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

Abstract

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.