Tetracene Dimers: A Platform for Balancing Intramolecular Up- and Down-Conversion

Abstract

Photon energy conversion can be accomplished in two opposing manners, down conversion (i.e., singlet fission, SF), and up-conversion (often referred to as triplet-triplet annihilation up-conversion, TTA-UC), both of which have the potential to help overcome the Shockley-Queisser limit of single-junction solar cells. Inter-chromophore electronic coupling plays opposing roles in these two processes. The energies of the lowest singlet excited state and twice the triplet excited state are comparable in tetracene dimers, which exhibit both up- and down-conversion. Here, we have designed meta-phenylene- and 1,3-diethynyladamantyl-linked tetracene dimers with different electronic coupling to probe the interplay between intramolecular SF (intra-SF) and intramolecular TTA-UC (intra-TTA-UC) viasteady-state and time-resolved absorption and fluorescence spectroscopy. In addition, temperature-dependent measurements are carried out to shed light on the thermal effects on intra-SF and intra-TTA-UC. We have also used a Pd-phthalocyanine as photosensitizer to enable intra-TTA-UC in the two dimers via indirect photo-excitation in the near-infrared. These results not only reveal the interplay between intra-SF and intra-TTA-UC, but also provide guidelines and strategies for the design of photon down- and up-converting materials.