Two‐Photon Absorption Cross Sections of Dithienothiophene‐Based Molecules

We performed nonlinear transmission measurements and quantum‐chemical calculations on dithienothiophene (DTT)‐based molecules to gain insight into the effect of acceptor and donor groups on two‐photon absorption (TPA) properties. The TPA intensity showed dispersion characteristics of the single‐photon absorption spectrum. When the molecules included an asymmetric donor‐acceptor pair, the single‐ and two‐photon absorption maximum wavelengths were red‐shifted more than when the molecules had a symmetric donor‐donor structure. We interpreted this result as indicating that the S 2 state plays the dominating role in the absorption process of molecules with a symmetric structure. The experimental TPA δ values at the absorption peak wavelength showed a dependence on the structural variations. We found the self‐consistent force‐field theory and Hartree‐Fock Hamiltonian with single configuration interaction formalism to be valid for evaluating TPA δ. Although the quantum‐chemical calculations slightly underestimated the experimental δ values obtained from nonlinear transmission measurements, they reasonably predicted the dependence of the δ value on the structural variations. We confirmed the role of molecular symmetry by observing that donor‐donor substituted structure gave the highest experimental and theoretical TPA δ values and that the donor‐acceptor substituted structure showed a greater red‐shift in the TPA absorption maximum wavelength. Overall, the theoretical δ values of DTT‐based molecules were in the order of 10 −−46 cm 4 · s · photon −1 and are higher than that of AF‐50 by nearly two orders of magnitude.