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 N-alpha (alpha = 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. (C) 2004 Elsevier B.V. All rights reserved.