Diphenyldihydropentalenediones: Wide Singlet–Triplet Energy Gap Compounds Possessing the Planarly Fixed Diene Subunit

2,2,5,5-Tetramethyl-3,6-diphenyl-2,5-dihydropentalene-1,4-dione (PD-H) and its dimethoxy (PD-OCH3) and bis(trifluoromethyl) derivatives (PD-CF3) were developed as a new class of compounds possessing a wide excited singlet–triplet energy gap. The PD derivatives would also have a high energy level of the triplet-excited state (ET) due to the planarity of the fused-diene subunit. The results of photophysical studies revealed that the energy level of the singlet-excited state (ES) and ET of PD-H are 2.88 and 1.43 eV, respectively. These values indicate that PD-H has the energy relationship, ES > 2ET, required for it to be a singlet fission (SF) material. Moreover, the introduction of electron-donating or -withdrawing groups on the benzene rings in PD-H enables fine-tuning of ES and ET. The results of transient absorption spectroscopic studies show that PD-H, PD-OCH3, and PD-CF3 in CH2Cl2 have respective T1 lifetimes of 71, 118, and 107 μs, which are long enough to utilize its triplet exciton in other optoelectronic systems. These findings suggest that the PDs are potential candidates for SF materials with high ET levels.