Heteroatom induced contrasting effects on the stimuli responsive properties of anthracene based donor–π–acceptor fluorophores†
Two donor–π–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 π-stacking, whereas in ABT, the ring overlap is more significant making the π-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.