Multi-functional supramolecular building blocks with hydroxy piperidino groups: new opportunities for developing nonlinear optical ionic crystals†
We report on multi-functional supramolecular building blocks for obtaining a non-centrosymmetric crystal structure, which is the essential requirement for achieving a macroscopic second-order nonlinear optical activity. The supramolecular building blocks PM (piperidin-4-ylmethanol) and PO (piperidin-4-ol) consist of a piperidino group acting as a strong electron-donating group and a hydroxyl group acting as a strong hydrogen-bonding group, which also may act as a ‘site-isolation’ group. The quinolinium crystals PMQ-T and POQ-T that introduce PM and PO blocks, respectively, exhibit a non-centrosymmetric crystal structure with nonlinear optical activity, while analogous quinolinium DAQ-T crystal with the widely used N,N-dimethylamino (DA) group exhibits a centrosymmetric crystal structure without nonlinear optical activity. Compared to the DAQ-T crystal, the PMQ-T and POQ-T crystals exhibit a lower tendency to form a hydrate phase in the crystalline state. The PMQ-T crystal exhibits a higher molecular optical nonlinearity in the crystalline state than the DAQ-T crystal, which is attributed to the ‘site-isolation’ effect resulting from the bulky PM block. In addition, the PMQ-T crystal exhibits large macroscopic optical nonlinearities with more than 3 times higher off-diagonal effective hyperpolarizability than that of benchmark stilbazolium crystals. Therefore, multi-functional supramolecular building blocks PM and PO can give new opportunities for developing non-centrosymmetric ionic crystals for nonlinear optical applications.