Two-photon absorption properties of symmetric substituted diacetylene: drastic enhancement of the cross section near the one-photon absorption peak

Two-photon absorption (TPA) spectra are presented for newly synthesized bis(pyridylstyryl)-substituted diacetylene and related methyl triflate derivatives. The TPA cross section of a derivative (MPPBT) was found to increase drastically with decreasing the incident wavelength below 650 nm and to reach (2.42±0.46)×10−47 cm4 s molecule−1 photon−1 at 571 nm, in addition to the weaker, lowest energy TPA peak centered at 764 nm. A four-state model was needed to account for the observed spectral shape, suggesting that the drastic enhancement is due to one-photon resonance enhancement not only of the lowest energy TPA transition but also of a higher energy two-photon transition.