A typical sensitizer, PtOEP, was combined with DPA2BPH
4 to construct the electric field-responsive TTA-UC system. The individual absorption and emission spectra of DPA2BPH
4 and PtOEP in acetonitrile are depicted in Fig. S12 (Supporting information). Upon selectively exciting PtOEP of the TTA-UC system by 532 nm light, the phosphorescence of PtOEP was significantly quenched by DPA2BPH
4, accompanied by the appearance of typical upconverted emission from the DPA chromophore. The upconverted emission increased while the phosphorescence of PtOEP decreased as the DPA2BPH
4 concentration increased, clearly demonstrating concentration-dependent TTA upconversion. The upconversion emission and quantum yield reached a plateau with DPA2BPH
4 of greater than 200 µmol/L (Figs. S13 and S14 in Supporting information). When a voltage of 10 V was applied, the luminescence near the anode turned red and the blue emission near the cathode intensified under the radiation of 365 nm light (
Fig. 3a), and a clear boundary in the middle between to electrode appeared. The emission change is attributed to the electrophoretic shift of the cationic DPA chromophores toward the cathode, leading to more quenching of the phosphorescence of PtOEP. The applied voltage had no effect on the phosphorescence distribution in the control experiment with only PtOEP (Figs. S15 and S16 in Supporting information), further confirming that the emission change resulted from the electrophoresis of DPA2BPH
4. The effect of electric field on the upconversion emission was observed under the excitation with a 532 nm laser (200 mW/cm
2). The emission of TTA-UC solution presented the superposition of the blue upconversion emission and the red phosphorescence of PtOEP, which was visible as magenta emission. The DPA2BPH
4/PtOEP system displays a prolonged upconverted emission lifetime of 17 µs in contrast to DPA2BPH
4 directly excited by UV laser (5.2 ns, Fig. S8), which is characteristic of delayed fluorescence arising from TTA of the long-lived triplet (Fig. S17 in Supporting information). In the absence of electric field, the upconversion emission and the phosphorescence seen behind a 470 nm and a 660 nm bandpass filter, respectively, were even along the laser pathway (
Fig. 3b). After applying the electric field, the upconversion emission near the cathode became more intense while the phosphorescence near the anode became more evident. The mixed color in the corresponding area seen with the naked eye changed as well. The emission spectra collected in the cathode areas gave a
ca. 20% increase of upconversion emission and a decrease of phosphorescence under the electric field, whereas the anode area showed the opposite change (
Fig. 3c). In the absence of electric field, the QY of the DPA2BPH
4 (60 µmol/L)/PtOEP (10 µmol/L) is 1.16%. After applying the voltage, the quantum yields in the cathode and the anode region are estimated to be 1.43% and 0.87%, respectively, based on the percentage of the emission enhancement and reduction.