Os(II) Carbonyl Complexes Bypassing Intersystem Crossing for Visible-to-UV Upconversion at 532 nm

Triplet-triplet annihilation (TTA) upconversion from visible to ultraviolet (UV) light is highly desirable for driving challenging photochemical reactions, yet its excitation wavelength has been largely limited to the blue region (< 480 nm) due to the substantial energy loss associated with intersystem crossing (ISC) in conventional photosensitizers. In this study, we report two novel osmium (Os) carbonyl complexes, Os-teph-CO and Os-tebp-CO, which feature direct S₀ → T₁ absorption, thereby completely eliminating ISC-related energy losses. By introducing carbonyl ligands, the triplet energy levels of these complexes are significantly raised to match the UV annihilator 1,4-DTNA. Consequently, green-to-UV TTA upconversion is achieved with an excitation wavelength extended to 532 nm, which is the longest reported to date for visible-to-UV TTA upconversion. The systems exhibit upconversion quantum yields of up to 2.5% (maximum value is 50%), large anti-Stokes shifts of 0.99 eV, and good photostability. The practical utility of this upconversion system is further demonstrated by visible‑light‑driven photoinitiated radical polymerization, where the upconverted UV photons successfully initiate free‑radical polymerizations. This work establishes Os carbonyl complexes as a new class of photosensitizers that overcome the traditional ISC bottleneck, opening the door to longer-wavelength excitation (potentially up to 570 nm) for visible-to-UV TTA upconversion.