Annihilation Versus Excimer Formation by the Triplet Pair in Triplet-Triplet Annihilation Photon Upconversion

Abstract

The triplet pair is the key functional unit in triplet-triplet annihilation photon upconversion. The same molecular properties that stabilize the triplet pair also allow dimers to form on the singlet energy surface, creating an unwanted energy relaxation pathway. Here we show that excimer formation most likely is a consequence of a triplet dimer formed before the annihilation event. Polarity-dependent studies were performed to elucidate how to promote wanted emission pathways over excimer formation. Furthermore, we show that the yield of triplet-triplet annihilation is increased in higher-viscosity solvents. The results will bring new insights in how to increase the upconversion efficiency and how to avoid energy-loss channels.

Scheme 1

(a) The proposed mechanism for excimer formation in TTA-UC. (b) Proposed energy conversion diagram for excimer formation in TTA-UC.

Figure 1

(a) Molecular structure of perylene, PdOEP, and PtTBTP. (b) Normalized absorption and emission spectra of perylene, PdOEP, and PtTBTP. (c) Normalized emission spectra of 10 μM PdOEP (excited at 532 nm) or PtTBTP (excited at 617 nm) and 1 mM perylene in THF.

Figure 2

Quantum yield of (a) monomer emission (Φ_M) and (b) excimer emission (Φ_E) in different solvents with 10 μM PtTBTP and 1 mM perylene. Hex, Oct, CyHex, Dodec, Hexadec, Tol, and THF stand for hexane, octane, cyclohexane, dodecane, hexadecane, toluene, and tetrahydrofuran, respectively. Reported values are an average of three individual measurements.

Figure 3

(a) Emission spectra of mixtures of 10 μM of PtTBTP and 10, 20, 40, 100, 200, 400, 600, or 1000 μM of perylene in deoxygenated THF solution excited at 617 nm. (b) Quantum yield of monomer and excimer emission. (c) Ratios between excimer and monomer emission quantum yields. Experimental data are labeled with circles (excited at 617 or 380 nm), and lines show modeling using mechanisms 1–3.

Figure 4

Transient absorption spectra of (a) 10 μM PtTBTP or (b) 10 μM PtTBTP and 1 mM perylene in deoxygenated THF after excitation at 617 nm.

Figure 5

(a) Transient absorption decays at 485 nm of 2 μM PtTBTP and 1 mM perylene in deoxygenated alkane solvents. (b) The calculated intrinsic triplet perylene decay rate constants vs solvent viscosity.