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

Abstract

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, transitioning from the blue emission of NDSA-2H (λmax 408 nm; τ 1.26 ns; Φ 10.2%) to the dark cyan emission of P1 (λmax 490 nm; τ 4.96 ns; Φ 31.3%) and the green emission of P2 (λmax 545 nm; τ 5.26 ns; Φ 24.1%). Structural investigations reveal proton transfer between crystal components in both forms, with P1 existing as a monohydrate. The 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 significant detail, whereas P2 shows mild mechanochromic behavior. Variable-temperature diffraction studies demonstrate these materials' exceptional thermal phase stability, along with lattice compression and crystalline phase improvement, which result in red-shifted emission and a striking enhancement in quantum efficiency up to 67% upon heating.