Ping-Pong Energy Transfer in a Boron Dipyrromethane Containing Pt(II)–Schiff Base Complex: Synthesis, Photophysical Studies, and Anti-Stokes Shift Increase in Triplet–Triplet Annihilation Upconversion

A boron dipyrromethane (BDP)-containing Pt­(II)–Schiff base complex (Pt-BDP), showing ping-pong singlet–triplet energy transfer, was synthesized, and the detailed photophysical properties were investigated using various steady-state and time-resolved transient spectroscopies. Femtosecond/nanosecond transient absorption spectroscopies demonstrated that, upon selective excitation of the BDP unit in Pt-BDP at 490 nm, Förster resonance energy transfer from the BDP unit to the Pt­(II) coordination center occurred (6.7 ps), accompanied by an ultrafast intersystem crossing at the Pt­(II) coordination center (<1 ps) and triplet–triplet energy transfer back to the BDP moiety (148 ps). These processes generated a triplet state localized at BDP, and the lifetime was 103.2 μs, much longer than the triplet-state lifetime of Pt-Ph (3.5 μs), a complex without the BDP moiety. Finally, Pt-BDP was used as a triplet photosensitizer for triplet–triplet annihilation (TTA) upconversion through selective excitation of the BDP unit or the Pt­(II) coordination center at lower excitation energy. An upconversion quantum yield of up to 10% was observed with selective excitation of the BDP moiety, and a large anti-Stokes shift of 0.65 eV was observed upon excitation of the lower-energy band of the Pt­(II) coordination center. We propose that using triplet photosensitizers with the ping-pong energy-transfer process may become a useful method for increasing the anti-Stokes shift of TTA upconversion.