Fluorene-based metal-ion sensing probe with high sensitivity to Zn ²⁺ and efficient two-photon absorption

The photophysical, photochemical, two-photon absorption (2PA) and metal ion sensing properties of a new fluorene derivative (E)-1-(7-(4-(benzo[d]thiazol-2- yl)styryl)-9,9-bis(2-(2-ethoxyethoxy)ethyl)-9H-fluoren-2-yl)-3-(2-(9,10,16,17, 18,19,21,22,23,24-decahydro-6H dibenzo[h,s][1,4,7,11,14,17] trioxatriazacycloicosin-20(7H)-yl)ethyl)thiourea (1) were investigated in organic and aqueous media. High sensitivity and selectivity of 1 to Zn ²⁺ in tetrahydrofuran and a water/acetonitrile mixture were shown by both absorption and fluorescence titration. The observed complexation processes corresponded to 1:1 stoichiometry with the range of binding constants ∼(2-3) - 10⁵ M^-1. The degenerate 2PA spectra of 1 and 1/Zn ²⁺ complex were obtained in the 640-900 nm spectral range with the maximum values of two-photon action cross section for ligand/metal complex ∼(90-130) GM, using a standard two-photon induced fluorescence methodology under femtosecond excitation. The nature of the 2PA bands was analyzed by quantum chemical methods and a specific dependence on metal ion binding processes was shown. Ratiometric fluorescence detection (420/650 nm) provided a good dynamic range (10^-4 to 10^-6 M) for detecting Zn ²⁺, which along with the good photostability and 2PA properties of probe 1 makes it a good candidate in two-photon fluorescence microscopy imaging and sensing of Zn ions.