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

Important applications of photon upconversion through triplet-triplet annihilation require conversion of near-IR photons to visible light. Generally, however, efficiencies in this spectral region lag behind bluer analogues. Herein we consider potential benefits from a conformationally well-defined covalent dimer annihilator TIPS-BTX in studies that systematically compare function to a related monomer model TIPS-tetracene (TIPS-Tc). TIPS-BTX exhibits weak electronic coupling between chromophores juxtaposed about a polycyclic bridge. We report an upconversion yield ϕUC for TIPS-BTX that is more than 20× larger than TIPS-Tc under comparable conditions (0.16%). While the dimer ϕUC is low compared to bluer champion systems, this yield is amongst the largest so-far reported for a tetracenic dimer system and is achieved under unoptimized conditions suggesting a significantly higher ceiling. Further investigation shows the ϕUC enhancement for the dimer is due exclusively to the TTA process with an effective yield more that 30× larger for TIPS-BTX compared to TIPS-Tc. The ϕTTA enhancement for TIPS-BTX relative to TIPS-Tc is indicative of participation by intramolecular multiexciton states with evidence presented in spin statistical arguments that the 5TT is involved in productive channels. For TIPS-BTX we report a spin-statistical factor f = 0.42 that matches or exceeds values found in champion annihilator systems such as DPA. At the same time, the poor relative efficiency of TIPS-Tc suggests involvement of non-productive bimolecular channels and excimeric states are suspected. Broadly these studies indicate that funneling of photogenerated electronic states into productive pathways, and avoiding parasitic ones, remains central to the development of champion upconversion systems.

Preparation of a Rigid and Nearly Coplanar Bis-tetracene Dimer through an Application of the CANAL Reaction.

A rigid tetracene dimer with a substantial interchromophore distance has been prepared through an application of the recently developed catalytic arene-norbornene annulation (CANAL) reaction. An iterative cycloaddition route was found to be unsuccessful, so a shorter route was adopted whereby fragments were coupled in the penultimate step to form a 13:1 mixture of two diastereomers, the major of which was isolated and crystallized. Constituent tetracene moieties are linked with a rigid, well-defined bridge and feature a near-co-planar mutual orientation of the acenes.

Triplet-Fusion Upconversion Using a Rigid Tetracene Homodimer.

We demonstrate that a structurally rigid, weakly coupled molecular dimer can replace traditional monomeric annihilators for triplet fusion upconversion (TUC) in solution by observing emitted photons (λ = 540 nm) from a norbornyl-bridged tetracene homodimer following excitation of a triplet sensitizer at λ = 730 nm. Intriguingly, steady-state spectroscopy, kinetic simulations, and Stern-Volmer quenching experiments show that the dimer exhibits qualitatively different photophysics than its parent monomer: it is less effective at diffusion-mediated triplet exciton transfer, but it fuses extracted triplets more efficiently. Our results support the development of composite triplet-fusion platforms that go beyond diffusion-mediated triplet extraction, ultimately circumventing the concentration dependence of solution-phase TUC.

Using Structurally Well-Defined Norbornyl-Bridged Acene Dimers to Map a Mechanistic Landscape for Correlated Triplet Formation in Singlet Fission.

Structurally well-defined TIPS-acetylene substituted tetracene (TIPS-BT1') and pentacene (TIPS-BP1') dimers utilizing a [2.2.1] bicyclic norbornyl bridge have been studied-primarily using time-resolved spectroscopic methods-to uncover mechanistic details about primary steps in singlet fission leading to formation of the biexcitonic 1TT state as well as decay pathways to the ground state. For TIPS-BP1' in room-temperature toluene, 1TT formation is rapid and complete, occurring in 4.4 ps. Decay to the ground state in 100 ns is the primary loss pathway for 1TT in this system. For TIPS-BT1', the 1TT is also observed to form rapidly (with a time constant of 5 ps), but in this case it occurs in concert with establishment of an excited-state equilibrium ( K ∼ 1) with the singlet exciton state S1 at an energy of 2.3 eV above the ground state. The equilibrated states survive for 36 ns and are lost to ground state through both radiative and nonradiative pathways via the S1 and nonradiative pathways via the 1TT. The rapidity of 1TT formation in TIPS-BT1' is at first glance surprising. However, our analysis suggests that the few-parameter rate constant expression of Marcus theory explains both individual and comparative findings in the set of systems, thus establishing benchmarks for diabatic coupling and reorganization energy needed for efficient 1TT formation. Finally, a comparison of TIPS-BT1' with previous results obtained for a close constitutional isomer (TIPS-BT1) differing in the placement of TIPS-acetylene side groups suggests that the magnitude of exchange interaction in the correlated triplet manifold plays a critical role dictating 1TT yield in the tetracenic systems.

Modular Synthesis of Rigid Polyacene Dimers for Singlet Fission.

An improved, modular synthesis of rigid, geometrically well-defined, alkyne-substituted tetracene (1) and pentacene (2) dimers is reported. The synthesis is rooted in sequential Diels-Alder reactions of a norbornyl tetraene with triisopropylsilylacetylene-substituted (TIPS-acetylene) quinone dienophiles. The incorporation of solubilizing and stabilizing TIPS-acetylene groups early in the synthesis affords a mild and reliable route, providing access, for the first time, to norbornyl-bridged pentacene dimers. A preliminary exploration of the excited state behavior of these molecules is also described.

Synthesis of Geometrically Well-Defined Covalent Acene Dimers for Mechanistic Exploration of Singlet Fission.

We report the first synthesis of norbornyl-bridged acene dimers (2 and 3) with well-defined and controlled spatial relationships between the acene chromophore subunits. We employ a modular 2-D strategy wherein the central module, common to all our compounds, is a norbornyl moiety. The acenes are attached to this module using the Diels-Alder reaction, which also forms one of the acene rings. Manipulation of the Diels-Alder adducts provides the desired geometrically defined bis-acenes. The modular nature of this synthesis affords flexibility and allows for the preparation of a variety of acene dimers, including functionalized tetracene dimers.