(Invited) Vibronic Effect of Donor-Acceptor Interaction Determines Fate of Mutiexciton Spins Generated By Singlet Fission
Authors: Kobori Yasuhiro , Fuki Masaaki , Nakamura Shunta , Hasobe Taku 
Published: July 2021

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit with power conversion efficiencies (PCE) ~33 % of single junction organic solar cells (OSC) 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. The intramolecular SF (iSF) dynamics have widely been studied using the transient absorption spectroscopic methods together with theoretical modeling taking into account the vibronic effects in the ultrafast regimes. The correlated intermolecular triplet pair 1(TT) generated with the singlet character is known to be converted to the quintet (Q) state as 5(TT). These TT pairs separate into individual triplets as the T+T state. In the previous study, we reported the iSF dynamics and subsequent separations to T+T in the pentacene dimers (PcD) bridged by a phenylene at ortho and meta positions by a combination of the ultrafast transient absorption spectroscopy and the time-resolved electron paramagnetic resonance (TREPR) method.1

Before the spin-conversions to the 5(TT) state, the initially populated 1(TT) is strongly coupled in the four unpaired spins of the two triplet excitons, i.e. the entangled spin-state by large spin-spin exchange coupling (J) with the large orbital overlap produced at femtoseconds regions. The modulations of the J-couplings are thought to be essential for the 1(TT) -> 5(TT) conversions. In an iSF system, we interpreted that the quintet ESPs were caused by the anisotropic spin conversions due to singlet-quintet spin-relaxations to the five sublevels at m
S = +2, +1, 0, -1 and -2 in accordance with the anisotropy of the ZFS coupling which is highly dependent on the relative orientation of one triplet exciton (TB) with respect to the other triplet (TA) in the TATB multiexciton.2 Internal fluctuations in the J-coupling were concluded to be crucial on the quintet generations. Moreover, we reported the treatment of the EPR transitions in the weakly coupled T+T states in the framework of the spin-correlated triplet pair (SCTP) for the very weak exchange coupling including the superpositions of the nine basis functions of 5(TT) - 3(TT) - 1(TT), as treated in the spin-correlated radical pair model. 3,4

The above pioneering interpretations of the ESP mechanisms may pave a new avenue to clarify how the molecular conformations and their motions drive the spin-conversions and the ultimate T+T generations, both of which are essential but unknown for the photon-to-energy conversion process. In particular, because the J-coupling is anticipated to be sensitive to the TATB conformations, it is expected that the specific molecular motions or phonon modes may play roles for the ultimate T+T de-coupling, as the vibronic effects. So far, details of the TATB geometries, the vibration modes and frequencies, and the electronic couplings are unresolved in the iSF systems for generating the quintet multiexcitons and the subsequent T+T dissociations.
We have characterized the molecular geometries, conformation dynamics and J-couplings of the intramolecular multiexcitons after the photoinduced iSF with the proposal of the density matrix analysis of ESP observed by the TREPR method. It is concluded that the J-coupling is determined by the donor-acceptor electronic coupling in the multiexciton and that terahertz TA-TB conformation dynamics plays a significant role for generating the quintet multiexcitons and for the individual T+T states caused by the J-fluctuation in the covalently linked systems.4

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