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, ES...