Studies of Two-Photon Property of Intensely Luminescent Alkynyl−Phosphine Gold(I)−Copper(I) Complexes

Two-photon absorption (TPA) spectra of a series of highly symmetric, strong phosphorescence Au(I)-Cu(I) complexes 1-7 bearing alkynyl-phosphine ligands in solution were obtained over the biological windows (760-840 nm) by the two-photon-induced emission technique under femtosecond laser excitation. The resulting TPA cross sections are further confirmed by the open-aperture Z-scan method, which offers the advantage of reducing the excited-state reabsorption for these long-lived phosphorescence dyes. The results of these two methods are mutually in good agreement, showing that complexes 1-7 exhibit moderately strong TPA cross section, for example, approximately 710 GM at 760 nm for 1 in CH2Cl2. Owing to the highly centrosymmetric property, the resulting trend in TPA values for 1-7 can be rationalized by the unit of X-group-alkynyl-heterometallic-core-alkynyl-X-group fragment, categorized as A-pi-D-pi-A or D-pi-A-pi-D structural motif, which renders large changes in polarization upon excitation and provides significant near-IR TPA strength.