Fig. shows the TREPR spectra of perylene diimide (PDI)-tetrathiaryl trityl radical dyad linked with biphenyl group (PDI-biph-trityl) in d-toluene solution at room temperature. As reported previously, the spectra exhibit microwave absorption (A) and emission (E) spin polarization. The broad A/E/A/E pattern is generated together with the sharp emissive center EPR line. The spectra were explained by the excited quartet state in the triplet-radical pair of 4 ( PDI*-biph-2 R•) generating A/E/A/E fine structure by sublevel transitions between |mS = ±1/2 > (Q ±"/$ ) and |mS = ±3/2 > (Q ±%/$ ), while the center sharp line was attributed to the sublevel transitions between |mS = +1/2 > (Q &"/$ ) and |mS = -1/2 > (Q '"/$ ). The observation of the fine structure by the ZFS interaction in the fluid solution denotes that the isotropic rotation correlation time (t) of this molecule is larger than the inverse value of the ZFS constant of D ~ 700 MHz). From this, t > 1 ns is concluded. This slow molecular rotation is interpreted by the bulkiness of the present dyad molecule in Fig. . However, quick intramolecular conformation change is plausible due to the flexibility in the linker, as reported previously. Effect of this anisotropic motion will be invoked to understand the spin polarization (red lines in Fig. ). Fig. shows time profiles of the TREPR signals at 350 mT and 340 mT. The quick singal rises are consistent with a reported rapid enhanced intersystem crossing (EISC) in 9 picosecond to generate results suggest that the deactivation of the quintet and doublet SCTRP is quick due to equilibrium between 4 ( 3 PDI*-biph-2 R•) and 2 ( 3 PDI*-biph-2 R•) to generate the excited doublet state (D1) by the process of 2 ( 3 PDI*-biph-2 R•) -> PDI-biph-2 R•*. This is in highly contrast with reported observations of the long-lived quartet species in the chromophore-radical linked systems at cryogenic conditions, indicating importance of the anisotropic molecular motions for the efficient interconversion between the excited doublet and quartet states at room temperature. The above model is summarized in Fig. . In previous studies on intramolecular singlet fissions in linked chromophore systems, the singletquintet interconversion has been explained by activation motions of the TT states: anisotropic conformation changes in the triplet exciton can cause modulations of the ZFS and spin-spin exchange (J) interactions to induce the 1 (TT) -> 5 (TT) conversions. Such flexibility can also conduct the 1 (TT) generation from the weakly coupled triplet pairs (T+T) for the triplet-triplet annihilation to induce the delayed fluorescence. Thus, we propose a theoretical treatment of the analysis of spin polarization of spin-correlated triplet-radical pair (SCTRP).