We discuss coherent control of the centre of mass motion of complex molecules by de Broglie interferometry. We describe an experiment to couple the dynamics of internal state population of complex molecules to their centre of mass motion. We discuss how this can be used to probe state population and transition, especially the photo-switching of flourinated di-azobenzene molecules between their cis- and trans-configuration. We propose an experiment to photo-isomerise complex di-azobenzene molecules in the gas-phase, including the selective detection of molecules in different conformations. In addition we discuss possible ways of optimising the conformation detection through cooling, and optical techiques.