Two-photon mediated three-photon fluorescence: Lessons from a quinoidal oligothiophene dimer

Detailed here is the use of NIR femtosecond pulsed excitation to drive three-photon absorption (3PA) in a cyano-terminated quinoidal oligothiophene (QOT) dimer to the exclusion of all other fluorescing processes, resulting in 3PA emission bright enough to be visible by eye. Through steady-state, multiphoton, and ultrafast transient spectroscopy, it is shown that despite competing nonlinear optical processes, such as an available two-photon transition (2PA) and excited-state absorption (ESA), emission characteristics remain an I³ process explicitly due to 3PA. Specifically, we will show the viability of a two-photon transition at the pump wavelength and strong ESA; neither occurs to a significant degree in this QOT system when pump energies match both a virtual state near resonant with a viable 2PA state and a higher-lying 3PA energy level. This study provides a clear method for evaluating multiband 3PA emitters, which bypasses much of the ambiguity observed in purely absorption based studies where 2PA, ESA, and 3PA may all contribute to signal and are difficult to distinguish. Additionally, this study introduces a rigid, molecular wire-like thiophene oligomer as a strong nonlinear optical responsive material with gross response changes based on small changes in IR excitation.