Multi-color emission and temperature promoted luminescence efficiency of stimuli-responsive stoichiomorphic ionic co-crystals†

Stimuli-responsive organic solid-state luminescent crystals are advanced materials with potential applications in sensing, detection, recording, data storage, optoelectronics, displays, and security technologies. This study reports the synthesis of cocrystal-salt stoichiomorphs of organo-sulfonate systems: 1,5-naphthalene disulfonic acid (NDSA-2H) combined with 0.5 (P1) and 1.0 (P2) equivalents of 1,2-di(4-pyridyl) ethylene (4,4′-BPE). These systems exhibit remarkable solid-state emission tuning, changing from the blue emission of NDSA-2H (λ_max 408 nm; τ 1.26 ns; Φ 10.2%) to the dark cyan emission of P1 (λ_max = 490 nm; τ_av = 13.68 ns; Φ_PL = 31.3%) and the green emission of P2 (λ_max = 545 nm; τ_av = 5.26 ns; Φ_PL = 24.1%). Structural investigations reveal proton transfer between crystal components in both forms, with P1 existing as a monohydrate. This study highlights rare hetero-aggregation-induced tunable emission in these solid forms, supported by optical, dynamic light scattering (DLS), microscopy, and powder X-ray diffraction analyses. P1 exhibits reversible thermo-fluorochromic and irreversible mechano-thermo-fluorochromic properties, explored in detail, whereas P2 shows only mechanochromic behavior. In-depth lifetime and variable temperature emission studies indicate lattice rigidity resulting upon applying external stimuli, augmented optical performance by suppressing non-radiative pathways. Variable-temperature diffraction and photoluminescence (PL) studies demonstrate the exceptional thermal phase stability, lattice compression, and crystalline phase improvement of these materials, resulting in red-shifted emission and a striking enhancement in quantum yields upon heating.