Exceptional Thermo-Mechano-Fluorochromism and Nanomechanical Analysis of Mechanically Responsive J and H Type Polymorphic Systems
Poonam Deka, Susobhan Das, Parishmita Sarma, Kalyan Jyoti Kalita, Ratheesh K. Vijayaraghavan, C. Malla Reddy, Ranjit Thakuria
Abstract
Mechanically responsive organic crystals with thermo-mechano-fluorochromic behavior are relatively scarce. A polymorphic J and H aggregate system with such mechanosalient properties has potential importance for the understanding of structure-mechanochromic luminescence. Herein, we not only carefully investigated the nanomechanical properties of a polymorphic organic fluorophore of (biphenyl-4-ylmethylidene) propanedinitrile but also demonstrated their exceptional thermo-mechanochromic luminescence. Both the polymorphs were characterized using single crystal X-ray diffraction, solid-state UV, SSFL, nanoindentation, Hirshfeld surfaces, energy frameworks, powder X-ray diffraction, and differential scanning calorimetry thermograms. Relatively weak π–π stacking interactions present in the crystal structures govern the hypsochromic shift on mechanical grinding. Density functional theory calculations provide tremendous support to shed light on the origin of the distinct photophysical properties of the two polymorphs.