The present invention generally relates to various photoreactions, including reactions generally based on triplet-triplet annihilation upconversion. One aspect of the present invention is directed to systems and methods for absorbing energy (e.g., from a photon) in a photo sensitizer, transferring that energy by triplet-triplet energy transfer to an annihilator to produce a higher energy state via upconversion, then transferring that energy to cleave a cleavable or other active moiety, for instance, in order to cause the release of a releasable moiety. The energy may be transferred to the moiety via Forster resonance energy transfer. In some cases, these may be contained within a suitable carrier material, for example, a particle or a micelle. Such systems and methods may be used in a variety of applications, including various biological or physical applications. For example, such systems and methods may be useful for delivering drugs or other releasable moieties to regions of the body which may be affected by too much light, such as the eye. Other aspects of the present invention are generally directed to methods for making or using such systems, kits including such systems, or the like.
