The non-emissive behaviour of 2-(phenylazo)pyridine, azpy, observed in the fluorescence experiment was explained by investigating its photoisomerisation mechanism. By using the Density Functional Theory and Time-dependent Density Functional Theory (DFT/TDDFT) methods, the two-dimensional potential energy surfaces in the ground state, and the first and second excited states which are relevant to the trans-to-cis isomerisation were constructed. The pathways related to the inversion, rotation and their concerted motion of the phenyl and pyridyl groups around N=N bond were considered. The energy profiles depend not only on the type of pathways but also on the moving group. The concerted-inversion pathways show the possibility of S 1/S 2 and S 0/S 1 conical intersections. The relaxation through such intersections should increase the non-radiative transition rate to the extent that the compound does not emit fluorescence. © 2011 Canadian Society for Chemical Engineering.