Tuneable solid-state emitters based on benzimidazole derivatives: Aggregation induced red emission and mechanochromism of D-π-A fluorophores

Organic fluorophores with photoluminescent properties in the solid state are widely applied in modern technology. Of special interest are those molecular materials capable of luminescent "switching" when exposed to external stimuli. However, the development of solid-state molecular systems capable of luminescence tuning over a broad spectral range, especially in the red region, remains a major challenge. In this work an aggregation induced emission phenomenon (ME) is shown as a favourable strategy for design and development of novel solid-state emitters, especially in combination with excited-state proton transfer reactions (ESIPT). In general, the demanding and complex synthesis of ME molecular systems (commonly large rotor molecules with luminescence in the blue and green spectral region) is known to be a major drawback of this approach. We present a systematic study of the spectral properties of five AIE- and ESIPT-based solid-state emitters comprising facile synthesis of benzimidazole (BI) derivatives, a new class of candidates for solid state fluorophores. The five BI-based Schiff bases (compounds 1-5) were characterised in solution and solid state using spectroscopic and diffraction methods. All compounds are D-pi-A structures with bulky aromatic groups that can undergo twisted molecular conformational change. The presence of the -OH group was found to cause excited state intramolecular proton transfer (ESIPT) while different substituents on the pi-conjugated backbone allow tuning of solid-state emission from blue to red. The compounds tend to aggregate in water, exhibiting new red shifted emission bands. Compounds 1-5 display very weak emission in solutions, while in solid-state the emission is enhanced to various levels as a result of aggregation induced emission phenomena (ME). Since AIE can easily be disrupted, the spectral properties of solid-state fluorophores are sensitive to external influence, such as mechanical or chemical stimuli. Reversible mechanochromic luminescence induced by mechanical grinding and subsequent chemical fuming and recrystallisation, is observed for fluorophores 2-5 with wavelength shifts of 16 nm, 19 nm, 26 nm and 48 nm, respectively. Fluorophore 5 shows red fluorescence with mechanochromic hysteresis. These results are discussed from a structural point of view and confirm the applicability of the dyes as mechanochromic materials.