Exploring Intermolecular Singlet Fission in Pyrene-Fused Azaacenes Film

Singlet fission (SF), a process that involves the conversion of one singlet exciton (S₁) into a pair of triplet excitons (T₁), holds great technological promise for photovoltaics. Typical SF materials necessitate that the singlet energy (E_S1) be at least twice that of the triplet energy (E_T1) or that, equivalently, E_S1 be less than twice the singlet-triplet energy splitting (ΔE_S1-T1). Based on these design principles, we have developed a series of linearly arranged pyrene-fused azaacenes with varying conjugation lengths, featuring four, five, and six aromatic rings. These molecules exhibit distinct excited-state dynamics in both solution and films. With increasing conjugation length, the E_S1 values decrease correspondingly from 2.51 to 2.09 eV. Time-resolved spectroscopy and quantum chemistry calculations reveal that ΔE_S1-T1 increases from 0.38 to 1.13 eV, which is attributed to the increasing molecular frontier orbital overlaps. These systematic trends result in an intersystem crossing in the short-conjugated backbone (four aromatic rings). Conversely, the long-conjugated backbone of six aromatic rings exhibits an ultrafast intermolecular SF process. These findings regarding this newest addition to the collection of SF materials provide guidance on the construction of molecules for desired optoelectronic properties.