1. Crystal Engineering of Polar Solids as Second-Order Nonlinear Optical Materials.

By taking advantage of the ability of metal-ligand coordination in counteracting unfavorable centric interactions (e.g., dipole-dipole repulsions), our group has explored the rational synthesis of polar solids based on polymeric metal-organic coordination networks. The simplification of acentricity control on the coordination networks via “dimensional reduction” is the central theme of our supramolecular engineering approach towards polar solids. We have rationally constructed polar polymeric coordination networks containing NLO-active chromophoric building blocks based on three persistent structural motifs, i.e., a 3-D diamondoid network; a 2-D grid structure; and a 2-D network composed of “basic” trinuclear carboxylates with 3-fold rotational symmetry. In all of the three approaches, controlling the acentricity of polymeric coordination networks has been reduced from a very difficult 3-D problem to either (i) a choice of the ligands of suitable lengths or (ii) a much simpler 1-D problem.

For example, when unsymmetrical bridging ligands such as p-pyridinecarboxylates are used to link divalent d¹⁰ metal centers (for transparency consideration), each metal center can coordinate to two pyridyl nitrogen atoms and two carboxylate groups to have a maximum C_2V symmetry, and therefore will not possess a center of symmetry (Scheme I). Owing to the pseudo-tetrahedral connectivity of the metal centers, they can act as connecting points for a diamondoid network. The unsymmetrical nature of the bridging ligands ensures that each diamond net is acentric. However, due to the large metal-metal separations in these coordination networks, interpenetration of independent diamond nets is unavoidable (Fig. 1). Such interpenetration presents a potential problem for the crystal engineering of acentric solids because two diamond nets can be related by a center of symmetry. To overcome this obstacle, rigid bifunctional linking ligands of various lengths have been prepared and used to construct diamondoid coordination networks with various degrees of interpenetration (defined as the number of independent diamond nets). This work led to an understanding of the correlation between the degree of interpenetration and ligand length (Fig 2). More importantly, because a center of symmetry can only (but not necessarily) be introduced in diamondoid networks with an even-number-fold interpenetration, ultimate acentricity control can be exerted on these metal-organic diamondoid networks by choosing the ligands of appropriate length. The electronic asymmetry of p-pyridinecarboxylate bridging ligands provides the molecular basis for the bulk second-order optical nonlinearity (c²), and some of these solids possess c² comparable to that of technologically important lithium niobate.

We have also successfully synthesized a variety of NLO-active acentric coordination networks with 2-D grids and octupolar building motifs (Fig. 3). Some of these solids exhibit powder SHG efficiency higher than LiNbO₃. Our recent article in Acc. Chem. Res. (2002, 35, 511-522) highlights some of these results.

Representative Publications:

1. “Nonlinear Optically Active Zinc and Cadmium p-Pyridinecarboxylate Coordination Networks.” Ayyappan, P.; Sirokman, G.; Evans, O.R.; Warren, T.H.; Lin, W. submitted to Inorg. Chem.

2. Nonlinear Optically Active Polymeric Coordination Networks Based on Metal m-Pyridylphosphonates.” Ayyappan, P.; Evans, O.R.; Cui, Y.; Wheeler, K.A.; Lin, W. Inorg. Chem. 2002, 41, 4978-4980 [pdf].

3. “Crystal Engineering of NLO Materials Based on Metal-Organic Coordination Networks.” Evans, O.R.; Lin, W. Acc. Chem. Res. 2002, 35, 511-522 [pdf].

4. “New Open Frameworks Based on Metal Pyridylphosphonates.” Ayyappan, P.; Evans, O.R.; Foxman, B.M.; Wheeler, K.A.; Warren, T.H.; Lin, W. Inorg. Chem. 2001, 40, 5954-.5961 [pdf].

5. “Crystal Engineering of NLO Materials Based on Interpenetrated Diamondoid Coordination Networks.” Evans, O.R.; Lin, W. Chem. Mater. 2001, 13, 2705 [pdf].

6. “Rational Design of NLO Materials Based on 2D Coordination Networks.” Evans, O.R.; Lin, W. Chem. Mater. 2001, 13, 3009-3017 [pdf].

7. “Self-Assembled Multilayer Films: Second-Order Nonlinear Optical Applications.” Lin, W.; Evans, O.R. in Encyclopedia of Materials Science and Technology, Pergamon, Amsterdam, 2001.

8. “NLO-Active Zinc(II) and Cadmium(II) Coordination Networks with 8-fold Diamondoid Structures.” Lin, W.; Ma, L.; Evans, O.R. Chem. Commun., 2000, 2263-2264 [pdf].

9. “Towards Rational Synthesis of Polar Solids. Synthesis and X-ray Structures of Cadmium(II) meta-Pyridinecarboxylate Coordination Polymers." Evans, O.R.; Lin, W. J. Chem. Soc., Dalton Trans. 2000, 3949-3954 [pdf].

10. “A Novel Octupolar Metal-Organic NLO Material Based on a Chiral 2D Coordination Network”. Lin, W.; Wang, Z; Ma, L. J. Am. Chem. Soc. 1999,121,11249-11250 [pdf].

11. “Crystal Engineering of Acentric Diamondoid Metal-Organic Coordination Networks " Evans, O.R.; Xiong, R.-G.; Wang, Z.; Wong, G.K.; Lin, W. Angew. Chem. Int. Ed. Engl., 1999, 38, 536-538 [pdf].

12.“Supramolecular Engineering of Chiral and Acentric 2D Networks. Synthesis, Structures, and Second-Order Nonlinear Optical Properties of Bis(nicotinato)zinc and Bis{3-[2-(4-pyridyl)ethenyl]benzoato}cadmium.” Lin, W; Evans, O.R.; Xiong, R.-G.; Wang, Z. J. Am. Chem. Soc., 1998, 120, 13272-13273 [pdf].