Planarisation or a twist? Using steric engineering to unlock the origin of mechanofluorochromic red-shifts

Engineering the ground-state orientations of donor and acceptor groups through steric control of fluorophore conformations is an effective strategy for manipulating molecular electronics and, in turn, their emissive properties. Where strong emission is retained in the crystalline state, a correlation of structure with photophysical properties can be made, as is the case for the five pyridinium betaines reported herein. Our findings provide strong evidence that an increase in dihedral angle between N,N-diphenylamino donor and pyridinium acceptor induces a notable red-shift in emission maximum, with the mechanofluorochromic response also correlated with the same process. This research aims to address the oft-invoked explanation that planarisation induces red-shifted emission, highlighting that this effect is not universal and that systematic studies are essential. Further, this elegant steric engineering approach may be applied to other mechanochromic systems to determine the nature of their geometry changes.