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.