“…Low-energy photons can be converted to higher-energy photons by a photon up-conversion process; such photon up-conversion has attracted interest for using low-energy photons in the solar spectrum, in organic light-emitting diodes, and in bioimaging, among other applications. − Recently, among various photon up-conversion processes, triplet–triplet annihilation (TTA) up-conversion has been intensively studied for its efficient up-conversion under low excitation power densities. ,− ,− Although efficient TTA up-conversion in solutions has been reported, ,− TTA up-conversion in solid matrixes is preferred for practical applications. ,− ,,− TTA in solutions occurs when excited molecules are brought into a close-contact state by diffusion at room temperature; when the molecules are excited by pulsed light, the fluorescence decay from the excited singlet state generated by the TTA shows relatively simple exponential kinetics, with a typical lifetime of one-half the natural lifetime of the triplet exciton. ,− Even the exceptional case of nonexponential kinetics can be theoretically interpreted on the basis of the bulk second-order reaction influenced by the first-order decay originating from the natural lifetime of the triplet exciton. ,− In solid matrixes, the emission decay kinetics after pulsed-light excitation is more complex because it depends on the excitation light intensity and the temperature. The complexity of the TTA kinetics in solid matrixes compared with that in solutions might originate from processes such as the slow mixing of triplet excitons by migration,the strong dependence on the initial distribution of triplet excitons, and the anisotropic random walk of excitons, but the process underlying the different TTA kinetics has not yet been elucidated.…”