Multistimuli and fingertip-triggered luminescence switching: a five-colored ink-free rewritable secured platform with strongest red emission

Abstract

Strategic designs to develop highly sensitive stimuli-responsive photoluminescence (PL)-switching materials are vastly preferred, but hard to achieve. This work describes the design and straightforward economic synthesis of small, conformationally twisted, and thermally stable π-conjugates as solid-state emitters with highly sensitive stimuli-responsive properties, including brightest-red emission. A metal-free, economic and facile synthetic route affords the π-conjugates conveniently at room temperature in a large quantity. The rigid carbazole and flexible diphenylamine rotors harvest four to five distinct bright emission colors that are reversibly switchable with high contrast in response to various external stimuli by tuning the conformational mobility and intermolecular interactions. Variations in the molecular conformation and arrangement in diphenylamine offer two controlled polymorphs with blue and green emissions. The blue polymorph displays highly sensitive PL-switching [blue to greenish-yellow (Δλ = 68 nm)] by fingertips rubbing, whereas the other polymorph needs rigorous grinding. These polymorphs are not interconvertible, but reach identical molecular phases with an almost intact brightness under the same stimuli. A remarkable protonation/deprotonation-induced, reversibly redshifted PL-switching with enhanced quantum yield (Φ_f) was observed for all. The access of the brightest red-emitting small molecules with Φ_f = 63–64% is noteworthy via the protonation process and unique. The highly intense stimuli-responsive emission change and sensitivity differences in these molecules were elucidated by analyzing the molecular twisting ability and packing, powder X-ray diffraction pattern, and differential scanning calorimetry. The lifetime was measured to illustrate a dominant radiative relaxation pathway to describe the high quantum yield. Furthermore, theoretical support is provided to explain the sensitivity of the blue polymorph by determining the favorable optical bandgap and dimer formation energy. Even the tetramer unit for the blue polymorph also exhibited a greater number of non-covalent interactions, which was further quantitatively deciphered by Hirshfeld surface analysis. Such sensitive stimuli-responsive multicolored emissive materials can be utilized for various applications in optical recording devices and haptic sensing. Finally, these solid-state emitters were employed to fabricate a portable device for optical recording as an efficient multicolored ink-free writing platform.