Cooperative enhancement of two-photon absorption cross sections in three-branched oligofluorene with boron center

A comparative study of one-photon absorption (OPA) and two-photon absorption (TPA) properties of three-branched oligofluorenes with boron center (CBTFn) and two-branched (CBDFn) as well as one-branched (CBSFn) counterparts is presented based on the correlated quantum chemical calculations. The roles of boron center in the multi-branched oligofluorenes on OPA and TPA properties are discussed first. Theoretical calculations results reveal that the maximal two-photon absorption cross sections (δmax) values of CBSFn, CBDFn and CBTFn increase obviously with the increase of conjugation length and branch number. Among them, CBTFn has the largest δmax value and maintains good transparency when n increases. The comparisons of δmax values with simplified exciton model show that the effective electronic coupling between boron center and peripheral fluorene is relatively strong. In addition, the cooperative coupling in CBTFn is larger than that in CBDFn. Therefore, CBTFn is a kind of promising TPA material for optical power limiting.