A-π-D-π-A pyridinium salts: synthesis, crystal structures, two-photon absorption properties and application to biological imaging

Three different pyridinium salts with I−, BPh4− or [Eu(TTA)4]− anions were synthesized and fully characterized. The crystal structures and 1H-NMR spectra of the dipyridinium cations showed that the coplanarity of the cation was controlled by the type of anion. Their one- and two-photon absorption properties were studied in combination with their structure. The two-photon absorption properties were shown to be tunable by the counter-anions. A pyridinium salt containing L[Eu(TTA)4]2 was selected as a marker for use in fluorescence microscopy to image living cells. This salt clearly displayed the cell structure and was used successfully in two-photon excited fluorescence microscopy.