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-OCH) and bis(trifluoromethyl) derivatives (PD-CF) 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 ( ) due to the planarity of the fused-diene subunit. The results of photophysical studies revealed that the energy level of the singlet-excited state ( ) and of PD-H are 2.