Mechano- and Thermo-responsive Luminescence of Crystalline Thienylbenzothiadiazole Derivatives: Stepwise Hypsochromic Switching of Near-Infrared Emission

Organic crystals that exhibit mechanochromic luminescence (MCL) switch their emission colors in response to mechanical stimuli, and the original emission colors usually recover upon heating the mechanically changed state. Contrary to the recent enthusiastic investigations of MCL crystals, only a limited number of organic crystals transform to another state showing a third emission color upon sequential exposure to mechanical and thermal stimuli. The development of near-infrared (NIR) emissive organic crystals is another challenge. Although several NIR-emissive crystals that can respond to mechanical stimuli have been developed, further investigations are still required for the multi-stimuli-responsive emission involving the NIR region. In this study, we investigate the mechano-responsive properties of crystalline thienylbenzothiadiazole derivatives. The maximum emission wavelengths of these crystals are observed between blue-green to NIR (502-731 nm), which depend on the substituents R-1 and R-2 on the thiophene and benzothiadiazole rings, respectively. All derivatives shift the emission wavelength in the hypsochromic direction upon grinding. Moreover, derivatives having a carbonyl group on R2 display further hypsochromic shifts of their emission bands by thermal annealing of the ground states. Specifically, a stepwise hypsochromic shift of the emission from the NIR region has been achieved for a derivative bearing phenyl and formyl groups (R-1 = Ph, R-2 = CHO). Powder X-ray diffraction analysis, differential scanning calorimetry, and spatially resolved fluorescence microscopy have suggested that the stepwise emission-color switching should be attributed to the crystal-size-dependent luminescence of the carbonyl-substituted derivatives. The present study can be expected to provide useful insights into developing diverse multi-stimuli-responsive luminescent organic crystals and thus accelerate the practical applications of luminescent sensor systems.

Automated summary

This study investigates the mechano-responsive properties of crystalline thienylbenzothiadiazole derivatives, which exhibit emission colors ranging from blue-green to NIR depending on the substituents on the thiophene and benzothiadiazole rings. Grinding causes a hypsochromic shift in emission wavelength for all derivatives, with further shifts observed in derivatives with a carbonyl group on R2 upon thermal annealing of the ground states. Stepwise emission-color switching in the NIR region is achieved for a derivative bearing phenyl and formyl groups, attributed to crystal-size-dependent luminescence.