Diphenyldihydropentalenediones: Potential Singlet Fission Materials with High Triplet Energy Levels

2,2,5,5-Tetramethyl-3,6-diphenyl-2,5-dihydropentalene-1,4-dione (PD-H) was developed as a new molecular framework for singlet fission (SF) materials. It was proposed that PD-H and related substances would have high energy level of the triplet-excited state (ET) as a result of the enforcement of molecular planarity through incorporation of ring-fusion constrained diene and avoidance of resonance caused by the presence of amide moieties in the diketopyrrolopyrrole core. Three members of this family (PDs), PD-H, PD-OCH3 (the 4-methoxyphenyl derivative), and PD-CF3 (the 4-(trifluoromethyl)phenyl derivative), were synthesized. The results of photophysical studies revealed that the energy level of 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 material. Moreover, introduction of electron-donating or -withdrawing groups on the benzene rings in PD-H enable fine-tuning of ES and ET while maintaining ES > 2ET. The results of transient absorption spectroscopic studies show that PD-H, PD-OCH3, and PD-CF3 in CH2Cl2 have respective T1 lifetimes of 54, 94, and 108 microsec, which are long enough to utilize its triplet energy as an excitation energy in other optoelectronic systems. These findings suggest that diphenyldihydropentalenediones are potential candidates for SF materials with high ET levels.