(Invited) Electron Spin Polarization Generated by Transport of Spin-Entanglements in Singlet Fissions
Authors: Kobori Yasuhiro 
Published: May 2020

Applications of singlet fission (SF)1 are expected to exceed the Shockley–Queisser theoretical limit because two separated triplet excitons (T+T) can be produced from one excited singlet state (S1S0) sharing its excitation energy with a neighboring ground-state chromophore. Subsequent two charge-separations (CS) at the D:A domain interfaces after individual triplet-exciton diffusions may cause the boost of the photocurrent density of the OSCs. Applications of the SF-induced triplet generations are also promising for boosting the organic light emitting diode (OLED) efficiency. Several studies clarified the initial multiexciton (TT) generation mechanisms from the S1S0 states. More importantly, elucidation of TT T+T dissociation mechanism is highly desired. The dissociation time constants ranging from sub-picosecond to microsecond regions are reported. However little is known how the T+T dissociation or the decoupling2 occurs from the strongly coupled TT states.

The strongly coupled singlet TT states are known to be initially generated from the S1S0 states. Generations of the quintet state in the triplet–triplet pair3-7 have been reported and are thought to be essential for preventing the loss of the SF-born multiexciton through the singlet channels. Using the time-resolved electron paramagnetic resonance (TREPR) method for TIPS-tetracene thin films, Weiss et al.
8 characterized the SF-induced strongly-coupled quintet of 5(TT). The quintet electron spin polarization (ESP) was detected as the microwave absorption (A) and emission (E) in 5(TT) for frozen aggregates of the SF-materials. The ESPs were interpreted using the sublevel-selective quintet conversions by the zero-field splitting (ZFS) interaction at the singlet-quintet level-crossings in the presence of the negative exchange coupling (J) during triplet exciton-diffusion and subsequent re-encounter in the highly disordered region, causing modulation of the J-coupling to result in spontaneous 5(TT) generations.4 Interestingly, transport of spin-entangled multiexcitons by the dissociative exciton diffusion have been observed using an ultrafast transient absorption imaging of pentacene crystals and by the magnetic field effect on the fluorescence time-profiles. Although such transportation of spin-entanglements is key to elucidate the dissociation mechanism and to apply in quantum information science, no direct evidence for the transportation of the spin-entanglements has been examined in the separated T+T states with the singlet and quintet characters.

The present study thus focuses on how the spin-entanglements develop and affect the direct TREPR detection of the T+T state. The transport of the quantum entanglements was demonstrated in thin films. When the triplet excitons are separated from each other to result in a negligible magnitude of the exchange coupling (J 0), spin correlations in the triplet-triplet pair may occur to induce diagonalized nine spin-states as the quantum superpositions of the singlet-triplet-quintet character through the spin-spin dipolar couplings of the individual triplet species. This is quite similar to the spin-correlated radical pair (SCRP) model9, 10 in the photoinduced long-range charge-separation system generated by singlet or triplet precursors, causing the superpositions of the singlet-triplet characters when the J-coupling is small in the radical pairs. We herein demonstrate that the transportations of the SF-induced 5(TT) and 1(TT) characters can be distinguished on the T+T dissociations using the ESPs of correlated T-T states by applying TREPR spectroscopy to thin films of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn).

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