Multipolar second-order nonlinear optical (NLO) materials have become a topic of intense interest owing to the finding that donor-acceptor-substituted molecules with threefold rotational symmetry (octupolar chromophores) can have an improved transparency/optical nonlinearity tradeoff when compared with traditional dipolar chromophores. [1] While octupolar nonlinearity has been achieved at the molecular level, [2, 3] its demonstration in multidimensional crystalline systems remains challenging. [3][4][5] Studies on the assembly of metal-organic frameworks (MOFs) from molecular building blocks, on the other hand, have uncovered methods to build extended structures with novel topologies and exploitable functions. [4][5][6][7] In principle, octupoles can be expressed in cubic, tetrahedral, and trigonal symmetries. [3] Trigonal molecules or bridging ligands are thus very useful building blocks for the construction of higher-dimensional octupolar NLO-active MOF materials, but the products normally do not adopt acentric trigonal structures or have weak NLO responses. [7] We recently prepared serendipitously an organically templated 3D octupolar MOF exhibiting very strong and cation-dependent second harmonic generation (SHG) responses. [5c] We have now developed a strategy for the rational synthesis of 3D octupolar materials by carrying over both threefold symmetry and chirality of organic bridging ligands into MOFs through coordination assembly. To this end, we synthesized rigid trigonal tris(4-pyridylduryl)borane (L) featuring a three-coordinate boron chromophore. [8] The tridurylborane core adopts a propellerlike conformation to minimize repulsive interactions between its phenyl rings, and therefore it is helically chiral (D and L isomers) with D 3 octupolar symmetry as shown in Scheme 1. The vacant and low-lying 2p p orbital on the boron center makes threecoordinate organoboron derivatives good electron acceptors that have versatile potential applications in organic lightemitting diodes and nonlinear optics. [8] Herein we present the assembly of chiral octupolar metal-organoboron frameworks with exceptional (14,3) topology [9] which display significant and, more interestingly, anion-tunable SHG responses.
Racemic L was synthesized in 70 % yield by Suzuki coupling between 4-pyridylboronic acid and tris(iododuryl)borane, which was obtained in four steps in good overall yield from readily available 1,2,4,5-tetramethylbenzene (Scheme 1). Ligand L was characterized by 1 H and 13 C NMR spectroscopy and ESI mass spectrometry. Single crystals of eight compounds with the general formula [MX 2 L]•G [M = Cd, X = Cl, Br, I, NO 3 , OAc, ClO 4 (1 a-1 f), MX 2 = CuCl 2 (2), CoCl 2 (3); G = 2 H 2 O for 1 and 2 and 3/ 2 CH 3 OH•H 2 O for 3] were readily obtained in good yields by heating a mixture of MX 2 •n H 2 O and L in dimethylsulfoxide Scheme 1. a) Synthesis of L. b) Two isomers of L and their space-filling depictions.