Chiral Octupolar Metal–Organoboron NLO Frameworks with (14,3) Topology

Abstract

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

The Synthesis and One‐ and Two‐Photon Optical Properties of Dipolar, Quadrupolar and Octupolar Donor–Acceptor Molecules Containing Dimesitylboryl Groups

“…[17] Several molecules containing B(Mes) 2 groups have been shown to exhibit large two-photon absorption (TPA) cross-sections up to 1340 GM. [18,19] TPA is defined as the electronic excitation of a molecule induced by simultaneous absorption of two photons.…”

Multidimensional potential of boron-containing molecules in functional materials

“…Starting from [B 12 R 12 ] •-, the formal replacement of one BR by an equivalent [CR] + function not only removes the symmetry and thus the orbital degeneracy, it also changes the redox potential and leaves a neutral radical species such as [CB 11 Me 12 ] •-(g = 2⋅0037) as described by Michl and coworkers. 64 Compound [CB 11 Me 12 ] •-has been discussed in connection with its synthetic potential as an oxidant, 64 whereas related anion radical species [1,2-Ph 2 -1,2-C 2 B 10 H 10 ] •-and [1,2-(CH 2 ) 3 -1,2-C 2 B 11 H 11 ] •-have been reported recently.…”

“…64 Compound [CB 11 Me 12 ] •-has been discussed in connection with its synthetic potential as an oxidant, 64 whereas related anion radical species [1,2-Ph 2 -1,2-C 2 B 10 H 10 ] •-and [1,2-(CH 2 ) 3 -1,2-C 2 B 11 H 11 ] •-have been reported recently. 65,66 Higher symmetry was established EPR spectroscopically for the [C 4 B 8 R 4 H 8 ] •-radical ion (R = SiMe 3 ) 67 which was obtained by reduction of the cuboctahedral precursor.…”

“…in the use of boryl groups for optoelectronic effects (sensing, NLO behaviour). 10,11 The accommodation of one extra unpaired electron, e.g. through oneelectron reduction or oxidation of suitable precursors, is also a viable option, leading to spin-bearing and thus magnetically interesting systems.…”

“…through oneelectron reduction or oxidation of suitable precursors, is also a viable option, leading to spin-bearing and thus magnetically interesting systems. 2,12 Following the electron acceptor capability of boron as apparent from the electronic 10,11,13 and elec- Scheme 1. trochemical 14 substituent effects of boryl substituents we start here by presenting selected examples of corresponding spin-carrying organoboron systems. 2,12 Considering the characteristical cluster chemistry of oligoboranes, carboranes, and metallacaboranes with their electron number rules 15 and their enormous potential in Boron Neutron Capture Therapy (BNCT) of malignant tumours e.g.…”