Theoretical investigation on the one- and two-photon absorption properties of multi-branched oligomers with truxenone center and fluorene branches

A comparative study of electronic structure, one-photon absorption and two-photon absorption (TPA) properties of octupolar oligofluorenes and their dipolar counterparts is presented by using AM1 and ZINDO-SOS methods. The investigation of the structure–property relationship of these materials has indicated differences between octupolar oligofluorenes and dipolar ones. The results indicate that the conjugated chain length importantly affects the magnitude of TPA cross-section. The dipolar molecules reach a saturation limit for energy gap between HOMO and LUMO versus the number of fluorene rings more quickly than octupolar systems. The maximum TPA cross-section increases in proportion to Nα (α = 3.16 and 2.77), where N = 2–6 is the number of fluorene moieties in dipolar molecules and each arm of octupolar analogs. Because of the weak coupling among the branches of the octupole, an increase by a factor of less than 3 of the TPA cross-section of the octupolar molecule with respect to the dipolar molecule occurs.