High contrast mechanochromic and thermochromic luminescence switching by a deep red emitting organic crystal

In this study, we report a deep red emissive organic crystal that displays high contrast fluorescence switching under mechanical and thermal stimulation. Upon mechanical grinding, the pristine red emissive crystals of 1R (λ_em = 667 nm) transformed into green emissive crystalline powder (λ_em = 550 nm) with a remarkable hypsochromic shift of wavelength, Δλ = 117 nm. The melting of 1R leads to a green emissive amorphous solid (λ_em = 555 nm) with a wavelength shift of Δλ = 112 nm. Upon fuming with different solvents, the amorphous melt solid transforms into different polymorphs having distinct emission characters. The structural relationship between different emissive states is investigated with the aid of X-ray diffraction and other spectroscopic studies, which clearly demonstrate the roles of ordered molecular packing and intermolecular interaction in determining their diverse optical response.