Heavy-Atom-Free Red-to-Yellow Photon Upconversion in a Thiosquaraine Composite

We demonstrate a completely heavy-atom-free red-to-yellow triplet–triplet annihilation (TTA) photon upconversion system using a thionated squaraine sensitizer, both in fluid solution and in a solid-state composite architecture. Previous works have shown that thionation introduces sulfur nonbonding (n) orbitals that invert the native energy ordering of the ππ*(S1) and nπ*(S2) singlet transitions on the squaraine core, opening a channel for efficient intersystem crossing in the thiosquaraine without relying on the heavy-atom effect, as expected based on El-Sayed’s rule. Our thiosquaraine [2-(4-(dibutylamino)phenyl)-4-(4-(dibutyliminio)cyclohexa-2,5-dien-1-ylidene)-3-thioxocyclobut-1-enethiolate] exhibits an intense red absorption band, no measurable room-temperature fluorescence, and a native triplet lifetime on the order of 20 μs. This triplet is readily quenched (kQ = 1.4 × 109 M–1 s–1) upon sensitizing the triplet excited state of rubrene as a model upconversion emitter. We observe a 0.27 eV anti-Stokes shift, with selective 685 nm excitation of the thiosquaraine resulting in upconverted rubrene fluorescence centered at 570 nm. The system shows an upconversion quantum efficiency of ∼1.5% in deaerated toluene solution. This quantum efficiency is defined based on a maximum 50% quantum efficiency for TTA upconversion. This system exhibits upconversion under filtered (650 nm long-pass) simulated solar illumination and an intensity transition from quadratic to linear optical power dependence at ∼150 W/cm2 under 685 nm laser diode illumination. We also apply this thiosquaraine system to demonstrate red-to-yellow photon upconversion in a solid-state polymer composite, a prerequisite for light-harvesting device integration. In contrast with traditional TTA upconversion photosensitizers incorporating cost-prohibitive precious metals or photolabile arylhalide groups, we present an easily tunable squaraine dye that serves as a promising red-absorbing heavy-atom-free upconversion sensitizer for increased scalability and photostability.