Photochemical upconversion and triplet annihilation limit from a boron dipyrromethene emitter

Abstract

Non-coherent sensitized red-to-green upconversion has been achieved utilizing platinum(II) tetraphenyltetrabenzoporphyrin (PtTPTBP) as the triplet sensitizer and a nearly quantitatively fluorescent meso-(2,6-dichloropyridyl)-substituted boron dipyrromethene (Cl₂PyBODIPY) chromophore (Φ = 0.99 in toluene) as the energy acceptor/annihilator in deoxygenated toluene. Dynamic Stern–Volmer analysis revealed that PtTPTBP phosphorescence as quenched by Cl₂PyBODIPY occurs with a K_SV of 108 000 M⁻¹, yielding a triplet–triplet energy transfer rate constant of 2.3 × 10⁹ M⁻¹ s⁻¹. Using a non-coherent red light-emitting diode excitation source centered at 626 nm, the incident power dependence responsible for generating singlet BODIPY fluorescence in the green was shown to traverse quadratic to linear regimes, the latter being achieved near 60 mW cm⁻². These data were consistent with a photochemical upconversion mechanism being responsible for generating singlet fluorescence from the Cl₂PyBODIPY chromophores through sensitized triplet–triplet annihilation (TTA). Integrated delayed fluorescence transients were utilized to reveal the TTA efficiency for the Cl₂PyBODIPY chromophore and saturated near 46%, representing the lower limit for the TTA process. Kinetic modelling of the delayed fluorescence transient produced from 1.5 mJ laser pulses (λ_ex = 615 nm) revealed a maximum limiting TTA efficiency of 64% for this upconverting composition, implying that this is indeed an extremely relevant acceptor/annihilator composition for photochemical upconversion.