(4Z)-4-(4-N,N-dimethylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5H -imidazolin-5-one (DMPI), an analog of the green fluorescent protein (GFP) luminophore has been investigated by steady state absorption and emission spectroscopic studies. The polarity and viscosity dependent fluorescence solvatochromic studies along with time dependent density functional theory (TDDFT) calculations support a molecular rotor property of DMPI and not the 'hula-twist mechanism'. The results suggest that the protein environment around the GFP luminophore could be playing a similar role by increasing the viscosity around the luminophore leading to the enhanced quantum yield of GFP. (C) 2010 Elsevier B. V. All rights reserved.