Singlet fission (SF) is expected to exceed the theoretical limit of the solar cell efficiency. Quintet (Q) state generation in triplet-triplet pair is essential for preventing the unwanted loss of SF-born multiexciton through singlet channels, although little is known on the primary multiexciton spin dynamics following the intermolecular SF. In this study, time-resolved EPR revealed the intermolecular multiexciton dynamics, energetics and geometries in aggregated 6,13-bis(triisopropylsilylethynyl)pentacene and 2-phenyl-6,11-bis(triisopropylsilylethynyl)tetracene in diluted frozen solution. We have demonstrated sublevel selective generations of excited quintet states (|Q⟩, |Q⟩ and |Q⟩) by singlet-quintet (SQ) mixings during triplet-exciton diffusions within geminate multiexcitons. The present fundamental characteristics of the quintet generations shows strong impact of coexistence of molecularly ordered "hot spot" and disordered regions for exergonic SQ mixings driven by entropy, thereby paving a new avenue for rational designs of organic devices with controlled multiexciton dynamics by optimizing film morphologies.

An adamantane-linked tetracene dyad (Tc-Ad-Tc) undergoes exergonic intramolecular singlet fission (SF), producing longlived (t = 175 µs) and high-energy (2 ´ 1.03 eV) multiexciton. Time-resolved 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 formation of syn-and anti-conformers in the quintet, i.e. 5 ( 3 Tc-Ad-3 Tc)*. The quintet generation requires small conformational motion to induce singlet-quintet spin relaxation. The presence of aliphatic linkages, like the rigid adamantane group, may enable effective conservation of intrinsic high S1 and T1 levels of the original monomers, moderate bridge-mediated s-p interaction leading to exergonic intramolecular SF involving 1 Tc*-Ad-Tc ® 1 ( 3 Tc-Ad-3 Tc)*, and prevention of undesirable triplet-triplet annihilation, finally result in long-lived and high-energy multiexciton.

<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 ¹(³Tc–Ad–³Tc)* -> ⁵(³Tc–Ad–³Tc)* 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 S₁ and T₁ levels of the original monomers, and moderate bridge-mediated σ–π interaction leading to exergonic intramolecular SF involving ¹Tc*–Ad–Tc -> ¹(³Tc–Ad–³Tc)*.</div><div><br></div>

The singlet fission (SF) is expected to be powerful tool for exceeding the theoretical limit of the power conversion efficiency on the organic solar cells. However, little is known on the intermolecular SF mechanism in the solid state. In the present study, we have investigated intermolecular SF in 2-phenyl-6,11-bis(triisopropylsilylethynyl)tetracene (TIPS-Ph-Tc) in frozen solutions using a time-resolved electron paramagnetic resonance measurement at a low temperature. SF-born quintet states (Q) and subsequent dissociations into two triplet states (T + T) were detected for a diluted solute concentration of 10 -4 M in dichloromethane. The microsecond triplet dissociation was found to occur due to the amorphous morphology in the frozen aggregate, suggesting that Q is generated as a trapped state. Furthermore, it was also suggested that the T + T dissociation follows the Q generation due to a T-T repulsion, whose energy were determined by the negative exchange couplings in the triplet pairs, (TT).

<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 ¹(³Tc–Ad–³Tc)* -> ⁵(³Tc–Ad–³Tc)* 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 S₁ and T₁ levels of the original monomers, and moderate bridge-mediated σ–π interaction leading to exergonic intramolecular SF involving ¹Tc*–Ad–Tc -> ¹(³Tc–Ad–³Tc)*.</div><div><br></div>