Triplet-triplet annihilation photon-upconversion in hydrophilic media with biorelevant cholesteryl triplet energy acceptors

We report two new biorelevant cholesteryl-based triplet energy acceptors, derivatives of DPA or 9,10-diphenylanthracene (C–DPA and C₂–DPA). Using two different triplet sensitizers: QDN (E_T ≈ 1.67 eV in PEG200) and PdTPP (E_T ≈ 1.78 eV in PEG200), we were able to achieve both endothermic (with QDN) and exothermic (with PdTPP) triplet sensitization of DPA, C–DPA and C₂–DPA in hydrophilic PEG200 media. While the maximum rate of triplet energy transfer (TET) was achieved with PdTPP and DPA (k_TET = 4.7 × 10⁷ M⁻¹ s⁻¹), for the cholesteryl-based acceptors, we found that the kinetic of the TET process was dependent upon the concentration of the acceptor. For PdTPP/C–DPA pair, the rate of the dynamic triplet energy quenching was k_TET = 1.9 × 10⁷ M⁻¹ s⁻¹; however, at higher concentrations of the quencher, the system reached a stationary state due to formation of self-assembled sub-domains of C–DPA that likely slowed the TET process. It was also found that this aggregation of C–DPA in PEG200 led to a 3.5 folds increase in the I_th compared to 133 mW cm² for DPA. Subsequently, we estimated the Φ_UC for these donor/acceptor pairs: QDN/DPA, PdTPP/DPA, and PdTPP/C–DPA. With respect to the estimated threshold intensity (I_th), we found that the quantum yields of TTA-UC were 2 ≤ QY_UC≤12%.