A theoretical study on the efficient reversible redox-based switching of the second-order polarizabilities of two-dimensional nonlinear optical-active donor-acceptor phenanthroline-hexamolybdate.

The Lambda-shaped phenanthroline-hexamolybdate compounds that are based on the reversible Mo-centered redox process were investigated. The attachment of hexamolybdate terminals to phenanthroline by a pi-conjugated phenylamine bridge generated the organic ligand-centered or Ni-centered HOMO and the transition metal Mo-centered LUMO. The population in HOMO and LUMO predicted the reversible Mo(VI/V) redox process and the ligand-to-metal charge transfer (LMCT) to a polyanion acceptor, which consequently evoked a significant second-order nonlinear optical (NLO) response. Moreover, the electron transition of these compounds exhibited a large beta(zyy) tensor along the y-axis, which confirms a promising two-dimensional (2D) character with sizable anisotropy values. Interestingly, the addition of electrons into the high-valence Mo atom in the hexamolybdate acceptor evoked dramatic enhancements in the NLO response for the reduction states in contrast to the response of the corresponding oxidation states. The reduction states in system I exhibited second-order NLO responses about 200 times larger than the oxidation states. In addition, the attachment of the Ni atom in compound IIa(red) enhanced the NLO response to nearly 1019 times greater than the response of the corresponding oxidation state compound IIa. The Ni atom as the electron donor plays an important role in the major electron transition for the reduction states in system II. Therefore, the NLO response of such compounds can be reversibly switched through the transition metal Mo(VI/V) redox that is effectively coupled with the LMCT transition. Thus, the NLO activity can be controlled by a one-electron redox process, and the redox-active phenanthroline-hexamolybdate compounds are promising candidates for 2D redox-switching NLO materials in novel optoelectronic applications.