Nonconjugated Alkyl Chains in Ionic Molecular Rotor Fluorophores: An Effective Modulating Moiety for Amyloid-β Probes

Developing molecular structural tuning strategies for ionic molecular rotor (MR)-based β-amyloid (Aβ) fluorescent probes is essential for balancing high sensitivity, signal-to-background ratio (SBR) in wash-free mode, and blood–brain barrier (BBB) permeability. Compared with the widely used adjustment strategy on the electron donor, π-bridge, and electron acceptor, the nonconjugated hydrophobic part that can directly affect the lipophilicity and dispersion of the whole ionic MRs has been omitted. Herein, we developed a series of diarylbutadiene ionic MRs with variable alkyl chains substituted on the nitrogen atom of pyridinium (DM-Cn), and unveiled the relationship between the length of alkyl (ethyl to undecyl) and the spectral response, binding process, and imaging performance. The N-alkyl extension enhanced the sensitivity, strengthened the binding affinity, and facilitated BBB permeability; however, it hindered achieving a high SBR in the wash-free mode. We successfully deployed three representatives (DM-C2, DM-C7, DM-C11) for both in vitro and in vivo imaging of the Aβ plaques in AD mice, among which DM-C7 with a finite extended N-alkyl chain displayed the most satisfactory and balanced sensing performance. The findings of this study establish a decisive structure–performance relationship for ionic MR-based Aβ probes, which can facilitate the on-demand design of new probes and imaging applications.