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

Abstract

Structurally well-defined TIPS-acetylene substituted tetracene (TIPS-BT1') and pentacene (TIPS-BP1') dimers utilizing a [2.2.1] bicyclic norbornyl bridge have been studiedprimarily using time-resolved spectroscopic methods -to uncover mechanistic details about primary steps in singlet fission leading to formation of the biexcitonic 1 TT state as well as decay pathways to the ground state. For TIPS-BP1' in room temperature toluene, 1 TT formation is rapid and complete, occurring in 4.4 ps. Decay to the ground state in 100 ns is the primary loss pathway for 1 TT in this system. For TIPS-BT1', the 1 TT 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 S 1 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 non-radiative pathways via the S 1 and non-radiative pathways via the 1 TT. The rapidity of 1 TT formation in TIPS-BT1' is at first glance surprising. However, our analysis suggests that the fewparameter 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 1 TT 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 1 TT yield in the tetracenic systems.

Multiexcitonic Triplet Pair Generation in Oligoacene Dendrimers as Amorphous Solid‐State Miniatures

“…At the same time, negligible spectral changes from time‐resolved fluorescence spectra were found, which clearly demonstrates the fluorescence originates solely from the bright S 1 state. Quasi‐equilibrium between local excitonic states (S 1 ) and multiexcitonic triplet pair states ( 1 TT) accounts well for the above experimental findings achieved by biphasic SF [65] . As biphasic excited‐state population shifts to multiexcitonic states and the geometric feature of QP1 whose each TIPS‐pentacene constituent share similar circumstances for SF processes are considered, two possible mechanisms revealed in the solid‐state are excluded from interpreting biphasic SF processes in QP1 .…”

Conversion between triplet pair states is controlled by molecular coupling in pentadithiophene thin films

“…We conjecture that a combination of atypical crystal structure and a large amount of driving force for SF have made the transition between (TT) manifolds clearly optically distinguishable for the rst time. Whereas most prior observations of (TT) spectra involved signicant mixing with the S 1 state, [33][34][35] the large exothermicity of SF in PDT more clearly separates the associated S 1 vs. TT transitions and reduces this mixing, which is more likely with the closer S 1 /TT resonance in pentacene derivatives. 8,13,36 This prior work has suggested that transitions from 1 (TT) to S n states, energetic companions to the S 1 -S n transitions, dominate the near-IR portion of the pentacene spectrum, but for PDT the roughly 0.5 eV energy separation between (TT) and S 1 removes any expected correlation between (TT) ESA and the S 1 -S n spectra.…”

Singlet fission for quantum information and quantum computing: the parallel JDE model

“…In this work we consider singlet fission between chromophores in two classes of systems that are widespread in the literature: covalently linked organic dimer molecules 26 and organic crystals comprised of chromophore pairs 14 . As is often the case, the selection rules governing the quantum relaxation phenomena depend sensitively on molecular symmetries.…”