Two‐Photon Absorption Switches Based on Protonation of Pyrimidine Derivatives

Abstract The photophysical and two‐photon absorption (TPA) properties of five push‐pull pyrimidine derivatives were studied upon protonation with trifluoroacetic acid (TFA). The pyrimidine heterocycles have been used as the protonation sites and electron acceptors while methoxy groups were employed as electron donors. Steady state and fluorescence time‐resolved spectroscopy from femtoseconds to nanoseconds have been used together with the two‐photon excited fluorescence method. Protonation with TFA resulted in red‐shifted absorption and fluorescence spectra as well as to an acceleration of the excited state dynamics. The TPA properties showed a remarkable enhancement upon protonation with a ten‐times increase of the TPA cross sections due to an increased intramolecular charge transfer. Our results demonstrate the potential of pyrimidine chromophores to be used as protonation tunable non‐linear optical chromophores.