A Europium Complex With Excellent Two-Photon-Sensitized Luminescence Properties

We report the synthesis and excellent two-photon-sensitized luminescence properties of a new complex [Eu(tta)3dmbpt] (tta = henoyltrifluoroacetonate; dmbpt = 2-(N,N-diethyl-2,6-dimethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine) that exhibits the highest efficiency of lanthanide luminescence when excited by near-infrared (NIR) laser pulses (action cross section of two-photon-excited fluorescence d ×UF: 85 GM at 812 nm and 56 GM at 842 nm; 1 GM= 10–50 cm4 s photon–1 molecule –1). Compared to a previously reported [Eu(tta)3dpbt] complex, (dpbt = 2-(N,N-diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol- 1-yl)-1,3,5-triazine), [Eu(tta)3dmbpt] has two excess methyl groups at the 2,6-positions of the phenyl ring. Crystallographic data of dmbpt show that the 2,6-dimethyl substitutes bring about a significant twist in the conformation of the diethylamino group compared to that in dpbt, which severely influences the conjugation in the ground state between the electron lone pair of N in the –N(CH2–)2 moiety and the aromatic electron system in dmbpt. The large two-photon absorption (TPA) cross section of dmbpt is mainly derived from its large static dipole moment difference between the S0 and the S1 states, which is partly responsible for the high capability of two-photon-sensitized luminescence of [Eu(tta)3dmbpt]. The broader two- and single-photon excitation windows and the superior two-photon-sensitized luminescent properties in the long-wavelength NIR region of [Eu(tta)3dmbpt] compared to [Eu(tta)3dpbt] are also explained according to the calculated results and twisted structure.