Flexible Chiral Molecular Crystal with Reversible Single‐Crystal‐to‐Single‐Crystal Phase Transition for Second Harmonic Generation

Abstract Flexible molecular single crystals, characterized by their softness and lightweight, have emerged as a novel crystalline material. One of the current research hotspots is to explore their potential application within the optoelectronic field. Here, a mechanically flexible nonlinear optical (NLO) crystal is reported, with excellent robustness for reversible single‐crystal‐to‐single‐crystal (SCSC) phase transition under thermal stimulation. The single crystals can be plastically bent with application of mechanical force. Moreover, they show a nearly defect‐free reversible SCSC phase transition over 300 cycles. The variable temperature single crystal X‐ray diffraction (SCXRD) combined Raman spectroscopy demonstrates that the rotation of the tert‐butyl side chain triggers the reversible phase transition. Both two forms have strong second harmonic generation (SHG) attributed to their polar spatial arrangements, although their SHG intensities are different. Additionally, the bent crystals still have SHG activity, however, deformation leads to a different polarization‐dependent SHG intensity compared to the undeformed (straight) regions. This work integrates the mechanical flexibility and SCSC phase transition in NLO molecular crystals and paves the way for the design of novel flexible NLO materials.