The electron binding energies for the trans and cis conformers of azobenzene (AB), a prototypical photoswitch, were investigated by electron propagator theory (EPT). The EPT results are compared with data from photoelectron and electron transmission spectroscopies and complemented by the calculation of the differences between vertical and adiabatic ionization energies and electron affinities of the AB conformers. These differences are discussed in terms of the geometry changes associated with the processes of ionization and electron attachment. The results pointed out a major difference between these processes when we compare trans-AB and cis-AB. For trans-AB, electron attachment leads to a small geometry change, whereas for cis-AB, it is the ionized structure that keeps some similarity with the neutral species. We emphasize the interest of the present results for a better understanding of recent experiments on the dark cis-trans isomerization in different environments, specifically for azobenzenes in interaction with gold nanoparticles, where the proposed cis-trans isomerization mechanism relies on electron transfer induced isomerization.