Ultrafast Intramolecular Singlet Fission in Homoconjugated Oligomers of Azapentacene with Rigid Connecting Unit

A novel series of homoconjugated oligomers of azapentacene, linked via the rigid connecting unit bicyclo[2.2.2]octane bridge is presented. The investigation delves into intramolecular singlet fission (iSF) within these dimeric and trimeric frameworks, employing various time-resolved spectroscopies and quantum chemical calculations. Combining the merits of both through-space and through-bond couplings between the constituent chromophores within these rigid homoconjugated structures, iSF proceeds within a few picoseconds, exhibiting a 2-3 orders of magnitude acceleration compared to traditional covalently linked oligomers and achieving an iSF efficiency approaching unity. In the homoconjugated trimer, the larger interplanar angle between neighboring chromophores leads to a relatively slower iSF rate compared to that of the homoconjugated dimer. Furthermore, the active involvement of the charge transfer state is demonstrated to accelerate the iSF process in the trimer, while exerting no influence on the iSF process in the dimer. The study provides novel avenues for investigating the influence of molecular geometry on singlet fission and reveals the crucial role of the interplay between spatial and covalent coupling in facilitating intramolecular singlet fission. A novel class of rigid homoconjugated oligomers of azapentacene is designed and synthesized to investigate the impact of geometric design on intramolecular singlet fission (iSF). By combining through-space and through-bond couplings, iSF occurs rapidly within picoseconds, significantly outperforming traditional covalently linked structures. The trimer demonstrates enhanced iSF acceleration in polar solvent through charge transfer involvement. image