Insight into the Nonlinear Absorbance of Two Related Series of Two-Photon Absorbing Chromophores

Abstract : A comprehensive photophysical study has been carried out on two series of two-photon absorbing dyes. Looking at a series of structurally related chromophores provides insight into what controls the linear and nonlinear photophysical properties. These materials consist of an electron withdrawing benzothiazole group connected to a diphenylamine via a fluorene group. Within the series there are one-arm (dipolar), two-arm (quadrupolar), and three-arm (octupolar) versions of each dye. The one photon properties were studied using steady-state absorption, steady-state and time resolved emission, femtosecond transient absorption, and nanosecond transient absorption. The two-photon properties were studied using femtosecond Z-scan and nanosecond nonlinear transmittance. On the basis of a two photon assisted excited state absorption model we show that the excited state absorption from both the singlet and triplet excited states contributes to the overall nanosecond nonlinear absorption in these materials. In this study we determined that increasing the number of branches from one to three on the chromophore results in an increase in the overall nanosecond nonlinear absorption due to an increase in the excited state absorption.