Understanding the One-Photon Photophysical Properties of a Two-Photon Absorbing Chromophore

Abstract

A comprehensive one-photon photophysical study has been carried out on AF455, a known two-photon absorbing dye. AF455 is composed of an electron-accepting center with three arms that consist of a π-conjugated group with an electron-donating group at the terminal end of each arm. The objective of this work is to understand the one-photon excitation photophysical properties so that this knowledge will be carried into understanding the two-photon absorption properties. This was done by utilizing steady-state absorption, steady-state and time-resolved emission, femtosecond pump−probe, and nanosecond laser flash photolysis techniques. Through this study it was determined that AF455 undergoes an excited-state intramolecular charge transfer (ICT) upon absorption of a single photon. The extent of ICT stabilization is dependent on the solvent polarity, with increasing ICT stabilization in more polar solvents. The formation of the triplet excited state is small (<8%) and is effected only slightly by a change in the solvent polarity. With an increase in polarity the fluorescence quantum yield decreased and internal conversion was found to become more competitive. On the basis of a two-photon assisted excited-state absorption model we tied in the measured one-photon photophysical properties to understand why the nanosecond effective two-photon absorption cross section is much larger than the femtosecond intrinsic two-photon absorption cross section. We found that AF455 exhibits triplet excited-state absorption in the region of 800 nm that leads to enhancement of the nanosecond effective two-photon absorption cross section.