Photo-switching in multi-stimuli-responsive low Z′-high Z′ co-crystal polymorphs†

Emission switching in crystalline materials, a phenomenon uncommon for multi-component polymorphic crystals, is an intriguing with a wide range of applications in optics and optoelectronics. We reported co-crystal polymorphs obtained by the crystallisation of o-arsanilic acid (2-ABAA-2H) and 4,4′-bipyridyl (4,4′-BPY) in different solvent systems. Structural studies established that the crystal forms exist as low Z′ (=1), Z′′ (=3) [(4,4′-BPY-2H)²⁺(2-ABAA-H)₂⁻] (1) and its high Z′ (=6), Z′′ (=18) [(4,4′-BPY-2H)₆²⁺(ABAA-H)₁₂⁻] (2) forms and undergo proton transfer between crystal forms to exist as ionic solids. Form 1 is a green emitter (λ_max 512 nm, ϕ 2.6%, τ 2.5 ns), while 2 is non-emissive. Mechanochromic studies establish the retention of green emission in the 1G form and emission turn-on in the 2G form. The ground forms respond to basic ammonia fumes by undergoing emission tuning to cyan and intense blue in 1-NH₃ and 2-NH₃, respectively. Thin films of both 1 and 2 exhibit green emission when prepared in a range of solvents, implying the retention of their microcrystalline phases upon dissolution. Aggregation-induced emission (AIE) studies of the products are reported and supported through FE-SEM and DLS studies. Structural studies indicate that slipped π–π interactions in the lattice of 1, form J-type aggregates, and are responsible for its solid-state emission, while 2, devoid of notable π–π interactions, is non-emissive due to loose crystal packing, plausibly leading to vibrational quenching.