Abstract:
Two donor–p–acceptor type fluorophores consisting of anthracene as the donor and benzoxazole (ABO)
or benzothiazole (ABT) as the acceptor are synthesised. Both fluorophores exhibit excellent solid state
luminescence, and respond to external stimuli such as mechanical force (mechanochromism) and protons
(acidochromism). Though the differences between the two molecules are marginal, their stimuli responsive
behaviours are contrasting. ABO shows stable mechanochromism but the protonation induced colour
change is unstable. On the other hand, ABT shows stable acidochromism but luminescence changes
induced by mechanical stress are reverted back quickly. A detailed study reveals that the variance in the
heteroatoms in benzoxazole/benzothiazole moieties (oxygen and sulphur, respectively) significantly affects
both the intra- and intermolecular electronic interactions of the molecules resulting in such observations.
In the case of ABO, the benzoxazole moieties interact through weak edge-to-edge p-stacking, whereas in
ABT, the ring overlap is more significant making the p-stacking a stronger face-to-face type, which helps
in the restoration of the molecular ordering in ABT, which is energetically more viable. Regarding the
acidochromic properties, the presence of more electronegative oxygen (compared to sulphur) in ABO
draws electrons efficiently towards it making the nitrogen less basic and the complex formed between
ABO and TFA less stable. On the other hand, electron density on the nitrogen in ABT might be higher
compared to ABO due to the presence of less electronegative sulphur leading to the formation of a stable
complex between ABT and TFA.
