NIAD-4 is a novel class of fluorescent probe, having good blood-brain barrier permeability, specificity to amyloid-β plaques, and "turn-on" bright far-red emission upon Aβ binding. NIAD-4 has a structure geometry that is readily planarized and results in a red shift upon binding to Aβ. The highly polarizable bridge provides a substantial bathochromic shift in both absorption and emission spectra of the agent.