Novel two-photon-absorbing, 1,10-phenanthroline-containing π-conjugated chromophores and their nickel(ii) chelated complexes with quenched emissions

Abstract

Novel 1,10-phenanthroline-containing π-conjugated chromophores with varied electron donors were synthesized and characterized together with their corresponding nickel(II) chelated complexes. The linear absorption maxima for these chromophores could be tuned from 357 to 415 nm by using different electron donors. The 1,10-phenanthroline-containing π-conjugated chromophores exhibit strong fluorescence in solutions upon one- or two-photon excitation. The fluorescence of these chromophores can be quenched or partially quenched by some metal ions due to an excellent metal ion chelating ability of the 1,10-phenanthroline backbone. The nickel(II) chelating complexes show quenched emission, both in dilute and concentrated solutions, compared to their metal ion-free chromophores. Their two-photon absorption (TPA) spectra are obtained using a newly developed multi-photon spectroscopic technique which utilizes a single strong femtosecond white-light continuum generation beam. The two-photon absorption spectra show that the TPA band for these compounds can be tuned by choosing different electron donors. The maximum TPA cross-section σ₂ values for the three metal ion-free chromophores are variable from 0.58 to 2.26 × 10⁻²⁰ cm⁴ GW⁻¹ (±15%). The metal ion-free chromophore-containing dihexylamino groups (9c) has a larger TPA cross-section σ₂ value compared to the chromophores containing alkyloxy groups or alkylthio groups. When the metal ion-free chromophores coordinate with the nickel(II) ion, the resulting complexes do not lose their excellent two-photon-absorbing ability. The nickel(II) chelated complexes display red-shifted two-photon absorption bands compared with their metal ion-free chromophores.