Nonlinear Optical Crystals Designed with 4-Nitrophenolate Chromophores: An Engineering Route using a Multidipolar Chromophore, 3-Hydroxy-2,4,6-Trinitrophenolate

4-Nitrophenol derivatives are interesting chromophores for nonlinear optics (NLO), being typical donor-acceptor system connected by π conjugation. Due to the presence of the acidic phenolic proton, they also readily form salts with suitably selected organic and metallic bases. The so-formed 4-nitrophenolates have increased molecular hyperpolarizabilities (β). The phenolate and nitro oxygens are strong hydrogen bond acceptors and the nitro group has a tendency to coordinate with metal centers. This opens up two different engineering routes to build NLO materials from these chromophores: (a) organic-organic and (b) metal-organic salts. This paper reviews the crystal packing and nonlinear optical (NLO) efficiency in nitrophenolate crystals engineered through these routes and presents the crystal structure of a new NLO efficient organic-organic salt, 1-hydroxy-4-methylpyridinium 3-hydroxy-2,4,6-trinitrophenolate (P21). In this crystal both the multidipolar anion and dipolar cation are NLO chromophores. The anions form herringbone mediated chain with intra and intermolecular O-H…O hydrogen bonds and the cations are attached to them through strong O-H…O− hydrogen bonds.