Facile activation of an unconjugated zwitterionic squaraine dye for tunable fluorescence and morphology behaviors†

The regulation of organic microcrystals with different fluorescence and morphological behaviors is pivotal for their applications; however, it is still complex to implement. Herein, we present the facile preparation of tunable light-emitting ionic organic microcrystals by the in situ self-assembly of a non-emissive squaraine dye (SQH) with various amines (NH₂Et, NHEt₂, NEt₃) or NEt₄OH. These behaviors primarily arise from the activation of conjugation in the surprisingly unconjugated configuration of zwitterionic SQH, which forms anionic SQ⁻ accompanied by ammonium counter ion in the microcrystals. The fluorescence quantum yields of these ionic microcrystals ranged from 18.6% to 68.2%, along with a change from two-dimensional to one-dimensional morphology, which is closely associated with their distinct cation–anion arrangements. Notably, the SQ–NH₂Et₂ microcrystals exhibited reversible vapoluminescence behavior at room temperature, enabling easy cryptographic application.