Searching Building Blocks with Large Excited-State Gaps in Five-memebered Heterocyclic Rings by Aromaticity Strategy

Organic molecules with large gap between the excited singlet/triplet states can be applied in hot exciton emission and singlet fission to beat the spin statistics limit in optoelectronic devices. Herein, a novel strategy is proposed for achieving large triplet-triplet gap (E(T1T2)) and singlet-triplet gap (E(S1T1)) by manipulating the aromaticity of the low-lying excited states. Partially conjugated five-membered heterocyclic rings are found to naturally have low E(T1) owing to high aromaticity obeying Baird’s Rule. Utilizing such ring (pyrazoline) as a bridge and selecting various donor and acceptor moieties, numbers of derivatives have been theoretically designed with tunable emission colors, significantly large E(T1T2) and E(S1T1), and satisfying E(T2)>E(S1)≥2E(T1). The ultrafast spectroscopy and sensitization measurements for one of them with blue fluorescence (named TPA-DBPrz) confirm the calculated prediction. This work opens a new avenue and provides molecular units to develop high-efficiency optoelectronic materials.