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 8 values obtained from nonlinear transmission measurements, they reasonably predicted the dependence of the 8 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 8 values and that the donor-acceptor substituted structure showed a greater redshift in the TPA absorption maximum wavelength. Overall, the theoretical 8 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.