Novel NIR DDNP-type solvatochromic and fluorogenic probes for amyloid β detection

Selective near-infrared fluorescent probes for amyloid β aggregates are of considerable interest as molecular imaging tools for the diagnosis and study of Alzheimer's disease. Herein, we describe the rational design and synthesis of DDNP-type donor-π-acceptor fluorophores incorporating a fixed N,N-dimethylamino donor group, while systematically modifying seven aromatic π-bridges and four electron-withdrawing acceptor groups to investigate structure-property relationships. The library of 26 probes is a result of multi-step synthetic routes and exhibits pronounced solvatochromic behaviour together with strong fluorogenic responses. Compounds' photophysical properties, including absorption and emission maxima, Stokes shifts, and quantum yields, are systematically evaluated in five solvents of varying polarity. The study reveals clear structure-property relationships, demonstrating the influence of π-bridge length and electron-acceptor strength on the optical characteristics, including near-infrared emission. Selected fluorophores are further assessed for their affinity towards amyloid β fibrils through a combination of in vitro experiments and in silico molecular docking studies. Among the compounds investigated, the N,N-dimethylamino-stilbene-malononitrile derivative emerges as the lead candidate, showing an optimal balance of favourable photophysical properties and strong amyloid β binding affinity. Subsequent evaluation using brain tissue obtained post mortem from a patient with Alzheimer's disease demonstrates selective staining of amyloid β plaques with minimal background fluorescence.