We next sought to establish TTA-UC as the subsequent step to TTEnT via photoexcitation of PdPc at 730 nm and recording the up-converted fluorescence from the tetracene derivatives annihilators in the range of 500-650 nm. PdPc and annihilator concentrations were held constant at 4.5 × 10 -5 and 2.5 × 10 -4 M, respectively. The power-law dependence of the integrated up-converted fluorescence intensity versus the incident photon power is the hallmark of TTA-UC. At low incident power densities, some PdPc triplet excited states will not collide with an annihilation 60 partner, but will decay non-radiatively. Under these ). At annihilator concentrations as 15 low as 1.0 × 10 -5 M, where diffusion-controlled inter-TTA-