Abstract
We report the synthesis and excellent two‐photon‐sensitized luminescence properties of a new complex [Eu(tta)~3~dmbpt] (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 δ × Φ~F~: 85 GM at 812 nm and 56 GM at 842 nm; 1 GM = 10^–50^ cm^4^ s photon^–1^ molecule^–1^). Compared to a previously reported [Eu(tta)~3~dpbt] complex, (dpbt = 2‐(N,N‐diethylanilin‐4‐yl)‐4,6‐bis(3,5‐dimethylpyrazol‐1‐yl)‐1,3,5‐triazine), [Eu(tta)~3~dmbpt] 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(CH~2~–)~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 S~0~ and the S~1~ states, which is partly responsible for the high capability of two‐photon‐sensitized luminescence of [Eu(tta)~3~dmbpt]. 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)~3~dmbpt] compared to [Eu(tta)~3~dpbt] are also explained according to the calculated results and twisted structure.