Phenothiazine-Based Mechano-Fluorochromic Gels and Solids: Superhydrophobic Surface Formation and Crystal-to-Crystal Phase Transition

Organic materials exhibiting mechano-fluorochromic behavior in their gel state are extremely rare in the literature. Similarly, examples of mechano-fluorochromic organic materials with crystal-to-crystal phase transitions have not been reported until date. Herein, we achieved these two remarkable properties in phenothiazine derivatives S1 and S2[(E)-N'-((10-ethyl-10H-phenothiazin-3-yl)methylene)-3,4,5-tris(hexadecyloxy)benzohydrazide and (E)-N'-((10-ethyl-10H-phenothiazin-3-yl) methylene)isonicotinohydrazide)], where S1 formed mechano-luminescent xerogel and S2 exhibited the rare feature of crystal-to-crystal phase transition with mechano-luminescence behavior. Study of the photophysical properties of S1 and S2 indicated the suppression of twisted intramolecular charge transfer (TICT) upon self-assembly, leading to blue-shifted emission from S1 and S2. While both S1 and S2 exhibited the propensity to aggregate and disclosed the aggregation induced emission (AIE) behavior in a binary solvent (THF/H2O) mixture, only S1 resulted in a robust gel, which was further utilized for developing a superhydrophobic surface with a contact angle of 157.56 degrees. Upon mechanical grinding, both pristine sample and xerogel of S1 showed an emission color change from light green (505 nm) to yellowish green (530 nm), whereas S2 exhibited an emission color change from cyan blue (480 nm) to green (535 nm). Furthermore, both fluorophores exhibited red-shifted emission under application of increased pressure. Mechanistic studies indicated that grinding resulted in crystal to amorphous phase transition in S1, whereas grinding in S2 demonstrated the rare feature of crystal-to-crystal phase transition. The mechano-fluorochromic (MFC) natures of S1 and S2 were supported by single point energy calculations using density functional theory (DFT).