Decoding emission pathways in solid-state salicylaldimines

This study examines six salicylaldimines in their crystalline form, analyzing their photoluminescent properties at both room and liquid helium temperatures (5 K). Obtained results provide a solid-state perspective on the photophysical characteristics of a class of donor-acceptor compounds that were previously analyzed in different solvents (Journal of Molecular Liquids, 2021, 343, 117532). Consistent with measurements performed in solution, four donor-acceptor salicylaldimines (1–4) exhibit solely LE emission, while analogous naphthyl derivatives (5 and 6) exhibit ESIPT occurrence in the solid state. This finding correlates strongly with the estimated height of the ESIPT reaction barrier computed at the ADC(2)/cc-pVDZ level of theory. Consequently, 1–4 deviate from the traditional classification of salicylideneanilines into thermochromic or photochromic materials. In contrast, 5 and 6 show weak dual fluorescence with the dominant ESIPT emission. The crystal packing restricts large-amplitude motions, resulting in quantum yields over one order of magnitude higher than those in solution. Interestingly, the anisotropy of the polycrystal favors the antiparallel stacking of 1–5, which promotes the population of a dark CT state upon excitation, which competes with the LE emission at low temperature, manifested by the observed growth of LE fluorescence intensity shortly after excitation.