Fluorescence intensity and quantum yield of organic dyes often depend on their internal rotation in the excited electronic state. NIAD-4 is a novel amyloid-binding dye and a possible substitute for Thioflavin T in amyloid tagging. Its structure suggests that it should act like a molecular rotor whose photophysical properties are strongly governed by internal rotation, although little is known about the photophysics of this molecule to date. We investigated the molecular rotor property of NIAD-4 by measuring the viscosity dependence of fluorescence intensity against the molecular rotor model. The origin of torsion-dependent fluorescence switching of NIAD-4 is discussed. (C) 2015 Elsevier B.V. All rights reserved.