Pressure induced the largest emission wavelength change in a single crystal

The three molecules of SP-4-An-2, SP-4-An-9 and SP-5-An-9 were synthesized and their fluorescent mechanochromism was studied in their pristine crystalline state and single crystal form, respectively. The molecules have small structure dissimilarity but show quite different responsive behavior to anisotropic grinding and isotropic hydrostatic pressure either in the pristine crystallites or in the single crystals. Manual grinding transfers the ordered pristine crystallites phase to amorphous state, however, it can hardly shift the emission to the longer wavelength, suggesting the ring-opening isomer is rare in the system. As a comparison, hydrostatic pressure compels the anthracene units in the single crystals to be closer with enhanced π-π interaction, and induces the isomerization of spiropyran to be a more flat and conjugated merocyanine (MC) form, resulting in the gradually red-shifted emission and red fluorescence. In particular, the single crystal of SP-4-An-2 achieves a high contrast multicolored change and the largest emission red-shift over 190 nm. These results show the dependency of mechanochromic property on the molecular structure, i.e. marked position of anthracene-to-anthracene, and anthracene-to-spiropyran frameworks based on in-situ observation with the diamond anvil cell (DAC), which is in sharp contrast to the powder crystals and ambient fluorescence measurements.