Bioinspired Orientation of β-Substituents on Porphyrin Antenna Ligands Switches Ytterbium(III) NIR Emission with Thermosensitivity.

"Configurational isomerism" is an important approach found in naturally occurring chlorophylls to modulate light harvesting function without significant structural changes; however, this feature has been seldom applied in design of antenna ligands for lanthanide (Ln) sensitization. In this work, we introduced a bioinspired approach by orientation of β-dilactone moieties on porphyrinates, namely cis-/trans-porphodilactones, to modulate the energy transfer process from the lowest triplet excited state of the ligand (T1) to the emitting level of ytterbium(III) (2F5/2, Yb*). Interestingly, near-infrared (NIR) emission of Yb(III) could be switched "on" by the cis-porphodilactone ligand, while the trans-isomer renders Yb(III) emission "off" and the ratio of quantum yields is ∼8. Analysis of the structure-photophysical properties relationship suggests that the significant emission difference is correlated to the energy gaps between T1 and Yb* (1152 cm-1 in the cis- vs -25 cm-1 in the trans-isomer). More interestingly, due to back energy transfer (BEnT), the Yb(III) complex of cis-porphodilactone exhibits NIR emission with high thermosensitivity (4.0%°C-1 in solution and 4.9%°C-1 in solid state), comparable to previously reported terbium (Tb) and europium (Eu) visible emitters, in contrast to the trivial emission changes of the trans-isomer and porphyrin and porpholactone analogues. This work opens up new access to design NIR emissive Ln complexes by bioinspired modification of antenna ligands.