Linear and Nonlinear Optical Properties of Ramified Hexaazatriphenylenes: Charge Transfer Contributions to the Octupolar Response

Four new HAT derivatives with octupolar symmetry have been synthesized. Their linear and nonlinear optical, photophysical, and electrochemical properties have been rationalized in terms of donor-acceptor interactions. We get further insights in the relationship between experimental first hyperpolarizability and two-photon absorption cross section: the delta/beta ratio is connected with the electron donor-acceptor strength between the HAT core and the peripheral groups but also with the ethylene/acetylene bridges. This structure-property relationship can be used to design octupoles with more efficient nonlinear responses. A HAT derivative based on the fusion of three HAT cores and without any defined donor-acceptor pattern is studied in comparison with the charge transfer HAT versions. A complete study of the electronic structure of the five molecules in the context of the origin of the relevant optical properties is carried out combining several spectroscopic techniques and quantum chemistry.