Solution-Processable, Solid State Donor–Acceptor Materials for Singlet Fission

Abstract

The exploitation of singlet fission (SF) materials in optoelectronic devices is restricted by the limited number of SF materials available and developing new organic materials that undergo singlet fission is a significant challenge. Using a new strategy based on conjugating strong donor and acceptor building blocks, the small molecule (BDT(DPP)2) and polymer (p-BDT-DPP) systems are designed and synthesized knowing that bisthiophene-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) has a low lying triplet energy level, which is further confirmed by time-dependent density functional theory (TD-DFT) calculations. TD-DFT and natural transition orbital (NTO) analysis are conducted to gain insight into the photophysical properties and features of excited states in BDT(DPP)2, respectively. Femtosecond and nanosecond transient absorption spectroscopies are used to investigate the excited state kinetics in the synthesized compounds. Fast formation of triplet pairs in thin film of p-BDT-DPP and BDT(DPP)2 and the equilibrium formation of correlated triplet pairs and S1 from triplet–triplet annihilation in solution of BDT(DPP)2 are further evidence of SF in these compounds. The short triplet lifetime, as a result of fast biexcitonic recombination, provides additional support for triplet pair formation through singlet fission.