Supermolecular-Chromophore-Sensitized Near-Infrared-to-Visible Photon Upconversion

Abstract

Selective near-IR (NIR) excitation (780 nm) of the conjugated supermolecule ruthenium(II) [15-(4′-ethynyl-(2,2′;6′,2′′-terpyridinyl))-bis[(5,5′,-10,20-di(2′,6′-bis(3,3-dimethylbutoxy)phenyl)porphinato)zinc(II)]ethyne][4′-pyrrolidin-1-yl-2,2′;6′,2′′-terpyridine] bis(hexafluorophosphate) (Pyr₁RuPZn₂) in solutions containing N,N-bis(ethylpropyl)perylene-3,4,9,10-tetracarboxylicdiimide (PDI) or tetracene gives rise to a substantial anti-Stokes energy gain (PDI, 0.70 eV; tetracene, 0.86 eV). Experimental data clearly demonstrate that this upconverted fluorescence signal is produced via Pyr₁RuPZn₂-sensitized triplet−triplet annihilation (TTA) photochemistry. The TTA process was confirmed by the quadratic dependence of the integrated ¹PDI* emission centered at 541 nm derived from 780 nm laser excitation. The T₁→T_n excited state absorption decay of Pyr₁RuPZn₂, monitored at 900 nm as a function of PDI concentration, revealed Stern−Volmer and bimolecular quenching constants of 10 048 M⁻¹ and 5.9 × 10⁸ M⁻¹ s⁻¹, respectively, for the PDI triplet sensitization process. The T₁→T_nPDI extinction coefficient at 560 nm (ε_T = 6.6 × 10⁴ M⁻¹ cm⁻¹) was determined through the triplet energy transfer method utilizing anthracene as the donor chromophore. ³PDI* transient triplet absorption dynamics observed as a function of 485 nm incident nanosecond pump laser fluence demonstrate a bimolecular ³PDI*−³PDI* TTA rate constant (k_TT = 1.0 ± 0.2 × 10⁹ M⁻¹ s⁻¹). The maximum quantum yield of the supermolecule-sensitized PDI upconverted emission (Φ_UC = 0.0075 ± 0.0002) was determined relative to [Os(phen)₃][PF₆]₂ at an incident laser power of 22 mW at 780 nm. This study successfully demonstrates NIR-to-visible photon upconversion and achieves a new record anti-Stokes shift of 0.86 eV for sensitized TTA, using the supermolecular Pyr₁RuPZn₂sensitizer. The stability of the Pyr₁RuPZn₂/PDI chromophore combination is readily apparent as continuous irradiation at 780 nm produces 541 nm centered fluorescence with no significant decrease in intensity measured over time domains exceeding several hours. The molecular components of these NIR-to-vis upconverting compositions illustrate that substantial anti-Stokes energy gains via a TTA process can be effortlessly realized.