As a particle and its antiparticle annihilate each other a huge amount of energy is released, and no mass is left. This energy always comes in the form of force mediating particles (photons, gluons). Can´t we say that this is the definition of pure energy because all the mass has disappeared? If a photon has energy, then according to the definition it is its ability to do work. But what´s the difference between the ability of a photon to to work and the ability to do work for a particle with electric charge? A photon doesn´t do work as an electron does in an electric field. The photon can´t move against a force, as the photon itself is a particle that makes up a force field. The electron cán move aginst a force thereby acquiring potential energy. The photon just disappears because in can be absorbed by the particles that produce the field.
Maybe I just want to say that a photon (or a gluon) ís energy while particles of matter possess energy.