18-Crown‑6 Coordinated Metal Halides with Bright Luminescence and Nonlinear Optical Effects

The crown-ether coordination compounds
ZnX2(18-crown-6), EuX2(18-crown-6)
(X: Cl, Br, I), MnI2(18-crown-6), Mn3Cl6(18-crown-6)2, Mn3I6(18-crown-6)2, and Mn2I4(18-crown-6)
are obtained by ionic-liquid-based synthesis. Whereas MX2(18-crown-6) (M: Zn, Eu) show conventional
structural motives, Mn3Cl6(18-crown-6)2, Mn3I6(18-crown-6)2, and Mn2I4(18-crown-6) exhibit unusual single MnX4 tetrahedra coordinated to the crown-ether
complex. Surprisingly, some compounds show outstanding photoluminescence.
Thus, rare Zn2+-based luminescence is observed and unexpectedly
efficient for ZnI2(18-crown-6) with a quantum yield of
54%. Unprecedented quantum yields are also observed for Mn3I6(18-crown-6)2, EuBr2(18-crown-6),
and EuI2(18-crown-6) with values of 98, 72, and 82%, respectively,
which can be rationalized based on the specific structural features.
Most remarkable, however, is Mn2I4(18-crown-6).
Its specific structural features with finite sensitizer–activator
couples result in an extremely strong emission with an outstanding
quantum yield of 100%. Consistent with its structural features, moreover,
anisotropic angle-dependent emission under polarized light and nonlinear
optical (NLO) effects occur, including second-harmonic generation
(SHG). The title compounds and their optical properties are characterized
by single-crystal structure analysis, X-ray powder diffraction, chemical
analysis, density functional theory (DFT) calculations, and advanced
spectroscopic methods.