A new two series of phenylazo indole dyes was synthesized and the structures of the dyes were confirmed by UV vis, FT-IR, HRMS and H-1/C-13 NMR spectroscopic techniques. Five of these dyes (I, I', II', HI and were also characterized in solid state by using single crystal X-ray diffraction studies besides other spectroscopic techniques. The geometries of the azo and hydrazone tautomeric forms of the dyes were optimized by using Density Functional Theory (DFT). In addition, the effects of the donor and acceptor groups on the azo and hydrazone forms of the dyes were evaluated experimentally and theoretically. The results indicate that the phenylazoindole dyes derived from 2-phenyl indole as coupling component exist as azo form in solution, gas phase and solid state. (C) 2014 Elsevier B.V. All rights reserved.