer by binding with the negatively charged phenolate group of the zwitterionic form.
As discussed before, a characteristic absorption appears at 551 nm after the irradiation of ultraviolet light (365 nm).
As can be seen from Fig. 7, some changes are observed in the UVeVIS spectra of photo-responsive unit upon complexation with different divalent metallic ions.
The locations of the maximum absorption peak stay almost as same as MC when it is complexed with Cu2þ, Mn2þ and Hg2þ.
The location, however, moves from 551 nm to 526 nm upon complexation with Co2þ.
The significant hypsochromic shift in absorbance maximum is due to a disruption in planarity of the trans-MC during the complexation.
Fig. 8 displays the obvious color change of the solution from purple to claret-red after complexation with Co2þ. The high selective and sensitive chelation of P(SPMA-co-MAA) toward Co2þ over other metal ions may be explained as follows [22,37e39].
On one hand, the formation of networks structure by intramolecular hydrogen bond between carboxyl of MAA and phenolate oxygen group hinders the metal ions to contact with merocyanine.
Furthermore, the MAA unit is prone to cooperate with the phenolate anion of the open merocyanine in the chelation of metal ions.
On the other hand, the relative large radius of Hg2þ leads to strong steric hindrance, which seriously impedes its coordination with the copolymer system.
The valence shell configuration of Mn2þ is 3d5 and its 3d orbits are in the half-full state, resulting in the relatively stable chemical property.
So it is difficult for Mn2þ with the weak electronegativity to complete with the strong intramolecular hydrogen bond.
The radius and electronegativity of Cu2þ are very close to those of Co2þ.
However, the valence shell configuration of Cu2þ is 3d9, and there is only one orbit available for coordination.
Meanwhile, the valence shell configuration of Co2þ is 3d7 and there are three orbits available, which contributes to the particularly high thermodynamic affinity of Co2þ for the polymeric system. The above factors lead to the remarkable specific complexation of the merocyanine-MAA system toward Co2þ over the other transition-metal ions.
This unique characteristic makes this spiropyran-based random copolymer appropriate for selectively colorimetric sensing of Co2þ.
Moreover, this interaction is reversible, and ring-closed form is able to recover under visible light irradiation, generating a molecular system that may be applied in reversible ion sensing.
From the curve 3 in Fig. 9 we know, Co2þ is released from the complexation
and the original spiropyran isomer comes into being again upon the irradiation of visible light.
It is found that this process can be repeated by irradiating the solution with UV and visible light respectively in the presence of Co2þ.
This distinctive character in the spectra response of merocyanine to different divalent metal ions may be potentially used in a recyclable colorimetric sensor which can selectively recognize Co2þ.