Insights into the mechanochromism of spiropyran elastomers†
Colourless polymeric samples comprising mechanochromic spiropyrans (SPs) rapidly appear coloured under external pressure, due to their transition from ring-closed SP to ring-open merocyanine (MC). Here, we design a new spiropyran with three attachment points which was systematically compared with two widely used spiropyrans having two attachment positions in the same PDMS matrix on their mechanochromic properties, to determine whether the geometric effect or electronic effect dominates the force-responses. The attachment positions on the chromene ring on the three spiropyrans are identical, with varying attachment positions on the indoline ring. RGB colour analysis and in situ absorption measurements were used to characterize the optical changes and density functional theory calculations were used to better understand the isomerization process. The results demonstrated that by changing the attachment positions of SPs, both the electronic and geometric effects influenced mechanochromism with geometry playing a more dominant role.