The main framework of theories and experiments on energy transfer were elaborated in the middle of the last century by studying organic crystals of, for example, anthracene or tetracene. In this period, the description of phenomena such as annihilation of excited states, delayed fluorescence, and magnetic-field effects established the basis of our current understanding of excited-state interactions between chromophores (see for example refs.[1, 2]). Recently,[3, 4] advances in organic synthesis of multichromophoric systems and the development of new ultrasensitive fluorescence-detection techniques, that is, single-molecule spectroscopy (SMS), has encouraged investigations of excited-state interactions between chromophores on a molecular scale, focusing on different interaction regimes depending on the special interrelation of chromophores in such multichromophoric systems.

Single-molecule spectroscopy is complementary to ensemble measurements but offers the additional advantage of being able to measure distributions of properties instead of averages and allows one to reveal rare events. One such rare event is the annihilation of one exciton after absorption of two photons during the excited-state lifetime, a process generally termed singlet–singlet annihilation (SS annihilation). So far, SS annihilation was revealed by a laser-intensity-dependent additional short component in fluorescence up-conversion and transient absorption measurements.[5, 6] Furthermore, it is often assumed that double excitations play only a minor role in the photophysics of multichromophoric systems under relatively