Thermally induced transition from ordinary phosphorescence to delayed fluorescence in two kinds of luminescent copper(i) complexes is comprehensively investigated by using variable-temperature time-integrated and time-resolved photoluminescence measurements as well as model analysis. A pronounced impact of the molecular structure on exciton transfer from the lowest excited triplet spin states to the singlet spin states with higher energy is firmly demonstrated. Moreover, several fundamental photophysical processes including triplet localization, triplet harvesting, and reverse intersystem crossing are explored using theoretical models. Temperature dependence abnormalities of the emission intensity are quantitatively interpreted. Raman spectral characterization and theoretical calculations of vibronic emission transitions reveal that the molecules' thermal vibrations play an essential role in the fluorescence process.