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 Mn2I6 (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 (18crown-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.

Automated summary

A series of transition metal crown-ether coordination compounds were synthesized using ionic liquids, showing both conventional and unusual structural motifs. Some of these compounds exhibit outstanding photoluminescence properties, with high quantum yields observed for compounds like ZnI2, Mn2I4 (18-crown-6)(2), EuBr2, and EuI2. The optical properties and structural features of these compounds were characterized through various analytical techniques, revealing unique and efficient luminescent behavior.