The structural, electronic, and optical properties of ladder-type polyheterofluorenes: a theoretical study

The ladder-type polyheterofluorenes were investigated theoretically by using density functional theory (DFT) to reveal their optical and electronic properties for applications in organic optoelectronic devices. The incorporation of heteroatoms (B, Si, Ge, N, P, O, and S) into the ladder-type highly fused polyfluorene backbone can influence and modify the optoelectronic properties significantly. The functionalization on the heteroatoms allows for facile derivation and incorporation of substitutes to further tune the properties. Small geometry variations between the ground, anionic/cationic, the first excited singlet and triplet states were observed due to the very rigid ladder-type coplanar backbone. Ladder-type polycarbazole was predicted to have the highest HOMO and LUMO energy levels, polyphosphafluorene oxide have the lowest HOMO energy level, polyborafluorene have the lowest LUMO energy level and bandgap, and polysulfafluorene has the highest bandgap and triplet energy. The ladder-type carbazole and borafluorene show the highest hole and electron injection abilities respectively; while sulfafluorene has the highest electron transfer rate. Most ladder-type heterofluorenes show bipolar charge transport character suggested by the reorganization energy. All of them have significantly short effective conjugation length in comparison with linear conjugated polymers. Their absorption and emission spectra were also simulated and discussed. The diversified electronic and optical properties of the ladder-type polyheterofluorenes with the different incorporated heteroatom and the substituent on it indicate their broad potential applications in organoelectronics.