Impact of tert-butyl on mechanofluorochromism of triaryl-substituted phenothiazine derivatives

Abstract

Two phenothiazine derivatives, PTCNB and PTCNTBB, were designed and synthesized to confirm whether triaryl-substituted phenothiazine has mechanofluorochromic (MFC) properties and investigate the impact of tert-butyl (t-butyl) as a steric hindrance group on the molecular response to force stimuli. It was observed that the introduction of the t-butyl moiety did not significantly alter the molecular photophysical properties in the solution state due to its minimal effect on molecular π-conjugation. However, the presence of t-butyl strongly influenced their photoluminescence behavior and force-stimulus response in the solid state. PTCNB without t-butyl exhibited orange fluorescence in its pristine solid, which transformed into red fluorescence upon grinding, accompanied by a spectral shift of 23 nm. Conversely, PTCNTBB with t-butyl displayed green fluorescence in the as-synthesized solid and subsequently emitted orange fluorescence under force stimuli, demonstrating a more pronounced color variation. The corresponding spectral shift value was measured to be 58 nm. Remarkably, both ground solids regained their original fluorescence colors after vapor fuming, indicating a reversible MFC process. XRD patterns suggested that the application of force stimuli disrupted the intermolecular ordered packing while the fuming process restored ordered stacking, leading to the recovery of fluorescence emission. Absorption spectral changes implied that the presence of t-butyl induced loose packing and weak π–π interactions in PTCNTBB's initial solid, resulting in a short-wavelength fluorescence. Under force stimuli, the intermolecular interaction between PTCNTBB molecules was significantly enhanced, leading to an obvious redshift in the fluorescence spectra. In contrast, due to the absence of bulky steric groups, intermolecular packing between PTCNB molecules was relatively dense in its pristine solid and then applying pressure did not cause significant changes in a π-packing arrangement; hence only a small spectral shift occurred.