(Invited) Geometry and Dynamics of Quintet Multiexciton Studied By Time-Resolved EPR
Authors: Kobori Yasuhiro , Nagashima Hiroki , Kawaoka Shuhei , Akimoto Seiji , Tachikawa Takashi , Matsui Yasunori , Ikeda Hiroshi 
Published: February 2020

Singlet fission (SF) is a reaction that one singlet exciton separates to two triplet excitons. Theoretical limit of the photovoltaic efficiency (Shockley–Queisser limit) can be exceeded by the SF because of a potential of generating two charge-separations from the one high-energy photon. The SF reactions have been observed in organic molecules, tetracene (Tc), pentacene (Pn) and their derivatives. 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) and 5,12-bis(triisopropylsilylethynyl)tetracene (TIPS-Tc) are representative molecules and their SF efficiencies are high (up to 200 %). Clarification of the mechanisms and guidelines for designing the SF materials are highly required in the solid state.

In the solid state, large chromophore couplings were reported for SF because Pn molecules are highly ordered to form the p-stacking and herringbone structures. Recent study however showed an efficient SF in an amorphous environment.[1] It is particularly important for the OPV applications that the two triplets are efficiently separated from the correlated TT pair without going back to the S1S0and S0S0 pairs. Since these pairs are initially the singlet states, efficient production of the quintet states (5TT pair) is essential for preventing the unwanted singlet deactivations. However, SF-born spin-conversion mechanism is not fully understood.

The time-resolved electron paramagnetic resonance (TREPR) spectroscopy is a powerful tool to detect unpaired spins and to investigate their electronic structures, geometries and dynamics. Recently, TREPR spectra were reported on the SF-born quintet states as correlated TT pairs.[2] The spin polarized EPR spectra were interpreted by initial population of the quintet states at a sublevel of m
S= 0, i.e. |5TT0> |Q0>. The origin of the Q0population is however unclear. When the SF occurs via the spin-conservation, the quintet generation is required to be accompanied by the singlet–quintet (SQ) mixing from the singlet multiexciton. The |1TT> –> |5TT0> (SQ0) mixing is allowed when the energy difference (|E
S– E
Q| between Q0and S is smaller than the size of the zero-field splitting (ZFS) interaction (D) of the individual triplet. Nevertheless, it is reported that |1TT> is highly stable with respect to |5TT0>; the exchange coupling 6J(ΔE
SQ= E
S– E
Q) is stronger than D.[2a]It was hypothesized that an intermediate TT pair with weak Jwould play a role for the Q0populations. Since deactivations occur not from the 5TT but from the 1TT in the multiexciton, clarification of the 5TT formation mechanism is a key to the OPV applications. Reported time scales (typically in order of fs – ps) of the SF and the triplet-exciton migrations are however too short to track the SF-induced spin-conversion and dissociation those of which are hidden in the multiexciton dynamics.

We herein prepared disordered aggregates of TIPS-Pn and of 2-phenyl-6,11-bis(triisopropylsilylethynyl)tetracene (TIPS-Ph-Tc). The disordered aggregates were produced by freezing diluted solutions in dichloromethane (CH2Cl2). This will inhibit the rapid exciton process, allowing us to unveil natures of the multiexcitons in inhomogeneous environments of the solid-states that generally possess both the crystalline and disordered domains. The spin-state dynamics and its related energetics were clarified by a combination of the TREPR and the picosecond fluorescence measurements. We have demonstrated sublevel selective populations in the excited quintet states (|5TT0>, |5TT-1> and |5TT-2>) of triplet-triplet (TT) pairs, manifesting that the SF-born quintet state is detected as strongly-coupled trapped TT-pairs generated by singlet-quintet interconversions during the triplet-exciton diffusion within hot geminate multiexcitons in the presence of certain exchange couplings. The present fundamental characteristics on the quintet generations demonstrate the importance of controlling both molecular p-stacking “hot spot” and disordered “trapping area” for rational designs of the efficient SF systems with preventing unwanted triplet fusion processes.[3]

References

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[3] H. Nagashima, S. Kawaoka, S. Akimoto, T. Tachikawa, Y. Matsui, H. Ikeda, Y. Kobori J. Phys. Chem. Lett.
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