Tetracene Dimers: A Platform for Intramolecular Down- and Up-conversion

Abstract

Photon energy conversion can be accomplished in many different ways, including the two opposing manners, downconversion (i.e., singlet fission, SF) and up-conversion (i.e., triplettriplet annihilation up-conversion, TTA-UC). Both processes have the potential to help overcome the detailed balance limit of singlejunction solar cells. Tetracene, in which the energies of the lowest singlet excited state and twice the triplet excited state are comparable, exhibits both down-and up-conversion. Here, we have designed meta-diethynylphenylene-and 1,3-diethynyladamantyl-linked tetracene dimers, which feature different electronic coupling, to characterize the interplay between intramolecular SF (intra-SF) and intramolecular TTA-UC (intra-TTA-UC) via steady-state and time-resolved absorption and fluorescence spectroscopy. Furthermore, we have used Pd-phthalocyanine as a sensitizer to enable intra-TTA-UC in the two dimers via indirect photoexcitation in the near-infrared part of the solar spectrum. The work is rounded off by temperature-dependent measurements, which outline key aspects of how thermal effects impact intra-SF and intra-TTA-UC in different dimers.

Enhancing NIR-to-visible upconversion in a rigidly coupled tetracene dimer: approaching statistical limits for triplet–triplet annihilation using intramolecular multiexciton states

“…In related rigid tetracenic dimers such as TIPS-BT1 62 and TIPS-BT1′, 63 where the acene chromophores are separated by a single norbornyl bridge, we have observed formation of the multiexciton singlet 1 TT in equilibrium with S 1, but this equilibrated set of states decays to the ground state without observation of isolated T 1 . However, other non-rigid tetracenic dimer systems have shown long-lived T 1 populations that may be presumed to originate from the multiexcitonic triplet ( 2S+1 TT) manifold, likely following internal conversion via the 3 TT, 56,67,69 something that is more commonly seen in pentacene-based dimers ( e.g. ref.…”

“…First, the greater than 20× improvement in ϕ UC compared to TIPS-Tc is more than 2× larger than the advantage reported for a non-rigid dimer compared to the same monomer. 47,56 This could reflect a situation where dimer rigidity prevents intramolecular 5 TT → 3 TT → T 1 loss channels while allowing for 5 TT → 1 TT → S 1 productive channels. Second, we calculate a spin-statistical value of f = 0.42 for TIPS-BTX with evidence that f would increase in studies with higher concentrations of the dimer.…”

“…Finally, Guldi, Tykwinski, and coworkers recently explored two non-rigid tetracene dimer types with either a meta-substituted diethynylphenylene bridge or a 1,3-diethynyladamantyl bridge relative to monomer models with bridge-specific substituents. 56 They also found that the dimers are brighter TTA-UC emitters compared to the respective monomer models at comparable annihilator concentrations, with relative ratios of approximately 4 and 3 for the phenylene and adamantyl comparisons, respectively. They have also considered upconversion yields ϕ UC , as we do here, and at the highest annihilator concentrations they consider, found ratios of 3.7 and 2.8 favoring the dimers for the phenylene and adamantyl comparisons, respectively.…”

“…We and others have considered potential benefits within dimers and oligomers comprising covalently fused annihilators. 26,33,47,49–57 In principle, intramolecular multiexciton states in such systems could participate in excited state dynamics, perhaps in productive ways. 33 From a technological perspective dimers and oligomers are also interesting in settings where one cannot rely on diffusion to bring pairs of triplet annihilator species together, for example at nanomaterial/molecular interfaces where the nanomaterial functions as a triplet sensitizer to the bound species.…”

Tetracene cyclophanes showing controlled intramolecular singlet fission by through-space orientations

“…5 Many examples of covalently-bridged acyclic acene dimers for intramolecular SF (ISF) have been reported, so far. 6–19 We recently demonstrated the importance of a new structural parameter: “conformational flexibility”, because the structural change from TT to T 1 + T 1 is required in addition to the conventional “electronic coupling” associated with the ISF rate constants. 5 In intramolecular reactions of covalently linked acyclic dimers ( e.g.…”