Molecular Conformation- and Packing-Controlled Excited State Intramolecular Proton Transfer Induced Solid-State Fluorescence and Reversible Mechanofluorochromism

Excited state intramolecular proton transfer (ESIPT) materials are highly desirable because of their unique solid state fluorescence. Schiff base molecules with appropriate hydrogen donor functional group forms intramolecular H-bonding, a basis for ESIPT, however, only few molecules show solid state fluorescence. Herein, carbazole based Schiff base molecules 1 (2-((E)-(4-(9H-carbazol-9-yl) phenyl-mino) methyl) phenol), 2 (2-(( E)-(4-(9H-carbazol-9-yl) phenylimino) methyl)-5-methoxyphenol), 3 (2-((E)-(4-(9H-carbazol-9-yl) phenyl-imino) methyl)-5-(diethylamino) phenol) and 4 (1-((E)-(4-(9H-carbazol-9-yl) phenylimino) methyl) naphthalen-2-ol)) were synthesized and ESIPT induced solid state fluorescence were correlated with solid state structural conformation and molecular packing. 2 and 4 exhibited solid state fluorescence whereas 1 and 3 did not show any fluorescence even though both 1 and 3 showed strong intramolecular H-bonding. Further structural comparison revealed that not only the intramolecular H-bonding facilitated ESIPT, the molecular conformation and resulting molecular packing also plays important role on the solid state fluorescence. Computational studies have also been performed to get the insight on the role of molecular conformation and organization. The non-planar twisted molecular conformation of 2 and 4 showed reversible mechanofluorochromism (MFC). Powder X-ray diffraction indicates that hard crushing converted crystalline to amorphous/partial amorphous phase whereas heating converts amorphous to crystalline. Thus the present study provides a structural insight for developing ESIPT based new Schiff base materials.