Distinct Mechanoresponsive Luminescence, Thermochromism, Vapochromism, and Chlorine Gas Sensing by a Solid-State Organic Emitter

In
this study, we report a synthetically simple donor–acceptor (D–A)-type organic solid-state
emitter 1 that displays unique fluorescence switching
under mechanical stimuli. Orange and yellow emissive crystals of 1 (1O, 1Y) exhibit an unusual “back
and forth” fluorescence response to mechanical force. Gentle
crushing (mild pressure) of the orange or yellow emissive crystal
results in hypsochromic shift to cyan emissive fragments (λem = 498–501 nm) with a large wavelength shift Δλem = −71 to −96 nm, while further grinding results
in bathochromic swing to green emissive powder λem = 540–550 nm, Δλem = +40 to 58 nm.
Single-crystal X-ray diffraction study reveals that molecules are
packed by weak interactions, such as C–H···π,
C–H···N, and C–H···F,
which facilitate intermolecular charge transfer in the crystal. With
the aid of structural, spectroscopic, and morphological studies, we
established the interplay between intermolecular and intramolecular
charge-transfer interaction that is responsible for this elusive mechanochromic
luminescence. Moreover, we have also demonstrated the application
of this organic material for chlorine gas sensing in solid state.