A Study of Redox Mediator Types and Their Quenching Effects on the Triplet-Triplet Annihilation Photon Upconversion Process
Journal of Physical Chemistry C
Photon upconversion based on sensitized triplet-triplet annihilation has garnered a great deal of attention for various applications, such as solar photon harvesting, photochemistry, and biological imaging. Its implementation, however, remains limited in part due to the presence of undesirable excited-state quenching by redox-active species. Herein, this process and its constituent steps are studied in the presence of three classes of redox mediators, which specifically are as follows: iodide-triiodide (I-/I3-), cobalt(II/III) [Co(II/III)], tris-bipyridine [Co(bpy)32+/3+], and tris(p-anisyl) amine (TPAA0/+). Both the mediators’ classes and concentrations were shown to have a significant effect on the quenching of upconverted emission, which is seen to result largely from the quenching of the sensitizer triplet excited state. Our findings revealed that the small organic and heavy element-free redox mediator, TPAA0/+, showed the lowest quenching rate constant (3.73 × 105 M-1 s-1) of the sensitizer triplet states, which is 2 and 3 orders of magnitude lower than that of I-/I3- and Co(bpy)32+/3+, respectively. We also found that the quenching of the sensitizer triplet states is attributed to the charge and energy transfer as well as paramagnetic quenching based on tests in the presence of just the Co(II) species. The findings reported herein represent an important step toward minimizing the TTA-UC quenching by minimizing the quenching effects using organic and heavy element-free redox mediators. This makes it promising for future applications such as the intermediate band dye-sensitized solar cell architecture, which is a promising approach to effectively utilize low-energy photons.
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Australian Research Council