Metal-ion sensing fluorophores with large two-photon absorption cross sections: Aza-crown ether substituted donor-acceptor-donor distyrylbenzenes

Chromophores based on a donor-acceptor-donor structure possessing a large two-photon absorption cross section and one or two mono-aza-15-crown-5 ether moieties, which can bind metal cations, have been synthesized. The influence of Mg²⁺ binding on their one- and two-photon spectroscopic properties has been investigated. Upon binding, the two-photon action cross sections at 810 nm decrease by a factor of up to 50 at high Mg²⁺ concentrations and this results in a large contrast in the two-photon excited fluorescence signal between the bound and unbound forms, for excitation in the range of 730 to 860 nm. Experimental and computational results indicate that there is a significant reduction of the electron donating strength of the aza-crown nitrogen atom(s) upon metal ion binding and that this leads to a blue shift in the position as well as a reduction in the strength of the lowest-energy two-photon absorption band. The molecules reported here can serve as models for the design of improved two-photon excitable metal-ion sensing fluorophores.