Long-lived Triplet Excitons Formed by Exergonic Intramolecular Singlet Fission of an Adamantane-linked Tetracene Dyad

Abstract

<div>An adamantane-linked tetracene dyad (Tc–Ad–Tc) undergoes exergonic intramolecular singlet fission (SF), producing longlived (τ = 175 μs) and high-energy (2 x 1.03 eV) multiexcitons. Timeresolved absorption, fluorescence decay, and electron paramagnetic resonance (EPR) spectroscopic analysis revealed that the long-lived triplet species is generated in this system via correlated triplet pair having singlet and quintet characteristics. Time-resolved EPR analysis revealed that conversion of <sup>1</sup>(<sup>3</sup>Tc–Ad–<sup>3</sup>Tc)* -> <sup>5</sup>(<sup>3</sup>Tc–Ad–<sup>3</sup>Tc)* requires small conformational dynamics accompanied by molecular motion. Analysis of the geometries of the quintet states shows that formation of the long-lived multiexciton is enabled by precise and close alignment of the tetracene moieties, which leads to their moderate interaction in the singlet excited state, while triplet–triplet annihilation is prevented by quintet generation. The presence of aliphatic linkages, like the rigid adamantane group, might enable effective conservation of intrinsic S<sub>1</sub> and T<sub>1</sub> levels of the original monomers, and moderate bridge-mediated σ–π interaction leading to exergonic intramolecular SF involving <sup>1</sup>Tc*–Ad–Tc -> <sup>1</sup>(<sup>3</sup>Tc–Ad–<sup>3</sup>Tc)*.</div><div><br></div>

Steady-State Fluorescence Signatures of Intramolecular Singlet Fission from Stochastic Predictions

“…We now extend our consideration to six different iSF-dyad model systems with varying iSF yields as reported from TA studies (see Table 2 for labeling assignments and associated parameter values for each dyad). 37,45,47,48 Molecular structures of each dyad and complete rates and reaction schemes may be found in the Supporting Information, which are based on published unimolecular deterministic models. Briefly, we source models for pentacene dyads (PD) and a tetracene dyad (TD), which a "NC" in source.…”

“…Model identical with PD-A with addition of singlet solvent relaxation. 47 f "D" in source. Model identical with PD-A with addition of singlet solvent relaxation.…”

“…Model identical with PD-A with addition of singlet solvent relaxation. 47 are further designated with a letter as shown in Table 2 (i.e., PD-A). Figure 4 displays simulation results for these systems using our stochastic approach.…”

Supramolecular-scaffold-directed two-dimensional assembly of pentacene into a configuration to facilitate singlet fission

“…However, to realize efficient SF in the solid state, two requirements should be satisfied. First, acene chromophores need to be placed in close proximity to each other for sufficient electronic coupling 30,[45][46][47][48][49][50][51] . Second, the environment around the chromophores needs to be designed to allow them to undergo conformational changes 30,[45][46][47][48][49][50][51] , so that the orbital overlap within (T1T1)* is reduced, suppressing unfavorable T1T1 annihilation and facilitating the dissociation of (T1T1)* into two free triplets.…”

“…First, acene chromophores need to be placed in close proximity to each other for sufficient electronic coupling 30,[45][46][47][48][49][50][51] . Second, the environment around the chromophores needs to be designed to allow them to undergo conformational changes 30,[45][46][47][48][49][50][51] , so that the orbital overlap within (T1T1)* is reduced, suppressing unfavorable T1T1 annihilation and facilitating the dissociation of (T1T1)* into two free triplets. In view of their geometrical features, 2D assemblies of acenes induced by tripodal triptycenes would be interesting as a motif for efficient SF materials.…”