Mechanofluorochromic properties of fluorescent molecules based on a dicyanomethylene-4H-pyran and indole isomer containing different alkyl chains via an alkene moduleElectronic supplementary information (ESI) available: UV-vis spectra, fluorescence spectra, photophysical data, XRD curves, and other additional information. CCDC 1554482-1554487. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7ra06951k

A series of 3-, 4-, and 5-position indole-substituted dicyanomethylene-4 H -pyran ( I3PM , I4PM , and I5PM ) derivatives with different alkyl chains were synthesized and their mechanofluorochromic (MFC) properties were investigated. Compared with the alkyl chains, the isomerization of the indole unit plays a more important role in regulating the MFC properties of these compounds. The increase of the alkyl chain length and the isomerization of the alkyl chain are favorable to the effective mechanofluorochromism of the I3PM derivatives, however, the former exhibits the opposite effect on that of the I4PM derivatives. The opposite alkyl length-dependent MFC behavior is closely related to the degree of distortion of the molecular conformation, namely the more twisted the molecular conformation, the more pronounced the MFC phenomenon. In particular, the introduction of the alkyl chains is completely detrimental to the MFC properties of the I5PM derivatives. X-Ray diffraction experiments indicate that the MFC properties of some of these compounds should be mainly attributed to the transformation from a crystalline state to an amorphous state because the weak intermolecular interactions and loose molecular packing modes in their solid-state samples can be easily damaged under external force stimuli as revealed by the single crystal X-ray diffraction analysis. The length and isomerization of the alkyl chains exhibit different effects on the mechanofluorochromic properties of the dicyanomethylene-4 H -pyran derivatives when the substitution position of the indole unit is different.