Preparation, characterization and third order optical nonlinearities of looped covalently bonded Anderson-type polyoxometalate-porphyrin hybrids

Abstract

Recently, combination of porphyrin and polyoxometalates in various interaction modes proved to be the basis of opening new avenues in the field of optical materials. Herein, two new looped covalently bonded Anderson-type polyoxometalate-porphyrin hybrids, having molecular formulae as [(C4H9)4N]6[{C42H26N6(CO)2(NH)2C2(OCH2)6(MnMo6O18)2{(OCH2)6C2(NH)2(CO)2N6H26C42}]⋅2CH3CN (denoted Di-NTPP@Di-POM), [(C4H9)4N]6[{C44H28N4(CO)2(NH)2C2(OCH2)6}(MnMo6O18)2{(OCH2)6C2(NH)2(CO)2N4H28C44}] (denoted Di-TPP@Di-POM), were synthesized by the reaction of [N(C4H9)4]4[α-Mo8O26], Mn(OAc)3∙2H2O with two porphyrins 4,4'-(10,15-di(pyridin-4-yl)porphyrin-5,20-diyl)bis(N-(1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl)benzamide) (denoted DiTris@NTPP) and 4,4'-(10,15-diphenylporphyrin-5,20-diyl)bis(N-(1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl)benzamide) (denoted DiTris@TPP), respectively. The hybrids were clearly characterized by spectroscopic (NMR, 2D-NOESY NMR, FT-IR, UV–visible, fluorescence) techniques, elemental analyses and electron spray ionization mass spectrometry (ESI-MS) techniques. Prominently, remarkable enhancement of the non-linearity of the hybrid Di-TPP@Di-POM (consisting of two POM moieties and two phenyl-containing porphyrin moieties) over its corresponding porphyrin precursor was observed under laser irradiation indicating that integration of electron-accepting POM moiety and electron-donating Di-TPP moiety can significantly enhance the NLO response of the porphyrins, while Di-NTPP@Di-POM (consisting of two POM moieties and two pyridyl-containing porphyrin moieties) has larger molecular second hyperpolarizability than DiTris@NTPP, the contribution per porphyrin moiety and POM moiety in Di-NTPP@Di-POM is slightly smaller than that in DiTris@NTPP due to the offset effect in a certain degree because both the POM moieties and the pyridyl groups have electron-withdrawing characters. It is also found that the HOMO-LUMO gap (Eg) values of the hybrids that were obtained according to the absorption edges of UV–vis. spectra decreased in an order of Eg(DiTris@TPP) > Eg(DiTris@NTPP) > Eg(Di-TPP@Di-POM) > Eg(Di-NTPP@Di-POM), whilst, the molecular second hyperpolarizability γ increased as follows γ(DiTris@TPP) ˂ γ(DiTris@NTPP) ˂ γ(Di-TPP@Di-POM) ˂ γ(Di-NTPP@Di-POM). Meantime, it is also found that fluorescence intensity was decreased by 90%, 85% for the hybrids Di-NTPP@Di-POM, Di-TPP@Di-POM, respectively, as compared to their porphyrin precursor molecules DiTris@NTPP and DiTris@TPP, respectively, due to energy and/or charge transfer from porphyrin moieties to Anderson type polyoxometalate moieties.