Internal conversion, an electron-spin conserved non-radiative loss pathway, is a process of great importance and interest, mediated by non-adiabatic, or vibronic, interactions (the electronic-vibrational coupling between two electronic states). This loss mechanism interplays and competes with every other mechanism of interest; vast and ranging, they can be tracked to all the major fields of science. This includes photon harvesting mechanisms and applications , charge and proton transfer in biological systems , energy transfer between excited states , and thermally activated delayed fluorescence and phosphorescent systems .