Luminescence modulated by molecular conformation in stimuli-responsive polymorphs of dibenzothiaborinine dioxide-pyridylphenolate complex

Novel spirane organoboron complex, SO2-pp, composed of highly electron-accepting dibenzo[b,e][1,4]thiaborinine 5,5-dioxide scaffold and 2-(2-pyridyl)phenolate ligand exhibits moderate-to-strong fluorescence in solution and solid state. Photophysical properties of this system strongly depend on the conformation of the molecule. Specifically, due to the semi-labile nature of organoboron moiety as well as the asymmetric character of the ligand, two distinctive conformations can be distinguished. Their occurrence in the solid state is regulated by the crystallization conditions leading to the formation of two polymorphic phases exhibiting sky blue (SO2-pp-α, λ = 446 nm, Φ^F = 70 %) and green (SO2-pp-β, λ = 489 nm, Φ^F = 36 %) emission, respectively. Their crystal structures, stability and photophysical properties were comprehensively studied by DSC, single-crystal XRD and variable-temperature PXRD. Periodic DFT calculations showed that the lattice energy of SO2-pp-β polymorph is by 28 kJ‧mol⁻¹ more favoured (7 kJ‧mol⁻¹ per molecule, optimization in periodic boundary conditions) owing to more efficient intermolecular interactions in the crystal structure. In contrast, the molecule of the kinetic SO2-pp-α phase adopts more stable conformation (energy difference of 4.7 kJ‧mol⁻¹, single-molecule DFT optimization) which can be ascribed to the formation of intramolecular C–H⋯O hydrogen bond interaction. Both forms show sensitive tricolor reversible response under mechanical and temperature stimuli. Under grinding, they gradually turn into cyan (λ = 479 nm) amorphous phase (SO2-pp-A). This process can be reversed by heating or exposure to CHCl₃ vapours.