Building symmetric two-dimensional two-photon materials.
Two-dimensional multi-annulenic carbon networks are important molecules with possible applications in optoelectronic devices and nonlinear optics. Investigations of two-photon absorption (TPA) cross sections have been carried out in a series of annulenes with a basic building block approach and variable symmetries. Enhancement of the TPA cross section has been observed with an increase in number of building blocks and order of symmetry. Evaluations of the ground-state transition dipole moment and chromophore density are not sufficient to explain the observed enhancement. Estimates of excited-state transition dipole moments made by femtosecond transient absorption measurements are able to successfully predict the observed trend in TPA cross section. It has been observed that the symmetry of the molecule plays a vital role in enhancing the TPA cross section by virtue of increasing the excited-state transition dipole moment.